Review of Energy-Efficient Pneumatic Propulsion Systems in Vehicle Applications

Abstract

This review comprehensively presents the development of energy-efficient pneumatic propulsion systems (PPSs) in road vehicle applications, which are classified as green vehicles. The advantages and disadvantages of PPSs were presented, and PPSs were compared with combustion propulsion systems (CPSs) and electric propulsion systems (EPSs), as well as their power-to-weight ratios (PWRs), energy densities, and CO₂ emissions. The review of compressed air vehicles (CAVs) focuses on their historical development and future prospects. This review discusses the use of PPSs with compressed air engines (CAEs) as an alternative propulsion system in green vehicles, providing a simple, energy-saving, and environmentally friendly solution. This review also discusses hybrid air propulsion, which, when combined with internal combustion engines (ICEs) or electric motors (EMs), offers innovative energy-efficient propulsion systems that are more economical than conventional hybrid propulsion systems. This review focuses on recent advances in lightweight air vehicles that improve vehicle handling, increase efficiency, and reduce propulsion energy consumption. Discussion of the study results concerns the use of PPSs in a three-wheeled rehabilitation tricycle (RTB). A comprehensive computation model of the RTB was presented, and the key performance parameters crucial to its operation were analyzed. The results of the RTB simulation were verified through field tests.

1. Introduction

Transportation and new green and sustainable propulsion systems are crucial components of the EU’s energy and climate strategy. The Green Cars Initiative, which is part of the European Economic Recovery Plan endorsed by the European Council, focuses on promoting the creation of new and sustainable transportation propulsion systems. The European Commission is currently supporting the development of alternative fuels and propulsion technologies [1]. This initiative includes vehicle propulsion systems that include biofuels, in liquid or gas form, hydrogen and fuel cell technologies, electric vehicles (EVs), hybrid vehicles (HVs), and other alternative vehicle propulsion systems. To align with the EU goal of achieving climate neutrality by 2050 by reducing carbon dioxide emissions and adopting more environmentally friendly transport options and propulsion systems in green vehicles, there is a plan to gradually eliminate combustion engine vehicles. Starting in 2035, the registration of new passenger cars and vans with combustion engines will be prohibited, although there will be certain exceptions for vehicles that run on synthetic or hydrogen fuels. Green propulsion systems (GPSs) refer to propulsion technologies designed to minimize environmental impact, typically using less harmful propellants or alternative energy sources [2]. They represent an expanding field of research and development, especially in the automotive sector, as traditional combustion engines pose safety and environmental risks. Green vehicles using compressed air are the best option that provides the most comprehensive solution to the current urban pollution problems in a simple, economical, and inoffensive way [3]. They are environmentally friendly, easy to drive, and relatively inexpensive to exploit and maintain.

Propulsion is a term used to describe the action or process of applying force in a way that causes vehicles to change motion. This is derived from the Latin word “propellere,” where pro means before and “pellere” means drive. It is used to propel various modes of transportation, including vehicles, airplanes, and maritime vessels, and others. Propulsion systems are made up of two parts: a mechanical power source and a propeller that transforms the power into a propulsion force. The power of a combustion vehicle is generated by burning gasoline, and the propulsion system consists of internal combustion engines (ICE), a drivetrain, and wheels. Electric vehicles (EVs) are powered by batteries, with an electric motor, a drivetrain, and wheels being its propulsion system. Pneumatic propulsion systems (PPSs) use compressed air to generate thrust and have various applications in transportation and automobiles. A PPS is a cleaner alternative to other propulsion methods, particularly in road transport, such as vehicles and in cars, and maritime transport, such as ferries and vessels. In [4], the review of the literature presents the advantages and challenges of PPSs in vehicles, highlighting its potential as a sustainable transportation solution while acknowledging limitations such as range restrictions, safety concerns, and infrastructure requirements. The simplicity of PPS design and construction leads to increased reliability and reduced maintenance needs. PPSs can provide instant power and acceleration, making them suitable for applications that require rapid bursts of speed or force. APPS is well-suited for applications that require cost-effectiveness, safety, and environmental friendliness, particularly when air is readily available. It is crucial to investigate PPSs due to their use of clean, renewable energy, energy storage, and rapid energy transfer capabilities. PPSs are environmentally friendly, cost-effective, affordable, easy to construct, safe, and compatible with compressed air systems (CASs). They can also use renewable energy sources (RES). Additionally, the materials used to make PPSs can be almost entirely environmentally friendly and recyclable.

Advantages of PPSs:

Energy expanders: In PPS, various expanders can be used to transform compressed air energy into mechanical propulsion energy, such as air motors (AMs), pneumatic cylinders (PVs), air turbines (ATs), and CAEs. Air expanders are inexpensive in terms of cost, maintenance, and operation.

Cost-effective: Compressed air is often an inexpensive and readily available energy source, especially when air can be sourced from local compressors.

Environmentally friendly: Compressed air is a cleaner alternative to fossil fuels with minimal emissions when renewable energy sources are used for air compression.

Potential hybridization: Compressed air can be used in conjunction with other power sources, such as combustion, hydraulic, and electric.

Environments safety: PPSs are often safer in hazardous environments because of the non-flammable and non-explosive nature of compressed air.

Compact and powerful: PPSs can be designed to be relatively compact and powerful for their sizes.

Disadvantages of PPSs:

Energy density: Compressed air has a lower energy density than other energy sources, such as gasoline or electricity, limiting the range and power output of PPSs.

Efficiency: The overall efficiency of a PPS that uses compressed air energy storage is indeed quite low, typically around 7%. The widespread adoption of PPS technology faces a significant challenge because of its low efficiency.

Propulsion start-up: One of the significant disadvantages of PPS is the increased loads on the crank mechanism during propulsion start-up.

Thermodynamic processes: PPSs operate under the conditions of thermodynamic processes because air heats up when compressed in tanks and cools down after expansion in expanders. Both compression and expansion must proceed near the isothermal limit. This can only be done with multistage compression and expansion processes with heat exchangers.

Recharging time: The loading of compressed air tanks (CATs) to high pressure can be problematic due to the limited availability of high-pressure compressors, potentially disrupting the operations of the PPS.

Noise: Although air expanders can be noisy, noise levels can be mitigated with proper design.

Air humidity: The need for complete dehydration of compressed air, if humidity subsists in compressed air, causes the pneumatic valve and expanders to stop due to inter-icing.

Tank safety: Pressure tanks in road transport must adhere to strict standards and regulations, which are governed primarily by the National Transportation Technical Supervision (in Polish TDT), and meet the requirements of the EU Pressure Equipment Directive (PED) [5].

PPSs in the automotive segment can be used in urban vehicles, especially for short and predictable routes, hybrid vehicles to improve fuel efficiency and reduce emissions, and light vehicles such as scooters, motorcycles, quadricycles, three-wheelers, and two-wheelers for utility, recreational, rehabilitation, and other applications. In [6], a technical potential assessment of PPSs as an alternative to conventional vehicle propulsion technologies is presented. Although air power is limited by operating pressure and storage volume, its use in the application of land and maritime vehicles can be justified if the operation of the PPS is limited to a short-term and fixed travel distance. The travel range of CAVs provides predictability of important operational parameters, such as CAT sizes, thermal and mechanical efficiency, operating pressures, and propel mechanism.

This review addresses gaps in the current literature by presenting the current status and prospects for the development of the PPS, which is still largely in the prototype and research phases, with limited commercial availability. The review presents the future of PPS systems in vehicles, which are evolving, and ongoing research is focused on increasing their energy efficiency and integrating them with emerging technologies such as autonomous, hybrid, and electric vehicles. Compared with other monothematic reviews, this review highlights significant advances in areas such as air compression, storage, and expansion, and compatibility with CAS using RES, which are essential to improve the energy efficiency of PPS systems and make them more competitive with other propulsion technologies. PPS systems are inefficient in energy conversion, so research must be carried out to develop better and more efficient solutions.

1.1. PPS and CPS Comparison

A CPS is a propulsion mechanism that utilizes ICE and generates thrust using the heat produced by the controlled burning of fuel. ICEs and air AMs differ in their operating principles, efficiency, and potential applications. ICEs, whether they run on gasoline or diesel, deliver much higher power and energy densities than compressed air systems. This means that they can produce more power from a smaller and lighter unit and offer a longer range of operations. Although AMs efficiently convert compressed air into mechanical energy, they are limited by the relatively low energy density of compressed air. This results in shorter operating durations and restricted power output for a given device size and weight. Vehicles powered solely by compressed air have a significantly shorter range than those powered by ICE. The necessity of carrying large and heavy air tanks to achieve a reasonable range limits their practicality for everyday use. However, compressed air can be integrated into a hybrid system to support a combustion engine, and the primary source of range and power remains the ICE.

This PPS technology has the potential to replace conventional CPS, with the benefit of avoiding tailpipe emissions. Compressed air is stored in high-pressure CAT, similar to the storage of fuel in tanks in conventional combustion vehicles (CVs). Instead of fuel combustion, CAT uses compressed air to expand and employs power rotating or linear pneumatic actuators (motors and cylinders). At a pressure of 35 MPa, the CAT can only store about 0.5% of the energy of gasoline and only 1.5% of the energy of compressed natural gas (CNG) within a comparable volume. This is due to the much higher energy density of liquid fuels, such as gasoline, compared to the lower energy density of CAT [7].

Scientists are actively researching and developing alternatives to diesel engines, including PPSs, for various applications. In maritime propulsion, PPSs are a cleaner alternative to diesel or electric engines on ferries and other short-distance vessels and boats, offering the potential for reduced emissions and lower maintenance costs. In [8], the PPS was used as an environmentally friendly alternative to the current diesel engine on a ferry in Finland. The existing diesel engine system produces a maximum power of 250 kW on each side of the ferry and uses approximately 55,201 L of diesel annually. To meet the daily energy requirement of 3.58 GJ, the PPS features four 60 kW air motors on each side of the ferry, with an air consumption of 400 L/h, along with a 50 m³ CAT that is pressurized to 150 bars. The use of a 132 kW power compressor allowed the CAT to be charged in 6.2 h while maintaining efficient ferry boat operations. Economic and environmental assessments indicate that replacing the diesel engine with the PPSs could result in substantial annual savings in ferry operation amounted to USD 73,051, and a reduction of 120 tons of CO₂ emissions, with an expected payback period of 8 years. The use of PPSs helps reduce not only CO₂ emissions, but also nitrogen oxide emissions (NOX). When certain types of fuel are used, sulfur oxide (SOX) emissions are produced.

The energy efficiency of different car propellants varies significantly. Diesel engines, with a theoretical efficiency of 55–60%, and petrol engines, achieving around 20–40% in road vehicles, are less efficient than some alternatives such as EVs. The natural gas engine can offer an efficiency comparable to that of gasoline or diesel, especially when optimized for specific engine types. Biogas, when used in converted engines, can offer performance comparable to that of natural gas, with efficiency depending on the type of engine and biogas composition. Compressed air is typically not a highly efficient propellant for vehicles. Compressed air, while potentially emission-free, faces challenges as a result of the low energy density of compressed air and is not commonly used. The energy needed to compress air is significant, and the efficiency of air expanders is relatively low. CAVs are more suited to niche applications where their specific advantages, e.g., zero tailpipe emissions in certain environments, are prioritized.

1.2. PPS and EPS Comparison

PPSs have never attained significant popularity compared to EPS. PPSs are generally considered inferior to electric propulsion systems (EPSs) for several reasons, mainly because of their efficiency, control, and environmental impact. EPS is advantageous for applications that demand superior precise control, quiet operation, energy efficiency, and minimal pollutants emissions, but may face limitations in terms of the range and possibilities of energy storage. The choice between the PPS and EPS depends on the specific application requirements, considering factors such as range, power requirements, environmental concerns, and cost constraints.

The advantages and disadvantages of PPS compared to EPS are presented.

• The lower efficiency of PPSs is due to their dependence on compressed air, which is constantly compressed to maintain pressure even when the PPS is not in use. These continuous air compression and decompression processes lead to energy waste. EMs are highly efficient, converting a large percentage of electrical energy into mechanical energy, and can precisely control the draw power when required, resulting in a higher overall energy efficiency.
• AM and AT offer lower precision control than EM. The pressure of compressible air is more difficult to regulate than that of electricity, making it difficult to achieve fine-tuned movement and precise positioning. EMs provide excellent control and repeatability, allowing for precise movement and adjustment.
• PPSs generate more noise and vibration due to compressed air and mechanical components. EMs are generally quieter, especially when properly tuned, and generate minimal vibrations, thereby improving comfort and reducing wear and tear.
• Although PPSs do not produce emissions directly, the electrical energy required to compress air often comes from power plants that rely on fossil fuels, indirectly contributing to pollution. EPSs produce zero tailpipe emissions, contributing to cleaner air and reduced greenhouse gas emissions (GHG). EPSs offer zero-emission operation, making them ideal for environmentally sensitive areas. EPSs are equipped with electric batteries, the disadvantages of which include high purchase costs, limited range, long charging times, risk of overheating and inflammation, and potential disposal problems. Additionally, the materials used to manufacture batteries are expensive and, in some cases, toxic, and the process of obtaining and disposing of them can be challenging to the environment.
• PPSs require more frequent maintenance because air hoses, valves, tanks, motors, and fittings must be inspected and maintained. EPSs have fewer moving parts and generally require less maintenance than conventional systems.
• The advantages of PPSs include their diverse storage volumes, their ability to store at high pressure, rapid and inexpensive filling, easy storage and recovery of air energy, and long service life. The storage of compressed air for PPSs is less efficient than the storage of electricity in batteries. The EPS system can also use regenerative braking to recover energy during deceleration, further improving its efficiency. EPPs can be more expensive than PPSs, particularly when considering the cost of batteries and charging infrastructure.

In [9], the performance of the PPS was compared with that of an EPS applied to the propulsion of a ferryboat by measuring the thrust exerted by the boat on the load cell. The experimental results suggest that a PPS is a viable alternative to an EPS, but it has a higher thrust of 104 N vs. 99 N for EPS. Four compressed air tanks with a 40 L capacity can be replaced and provide an operational time equivalent to a 12 V, 18 Ah battery. A PPS provides significant advantages over other propulsion methods, such as CES and EPS, particularly for ferryboat applications with defined constant trip travel lengths.

In [10], the compressed air car (CAC) was compared with the common BEV, powered by grid electricity. CACs can compete with BEVs to replace gasoline cars. However, the life-cycle analysis of the compressed air car showed that CAC ranked lower than the BEV in energy-required greenhouse gas (GHG) emissions and life-cycle costs, even under very optimistic assumptions about performance. In fact, the BEV outperforms the CAC in all categories, because the uncertainty in technology specifications is significantly higher for CACs than for BEVs, which is a risk.

In the operation of a CAC, overall efficiency is a crucial factor. The optimal CAC is achieved when the maximum technical work can be produced with the least amount of technical work required for air compression in CAT. This indicates that both the compression and expansion processes should operate near the isothermal limit [11]. This can only be achieved through multistage compression and expansion techniques, along with heat exchangers, to remove or add heat to the compressed air process. Specifically, it is important to take into account the thermodynamics of the PPS, including factors like heat transfer, mechanical and aerodynamic losses, and electrical efficiencies, among others, must be taken into account.

BEVs and hybrid vehicles, including air-powered hybrid vehicles, offer different approaches to transportation, with BEVs relying solely on electricity and hybrids combining electric or air power with gasoline engines. Air powered hybrid systems use compressed air as a secondary power source in addition to gasoline. BEVs provide zero tailpipe emissions and potentially lower running costs, while hybrid vehicles offer an extended range and can be more affordable upfront. The best choice depends on individual needs and priorities, such as driving range, charging infrastructure, and budget.

Emissions: Hybrid vehicles generally produce lower emissions than traditional gasoline cars, but still produce some emissions, especially if the gasoline engine is frequently used.

Driving range: Hybrids offer a longer driving range than BEVs, as they can switch to gasoline engines for extended trips.

Charging: Mild hybrid vehicles typically do not require external charging because the gasoline engine and the regenerative braking system charge the battery.

Cost: Hybrid vehicles are generally more affordable than BEVs, but may have higher running and maintenance costs due to fuel consumption.

1.3. Comparison of an Energy Storage System

Energy storage systems (ESSs) are crucial for EVs and HVs and CAVs that provide the power needed for propulsion and other functions. Compressed air energy storage (CAES) systems using the CAT are relatively inefficient for single vehicles compared to the current EC. To date, CAES technology has shown great potential for stationary energy storage applications, but its use in sustainable mobility is currently being studied as a driving force for development. The energy density of various energy sources, including CASs, is a significant factor in determining the amount of energy that can be stored per unit of mass or volume. The higher the energy density, the more energy can be delivered from the battery to the EM, AM, or CAE, resulting in a longer range for the vehicle. At higher pressures, CAT has a much lower energy density than other energy sources used for transportation, such as rechargeable batteries and liquid and gaseous fuels. The CATs are heavier and have a poor energy density compared to the types of rechargeable batteries available for vehicle use, such as lead acid (Pb-Acid), nickel cadmium (NiCd), nickel metal hydride (NiMH), and lithium ion (Li-ion) batteries. The energy density of CAT is only 0.5% that of gasoline and 1.5% that of compressed natural gas (CNG). The energy density of CAT is also lower than those of gasoline, liquefied petroleum gas (LPG), compressed natural gas (CNG), and hydrogen fuel cells (HFC). Figure 1 shows the relationship between the energy density per volume and the energy density per mass for various energy sources on a logarithmic scale [12].

A review of the literature compared six vehicle propulsion technologies that, according to the principles of sustainable transport development, can have a significant impact on the economy during global crises [13]. In the case of CACs, they offer lower transportation costs (economic and social crises), no life-cycle emissions, and improved air quality (environmental and health crises). In [14] the thermodynamic energy efficiency of CAT was studied. In the CAT energy efficiency evaluation, the various relationships were developed.

CAVs do not produce CO₂ emissions from their tailpipes. Total CO₂ emissions of a compressed air vehicle are influenced by the amount of compressed air generated. If the electricity used to power the air compressor originates from a source that emits CO₂, such as a coal-fired power plant, the overall system will still have a carbon footprint. If electricity is generated from renewable sources, overall emissions are significantly lower. In

Table 1. Comparison of relative CO₂ emissions of different vehicle energy sources [15].

Type of Vehicle	Primary Energy, kWh/km	Emission CO2, g/km
CV	1.018	202
BEV	0.255	86
HFCV	0.206	190
LPG	0.750	171
CNG	1.170	167
CAV	0.188	174

, the specific energy consumption and CO₂ emissions of cars powered by different energy sources throughout their life cycles are given based on a distance of 1 km traveled based on average EU data [15].

Hydrogen and compressed air are both alternative cleaner sources of energy for CVs and EVs, but differ significantly in their technology and applications. A hydrogen fuel cell vehicle (HFCV) or hydrogen fuel cell electric vehicle (HFCEV) is a battery electric vehicle (BEV) that generates its own electricity through an electrochemical reaction between hydrogen gas and oxygen from the atmosphere. This process powers electric motors to drive the wheels and produces water and heat as the only by-products, resulting in zero harmful tailpipe emissions. HFCEVs offer quick refueling times (around 5 min) and long driving ranges, similar to conventional gasoline-powered vehicles, making them a potential clean alternative for transportation. HFCV offers a longer driving range, comparable to traditional gasoline vehicles. HFCVs are currently more expensive because of cost of the technology and fuel cells. Hydrogen refueling infrastructure is very limited and expensive to build.

The most common new-energy vehicles are BEVs and HFCVs. In contrast to vehicles powered by ICEs, BEVs and HFCVs generate almost zero emissions of exhaust gases and carbon during operation. Unlike EVs or HFCVs, CAVs are affordable and have a performance rate PWR of up to 0.0373 kW/kg [16]. The performance PWRs of BEVs vary significantly depending on the vehicle’s design and application, and in some BEVs, the PWR can range from around 0.08 kW/kg to more than 20 kW/kg. The example Tesla Model 3 has a PWR of approximately 6.26 kW/kg, and the Porsche Taycan has a PWR of approximately 7.5 kW/kg. A typical CV has a PWR generally between 0.05 kW/kg and 0.40 kW/kg.

Table 2. Comparison of the basic features of various types of vehicles (based on internet data).

Type Vehicle	Fuel Economy	Travel Range	Production Cost	Reduction CO2
CV	0.165 kWh/km	Long	Medium	0%
Biodiesel	0.61 kWh/km	Long	Medium	100%
BEV	0.30 kWh/km	Medium	Hight	17–30%
CAV	0.075 kWh/km	Short	Low	100%

compares the basic features of several types of vehicles.

Unconventional energy storage methods include hydrogen energy storage, supercapacitors (SC), and semiconductor (SMC) storage; however, they differ significantly in their storage technology and applications. Hydrogen storage can involve physical storage of gas or liquid or chemical storage in materials such as metal hydrides. Solid-state batteries use a solid electrolyte instead of a liquid, potentially offering a higher energy density and improved safety. Supercapacitors, also known as ultracapacitors, store energy electrostatically and provide a high-power output, but have a lower energy density than batteries. A key trend is the integration of different energy storage technologies to maximize their strengths. For example, hydrogen energy storage can be paired with batteries or ultracapacitors to create hybrid systems that offer high energy density and fast power delivery. This can lead to more efficient and reliable energy storage solutions for various applications. Hydrogen energy storage and supercapacitors can be integrated into hybrid energy storage systems (HESSs). Integration of hydrogen energy storage, supercapacitors, and semiconductor storage into vehicles offers a path toward more efficient and sustainable transportation. HFCs can serve as a primary power source, whereas batteries and supercapacitors can handle peak power demands and regenerative braking, and semiconductor storage can potentially be used for advanced control systems and data management. To enhance the system performance and extend the lifespan of individual energy storage systems (ESSs) in the hybrid energy storage system (HESS), it is essential to have an effective and reliable energy management system (EMS) along with appropriate control measures. The EMS is responsible for the power-sharing strategy that ensures that the HESS operates effectively, while the basic control mechanism enables each ESS to receive the appropriate power flow as dictated by the EMS. This is a dynamic field of study.

To improve the comparability of the ESSs,

Table 3. Technical indicators to compare ESS evaluations [17].

ESSs	Power Rating kW	Discharge Time	Energy Density Wh/kg	Energy Cost ∈/kWh	Lifetime Efficiency
Micro-scale CAES Battery NiCd Battery Li-ion HFC SC SMC	1–50 1.0–100 1.0–100 1–50,000 10–1000 10–10,000	max 30 min max 1 h max 1 h 14–24 h msec to minute msec to seconds	20–50 40–60 75–250 800–10,000 0.1–15 0.5–5	10–120 200–1000 200–1800 1–15 300–4000 700–7000	70–80% 60–91% 85–100% 20–50% 90–95% 80–90%

provides technical indicators for the comparability of the assessment.

The volume of energy sources is the central part of the energy storage and the traction range of vehicles. In the case of vehicles, the goal is to store large amounts of energy in the smallest possible physical space. The comparison of the volume in L of the energy source (CAT, Irving, TX, USA, fuel tank, battery) required for a range of 150 km in vehicles of 900 weight with propulsion systems such as PPSs, CPS and BEV type is shown in Figure 2 [18]. In the case of BEV, the weight of the battery was converted to volume from the energy density per volume.

Summary

The energy indicators for PPSs and energy storage in CAT are significantly worse than those of other green propulsion systems used in vehicles. This is primarily due to the low energy density of compressed air, which means that a large volume of air is needed to store a relatively small amount of usable energy. The energy efficiency of a PPS depends on the CAT energy, which is a product of its volume and pressure. The size of the CAT is limited by the design of the vehicle. The larger CAT sizes can be found in the the CAV, the smaller in tricycles, and the smallest in the bicycles. Charging the CAT at high pressures of 20–30 MPa is limited by the availability of high-pressure compressors, which are most often used by fire departments and diving clubs. Petrol stations have compressors with low compression pressures, up to a maximum of 1 MPa.

To improve the energy storage capacity of air tanks, several strategies can be used, such as increasing pressure, optimizing tank design, and exploring advanced technologies such as liquid air energy storage (LAES). Energy storage capacity can be increased by compressing more air into the same volume using materials that can withstand higher pressures. The energy density can be improved by carefully designing the shape and volume of the tank. For example, spherical tanks have a better strength-to-volume ratio than cylindrical tanks. Improving the efficiency and capacity of energy storage, especially in adiabatic CAES systems, can be achieved by properly insulating the tank during compression and expansion, reducing heat loss. Selecting materials that have a high strength-to-weight ratio allows the construction of larger tanks with reduced structural weight, leading to increased storage capacity. Enhancing the storage capacity of compressed air in CATs decreases the need for compressor operation, subsequently reducing their operating expenses. Air compressors can be powered by renewable energy sources such as solar panels, wind turbines, hydroelectric systems, tidal energy, and wave energy turbines.

Research gaps in PPSs concern energy storage and density. There is still a need for better energy management systems to optimize the use of limited compressed air capacity. There is a lack of adequate modeling tools to assess the reliability of components and the overall system prior to an extensive experimental test. Integrating PPSs into existing vehicle architectures, including power transmission and structural components, requires further research and development.

Key questions about the development of energy-efficient PPSs include the following.

• How can application performance be optimized, especially when using heat exchangers and micro-compressed air storage?
• How to determine the optimal air pressure for storage in CAT considering factors such as energy density and storage capacity?
• What are the most accurate methods to model air compression and expansion in different thermodynamic scenarios?
• What are the best optimization methods to minimize energy losses and transfer energy?
• How many stages of compression and expansion should be used to obtain an efficient PPS?
• What numerical analysis of PPSs can be used to validate a small-scale experiment for comparison with real data?
• What advanced forecasting and predictive analysis methods can be applied in the context of energy storage in PPS systems to improve operational efficiency?
• What external factors (e.g., changes in temperature, humidity, wind) have the greatest impact on PPS system performance, and how can real-time operating parameters be adjusted to accommodate these changes?
• What modern CAT tank materials can be used to reduce degradation and improve long-term energy storage?

2. Materials and Methods

CAVs, also known as air-powered vehicles (APVs) or, less commonly, pneumatic vehicles (PVs), use compressed air as their primary propellant source.

Notice:

In this article, there is a reference to vehicles and cars. Vehicles can include anything that transports people or goods, such as trucks, buses, trains, ships, boats, motorcycles, tractors, airplanes, and all kinds of bicycles. A car is a specific type of vehicle. The speed, power, comfort, and safety of PVs are the same as or better than those of vehicles currently in use.

Air cars (ACs), also called compressed air cars (CACs), are air-powered cars (APCs) that do not use the conventional fuels present in modern automobiles. Although the idea of APVs and APCs has existed for a long time, the achievement of practical and commercially viable cars presents significant challenges. Inventors have been working on APVs and APCs for nearly two centuries now. Charles B. Hodges pioneered air-powered engines (APP) in 1896, selling hundreds of locomotives through the H. Hodges Company founded by Porter. The application of pneumatic power in transportation has been around since the middle of the 19th century. In 1840, French inventors Andraud and Tessie of Motay were said to have tested a car driven by a pneumatic motor on a track. Engines that utilize compressed air to operate piston engines have also been investigated. The initial instances consisted of mining locomotives and streetcars that operated on pneumatic power, using Polish engineer Mekarski pneumatic engines (air engine). In 1925, the first gasoline-powered car was converted to run on air. Bob Neal filed a patent application for a pneumatic engine in 1934. In 1934, Johannes Wardenier, a Dutch native, claimed to be the inventor of the first AC. In the 1970s, there was a revival of initiatives aimed at creating functional public transportation that included both air trucks and air cars.

The most recent history of CAC development is from the first 20 years of this century. In 2007, Tata Motors introduced the MDI CityCat developed by Guy Nègre as the first commercial CAC [19]. As of 2009, two more MDI CAC models have been showcased. Certain CAC prototypes were designed to achieve considerable range and fast refueling times. In 2011, a small pencil-shaped rocket called KU:RIN (3.5 m long and 0.8 m wide) powered by compressed air broke the speed record of 130 km/h over a distance of 3.2 km at the Japan Automobile Research Institute (JARI) test facility [20]. In 2014, the French company PSA presented a concept model for the unique Citroën C4 Cactus Airflow 2L hybrid, which uses a three-cylinder combustion engine and an air motor. This hybrid can operate in one of three modes: combustion, air, or combined. Citroën claims that this powertrain achieves a fuel consumption of 2 L per 100 km [21].

Although CACs have a long history and are still being studied, they face obstacles such as the low energy density of compressed air and the need for effective air compression and storage solutions. To develop a safe, lightweight and cost-effective CAC, it is necessary to master the control of compressed air parameters such as temperature, energy density, input power requirements, and energy expansion [22]. Additionally, CAT charging compressors are not as common as petrol stations or EC charging stations. Research is continuously carried out to improve the range, performance, and infrastructure of CAVs.

The PPSs used in CAVs (PVs, ACs) contain a CAT that powers an AM, PC, or CAE, which then propels the wheels. The functional diagram of the application of CAVs, which consists of a high-pressure CAT, pneumatic control valves (PCVs), and an AM, is shown in Figure 3. The expansion of air in the AM results in the potential energy of compressed air being converted into mechanical energy for wheel rotation.

In PPSs and rotary AMs, a natural rotary propulsion is more commonly used in vehicles. In rotary AM, compressed air energy is transformed into mechanical rotary energy. The most well-known AM mechanisms are vanes, pistons, gears, and turbines [23]. Among the biggest advantages of AMs is compactness, as they have little space and are small in weight. They also meet the explosive atmosphere (ATEX) specifications of French Atmosphères Explosibles. AMs have a long durability and the possibility of use even in extreme conditions. They are relatively easy to install and assemble, have a high load capacity, and can easily control air pressure or flow. They can also be reversible in versions for use in two rotation directions.

In [24], a new innovative concept of a PPS is present in the case of a small urban car for four passengers. For an average speed of 50 kph under urban conditions, the estimated urban car ride is 75 km. The PPS of the urban car consists of a high-pressure CAT, a pressure regulator, a low-pressure CAT, and a PM with stepped torque adjustment.

The slider crank mechanism and the chain mechanism are crucial in converting the reciprocating motion of the PC piston into the rotational motion needed for the vehicle to move. The reciprocating piston of the double acting PC is converted by means of a slider crank mechanism or chain mechanism to the rotational motion of vehicles on wheels [25]. Solenoid valves help to operate the double-acting pneumatic actuator [26]. The reciprocating motion mechanism, commonly found in ICEs, utilizes a connecting rod to connect the piston to a crankshaft.

In [27], the research showed that PV called pressurized three-wheel drive vehicles use AM with a torque of 677 Nm, a rotation speed of 300 rpm (r/min), a working pressure of 6.2 bar, an air consumption of 340 L/min, and will produce enough power for speeds of approximately 15–20 kph (km/h). This PV will have a built-in air compressor for plug-in power. After a fast drive, we can quickly load the CAT. In [28] the impact of rotation speed, torque, and regulated pressure on performance, efficiency, and energy conversion efficiency on PM was studied. This study analyzes the efficiency of energy and the efficiency of converting PM energy into CAV performance and economy under various road conditions. According to a review [29], CAVs are in many respects comparable to EVs, but energy is stored in the CAT instead of in batteries. A significant disadvantage of PPSs in CAV is their limited range, which is derived from the small capacity of CAT due to its low energy density [30]. The AM efficiency is quite poor because the compressed air must be cooled before being stored in the CAT. The most effective method to improve AM efficiency is to utilize ambient heat to heat the working medium and the engine cylinders. Another approach to improving the performance of PPSs is to structure the expansion to capitalize on significant drops in enthalpy at the beginning of the process (when the reservoir pressure is highest) and to utilize smaller drops when the tank is low. The benefits of PPS energy efficiency can also be achieved by properly charging the CAT using cooling during the compression stage, rather than exclusively after compression. In addition, in this case, it is possible to use two-stage compression, which will increase the energy density of the CAT.

2.1. Commercialization of CAVs

Commercialization of CAVs was affected by their advantages and disadvantages.

Advantages of CAVs:

The main advantage of eco-friendliness is the absence of exhaust emissions, which makes them a more green and sustainable transportation choice.

Maintenance expenses can be reduced due to the less complex nature of PPSs.

Research is being conducted to improve the efficiency of storage in CAT and the expansion and conversion of air energy in AM.

Disadvantages of CAVs:

The range is limited by the storage capacity of the CAT and the energy efficiency of the PPSs.

In the refueling infrastructure, a network of compressor air stations is required.

The exhaust of compressed air and AM work can be loud but can be mitigated with proper design silencers.

Globally, the CAV market is segmented by propulsion type (single and dual propulsion modes), vehicle type (passenger cars and commercial vehicles), and geographic coverage [31].

The single propulsion segment uses compressed air energy, while the dual propulsion mode uses compressed air energy and fuel engines (such as gasoline, gas oil, biodiesel, gas, liquidized gas, ecological fuel, and alcohol). Compressed air propulsion operates at speeds below 50 kph in urban areas. At speeds exceeding 50 kph, the propulsion changes to fuel propulsion for the city and the open road. Single-propulsion mode for CAVs is expected to dominate the global market because it uses compressed air exclusively for propulsion. Furthermore, developing countries and cities have a higher demand for emission-free vehicles, which is expected to hold a significant share of the CAV market.

The passenger car segment holds a major share of the global CAV market due to improvements in urban and suburban emissions regulations. Furthermore, the increasing demand for compact and lightweight vehicles is expected to boost the global CAV market.

The CAV market is geographically divided into North America, Europe, Asia-Pacific, and other regions. The Asia-Pacific region represents the largest portion of the global market and is expected to maintain its leading position in the coming years. This area is characterized by several developing nations, such as China, India, Indonesia, and Malaysia, which play an important role in the automotive industry. Various government programs aimed at environmental protection have increased the demand for CAVs in the area. Additionally, this region is the largest exporter of CAVs due to its low raw material and production costs, affordable labor, and advanced technology infrastructure. Europe and North America are also experiencing rapid growth in global markets. The growing recognition of the importance of a clean and pollution-free environment, combined with the advancement of emission-free vehicles, is expected to drive the demand for CAVs.

The key market driver is the growing demand for alternative CAVs, which is expected to drive market growth. The key market restriction is the low speed of the CAVs, along with the lack of refueling infrastructure, which can hinder market growth. The size of the CAV market was valued at USD 0.2 billion in 2023. The CAV market industry is projected to grow from USD 0.31 billion in 2024 to USD 6.5 billion in 2032, showing a compound annual growth rate (CAGR) of 45.82% during the forecast period (2024–2032) [32].

Key companies involved in the global CAV market include Motor Development International SA, Tata Motors (Mumbai, India), Groupe PSA (Paris, France), and Magnetic Air Car, Inc. (San Jose, CA, USA).

Motor Development International SA (MDI) is a Luxembourg-French company that utilizes the piston engine to design mobility products such as AirPod. The AirPod car is controlled by the driver using a joystick and two passengers can sit facing back in the back seat. The AirPod car has the following specifications: a weight of 220 kg for the car, a weight of CAT of 80 kg, a compressed air pressure of 350 bar, a maximum speed of 45 kph, and a range of 220 km [33]. Filling the CAT takes approximately three minutes, using only 1.5 euros of electricity. Although the AirPod project has been developed and tested, including a prototype, it has not yet been widely commercialized. India’s Tata Motors collaborated with MDI in a partnership to produce the AirPod vehicle. Tata Motors has successfully completed the first phase of the ambitious AirPod project, and the second stage of AirPod 2.0 is underway. Emerging reports indicate that AirPod 2.0, powered by compressed air technology, will become reality in three years. [33]. According to public data from a small-scale AirPod 2.0 vehicle, a tank with a volume of 300 L and a pressure of 30 MPa can power this car for more than an hour [34]. AirPod 2.0 and other vehicle concepts have been promising a production version for more than two decades, but progress seems to be moving slowly.

MDI Enterprises SA (Nice, France) presented the invention of the CityCat car, which runs on compressed air. The car has a range per tank of approximately 200 km at a maximum speed of 110 kph, and the fuel costs almost nothing [35]. CityCat comprises a polyurethane body shell fitted onto a simple rust-free chassis made of aluminum tubes. Underneath the chassis are three long carbon fiber tubes (instead of a tank) that contain 300 L of air pressurized at 300 bar. The 800 cc 25HP piston engine works by taking in outside air, compressing it to 20 bar, and heating it to 400 °C. Pressurized air is injected into the engine, forcing the piston down and the crankshaft around. When the CityCat car brakes, the movement of the wheels drives a compressor, which compresses air into the storage tanks and helps restore some of the lost pressure. Tanks can be quickly refueled using a special compressor, a process that takes three minutes, or using a compressor at home, which takes two hours. The price of CityCat will be in the range of 14,000 euros. The car will be tested for safety by French auditors. This car will be sold by ZevCat, a US company based in California.

The comparison CityCat car with BECs is introduced in

Table 4. Comparison of the CityCat car with BEC (based on internet data).

Data Comparison	Nissan EV	Toyota Rave EV	CityCat
Fuel type	Electric	Electric	Compressed air
Energy consumption	236 Wh/km	204 Wh/km	198 Wh/km
Annual energy cost	€331	€391	€220
Range	250 km	220 km	200 km
Top speed	180 kph	160 kph	110 kph
Recharge time	4 h	5 h	3 min (2 h home)

.

Tata OneCat is a concept compressed air car built in cooperation between the Indian company Tata Motors and MDI, which was first presented at the New York International Auto Show in 2008 [36]. The vehicle is powered by a piston engine operating on compressed air at a pressure of 300 bar from CAT made of glass fiber. According to designers, the series model could reach speeds of up to 100 kph, and a single CAT load using a compressor from the plug-in in approximately 4 h would be sufficient for a 200 km travel. The construction of cars is very light (320–380 kg, depending on the version) due to the use of a glued aluminum frame and a body made of glass fiber. The vehicle can carry 3–6 people, depending on the version.

Christchurch-based Air Future Ltd., along with its Australian subsidiary company, has the license rights to manufacture AirPod, AirCity, OneCat and in Australia, New Zealand, and the Pacific Islands.

Hybrid Air technology from PSA Peugeot Citroen is developing the trend for hybrid technology powertrains by combining a petrol engine, hydraulic system, and compressed gas energy storage reservoir [37]. This technology is called Hybrid Air, although its actual role is only as a compressible fluid, N₂ (nitrogen) to support the conversion of hydraulic power. The main features of Hybrid Air technology use a hydraulic pump to compress the gas and store it in a high-pressure reservoir that acts as a hydro-pneumatic accumulator. The energy of compressed gas stored in the accumulator is used to power a hydraulic motor that independently propels the vehicle. The lower cost and easier recycling assume that the Hybrid Air system will be cheaper to produce and easier to recycle than hybrid electric systems. Potential environmental benefits The Hybrid Air aims to achieve CO₂ emissions similar to those of electric hybrids at lower costs and in a more environmentally friendly production and disposal process. Hybrid Air Technology aims to create a more ecological and economical drive by combining a combustion engine with a compressed gas-based hydraulically assisted hybrid system. Hybrid Air Technology predicts a 45% decrease in your monthly fuel bills. Due to hybrid air technology, cars such as Citroen C3 or Peugeot 208 will emit CO₂ only 69 g/km. However, despite the PSA manufacturer’s statement, Hybrid Air was never produced. Due to the financial difficulties of the PSA and the lack of additional investors, the work was stopped.

The Australian company EngineAir has developed a vehicle featuring an innovative and environmentally friendly Angelo Di Pietro rotary engine, which provides the necessary power and torque for stationary and mobile applications without relying on combustion fuels or harmful batteries [38]. The Di Pietro engine provides immediate torque from zero RPM and allows for precise control, guaranteeing smooth starts and managed acceleration, with an impressive efficiency rating of 94.5%. Furthermore, the engine operated without vibrations and exhibited minimal friction, allowing it to operate effectively at a low supply pressure of approximately 0.07 bar. The speed and torque of the engine are controlled by a throttle or pressure valve located in the engine intake. The timing of air intake and exhaust was controlled using a slotted timer attached to the output shaft, which rotated at the same speed as the engine. Motor engine parameters can be adjusted by altering the timing of air entry into the engine compartment. A prolonged intake period allowed greater airflow into the chamber, resulting in a higher torque. The reduced duration of the intake restricts airflow, allowing air in the chamber to expand efficiently. This enables the use of compressed air to generate increased torque and power output based on the vehicle operating mode.

Saint Hilaire Canadian inventors developed zero-pollution cars using Quasiturbine [39]. The basic single-rotor Quasiturbine engine is made up of an oval housing that surrounds a four-sided articulated rotor that rotates and moves inside it.

Indian scientists have developed a turbine engine that can be used effectively as the main drive of light vehicles and motorcycles using high-pressure compressed air as the propulsion force at ambient temperature [40].

Engineers from Toyota’s parent company, Toyota Industries Corporation, have achieved a remarkable speed of 80.3 kph with a car that runs solely on air [41]. However, you are more likely to encounter this technology in a hybrid vehicle than in a standalone one. The engine uses compressed air to expand and activate the pistons, which ultimately propels the vehicle forward. A CAT supplies compressed air ‘fuel’ to power the car.

In [42], a research study was conducted in which a prototype of a CAV was constructed and tested at the Ontario Technical University located in Oshawa. The system features an innovative compressed air set-up combined with phase change materials (PCMs) designed for heat recovery. The engine’s maximum torque ranges from 21 to 44 Nm as the shaft speed changes from 400 to 1300 rpm, while the minimum torque varies between 3 and 13 Nm within the same speed range. The additional battery power is 2.18 kW, while the AT work output measures 1.25 kW, using recovered air to supply 36% of the heat necessary to heat the PCM heat exchanger. The total motor work used during the operation is 18.36 kW, which can be considered to be the suitable capacity for the powertrain of a compact city vehicle. The compressed air vehicle has an energetic efficiency of 59.5% and an exergetic efficiency of 51.0%.

According to press reports, magnetic air motors (MAMs) of San Jose, California, filed a patent for this propulsion system in 2008, and the patent was granted in 2011, creating a prototype of a fuelless car that used compressed air and magnets to power the vehicle [43]. The battery starts a magnetic motor that runs a compressor that loads two 10-gallon CATs with a pressure of 200 psi. The compressed air then runs four air-bearing turbochargers, which, according to the company, can reach high revolutions per minute and do not require maintenance. Compressed air is fed to a pneumatic torque converter, which transfers the power generated by the cyclic pneumatic mechanism to a transmission and then to vehicle wheels.

Summary

The commercialization of CAVs has not been carried out as planned or expected. In the first 20 years of this century, there was much optimism for the advancement of a green-friendly vehicle that uses compressed air. MDI has designed AirPod and other vehicles, but not on a commercial scale. Various CAV designs were developed before the mass production of electric and hybrid cars, as is currently the case. There were serious announcements about the production of the PSA Hybrid Air car, which was a hydraulic combustion hybrid, not an air hybrid. As a result of the current state of development in the automotive industry, there is no prospect of mass production of CAVs. This review describes the development periods of APVs (the first two decades of this century); currently, no commercially planned APVs are in production.

2.2. CAV Propelled by CAE

A promising direction in the development of CAVs is the use of compressed air to power ICE, which led to the creation of CAE. CAE is an emission-free piston engine that uses compressed air to operate the combustion engine, thereby eliminating the use of fossil fuels. CAE can be used to propel personal cars, used cars, motorcycles, trams, locomotives, ferries, and other land or maritime vehicles. Figure 4 shows the functional diagram of the PPSs with CAE.

CAE was invented by Guy Nègre, a French engineer who started MDI in 1991 [44]. The CAE is a 2, 4 or 6 cylinder that can dual-energy engine that runs on both compressed air and regular fuel. Based on this CAE, the MDI company has developed a CAV called AirCar, and there are four different vehicles based on this air car technology, including the Open Top One FlowAir, the three-seater Mini FlowAir the Truck-like City FlowAir, and the Multi-FlowAir public transport concept vehicle [MDI].

The benefits of CAE are as follows.

Compressors use electricity to compress air, which is relatively cheaper and more common. The temperature of the exhaust gas is slightly lower than the atmospheric temperature, which helps to control global warming and limit temperature increases caused by other factors.

• Smooth operation due to minimal wear and tear of the components;
• There is no possibility of knocking; no need for cooling systems;
• Spark plugs or complex fuel injection systems; no use of expensive fossil fuels, as free air is taken and compressed; less wear and tear, and thus a lower maintenance cost.

Due to its advantages and disadvantages, CAE technology is certainly a futuristic propulsion system for vehicles. For the practical application of CAE to vehicles, several technical problems must be addressed.

• The expansion of air under pressure causes it to cool, resulting in a decrease in the energy of the CAE. The efficiency of CAE operation is increased by the use of environmental heat.
• Cold air from exhaust (−15 °C), which can also be used for air conditioning in a car.
• The compressor air in the CAT increases its temperature and if this heat is not recovered, energy losses occur and propulsion efficiency decreases.
• Storage of air under high pressure requires a strong CAT, which, if not made of expensive carbon fiber composites, will be heavy in a steel tank, reducing propulsion efficiency.
• Energy recovery during braking by compressing air generates heat that must be recovered for better propulsion efficiency.

Advantages of CAE:

They are environmentally friendly and produce zero tailpipe emissions, making them a potentially cleaner option, particularly in urban areas.

They require less maintenance than traditional engines and do not involve complex combustion.

The cost effectiveness and simplicity of the design can lead to lower production and maintenance costs.

Existing internal combustion engines can be converted to run on compressed air, offering a way to upgrade existing combustion vehicles to more sustainable alternatives that use compressed air.

They can be integrated into hybrid systems, potentially improving fuel efficiency and reducing traditional engine emissions.

Enhanced safety without the dangers of high temperatures and combustion.

If the compressor is powered by renewable energy sources (RES), it can contribute to a more sustainable transportation system.

For the same power output, a CAE can be smaller and lighter than a comparable ICE.

Disadvantages of CAE:

Compressed air has a lower energy density than gasoline or battery power, limiting the range and power output of vehicles.

CAEs typically have a lower efficiency than electric motors (EM) or ICEs.

Range limitations and the limited storage capacity of compressed air tanks restrict the distance that a vehicle can travel on a single charge.

Compression time in compressor and tank charging can be time-consuming, particularly with conventional air compressors.

As it is used, the pressure in the tank decreases, which can potentially affect engine performance.

Although not as noisy as some combustion engines, they can produce noise and vibration, which is generally less than that of conventional ICE.

In [45], the principle and unique characteristics of CAE, their operational characteristics, and the current state of research are explained, with special emphasis on their limitations. The study presented a comprehensive overview of research advances made in the field of propulsion systems that utilize conventional PPSs in CAEs. In [46], innovative thermodynamic cycles have been developed for car engines to harness braking energy, such as compressed air, allowing this energy to be utilized during subsequent acceleration. They can be used with all kinds of automobile engine. Every four-stroke cycle consists of two power strokes: one powered by compressed air and the other by combustion gases. It is also possible to change the engine’s operation from a four-stroke cycle to a two-stroke cycle during acceleration, enabling a decrease in engine displacement.

There are possibilities to use CAEs as reciprocating piston engines (RPE), rotary motors: vane air motor, air scroll expander, eccentric rotor air motor, and rotary piston air engine. The rotary vane air motor has been identified as suitable for implementation as a primary motor in motorcycles [47]. The maximum power output of the CAE vane is achieved at 3.977 kW (5.50 HP) with an injection pressure of 6 bar, a rotation speed of 2500 rpm, and different rotor-casing diameter ratios. An RPE is a type of engine that converts the potential energy of compressed air into mechanical work, using a piston that moves back and forth within a cylinder, similar to a traditional ICE [48]. Compressed air flows into the cylinder through an intake valve. The piston changes direction, compressing the air inside the cylinder. The compressed air expands, driving the piston and transforming the energy into mechanical movement, rotational, or linear. The expanded air is released from the cylinder through an exhaust valve.

In the study [49], the design and performance of a three-wheel APC was presented for one person. The design uses a four-vane CAE with a maximum rotation of 3000 rpm, equipped with an air tube capacity of 6 m³, and a maximum pressure of 150 bar. An operating pressure of 80 bar produces a power of 2.74 HP. An air-powered car with a light construction can drive stably and has a simple operating mode. It has low manufacturing and maintenance costs and does not produce exhaust emissions. However, there is still a loud noise from the air outlet, which is a disadvantage. In [50], an innovative vaned-type air turbine was tested and developed, which has minimal losses and a high efficiency range of 72–97%, significantly higher than a standard ICE. The scroll air expander has a structure similar to the scroll compressor and has an orbital fixed scroll placed at an angle of 180° [51]. The efficiency of the scroll air expander ranges from 40% to 66% at input pressures of 2 to 4 bar [52]. An eccentric rotary air motor consists of embedded vanes placed within the housing, creating closed chambers between the vanes, the inner wall of the housing, and the outer wall of the rotor [53]. This motor uses its distinctive structural features to improve starting torque and reduce friction losses compared to rotary vane motors, which can be effectively used in a variety of applications, such as Formula Racing cars, motorcycles, forklifts, and transport vehicles. The rotary piston air engine can be viewed as a form of reciprocating CAE, featuring a design with several reciprocating cylinders arranged radially around a central crankshaft [54]. The engine functions by sequentially allowing compressed air to flow into each cylinder, causing the pistons to move, resulting in the rotation of the crankshaft. The possible drive range of the hybrid car with rotary piston air engine with a maximum engine power of 8.75 kW at 850 rpm and a maximum torque of 127.54 Nm at 400 rpm was analyzed. On a cylinder with a volume of 100 L, an initial pressure of 35 MPa and a final pressure of 2 MPa, the hybrid car can travel about 26 km.

In [55], the focus is on a multi-faceted analysis of pneumatic drives with regard to their application in vehicle propulsion. In cars and motorcycles, PPSs can contain piston-type CAEs, which are the result of the conversion of two- and four-stroke ICE. The PPS involves the storage of compressed air under high pressure in the CAT, the expansion of air in the CAE to drive the pistons, and the conversion of the potential energy of the compressed air into mechanical energy that can be utilized to propel vehicles.

CAEs can be integrated into current ICEs. CAEs provide a potentially clean and efficient alternative to conventional ICE, especially in applications where low emissions and cost efficiency are important. Although not widely used, ongoing research and development is investigating its potential, particularly in vehicle propulsion. CAEs provide benefits such as zero emissions during use and low maintenance requirements. However, they face issues related to energy density, range restrictions, and the need for effective and rapid air compression [56].

In the study [57], the thermodynamic characteristics and analysis of the efficiency of a CAE were presented. Intake pressure and rotational speed can affect CAE performance, and the duration of the injection and cooling process can affect CAE thermal efficiency. The maximum CAE efficiency measured is found to be less than 30% under all working conditions, while the predicted efficiency can be higher. In [58], an experimental study of two-stroke CAE was presented at a pressure of 8 bar, which was created by modifying the cam-gear system of a four-stroke 100 cc ICE. An experimental analysis was performed on the modified CAE to determine its performance characteristics, such as brake power, mechanical efficiency, indicated power, and torque. In [59], a review of the literature was conducted to analyze the effects of various parameters on CAE, such as pressure, CAT capacity, engine stroke count, number of cylinders, CAT, and number of intake and exhaust ports. In [60], a CAE was developed by modifying a single-cylinder, 4-stroke, spark ignition (SI) engine by replacing the spark plugs with compressed air valves, in which compressed air acts as a fluid in a four-stroke engine. In [61], an experimental investigation of a 100 cc four-stroke air-cooled spark ignition (SI) engine modified to two-stroke CAE is presented, which can power light utility vehicles (LUV). The modification of the four-stroke SI engine to a two-stroke CAE involved the design of a new cam system driven by the crankshaft. The result was established that the engine can operate well at a pressure of 5 to 9 bar. Although this technology is still in its infancy, it has the potential to become commercial LUV propulsion. Work on the conversion of an SI engine in a CAE for a three-wheeled vehicle is presented in [62]. This three-wheeled vehicle is commercially available in a version that is capable of traveling very short distances. The maximum calculation power for air motors is 50% lower than that of the basic ICE version [63]. Research results [64] show that ICE, after modification, can operate as a compressed air engine at an air pressure of 5 to 9 bar, a power output of 0.96 kW, and a torque of 9.9 Nm.

In [65], the concept of motorcycle drives was presented by modifying a conventional 100 cc four-stroke piston type ICE with a two-stroke CAE. To stabilize the air energy sources, two air buffers with a capacity of 9 dcm³ and a pressure of 250 bar were used. The CAE equipped motorcycle can operate at a maximum speed of 38.2 kph and a ride distance of up to 5 km. CAE can be used as a secondary power system to start or regenerate motorcycle ICE. In [66], modifications of motorbike engines are presented, such as the 150 cc four-stroke petrol Dayun natural gasoline engine, which aims to achieve a CAE speed of 300 rpm at 8 bar air pressure, with a torque of 7.8 Nm and a power of 245 W. However, the efficiency of this CAE is only 9.6%, which hinders its commercialization. More work is required in CAE to reduce the fuel and gas emissions of motorcycle engines. A modified natural gas station is also planned as a compressed air station. In [67] shown how a regular scooter can be converted to a compressed air Puch moped equipped with CAE and CAT. The maximum speed of the Puch moped was approximately 29 kph and could cover a distance of only 11.2 km before the compressed air pressure of the CAT was discharged. The O2Pursuit compressed air powered motorbike was designed by students in industrial design at the Royal Melbourne Institute of Technology (RMIT). Instead of a regular gasoline engine, the O2Pursuit uses an Australian-designed CAE developed by EngineAir Australia [68]. The bike is approximately the size of a regular 250 cc motocross two-wheeler and uses several body components donated by Yamaha Australia. In [69], the feasibility of using compressed air powertrains in mining rescue vehicles is examined. In [69], the feasibility of applying a compressed air powertrain to mine rescue vehicles is investigated, based on the simulation and preliminary tests of a single cylinder air powered engine prototype, the concept of a compressed air powertrain (CAP) for a rescue vehicle is proposed. In [70], the workings of the compressed air vehicle (CAV) are presented as a quad bike, powered by a CAE. The HONDA GX 160 motorcycle engine has undergone several structural changes and has been modified for CAE. For PPSs containing a CAE capacity of 300 cc and CAT capacity of 13 L at a pressure of 10 bar, the quad moves at an average speed of 5 kph. In [71] a CAE-based model of a motorbike pneumatic system was investigated. In this study, a theoretical model of the thermodynamic cycle of an air engine was developed, and the indicated CAE power, driving speed, fuel consumption, and friction force were estimated. To increase the performance and efficiency of CAE, some recommendations can be made, such as: testing CAE at different intake pressures above 20 bar, creating a real thermodynamic model for CAE to optimize its performance based on the operating conditions, managing the engine exhaust gas temperature, and preventing ice formation around the cylinder liner, using a heat exchanger and lighter materials for the construction of CAE.

Summary

Although CAEs have been used for a long time, they have not achieved widespread commercial success in CAVs. The use of CAEs has been limited to trams, locomotives, cars, and motorcycles. Current research is focused on developing CAE systems that are more efficient for hybrid vehicles. The use of CAEs offers a promising path to achieve greener and more sustainable transportation; however, significant technological advances are required to address existing limitations and transform them into a viable alternative to traditional propulsion systems. It is important to note that CAE technology can be used to propel vehicles during periods of crisis, such as lack of fuel access, war, natural disasters, embargoes, and trade restrictions. CAE is being developed by companies such as EngineAir Pty Ltd. (Melbourne, Australia), Honda Motor (Tokyo, Japan), Matrix Comsec Pvt (Gujarat, India), Phinergy (Kfar Saba, Israel), and Stellantis NV (Amsterdam, The Netherlands).

2.3. Hybridization of PPS

The principle behind air hybrid vehicles (AHV) is the same as that of hybrid electric ones. They use two energy sources, fuel and pressurized air, to propel the vehicle. Hybridization involves combining a PPS with traditional ICE or potentially other power sources, such as EM, to improve the propulsion efficiency of the vehicle and potentially reduce exhaust emissions. Air hybrid (AH) or pneumatic hybrid (PH) is defined as a vehicle propulsion system, such as pneumatic hybrid vehicles (PHV) or hybrid air cars (HAC), that combines ICE or EM with compressed air technology, allowing energy recovery through regenerative braking and significant reductions in fuel consumption and pollution. AH and PH offer novel propulsion methods and present a more straightforward and cost-effective concept than other hybrid systems. For many years, automotive engineering researchers have focused on developing new and highly efficient hybrid cars powered by two energy sources: compressed air and ICE, or compressed air and EM. Figure 5 shows a functional diagram of the parallel air-combustion hybrid (ACH) and parallel air-electric hybrid (AEH).

Figure 5 shows a pneumatic reversible unit operated as an AM and compressor, which is connected to two CATs, one with low pressure and the other with high pressure. When the vehicle slows down, the AM functions as a compressor, transforming the kinetic energy of the vehicle into the potential energy of compressed air, which is stored in the CAT. This function is also known as regenerative braking.

The hybrid configurations of ACH and AEH are more efficient than the PPS itself, which uses compressed air to propel the vehicle. Hybrid propulsion has four operating modes: gasoline (electric), compressed air, combined power, and brake energy recovery.

In [72], a new concept of an AEH has been designed that uses a main CAT and a sub-CAT, a battery, and a kinetic energy recovery system. If this hybrid system is to operate in CAV mode rather than in BEV mode, the charged battery should constitute half of the total CAT energy. The AEH operates in three propulsion modes: AM main, AM sub with kinetic energy recovery, and EM to assist AM acceleration. In [73], the ACH was preset, in which the regeneration of kinetic energy while braking was stored as compressed air and then utilized to aid vehicle acceleration. Unlike electric hybrids, the ACH operates without the need for an additional propulsion system. This method offers a notable improvement in fuel efficiency without the complications associated with electric hybrids. The study examines the fuel efficiency possibilities of an ACH by showcasing the modeling results of a 2.5 L V6 spark ignition engine that features an electrohydraulic camless valvetrain, employed in a 1531 kg passenger vehicle. Covers alterations made to the engine, the thermodynamic aspects of different operating modes, and the simulation of the vehicle’s driving cycle. The ACH modeling indicated a 64% improvement in fuel economy compared to the baseline vehicle during city driving and a 12% improvement on the highway. This can be achieved without decreasing the vehicle’s weight to offset the added hardware or lowering the engine displacement. In [74], the key distinction between the ACH presented and previous ACH designs is the incorporation of a low-pressure CAT that replaces an atmosphere as a source of low air pressure. This modification allows for extremely high torque in both compressor mode and AM mode. The ACH system was designed for the regenerative brakes of a city bus stopping at maximum speed in city traffic. When this criterion was applied to a 15000 kg bus, the required high-pressure tank capacity was 140 L and the low-pressure tank capacity was 600 L. The total reduction in fuel consumption was estimated to be 23% for urban driving.

In [75], an extensive overview of compressed air hybrid (CAH) technology applied to passenger and commercial vehicles was provided, following its development from its inception to the present. This article focuses on the design of CAH, its components, recent discoveries, technological advances, and the advantages and disadvantages of the system. The evaluation also includes the latest CAH prototype, which has been tested. According to research, the existing literature indicates that the CAH system is effective. However, additional CAH research should focus on addressing certain issues, including improving energy efficiency and optimizing lightweight system designs. The development of CAH in passenger vehicles is still in its early stages and there are many areas to investigate. If CAH technology proves to be effective, it will undoubtedly improve future energy efficiency, lead to cost savings, and decrease pollution levels. In [76], the design, fabrication and simulation of a compressed air hybrid vehicle (CAHV) powered by two forms of energy, pneumatic and electrical are presented.

Combining a conventional ICE with a PPS storage system is an interesting approach to achieve a hybrid pneumatic powertrain (HPP) with lower fuel consumption [77]. The idea is to use compressed air as an energy storage medium instead of batteries, which can be more durable and potentially cheaper. Hybridization can be achieved using HPP as an alternative solution. The primary advantages of HPP are cost-effectiveness and direct torque transmission. HPP suits for urban driving and mild hybridization of cars. An efficiency improvement of 50% has been achieved for urban driving and C-segment vehicles. CO₂ emissions in the urban cycle are very low, only 51 gCO₂/km.

The world’s first fully functional hybrid pneumatic engine (HPE) has been achieved through extensive fundamental research at ETH Zurich. Pneumatic hybridization of gasoline engines primarily aims to maximize down-sizing and increase drive ability with minimal additional cost. Compared to engines with the same maximum power, the engine system presented is up to 35% more efficient in the New European Driving Cycle (NEDC). NEDC is a standard test procedure used to measure fuel consumption and emissions in vehicles. The NEDC test simulates urban and interurban driving conditions and is performed on a chassis dynamometer. NEDC is being replaced by more realistic Worldwide Harmonized Light Vehicle Test Procedures (WLTP) at present [78].

In [79], it was shown that pneumatic motor mode can be used to start HPE quickly enough to justify the use of the start/stop function. To improve the energy efficiency of conventional vehicles that use internal combustion engines, the HPE concept was presented, including regenerative braking and compressed air-assisted starting functions [80]. In regenerative braking mode, the engine functions as a two-stroke compressor, transforming the kinetic energy of the vehicle into compressed air. In the two-stroke air motor mode, a throttle valve in the intake manifold is utilized to restrict the entry of ambient air, while an electropneumatic solenoid valve regulates the injection of compressed air. The result of the urban driving cycle simulation indicates that a light-duty vehicle equipped with a pneumatic hybrid system can achieve an 8% reduction in fuel consumption.

A PHV that uses pneumatic hybrid engines (PHE) blends conventional ICE with a compressed air system to improve fuel efficiency and reduce emissions. During deceleration of the vehicle or brakes, the ICE acts as a compressor, using the kinetic energy of the vehicle to compress air into a high-pressure CAT. An air motor is powered by compressed air, which can help the ICE accelerate or, in some cases, run the vehicle independently over short distances. Researchers at Lund University have explored the potential of PHE, which showed a 30% reduction in fuel consumption compared to conventional gasoline engines [81]. The functional PHE they developed resulted in fuel consumption reductions of up to 58% in a conventional bus converted to a pneumatic hybrid. In [82] a detailed technological development of PHV based on PHE technologies was presented using different pneumatic hybridization methods, with the aim of providing a detailed overview of the advantages and limitations of different pneumatic hybridization methods. In [83], a lightweight AEH (approximately 200 kg) was designed using wind as an additional energy source to charge the battery, thus reducing dependence on conventional fuel sources. The PPS includes a compressor, a CAT, and an actuator, which is a pneumatic cylinder that supports the propulsion of the car by regulating the pressure.

Although all existing hybrid power engines can reduce exhaust emissions and fuel consumption, they still struggle to achieve a stable and optimal operating condition right after ignition, resulting in low thermal efficiency. To address the challenges mentioned above, a new idea has been proposed: a hybrid pneumatic power system (HPPS) that stores “flow work” rather than the electrochemical energy found in batteries [84]. The text you provided is extremely brief and does not contain enough context or content to be meaningful. This groundbreaking power system not only guarantees that the internal combustion engine operates at peak efficiency, but also utilizes the exhaust flow to drive the vehicle. Enhancing the internal combustion process and reusing exhaust energy can boost a vehicle’s efficiency from 15% to 33%, resulting in a total improvement of 18%.

In [85], the study focused on the overall effectiveness of a hybrid pneumatic power system (HPPS). The HPPS was tested under various conditions, including different ICE speeds, fuel consumption levels, compressor speeds, CAT pressure, and AM efficiency. Optimal operating conditions for the HPPS were established and the overall efficiency of the HPPS was assessed under these conditions. The experimental findings indicate that the overall efficiency of the HPPS can reach 45.3%. Additionally, the HPPS has the potential to reduce fuel consumption by 38% in optimal performance.

In [86], a novel type of hybrid pneumatic combustion engine (HPCE) was introduced that uses compressed air injection for improved performance. HPCE captures energy wasted during ICE braking to improve ICE performance and achieve better fuel efficiency. A mathematical model for the HPCE was developed that incorporates a supercharged ICE and an air tank. The findings indicate that the air injection booster system significantly improved the steady-state performance of the HPCE. At a speed of 1900 rpm and full load, using air injection boosting at a pressure of 0.5 MPa can elevate the engine torque from 1039 to 1057 Nm and power from 206.9 to 210 kW. This study examined the impact of air injection parameters, revealing that improved performance can be obtained with higher air tank pressure and larger injection hole diameters.

In [87], a new compression strategy was created for hybrid air engines (HAE), significantly improving the efficiency of traditional hybrids. The novel HAE uses an innovative compression method that involves two air tanks to increase air pressure while the engine is in compressor mode. The experimental findings demonstrate that the new HAE configuration outperforms the traditional single-tank system in terms of energy storage. The newly throttle-based approach to HAE torque control was simulated and validated by experiments. The fuel efficiency achieved during a drive cycle with a double tank HAE was assessed and compared with that of a single tank HAE.

A pneumatic hybrid electric vehicle (PHEV) consists of pneumatic and electric sources, a component for compressing and storing air, a pneumatic engine and an electric engine [88]. A pneumatic engine produces mechanical power from compressed air, and an electric engine generates mechanical power from an electric source. The power transmission system delivers power from both pneumatic and electric engines. The control system manages the operation of each pneumatic and electric engine based on a predefined torque range. This PHEV approach offers potential benefits, such as increased fuel efficiency, reduced emissions, and potentially lower costs, compared to a vehicle with a purely electric or combustion engine.

Energine Corp., a Korean company, has developed a prototype PHEV that operates with an electric motor and compressed air [89]. This PHEV is engineered to utilize compressed air for propulsion until it reaches a speed of approximately 15 mph, at which point the electric motor takes over.

In [90], a hybrid technology with energy storage systems (ESS) was presented in different non-electrical vehicles, including four basic pressure layouts using compressed air and hydraulic systems. And to more accurately evaluate the performance of these hybrid systems in vehicles, the next step is to perform an integrated simulation of the ESS with the vehicle dynamics system.

Summary

Hybrid technology shows promising development prospects in road transportation. Hybrid vehicles can play a crucial role in the transition to full BEVs by offering more accessible and affordable alternatives. Hybrid vehicles, including compressed air hybrid vehicles, can serve as a stepping stone towards improvements in propulsion systems and emissions reduction compared to traditional vehicles. AHs (PHs) are ideal for vehicles operating in urban traffic, where frequent braking and acceleration cycles offer opportunities for energy regeneration and efficient use of compressed air. Air hybridization can be a suitable option for mild hybridization, where it complements traditional ICE rather than completely replacing it. The compressed air energy storage (CAES) system can effectively capture and regenerate braking energy, thus improving the overall efficiency of vehicle propulsion. The CAES system contains dual CATs (e.g., high-pressure and low-pressure) to improve energy conversion efficiency and braking performance. AH can be integrated with other propulsion sources, such as ICE, EM, or HFC, to further improve vehicle performance. To overcome the public perception of AH, it is necessary to demonstrate its benefits and address concerns regarding its performance and reliability. A particularly useful solution in PHVs is to charge high-pressure CATs using a hydraulic air compressor, which converts the hydraulic motor power driven by the ICE vehicle into high-pressure compressed air in the CAT. The lightweight and space-saving design of the hydraulic air compressor enables continuous CAT charging, making it ideal for mobile applications. Therefore, efforts must be made to develop hybrid drives that combine high-pressure hydraulics, compressed air energy storage, and electric drive systems. Certain automotive manufacturers are exploring the possibility of using compressed air storage energy in hybrid vehicles, which could lead to the development of more advanced and energy-efficient vehicles.

2.4. The Use of PPSs in Light Vehicles

The latest trend in personal mobility is the development of lightweight vehicles; however, light electric vehicles (LEVs) are unrivaled in this segment [91]. The development of lightweight pneumatic vehicles (LPVs), often called lightweight air vehicles (LAVs), has the greatest potential as a popular means of short-distance transportation, such as utility, recreational, rehabilitation, cargo, competition, and rickshaws. The benefits of LAVs include better vehicle handling, greater vehicle efficiency, and reduced energy consumption for propulsion. Due to the cumbersome installation of PPSs, three- and four-wheeled LAVs are the best solution. Traditional LAVs, such as two- and three-wheeled vehicles, have different designs, differ primarily in their wheel configuration, and are steered systems that affect their stability, handling, and overall riding experiences. Two-wheeled vehicles, such as motorcycles and bicycles, rely on the rider’s balance and skill to stay upright, whereas three-wheeled vehicles, or trikes, offer inherent stability owing to their extra wheel. This fundamental difference in design leads to variations in how they are ridden and the types of riders who prefer them. The three-wheeled layout can vary, with common configurations including a “delta” (one wheel in front, two in back) and a “tadpole” (two wheels in front, one in the back). The PPS in light air vehicles is important for their operation, with different accepted propulsion mechanism solutions based on AM and PC propulsions. AM propulsions are more interesting for the propulsion of light-duty vehicles due to their high torque relative to mass. However, the control system and the cost-effectiveness of PPSs using PC propulsion that can be easily coupled to a bicycle wheel should be considered. The PC propulsion was operated using a crank mechanism that converted the linear reciprocation of the piston rod into the rotational motion of the rear wheel of the bicycle. Figure 6, Figure 7 and Figure 8 show the functional diagrams of the propulsion mechanisms used in an air bike, an air trike, and an air quad.

In [92], the PPS solution is proposed for LAV and tracked vehicles. The proposed vehicle has a drive wheel propelled by a reciprocating pneumatic prime mover attached to the vehicle structure. In some embodiments, each drive wheel is propelled by a different single pneumatic prime mover that is operative to turn the driving wheel through a one-way crank arm and clutch. The other arrangement involves converting the reciprocating motion of the prime mover to rotation motion directly or indirectly through a supplementary drive device or a transaxle comprising a steering device.

In [93], the design, production and evaluation of an LAV are discussed. The findings of this stage of LAV development focus on aligning decompressed air with the demand for the LAV ride cycle. Compressed air powers an AT connected to an electrical generator, which in turn powers the EM to drive the vehicle. Power requirements are defined in relation to the thrust of the vehicle resulting from the anticipated operational mode. The prototype traveled 180 m successfully at a speed of approximately 3.5 kph with an air pressure of only 6 bar. Improvements in design are in progress, with the aim of achieving direct mechanical load transmission to the wheels without relying on an EM and using CAT with greater capacity.

In [94] a general idea of a pneumatic propulsion/driving system was developed as a demonstrator, which should be used to drive an LAV (such as a bicycle, tricycle, or scooter) over short distances. The proposed PPS comprises a CAT with pressure up to 300 bar, pneumatic control elements (valves), and a PM with a power of 20–30 W, which is capable of moving a light vehicle over a distance of up to 20 km, a speed ranging from 19 to 22 kph, and for at least 5 min. In [95], the operation of a pneumatic power LAV using a double acting PC that is used as a slider crank mechanism. This PPS was created to replace battery-powered industrial vehicles.

As part of the Zephyr AGH University of Science and Technology project in Krakow (Poland), students from the Ignis and Nova Energia research groups have created an eco-friendly low-emission LAV [96]. The project aims to find optimal power solutions for such vehicles and to demonstrate the technology.

Students in the Department of Robotics at the Technical University of Kosice, Slovak, have participated in the International Pneumobil 2020 competition [97]. The International Pneumobil competition is a race of compressed air-driven vehicles constructed by technical university students. They have created the third concept of the pneumatic car to meet the demands of competition. The car has a length of 2.4 m and a width of 1.5 m, and a weight of around 150 kg. The engine consists of three pneumatic pistons connected to a single crankshaft. Compressed air is pumped into these piston engines, which causes a rotary motion of the engine, and is transmitted to the rear drive wheel via a gearbox.

In [98], a four-cylinder AM and CAES that can be used in LAV is analyzed. In [99], the idea of an air bike (AB) is presented, which is built on the mechanical framework of the bike and is powered by an AM. In the project, the assumptions assumed a CAT capacity of 45 dm3 and an AM power of 250 W. In the low-pressure mode at 1 MPa, the air bike can ride for 3 min, while in the high-pressure mode at 20 MPa it can ride for one hour.

In [100], the AB power mechanism was constructed, which consists of the following elements: air tank, four-way and two-position (4/2) direction valve, double-acting pneumatic cylinder and simple belt drive. From the calculations, we can see that for a maximum pressure of 9.25 bar in the air tank, the resulting propulsive power is 10.96 W, which allows for a speed of 40 kph for short-distance travel. In [101], the study focused on the use of air energy as an alternative energy source for a powered wheelchair, called PneuChair, which is used in a water park. In [102], the project focuses on the design, fabrication, and development of a rear wheel-drive pneumatic tricycle (PT), which is useful for people with disabilities. This PT is equipped with a pressure regulator, air compressor, air tank, pneumatic ratchet, and chain sprocket transmission drive. Although the PT is still in its early stages of development, it holds great promise and provides scope for further research.

In [103], the operation of the PT is similar to that of a typical PPS: compressed air from the compressor is passed into the control system, which then propels the piston rod, pushing the gear and the pinion connected to the wheels. In [104], the conceptual design of the PT was demonstrated, which operated quietly and smoothly and gave users the feeling of being in control of the vehicle. Ergonomics evaluation also demonstrated that PT is easy to use in normal use situations, including situations that involve obstacles, for a broad cross section of users. PT is compared favorably to other types of vehicles, such as bicycles and mopeds, especially in terms of stability. In [105], the main objective of the project is to design and manufacture a pneumatically operated PT for disabled people. The cylindrical steel CAT is designed to store highly compressed air within its maximum air pressure of 22 bar.

In [106], the air hybrid bicycle (AHB) is presented as a unique combination of electrical, human, and compressed air energy that is used simultaneously to improve the speed and range of the bicycle. This bicycle is expected to be more energy efficient than electric vehicles at lower speeds. The hybrid pneumatic bicycle also has some advantages, such as customizing the acceleration and a wider speed range. In addition to that, the pneumatically powered bicycle can be recharged manually or at petrol stations, which is much faster than recharging a battery and costs much less than petrol, diesel, or any other form of combustible fuel.

In [107], the traditional tricycle has been converted to an air hybrid tricycle (AHT) with pedaling and pneumatic aid option using double acting cylinders, significantly reducing the muscle power required by the user and maintaining momentum for longer periods of time. The sustainability of momentum is the main focus of ATH, which is a consequence of low muscle and pneumatic power, resulting in less frequent pedaling by the driver. AHT can be propelled by pedals or compressed air. AHT is an inexpensive and environmentally friendly means of transport.

In [108], the use in AHT was considered to incorporate two different ways to power an air compressor for PPSs and propel the AHT, from the lithium-ion battery and solar power. This dual power system will enable substantially longer distance power assistance ATH, in addition to regenerating power from pedal energy (human energy).

Summary

The popularity of lightweight wheeled vehicles, such as bicycles, tricycles, and quads, is increasing as an alternative choice for independent short-distance travel and for recreational and rehabilitation purposes. The use of LAVs presents a promising but still developing alternative to electric personal transportation, particularly in urban areas. Although LAVs offer potential benefits, such as zero emissions and potentially lower operating costs, they face challenges related to energy density and efficiency, which currently limit their range and performance compared to LEVs. LAVs may seem like a distant dream, but their environmentally friendly nature still attracts public interest, especially in the most populous countries that still live in villages (Asia), where transport is usually by light vehicles such as bicycles, tricycles, mopeds, quad bikes, or motorbikes. Air-powered bikes are popular projects among university students, inventors, and tinkerers. Currently, there are no commercially available air bikes because the range of a relatively low-pressure air-powered bicycle is limited.

2.5. Comparing LAV and LEV

Light vehicles have relatively low energy and space requirements, making them ideal for urban settings and for promoting green and sustainable transport. LAVs and LEVs are alternative vehicles that differ significantly in their mechanism of action and benefits. LEVs are compact e-vehicles intended for short-range transportation. They represent an expanding segment of the e-vehicle industry, which includes different types of transportation, such as e-bikes, e-scooters, electric two-, three-, and four-wheeled vehicles, skateboards, Segway, and other compact personal mobility.

LAVs and LEVs are steadily taking over urban car trips of less than 8 km, making up a significant portion of trips around the world. The use of traditional vehicles may also decrease in rural areas. LAVs and LEVs broaden the potential user base to include the elderly and those with limited mobility, while promoting the growth of what is known as green tourism. LEVs are common and have advantages such as longer range and easier operation, whereas LAVs are more niche and focus on environmental friendliness and lower running costs, eliminating the need for electricity and fuel consumption. LAV offers a sustainable and low-cost transportation option, while LEV provides a practical and accessible solution for a wider range of users. The markets for LAVs and LEVs are growing rapidly due to the increasing awareness of their advantages and the ongoing technological advancements that improve their performance and reduce costs. This expansion is driven by factors such as greater awareness of environmental issues, the demand for sustainable transportation alternatives, and the search for convenient and affordable mobility solutions.

Advantages of LAV:

They are environmentally friendly because they do not produce emissions, making them a sustainable vehicle choice.

Lower running costs are required for this. After the initial investment is made, there are no fuel or electricity costs involved.

Compressed air can be replenished quickly at dedicated stations, which could potentially offer a range longer than that of some electric vehicles.

Disadvantages of LAV:

The niche technology of LAV is less common than that of LEV and may be more difficult to buy or repair.

The availability of parts and expertise for LAV may be less than that for LEV.

High-pressure tanks can pose a danger if not handled properly, which is why there are safety concerns.

The speed of the LAV may not be as fast as that of the LEV or traditional bicycles.

Advantages of LEV:

LEVs can travel longer on a single charge depending on the battery capacity than LAVs.

The availability of LEB in various models and configurations is higher.

Electric components are generally easier to maintain and repair than pneumatics.

Disadvantages of LEV:

LEVs are heavier than LAVs due to battery and EM propulsion.

LEV requires regular charging, which may be uncomfortable for some users.

Batteries have a limited lifespan and must be replaced regularly.

LEVs have the potential to be more expensive than LAVs, depending on their features and battery capacity.

3. Discussion of a Prototype of an Air Tricycle Bike

This study discusses a comprehensive computational model and the results of field tests of an air tricycle bike (ATB), which is also known as a rehabilitation tricycle bike (RTB) due to its intended use. RTB will be used for the rehabilitation of patients with neurological diseases, strokes, movement disorders of the lower limbs, the elderly, obesity, after traffic accidents and other conditions. RTB effectively supports therapeutic exercises for patients and those that improve overall health. Environmentally friendly RTB is an excellent rehabilitation device for use in medical centers (hospitals), social care centers, and patient homes. Due to its simple and low-cost structure, the RTB can be purchased for home training without the need for a specialist such as physiotherapists. The RTB will be equipped with devices that support therapeutic procedures and monitor the patient’s physical condition, such as checking blood pressure, heart rate, and breathing intensity, which are not within the scope of this study. RTB would allow faster and more efficient distance coverage without adverse effects on the user while maintaining any potential benefits of cardiorespiratory training. Heart rate (HR) can be monitored by measuring the number of beats per minute (bpm), which is the heart response to the training load and an indicator of the patient’s physical fitness. It is possible to assess the amount of energy the body uses, measured in calories, to cover the required distance. The benefits of RTB also include improved passive range motion and reduced abnormal muscle tone, spasms, and pressure sores. The use of RTB has a wide range of social benefits for users, including increased independence, self-esteem, and social status; enhanced communication, access, quality of life, and functional range; and improved participation in daily activities (ADL) and psychological well-being.

The three-wheeler RTB is a good and safe alternative for adults, seniors, and disabled people who cannot ride a regular two-wheeler bike. The RTB was specifically designed for those with special needs who require support while sitting and a stable and comfortable ride. The RTB has two wheels at the rear and one at the front, and a high seat. The RTB can be ridden in three modes: pedaling without PPSs, pedaling with PPSs, and using only PPSs. The PPS propels two wheels to the rear axle of the RTB. The RTB was designed, constructed, manufactured and tested in a research project POIR.01.01.01-00-0013_19 within the framework of the Smart Growth Operation Program 2014–2020, conducted by the authors with the participation of an industrial partner [109]. Figure 9 shows the prototype RTB from the sides and rear, where the PPS was installed [110].

Our RTB from Kielce University of Technology in Kielce, Poland, was exhibited at the Kielce Fair during its participation in the KIELCE BIKE-EXPO 2022 bicycle show. show. The assumptions of the RTB design include a curb weight of up to 30 kg, a weight with a load of 150 kg, a minimum driving range of 15 km, a speed greater than 15 kph, a wheel diameter of 20–24 in., a wheel speed of 200–300 rpm, a torque rating of 35–50 Nm, a power rating of 180–250 W, and overall dimensions: width of up to 80 cm and length of up to 180 cm.

The traction parameters of the designed RTB were selected using the comprehensive PPS computation model, which consists of interconnected series pneumatic components, such as a CAT, a PCV block, and an AM. The computational model of the two high-pressure CATs considered the instantaneous expansion of air during its discharge process through the AV block with series-connected valves. The computational model of the AV block considered the compound values of the flow parameters, such as sonic conductance and the critical pressure ratio, in the mass flow rate under choked and subsonic conditions. The AM computational model considered catalog parameters, measurement parameters, torque and power characteristics, and calculated parameters such as operating pressure using air expansion power. A PPS computation diagram for the RTB is shown in Figure 10.

The proposed comprehensive computational model of the PPS can be used to calculate the traction parameters for all categories of two-wheeled cycles (bicycles), three-wheeled cycles (tricycles), and four-wheeled cycles (quadricycles), which are powered by human muscles with pneumatic power assistance.

3.1. Computational Model of a PCV Block

The PPS contains a PCV block consisting of several different series-connected valves to regulate and reduce pressure, shut-off, throttle flow, and other control functions. The series interconnection of n single valves in PCV block is shown in Figure 11.

In the single PCV calculation model, according to the standard, two flow parameters are recommended: sonic conductance C and critical pressure ratio b. The flow parameters C and b were measured and reported by the PCV manufacturer on the data sheet. The flow parameters C and b are determined when the air flow reaches the sonic flow condition (choked flow). Compound flow parameters C and b for the PCV block were calculated from the C_i and b_i parameters of the individual valves [111]. It starts with the calculation of the compounded value of the flow parameters C and b for the first pair of valves, with the flow parameters C₁ and the b₁ for the first valve and flow parameters C₂ and b₂ for the second valve. The methods to calculate the compound value of the flow parameters C and b depend on the ratio of the critical flows expressed by factor K₁₂,

$${{\displaystyle K}}_{12}={\displaystyle \frac{{{\displaystyle C}}_1}{{{\displaystyle C}}_2}}$$

(1)

The different values of K₁₂ indicate the following method to calculate the compound values of the sonic conductance C₁₂.

For K₁₂ ≤ 1, the compound value of C₁₂ is equal to C₁, that is C₁₂ = C₁,

For K₁₂ ≈ 1, the compound value of C₁₂ results for C₁ and C₂ is as follows:

$${{\displaystyle C}}_{12}=\sqrt[3]{{\displaystyle \frac{{{\displaystyle C}}_1^3{{\displaystyle C}}_2^3}{{{\displaystyle C}}_1^3+{{\displaystyle C}}_2^3}}}$$

(2)

For K₁₂ ≥ 1, the compound value of C₁₂ is equal to C₂, that is C₁₂ = C₂.

After calculating C₁₂, the flow parameter b₁₂ is given by the formula,

$${{\displaystyle b}}_{12}=1-{{\displaystyle C}}_{12}^2{\displaystyle \sum _{i=1}^2\left(\frac{1-{{\displaystyle b}}_i}{{{\displaystyle C}}_i^2}\right)}=1-{{\displaystyle C}}_{12}^2\left[\left({\displaystyle \frac{1-{{\displaystyle b}}_1}{{{\displaystyle C}}_1^2}}\right)+\left({\displaystyle \frac{1-{{\displaystyle b}}_2}{{{\displaystyle C}}_2^2}}\right)\right]$$

(3)

The same computational methods for compounded values of the flow parameters were applied for subsequent valve pairs, that is, C₁₂, C₃, b₁₂ and b₃, etc.

Mass flow rates in kg/s through the PCV block under choked and subsonic flow conditions are as follows [112],

$$\left\{\begin{array}{l}{{\displaystyle q}}_{mc}=C {\rho }_N\sqrt{{\displaystyle \frac{{{\displaystyle T}}_N}{T}}}p\hspace{1em}\mathrm{choked} \mathrm{flow} \mathrm{conditons} \mathrm{for} {{\displaystyle p}}_M/p\le b\\ {{\displaystyle q}}_{ms}={{\displaystyle q}}_{mc}\sqrt{1-{\left({\displaystyle \frac{{{\displaystyle p}}_M/p-b}{1-b}}\right)}^2} \mathrm{subsonic} \mathrm{flow} \mathrm{conditons} \mathrm{for} {{\displaystyle p}}_M/p>b\end{array}\right.$$

(4)

where q_mc and q_mc are the mass flow rate through the PCV block under choked and subsonic flow conditions, p is the pressure in the CAT, T is the temperature in the CAT, p_M is the operating pressure in the AM, C is the compound sonic conductance, b is the compound critical pressure ratio, T_N and ρ_N are the normal temperature and density according to the reference atmospheric normal conditions ANR (Atmosphere Normale de Reference): T_N = 293.15 K, p_N = 100 kPa, ρ_N = 1.18 kg m⁻³, R_N = 288 J kg⁻¹ K⁻¹ at 65% RH (Relative Humidity) [113].

In PPSs, the volumetric flow rate in m³/s (m³/h) or L/min (LPM) for the data sheet parameters is the measure of flow through the PCV and is a crucial factor in determining air consumption and rotation speed in AM. Volume flow rates through the PCV block under choked and subsonic flow conditions are as follows.

$$\left\{\begin{array}{l}{{\displaystyle q}}_{vc}={\displaystyle \frac{{{\displaystyle q}}_{mc}}{{\rho }_N}}=C\sqrt{{\displaystyle \frac{{{\displaystyle T}}_N}{T}}}p\hspace{1em}\mathrm{choked} \mathrm{flow} \mathrm{conditons} \mathrm{for} {{\displaystyle p}}_M/p\le b\\ {{\displaystyle q}}_{vs}={{\displaystyle q}}_{vc}\sqrt{1-{\left({\displaystyle \frac{{{\displaystyle p}}_M/p-b}{1-b}}\right)}^2} \mathrm{subsonic} \mathrm{flow} \mathrm{conditons} \mathrm{for} {{\displaystyle p}}_M/p>b\end{array}\right.$$

(5)

The parameters of the single-stage GCE FMD 530 pressure regulator are the following: inlet pressure 300 bar; outlet pressure range 0.5 to 6 bar; and sonic conductance C₁ = 2.31 10⁻⁸ m³/sPa determined from the nominal flow q_n = 500 L/min. The parameters of the AVENTICS 3/2 way directional spool valve were as follows: working pressure of 10 bar, nominal flow q_n = 550 L/min, sonic conductance C₂ = 2.55 10⁻⁸ m³/sPa and critical pressure ratio b₂ = 0.28. The proportional pressure regulator parameters of FESTO VPPE were as follows: set voltage signal u = 0–10 V, input pressure 6–8 bar, pressure regulation range 0.15–6 bar, and sonic conductance C₃ = 1.45 10⁻⁸ m³/sPa determined from nominal flow q_n = 310 L/min. Calculations of the compound value of the sonic conductance C = 1.208 10⁻⁸ m³/sPa and the critical pressure ratio b = 0.35 were performed using the online version of the SMC software version number 2.2.09 [114].

The flow parameters C and b are recommended by the ISO 6358 standard, which specifies the requirements for the test stand, the test procedure, and the presentation of the results [115]. The flow rate characteristics of the PCV block, which were determined based on the compound flow parameters C and b, were verified using the measurement data obtained on the measuring stand [116]. Measurement data were used to determine flow parameters using the following equations:

-

sonic conductance,

$$C={\displaystyle \frac{{{\displaystyle q}}_{vm}}{p_{1m}}}\sqrt{{\displaystyle \frac{{{\displaystyle T}}_{1m}}{T_N}}}$$

(6)

where q_vm is the volumetric flow rate measured at critical flow, p_1m, T_1m are the upstream pressure and temperature measured at critical flow,

-

critical pressure ratio,

$$b={\displaystyle \frac{p_{2m}}{p_{1m}}}$$

(7)

where p_2m is the downstream pressure measured in the critical flow.

Figure 12 shows the ratio of the volumetric flow rate of the PCV block obtained from the computational data using C and b and the measured data.

The problem of determining the critical pressure ratio b_m based on the measurement data was solved using a fitting function,

$${{\displaystyle F}}_i({{\displaystyle X}}_i,b)=\sqrt{1-{{\displaystyle \left(\frac{{{\displaystyle X}}_i-b}{1-b}\right)}}^2}$$

(8)

where X_i is the i-th pressure ratio for the measurement data,

$${{\displaystyle X}}_i={\displaystyle \frac{{{\displaystyle p}}_{mi}}{{{\displaystyle p}}_{ma}}}$$

(9)

where p_ma is the average pressure measurement data.

To determine the critical pressure ratio b_m, the square function fit (LSF) was minimized using the MATLAB lsqcurvefit function to set the initial point b = 0.35 of the computational models.

$$LSF(b)=\min {\displaystyle \sum _{i=1}^n{{\displaystyle \left({{\displaystyle Y}}_i-{{\displaystyle F}}_i({{\displaystyle X}}_i,b)\right)}}^2}$$

(10)

where Y_i is the i-th flow rate ratio for measurement data,

$${{\displaystyle Y}}_i={\displaystyle \frac{{{\displaystyle q}}_{vmi}}{{{\displaystyle q}}_{v\max }}}$$

(11)

where q_vmax is the maximum flow rate measurement data.

From the numerical solution, the best-fit value of the critical pressure ratio b_m = 0.376 was obtained.

The relative fit error FE_i was determined using a formula based on flow rate measurement data.

$${{\displaystyle FE}}_i=\left|{\displaystyle \frac{{{\displaystyle F}}_i({{\displaystyle X}}_i,b)-{{\displaystyle Y}}_i}{{{\displaystyle Y}}_{\max }}}\right|100\%$$

(12)

The relative fit error of the flow rate measurement data is shown in Figure 13.

3.2. Computational Model of the CAT Discharge Process

When the CAT polytropic discharge process occurs for a constant volume V, the mass flow rate q_m through the PCV block is expressed as follows [117]:

$${{\displaystyle q}}_m=-{\displaystyle \frac{V}{nRT}}{\displaystyle \frac{dp}{dt}}$$

(13)

where V is the volume of the CAT, p and T are the instantaneous pressure and temperature in the CAT, R is the gas constant (individual for air) and n is the polytropic exponent, which was experimentally measured.

The polytropic exponent n was calculated using the logarithmic formula based on the pressure and temperature measurements [118],

$${\displaystyle \frac{n}{n-1}}={\displaystyle \frac{\ln ({{\displaystyle p}}_M/{{\displaystyle p}}_s)}{\ln ({{\displaystyle T}}_M/{{\displaystyle T}}_s)}}={{\displaystyle K}}_n\Rightarrow n={\displaystyle \frac{{{\displaystyle K}}_n}{{{\displaystyle K}}_n-1}}$$

(14)

where K_n is the slope of the logarithmic characteristics, T_s is the initial storage temperature, p_s is the initial storage pressure in CAT, T_M is the discharge temperature in CAT, and p_M is the operating pressure in AM.

The RTB ride was carried out using two SAFER air cylinders (scuba tanks), manufactured with high-pressure aramid and carbon fiber, with a capacity of V = 2 × 9 L =18 L, a storage pressure of p_s = 300 bar, and an operating pressure of p_M = 4 bar in AM.

During expansion of the polytropic discharge of a CAT, the storage pressure p_s drops to the operating pressure p_Min AM. The useful energy stored in the CAT decreases with pressure drop according to the following equation,

$$E_S={\displaystyle \frac{n}{n-1}} {{\displaystyle p}}_{s}V\left[1-{\left({\displaystyle \frac{p_M}{{{\displaystyle p}}_s}}\right)}^{{\displaystyle \frac{n-1}{n}}}\right]\to \mathrm{for}\to p_{\mathrm{s}}\ge p\ge p_{\mathrm{M}}$$

(15)

The useful energy stored in relation to the CAT volume is the useful energy density stored, which can be expressed by the following equation.

$$E_{Sd}={\displaystyle \frac{{{\displaystyle E}}_S}{V}}={\displaystyle \frac{n}{n-1}} {{\displaystyle p}}_s\left[1-{\left({\displaystyle \frac{p_M}{{{\displaystyle p}}_s}}\right)}^{{\displaystyle \frac{n-1}{n}}}\right]\to \mathrm{for}\to p_{\mathrm{s}}\ge p\ge p_{\mathrm{M}}$$

(16)

Figure 14 shows the curves of the useful energy stored and the useful energy density stored in the CAT from the stored pressure (30 MPa) to the operating pressure (0.4 MPa), adopting an energy unit conversion of K_e = 2.778 $\times {10}^{-7}$ from J to kWh.

3.2.1. Solution for Choked-Flow Conditions in the CV Block

Under choked flow conditions through the CV block in the CAT, the pressure drops from the storage pressure p_s to the intermediate pressure p_in = b p_s in the time interval between the start time t_s (assuming t_s = 0) and the time t_in. According to these conditions, Equation (16) can be written as follows,

$${\displaystyle \underset{{{\displaystyle t}}_s}{\overset{{{\displaystyle t}}_{in}}{\int }}dt}=-{\displaystyle \frac{V}{nRT}}{\displaystyle \underset{{{\displaystyle p}}_s}{\overset{{{\displaystyle p}}_{in}}{\int }}\frac{dp}{{{\displaystyle q}}_m(p)}}=-{{\displaystyle \tau }}_{in}{\displaystyle \underset{{{\displaystyle p}}_s}{\overset{{{\displaystyle p}}_{in}}{\int }}\frac{dp}{p}}$$

(17)

where τ_in is the time constant of the CAT discharge under choked flow conditions in the CV block,

$${{\displaystyle \tau }}_{in}={\displaystyle \frac{V}{nR{{\displaystyle T}}_{in}C{\rho }_N\sqrt{{{\displaystyle T}}_N/{{\displaystyle T}}_s}}}$$

(18)

where T_in is the temperature of the CAT discharge under choked flow conditions, determined from the polytropic process,

$${{\displaystyle T}}_{in}={{\displaystyle T}}_s{{\displaystyle \left({{\displaystyle p}}_{in}/{{\displaystyle p}}_s\right)}}^{(n-1)/n}$$

(19)

From Equation (17), the calculated time interval Δt_c of CAT discharge under choked flow conditions in the CV block is as follows:

$${{\displaystyle \Delta t}}_{in}={{\displaystyle t}}_{in}-{{\displaystyle t}}_s={{\displaystyle \tau }}_{in}\ln \left({\displaystyle \frac{{{\displaystyle p}}_s}{{{\displaystyle p}}_{in}}}\right)$$

(20)

From Equation (19), the CAT discharge pressure p(t) is obtained in the range from p_in to p_c and the time interval Δt_c,

$$p(t)={{\displaystyle p}}_s{{\displaystyle e}}^{-t/{{\displaystyle \tau }}_{in}}\hspace{1em}\mathrm{for}\hspace{1em}{t}_s\le t\le t_{in}$$

(21)

3.2.2. Solution for the Subsonic Flow Condition in the CV Block

Under subsonic flow conditions through the PCV block, the CAT pressure drops from the intermediate pressure p_in to the AM pressure p_M in the time interval between the intermediate time t_in and the CAT discharge time t_d. According to these conditions, Equation (5) can be written as follows,

$${\displaystyle \underset{{{\displaystyle t}}_{in}}{\overset{{{\displaystyle t}}_d}{\int }}dt}=-{\displaystyle \frac{V}{nRT}}{\displaystyle \underset{{{\displaystyle p}}_{in}}{\overset{{{\displaystyle p}}_M}{\int }}\frac{dp}{{{\displaystyle q}}_m(p)}}={{\displaystyle \tau }}_d\left(1-b\right){\displaystyle \underset{{{\displaystyle p}}_{in}}{\overset{{{\displaystyle p}}_M}{\int }}\frac{dp}{\sqrt{{{\displaystyle p}}^2{{\displaystyle \left(1-b\right)}}^2-{\left({{\displaystyle p}}_M-bp\right)}^2}}}$$

(22)

where τ_d is the time constant of the CAT discharge under subsonic flow conditions in the PCV block,

$${{\displaystyle \tau }}_d={\displaystyle \frac{V}{nR{{\displaystyle T}}_{d}C{\rho }_N\sqrt{{{\displaystyle T}}_N/{{\displaystyle T}}_s}}}$$

(23)

where T_d is the CAT discharge temperature under subsonic flow conditions, determined from the polytropic process,

$${{\displaystyle T}}_d={{\displaystyle T}}_{in}{{\displaystyle \left({{\displaystyle p}}_M/{{\displaystyle p}}_{in}\right)}}^{(n-1)/n}$$

(24)

From Equation (22), it is possible to calculate the time interval Δt_s of CAT discharge under subsonic flow conditions in the PCV block using the integral function solution.

$${{\displaystyle \Delta t}}_d={{\displaystyle \tau }}_d\left(1-b\right){\displaystyle \underset{{{\displaystyle p}}_{in}}{\overset{{{\displaystyle p}}_M}{\int }}\frac{dp}{\sqrt{{{\displaystyle p}}^2{{\displaystyle \left(1-b\right)}}^2-{\left({{\displaystyle p}}_M-bp\right)}^2}}}={{\displaystyle \tau }}_d\left(1-b\right){{\displaystyle f}}_q$$

(25)

where f_q is the approximate integral of the function solved from p_in to p_M using a Matlab quad numerical function based on the recursive adaptive Simpson quadrature,

$${{\displaystyle f}}_q={\displaystyle \underset{{{\displaystyle p}}_{in}}{\overset{{{\displaystyle p}}_M}{\int }}\frac{dp}{\sqrt{{{\displaystyle p}}^2{{\displaystyle \left(1-b\right)}}^2-{\left({{\displaystyle p}}_M-bp\right)}^2}}}$$

(26)

The total discharge time t of the CAT is determined by summating the time ranges of Δt_in and Δt_d from Equations (19) and (25),

$$t={{\displaystyle \Delta t}}_{in}+{{\displaystyle \Delta t}}_d={{\displaystyle \tau }}_{in}\ln \left({\displaystyle \frac{{{\displaystyle p}}_s}{{{\displaystyle p}}_{in}}}\right)+{{\displaystyle \tau }}_d\left(1-b\right){{\displaystyle f}}_q$$

(27)

From (17), the derivative of the CAT discharge pressure dp/dt is determined in the pressure range of <p_in, p_M>, which was numerically solved using the Runge–Kutta function ode45 in Matlab R2024,

$${\displaystyle \frac{dp}{dt}}=-{\displaystyle \frac{p}{{{\displaystyle \tau }}_d}}\sqrt{1-{\left({\displaystyle \frac{{{\displaystyle p}}_M/p-b}{1-b}}\right)}^2}\hspace{1em}\mathrm{for}\hspace{1em}{t}_{in}\le t\le t_d$$

(28)

3.3. Computational Model of an AM

A method was proposed to select the AM for PPSs on the basis of the parameter specifications and static characteristics provided in the manufacturer’s data sheet.

The linear equation for the dimensionless static characteristic of the torque versus the rotational speed of the AM is expressed as follows:

$${\displaystyle \frac{{{\displaystyle T}}_M}{{{\displaystyle T}}_{MS}({{\displaystyle p}}_M)}}=1-{\displaystyle \frac{{{\displaystyle n}}_M}{{{\displaystyle n}}_{MF}({{\displaystyle p}}_M)}}\Rightarrow {{\displaystyle T}}_M={{\displaystyle T}}_{MS}({{\displaystyle p}}_M)\left(1-{\displaystyle \frac{{{\displaystyle n}}_M}{{{\displaystyle n}}_{MF}({{\displaystyle p}}_M)}}\right)$$

(29)

where T_M is the torque, n_M is the rotational speed, p_M is the operating pressure, T_MS is the stall torque for n_M = 0, and n_MF is the free rotational speed (idle) for T_M = 0.

The mechanical power of the AM, after considering the static torque characteristic according to (29), is as follows:

$${{\displaystyle P}}_M={\displaystyle \frac{\pi }{30}}{{\displaystyle n}}_M{{\displaystyle T}}_M={\displaystyle \frac{\pi }{30}}{{\displaystyle n}}_M{{\displaystyle T}}_{MS}({{\displaystyle p}}_{Mi})\left[1-{\displaystyle \frac{{{\displaystyle n}}_M}{{{\displaystyle n}}_{MF}({{\displaystyle p}}_{Mi})}}\right]$$

(30)

The maximum mechanical power P_Mmax of the AM is defined as the nominal rotational speed n_Mnom equal to 50% of the free rotational speed n_MF, and the nominal torque T_Mnom equals 50% of the stall torque T_MS. The dimensionless equation for the mechanical power of AM versus the rotational speed is as follows:

$${\displaystyle \frac{{{\displaystyle P}}_M}{{{\displaystyle P}}_{M\max }}}={\displaystyle \frac{1}{0.25}}{\displaystyle \frac{{{\displaystyle n}}_M}{{{\displaystyle n}}_{MF}({{\displaystyle p}}_M)}}\left[1-{\displaystyle \frac{{{\displaystyle n}}_M}{{{\displaystyle n}}_{MF}({{\displaystyle p}}_M)}}\right]$$

(31)

The air expansion power of AM is determined for the adiabatic expansion process, the operating pressure p_M decreasing to the atmospheric pressure p_a,

$${{\displaystyle P}}_A={\displaystyle \frac{\kappa }{\kappa -1}}{{\displaystyle q}}_M{{\displaystyle p}}_M\left[1-{{\displaystyle \left(\frac{{{\displaystyle p}}_a}{{{\displaystyle p}}_M}\right)}}^{{\displaystyle \frac{\kappa -1}{\kappa }}}\right]$$

(32)

where κ is the adiabatic index, q_M is the air consumption of AM,

$${{\displaystyle q}}_M={\displaystyle \frac{1}{{{\displaystyle \eta }}_q}}{\displaystyle \frac{1}{60}}{{\displaystyle V}}_M{{\displaystyle n}}_{Mn}{\displaystyle \frac{{{\displaystyle p}}_M}{{{\displaystyle p}}_a}}$$

(33)

where V_M is the displacement of AM, n_Mn is the nominal rotational speed, η_q is the flow efficiency of AM.

The maximum mechanical power P_Mmax of the AM after considering (28) and (32) takes the following form:

$${{\displaystyle P}}_{M\max }={\displaystyle \frac{{{\displaystyle \eta }}_m}{{{\displaystyle \eta }}_q}}{{\displaystyle P}}_A={\displaystyle \frac{{{\displaystyle \eta }}_m}{{{\displaystyle \eta }}_q}}{\displaystyle \frac{\kappa }{\kappa -1}}{\displaystyle \frac{1}{60}}{{\displaystyle V}}_M{{\displaystyle n}}_{Mn}{{\displaystyle p}}_a\left[{{\displaystyle \left(\frac{{{\displaystyle p}}_M}{{{\displaystyle p}}_a}\right)}}^2-{{\displaystyle \left(\frac{{{\displaystyle p}}_M}{{{\displaystyle p}}_a}\right)}}^{{\displaystyle \frac{\kappa +1}{\kappa }}}\right]$$

(34)

where η_m is the power efficiency of AM, including flow efficiency and mechanical efficiency.

The nominal torque T_Mn at the maximum mechanical power P_Mmax of the AM is as follows:

$${{\displaystyle T}}_{Mn}={\displaystyle \frac{30}{\pi }}{\displaystyle \frac{{{\displaystyle P}}_{M\max }}{{{\displaystyle n}}_{Mn}}}$$

(35)

Equation (6) was transformed into a polynomial function as follows:

$${{\displaystyle \left(\frac{{{\displaystyle p}}_M}{{{\displaystyle p}}_a}\right)}}^2-{{\displaystyle \left(\frac{{{\displaystyle p}}_M}{{{\displaystyle p}}_a}\right)}}^a-{\displaystyle \frac{{{\displaystyle P}}_{Mmax}}{{{\displaystyle K}}_M}}=0$$

(36)

where a is the exponent, a = (κ + 1)/κ, and K_M is a constant:

$${{\displaystyle K}}_M={\displaystyle \frac{{{\displaystyle \eta }}_m}{{{\displaystyle \eta }}_q}}{\displaystyle \frac{\kappa }{\kappa -1}}{\displaystyle \frac{1}{60}}{{\displaystyle V}}_M{{\displaystyle n}}_{Mn}{{\displaystyle p}}_a$$

(37)

Equation (36), as a polynomial function, was numerically solved with respect to the variable X = p_M/p_a and the constant K = P_Mmax/K_M using the Matlab fsolve function,

$$fun(X)={{\displaystyle X}}^2-{{\displaystyle X}}^a-K=0$$

(38)

From the solution (38), the operating pressure AM is obtained, which is p_M = X p_a.

The static characteristics of the torque and power of the GLOBAL RM012 air motor as a function of the rotational speed, determined using the nominal catalog and measured parameters, are shown in Figure 15.

The measured parameters of the GLOBAL RM012 radial piston air motor rotating clockwise (CW) at an operating pressure p_M = 4 bar are as follows: rotational speed n_Mm = 124 rpm, torque T_Mm = 5.1598 Nm, mechanical power P_Mm = 68.2998 W, and air consumption q_Mm = 120 L/min. Based on the static characteristics of the GLOBAL RM012 air motor at an operating pressure p_M = 4 bar, the nominal parameters were determined as follows: torque T_Mn = 3.315 Nm, rotation speed n_Mn = 300 rpm, maximum mechanical power P_Mmax = 104.1438 W, and air consumption q_Mn = 180 L/min [119].

When selecting the RTB parameters, the following key parameters were considered: CAT capacity, which affects range; air consumption, power and torque characteristics of AM, which determine ride dynamics; motor type and location (hub or central) for terrain; and RTB weight, which affects maneuverability and transportability. Attention was also paid to the quality of the braking system, derailleurs, and other mechanical components that ensure the comfort and durability of the ride.

3.4. Comparison of Computation Results with the Field Test

The rotation speed of the RTB wheel n_w = n_M/i = 20.66 rpm was determined based on the chain transmission ratio obtained from the sprocket ratio of i = 6. For wheels with a diameter of D = 0.61 m, the RTB speed was determined to be v = π² D n_w/30 ≈ 4 m/s, which is 14.4 kph. The performance of the RTB ride was presented in PPS mode using the adopted parameters.

The discharge characteristics of the CAT were determined for the case considered when the CAT was charged at a pressure of 300 bar and discharged at an operating pressure of 4 bar in AM. The discharge chart of the CAT through the PCV block to the AM operating pressure is shown in Figure 16. The critical point that separates the choked (sonic) and subsonic flow conditions through the PCV during the discharge of the CAT is marked on the chart.

The computational model shows the parameters of the RTB ride: speed of v = 14.92 kph, time of t = 2011 s = 33.52 min, and mean distance of S = v t = 8.33 km. The RTB field test in PPS mode revealed that the distance traveled was 7.93 km and the travel time was 35 min, resulting in an average speed of 13.6 kph [120]. The results of the simulations and field tests were similar; therefore, it can be assumed that the comprehensive computational model of the PPSs will also be accurate for other driving modes.

RTB meets international safety requirements, such as the ISO 4210 standard [121], which specifies requirements for the safety and performance of bicycles and their components. Covers the design, assembly, and testing of bicycles, including city, trekking, mountain, racing, and folding bicycles. This standard aims to ensure that bicycles are safe to use and meet specified performance criteria. Furthermore, the CAT used in RTB is in accordance with the PED Directive [122], which governs its design, manufacture, operation, and maintenance to ensure safe use of pressure vessels.

Summary

The purpose of this study was to create a new PPS in RBT to achieve practical benefits in terms of speed and distance riding. To achieve this goal, a comprehensive computational model for a PPS consisting of a series of interconnected pneumatic components, such as the CAT, the PCV block, and AM was developed. The comprehensive computational model of PPSs uses the original numerical solution methods of the CAT discharge process with instantaneous air expansion and mass flow rate through the PCV block under choked and subsonic conditions. The operating pressure was calculated using the torque, power characteristics, and compressed air expansion energy in the AM. The results of the PPS simulation obtained using a comprehensive computational model for the parameter values selected from the catalogs and those obtained from the measurements were verified during the RTB test under riding-field conditions. Field tests confirmed that the RTB traction parameters could be selected using a comprehensive computational model of PPSs. The PPSs can also be useful for other categories of bicycles and can be further improved and adapted to modern air vehicles.

4. Future-Proof Energy Efficiency of PPS

The overall energy efficiency of the PPSs depends on the efficiency of converting energy from compressor to CAT, the efficiency of the processes of energy storage in CAT; the efficiency of energy transfer from CAT to expander (EX) as AM. The concept of future-proof energy-efficient PPSs is shown in Figure 17.

Future-proof energy-efficient PPSs will need to use innovative technologies, such as renewable energy sources, energy recovery methods such as regenerative brakes, heat recovery and exchange, the development of highly efficient energy storage in CATs, high-efficiency expanders, and advanced control strategies. Future-proof energy-efficient PPSs can be powered by a hybrid energy system (HES) and includes heat exchangers (HEXs). HES integrates two or more energy sources, such as renewable energy sources (RES), recovered energy, thermal energy, and diesel energy, with complementary power generation profiles to drive the compressor on-board, as well as a micro-scale compressed air energy storage (MS-CAES), to ensure uninterrupted-power PPSs with greater efficiency and stability. This solution ensures is independent of the charging infrastructure.

Heat exchange technology improves the energy efficiency of APVs by improving thermal management by using components such as HEXs to reduce energy loads and improve overall PPS performance. The thermodynamic efficiency of PPSs can be increased by thermal energy storage (TES).

First, the total performance and efficiency of PPSs can be increased by HEX, high-temperature heat exchange (HT-HEX) when air is compressed, and low-temperature thermal energy storage (LT-TES) when air is expanded. The HT-HEX system is used to cool compressed air, and the LT-HEX system is used to heat expanded air.

Second, high-temperature thermal energy storage (HT-TES) is obtained from high-temperature heat exchange (HT-HEX) and low-temperature thermal energy storage (LT-TES) is obtained from low-temperature heat exchange (LT-HEX). Integrating PPSs with the HEX system creates a complex trigeneration system that combines cooling, heating, and thermal energy storage. The integration of PPSs with HEX reduces energy losses and improves the efficiency of vehicle propulsion systems.

All elements and modules of the PPS prototype will be scaled to vehicle applications, including electric motor (M), air compressor (C), CAT, and expanders such as AM or AT. The scaling of PPSs takes into account the parameters of energy conversion from HESs to PPSs by individual modules such as the M-to-C, C-to-CAT through the inlet valve (AVin), CAT-to-AM (AT) through the outlet valve (AVout), and AM(AT) to wheels. Based on the energy conversion, the construction and operational parameters of all elements of the PPSs are selected: M, C, CAT, AVin, AVout, AM. Expert systems will be used to select elements and modules for the scaled and integrated PPS prototype. An expert system will leverage knowledge and rules related to the design of the PPS prototype to provide advice to optimize and predict the capabilities for selecting the components of the PPS prototype. An expert system can use algorithms to find the best combination of parameters for a PPS prototype to maximize efficiency, minimize costs, and meet specific operational requirements. The expert system could help determine the optimal design of the PPS prototype, considering factors such as storage capacity, compression/expansion methods, thermal management, and integration with other power sources. An expert system could recommend specific compressors, expanders, storage tanks, air motors, air valves, and other components based on their performance characteristics, cost, and compatibility with the overall system.

Development of the PPS energy-efficient concept based on the following principles.

• PPSs will be optimized for energy efficiency, especially when applied to vehicles.
• Determine the optimal air pressure for storage in CAT considering factors such as energy density and storage capacity.
• Adoption of accurate methods to model air compression and expansion in different thermodynamic scenarios.
• The best optimization methods should be adopted to minimize energy losses and transfer energy.
• The evaluation of many stages of compression and expansion in CAT should be used to obtain an energy-efficient PPS system.
• The numerical analysis of PPSs can be used to validate an experiment for comparison with real data.
• The advanced forecasting and predictive analysis methods adopted can be applied in the context of energy storage in PPSs to improve operational efficiency.
• Evaluation of external factors (eg changes in temperature, humidity, and wind) that have the greatest impact on PPS performance.
• Evaluation of modern CAT materials, which can reduce degradation and improve the efficiency of long-term energy storage.

Currently, the experimental version of the prototype PPS is based on the development, testing, and validation of novel construction concepts and control strategies to optimize the energy efficiency and integrity of energy storage in the CAV.

The authors are applying for a grant from the European Funds for Smart Economy Programme (EFSE) for the time period 2021–2027 on the topic of “New approaches to energy-efficient PPSs for applications of light vehicles powered by air.”

5. Conclusions

Energy-efficient use of PPSs in vehicles involves optimizing compressed air energy, improving propulsion system design, and employing advanced propulsion strategies. This can result in a decrease in energy consumption, operating costs, and environmental impacts.

Vehicles that use PPSs are still largely in the prototype and research stages, with limited commercial availability and high costs. Although they offer potential benefits such as zero tailpipe emissions and the possibility of integrating with renewable energy sources, challenges related to their efficiency, range, and storage capacity have hindered their widespread adoption. The transition to greener and more sustainable PPSs, which are used as both a primary power source and an auxiliary power unit for vehicles, can lead to technological advances in improving their overall efficiency and the adoption of zero-carbon-emission solutions. Although some companies have developed prototypes and announced production plans, there are not many commercially available CAVs. Optimizing PPSs, hybrid solutions (combining compressed air with other energy sources), and improving energy storage efficiency are current research and development areas. Investigating PPSs in CAVs is crucial because of their use of clean and renewable energy, their ability to store energy, and their rapid energy transfer capabilities. They are also environmentally friendly, cost-effective, affordable, simple to construct, safe, and compatible with compressed air systems (CASs) that use renewable energy sources (RES).

The future of PPSs in vehicles is evolving, and ongoing research is focused on improving their energy efficiency and integration with emerging technologies such as autonomous, hybrid and electric vehicles, making PPSs a potential component of a cleaner, greener, and more sustainable transportation future. Autonomous, hybrid, and electric vehicles are three key trends shaping the future of the automotive industry, often intertwining and influencing one another. Electric vehicles focus on replacing traditional combustion engines with electric powertrains, while hybrid vehicles involve combining combustion and electric powertrains in a single vehicle. Autonomous vehicles, or vehicle automation, involve the development of systems that allow vehicles to operate with minimal or no human intervention.

PPSs can play an important role in promoting cleaner, greener, and more sustainable transportation, particularly when integrated with renewable energy sources. However, substantial progress in fields such as air compression, storage, and expansion is essential to improve the energy efficiency of PPSs and increase their competitiveness with other propulsion technologies. PPSs do not work efficiently for energy conversion, which means that we must wait for better and more effective solutions to be developed. Therefore, the future of air-powered vehicles is not promising at present. This is unfortunate because the rare earth metals used in electric vehicles are less available and much more polluting than most people realize.

Air cars (ACs) policy frameworks are still non-existent, generally relying on general regulations and a focus on promoting cleaner transportation and sustainable mobility. Current frameworks prioritize emission reductions and frequently offer incentives for the adoption of clean vehicles, such as tax breaks and financial aid. Some countries are exploring mandates for zero-emission vehicles and significant reductions in greenhouse gas emissions. Regulations also address vehicle safety certifications, especially for high-pressure compressed air tanks used in APVs.

The concepts of future-proof and energy-efficient PPS with HESs, HEX, on-board compressors and MS-CAES systems were presented. This solution ensures uninterrupted-power PPSs with greater efficiency and stability and is independent of the charging infrastructure. PPSs can then be applied to road, maritime, and military vehicles for long-distance transportation.

This review describes the development periods of APVs (the first two decades of this century). Currently, there are no commercial APVs in production. Research work in CAE focuses on designing more efficient engines through modifications to ICEs, optimizing valve timing, and developing innovative multistage expansion systems. Niche solutions include LAVs, such as bicycles, tricycles, and quads, which are manufactured by student groups, start-ups, or are developed within the framework of research projects such as our RTB.

Future research on PPSs will focus on the fundamental principles within concepts and feasibility studies.

• The basic problem to be solved is the development of modular, scalable and trigenerative PPSs that will ensure efficient propulsion of CAVs.
• The solution to the modularity problem of PPSs consists of modules for the selection of energy conversion, air compression and expansion, and heat exchange, which meet the requirements of the optimal energy efficiency of the CAVs.
• The solution to the PPS scalability problem is to adapt its applications to various CAV vehicles.
• The problem of integrating PPSs with HRX and TES must be solved to reduce the energy loss of the trigeneration system and improve the energy conversion and overall CAV propulsion efficiency of the CAV.
• Solution of the problem of multi-objective optimization of energy storage processes in CAT in real time, based on measurement systems and computational algorithms using artificial intelligence (AI) tools.
• It is imperative to devise solutions that will mitigate the impact of material degradation on PPS components, particularly CAT, thus improving their longevity and efficiency over time.