The Comprehensive Effects of Nano Additives on Biodiesel Engines—A Review

Abstract

In modern society where fossil fuel prices are increasing and environmental issues are becoming more severe, biodiesel, as a new type of clean fuel, is receiving increasing attention. Biodiesel has the advantages of renewability, environmental friendliness, and good fuel properties, demonstrating broad application prospects. However, the use of biodiesel also faces some challenges, such as higher density and kinematic viscosity, lower calorific value, etc. The application of nanoparticles in biodiesel engines helps to achieve the goal of clean fuel. In terms of fuel characteristics, nanoparticles increase the calorific value, cetane value, and flash point of the fuel, improving combustion efficiency and safety, but increasing density may affect combustion. The use of nanoparticles can promote micro explosions and secondary atomization of fuel, improve combustion characteristics, and increase cylinder pressure, heat release rate, and brake thermal efficiency while reducing fuel consumption. Nanoparticles reduce HC and CO emissions, improve combustion through higher oxygen and reaction area, and reduce incomplete combustion products. On the contrary, nanoparticles also increase CO₂ emissions because better combustion conditions promote oxidation reactions. For NO_X emissions, some nanoparticles lower the combustion temperature to reduce emissions, while others increase emissions. Comparison shows that all nanoparticles offer varying degrees of improvement in engine performance and emissions, but the improvement provided by TiO₂ nanoparticles is significantly better than that of other nanoparticles. In the future, the synergistic effect of multiple nanoparticles should be explored to further improve performance and reduce emissions, achieving effects that cannot be achieved by a single nanoparticle.

1. Introduction

Diesel engines play a crucial role in modern industry and transportation. Their high efficiency and durability make them a major source of power in various fields such as heavy machinery, freight trucks, agricultural equipment, irrigation pumps, and ships [1,2]. The superior performance of diesel engines greatly improves productivity, and they perform well under extreme working conditions, significantly extending the service life of equipment. In addition, diesel engines have relatively high fuel efficiency, which allows them to maintain low fuel consumption even during prolonged high-load operation. However, the widespread use of diesel engines has also brought environmental and health challenges. The nitrogen oxides (NO_X), carbon dioxide (CO₂), and particulate matter (PM) emitted by diesel engines are one of the main sources of air pollution, posing a serious threat to the atmospheric environment and human health [3,4,5]. Nitrogen oxides can form photochemical smog in the atmosphere, while particulate matter can directly enter the human respiratory system, causing or exacerbating health problems such as respiratory and cardiovascular diseases. CO₂ can absorb and re-radiate infrared radiation emitted from the Earth’s surface, leading to an increase in heat in the atmosphere and causing global warming and climate change. Therefore, CO₂ is considered one of the main driving factors of the greenhouse effect. These emissions not only affect urban air quality but also have a negative impact on global climate change. In addition, with the development of the global economy and population growth, the consumption rate of fossil fuel resources is constantly accelerating, leading to a gradually tight supply of fossil fuels such as oil [6]. The high dependence of diesel engines on fossil fuels poses a risk of fuel shortage, forcing us to re-examine the sustainability of energy use. In this context, finding alternative solutions to clean and renewable energy is particularly urgent [7,8]. Through technological innovation and policy guidance, promoting the development and application of new energy technologies can help reduce dependence on fossil fuels, reduce environmental pollution, and ensure long-term stability of energy supply.

Biodiesel, as a new type of clean fuel, is receiving increasing attention. As shown in Figure 1, biodiesel is mainly made from vegetable oil, animal fat, or waste edible oil through transesterification, and its sources are abundant and renewable.

Compared with traditional fossil fuels, biodiesel has shown significant advantages and feasibility in multiple aspects [10]. Firstly, from the perspective of sources, the raw materials for biodiesel are extensive and sustainable. Plant oils such as soybean oil, rapeseed oil, palm oil, animal fat, and waste fats from the catering industry can all be used to produce biodiesel [11,12]. These raw materials not only can be obtained through agricultural production but also effectively utilize waste resources and reduce environmental pollution. Secondly, biodiesel has significant environmental benefits, and the use of biodiesel can significantly reduce harmful emissions [13,14]. Compared with traditional diesel, biodiesel has an inhibitory effect on greenhouse warming on the Earth because the plants used to produce biodiesel absorb a large amount of CO₂ during their growth process, achieving relative carbon neutrality [15,16]. Moreover, compared to diesel, the chemical structure of biodiesel contains fewer carbon chain structures, which will help reduce the emissions of carbon oxides from compression ignition engines [17,18]. In addition, the sulfur oxides (SO_X) and PM generated during the combustion of biodiesel are greatly reduced, which helps to improve air quality and reduce the formation of acid rain and haze [19]. From the perspective of fuel characteristics, biodiesel has good combustion and lubrication performance. Biodiesel has a higher flash point, better safety, and is less likely to cause fire accidents. It has strong lubricity, which can reduce the wear of engine components and extend the service life of the engine. In addition, biodiesel can be mixed with traditional diesel, is compatible with existing engines, and does not require large-scale modifications, making it easy to promote and apply [20,21]. Biodiesel, as a new type of clean fuel, is renewable, environmentally friendly, and has good fuel characteristics, demonstrating broad application prospects [22].

However, the feasibility of biodiesel also faces some challenges. Firstly, the production cost of biodiesel is still higher than that of traditional diesel, mainly due to limitations in raw material costs and production processes [23]. With technological progress and the realization of economies of scale, production costs are expected to gradually decrease. Secondly, biodiesel has poor low-temperature performance and may experience solidification under cold climate conditions [24], which needs to be addressed through improved processes and the addition of anticoagulants. In terms of combustion, the high viscosity of biodiesel directly affects its application in engines, as high viscosity can affect fuel injection and atomization, further affecting combustion efficiency [25,26]. Some research results indicate that the high oxygen content of biodiesel promotes the generation of exhaust gases such as NO_X, resulting in higher NO_X emissions.

In existing research, the method of improving the combustion performance of fuel by changing its basic properties through physical, chemical, or other means is called fuel modification technology [27]. Adding nanoparticles, alcohols, water, and hydrogen to fuel as fuel additives is a common method of fuel modification technology [28,29,30,31]. Among them, metal and non-metal nanoparticles have attracted much attention due to their diverse types, high reaction area, high heat transfer ability, high oxygen content, high catalytic activity, and high redox activity [32,33]. Figure 2 shows the advantages of some nanoparticles as fuel additives.

Scholars from multiple countries have found that the use of nanoparticles in biodiesel effectively increases the reaction area, oxygen content, and reaction rate of the fuel, playing a positive role in the combustion, performance, and emissions of biodiesel engines. A higher reaction area and combustion rate improve the mixing degree of fuel and air, allowing the fuel to be fully burned. The high oxygen content accelerates the oxidation reaction rate at high temperatures, promotes the binding of N and C atoms with oxygen atoms, and leads to a certain upward trend in NO_X and CO₂ emissions. However, some research results also show that metal nanoparticles can absorb oxygen to reverse-convert NO_X and CO₂, thereby reducing NO_X and CO₂ emissions [35]. As is well known, compared to diesel, biodiesel has a higher density and kinematic viscosity, which in turn affects injection and atomization conditions, greatly affecting engine performance and emissions. The addition of nanoparticles can greatly improve injection and atomization conditions. Nanoparticles increase the thermal conductivity of fuel, allowing it to absorb heat faster and more evenly at high temperatures, forming a larger temperature gradient. This temperature gradient causes a significant local vapor pressure difference inside the fuel, causing it to reach a state of overheating and expansion, promoting the rupture of droplets and triggering micro explosions. After the micro explosions, many fuel droplets containing nanoparticles are formed to promote secondary atomization. This phenomenon causes fuel droplets to rapidly rupture into smaller particles, increasing surface area, facilitating thorough mixing of fuel and air, improving combustion stability and uniformity, improving engine performance, and reducing fuel consumption [36,37,38,39].

In addition, many scholars have combined nanoparticles with other types of additives to explore the combined effects of various additives on biodiesel engines. When nanoparticles are mixed with additives such as ethanol, hydrogen, and water, the advantages of high thermal diffusion rate and low combustion temperature of ethanol, water, and hydrogen, combined with the high reaction area of nanoparticles, have resulted in better combustion performance and emissions [40,41,42].

As shown above, nanoparticles play a positive role in improving the combustion characteristics of biodiesel. With the increasing demands of modern society for its power, environment, and new energy, nano biofuels play a crucial role and have broad application space. In existing research, there have been many studies on nano-biofuels, but there is a lack of comprehensive summary of nano-biofuels, making it difficult to intuitively understand their development process. In order to make up for this deficiency and facilitate other scholars to have a more intuitive understanding of the application of existing nano-fuel technology in compression ignition engines, this article comprehensively reviews and organizes the latest applications and discoveries of nano-biofuels in recent years. This article is divided into five parts. The first part introduces the inevitability and superiority of the application of biofuels and nanoparticles under the changing times. The second part introduces the application of nanoparticles in the production, storage, and preparation of biofuels. The third and fourth parts respectively review and analyze the impact of nano-biofuels (including the combined effects of nanoparticles and other additives) on the combustion, performance, and emissions of compression ignition engines. The fifth chapter summarizes the entire article and analyzes future research directions. We hope that the analysis in this article can help relevant researchers and practitioners further understand the issues and prospects of nanotechnology in the combustion performance and emission management of biodiesel engines and better pursue environmentally friendly and efficient biodiesel engine technology, as well as providing assistance for research and development in related fields.

2. The Effect of Nanoparticles on Biodiesel

2.1. The Impact on the Characteristics of Biodiesel

Mixing different nanoparticles with biodiesel can alter the properties of the fuel, including kinematic viscosity, density, calorific value, cetane number, and flash point.

Table 1. Characteristics of different fuels.

Biofuel Type	Blend Type	Density (kg/m3)	Kinematic Viscosity (cSt)	Calorific Value (MJ/kg)	Cetane Number	Flash Point (°C)	Ref.
Terminalia bellirica biodiesel	B20 (20% biodiesel + 100% diesel)	872.6	4.22	40.319	62	142	[43]
	+50 ppm BZnFMO	875.3	4.28	40.858	62	140
	+75 ppm BZnFMO	877.7	4.32	41.065	62	138
Palm oil biodiesel	B100	883.3	4.3	39.6	52	140	[44]
	+45 ppm CeO2	891	4.41	40.95	57	122
Mahua oil biodiesel	B20	850	2.62	41.114	/	43	[45]
	+25 ppm TiO2	852	2.7	41.168	/	40
	+100 ppm TiO2	858	2.74	41.283	/	41
Mesua ferrea biodiesel	B20	872	3.5	39.66	58	152	[46]
	+60 ppm Cr2O3	876	3.61	40.34	60	147
	+100 ppm Cr2O3	882	3.73	40.93	61	138
Guizotia abyssinica (L.) biodiesel	B10	820.1	3.21	43.0	46	75	[47]
	+100 ppm AI2O3	834	2.91	45.0	53	79
	+100 ppm TiO2	844	2.98	44.3	52	81
Raw linseed oil biodiesel	B20	834.2	3.36	40.03	49	107	[48]
	+50 ppm Co3O4	836	3.46	43.16	49.7	109
	+100 ppm Co3O4	838	3.56	43.27	50.4	110
Waste frying oil biodiesel	B30	852	4.9	45.29	/	76	[49]
	+300 ppm ferrocene	853	4.85	/	/	76
Waste frying oil biodiesel	B10	844.6	3.17	41.89	51.147	>100	[50]
	+100 ppm TiO2	844.7	3.19	42.73	52.134	>100
	+100 ppm AI2O3	844.7	3.19	43.05	51.983	>100
	+100 ppm SiO2	844.7	3.19	42.89	51.676	>100
Canola oil biodiesel	B20	839.25	3.214	/	/	80	[51]
	+50 ppm quantum dots	840.43	3.236	/	/	77
	+100 ppm quantum dots	840.36	3.23	/	/	77
Jojoba biodiesel	B20	845.36	3.59	41.93	/	71	[52]
	+25 ppm CuO	858.15	3.68	41.22	/	66
	+75 ppm CuO	871.17	3.87	41.66	/	63
Pithecellobium Dulce seed-derived biodiesel	B20	838	3.22	41.9	55	65	[53]
	+100 ppm Groundnut shell nanoparticle	840	3.20	42.1	54	64
parsley biodiesel	B20	901	4.01	40.24	54.2	108	[54]
	+50 ppm SiO2	907	4.08	41.38	57.2	120
	+100 ppm SiO2	918	4.14	42.53	62.8	127

summarizes the effects of nanoparticles on fuel properties in some studies.

As shown in Table 1, the experiment shows that as the amount of nanoparticles added increases, the density of the fuel usually increases. This is because nanoparticles themselves typically have a high density, and when they are dispersed into the fuel, it increases the density of the entire fuel system. However, the kinematic viscosity of fuel exhibits a complex trend of change, which may be lower or higher than the base fuel. This mainly depends on the relationship between the dynamic viscosity of the fluid and the fluid density [55]. Some nanoparticles have unique lubrication properties or undergo special surface treatments, which can significantly reduce the intermolecular forces within the fuel, reduce intermolecular friction, and thus lower the kinematic viscosity of the fuel. For example, some nanoparticles are surface-modified with lipophilic groups, which can form a lubricating layer between fuel molecules, reduce intermolecular adhesion and friction, and thus achieve the effect of reducing viscosity. Biodiesel with lower kinematic viscosity has significant advantages in practical applications. Firstly, lower kinematic viscosity facilitates the atomization process of fuel. A good atomization effect means that the fuel can be more evenly dispersed in the combustion chamber, forming small fuel droplets and increasing the contact area between the fuel and air, thereby improving combustion efficiency. Secondly, lower kinematic viscosity can also reduce the resistance of fuel in transportation pipelines and injection systems, improve fuel flow performance, and ensure the stability and accuracy of fuel injection.

Calorific value is the total amount of heat released per unit volume of fuel under complete combustion, which directly affects engine efficiency. In almost all studies, the addition of nanoparticles increases the calorific value of the fuel compared to the base fuel. Firstly, many nanoparticles themselves have high calorific values. For example, some metal nanoparticles can release a large amount of thermal energy during the combustion process, directly increasing the overall calorific value of the blends. Secondly, nanoparticles can serve as catalysts during the combustion process, providing more reaction area. Due to their extremely small size and relatively large surface area, nanoparticles allow fuel molecules to react more fully on the surface of the nanoparticles. Nanoparticle catalysts promote the decomposition and oxidation of fuel molecules by reducing the activation energy of combustion reactions, thereby accelerating the combustion process and increasing the calorific value of fuel [56].

The cetane number has a great impact on the ignition performance of fuel. The higher the cetane number, the shorter the ignition delay of the fuel, with a corresponding improvement in ignition performance. As the nanoparticles are mixed into the base fuel, the cetane number of the fuel increases to some extent. Even with the addition of nanoparticles, there is no significant improvement in the flash point of the fuel, resulting in a decrease. However, the impact on biodiesel fuel is relatively small because compared to diesel, biodiesel has a higher flash point, which can ensure the safety of transportation and storage.

2.2. Application of Nanoparticles in Biodiesel Production

At present, transesterification is the main way to produce biodiesel, which mainly changes the basic properties of animal and plant fats through complex chemical reactions between fats, methanol, and homogeneous or heterogeneous catalysts, making them similar to diesel to the greatest extent possible. Transesterification can significantly reduce the inherent high viscosity and density of animal and plant oils, making them more suitable for use as fuel.

Although transesterification has significant advantages in biodiesel production, it still faces some challenges in practical applications. Firstly, saponification reactions are a common issue. When the content of free fatty acids in fats is high, catalysts may react with these fatty acids to produce soap, leading to a decrease in reaction efficiency and a decrease in biodiesel production. Excessive free fatty acids are also a challenge. High levels of free fatty acids not only lead to saponification reactions but also increase the difficulty of subsequent separation and purification. Secondly, when using vegetable oil and waste edible oil to produce biodiesel, the vegetable oil and waste edible oil usually contain unsaturated fatty acids, which are easily oxidized by oxygen in the air, leading to oil rancidity. In the production of vegetable oil and waste edible oil biodiesel, hydrogenation treatment can partially or completely convert the double bonds of unsaturated fatty acids into saturated bonds, thereby improving the stability of the oil and prolonging its storage and service life. In addition, the separation of additives during the reaction process is difficult, especially when using homogeneous catalysts, where the reactants contain a large amount of catalyst, unreacted methanol, and by-products, all of which require effective separation and recovery treatment [57,58,59]. To overcome these difficulties, researchers from various countries have added nanoparticles as additives to the production of biodiesel. Elsie Bet Moushoul et al. [60] prepared and used CaO based/Au nanoparticles as heterogeneous catalysts for biodiesel production. After research and comparison, they found that the quality of biodiesel samples was higher when using CaO-AuNPs catalysts, and the catalyst could be reused more than 10 times without losing catalytic activity. In the study by Binta Hadi Jume et al. [61], GO@ZrO₂-SrO nanocatalysts were used as heterogeneous catalysts for ester exchange reactions; GO@ZrO₂-SrO nanocatalysts have the advantages of multiple reusability, fast mass transfer, low cost, and high productivity. When reacted at 120 °C for 90 min in a molar ratio of 1:4, the productivity can reach up to 91%. Alex Tangy et al. [62] mentioned in their review that strontium oxide is a highly active and reusable solid alkaline catalyst. They believe that the strong activity exhibited by SrO due to its strong alkalinity has a promoting effect on biodiesel production. Koberg Miri et al. [63] also expressed similar views. Basir Maleki et al. [64] synthesized αFe₂O₃ 1−x/ZnO_X and αFe₂O₃ nanoparticles by a sol-gel method. In the transesterification reaction, they found the best combination at x = 47.24 wt%, at which the biodiesel production rate was about 94.21%. The catalytic activity was still maintained after seven consecutive uses and it was more stable than simple αFe₂O₃ nanoparticles. Bishwajit Changmai et al. [65] studied a highly alkaline magnetic nanocatalyst for the production of biodiesel. The catalyst has a solid core–shell structure which not only enhances surface performance but also greatly improves stability. They found that the maximum production rate of biodiesel using this catalyst can reach 98%. In addition, the catalyst has magnetism and can be easily recovered through the use of external magnets and other methods while still maintaining high reactivity after nine repetitions. Adib Bin Rashid [66] also expressed the same viewpoint when studying magnetic nanocatalysts.

2.3. Preparation of Nanoscale Biofuels

Nano biodiesel fuel, also known as nanofluid, is a blend made by dispersing nanoparticles into biodiesel fuel through certain methods. This fuel combines the advantages of nanotechnology and biodiesel, aiming to improve fuel performance and reduce pollution emissions. The nanoparticles used in the preparation of nano-biodiesel fuel can be divided into metallic and non-metallic nanoparticles based on their material types, such as CeO₂, SiO₂, CuO, AI₂O₃, carbon nanotube (CNT), and carbon nanoparticle (CNP). The method for preparing some nanoparticles is shown in Figure 3.

The synthesis process of nanoparticles is mainly achieved through chemical, physical, and mechanical means. Mechanical means refer to using cutting, grinding, high-voltage impact, and other methods to cut larger materials to the nanoscale; physical and chemical means include the sol-gel method, laser ablation method, chemical vapor deposition, combustion method, nanoprecipitation, microwave radiation, and hydrothermal method [68,69,70,71,72].

At present, the most commonly used method for preparing nano biodiesel fuel is the two-step method. This method involves preparing nanoparticles and then uniformly mixing these particles into diesel fuel. During the mixing process, techniques such as mechanical stirring, magnetic stirring, and ultrasound are commonly used to ensure the uniform distribution of nanoparticles in biodiesel. Compared to the one-step method, the two-step method has greatly improved the chemical stability of nano biofuels. In addition to long-term physical stirring, the use of chemical dispersants and agents is also one of the commonly used methods [27]. These dispersants form a long circular covering structure on the surface of nanoparticles [73], which can effectively prevent mutual attraction and aggregation of nanoparticles, ensure their uniform distribution in biodiesel, and maintain fuel stability and excellent performance [74]. Medications prevent aggregation by increasing the interception ratio between nanoparticles. In addition, some advanced preparation technologies and processes, such as high shear stirring and microemulsion technology, have also been applied in the preparation of nano-biofuels to further improve their dispersibility and stability. This is undoubtedly very important because the aggregation of nanoparticles can lead to changes in fuel properties and reduce the heat transfer rate of nano biodiesel fuel and may lead to a decrease in fuel combustion efficiency, increase emissions of harmful substances, and even block engine parts, thereby causing engine failure [75,76]. Among the several nano-fuel preparation methods mentioned above, scholars from various countries will not use all of them in their research, as they believe this is sufficient to meet the research requirements [77].

N. Mohanrajhu et al. [78] used an ultrasonic device to mix Al (NO₃)₃ and graphene oxide nanoplates (GONP_S) with biodiesel, and the mixing process lasted for one hour. In addition, they also used a mechanical stirring device to stir the fuel, further increasing the stability of the nano-biodiesel fuel. Mishamo Tesfaye Lamore et al. [79] mentioned in their study that ultrasound technology is the most effective method for producing nano biodiesel fuel. They used a bath ultrasonic processor to uniformly disperse Al₂O₃ and CeO₂ nanoparticles into distilled water, and the reaction was carried out at a power of 120 W for 45 min. Subsequently, they used a mechanical stirring device to mix the previously obtained nanofluid with biodiesel fuel at a uniform speed of 1000 rpm, further producing emulsified fuel. Finally, to further ensure the stability of nano-biofuels, they added surfactants to prevent the precipitation of nanoparticles. Sarah Oluwabunmi Bitire et al. [80] dispersed TiO₂ nanoparticles into B20 parsley biodiesel using a 600 W power, 28 kHz/40 kHz dual frequency ultrasonic device, followed by stirring at a speed of 450 rpm for 60 min using a stirring device. The resulting nano biodiesel fuel was mixed uniformly without any precipitation of nanoparticles. Similarly, Mukul Tomar et al. [81] also used ultrasound, but when using ultrasound treatment, they placed containers containing fuel and nanoparticles in distilled water baths. Distilled water bath can provide a stable and uniform temperature environment, which helps to control the temperature during ultrasonic treatment. They believe that this has a positive effect on the mixing of nanoparticles. Finally, they added 0.5% non-ionic surfactant to further reduce the adsorption force between particles, prevent agglomeration, and improve the uniformity and stability of the experimental fuel. Harish Venu et al. [82] mixed Al₂O₃ nanoparticles into palm biodiesel using an ultrasonic device and a mechanical stirring device. In order to ensure the stability of the prepared fuel, they also added surfactants. The stability of the final prepared nano-biodiesel fuel exceeded 96 h. They emphasized that only after ensuring the stability of the nano-biodiesel fuel can it be put into the engine for experimentation. A. Murugesan et al. [83] also used an ultrasonic device to mix biodiesel and nanoparticles, and after testing, the stability of the prepared fuel exceeded 12 h. The same results were also reported in the study by A. Prabu et al. [84]. In the study by M. S. Gad et al. [85], it was mentioned that ultrasonic treatment is the most effective method to prevent agglomeration between nanoparticles. They prepared nano biodiesel fuel in three steps. Firstly, ultrasound was used to mix the nanoparticles with biodiesel fuel, and then a stirring device was used for stirring. Finally, surfactants were added. They pointed out in the report that the nano biodiesel hybrid fuel produced by the above method can maintain stability for more than two months. In addition, they analyzed the reasons for the agglomeration of nanoparticles in fuel. The cohesive force between particles forms surface tension, reducing the gaps between fuels. As the size and concentration of nanoparticles increase, the surface tension between particles increases. The function of surfactants is to form a film on the surface of nanoparticles, which reduces surface tension by increasing the repulsive force of static electricity. Similar to the above methods, R.S. Gavhane et al. [86] also prepared blends in three steps, namely ultrasonic–mechanical stirring–surfactant.

From the above review, it can be found that most researchers have used a three-step method to prepare nano biodiesel fuel. Firstly, an ultrasonic device is used for mixing for 30–90 min, and then further stirring is carried out in a mechanical stirring device to ensure that the nanoparticles are uniformly dispersed in the fuel. Finally, adding surfactants reduces surface tension, improves fuel stability, and reduces the probability of precipitation and agglomeration. It should be noted that when using an ultrasonic device to disperse nanoparticles, appropriate dispersion time has a positive effect. If treated excessively, it will have the opposite effect, that is, the nanoparticles in the fuel will re-aggregate [87,88,89]. Therefore, further research and improvement of the power and operating time of ultrasonic devices are crucial.

3. Engine Performance

Researchers from around the world have studied the effects of different nanoparticles on the performance of different biodiesel fuel engines. They have compiled these research results and believe that adding nanoparticles to biodiesel and biodiesel blends can improve engine performance.

Table 2. Combustion performance of engines with different fuels.

Fuel Type	BTE	BSFC	HRR	CP	Ref.
B30 + 100 ppm DPA	↑ 2.42% than B30	↓ 2.54% than B30	/	/	[90]
B30 + 50 ppm DPA + 50 ppm CeO2	↑ 5.74% than B30	↓ 6.35% than B30	/	/
B0 + 50 ppm ZnO	↑ 4.34% than B0	↓ 5.6% than B0	24.9 J/deg	↑ 1.82% than B0	[91]
B20 + 50 ppm ZnO	↑ 3.28% than B20	↓ 6.44% than B20	23.88 J/deg	↑ 0.46% than B20
B40 + 50 ppm ZnO	/	↓ 2.5% than B40	22.46 J/deg	/
B20 + 50 ppm RuO2	↑ 12.5%	↓ 5.55% than B20	/	/	[92]
B20 + 100 ppm RuO2	↑ 18.75%	↓ 6.7% than B20	/	/
B20	30.55%	0.3 kg/kWh	60.78 J/deg	51.19 bar	[93]
B20 + 25 ppm CuO	30.75%	0.3 kg/kWh	61.01 J/deg	53.34 bar
B20 + 50 ppm CuO	30.86%	0.297 kg/kWh	63.67 J/deg	53.99 bar
B20 + 75 ppm CuO	31.26%	0.295 kg/kWh	65.37 J/deg	53.24 bar
B100	24.11%	/	/	/	[94]
B100 + 50 ppm TiO2	24.99%	/	/	↑ 2.21% than Diesel mode
B100 + 100 ppm TiO2	25.77%	/	↑ 1.5% than Diesel mode	↑ 5.17% than Diesel mode
B100 + 150 ppm TiO2	26.98%	/	↑ 4.27% than Diesel mode	↑ 6.62% than Diesel mode
B20	29%	0.29 g/kWh	77.3 J/deg	72.8 bar	[95]
B20 + 50 ppm ZnO	31%	0.25 g/kWh	77.9 J/deg	73.4 bar
B20 + 100 ppm ZnO	33.1%	0.22 g/kWh	78.7 J/deg	75 bar
B20 + 50 ppm Ce2O3	↑ 2.6% than B20	↓ 5.9% than B20	↑ 1.2% than B20	↑ 3.6% than B20	[96]
B40 + 50 ppm Ce2O3	↑ 1.1% than B40	↓ 3.7% than B40	/	↑ 1.3% than B40
B25	↑ 13.9% than diesel	↓ 14.75% than diesel	↑ 5.7% than diesel	↑ 3.12% than diesel	[97]
B25 + 150 ppm CeO2	↑ 68.23% than diesel	↓ 41.23% than diesel	↑ 9.8% than diesel	↑ 5.72% than diesel
B25 + 150 ppm SiO2	↑ 49.73% than diesel	↓ 33.41% than diesel	↑ 12.6% than diesel	↑ 18.8% than diesel
B25 + 150 ppm TiO2	↑ 32.4% than diesel	↓ 25.62% than diesel	↑ 20.2% than diesel	↑ 23.4% than diesel

provides a detailed summary of these findings.

3.1. Cylinder Pressure

Fuel and air are mixed in the intake manifold and sucked into the cylinder, where they are compressed and burned, causing the gas to rapidly expand and form cylinder pressure. From the literature summary in Table 2, it can be seen that after adding nanoparticles to biodiesel, cylinder pressure shows a more or less increasing trend. In the study by Mohammed El Adawy [91], the effect of ZnO nanoparticles on the performance and combustion characteristics of biodiesel from waste edible vegetable oil was evaluated. He found that compared to fuel without nanoparticles, the addition of nanoparticles further increased the peak cylinder pressure of the fuel. He believes that the addition of nanoparticles causes micro explosions in the fuel, thereby promoting secondary atomization. In addition, the higher surface area of ZnO nanoparticles provides higher activity, which enables them to come into more effective contact with fuel and oxygen during combustion. At the same time, ZnO nanoparticles can release oxygen at high temperatures, provide additional oxidants, enhance the intensity and speed of combustion reactions, and burn more fuel within a certain period of time, further increasing cylinder pressure. T. Sathish et al. [95] also found the same phenomenon when adding ZnO nanoparticles to biodiesel, explaining that the high reaction area ratio, evaporation rate, and cetane number of nanoparticles combined affect the ignition characteristics of biofuels. In addition, nanoparticles optimize the heat transfer effect during fuel combustion. Akshay Jain et al. [94] investigated the effect of different concentrations of nanoparticles on the efficiency of Eichhornia Crassipes biodiesel engines under different engine load conditions. They found that compared to diesel, the higher viscosity and calorific value of biodiesel led to a decrease in peak cylinder pressure, but the addition of TiO₂ nanoparticles to the fuel significantly increased the peak cylinder pressure. They believe that this is the reason for the higher surface area and heat transfer coefficient of nanoparticles, and it is worth noting that biodiesel fuel with TiO₂ nanoparticles exhibits higher cylinder pressure than diesel. C. Dhayananth Jegan et al. [97] found that the combined effect of higher reaction area of TiO₂ nanoparticles and inherent oxygen in biodiesel improved combustion efficiency. They also believe that the heat transfer during the premixed combustion stage has a significant impact on the combustion of fuel, and the use of nanoparticles undoubtedly enhances the heat transfer effect. In the study by Aman Singh Rajpot et al. [98], it was found that nanoparticles effectively increase cylinder pressure by enhancing in-cylinder evaporation and combustion. S. Jaikumar et al. [99] mentioned that Cr₂O₃ nanoparticles can increase heat during the ignition process. They believe that dispersants promote the dispersion of nanoparticles in the fuel, enhance combustion sustainability, and thus improve combustion potential. In addition, higher surface area and oxygen content are also important factors in increasing cylinder pressure.

3.2. Heat Release Rate (HRR)

The heat release rate represents the sum of energy released by the fuel within a certain period of time, and a higher HRR can increase cylinder pressure and promote complete fuel combustion. In the research of many scholars, the addition of nanoparticles has a positive impact on HRR [69,100,101]. In the study by Mohammed El Adawy [91], it was found that HRR is influenced by fuel characteristics such as calorific value, kinematic viscosity, and density; therefore, B40 exhibits the lowest HRR. He believes that adding ZnO nanoparticles to fuel is a very effective method, whether in diesel or biodiesel. The addition of nanoparticles increases thermal conductivity and oxygen concentration, making fuel easier to burn and increasing the peak heat release. In addition, he believes that the micro explosions and secondary atomization caused by nanoparticles are also important factors. This process improves fuel evaporation and atomization, increases combustion temperature, greatly increases fuel utilization, accelerates fuel combustion speed, and increases heat release per unit time. Shiva Kumar et al. [102] believe that CeO₂ nanoparticles can improve fuel dispersion, prevent local enrichment of fuel, shorten fuel ignition delay, increase the mixing rate of air and fuel, and accelerate heat transfer rate, thereby promoting the early start of the combustion process and effective heat release. In the study by Soudagar et al. [103], it was mentioned that the reason why nanoparticles can improve HRR is that their molecular structure contains oxygen atoms and has a higher reaction area, which can promote better fuel combustion. Ramu Garugubilli et al. [33] also expressed similar views in their study. In addition, they also emphasized that substances such as surfactants have a significant improvement effect on HRR. When studying the effect of ZnO nanoparticles on biodiesel, Ashish Kumar Singh et al. [104] mentioned that nanoparticles have a significant impact on fuel injection speed and penetration rate. Nanoparticles promote better mixing of fuel with air, improve the consistency and uniformity of fuel distribution, promote uniform combustion during fuel combustion, and reduce the generation of fuel enrichment areas. In addition, ZnO nanoparticles also play a positive role in promoting early ignition and rapid combustion.

3.3. Brake Thermal Efficiency (BTE)

A higher BTE indicates a higher efficiency in converting fuel into useful work during the combustion process. The articles listed in Table 2 demonstrate the significant improvement in engine BTE when nanoparticles are mixed with biodiesel fuel. Vijay Kumar et al. [90] found, in their study of the effect of CeO₂ nanoparticles on biodiesel, that the fuel with added nanoparticles exhibited higher BTE compared to biodiesel without additives. They attribute this phenomenon to the catalytic effect of nanoparticles, and in addition, a higher area ratio is also an important factor in promoting better fuel combustion. In the article by Sangeetha Manimaran et al. [92], it was mentioned that the addition of nanoparticles increased the evaporation rate of fuel, improved the mixing effect between fuel and air, and thus promoted complete combustion of the blend. At the same time, nanoparticles accelerate the heat release during the combustion process, which increases the heat transfer rate. They believe that the increase in BTE is influenced by a combination of these factors. V. Meenakshi et al. [105] reported the effect of Ce₂O₃ nanoparticles on the performance of biodiesel engines. The chemical structure of Ce₂O₃ nanoparticles makes them carry a large number of oxygen atoms. These additional oxygen atoms increase the oxygen content in the combustion process, increase the fuel pressure, and then change the fuel spray pressure. At higher injection pressures, fuel atomization is better, oil droplet size is smaller, the combustion effect is enhanced, and combustion stability, uniformity, and flame penetration are increased. Xiujuan Liang et al. [106] argue that the variation of fuel BTE is influenced by the fuel’s calorific value, as fuels with higher calorific values can reduce fuel consumption and release more power during combustion. As shown in Table 1, after adding nanoparticles to the fuel, the calorific value of the fuel increases to a certain extent, which undoubtedly plays a positive role in the utilization of the fuel. B. Chetia et al. [107] also reported the same viewpoint. Meanwhile, they also pointed out that CeO₂ is a type of nanoparticle that combines oxygen storage, high catalytic activity, and high stability. The mutual influence of these characteristics enables more thorough fuel combustion and improves engine power output. The high active surface area of nanoparticles promotes better oxidation of fuel, shortens fuel evaporation time, and improves fuel stability. The heat dissipation of fuel is accelerated within the same combustion time, greatly improving the combustion efficiency of fuel [108]. The above research results further emphasize and prove the good effect of nanoparticles on BTE.

3.4. Brake Specific Fuel Consumption (BSFC)

As summarized in Table 2, adding nanoparticles to biodiesel fuel can increase the fuel calorific value, improve combustion conditions, and enhance fuel utilization, which is one of the effective means to improve fuel BSFC [90,93,94,97]. N. Mohanrajhu et al. [78] reported that under maximum load, compared to diesel and biodiesel fuels, the addition of Al (NO₃)₃ reduced BSFC by 3.57% and 6.89%, respectively. They believe that an increase in the oxidation degree of fuel will reduce fuel consumption. In addition, an increase in heat transfer coefficient can improve combustion, increase fuel utilization, and provide more energy per unit volume. In the study by Tewodros Taye Birhanu et al. [109], an improvement effect of nanoparticles on biodiesel BSFC was also reported. They found that after adding nano additives to the fuel, the calorific value of the fuel increased to a certain extent, and the fuel consumption required to release the same power decreased. In addition, additives also improve the atomization effect of fuel, that is, during the combustion process, the volume of fuel droplets decreases, the fuel distribution becomes more uniform, and the combustion effect is strengthened. Samet USLU et al. [110] also found that the relationship between Fe₃O₄ nanoparticles and biodiesel has a positive impact on fuel calorific value, which in turn affects engine performance. T. Sathish et al. [111] believe that the secondary atomization of ZnO nanoparticles has a significant impact on the changes in BSFC. Secondary atomization is caused by the micro explosion phenomenon of fuel. When nanoparticles are uniformly mixed in the fuel and participate in combustion in the cylinder, the fuel droplets undergo compression and high-temperature expansion to produce micro explosion phenomenon. Larger droplets are dispersed into more small droplets through micro explosions, thus achieving secondary atomization, avoiding local fuel enrichment, and increasing the contact area between fuel and air, thereby improving combustion efficiency and reducing BSFC. The study by Nagarajan Jeyakumar et al. [53] also reported the beneficial effects of micro explosions and secondary atomization caused by nanoparticles on engine BSFC. In addition, they also believe that the chemical structure of nanoparticles reduces the formation of carbon deposits and increases oxygen content, promoting the full combustion of fuel.

4. Exhaust Emissions

Nanoparticles have an improving effect on the emissions of biodiesel and its blends in CI engines, as they can promote secondary atomization and increase reaction area.

Table 3. Engine exhaust emissions when using different nanoparticles.

Fuel Type	CO	HC	CO2	NOX	Ref.
B30 + 100 ppm TiO2	↓ 12.46% than B30	↓ 8.63% than B30	/	↑ 1.84% than B30	[112]
B30 + 100 ppm CNT	↓ 8.5% than B30	/	/	↓ 3.92% than B30
Diesel	0.064 g/kWh	45 g/kWh	/	870 ppm	[92]
B20	0.06 g/kWh	40 g/kWh	/	900 ppm
B20 + 50 ppm RuO2	0.054 g/kWh	38 g/kWh	/	920 ppm
B20 + 100 ppm RuO2	0.05 g/kWh	34g/kWh	/	920 ppm
B20	↓ 11.12% than diesel	↓ 5.18% than diesel	/	/	[52]
B20 + 25 ppm CuO	↓ 12.87% than diesel	↓ 9.39% than diesel	↑ 1.7% than B20	↑ 0.4% than B20
B20 + 50 ppm CuO	↓ 13.15% than diesel	↓ 12.17% than diesel	↑ 3.4% than B20	↑ 0.7% than B20
B20 + 75 ppm CuO	↓ 11.5% than diesel	↓ 7.45% than diesel	↑ 5.16% than B20	↑ 1.8% than B20
EBD	↓ 27.36% than diesel	↓ 11.67% than diesel	↓ 16.99% than diesel	↓ 6.86% than diesel	[94]
EBD + 50 ppm TiO2	↓ 35.85% than diesel	↓ 24.56% than diesel	↓ 12.57% than diesel	↑ 3.05% than diesel
EBD + 100 ppm TiO2	↓ 34.9% than diesel	↓ 27.91% than diesel	↓ 8.01% than diesel	↑ 5.55% than diesel
EBD + 150 ppm TiO2	↓ 41.19% than diesel	↓ 31.8% than diesel	↑ 2.35% than diesel	↑ 8.06% than diesel
Diesel	/	0.5 g/kWh	/	5.02 g/kWh	[95]
B20	/	0.33 g/kWh	/	4.52 g/kWh
B20 + 50 ppm ZnO	/	0.31 g/kWh	/	3.67 g/kWh
B20 + 100 ppm ZnO	/	0.29 g/kWh	/	3.66 g/kWh
B20 + 25 ppm MWCNTs	↓ 19% than B20	↓ 11% than B20	/	↑ 21% than B20	[113]
B20 + 50 ppm MWCNTs	↓ 24.1% than B20	↓ 14.8% than B20	/	↑ 39.5% than B20
B20 + 100 ppm MWCNTs	↓ 7.5% than B20	↓ 4.7% than B20	/	↑ 13.5% than B20
Diesel	/			450 ppm	[114]
B20	/			496 ppm
B20 + 25 ppm TiO2	↓ 3.1% than B20	↓ 8.1% than B20		452 ppm
B30	/			452 ppm
B30 + 25 ppm TiO2	↓ 5.6% than B30	↓ 9.1% than B30		426 ppm
Diesel	9.1 g/kWh	/	11.68 g/kWh	6.2 g/kWh	[97]
B25	10.8 g/kWh	↓ 6.23% than diesel	14.28 g/kWh	/
B25 + 150 ppm CeO2	13.2 g/kWh	↓ 14.3% than diesel	18.18 g/kWh	6.09 g/kWh
B25 + 150 ppm SiO2	9.5 g/kWh	↓ 29.25% than diesel	18.9 g/kWh	4.29 g/kWh
B25 + 150 ppm TiO2	9.1 g/kWh	↓ 31.89% than diesel	/	2.87 g/kWh

summarizes the effects of different types of nanoparticles on exhaust emissions during combustion of biodiesel and its blends in engines.

4.1. Carbon Monoxide (CO)

As shown in Table 3, after adding nanoparticles to the fuel, the fuel exhibited lower CO emissions. Srinivasan Senthil Kumar et al. [115] found that the release of oxygen in biodiesel accelerates oxidation during combustion, resulting in better combustion of the fuel. After adding Al₂O₃ nanoparticles to biofuels, the combustion process undergoes catalytic oxidation reaction, which improves the mixing degree between the fuel and air. They believe that the combination of diesel–biodiesel–Al₂O₃ nanoparticles has a positive effect on reducing CO emissions. When Akshay Jain et al. [94] studied the effect of TiO₂ on biodiesel fuel, they also concluded that nanoparticles reduce emissions by enhancing combustion. Jong Boon Ooi et al. [113] studied the effect of MWCNTs nanoparticles on B20 biodiesel fuel and found that the addition of MWCNTs nanoparticles with a concentration of 50 ppm resulted in the lowest CO emissions. They believed that the nanoparticles increased the surface tension and density of the fuel, resulting in a larger fuel injection angle and wider shape, improved fuel atomization and air/fuel mixing, and ultimately reduced CO emissions. In the study by Dhayananth Jegan et al. [97], it was mentioned that TiO₂ nanoparticles reduce the viscosity of fuel, thereby improving combustion efficiency. In addition, they also believe that the larger surface area of nanoparticles promotes the generation of chemical reactions, shortens ignition delay, and promotes air/fuel mixing and combustion. Pankaj Mohan Rastogi et al. [52] believe that the presence of more oxygen in biodiesel enhances the combustion process of the fuel. In addition, CuO nanoparticles make the fuel atomization more uniform, increase the reaction area, and promote the reduction of CO emissions. M. Mofijur et al. [67] also believe that an increase in oxygen atoms is helpful in reducing CO emissions, but the reduction in coolant heat dissipation is also one of the important reasons. In summary, adding nanoparticles to biodiesel can reduce CO emissions, mainly by increasing oxygen content and reaction area.

4.2. Carbon Dioxide (CO₂)

Complete combustion within the engine cylinder is one of the common sources of CO₂. In the literature summarized in Table 3, compared to diesel or biodiesel blended fuels, the addition of nanoparticles resulted in an overall increase in CO₂. In the study by Pankaj Mohan Rastogi et al. [52], it was found that there is a relative balance between CO and CO₂ emissions. The addition of nanoparticles to B20 biodiesel fuel provides higher oxygen content and reaction area, enhances the combustion process, and leads to a slight increase in CO₂ emissions, but CO emissions show a decreasing trend. C. Dhayananth Jegan et al. [97] also found an increase in CO₂ emissions when adding nanoparticles to biodiesel blends. They believe that the increase in CO₂ emissions is the result of complete fuel combustion. In the case of complete combustion, CO emissions decrease, C atoms absorb additional O atoms for oxidation reactions, and nanoparticles provide oxygen, playing a catalytic role and promoting combustion. The oxidation mechanism maintains the balance between CO and CO₂ [116,117]. K. Simhadri et al. [45] studied the effect of TiO₂ nanoparticles on biodiesel and found the same situation. Akshay Jain et al. [94] reported that when a small amount of TiO₂ nanoparticles were added to biodiesel blend, the amount of CO₂ generated was lower than that of diesel fuel. However, as the concentration of nanoparticles increased, CO₂ emissions also increased and exceeded those of/diesel fuel. They explained that during combustion under high concentration of nanoparticles, the O atom in the nanoparticles oxidizes the C atom, which is consistent with the research results of other scholars. In addition, they also found that when using a biodiesel blend alone, CO₂ emissions were lower than diesel fuel. This may be due to the low hydrocarbon content of biodiesel [118].

4.3. Hydroxide (HC)

Similar to the formation principle of CO, HC is also a product of inefficient combustion. It is evident that adding nanoparticles to biodiesel or diesel has an important role in reducing HC emissions, as fully demonstrated in Table 3. Manzoore Elahi M. Soudagar et al. [119] reported that B20 biodiesel fuel has high viscosity, poor atomization effect, and led to an increase in HC emissions. After the addition of nanoparticles, there was a significant decrease in HC emissions, which proves the positive effect of Al₂O₃ nanoparticles in promoting combustion and reducing HC emissions. Nanoparticles act as oxidation catalysts during the combustion process, enhancing the combustibility of fuel in the cylinder and inhibiting the formation of HC [67]. In the study by S. Lalhriatpuia et al. [48], it was suggested that Co₃O₄ acts as an oxygen storage device, increasing the oxidation rate. At the same time, nanoparticles cause micro explosions, which rapidly rupture fuel droplets into smaller particles, promote secondary atomization of fuel, greatly increase fuel utilization, and accelerate fuel combustion speed. Osama Khan et al. [120] believe that the high catalytic activity of nanoparticles improves the atomization level of fuel and provides the optimal reaction area, with a significant decrease in HC emissions observed. They also stated that the combined effect of hydrogen and nanoparticles will further reduce HC emissions. In addition to the above situations, nanoparticles can alter the heat transfer performance of fuel by increasing its thermal conductivity, which promotes more complete combustion [121]. Shiva Kumar et al. [122] added iron-based nanomaterials to B20 blended fuels, and when the addition ratio was 1%, HC emissions were reduced by 22.9%. When the addition ratio was lower or higher than this ratio, the emission reduction effect on HC was reduced. When A. Mujtaba et al. [112] studied the effect of nanoparticles on the HC emissions of biodiesel fuel, they found that except for the fuel with added CNT nanoparticles, the HC emissions of other ternary blends showed a decreasing trend. The reason for the increase in HC emissions when adding CNT nanoparticles is believed to be the presence of carbon atoms in its atomic structure. They believe that using nano additives as fuel improvers can increase combustion temperature and heat release rate and reduce unburned emissions. In summary, most researchers believe that the reduction of HC emissions from biodiesel blends caused by nanoparticles is due to the increase in O atoms and, in addition, the higher interaction area of nanoparticles.

4.4. Nitrogen Oxides (NO_X)

NO_X is a product of efficient combustion, and its formation mainly depends on high temperature and high oxygen content. From Table 3, it can be seen that when nanoparticles are added to diesel or biodiesel blends, NO_X emissions increase and also decrease. M. A. Mujtaba et al. [112] reported that the use of CNT nanoparticles has a promoting effect on reducing NO_X emissions. They believe that CNT nanoparticles increase ignition timing, and the blend begins combustion when the piston reaches top dead center, reducing the mixture content during premixed combustion. Contrary to CNT nanoparticles, they found that TiO₂ nanoparticles improved combustion temperature and cylinder pressure, promoting NO_X emissions. M. S. Gad et al. [85] also reported the effect of CNT nanoparticles on reducing NO_X emissions. Srinivasan Senthil Kumar et al. [115] suggested that Al₂O₃ nanoparticles can reduce combustion temperature through thermal degradation between hydrocarbon molecules. In their study, fuels with added Al₂O₃ nanoparticles had the lowest NO_X emissions. T. Sathish et al. [95] stated that ZnO nanoparticles can improve thermal conductivity, enhance convective heat transfer in the cylinder, reduce combustion temperature in the cylinder, and reduce NO_X emissions. Under full load operation, the NO_X emissions of fuels with added ZnO nanoparticles with concentrations of 50 and 100 ppm are much lower than those of diesel and biodiesel blends. C. Dhayananth Jegan et al. [97] also believe that nanoparticles have a promoting effect on reducing NO_X emissions. They added three types of nanoparticles to B25 fuel and compared NO_X emissions. When using SiO₂ and TiO₂ nanoparticles, NO_X emissions were found to be significantly reduced. Harish Venu et al. [123] found that adding Al₂O₃ nanoparticles to fuel increases NO_X emissions under high- and full-load conditions, but this does not occur at low loads. They believe that under high loads, the cylinder pressure and combustion temperature increase while the combustion rate decreases. In addition, more fuel burns during the diffusion phase, and the combustion duration increases. Overall, some nanoparticles can promote heat transfer during combustion, resulting in a decrease in combustion temperature and NO_X emissions. However, some nanoparticles do not significantly improve heat transfer, promoting combustion while increasing combustion temperature and NO_X emissions.

5. Conclusions

Nanoparticles play an important role in improving engine performance and emissions, helping to achieve the development goal of clean fuels. This article reviews the application of different types of nanoparticles in biodiesel engines and compares and summarizes their effects on engine performance and emissions.

When adding nanoparticles to biodiesel, it can increase the calorific value, cetane value, and flash point, which not only improves fuel transportation and storage safety but also promotes engine combustion efficiency. But the density of nano-fuels also shows an increasing trend, which may have a counterproductive effect on combustion.

Nanoparticles improve combustion, increasing engine cylinder pressure, heat release rate, and brake thermal efficiency while reducing brake-specific fuel consumption. Because nano-fuels exhibit higher oxygen content and reaction area, in addition, nanoparticles also promote the micro explosion phenomenon of the fuel, allowing for secondary atomization of the fuel, increasing fuel utilization and significantly improving fuel combustion characteristics.

Nanoparticles have a significant inhibitory effect on the emissions of HC and CO. Most metal nanoparticles contain oxygen, which has a positive effect on combustion. More O atoms undergo oxidation reactions with C atoms, greatly reducing the emissions of incomplete combustion products. In the previous summarized research, the maximum reductions in CO and HC emissions compared to diesel were 41.19% and 31.89%, respectively, both of which were found when using TiO₂ nanoparticles.

The use of nanoparticles increases CO₂ emissions because more oxygen atoms and better combustion conditions promote the generation of oxidation reactions in the cylinder. The excess O atoms combine with C atoms, reducing CO emissions but increasing CO₂ emissions because CO and CO₂ are in a mutually balanced relationship. In some studies, when a small amount of TiO₂ nanoparticles were added, the amount of CO₂ generated was lower than that of diesel. CO₂ emissions decreased by 12.57% and 8.01% at 50 and 100 ppm, respectively. However, as the concentration of TiO₂ nanoparticles increased to 150 ppm, CO₂ emissions exceeded that of diesel. It is worth noting that in some studies, when only biodiesel blend was used, lower CO₂ emissions were observed compared to diesel because biodiesel has a lower hydrocarbon ratio.

When using nano-fuels, there is an increase or decrease in NO_X emissions. Some nanoparticles can improve the thermal conductivity of fuel, promote heat transfer during combustion, lower the combustion temperature in the cylinder, and lower NO_X emissions. In the research summarized in this paper, NO_X reduction occurred when using CNT, TiO₂, and ZnO nanoparticles, with the maximum reduction compared to diesel being 3.92%, 8.8%, and 27.1%, respectively. However, some nanoparticles do not significantly promote heat transfer. These nanoparticles not only improve combustion but also increase combustion temperature, promote the binding of N and O atoms, and increase NO_X emissions.

Overall, according to the literature, almost all nanoparticles offer varying degrees of improvement in engine performance and emissions, but the improvement of TiO₂ nanoparticles is significantly better than that of other nanoparticles. The authors of this article believe that TiO₂ nanoparticles have great potential in improving engine performance and emissions. In future research, the combined effects of two or several nanoparticles when used together in fuel can be explored. We should fully utilize the advantages of nanoparticles and pay more attention to research on improving engine efficiency and reducing emissions, especially in reducing CO₂ emissions.