Investigating the Effects of Environmentally Friendly Additives on the Exhaust Gas Composition and Fuel Consumption of an Internal Combustion Engine

Abstract

This article shows the effect of the addition of effective microorganisms and silver on the exhaust gas composition and fuel consumption. Exhaust emission standards are becoming increasingly stringent, which makes it difficult for engine manufacturers to meet them. For this reason, intensive work is underway to use alternative propulsion systems on ships, and for diesel engines, alternative fuels. Among other things, this applies to mixtures of petroleum-based fuels with vegetable oils and their esters. Unfortunately, their use, due to their physicochemical properties, can negatively affect the performance of the engine and the wear of its components. Therefore, the aim of this study was to see how additives of effective microorganisms in the form of ceramic liquid and tubes, and a silver solution and colloidal silver would affect some engine parameters, including the exhaust gas composition and fuel consumption. The authors are not aware of the results of previous research on this issue. The tests were carried out on a diesel engine for four types of green additives at concentrations of 2% and 5%, at different ranges of its load. The additives added to the diesel fuel were characterised, and the test stand was presented, along with the parameters of the tested fuel. The effect of additives on selected engine parameters, including fuel consumption, was presented. The characteristics of hourly fuel consumption and selected components of the exhaust gas, including nitrogen oxides, carbon monoxide and carbon dioxide as a function of the concentration of ecological additives are shown and analysed. It was found that the most beneficial additive that had a positive effect on the exhaust gas composition and fuel consumption was a silver solution in a 2% concentration. There was a decrease of up to 4% in the NO_x content of the exhaust gas, a decrease in carbon monoxide of more than 28%, a decrease in carbon dioxide of 4.6% and a decrease in fuel consumption of around 3% was achieved under the tested conditions. The use of these additives is an innovative solution that has a positive impact on reducing the emissions of harmful compounds into the atmosphere. In further research, it will be necessary to study the effect of this additive on the combustion process in the engine and the wear of its components, as well as to confirm the results obtained in real operating conditions.

1. Introduction

Climate change is one of the biggest challenges facing the world these days. Global warming, caused by increasing emissions of greenhouse gases into the atmosphere, is the result of a number of disturbing environmental events and is responsible for increasing average global temperatures, rising sea and ocean levels, more violent precipitation and expanding desert areas. These changes can pose a great danger to both humans and organisms in the world. There is evidence that they are the result of human activity, and the main example is the increase in the earth’s average annual temperature by approximately 1 °C relative to the period before the industrial revolution. The main reason for this problem is gas emissions from burning fossil fuels [1,2,3].

For example, the maritime industry is responsible for 2–4% of the world’s conventional fuel consumption, resulting in the emission of a large amount of greenhouse gases into the earth’s atmosphere. Ships primarily use low-quality fuel for propulsion, as its cost is responsible for 20–60% of the operating cost of the entire vessel [4,5].

Engines powered using low-quality fuel produce large amounts of toxic substances for the environment such as carbon dioxide, nitrogen oxides, sulphur oxides and others. Most ships under construction today use conventional propulsion using heavy fuel in the main engines and light fuel (diesel oil) in the generators [6,7,8].

Modern engines of this type are equipped with appropriate solutions to reduce the emission of toxic compounds into the atmosphere. As it becomes increasingly difficult to meet ever-tightening regulations on air emissions, advanced work is also being conducted on the use of alternative-fuel engines, as well as propulsion systems other than conventional ones [9,10,11]. For this reason, the share of fuels such as LNG, methanol, biodiesel, hydrogen and ammonia will be increasing [12,13].

Environmental issues related to the use of high sulphur fuel oil (HFO) are a major source of air pollution, acid rain and related climate change [14,15].

Therefore, the authors of [16] developed two process models for producing methanol and hydrogen from vacuum residues while minimizing sulphur and carbon emissions. These fuels are cleaner and more environmentally friendly than heavy fuel.

One alternative to HFO and MDO is the use of liquefied natural gas (LNG) as a fuel for ship and truck propulsion. Promoting the use of natural gas vehicles is considered one of the most important strategies for sustainable transportation. LNG contains virtually no sulphur and heavy metals such as cobalt, lead and mercury, so no sulphur oxides, dust or solid combustion wastes such as slag, ash or soot are formed during combustion. Compared to conventional fuels used on ships, the combustion of natural gas produces lower emissions of nitrogen oxides by approximately 85–90% and 15–25% of carbon dioxide compared to the exhaust emissions from the combustion of conventional fuels [17,18,19].

The possibility of using synthetic fuels, mainly from hydrogen and carbon dioxide, is also being considered to rid engines of their so-called carbon footprint.

E-fuels are gaseous or liquid fuels for internal combustion engines that are created by converting renewable electricity into chemically bound energy carriers. Examples include hydrogen, methane, methanol, oxymethylene ether (OME), dimethyl ether (DME) and Fischer–Tropsch fuels.

When burning e-fuels, CO₂ is emitted into the atmosphere, but a similar amount of carbon dioxide is used to produce them. For this reason, they are called climate-neutral fuels.

Among other issues, the reduction and control of particulate matter from the combustion of fossil fuels is also a problem. Therefore, various fuels are currently being studied as a potential substitute or additive for diesel and gasoline. One of them is precisely the study of the effect of the oxymethylene ether-3 (OME3) additive on the formation of carbon particles when mixed with ethylene [20,21]. These studies show that the volume proportion of soot indicates a reduction in the total number and size of soot particles, and further research is needed to more fully understand the effect of the additive.

In addition, the main problem with synthetic fuels is the cost of producing them, as production so far has been carried out on an experimental basis, so obtaining e-fuel is several times more expensive than petrol or diesel [22,23,24].

Another option is also to use hydrogen in, for example, fuel cells to produce electricity, which could then be used to power land and sea transport vehicles. Today, fuel cells (FCs) are widely regarded as highly efficient and non-polluting energy sources with higher energy efficiency than conventional technologies.

Proton exchange membrane fuel cells (PEMFCs) are seen as promising in transportation sectors due to their ability to start at low temperatures and minimal greenhouse gas emissions [25,26,27]. However, the drawbacks of these fuels of the future are now being highlighted, including the need to develop cleaner and cheaper production methods. The above may result in increased investment in this area and an increase in demand for these fuels.

Another way to meet tightening environmental regulations is to use alternative fuels—biofuels [28,29]. It can be considered that the combustion of vegetable oils and their esters and blends of these oils/esters with diesel fuels is well recognised [30,31].This applies to materials such as used motor oil, hazelnut oil, corn oil, soybean oil, sunflower oil, rapeseed oil and their esters [32,33,34].

Moreover, the use of agricultural waste is increasingly being considered as an attractive and widely available resource for the production of biogas slurry (BS) [35].

Unfortunately, there are several difficulties associated with the use of alternative fuel in diesel engines. These are related to adapting the engine to different combustion conditions. The use of alternative fuels leads to a change in the technical parameters of the internal combustion engine, and, in the vast majority of cases, the direction of these changes and the effects are unknown [36]. Accordingly, research is constantly being conducted to detect possible negative factors that can lead to the deterioration of technical performance and the premature wear of internal combustion engine components. Currently, research results are available which indicate that the use of, for example, fuel mixtures containing vegetable oil esters significantly reduces the toxicity of gases in an internal combustion engine, but this is associated with a decrease in the power and effective efficiency of the diesel engine [37,38]. This is one way to meet increasingly stringent standards related to environmental protection—the use of alternative fuels in the form of a mixture of diesel and cooking oils.

However, these oils have different physicochemical properties, so they can negatively affect the performance and wear of internal combustion engine components [39,40].

Due to toxic emissions coming from the exhaust of internal combustion engines, research is being conducted toward the use of non-metallic green additives for motor fuels. The authors of [41] proposed a less toxic and non-metallic diesel fuel additive in the form of nanographite and study its effect on the performance and regulated gas emissions of a four-stroke compression-ignition engine. According to the study, graphite nanoparticles are more environmentally friendly fuel additives than nitrogen oxides; however, their use is more recommended for large turbocharged ZS engines. Scientists have long been studying additives consisting of oxidised organic compounds that could help reduce the release of pollutants into the atmosphere when fossil fuels are burned. It is believed that diethyl carbonate (DEC), which consists of 40.6 percent oxygen by weight, could facilitate the clean burning of diesel fuels. With this additive, it will be possible to use biodiesel, consisting of various methyl and ethyl esters, in modern diesel and hybrid engines [42,43].

As the analysis presented here shows, the use of alternative fuels to petroleum-based fuels, especially for powering marine diesel engines, is not yet widely possible. This includes ammonia, hydrogen, methanol, biofuels with a high percentage of vegetable oils or their esters and synthetic fuels. These fuels have great potential as energy sources in new, cleaner powertrains. The use of these e-fuels has the potential to help convert today’s internal combustion engines from efficient CO₂-neutral to zero-emission powertrains. This will require the construction of structurally new engines or the adaptation of fuel properties to existing propulsion units, with a gradual trend of upgrading them. The use of pure vegetable oil without blending with diesel requires changes in engine design. One hundred percent rapeseed fuel can be used to power engines with different designs such as the Elsbett engine. Engines for military vehicles are already being produced that can run on diesel, gasoline, biodiesel or spirit.

Currently on ships, the exception is the so-called dual fuel (DF) engines adapted to burn not only petroleum fuel but also liquid natural gas (LNG). They have been used for several years for the main propulsion of ships carrying liquefied gases—on “gas carriers”, but their use is increasing. One of the problems to be solved is the universality of access to bunkering of this fuel in various ports around the world.

The prevailing view is that it is “the fuel to the engine, not the engine to the fuel” that should be adjusted. The chemical composition and characteristics of the fuel should be such that the user can use an engine of a certain design without incurring unnecessary expenses.

Taking the above into account, we decided to investigate the possibility of powering a diesel engine with MDO fuel with admixtures such as ionic and non-ionic silver, and effective microorganisms in liquid form and ceramic tubes with different percentages of them in MDO. Microorganisms and silver compounds are successfully used in other fields and are widely recognised as environmentally friendly. The proposed diesel fuel admixtures represent an innovative solution that have not previously been used in petroleum products, with a positive impact on environmental protection.

The main objective of this study was to demonstrate how these admixtures, with different percentages in the fuel, would affect selected engine parameters including the fuel consumption and exhaust gas composition. The tests were carried out on a Delfin diesel engine for four types of eco-additives at different engine load ranges. The results indicate that it is possible to feed the engine with MDOs with these admixtures, and over a period of several months no negative effect on the engine’s technical condition was found. It was shown that it is also possible to achieve the effect of decreasing the engine’s fuel consumption and reducing the content in the exhaust gas. Considering only the concentration of the 2% additive, the best results were obtained for the silver solution. After using this additive, the fuel consumption decreased by 4.5% compared to that of pure MDO. The silver solution was shown to be effective in reducing NO_x during engine operation under load, which, at a concentration of 2%, resulted in a 3.2% reduction in NO_x at a 20 kW load and a 4% reduction at a 30 kW load. The presence of a 2% silver solution in the fuel also resulted in a significant reduction in carbon monoxide in the exhaust by 28.7% at 20 kW and 25.3% at 30 kW. An additional advantage of using these additives may also be the reduction of biological degradation of the fuel, especially with longer storage.

2. Materials and Methods

2.1. Materials Used in the Research

Eurodiesel (MDO) manufactured by Lotos was used for this study. The tested fuel was characterised using selected parameters, which are summarised in

Table 1. Eurodiesel oil specification [44].

No.	Parameters	Value	Unit
1	Flash point	62	°C
2	Water content met. Karl Fisher	51	mg/kg
3	Solids content	6	mg/kg
4	Sulphur content	9.4	mg/kg
5	Cold filter blocking temperature	−15	°C
6	Density at 15 °C	0.8248	g/cm3
7	Kinematic viscosity at 40 °C	2.588	mm2/s
8	Cetane number	55.4	−
9	Lubricity (WSD)	215	μm
10	Fatty acid ester content	6.8	% [v/v]

[44].

Before starting the research, effective microorganisms (EM) were added to each tank in the form of liquid and ceramic tubes, as well as silver in the form of its solution and colloidal silver.

Effective microorganisms are specially selected from the smallest organisms on Earth, which consist of various strains of aerobic and anaerobic microorganisms including lactic bacteria, yeast, photosynthetic bacteria, moulds and actinomycetes [45]. EMs are not genetically modified and are completely environmentally friendly.

Effective microorganisms in the form of ceramic tubes are a special type of fermented clay inoculated with beneficial microflora. The clay is fired at high temperatures under anaerobic conditions. Ceramic is an inorganic material and does not contain metals. It is shaped and then hardened by firing at high temperatures of 1200–1300 °C. Ceramic tubes do not react chemically, are non-flammable and harder than steel. The content of effective microorganisms was 3% in each ceramic tube [46,47].

Effective microorganisms in the form of ceramic tubes in a designated amount (based on previous studies) were mixed with the fuel. As these tubes are in solid form, they do not dissolve and form an emulsion with the fuel. Due to their mass, they sink to the bottom of the tank and there the microorganism release processes begin. The fuel together with the ceramic tubes is stored for approximately 4 weeks. The effective microorganisms are released very slowly into the liquid, in this case the fuel. EM ceramics emit infrared waves, the longest rays in the light spectrum. The ceramic tubes are then removed from the fuel by filtering before the fuel is fed into the engine.

EM technology has enormous potential. It offers an opportunity to use these properties to solve global problems that are troubling current generations. Diesel additives in the form of effective microorganisms are widely available and used, among other things, in the following [47]:

−

Agriculture for soil fertilization, animal husbandry and in the processing of organic waste;

−

Environmental protection, including water treatment processes and treatment of domestic, municipal and industrial wastewater, as well as for treatment of contaminated water reservoirs;

−

Other industries.

Since their use in the above-mentioned area has the expected results, it is therefore necessary to study the effect of using effective microorganisms on petroleum products, which, when stored in tanks, undergo microbial decomposition to form sludge, among other things, and also contribute to tank corrosion. Solving this problem is very important because microbial contamination of diesel fuel occurs even in properly operated and maintained tanks where oils are stored. The tanks experience uncontrolled microflora growth and changes in the properties of the fuel to such an extent that they are often unsuitable for use in the engine and cause excessive wear on its components.

In diesel fuel, bacterial contamination can contribute to unstable fuel aging, but in general, the most important consequence is microbiologically induced corrosion of storage tanks and the piping system, and the formation of microbial coatings, which cause blockage of filters and fuel lines, and increase wear on pumps.

Given the current applications of effective microorganisms and their positive impact on solving microbial contamination problems, their use may have an impact on reducing or even combating microbial contamination in petroleum products. In addition to this, the smaller amount of contaminants in diesel fuel should have a positive impact on the smaller amount of harmful compounds in the exhaust gas and, consequently, on the environment as well, which is particularly important, since internal combustion engines are the predominant propulsion system for all types of transportation, as alternative propulsion is still under development.

Effective microorganisms were used for the study with the following composition (Figure 1): water 94%, Effective Microorganisms^®—3%, molasses—3%.

The second additive used in this study was silver in the form of colloidal silver (nanosilver) and silver solution (Figure 2).

Nanosilver is microscopic particles—silver ions—which are characterised by a distinct yellow colour. Silver nanoparticles are chemically insensitive and completely resistant to the harmful effects of UV radiation.

Colloidal silver consists of 80% silver particles and 20% silver ions. The factors determining the quality of colloidal silver are the content of silver particles and their active total surface area. In colloidal silver, the particles form a colloid and no protein substrate is needed to suspend them in water. The nanoparticles repel each other, keeping the colloid stable. Ionic silver is transparent, containing 90% silver ions and approximately 10% silver particles [48,49,50].

Colloidal nanosilver and silver solution were used for this study. Silver solution (transparent) was based on demineralised water and ionic silver with a concentration of 25 ppm. Colloidal silver (amber) was created on the basis of demineralised water with a non-ionic colloidal silver content of 25 ppm.

The effectiveness of using the bactericidal, fungicidal and virucidal properties of silver is very high. Silver broken down into nanoparticles has a large active surface area and huge biocidal potential. The effectiveness of nanosilver includes the elimination of more than 99.99% of bacteria, fungi, viruses and moulds. Nanosilver is able to attach to the cell membranes of bacteria and block their production of enzymes necessary for reproduction and growth. Good carriers of nanosilver are activated carbon fibres (ACFs). Silver is widely used in wastewater treatment plants to remove organic and inorganic contaminants due to its large surface area and rapid adsorption.

Silver nanoparticles can be obtained using chemical and physical methods. Physicochemical methods for obtaining nanosilver (less common than chemical) involve the use of microwave radiation, ultrasound, irradiation, mechanical grinding or various types of matrices (such as polymeric).

Among chemical methods, one can distinguish the reduction of Ag⁺ ions, electrochemical or photochemical methods. Silver nanoparticles can be obtained, for example, by reducing silver salts with methanol or ethylene and using the Tollens reaction, in which Ag⁺ ions are reduced with aldehyde or reducing simple sugars (such as glucose and galactose) or disaccharides (such as lactose and maltose). The most common reductants of silver ions are boron, hydrogen, citrates and ascorbates.

Silver nanoparticles are widely used because of silver’s antimicrobial properties, which have been known for centuries. Bringing silver down to a nanometer size increases the active surface area of the particles, so the biological and chemical activity of silver increases. This indicates the potential enhanced effectiveness of silver nanoparticles against microorganisms. Due to silver’s properties at the nanoscale—that is, its antibacterial, antifungal and antiviral effects, among others—it is currently used in products such as toothpaste, washing powders, cleaners, cosmetics, personal care products, air fresheners, clothing and products related to the food industry, as well as in electronic devices.

Due to such widespread use, the price of these additives is not exorbitant and results in a price increase of approximately 5%. With their widespread use by diesel and lubricating oil manufacturers, this effect on the price may be even lower. It can be expected that after using an additive in the form of a silver solution in an appropriate amount, lower fuel consumption will be obtained. This may have a positive impact on the final cost. Demonstrating this is one of the goals of the ongoing research.

We are not aware of the results of studies on the use of such additives and their impact on atmospheric emissions of harmful compounds and fuel consumption. In view of the above, the use of these additives is an innovative solution with a positive impact on reducing the emission of harmful compounds into the atmosphere. Further research will have to investigate the effect of this additive on the combustion process in the engine and the wear of its components, as well as confirm the results obtained in real operating conditions.

Figure 2. The commercial forms of silver solution and colloidal nanosilver [50].

Figure 2. The commercial forms of silver solution and colloidal nanosilver [50].

2.2. The Research Stand

The tests were performed on the SW 680/1 engine. This engine is a non-supercharged direct-injection designed by WSK Mielec (Mielec, Poland) under license from British Leyland. This engine has a power of 147 kW at 1500 rpm and is loaded with a GCPf94c/1, 60 kVA synchronous generator. The load can be continuously adjusted by changing the generator’s load from 0 to 40 kW of electrical power.

The engine is equipped with an injection apparatus consisting of an injection pump and CAV injectors [46]. The test stand used for the research with a WSK Mielec/British Leyland SW 680/1 Delfin engine determined certain limitations for the conducted research. These included, first of all, the limited range of the engine’s load on the alternator (max. 40 kW) and the impossibility of obtaining indicated parameters.

The choice of the test object and the timing of the tests were determined, among other things, by the considerable uncertainty that the fuels used would not cause significant side effects and lead to engine failure.

The engine is shown in Figure 3. The main engine specifications are shown in

Table 2. Engine technical particulars [51].

Parameters
Manufacturer	WSK Mielec/British Leyland
Type	SW 680/1 Delfin
Rated power (kW)	147
Cylinder number	6
Cylinder formation	Horizontal inline
Cylinder swept capacity (cm3)	1850
Rotational speed (rpm)	1500
Compression ratio	15.8
Fuel pump	Multi-element CAV
Injectors	CAV
Pump timing	30 degrees BTDC
Injector pressure	14.2/14.72 MPa
Operating oil pressure	0.38/0.41 MPa

.

Five types of diesel fuel were used in the tests, including one without any additives and four with additives of effective microorganisms in liquid form and ceramic tubes, as well as colloidal silver and silver solution. The fuel was stored in a 20 dm³ tank for four weeks first with an addition of 2% and then 5% of the total tank volume.

A study of the effect of combustion of individual diesel fuel samples (without and with additives) on engine performance parameters, including fuel consumption and exhaust gas composition, was conducted under the same conditions. Each test lasted 1 h of engine operation, of which 20 min were at idle, another 20 min when the engine was loaded with 20 kW of electrical power and another 20 min with a load of 30 kW.

Experimental studies were conducted according to the scheme shown in

Table 3. Schema of experimental tests of pure diesel and oil with additives at different concentrations and engine load levels.

Additive Concentration [0%]			Operating Parameters
Load	0 kW	Pure MDO
	20 kW
	30 kW
Additive Concentration [2%]
Load	0 kW	MDO + SS
	20 kW
	30 kW		Cooling water temperature [°C]
	0 kW	MDO + CN
	20 kW		Oil temperature [°C]
	30 kW
	0 kW	MDO + EM fluid	Exhaust gas temperature [°C]
	20 kW
	30 kW		Oil pressure [bar]
	0 kW	MDO + EM cer
	20 kW		Suction air pressure [kPa]
	30 kW
Additive Concentration [5%]			Instantaneous fuel consumption [dm3/h]
Load	0 kW	MDO + SS
	20 kW		NOx [ppm]
	30 kW
	0 kW	MDO + CN	CO [ppm]
	20 kW
	30 kW		CO2 [%]
	0 kW	MDO + EM fluid
	20 kW
	30 kW
	0 kW	MDO + EM cer
	20 kW
	30 kW

.

After the designated time, readings were taken of the exhaust gas composition using an exhaust gas analyser, with selected engine operating parameters measured using a computerised control and measurement system. In addition, fuel consumption was measured with a CONTOIL VZD 4 flow meter with a maximum permissible measurement error of the current value of ±0.5%. The results made it possible to assess the impact of environmental additives to diesel fuel on the aforementioned parameters. The results were read out using the control and measurement system working with the engine and the exhaust gas analyser MRU 95/2D (Figure 4).

Measurements were made with a Wimmer MRU 95/2 D analyser with an exhaust gas preparation station. The exhaust gas components were measured with an accuracy of ±1 ppm (CO, NO_x) and ±0.1% (CO₂) using electrochemical sensors. The measuring probe was placed in the flue gas exit pipeline for the duration of the measurement using a spigot made especially for this purpose.

Each measurement was carried out multiple times, while the paper presents the average result for each case in the form of graphs.

The conditions created in the laboratory correspond to the shipboard operation of marine power plant engines. However, the operating conditions for main propulsion engines are different. In this case, the emission of pollutants in the exhaust gas is influenced by so-called external conditions, which include sailing in waters with limited depth and channels, sailing during stormy weather and changes in hull draught (e.g., due to increased load or changes in water density). The engine operating conditions are also affected by changes in the ship’s motion (starting, acceleration, braking) set by the crew. Unfortunately, the laboratory test bench does not reflect the above conditions that occur during the operation of a marine engine, so it is necessary to confirm the results obtained in an engine operating under real conditions.

3. Results

During the tests, the basic engine operating parameters were recorded, which included the engine speed; the temperature of the cooling water, lubricating oil and exhaust gases; and the lubricating oil and engine intake air pressure. Based on the values read from the control and measurement system, it was found that they did not change significantly.

Statistical analysis was performed at a confidence level of 98%. The results obtained for fuel consumption without additives and with additives at different concentrations at a significance level of α = 0.02 showed no significant errors, as they fell within the lower and upper confidence intervals.

3.1. Testing of Hourly Fuel Consumption

Figure 5 and Figure 6 show the effect of the additives on hourly fuel consumption. Figure 5 shows a graph illustrating the fuel consumption trend of the three cases for better clarity, for which the greatest changes occurred in order to highlight how fuel consumption changes with changing load. Figure 6 refers to fuel consumption for the three loads of the test engine supplied with fuel with additives of different concentrations.

A trend graph was made for fuel without additives, for a 2% concentration of ionic silver and 5% effective liquid microorganisms. An analysis of the graph shows that the lowest value of fuel consumption was obtained for the fuel with ionic silver additives, but for these two mixtures it can be observed that as the load increases, the trend is downward compared to the consumption of pure fuel.

It can be seen from Figure 6 that when idling, the fuel consumption was 4.7 dm³/h. At a 2% additive concentration for colloidal nanosilver and effective microorganisms in each form, there was a decrease in fuel consumption to 4.6 dm³/h. The lowest value was obtained for the silver solution, after which the fuel consumption reached a value of 4.5 dm³/h.

When the concentration of the silver solution was increased to 5%, there was an increase in fuel consumption to 4.6 dm³/h.

Fuel consumption with the addition of ceramic effective microorganisms was at the same level as for pure diesel fuel, while for colloidal nanosilver and liquid effective microorganisms there was an increase in fuel consumption to 4.8 dm³/h. Again, the most beneficial additive was the silver solution used at a concentration of 5%, which reduced fuel consumption by up to 4.6 dm³. The consumption of fuel with the addition of ceramic effective microorganisms was the same as for fuel without any additive, i.e., 4.7 dm³.

At a 20 kW load, the diesel consumption without any additives was 10.2 dm³/h. With the addition of 2% liquid effective microorganisms, there was a slight increase in the fuel consumption to 10.3 dm³/h. For the other additives, the consumption decreased; for the effective microorganisms in the form of ceramic tubes it decreased to 10.1 dm³/h, for colloidal nanosilver it decreased to a level of 10 dm³/h and the largest decrease was observed for the silver solution, i.e., to 9.9 dm³.

At a 5% concentration there was also an increase in fuel consumption, as for idling. When the concentration was increased, the fuel consumption increased the most when liquid effective microorganisms were used, reaching 10.8 dm³/h. Colloidal nanosilver and ceramic effective microorganisms also increased the fuel consumption compared to pure diesel, to 10.5 dm³/h and 10.4 dm³/h, respectively.

The smallest increase in fuel consumption relative to pure fuel and fuel with a 2% additive concentration occurred when the silver solution was used. The fuel consumption in this case was 10 dm³/h. Again, the most beneficial additive was the silver solution, which reduced fuel consumption at a 20 kW load by approximately 3%.

For a 30 kW load, the fuel consumption without additives was 13.3 dm³/h. With the addition of 2% liquid effective microorganisms, there was an increase in fuel consumption to 13.5 dm³/h. The use of colloidal nanosilver caused no change and the fuel consumption remained at the same level as for pure diesel. For the effective microorganisms in the form of ceramic tubes, the fuel consumption dropped to 13.2 dm³/h. In contrast, the silver solution again proved to be the most effective, with fuel consumption dropping to 13 dm³/h after application.

The 5% addition of each type resulted in an increase in fuel consumption above the value obtained for pure fuel. The highest consumption was observed for the liquid effective microorganisms, which amounted to 13.9 dm³/h. A slightly lower consumption occurred after the use of colloidal nanosilver, which reached 13.8 dm³/h. Effective microorganisms in the form of ceramic tubes at higher concentrations resulted in an increase in the consumption of 0.2 dm³/h compared to pure diesel. The lowest increase in fuel consumption occurred with the silver solution to 13.4 dm³/h. Again, the most beneficial additive was the silver solution applied at a 2% concentration, after which the fuel consumption decreased by approximately 2.5%.

The development of internal combustion engines is largely determined by the increasingly stringent emission standards being introduced. Meeting the high requirements of these standards creates the need to optimise both the process of fuel combustion in the cylinder and to improve non-engine ways of reducing exhaust emissions. An equally important factor of particular importance to users is fuel consumption, the impact of which on operating costs is significant. The amount of fuel consumed, in turn, is directly related to CO₂ emissions, which are subject to significant restrictions in EU directives. These include such measures as the addition of catalyst-based additives to fuels, the use of fuel–water emulsion injection or the injection of water into the engine. The additives introduced into the test fuel consisted mainly of water (except for effective microorganisms in the form of ceramic tubes), so the fuel delivered to the cylinder was a fuel–water emulsion.

Water supplied to the cylinder of an internal combustion engine can have a reducing and inhibiting effect on the processes of formation of harmful substances in the combustion chamber, especially during the first stage of fuel combustion. This is because during this period, dehydrogenation of the fuel takes place, along with the accompanying processes of pyrolysis. The result of these processes is the formation of mainly particulate matter. Elements inhibiting the formation of soot particles are free OH radicals formed with the participation of oxygen. Their two renewal is intensified by such factors as the oxygen content, temperature and pressure in the combustion chamber. The reactions of OH radical formation can be written as follows [52,53]:

H₂O → OH + H

(1)

H₂O + O→ OH + OH

(2)

H₂O + H → OH + H₂

(3)

As can be seen, the presence of water in the combustion chamber can significantly affect the number of OH radicals. In the case of a ZS engine, in the first phase of mixing of fuel particles with air at a high temperature and pressure, CC and C–H bonds are broken. As a result of such processes, among other things, carbon-rich residues are formed, the rapid afterburning of which requires the presence of oxygen and a catalyst. A certain type of catalyst for these reactions can be steam, which, being above the critical parameters (374 °C and 221 bar) significantly increases its catalytic capacity [54].

Thus, supplying water to the cylinder accelerates carbon afterburning, but at the same time, due to the cooling properties of water, it lowers the maximum combustion temperature and thus leads to a reduction in nitrogen oxide emissions, the formation of which in the case of poor mixtures (compression-ignition engine) is largely dependent on the temperature prevailing in the combustion area.

As can be seen from the analysis of the data in

Table 4. Comparison of selected test results of physical, chemical and microbiological parameters for pure fuel and fuel with the additives.

	Flash Point [°C]	Fuel Density [g/cm3]	Water Content [%]	Bacterial Content [cfu/1 dm3]	Fungal Content [cfu/1 dm3]
Pure fuel	60.3	0.8239	0.0066	1.1 × 106	7.3 × 103
Fuel with silver solution	62.2	0.8242	0.0145	<1 × 102	<1 × 102
Fuel with colloidal nanosilver	61.6	0.8241	0.0078	<1 × 102	1.5 × 102
Fuel with EM fluid	60	0.8234	0.0109	<1 × 102	1.0 × 102
Fuel with EM cer	61.3	0.8240	0.0113	<1 × 102	1.0 × 102

, the highest amount of water in the fuel was observed precisely in the fuel doped with ionic silver, so the best results were obtained for this mixture. In addition, the fuel was also subjected to microbiological tests, from which it can be observed that the smallest amount of bacteria and fungi was obtained for the ionic silver additive. The problem of microbiological contamination is most common in tanks with longer storage periods.

The microbial communities inhabiting fuels used in high-pressure engines are very diverse. The microorganisms most commonly found in fuels such as diesel and biodiesel include mould fungi of the genus Cladosporium sp. [6], Pseudomonasaeruginosa bacteria [55] and sulphate-reducing bacteria such as Desulfovibrio [56].

From the microbiological studies conducted, it appears that the fuel with ionic silver was the least degraded, as there were the fewest bacteria and moulds in it. Consequently, the physical and chemical parameters of the fuel changed the least, and there was the least microbial contamination (sludge) in the fuel with ionic silver.

3.2. Testing the Composition of the Exhaust Gas

Figure 7 shows how the NO_x content of the exhaust gas changed with 0%, 2% and 5% concentrations of silver additives and effective microorganisms. For pure fuel at a 0 kW load, the NO_x content was 289 ppm. All additives at a concentration of 2% and a load of 0 kW resulted in a decrease in nitrogen oxides, with the largest decrease of more than 10% observed for the effective microorganisms in the form of ceramic tubes. This measurement needs to be checked to see if it is erroneous, as the other results obtained for other concentrations and additives contradict this.

For colloidal nanosilver and effective microorganisms, the NO_x content has a value of 266 ppm, a decrease of approximately 8%. In contrast, when the silver solution was used, the NO_x value dropped to 271 ppm, a decrease of only approximately 6% compared to NO_x emissions when burning pure MDO.

The 5% concentration of additives also influenced the decrease in NO_x emissions compared to the values obtained with MDO combustion. When the silver solution and colloidal nanosilver were used, the decrease was more than 15% (to a value of approximately 250 ppm). With regard to the liquid effective microorganisms, NO_x emissions at the higher concentration increased to 278 ppm and were approximately 4% lower compared to diesel without additives. In the case of EM in the ceramic form, the NO_x concentration was comparable to the value obtained with the 2% additive and was 259 ppm.

Figure 8 shows the change in NO_x content of the exhaust gas at different concentrations of silver additives and effective microorganisms for a 20 kW load. For the clean fuel at a 20 kW load, the NO_x content was 874 ppm. As with idling, the 2% additive caused a decrease in NO_x for all additives, but the largest was observed for the silver solution, which was 846 ppm, representing a decrease of 3%. For colloidal nanosilver, the NO_x content was 851 ppm (a decrease of approximately 2.5%), while the use of effective microorganisms resulted in the NO_x values dropping to 860 ppm for the liquid form and to 865 ppm for the ceramic form of the effective microorganisms (a decrease of approximately 1.5%). At a 5% additive concentration, there was a further decrease in the NO_x content of the flue gas for the addition of effective microorganisms—in liquid form to 822 ppm, in ceramic form to 825 ppm and with the silver solution to 830 ppm, while colloidal silver showed a similar effect as it did at a 2% concentration (852 ppm). At this load value, increasing the concentration of effective microorganisms resulted in a decrease in nitrogen oxides with a decreasing trend. The addition of colloidal silver showed an increasing trend.

The additives of effective microorganisms in liquid form and ceramic tubes had the best effect of reducing NO_x concentrations, with a decrease of approximately 6% after application.

Figure 9 shows the dependence of NO_x in the flue gas on the concentration of silver additives and effective microorganisms for a 30 kW load. For pure fuel, the NO_x content was 1222 ppm. With a 2% additive concentration there was also a decrease in NO_x for all additives. For the silver solution, a NO_x concentration of 1174 ppm was obtained, for the colloidal nanosilver this was 1200 ppm and for the liquid microorganisms this was 1220 ppm, while for the effective microorganisms in ceramic form a NO_x amount of 1190 ppm was observed. As with the 20 kW load, the largest decrease was observed for the 4% silver solution.

Again, the greatest impact on the decrease in NO_x concentration in the flue gas came from the effective microorganisms in both forms at a 5% concentration. For the effective microorganisms in both forms, the NO_x content was similar at around 1100 ppm. Thus, the decrease was approximately 11%. The silver solution and colloidal nanosilver showed no positive effect on the NO_x concentration in the flue gas for this loading, as the NO_x content was 1189 ppm and 1240 ppm, respectively.

In order to explain why there was a reduction in the exhaust gas content of nitrogen oxides, it is necessary to refer to the studies conducted on the subject. On the basis of data from the literature [57,58,59], it was indicated that emissions of NO_x (a mixture of NO, NO₂, N₂O, N₂O₃ and N₂O₄) increase with increasing combustion temperatures reaching up to 2400°C in marine engines. Oxides are formed in the combustion chamber due to the oxidation of atmospheric nitrogen. Primarily, nitrogen oxide is formed, and nitrogen dioxide is formed secondarily from the formed oxide. The contribution of fuel-derived nitrogen is negligible. The results of Sulzer’s [60] study of the increase in NO_x content in the exhaust gas at temperatures above 1500 K increases tenfold for every 100 K increase in temperature. Taking the above into account, a number of methods have been proposed and implemented to reduce NOx emissions, which results in lower combustion temperatures (e.g., feeding engines with fuel–water emulsion, exhaust gas recirculation, delaying fuel injection). On this basis, it can be argued that the tested additives could have influenced the reduction in the combustion temperature and, as a consequence, the reduction in nitrogen oxides. However, the test stand equipment did not allow the measurement of the combustion temperature. We are also not aware of a direct cause-and-effect relationship between the effect of the additive used and the combustion temperature in the cylinder, nor are there known publications in which this problem was described.

Another toxic compound found in exhaust gases is carbon monoxide, which is a product of incomplete combustion or combustion with insufficient air.

Figure 10 shows the dependence of CO in the exhaust gas on the concentration of silver additives and effective microorganisms for idling.

For pure fuel, the CO content was 434 ppm. With a 2% addition of silver solution, there was a significant decrease in CO to 364 ppm compared to other additives, a reduction of 16%. The addition in ceramic form of effective microorganisms resulted in a decrease in carbon monoxide to 389 ppm. For liquid effective microorganisms and colloidal nanosilver, the CO content was similar at 397 and 398 ppm, respectively. An addition of 5% in each case resulted in an increase in nitrogen oxides compared to the 2% addition, although this increase still did not reach the carbon oxides value that occurred for pure diesel. The highest value was found with the silver solution, which was 425 ppm. Slightly lower values were obtained for effective microorganisms in ceramic form (420 ppm) and colloidal silver (415 ppm). The lowest value was obtained for the liquid effective microorganisms, which was 398 ppm and is very similar to that of the 2% concentration.

Analysing the results obtained, it can be concluded that the best result was with the additive at a concentration of 2%. For all samples, the values were lowest in comparison to MDO combustion, and especially for the silver solution. Further increases in concentration appear to be pointless, as this brings the carbon monoxide content closer to the values as without any additives.

Figure 11 shows the CO content in the flue gas as a function of the concentration of silver additives and effective microorganisms for a 20 kW load. For pure MDO, the CO content was 935 ppm. Apart from this, the course of the characteristics was similar to that found for idling. With the 2% addition of silver solution, there was also a significant decrease in CO compared to other additives, to 667 ppm, which is almost a 29% reduction. The addition of colloidal nanosilver resulted in a decrease in carbon monoxide to 844 ppm. For liquid and ceramic effective microorganisms, the CO content was higher at 876 and 905 ppm, respectively.

An addition of 5% also resulted in an increase in CO in each case compared to the 2% addition. The use of silver at this concentration resulted in higher levels of carbon oxides compared to MDO. After using the silver solution, the amount of CO in the exhaust gases was 957 ppm, and for colloidal nanosilver this was 971 ppm.

For effective microorganisms, lower values were obtained than for silver and their content was at the level of 910 ppm for liquid effective microorganisms and 926 ppm for microorganisms in ceramic form. They are high enough that both the addition of silver and effective microorganisms are not worth using at this higher concentration. At a load of 20 kW, the best results were also obtained at a concentration of 2% for all samples. Therefore, increasing the additive concentration further is also not justified.

Figure 12 shows the dependence of CO in the flue gas on the concentration of silver additives and effective microorganisms for a 30 kW load. For pure MDO, the CO content here was 1615 ppm. Apart from this, the course of the characteristics was very similar to that found with lower loads. In this case, with the addition of 2% silver solution, there was also a significant decrease in CO compared to other additives, to a value of 1206 ppm, i.e., decreasing by over 25%.

The addition of colloidal nanosilver resulted in a decrease in carbon monoxide to a level of 1518 ppm. For the effective microorganisms in liquid form and ceramic tubes, the CO content was higher at 1616 and 1658 ppm, respectively. With a concentration of 5% of each additive, the CO values approached those obtained for MDO and were approximately 1600 ppm. It also follows from the above that even at higher loadings further increases in additive concentration are not effective.

Carbon monoxide is formed as a result of the incomplete combustion of hydrocarbons contained in the fuel. Its formation is also influenced by the combustion temperature and the heterogeneity of the fuel–air mixture in the engine combustion chamber. The likely reason for the decrease in CO in the exhaust gas is the improvement of the combustion process, as indirectly evidenced by the demonstrated reduction in fuel consumption. It can also be thought that there was a change in the combustion temperature, as mentioned earlier, with regard to the reduction of nitrogen oxides.

Another component of the exhaust gas examined was carbon dioxide, which has an impact on the greenhouse effect. In Figure 13, the idling curves can be seen, which, as for the other components of the exhaust gas, showed the lowest value for carbon dioxide at a 2% concentration of the silver solution.

For pure MDO without additives, the carbon dioxide content was 2%, while for the silver solution it was 1.8%, i.e., there was a 10% decrease in the CO₂ content. Furthermore, the addition of colloidal silver resulted in a decrease in carbon dioxide to a level of 1.9%. The other additives did not show such beneficial properties, as the use of liquid microorganisms did not change the CO₂ concentration compared to fuel without additives. Effective microorganisms in the form of ceramic tubes increased the CO₂ content of the exhaust gas to 2.1%. An additive concentration of 5% effective microorganisms in each form reduced the dioxide content compared to a concentration of 2%.For the liquid form, the value was 1.9% and for the ceramic this was 1.8%. Colloidal silver at this concentration resulted in a CO₂ concentration of 2%. The best effect was obtained for the silver solution, where there was a further decrease in the carbon dioxide concentration to 1.7%, which is 15% lower compared to pure diesel. In summary, in order to reduce the concentration of CO₂ in the exhaust gas, a silver solution of 5% would need to be added to the fuel.

The waveforms shown in Figure 14 were obtained for a load of 20 kW. A very similar effect of the additives as for idling is observed. The carbon dioxide content at this load was 4.7%. The best effect was obtained for the silver solution, with a 4.5% CO₂ level, which represents a 5% reduction in carbon dioxide. For colloidal silver, there was a decrease in CO₂ to 4.6%, while after using liquid effective microorganisms, the same result was obtained as for MDO without additives, i.e., 4.7%. As in the previous case, effective microorganisms in the form of ceramic tubes were the least effective, as there was an increase in the CO₂ content above that obtained for diesel without additives. A concentration of 5% of effective microorganisms reduced the carbon dioxide content compared to a concentration of 2% for effective microorganisms in liquid form, down to a value of 4.65%. For the silver solution, the resulting carbon dioxide value was the same as that found for the lower concentration, 4.5%. For the ceramic mould, the value was 4.7%, i.e., there was a decrease compared to the 2% additive concentration. Colloidal silver at this concentration increased the CO₂ value to 4.8%. The best effect was also obtained with the silver solution, but an increase in the additive concentration did not improve the results, so with this load the best result was obtained with the silver solution at a concentration of 2%.

The graphs shown in Figure 15 were obtained for a load of 30 kW. In this graph, we again observe similar characteristic curves to those obtained at lower loads. The carbon dioxide content at this load was 6.5%. The best result was also obtained for the silver solution, after which the carbon dioxide level dropped to 6.2%, a reduction of 5%. For colloidal nanosilver, there was a decrease in CO₂ to 6.4%. The use of effective microorganisms in liquid form and ceramic tubes was followed by an increase in carbon dioxide levels to 6.6% and 6.7%, respectively. The least effective microorganisms proved to be effective regardless of the type used, as here there was an increase in the oxide content above that obtained for diesel without additives. A concentration of 5% of effective microorganisms reduced the carbon dioxide content compared to a concentration of 2% for the silver solution alone, where the resulting carbon dioxide value was 6.4%. For the ceramic mould, the value was 6.5%, the same as for diesel without additives, while for the effective microorganisms in the ceramic mould there was an increase in carbon dioxide content to 6.6%, a higher value than for pure MDO. The addition of colloidal nanosilver at this concentration increased the CO₂ concentration to 6.8%, a higher value than that obtained when MDO was combusted without additives, and therefore a concentration of 2% is sufficient.

Carbon dioxide is produced during the complete combustion of hydrocarbon fuels and its content in the exhaust gas depends on the chemical composition of the fuel and the amount burned. In tests with the additive, it was shown that there was a noticeable decrease in fuel consumption, which probably resulted in a decrease in CO₂ emissions. The resulting changes were demonstrated under the described experimental run conditions. We are not aware of any publications in this field to which the results could be compared.

3.3. A Summary of the Result Analysis

In a summary of the analysis of the results obtained in

Table 5. The listing of exhaust gas components and fuel consumption: SS—silver solution, CS—colloidal silver, EM_fl—effective microorganisms in fluid form, EM_cer—effective microorganisms in ceramic form, down arrow—decrease in flue gas component relative to pure MDO, up arrow—increase in flue gas component relative to pure MDO.

NOx [ppm]								CO [ppm]
Engine Load	Pure Oil	Additives	Concentration 2%		Concentration 5%		Pure Oil	Additives	Concentration 2%		Concentration 5%
0 kW	289	SS	271	↓6.2%	251	13.2%	434	SS	364	↓16.2%	425	↓2.1%
		CS	266	↓8%	247	↓14.6%		CS	398	↓8.3%	415	↓4.4%
		EM_fl	266	↓8%	278	↓3.9%		EM_fl	397	↓8.5%	398	↓8.3%
		EM_cer	259	↓10.4%	259	↓10.4%		EM_cer	389	↓10.4%	420	↓3.3%
20 kW	874	SS	846	↓3.2%	830	↓5.1%	935	SS	667	↓28.7%	957	↑2.4%
		CS	851	↓2.6%	852	↓2.5%		CS	844	↓9.8%	971	↑3.9%
		EM_fl	860	↓1.6%	822	↓6%		EM_fl	876	↓6.3%	910	↓2.7%
		EM_cer	865	↓1.05%	825	↓5.6%		EM_cer	905	↓3.2%	926	↓1.0%
30 kW	1222	SS	1174	↓4%	1189	↓2.7%	1615	SS	1206	↓25.3%	1575	↓2.5%
		CS	1200	↓1.8%	1240	↑1.5%		CS	1518	↓6.0%	1601	↓0.9%
		EM_fl	1220	↓0.17%	1095	↓10.4%		EM_fl	1616	↑0.06%	1639	↑1.5%
		EM_cer	1190	↓2.62%	1105	↓9.6%		EM_cer	1658	↑2.7%	1701	↑5.3%
CO2 [%]								Fuel Consumption [dm3/h]
Engine Load	Pure Oil	Additives	Concentration 2%		Concentration 5%		Pure Oil	Additives	Concentration 2%		Concentration 5%
0 kW	2	SS	1.8	↓10.4%	1.7	↓15%	4.7	SS	4.5	↓4.5%	4.6	↓2.5%
		CS	1.9	5.0%	2	0%		CS	4.6	↓2.5%	4.8	↑4.6%
		EM_fl	2	0%	1.9	↓5.0%		EM_fl	4.6	↓2.5%	4.8	↑4.6%
		EM_cer	2.1	↑5.0%	1.8	↓10%		EM_cer	4.6	↓2.5%	4.7	0%
20 kW	4.7	SS	4.5	↓5.0%	4.5	↓4.5%	10.2	SS	9.9	↓3.0%	10	↓2.0%
		CS	4.6	↓3.0%	4.8	↑2.2%		CS	10	↓2.0%	10.5	↑3.0%
		EM_fl	4.7	0%	4.65	↓1.2%		EM_fl	10.3	↑1.0%	10.8	↑6.0%
		EM_cer	4.8	↑3.0%	4.7	0%		EM_fl	10.1	↓1.0%	10.4	↑2.0%
30 kW	6.5	SS	6.2	↓4.6%	6.4	↓1.6%	13.3	SS	13	↓2.3%	13.4	↑1.0%
		CS	6.4	↓1.8%	6.8	↑4.6%		CS	13.3	0%	13.8	↑3.8%
		EM_fl	6.6	↑1.8%	6.5	0%		EM_fl	13.5	↑1.6%	13.9	↑4.5%
		EM_cer	6.7	↑3.1%	6.6	↑1.6%		EM_cer	13.2	↓0.8%	13.5	↑1.6%

, the fuel consumption and selected components of the exhaust gas are listed for the various loads and additive concentrations mixed with diesel, showing the resulting percentage changes. The green colour represents the largest percentage decrease compared to the result obtained with MDO without additives. Red, on the other hand, in each case represents a value higher than that obtained for fuel without additives, indicating that the use of such an additive at a given concentration is pointless.

Analysing the results obtained, it can be concluded that for higher additive concentrations for both fuel consumption and exhaust gas composition, the changes were not as favourable as for a concentration of 2%.If there are too many additives, the engine thrust will decrease, which will lead to blocking of the hub.

Thus, considering only the additive concentration of 2%, the best results were obtained for the silver solution. After applying this additive, the fuel consumption dropped by 4.5% compared to the consumption for pure MDO. The decrease in NO_x when the engine was operated under load was shown to be effectively influenced by the silver solution, which, at a concentration of 2%, resulted in a reduction in NO_x by 3.2% at 20 kW load and by 4% at 30 kW load. The presence of a 2% silver solution in the fuel also resulted in a significant reduction in carbon monoxide in the flue gas by 28.7% at 20 kW and 25.3% at 30 kW.

4. Conclusions

The aim of this study was to see how the additives of effective microorganisms and silver would affect selected engine performance parameters including fuel consumption and exhaust gas composition. The tests were carried out for diesel fuel with 2% and 5% additives, for idling and two engine power loads—20 kW and 30 kW.

The additives were found to have no significant effect (neither positive nor negative) on selected engine operating parameters including the engine speed; the cooling water and lubricating oil temperature; the exhaust gas temperature; and the air intake pressure. They did, however, affect the hourly fuel consumption and exhaust gas composition.

The best fuel consumption results were achieved with a 2% silver solution. This addition caused fuel consumption to drop by up to 3%. Higher concentrations in most cases increased this consumption by up to 6% for liquid effective microorganisms.As part of the exhaust gas composition studied, the effects of the additives on the three most important components of the exhaust gas from an environmental point of view were analysed. These were nitrogen oxides (NO_x), carbon oxides (CO) and carbon dioxide (CO₂).

The decrease in NO_x when the engine is operated under load was shown to be effectively influenced by the silver solution, which at a concentration of 2% resulted in a reduction in NO_x, by 3.2% at 20 kW load and by 4% at 30 kW load.

It is probably due to the drop in temperature in the cylinder, as the intensive synthesis of oxygen and nitrogen occurs at very high temperatures. However, the mechanism of the effect of the additive used on heat generation in the cylinder and temperature is not known to the authors.

The presence of a 2% silver solution in the fuel also resulted in a significant reduction in carbon monoxide in the flue gas: by 28.7% at 20 kW and 25.3% at 30 kW. Increasing the concentration to 5% proved ineffective. This also applies to the other additives. Because carbon monoxide is produced by incomplete combustion of the hydrocarbons contained in the fuel. The likely reason for the decrease in CO in the flue gas is the improved combustion process, which is indirectly evidenced by the demonstrated decrease in fuel consumption. For carbon dioxide, the best results were also obtained with a silver solution of 2%. At idle, the reduction was up to 10% compared to pure oil. The higher the engine load, the lower the reduction, e.g., 4.6% for 30 kW. Carbon dioxide is produced during the combustion of hydrocarbon fuels and its content in the exhaust gas depends on the chemical composition of the fuel and the amount burned. Studies with the additive have shown that there is a noticeable decrease in fuel consumption, which is likely to result in a decrease in CO₂ emissions.

In summary, it can be concluded that in order to reduce the NO_x, CO and CO₂ content in the exhaust gas and achieve a noticeable decrease in hourly fuel consumption, a silver solution with a 2% concentration in MDO is the best additive. The above would also need to be confirmed in further tests on an engine operating under real operating conditions. It is also important that the fuels used with individual additives, regardless of their percentage of MDO, do not require structural changes to the fuel infrastructure.

In further studies, it will be necessary to investigate the effect of this additive on the combustion process in the engine and the wear and tear of its components, and to confirm the results obtained under real operating conditions.