Evaluation of energy efficiency of promising fuels for autotractor reciprocating engines

. The prospects for the use and reserves of petroleum fuel resources pose the problem of finding alternative fuel from non-oil sources for automotive and tractor engines. There is a problem of determining the most efficient type and source of production of such fuels. To do this, it is necessary to assess the energy efficiency of the systems for the production and use of fuel from the moment it is in the form of raw materials until it is converted into mechanical work. Such an approach to solving the problem makes it possible to evaluate all types of potential fuels, their main sources, as well as to evaluate the effectiveness of their use in automotive and tractor engines in a fuel-engine combination.


Introduction
When evaluating the energy efficiency of promising fuels, the results of studies of domestic and foreign experience were used. Before fuel from a type of raw material is used in an autotractor engine, it must go through several technical stages, i.e., the search for and extraction of raw materials from a source (drilling, mining, harvesting, etc.); processing of raw materials into fuel; distribution of fuel between consumers; fuel consumption by private transport consumers [1][2][3][4][5]. The total efficiency of these processes can be written as: Where: Z 1 -is the efficiency of the search and extraction of raw materials; Z 2 -is the efficiency of processing raw materials into fuel; Z 3 -is the efficiency of fuel distribution between consumers; Z 4 -is the efficiency of fuel use by private transport consumers.
The efficiency of the constituent members of equation (1) can be more or less significant individually, and the total efficiency, being the result of all these steps, can vary depending on the fuel, its source and the type of automotive engine. To reduce the amount of energy consumed by an auto tractor engine, it is necessary to maximize the efficiency of the entire process -from extraction to the final use of fuel in an internal combustion engine (ICE).
To simplify the assessment of the potentiality of systems, stages Z 1 and Z 2 were not considered by us, since they mainly include mechanical operations and, although they have a higher efficiency compared to other stages [5], they do not noticeably affect energy efficiency. Then, the total energy efficiency of potential fuels:

= •
(2) When evaluating the energy efficiency of potential fuels, the following issues were considered: energy resources and potential fuels; the efficiency of raw material processing in obtaining various alternative fuels from primary energy sources; fuel-engine system compatibility; fuel efficiency in various automotive and tractor engines; total energy efficiency, which makes it possible to determine the optimal fuel-engine combination from an energy point of view.

Research methods and results
Energy resources and potential fuels. The main sources that make it possible to obtain automotive and tractor fuels in the future are energy resources, which can be conditionally divided into two groups: 1) minerals -oil, natural gas, oil shale, coal; 2) non-fossils -biomass, nuclear energy, solar energy, hydraulic energy, geothermal energy, wind energy, tidal energy, etc.
The most likely sources for obtaining liquid automotive fuels are oil, oil shale, coal, biomass and nuclear energy. At this stage, oil and natural gas still play a major role in the energy mix of many countries.
However, due to the low volumetric energy density, the use of such gaseous fuels as hydrogen, light carbons, ammonia at this stage requires the solution of a number of problems, i.e. ensuring safety during transportation, storage, operation, impossibility of use on heavy vehicles, etc. [5][6][7].
Other types of non-fossil energy -solar energy, wind and tidal energy, geothermal energy and hydropower -are more appropriate to use for the production of electrical energy, so they can be considered together with nuclear energy. Table 1 and Fig. 1 present the main types of energy raw materials for the production of motor fuels [5,[8][9][10][11]. By appropriate processing of each of the sources presented in Table 1 and Fig. 1, various fuels can be obtained. However, not all types of fuels can be used in internal combustion engines. The data in Fig. 1 and Table 1 show that gasoline and diesel fuel can be obtained from several energy sources.
But, this requires a more complex processing process than when obtaining them from oil, so a large energy consumption is required.
The efficiency of processing raw materials into fuel is an important parameter in determining the energy efficiency of the fuel and the fuel-engine system.
The efficiency of raw material processing E is determined by the amount of energy required to obtain a given type of fuel, which varies depending on the nature of the source and the fuel itself [5,11]. Then approximately Where: Q T -is the total calorific value of the fuel, KJ; Q д -the amount of additional energy required in the processing and operation of auxiliary equipment for the generation of electricity, oxygen, etc.; Q C -is the total calorific value of the raw material for fuel production.
At the same time, we consider that the equipment that produces fuel is autonomous, i.e., it is provided with energy from the same raw materials that are used to produce fuel. Then the fuel obtained with minimal energy consumption will have a high energy efficiency and its use creates the possibility of increasing the efficiency of the fuel-engine system.
The efficiency of processing raw materials can be relatively easily assessed if there is a well-established technology, as well as information about the amount of raw materials needed for the mass production of fuel, the amount of fuel produced and raw materials for supplying energy to the main and auxiliary production equipment. However, in most cases, the existing equipment is not autonomous, since the oil feedstock may not always be the only source of energy for this equipment. Even with a proven technology, the available information cannot be used directly to determine the efficiency of oil conversion into fuel. Naturally, if the processing technology is newly developed, then the situation becomes even more complicated. In this case, the available information is applicable only to the laboratory, at best to experimental conditions. Information on the amount of feedstock used and fuel produced is currently only available for a few unrefined fuels, such as synthetic oil from coal or hydrogenated shale oil from oil shale. Data on the required energy costs for the processing process and the operation of auxiliary equipment are not available. Schemes for the production of fuel from a particular type of source are still at the stage of development and improvement. Therefore, the assessment of the efficiency of raw material processing is indicative and is based on the analysis of literature and other materials. This is necessary to analyze the influence of the source of raw materials, the processing process and the resulting product on the efficiency of processing raw materials into fuel. The influence of the source of raw materials on the quality of the resulting fuel is determined mainly by the presence of hydrogen and various impurities (sulfur, nitrogen, oxygen and water) in it. With an increase in the hydrogen content and a decrease in the amount of impurities, the efficiency of the process increases. The factor influencing the process is the rigidity of the processing mode -with its increase, the efficiency of converting raw materials into fuel decreases, and the quality of the fuel improves, i.e., a cleaner fuel is obtained. Stricter requirements for fuel quality and impurity content can also lead to a decrease in process efficiency.
When evaluating the efficiency of fuel processing from various potential sources, we assume that the fuel is obtained only from this source and only one type of fuel is produced. Obviously, when more than one type of fuel is obtained from a given source using a certain technology, the efficiency of processing may change.
The average values of the efficiency of processing raw materials into fuel from various sources (oil, oil shale, coal, biomass) are not the same. Each fuel, except ethanol and methanol, is a conventional fuel than a specific one with stringent requirements. For comparison, Fig. 2 shows the processing efficiency of crude oil as well. Fig. 2 shows the efficiency of obtaining fuel from a number of sources, such as coal, alcohols, oil shale, biomass, nuclear energy, synthetic oil, etc. Fig. 2 shows that the efficiency of obtaining fuels from various raw materials ranges from 17 % to 98%. At present, the production of alternative fuel from biomass and hydrogen fuel is of the greatest interest.
Biomass fuel. There are mainly two types of fuels that can be produced from biomass: alcohol (ethanol) and hydrocarbon liquids. The efficiency of obtaining ethanol is approximately 35%, hydrocarbon liquids -25%.
At present, commercial interest in biodiesel fuel has increased in the USA, Canada and EU countries, in particular in the technology of its production from vegetable oil. In the US, it is planned to replace commercial diesel fuel with biodiesel by 20% and use it in city buses, trucks and marine vessels. The use of biodiesel fuel is mainly associated with a significant reduction in the amount of harmful engine emissions (by 25-45%) and an improvement in the environment, since the sulfur content in biodiesel fuel is only 0.02% [1, 2, 6].
Biogas is a mixture of methane and carbon dioxide and is a product of methane fermentation of organic matter of plant and animal origin. The use of biogas as an alternative fuel improves the technical, operational and environmental performance of the engine.
Hydrogen as an alternative fuel. A possible type of autotractor fuel can also be hydrogen, obtained from water through the use of heat or electricity generated by a watercooled nuclear reactor. Such hydrogen, in turn, can combine with carbon oxides obtained from the decomposition of limestone, resulting in the formation of either methanol or liquid carbons. The efficiency of hydrogen production using atomic energy is 25-30%.
The use of hydrogen as a fuel for motor vehicles can make a significant contribution to the world's energy. Compared to gasoline and diesel fuel, hydrogen is more efficient and less polluting. At this stage of development, many automobile companies have experimental vehicles that run on hydrogen. Some firms offer combined solutions. For example, Mazda offers a car that has the ability to alternate fuel: gasoline and hydrogen. In the United States, tractors are produced in the engines of which a mixture of hydrogen and diesel fuel is used, which allows increasing engine power, environmental friendliness and reducing fuel consumption.
With the transition of vehicles to hydrogen fuel, environmental problems will be solved positively and forever. However, for this it is necessary to solve a number of problems, the main of which are: the need to use special ultra-tight containers for transporting and storing hydrogen; the need for huge energy costs to produce hydrogen by electrolysis of water; the need to create a developed network of filling stations in each settlement and along major highways, etc. In addition, the cost of hydrogen fuel is more expensive than the cost of fuels of petroleum origin.
Properties of promising fuels and assessment of the compatibility of the fuel-engine system. The efficiency of converting feedstock to fuel is one of many criteria for selecting potential fuels. The physical and chemical properties of fuels are also important as they determine the compatibility of the fuel with the engine. The engines of the future most likely include diesel engines, spark ignition engines with injector and layered charge distribution of the fuel mixture, gas turbines, Stirling engines, systems with electric batteries, etc. Specific requirements for the future fuel for these engines have not yet been developed. However, from the experience of working with petroleum fuels, certain requirements can be obtained, the most important of which should include approximately the same properties as for petroleum fuels, i.e., completeness of combustion, volatility, concentration of pollutants, toxicity, viscosity, operational safety, etc.
If the characteristics of combustion and volatility of the fuel are important in terms of economy and ease of starting the engine, then the content of sulfur, nitrogen, ash, atmospheric dust and water in it is undesirable, since they can increase toxicity and reduce the durability of the engine. After removing these impurities, alternative fuels become the same as oil ones. Viscosity and volumetric energy density, apparently, will not be of decisive importance for the normal operation of the engine, but they can lead to design changes in the engine necessary for storing, pumping and dosing fuel.
Matching fuel qualities and engine requirements is essential for the correct evaluation of future fuel-engine combinations. The fuel-engine system compatibility assessment concerns fuels derived from petroleum. Such an assessment is necessary for fuels from nonpetroleum sources, as it allows you to determine the efficiency of using alternative fuels and the overall energy efficiency of potential fuel-engine systems. But, the information available at this stage on the requirements for alternative fuels and engines is insufficient, therefore, until a sufficient number of alternative fuels from various sources become available, it is impossible to fully analyze the compatibility of the fuel-engine system. In this regard, we continue research work on these issues.
Evaluation of fuel efficiency in the engine. The analysis of this issue was considered on the example of a car engine, since it has the greatest application compared to tractor and other types of energy consumers. At the same time, we proceeded from the fact that the fuel efficiency (E I ) is determined by the amount of energy required to overcome the inertia forces and the dynamic resistance of the vehicle performing maneuvers (Wa), referred to the amount of fuel energy consumed by the vehicle during these maneuvers (W T ), i.e.
Fuel efficiency depends on several important factors, the main ones being the performance of the vehicle, the nature of the exhaust emission and the mode of operation, the secondary ones are the reliability, cost and acceptability for the consumer.
The fuel efficiency determined [5,11] for two existing engines -spark ignition and diesel, was 15 and 17%, respectively, i.e. diesel has a slightly higher efficiency than a spark ignition engine.
The fuel efficiency for all potential fuel-engine combinations, including each of the alternative engines, at this stage, as already noted above, cannot be accurately determined, since most of the alternative fuels of interest to us are not available, and in the required quantities. However, work on alternative fuels is being intensively carried out, and eventually the accumulated information will allow an accurate assessment of promising fuels and engines.
Thus, in order to determine the properties of alternative fuels, their compatibility with various potential engines, and to develop objective estimates of fuel-engine compatibility, research and scientific-practical work is necessary both in the automotive and tractor industry and in the energy sector. The result of this work is extremely important information needed to determine the most energy efficient fuel-engine system. Of course, the choice of such a system in the future is significantly influenced not only by energy, but also by economic and political factors, as well as issues of reducing environmental pollution.

Conclusions
1. Prospects for the use and resources of petroleum fuels pose the problem of finding fuel from non-oil sources for automotive and tractor engines. There is a problem of determining the most efficient type and source of fuel production. To do this, it is necessary to assess the energy efficiency of the systems for the production and use of fuel from the moment it is in the form of raw materials until it is converted into mechanical work. 2. The influence of the source of raw materials on the quality of the resulting fuel is determined mainly by the presence of hydrogen and various impurities (sulfur, nitrogen, oxygen and water) in it. With an increase in the hydrogen content and a decrease in the amount of impurities, the efficiency of the process increases. The factor influencing the process is the rigidity of the processing mode -with its increase, the efficiency of converting raw materials into fuel decreases, and the quality of the fuel improves, i.e., a cleaner fuel is obtained. 3. To determine the properties of alternative fuels, their compatibility with various potential engines, and to develop objective estimates of the compatibility of fuel and engine, research and scientific and practical work is needed, both in the automotive industry and in the energy sector.