Assessment of the functioning of biofuel-powered engines based on a multi-criteria analysis

. Reducing the emission of exhaust gas components into the natural environment from the transport sector is a priority. In recent years, the European Union has published many legal acts that regulate the emission of exhaust gas components. One of the known ways to reduce them is to use alternative fuels to power combustion engines. The paper presents an assessment of the functioning of drive units powered by mixtures of diesel oil and fatty acid methyl esters. The AHP (analytic hierarchy process) method was used for the analysis, which made it possible to determine the values of individual assessments. Based on the analysis, it was found that the lowest exhaust emission parameters were obtained for a fuel mixture containing 30% fatty acid methyl esters.


Introduction
The development of the transport sector has contributed to the increased use of nonrenewable energy sources and the dustiness of atmospheric air.The increasing use of road transport leads to environmental degradation, which has been noticed and regulated by legal regulations.The changes introduced in European and national law are aimed at limiting the negative impact of road transport on the natural environment.The European Commission has introduced Strategies whose main priority is to achieve sustainable development, which must be implemented by 2030, 2035 and 250.Sustainable transport concerns means of communication that minimize the emissions of substances that pollute the natural environment.Pollution from the transport sector is more dangerous than emissions from industry.This is due to their spread at low altitudes [1].The emission of automotive pollutants depends on many factors, which are presented in Figure 1.Introducing changes in the functioning of the transport sector, in accordance with the transport policy, is aimed at increasing public awareness of the threat posed by the constantly growing emission of toxic compounds from transport activities and its reduction.to a minimum.The negative impact of road transport on the environment and people includes [2,3,4]: • greenhouse gas emissions that contribute to climate change, • emissions of air pollutants, which negatively affect the natural environment and human health, especially in urban agglomerations, • occupying natural areas and interrupting their continuity in order to build road infrastructure, which contributes to the loss of biodiversity and noise emissions.Emission of air pollutants is measured by the concentration of individual exhaust gas compounds that are produced while driving, including: carbon oxides, nitrogen oxides, sulfur dioxides, particulate matter, lead and soot.Gases that are released into the atmosphere during the operation of combustion engines, interacting with sunlight, cause photochemical smog, especially in urban agglomerations.High air pollution occurs in places such as: underground parking lots, above-ground multi-storey parking lots, roads with heavy traffic, tunnels and around gas stations.In terms of the number of vehicles per population, Poland ranks second among the European Union countries [5].In total, there are approximately 25 million registered motor vehicles in Poland.The number of motor vehicles, divided into passenger cars, registered in Poland in 2016-2022 is presented in Figure 2.There are many practices that aim to reduce the negative impact of transport on the natural environment.The most important ones include legal regulations regarding ecological transport policy, protection of natural resources, environmental protection programs and pollution prevention.One of the solutions used is the use of renewable fuels.The use of renewable energy sources to power combustion engines makes it possible to become independent of the economy from fossil fuels and reduces the emission of toxic components found in exhaust gases.Crude oil deposits transformed into fuel under the influence of physical and chemical processes, 40% of them are used in the transport sector.There is a high demand for energy in the Polish energy sector, with little activity in modernizing the infrastructure necessary to produce it, which results in dependence on external suppliers.In 2020, the European Union standardized energy consumption regarding ecological obligations, which introduced changes such as reducing greenhouse gas emissions by 20% compared to 1990, reducing energy consumption by 20% compared to forecasts for the EU for 2020, and increasing use of renewable energy sources in transport up to 10%.
The use of fuels from renewable energy sources to power internal combustion engines is associated with high demand and a decreasing amount of fossil fuels.Vegetable oils have the widest applications for the production of biofuels.Alternative fuels of plant origin used to power compression ignition engines include: rapeseed oil, peanut oil, sunflower oil, soybean oil and animal fats [7].Fuels of plant origin, called biofuels, are subjected to chemical processes in order to obtain physicochemical properties similar to those of diesel oil.The physicochemical properties of biodiesel produced from rapeseed oil and diesel oil are presented in Table 1.For structural, technical, technological and economic reasons, rapeseed oil is the most commonly used.Biogas produced from waste from fruit and vegetable processing and from pyrolysis oil, i.e. a mixture obtained in the process of thermal decomposition of organic substances from used car tires, can also be used to power compression ignition engines [8].Another raw material used as a source of biofuels are microalgae [9,10,11].Municipal waste, sewage and residual waste from the agricultural industry can also be used to produce biofuels powering the diesel engine [12,13,14,15].Diesel engines can also be fueled with used vegetable oil, i.e. frying waste, but this requires the use of a dual-fuel system, where the start-up takes place on diesel oil [16,17,18].

Materials and methods
For the purposes of this work, an analysis of the literature regarding individual groups of parameters of combustion engines was performed.The features assigned to them have been defined as criteria for assessing drive units.The existence of these features of drive units is the basis for assigning them specific values.Assessment criteria are used to verify effectiveness and make the most effective decision possible.Based on the literature analysis, four groups of criteria for the evaluation of piston combustion engines were identified, such as: material criterion, economic criterion, utility criterion and ecological criterion.The selected most important criteria for assessing drive units are presented in Figure 3.In the case of compression-ignition engines, fuels based on rapeseed oil have significant ecological and economic advantages.The research material included mixtures of conventional fuels and biocomponents.For the compression ignition engine, these were mixtures of diesel fuel and RME fatty acid methyl esters (Table 2).Diesel oil was obtained directly from the refinery, so it was free of any biocomponents.An enriching additive was used in the BIO50 fuel mixture (a mixture of 50% diesel oil and 50% fatty acid methyl esters), whose task was to increase the cetane number and increase the lubricating properties of the fuel.The purpose of using fuel additives is to protect the engine and keep its fuel system clean.In this way, the wear of the drive unit is reduced and its operating parameters are improved.Fuels with such additives can be used in any internal combustion engine design.

BIO50+
The objects of the research were compression ignition engines.The drive units were selected due to their widespread use in road transport.The engine used in the research was a 1.6 HDi engine characterized by direct fuel injection using the Common Rail system with electromagnetic injectors.The power unit worked with a dual-mass flywheel and a turbocharger with variable blade geometry, which influenced its performance.The engine was not equipped with a particulate filter.The characteristics of the compression ignition combustion engine are shown in Figure 4. Based on the literature analysis [20,21,22,23,24,25,26,27,28,29], criteria were selected and subjected to research and evaluation.The operational parameters of combustion engines selected for operational tests are shown in Figure 5.The operational parameters presented in Figure 40 were tested while simulating road conditions on a chassis dynamometer with an eddy current brake.Multi-criteria analysis (MOA) was used to develop a model for assessing the functioning of the ignition engine.The multi-criteria analysis used consisted in ranking the assessed criteria in a specific order and determining their quality by assigning appropriate results to them.The use of the MOA method made it possible to define quantitative and qualitative criteria constituting a multi-criteria evaluation model, establish a system of weights for individual criteria, implement the process of calculating the total score for a given variant and interpret the received scores.

Analysis
The developed model compared the test results obtained for individual fuel mixtures.Conventional fuel was taken as the reference point.25 variants (configurations) were analyzed: fuel mixture -engine operating parameters.In order to compare the values of operational parameters of individual fuel mixtures to the reference point (in this case it diesel oil without the addition of a biocomponent), normalized values of vector components were determined.One-dimensional vectors marked with the variable X were random variables representing the values of individual operational parameters.The analyzed parameters reflect the assessment of a compression-ignition combustion engine, and the vector considered then took the form: Xi = < X1, X2,X3,X4,X5,X6,X7,X8,X9,X10 > (1) In order to standardize the results of the measurements, their values were set to <0÷10>.To standardize the values of individual operational parameters, it was necessary to determine the arithmetic mean of repeated measurements and to determine the minimum and maximum for a given parameter.The results obtained for the analyzed operational parameters and individual fuel mixtures, as well as the weight values of individual operational parameters and an explanation of the components of the form vector are presented in Table 3. Standardized results of measurements of the operational parameters of a compressionignition engine for individual mixtures of diesel oil and the RME biocomponent in relation to diesel oil are presented in a graphical interpretation in Figures 5-6.

Conclusions
Reducing the emission of combustion engines while maintaining their efficiency leads to the search for new solutions in their power supply.The use of fuels from renewable sources causes a decrease in exhaust gas components dangerous to the natural environment, which has been confirmed by many authors and in this work.The experiment showed that the addition of fatty acid methyl esters to diesel oil affects the exhaust gas components emitted by the tested compression-ignition engines.The analysis of selected parameters showed the highest results for the BIO10 mixture, while the lowest for BIO50.The research shows that the best mixture is a mixture containing 70% diesel oil and 30% fatty acid methyl esters.The assessment showed that nitrogen oxides and oxygen had the greatest impact on the composition of the fuel mixture and the regulation of the computer control system of the drive unit, while carbon monoxide was the least sensitive.It can be concluded that the addition of fatty acid methyl esters to diesel fuel has a positive effect on the reduction of exhaust gas emissions of the analyzed power unit.The obtained test results confirm the advisability of using alternative fuel together with the regulation of the computer engine control system.

Fig. 5 .
Fig. 5. Operating parameters of combustion engines selected for testing.

Table 2 .
Fuel mixtures used for testing compression ignition engines.

Table 3 .
Standardized values of vector components for individual values of operational parameters.