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This study discusses practical collection methods of cereal harvesting costs in different agricultural holdings in order to effectively manage combine harvester fleets, make economically reasoned decisions on the exploitation of combine harvesters, reduce harvesting costs and consequently the cost price of cereals. For this purpose, the author used work results of combine harvesters monitored by three randomly selected agricultural holdings, collected practical information on harvesting, analysed this information and provided assessments on the effectiveness of their combine harvester fleet. Evidently, not all combine harvester fleets and combines operate with the same efficiency, as their harvesting costs are different.

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Abstract. The use of fuel additives in engine fuels is a widespread tendency, and one which occurs on a worldwide basis. The most common area in the development of additives is for petrol and diesel fuel. The manufacturers claim that additives emulsify condensation water in the fuel tank, cleaning the fuel supply system and decreasing fuel consumption by improving the engine combustion process. Several additives have been developed for biofuels such as, for example, Valvoline VPS HD Diesel System Complete, which has been developed for biodiesel fuel. The manufacturers state that additives clean the fuel supply system and restore the engine power, which is something that could be reduced in cases in which biodiesel fuel is used. Moreover, soot emissions in exhaust fumes are decreased. According to the aforementioned points, fuel additives should have a significant impact on the engine combustion process in order to ensure improvements in the parameters that have been mentioned. In order to see the true picture, this article analyses the impact of biodiesel fuel with an additive, along with undiluted biodiesel fuel and diesel fuel, on the engine combustion process. More specifically, variations in the combustion pressure and combustion phases are being analysed. What is more, the heat release rate, heat release and cylinder working values are calculated, and the engine efficiency, fuel consumption, and exhaust emission levels are presented. The results indicate that this specific fuel additive does not have a significant impact on the engine combustion process.Keywords: diesel fuel, RME, compression ignition engine, heat release rate, heat release, exhaust emissions, exhaust fumes. IntroductionThe impact of fuel additives on internal combustion engines has been discussed in several studies [1][2][3][4][5][6][7][8][9][10]. The results show that there are several additives, which have an impact on the engine performance parameters and emission levels. The additives discussed in these studies are specific and have an impact on a certain parameter of the fuel such as, for example, the cetane number or oxygen content [5; 7]. The universal additives for which improved fuel properties have not been specified usually do not have an impact on the engine performance parameters [2]. Therefore, the effectiveness of using fuel additives remains disputable, and their impact on the engine should be studied. Fuel additives have been developed for biofuels such as, for example, biodiesel fuel; however, the effect of these additives on the engine performance parameters and exhaust emission levels remains lower than when compared to the use of diesel fuel [11]. This is caused by the different physico-chemical properties of biofuel in comparison to diesel fuel. The universal fuel additives for biofuels usually do not improve the engine performance parameters. There may, however, be a slight improvement in the engine exhaust emissions [1].

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