Abstract
Examining the influence of thermal conditions in the engine cylinder at the start of fuel injection on engine combustion characteristics is critically important. This may help to understand physical and chemical processes occurring in engine cycles and this is relevant to both fossil fuels and alternative fuels like biodiesels. In this study, six different biodiesel–diesel blends (B0, B10, B20, B40, B60 and B100 representing 0, 10, 20, 40, 60 and 100% by volume of biodiesel in the diesel–biodiesel mixtures, respectively) have been successfully tested in a cooperative fuel research (CFR) engine operating under a wide range of thermal conditions at the start of fuel injection. This is a standard cetane testing CFR-F5 engine, a special tool for fuel research. In this study, it was further retrofitted to investigate combustion characteristics along with standard cetane measurements for those biodiesel blends. The novel biodiesel has been produced from residues taken from a palm cooking oil manufacturing process. It is found that the cetane number of B100 is almost 30% higher than that of B0 and this could be attributed to the oxygen content in the biofuel. Under similar thermal conditions at the start of injection, it is observed that the influence of engine load on premixed combustion is minimal. This could be attributable to the well-controlled intake air temperature in this special engine and therefore the evaporation and mixing rate prior to the start of combustion is similar under different loading conditions. Owing to higher cetane number (CN), B100 is more reactive and auto-ignites up to 3 degrees of crank angle (DCA) earlier compared to B0. It is generally observed in this study that B10 shows a higher maximum value of in-cylinder pressure compared to that of B0 and B20. This could be evidence for lubricant enhancement when operating the engine with low-blending ratio mixtures like B10 in this case.
Highlights
Biodiesels are environmentally friendly alternative fuels and have important properties close to that of fossil diesel
In-cylinder pressure signals outputted in this study are averaged from 50 consecutive engine cycles
It is observed that the oxygen content in biodiesel has significant effects on the fuel auto-ignitability
Summary
Biodiesels are environmentally friendly alternative fuels and have important properties close to that of fossil diesel. Apart from controversies in some of the reported results as mentioned above, there are few key issues with respect to existing research on biodiesels: (i) the fuels are generally selected at random depending on their availability and without any reference to chemical and physical property variations amongst biodiesels derived from different feedstock; (ii) very few studies report on the basic auto-ignitability of biodiesels, a feature that may well be important considering how the influence of different thermal conditions in the cylinder at the start of injection (SOI) on the in-cylinder pressure development, heat release rate and ignition delay provides a better understanding of biodiesel auto-ignitability; and (iii) the definition of engine load condition is not consistent amongst the literature, a number of authors used similar fuel volume flow rates (mL/min) while some others used a similar amount of input energy (MJ/min) when testing auto-ignition engines with biodiesel blends and diesel. The injection timing and CR are varied in this study so that the thermal conditions at SOI are varied in a wide range
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