The potential of vegetable oils as an alternative to diesel fuel has already been proved. Nevertheless, the high viscosity of vegetable oils poses some problems concerning their use in unmodified vehicle diesels. In particular, distortion of the speed characteristics of a distributor fuel injection pump with a maximum-minimum speed mechanical governor when using sunflower oil-diesel oil blends has been observed. A diesel engine fuelled by a viscous blend could not develop its rated power. After fuel delivery was increased to retain the rated power, the engine was overfuelled at lower speeds and the smoke limit was exceeded. In the present paper, measures required to retain the characteristics of an injection pump fuelled by viscous blends are described. This can be done by readjusting the pump governor and changing the hydraulic characteristics of the fuel delivery system.

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Mustard Methyl Esters (further biodiesel) and regular diesel fuel were tested in direct injection diesel engine. Analysis of experimental data was supported by an analysis of fuel injection and combustion characteristics. Engine fuelled with biodiesel had increased brake specific fuel consumption, reduced nitrogen oxides emission and smoke opacity, moderate increase in carbon monoxide emission with essentially unchanged unburned hydrocarbons emission. Increase in fuel consumption was attributed to lesser heating value of biodiesel and partially to decreased fuel conversion efficiency. Analysis of combustion characteristics revealed earlier start of injection and shorter ignition delay period of biodiesel. Resulting decrease in maximum rate of heat release and cylinder pressure was the most probable reason for reduced emission of nitrogen oxides. Analysis of combustion characteristics also showed that cetane index determined by ASTM Method D976 is not a proper measure of ignition quality of biodiesel. Conclusion was made on applicability of mustard oil as a source for commercial production of biodiesel in Pakistan. Potentialities of on improving combustion and emissions characteristics of diesel engine by reformulating biodiesel were discussed.

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