Fuel composition impacts on processes in compression ignition engines.

摘要

Explorations of the impacts of fuel properties on the processes in compression ignition engines were performed. Major topics addressed are: (1) establishing relationships between the bulk modulus of compressibility, fuel injection timing in diesel engines, and NOx emissions; (2) identifying differences in autoignition chemistry between diesel fuel and two alternative fuels; and (3) assessing the effects of cetane number (CN) on homogeneous charge compression ignition (HCCI) engines. In the first study, bulk modulus correlated with start of injection timing (SOI) timing and NOx emissions, which are highest for biodiesel, followed by conventional diesel fuel, and lowest for Fischer-Tropsch (FT) diesel. FT diesel produced lower NOx emissions than expected based on SOI alone, and further analysis showed the timing of the maximum cylinder temperature produced a universal relationship with NOx emissions. In the second study, autoignition differences between conventional diesel, FT diesel, and methyl decanoate were investigated in a motored engine. Each demonstrated two-stage ignition, with a low temperature heat release (LTHR) event followed by main combustion. The magnitude of LTHR was highest for FT diesel, followed by methyl decanoate, with conventional diesel fuel last. Exhaust analysis for conventional diesel and FT diesel revealed LTHR produces high concentrations of aldehydes and carbon monoxide with only negligible carbon dioxide. Methyl decanoate differed by producing significant amounts of carbon dioxide during LTHR, a result of decarboxylation of the ester group, not oxidation. In the third study, the effects of CN on HCCI performance and emissions were investigated using fuels with a wide range of CN. Each fuel passed through a combustion phasing of maximum indicated mean effective pressure (IMEP), with an earlier phasing was required for the high CN fuels to prevent misfire, and LTHR magnitude decreased with decreasing CN. At their respective maximum IMEP phasing, all of the fuels performed comparably, but marked differences were observed at advanced and retarded combustion phasing. Increasing engine speed reduced the total LTHR, but the LTHR rate remained constant on a time basis, so the effect of higher engine speed is to reduce the time allowed for the reaction without changing the rate of reaction.