Characterization of the effect of fatty ester composition on the ignition behavior of biodiesel fuel sprays

The effect of fatty ester composition on the combustion behavior of biodiesel fuel sprays is investigated for soy-based methyl esters, canola-based methyl esters and canola-based butyl esters. The purpose of this study is to aid fuel engineering efforts by identifying molecular structure features of biodiesel which can influence the compression ignition process during direct biodiesel utilization in diesel engines. Combustion behavior is classified through ignition delays and maximum apparent heat release rates measured in a rapid compression machine, and these measurements have been taken at the low temperatures (676-815 K) and reduced oxygen concentrations (12% and 18%) corresponding to the low temperature combustion regime of advanced diesel engines. Ignition delay measurements show that the appearance of first heat release from the fuels is decreasingly sensitive to oxygen concentration as the reaction zone temperature is increased. Growth in the ester alkoxy chain length from one to four carbon atoms had minimal effect on the ignition delay. For tests in 12% oxygen, the increased polyunsaturated of soy methyl esters relative to canola methyl esters leads to longer ignition delays. This is not observed using 18% oxygen where physical transport processes are more important. The canola butyl esters exhibited the largest peak values for heat release rates, with the distinction becoming clear at reaction zone temperatures above 750 K. Normalization on ignition delay and input energy bases indicates that variation of the maximum apparent heat release rates among the fuels is primarily due to unique mixing times and fuel heating values.