In this paper, we examine the interaction between the bulk modulus of compressibility of various fuel samples and its effect on fuel injection timing. The fuels considered range from soy oilderived biodiesel, unrefined soybean oil, and paraffinic solvents to ultralow-sulfur and conventional diesel fuels. Both the impact on injection timing and the variation in the bulk modulus of compressibility are measured. The present work confirms that the higher bulk modulus of compressibility of vegetable oils and their methyl esters leads to advanced injection timing with in-line pump-line-nozzle fuel injection systems. This has been shown in the literature to contribute to the well-documented increase in NO x emissions with the use of biodiesel fuel. An opposite trend, a retarding of injection timing, is observed with paraffinic fuels, because they have a lower bulk modulus of compressibility than conventional diesel fuels. This supports the observation that paraffinic fuels such as Fischer-Tropsch diesel yield lower NO x emissions.

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This study explores the diesel injection and combustion processes in an effort to better understand the differences in NO x emissions between biodiesel, Fischer−Tropsch (FT) diesel, and their blends with a conventional diesel fuel. Emissions studies were performed with each fuel at a variety of static fuel injection timing conditions in a single-cylinder DI diesel engine with a mechanically controlled, in-line, pump-line-nozzle fuel injection system. The dynamic start of injection (SOI) timing correlated well with bulk modulus measurements made on the fuel blends. The high bulk modulus of soy-derived biodiesel blends produced an advance in SOI timing compared to conventional diesel fuel of up to 1.1 crank angle degrees, and the lower bulk modulus of the FT diesel produced a delay in SOI timing of up to 2.4 crank angle degrees. Compared to conventional diesel fuel at high load, biodiesel fuel blends produced increases in NO x emissions of 6−9% while FT fuels caused NO x emissions to decrease 21−22%. Shifts in fuel injection timing, caused by bulk modulus differences, were largely responsible for the NO x increases, but pure FT diesel produced lower NO x emissions than expected on the basis of SOI alone. Further analysis showed that no trends were seen between NO x and either ignition delay or maximum cylinder temperature, and only weak, or fuel-specific, relationships were seen between NO x and maximum heat release rate and the timing of maximum heat release rate. The timing of the maximum cylinder temperature, however, did produce a relationship with NO x emissions that was not dependent on fuel type.

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