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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.

The impact of a branched and unsaturated compound (diisobutylene) mixed with simple hydrocarbons such as nheptane and isooctane in binary blends on the autoignition behavior were investigated in a modified cooperative fuel research (CFR) engine at an equivlanece ratio of 0.5 and intake temperature of 120°C. From this test condition, a homogeneous charge of fuel and intake air can be achieved. The test fuels were prepared by addition of 5−20 vol % diisobutylene into n-heptane and isooctane. The engine compression ratio (CR) was gradually increased from the lowest point to the point where significant high temperature heat release (HTHR) was observed, and this point is also referred to as the critical compression ratio (CCR). Heat release analysis showed that each n-heptane blend had a noticeable low temperature heat release (LTHR), which was not observed in the isooctane blends. The gradual addition of diisobutylene into each primary reference fuel contributed to retarded high temperature heat release in these binary blends, increasing the in-cylinder temperature and decreasing formation of CO. The 15 and 20 vol % blends of diisobutylene in isooctane were not able to reach high temperature heat release in the CFR engine system under these test conditions. The fundamental ignition behavior such as CCR and calculated % LTHR show the impact of the presence of the C−C double bond on ignition reactivity. Species concentration profiles obtained in condensed products from the engine exhaust were measured via gas chromatrography−mass spectrometry and −flame ionization detector. The major intermediate species for each blend were captured at a compression ratio selected just before the high temperature heat release was observed. Most intermediate species were derived from n-heptane and isooctane, while diisobutylene rarely participated in forming any major species, with the exception of the formation of 4,4-dimethyl-2-pentanone. Addition of diisobutylene exhibited opposite trends with regard to the abundance of intermediate species when blended with n-heptane versus with isooctane, due to the different degree of oxidation reactivity of n-heptane in comparison to isooctane.

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