Effects of Cetane Number, Aromatic Content and 90% Distillation Temperature on HCCI Combustion of Diesel Fuels

The effects of cetane number, aromatics content and 90% distillation temperature (T90) on HCCI combustion were investigated using a fuel matrix designed by the Fuels for Advanced Combustion Engines (FACE) Working Group of the Coordinating Research Council (CRC). The experiments were conducted in a single-cylinder, variable compression ratio, Cooperative Fuel Research (CFR) engine. The fuels were atomized and partially vaporized in the intake manifold. The engine was operated at a relative air/fuel ratio of 1.2, 60% exhaust gas recirculation (EGR) and 900 rpm. The compression ratio was varied over the range of 9:1 to 15:1 to optimize the combustion phasing for each fuel, keeping other operating parameters constant.

The results show that cetane number and T90 distillation temperature significantly affected the combustion phasing. Cetane number was clearly found to have the strongest effect. An increase in cetane number or a decrease in the T90 distillation temperature advanced the combustion phasing. The cetane number effect is related to increased low temperature heat release (LTHR) with increasing cetane number. The T90 effect is primarily due to a change in the physical delay period associated with preparation of the fuel-air mixture. At a similar combustion phasing, the high CN fuels exhibited significantly longer combustion duration than the low CN fuels. The best fuel conversion efficiencies were generally achieved with the four low cetane number fuels (FACE No. 1-4) as optimized combustion phasing occurred at a higher compression ratio. However, the two fuels with high CN and low T90 (FACE No. 5 and 7) also produced high fuel conversion efficiencies, although at a slightly lower compression ratio. These two fuels (FACE No. 5 and 7) produced the lowest indicated specific HC (isHC) emissions. This is due to the combined effects of high fuel volatility and reactivity that resulted in a more efficient combustion process. FACE No. 8, which has high CN and high T90, produced notably higher isHC emissions when the combustion phasing was in the range of 0° to 10°CA, aTDC. FACE No. 6 and 8, which have high CN and high T90, produced higher isCO emissions when the combustion phasing was retarded. The isNO_x emissions were extremely low (below 0.008 g/kWh) for all fuels. Aromatic content did not directly affect the combustion phasing or emissions behavior.