The Effect of Fuel Properties on Low and High Temperature Heat Release and Resulting Performance of an HCCI Engine

A supercharged 4-cylinder engine was introduced to evaluate how fuel properties affect engine combustion and performance in homogeneous charge compression ignition (HCCI) operation. In this study, choosing from 12 hydrocarbon constituents, model fuels were mixed to have the same distillation but different octane numbers (RON=70, 80, 92). For each fuel, RON distribution against distillation is same to keep the same octane number in cylinder vapor during the air-fuel compression process. To confirm the appropriateness of model fuels and test procedures, regular gasoline (RON=90) was also included. From the combustion analysis it was clear that the low temperature heat release depends on fuel characteristics. RON92 fuel has a small low temperature heat release, and a high temperature heat release combusts slowly. RON92 fuel is considered to be a good anti-knocking fuel for HCCI, but at higher engine speeds, the high temperature heat release is excessively late resulting in frequent misfire. Although the higher octane number can yield better thermal efficiency, the engine could not be run at high speed. This trend was also evident with regular gasoline. Additionally, the start of low temperature heat release also depends on fuel characteristics. Both the phasing of the low temperature heat release and the amount released greatly influence the phasing of the high temperature heat release and the resulting engine performance. The effect of fuel chemical components on low temperature heat release was also investigated. To simplify the engine tests, three fuels, a base blend, a mixture of base blend+6.5% of n-heptane and a mixture of base blend+6.5% of toluene, were used. The heating value and the start crank angle of the low temperature heat release differ for these three fuels. Adding only 6.5% of either n-heptane or toluene against base blend changes the HCCI heat release and HCCI engine performances. Fuel components must be one of the most important factors for HCCI combustion.