The effect of fuel properties on HCCI engine combustion characteristics and performances

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 research octane numbers (RON=70, 80, 92). For each fuel, RON distribution against distillation is controlled to maintain the same in-cylinder history of octane number in the vapor phase during the air fuel compression process. To confirm the appropriateness of model fuels and test procedures, regular gasoline((w)=winter blend) (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 feel 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(w). Comparing the HCCI engine test results of regular gasoline((s) =summer blend) and a mixture of only paraffins yielding the same octane number, the heat release patterns differ This means that the fuel has a possibility to improve engine performance, and the distillation, vaporization characteristics and fuel components also affect HCCI combustion. The effect of fuel chemical components on low and high temperature heat release was investigated. To simplify the engine tests, 6 fuels, a base blend, a mixture of base blend+6.5% of n-hexane (n-paraffin), a mixture of base blend+6.5% of 2methylpentane (iso-paraffin), a mixture of base blend+6.5% of 4methyl-1pentene (olefin), a mixture of base blend+6.5% of cyclohexane (naphthene), and a mixture of base blend+6.5% of toluene (aromatic) here used. N-hexane, 2methylpentane, 4methyl-1pentene, and cyclohexane are all C-6 molecules but have different chemical structures. Toluene was used instead of benzene. The test results show that paraffin. blended fuels have a large low temperature heat release and the high temperature heat releases start earlier than olefin, naphthene, and aromatic blend fuels. Adding only 6.5% of chemical blend substantially changes the HCCI heat release and HCCI engine performance. PRF90.5, a fuel mixture consisting only of paraffins (iso-octane and n-heptane),exhibits earlier low temperature and high temperature heat releases. Fuel components must be one of the most important factors for HM combustion.