Influence of fuel properties on knock under lean combustion conditions

Internal combustion engines today are required to provide even higher levels of thermal efficiency. Raising the compression ratio and applying lean combustion are effective measures for increasing the thermal efficiency of internal combustion engines. However, raising the compression ratio gives rise to knock accompanied by pressure oscillations in the combustion chamber, thereby hindering the attainment of higher thermal efficiency. In short, it is necessary to have a technique for suppressing knock under engine operating conditions of a high compression ratio and lean combustion. The reaction characteristics of the fuel have a large effect on the occurrence of knock. Yet, the relationship between fuel properties and knock has still not been made entirely clear under conditions of a high compression ratio and lean combustion. Therefore, in this study a test engine was used to examine autoignition characteristics, pressure waveforms and other combustion characteristics under a condition of a high compression ratio, focusing in particular on fuel properties. As a result, it was found that, in addition to the fuel octane number, a fuel with a long ignition delay time in the high-temperature region is effective in suppressing knock accompanied by pressure oscillations.