A Study of HCCI Combustion Using a Two-Stroke Gasoline Engine with a High Compression Ratio

Abstract

In this study, it was shown that Homogeneous Charge Compression Ignition (HCCI) combustion in a 4-stroke engine, operating under the conditions of a high compression ratio, wide open throttle (WOT) and a lean mixture, could be simulated by raising the compression ratio of a 2-stroke engine. On that basis, a comparison was then made with the characteristics of Active Thermo-Atmosphere Combustion (ATAC), the HCCI process that is usually accomplished in 2-stroke engines under the conditions of a low compression ratio, partial throttle and a large quantity of residual gas. One major difference observed between HCCI combustion and ATAC was their different degrees of susceptibility to the occurrence of cool flames, which was attributed to differences in the residual gas state. It was revealed that the ignition characteristics of these two combustion processes differed greatly in relation to the fuel octane number. Specifically, a correlation was observed between the octane number and ignition timing in HCCI combustion that took place under a low level of residual gas, but no such relationship was seen for ATAC. External exhaust gas recirculation (EGR) and internal EGR were then separately applied to HCCI combustion conditions (a high compression ratio and WOT). It was found that the correlation between the fuel octane number and ignition timing diminished as the internal EGR rate was increased, with the ignition characteristics coming to resemble those of ATAC. When external EGR was applied, the ignition characteristics of HCCI combustion were maintained and a correlation was observed between the fuel octane number and ignition timing.