Experimental study on the effect of pre-ignition heat release on GCI engine combustion

Abstract

Interactions between fuel and engine are basic issues for gasoline compression ignition (GCI) engine, and recent engine experiment results show that the gasoline fuel with low-octane number RON = 60–80 is optimal for GCI engines. However, gasoline fuel with low-octane ratings exhibits significant low-temperature combustion behavior, which will significantly affect the entire combustion process. In this study, different toluene primary reference fuels (TPRF) with identical research octane number (RON) but various octane sensitivities were employed. Optimized start of injection (SOI) was obtained based on indicated mean effective pressure (IMEP), combustion stability limits and engine emissions. Under the optimized SOI, the role of fuel sensitivity in combustion processes was investigated. With addressing low-temperature heat release (LTHR) before main ignition, the effect of intake conditions and fuel sensitivity on combustion phasing was clarified. The results indicate that the LTHR only occurs when the SOI is sufficiently advanced, and an earlier injection timing can achieve stable combustion, higher IMEP and lower NOX emission. More obvious LTHR promotes the advancement of combustion phasing, and the magnitude of LTHR is negatively correlated with intake temperature but positively correlated with intake pressure. The effect of increasing LTHR magnitude on combustion phasing is similar to that of increasing intake temperature. Finally, the effect of octane sensitivity on LTHR was also studied, which shows that the impact of LTHR between different sensitivity fuels is mainly reflected in combustion phasing, and the onset of LTHR is more advanced for low sensitivity fuels.