Effect of degree of unmixedness on HCCI combustion based on experiment and numerical analysis

The purpose of this study was to gain a better understanding of the effects of in-cylinder gas temperature stratification on reducing the pressure-rise rate in HCCI combustion. HCCI combustion was investigated using an optically accessible engine and direct visualization of the combustion chemiluminescence. The engine was fueled with Di-Methyl Ether. Computational work was conducted on the gas compression and expansion strokes in HCCI engine with simple 0-dimensinal multi-zones model. When fuel inhomogeneous charging in experiment, maximum heat release rate decreased. Combustion duration got longer. Maximum pressure-rise rate decreased. Chemiluminescence, of which transition was identified from the side of intake valve to the side of exhaust valve, was observed. It is need for total moderate heat release to get local moderate combustion with not overall but continuous combustion in chamber. Computational result showed that difference of amount of heat release during LTR made temperature difference before HTR start and HTR start time lag. Initial fueling rate difference φ<inf>DME</inf> =0.3 was equivalent to difference of HTR start timing Δt=4.5ms. In more appreciate condition for reducing pressure-rise rate with 5-zones model, the difference φ<inf>DME</inf>=0.2-0.6 (total Δφ<inf>DME</inf>=0.4) and volume ratio is changed as higher fueling rate zone decreases volume. This condition got pressure-rise rate 3.1MPa/ms comparing homogeneous condition 6.0MPa/ms. Moreover same effect for reducing pressure-rise rate with only initial temperature difference was investigated. Result showed that initial temperature difference ΔT =20K is equivalent to fueling rate difference Δφ<inf>DME</inf>=0.4 for reducing maximum pressure-rise rate. It was found that degree of unmixidness about temperature difference plays a major role for reducing pressure-rise rate. And using large heat release during Low Temperature Reaction like Di-Methyl Ether, same effect of temperature difference was gotten with effect of fueling rate difference.