Study Of SI-CAI Hybrid Combustion In A Gasoline Engine

The spark ignition(SI) – controlled auto-ignition(CAI) hybrid combustion, as a special dual stage combustion mode, could effectively extend the operation range of CAI combustion and bridge the gap between tranditional SI and CAI combustion, which has become a promising gasoline combustion strategy for future practical application. In order to realize the full-load efficient gasoline combustion basing on the concept of SI-CAI hybrid combustion, the mechanism of the SI-CAI hybrid combustion and the effects of in-cylinder temperature, flow conditions and fuel/air equivalence ratio on controlling SI-CAI hybrid combustion process were studied using both experiments and simulations on a single cylinder CAI gasoline engine achieved by exhaust gas recompression strategy in this study. The relative studies covered the understanding of the hybrid combustion process, the effect of the engine control strategies and engine structure parameters.In order to simulate the SI-CAI hybrid combustion process accurately with high efficiency, a hybrid combustion model achieved by the ECFM3 Z and tabulated chemistry approach was developed and well validated by the corresponding experiments. The mechanism of the interacton between early flame propagation and auto-ignition was studied. In order to understand the effect of engine boundary conditions on SI-CAI hybrid combustion, the effect of coolant/wall temperature, intake temperature and intake valve parameters on the homogeneous stoichiometric SI-CAI hybrid combustion was studied using both experiments and simulations. The differeces of the in-cylinder thermal stratification caused by the coolant and intake temperature finally leads to higher sensitivity of the early combustion stage to intake temperature while higher sensitivity of later combustion stage to coolant/wall temperature. Overall, the intake temperature shows more significant effect on SI-CAI hybrid combustion compared to the coolant temperature. The macroscopic flow motions formed by the assymetric intake valve lifts show poor correlations with homogeneous stoichiometric SI-CAI hybrid combustion processes. The increase of the mean velocity in the central region and the average turbulent kinetic energy(TKE) of the whole cylinder would advance the mode transition from SI to CAI. On the contrary, the increase of the average temperature and the temperature inhomogeneity caused by the in-cylinder flow motion would inhibit the early flame propagation and in turn delay the mode transition timing.In order to enhance the controllability on SI-CAI hybrid combustion and extend the operating load range, a new combustion concept, named stratified flame ignition(SFI) combustion was developed. The effects of the spark timings, injection strategies, dilution strategies, piston shapes and compression ratios were investigated systematically. The shallow bowl pistons with low direct injection ratio render the direct injected fuel concentrated around the spark plug to enhance the control of the early stratified flame, while the leaner mixture formed by PFI at the peripheral region to limite the maximum pressure rise rate. Delaying the start of injection(SOI) timings and increasing the direct injection ratios could decrease the PRR of SFI combustion while also deteriorate the IMEP. In order to optimize the in-cylinder fuel/air equivalence ratio to enhance the early flame propagation, the effect of combined dilution stategies with both air and exhaust gas on the SFI combustion process were investigated. Compared to the baseline case achived by the homogeneious stoichiometric SI-CAI hybrid combustion with compression ratio of 10.66, the combined dilution strategy with direct injection ratio of 16% and compression raio of 14 could increase the IMEP of SFI combustion by 3.96% while reduce PRRmax by 43.88%, showing promising potential to elevate the combustion performance and extend load range.