Effect of the thermal stratification on SI–CAI hybrid combustion in a gasoline engine

In this research, experiments and CFD simulations were carried out to investigate the effect of thermal stratification on the SI–CAI hybrid combustion characteristics. Experimental results indicated that increasing both coolant temperature and intake temperature would enlarge the peak heat release rate and shorten the combustion duration, while the control capability offered by changing intake temperature was more notable. The relative sensitivity of changes among combustion characteristics (R) showed that the intake temperature has greater effect on the initial combustion whilst the coolant temperature has more significant effect on later combustion stage. Simulation results revealed that the heat transfer and residual gas distribution codetermine the thermal stratification pattern. The wall temperature has a significant effect on the end-gas auto-ignition process while showing little effect on the early flame propagation. In comparison, the increase of intake temperature would significantly enhance the early flame propagation and in turn advances the auto-ignition. The early auto-ignition (before MFB 65%) is significantly sensitive to the intake temperature whilst the sensitivity apparently decreases as the combustion proceeds because of the strong control capability of the wall temperature on the thermal stratification of the end-gas, which demonstrates the lower δ ¯ CA 90 for the intake temperature cases observed in the experiments.