Comparison of DME HCCI operating ranges for the thermal stratification and fuel stratification based on a multi-zone model

This work investigates the potential of in-cylinder thermal stratification and fuel stratification for extending the operating ranges in homogeneous charge compression ignition (HCCI) engines. The computations were conducted using both the single-zone model and the multi-zone model of the Senkin application of the CHEMKIN-II kinetics rate code, and kinetic mechanisms for dimethyl-ether (DME). The computational study was performed at three cases of charge stratification (Homogeneous, Thermal stratification, Fuel stratification) and included temperature sweeps (T = 330 ∼ 470K in incremental 10K) as a function of fueling sweeps (ϕ=0.1∼0.4 in incremental ϕ=0.05). This study shows that the potential for extending the high-load operating limit by adjusting the thermal stratification is very large. With appropriate stratification, even the knock cases can be combusted with low pressure-rise rates. It was also found that the thermal stratification plays a major role in producing the benefit of combustion-timing retard for slowing the combustion rate. Reduced chemical-kinetic rates with combustion retard are found to play a lesser role. It was found that fuel stratification offers good potential to achieve a staged combustion event with reduced pressure-rise rates. Therefore, fuel stratification has the potential to increase the highload limits. It was also shown to have good potential for improving combustion efficiency at low loads.