Thermal and chemical effects of the in-cylinder charge at IVC on cycle-to-cycle variations of DME HCCI combustion with combustion-phasing retard by external and rebreathed EGR

Combustion-phasing retard is an effective method to extend the high-load limit of homogeneous charge compression ignition (HCCI) engine, but it leads to unacceptable cycle-to-cycle variation. This study investigates the factors that lead to the increase of the cycle-to-cycle variations of HCCI combustion with combustion-phasing retard using dimethyl ether (DME). The experiment was conducted in a single-cylinder engine equipped with an exhaust gas recirculation (EGR) and a two-stage exhaust cam. The experimental results show that an increase of external EGR mass is accompanied with a combustion-phasing retard. The maximum pressure-rise rate decreases with combustion-phasing retard, but the cycle-to-cycle variations of HCCI combustion becomes unacceptable. For unstable operation, a partial-burn cycle is often followed by another partial-burn cycle, however well-burn cycle occurs ever for low in-cylinder charge temperature. This is assumed that the chemical effects of the residuals have to be responsible. To clarify the chemical effects of the residuals, the computation was performed using CHEMKIN with a detailed chemical-kinetic mechanism for DME. The computational results show that adding the residuals to the in-cylinder charge enhances the autoignition sufficiently to advance the combustion phasing in proportion to the amount of the residuals, and this contributes as well to the increase of IMEP.