The effect of diluent composition on homogeneous charge compression ignition auto-ignition and combustion duration

In this work, the effect of diluent composition is studied on the ignition timing and combustion duration of homogeneous charge compression ignition (HCCI) combustion. Full-cycle 3-D computational fluid dynamics (CFD) simulations were performed using two different dilution schemes, one which employs air dilution and the other which employs external exhaust gas recirculation (eEGR) dilution. Both cases used a fixed breathing strategy, and had the same fueling rate and engine speed typical of automotive applications. A premixed fuel-air mixture was used in the intake charge to avoid the fuel stratification effects associated with direct injection, while the valve events were selected to minimize the internal residual and its associated thermal and compositional stratification. With ignition timing held constant near top dead center (TDC) for both methods, there is a 10% increase in the combustion duration going from air to eEGR dilution. While the thermal stratification prior to ignition is similar for both methods, the eEGR-dilute case has an overall higher temperature throughout the charge. A reactivity stratification analysis, based on the distribution of ignition delays within the charge prior to ignition, showed nearly identical initial reactivity for the two methods, with the higher temperatures of the eEGR case compensating for this method's lower oxygen content. A quasi-dimensional multi-zone model was subsequently used to decouple the chemical kinetic and thermodynamic effects of eEGR on the combustion process. This analysis showed that the primary factor contributing to the longer combustion duration for the eEGR-dilute case is the lower ratio of specific heats (γ) in the unburned charge post-ignition which is thought to lower the rate of compression-induced heating of the unburned charge by the ignited and burning regions, leading to slower sequential auto-ignition.