The Impact of Low Octane Primary Reference Fuel on HCCI Combustion Burn Rates: The Role of Thermal Stratification

A new experimental method was developed which isolated charge composition effects for wide levels of internal exhaust gas recirculation (iEGR) at constant total EGR (tEGR) for homogeneous charge compression ignition (HCCI) combustion. The effect of changing iEGR was examined for both gasoline (research octane number (RON) = 90.5) and PRF40 at constant charge composition at multiple engine speeds. For this study, the charge composition was defined as the total mass of fresh air, fuel, and tEGR. Experimental results showed that for a given iEGR level, PRF40 had a reduced burn duration and higher maximum heat release rate (HRR) when compared with gasoline. PRF40 was found to have a nearly constant burn duration and HRR for a given load and CA50, largely independent of engine speed and iEGR level. Gasoline, for equivalent conditions, showed an increased burn duration at higher iEGR levels. When comparing PRF40 to gasoline at fixed combustion phasing and iEGR level, the increased HRR for PRF40 was correlated with reduced intake valve closing (IVC) temperatures. To examine the impact of thermal gradients (as distinct from fuel chemistry effects) due to IVC temperature differences, a multizone “balloon model” was used to evaluate experimental conditions. The model results demonstrated that when the in-cylinder temperature profiles between fuels were matched by adjusting wall temperature, the heat release rates were nearly identical. This result suggested the observed differences in burn rates between gasoline and PRF40 were influenced to a large degree by differences in thermal stratification and to a lesser extent by differences in fuel chemistry.