A new experimental method was developed which isolated charge composition effects for wide levels of internal EGR (iEGR) at constant total EGR (tEGR) for homogeneous charge compression ignition (HCCI) combustion. The effect of changing iEGR was examined for both gasoline (RON = 90.5) and PRF40 at constant charge composition. For this study, the charge composition was defined as the total mass of fresh air, fuel and tEGR.
From the experimental results, for a given iEGR level, PRF40 was found to have a reduced burn duration and higher maximum heat release rate (HRR) 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 increase in HRR was correlated with reduced intake valve closing (IVC) temperatures for the PRF40. To examine the impact of thermal gradients (due to IVC temperature differences) relative to fuel chemistry, a multi-zone ``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.