A new experimental method was developed which isolated charge composition effects for wide levels of internal EGR (iEGR) at constant total EGR (tEGR) for negative valve overlap (NVO) homogeneous charge compression ignition (HCCI) combustion. Using this method, the effect of changing iEGR was examined for both research grade gasoline (RON = 90.5) and PRF40 across multiple engine speeds and at constant charge composition. For this study, the charge composition was defined as the total mass of fresh air, fuel and tEGR. Comparison also was made between the two fuels at a fixed iEGR level to isolate, independent of compositional effects, the effect of a low octane fuel on HCCI burn rates.
From the experimental results, for all engine speeds, 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 speed, combustion phasing and iEGR level, the increase in HRR was found to coincide with reduced intake valve closing (IVC) temperatures necessary to maintain constant combustion phasing for the PRF40. The reduced IVC temperature for PRF40 reduced the thermal stratification in-cylinder compared with gasoline and may have been the cause of this change. To examine the impact of thermal gradients relative to fuel chemistry, a multi-zone “balloon model” was used to evaluate experimental conditions. The model used a reduced chemical kinetic mechanism for PRFs with PRF87 representing gasoline. The results of the model demonstrated that when the in-cylinder temperature profiles between PRF40 and PRF87 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.