The KIVA-CHEMKIN code and a reduced PRF mechanism were used to find optimized fuel blends and EGR combinations for HCCI operation at several engine loads. The study found that a minimum fuel consumption could not be achieved using either neat diesel fuel or neat gasoline alone, and that the optimal fuel reactivity required decreased with increasing load [13,14]. Fig. 8 shows the simulation results at 6 bar IMEP, and it is seen that when PRF 70 was used, the optimal combustion phasing could be achieved with no EGR. Thus, the reduced fuel reactivity could significantly reduce the burden on the air handling system. Furthermore, the ability to operate at low EGR rates may be beneficial for combustion phasing control during transient LTC operation. The optimal CA50 com- bustion phasing for 6 bar IMEP operation was found to be between 4 and 6o aTDC. Similarly, Fig. 9 shows simulation results at 11 bar
IMEP that indicates that low ISFC can be achieved using PRF blends ranging from 60 to 90, depending on the EGR level. The minimum ISFC coincided with a CA50 of ~6o aTDC. Furthermore, as load was increased, operation had to be shifted to higher EGR levels in order to reduce the rate of heat release and to meet the PPRR constraint. It was observed that, even with combustion phasing after TDC, the PPRR was excessive (~20e30 bar/deg) due to the volumetric heat release, and it was inferred that stratification may be useful to control the rate of heat release.