Understanding Fuel Stratification Effects on Partially Premixed Compression Ignition (PPCI) Combustion and Emissions Behaviors

Fuel stratification effects on the combustion and emissions behaviors for partially premixed compression ignition (PPCI) combustion of a high reactivity gasoline (research octane number of 80) was investigated using the third generation Gasoline Direct-Injection Compression Ignition (Gen3 GDCI) multi-cylinder engine. The PPCI combustion mode was achieved through a double injection strategy. The extent of in-cylinder fuel stratification was tailored by varying the start of second fuel injection timing (SOI_second) while the first fuel injection event was held constant and occurred during the intake stroke. Based on the experimental results, three combustion characteristic zones were identified in terms of the SOI_second - CA50 (crank angle at 50% cumulative heat release) relationship: (I) no response zone (HCCI-like combustion); (II) negative CA50 slope zone: (early PPCI mode); and (III) positive CA50 slope zone (late PPCI mode). Across the three zones, Zone II produced the best overall performance. In addition, a wider spray inclusion angle (130° vs. 100°) was found to allow for more retarded SOI_second, thereby resulting in stronger fuel stratification and enhanced control authority over CA50. Finally, closed-cycle combustion CFD analysis was performed to gain insight into the in-cylinder fuel-air mixing and combustion process in all three combustion zones. CFD analysis showed that a 130° spray inclusion angle led to improved fuel efficiency and emissions. It was also found that 350 bar fuel rail pressure helped reduce overmixing of the fuel, resulting in better ignitability of the gasoline and lower soot generated in the combustion chamber.