Utilizing boost and double injections for enhanced stratified-charge direct-injection spark-ignition engine operation with gasoline and E30 fuels

Abstract

This study systematically explores the effects of injection strategy and exhaust-gas recirculation on boosted stratified-charge operation of a direct-injection spark-ignition engine using gasoline and E30 fuels at absolute intake pressures in the range of 100–160 kPa. Nitrogen dilution is used to lower the intake mole fraction of oxygen to simulate the dilution effects of exhaust-gas recirculation. The engine is operated at 1000 r/min with an gross indicated mean effective pressure range of 380–680 kPa. In summary, significant reductions of engine-out NO_x and soot emissions can be achieved for stratified operation while still maintaining stable combustion (~2% gross indicated mean effective pressure variability) and high thermal efficiency (~40%). Adding exhaust-gas recirculation significantly suppresses NO_x emissions, not only by reducing flame temperatures but also by slowing down the combustion which effectively retards the combustion phasing. A well-designed closely spaced double-injection strategy is key to combustion stabilization and plays an important role to suppress soot emissions. Intake boost allows reaching higher loads with further reduced soot emissions. High-speed dual-camera flame imaging reveals key features of flame propagation for stratified operation with low NO_x emissions at high exhaust-gas recirculation levels. Heat release rate–based conditional analysis of flame images shows that compared to single injection, the use of double injections creates a spark plasma that is more stretched out, correlating with a more stable early combustion. It also demonstrates that double injections produce more symmetric flame propagation with smaller soot-containing regions, indicating more favorable fuel distributions. Stratified operation with gasoline and E30 fuels demonstrates similar engine performance and emission levels for double injections. These observations suggest that E0 (gasoline)–E30 fuel blends can be compatible with highly efficient stratified-charge spark-ignited operation. However, for a 50/50 double-injection strategy, E30 showed elevated smoke emissions for operation without intake boost, indicating that certain operating strategies can be adversely affected by the ethanol content of E30 fuel blends.