Evaluation of the Effects of Diesel Fuel Injection Timing in a High-Speed RCCI Engine at Low to High Load Conditions

Abstract

This numerical research mainly aimed to investigate the impacts of diesel fuel direct injection timing on the combustion characteristics, emission formation, and performance in a high-speed diesel-gasoline Reactivity Controlled Compression Ignition (RCCI) engine under low, medium, and high load operating conditions. The numerical achievements indicated that by late diesel injection timing (32 Crank Angle (CA) Before Top Dead Center (BTDC)), regions with a higher temperature and equivalence ratio were formed in the combustion chamber and caused a simultaneous increase in both Nitrogen Oxides (NOx) and soot emissions. On the contrary, early Diesel Injection Timing (DIT) under low load conditions concurrently reduced NOx and soot due to appropriate air-fuel ratio, more homogeneous air-fuel mixture formation as a result of longer Ignition Delay (ID) period, and the absence of high-temperature regions inside the combustion chamber. Also, when DIT was postponed under high load conditions, the combustion process became unstable and noise emission, as well as detonation tendency (sudden auto-ignition), were dramatically increased due to a considerable increase in in-cylinder maximum temperature and pressure rise rate. Furthermore, under low load conditions, because of low flame temperature and its incompetent propagation in the engine cylinder, a substantial amount of unburnt mixture (gasoline) was formed and accumulated at the center of the combustion chamber.