Partially premixed combustion (PPC) is used to meet the increasing demands of emission legislation and to improve fuel efficiency. With gasoline fuels, PPC has the advantage of a longer premixed duration of the fuel/air mixture, which prevents soot formation. In addition, the overall combustion stability can be increased with a longer ignition delay, providing proper fuel injection strategies.
In this work, the effects of multiple injections on the generation of in-cylinder turbulence at a single swirl ratio are investigated. High-speed particle image velocimetry (PIV) is conducted in an optical direct-injection (DI) engine to obtain the turbulence structure during fired conditions. Primary reference fuel (PRF) 70 (30% n-heptane and 70% iso-octane) is used as the PPC fuel. In order to maintain the in-cylinder flow as similarly as possible to the flow that would exist in a production engine, the quartz piston retains a realistic bowl geometry. The distortion caused by the complex shape of the optical piston is corrected by an advanced image-dewarping algorithm. The in-cylinder charge motion is evaluated and investigated over a range of crank angles in the compression and expansion strokes in order to understand the turbulence level, especially the late-cycle turbulence.
The results show the spatial and temporal development of the flow-field structures in the piston bowl. The PIV data, obtained in the vertical plan, are used to calculate the ensemble average velocity turbulent kinetic energy (TKE), cycle-resolved turbulence, and mean velocity of the instantaneous fluid motion.