Optical study of throttleless and EGR-controlled stoichiometric dual-fuel compression ignition combustion

This paper investigates using EGR instead of throttle to control the load of a stoichiometric dual-fuel dieseline (diesel and gasoline) compression ignition (SDCI) engine for reducing fuel consumption and emissions in future internal combustion engines. To investigate the combustion process in the SDCI mode, the flame development in a single-cylinder optical engine with a dual injection (PFI+DI system) has been recorded by high-speed imaging. The images are processed and analyzed, and the premixed blue flame is identified according to their RGB magnitudes. The effects of EGR and load conditions, injection pressure, injection timing, and split injection strategy on SDCI combustion are studied. An earlier injection strategy is found to be ideal for soot reduction; however, the ignition–injection decoupling problem results in difficulties in combustion control. A split injection strategy offers a flexible solution as simultaneous optimization for control of pressure rise rate, reduction of soot and increase of thermal efficiency. The premixed blue flame propagation speeds are correlated to the heat release data, and it is found that the flame propagation speeds are significantly higher than those in conventional spark ignition (SI) engines even with an EGR ratio higher than 40%.