Effect of injection strategy on smoothness, emissions and soot characteristics of PCCI-conventional diesel mode transition

Abstract

The mode transition from partially premixed compression ignition (PCCI) mode to conventional diesel combustion mode was studied in a single-cylinder heavy-duty diesel engine. The study focuses on fuel injection strategies during transitional cycles, to minimize the instabilities in the indicated mean effective pressure (IMEP) and emissions. The soot produced during the mode transition was collected using a fast sampling device, and its nano-structure was analyzed, to provide insight into its oxidative reactivity. During the mode transition from PCCI to conventional mode, the IMEP of some cycles is seen to exceed that of steady PCCI and steady conventional mode, due to the momentary combination of hot residual gases and transitional level of exhaust gas recirculation (EGR) inside the cylinder. The soot produced during the mode transition was also found to be more graphitized than that of PCCI and conventional mode, which indicates a lower oxidative reactivity. The systematic optimization of the injection strategy shows that the injection timing and the quantity should be preferably shifted to their final values in a gradual manner during earlier transitional cycles when the EGR level inside the cylinder is still high. The injection strategy that is finally achieved could reduce the coefficient of variation (COV) of the IMEP from 23% to 5.8% during the mode transition, which is deemed to be acceptable for diesel engine operation. Furthermore, unburned hydrocarbon emissions were reduced by 40% and the level of soot graphitization decreased, when the proper injection strategy was applied.