Influence of the number of injections on piston heat rejection under low temperature combustion conditions in an optical compression-ignition engine

摘要

New combustion concepts are being investigated to develop cleaner engines. One of the most promising is partially premixed combustion. The mechanisms of this combustion mode and its impact on performance and emissions have been studied in the previous years. Nevertheless, little research has been done from the point of view of heat transfer. In particular, the influence of the injection strategy on heat transfer is of great interest in partially premixed combustion. This work presents a method to calculate convective heat transfer to the piston. The method uses a combination of gas velocity models and experimental velocity data measured with the PIV technique. This method was applied to achieve the goal of studying the effect of the number of injections on heat rejection. First, the influence of the injection strategy on gas motion was examined. To do that, an analysis of the velocity components relevant to gas-surface convection was conducted, as well as of the resulting heat transfer coefficient. Next, heat flux results were discussed. The single injection strategy showed the highest heat transfer, followed by triple injection and double injection. Important instantaneous variations of heat flux were observed at different locations of the piston bowl. All findings were associated with concurring conditions of high gas velocity, density and temperature.