Experimental investigation on in-cylinder working process and thermal efficiency of ammonia/diesel low-carbon engine with pre-injection strategy

Ammonia, as a zero carbon and sustainable fuel, combined with traditional diesel fuel can provide a promising solution for reducing the engine environmental impact. However, ammonia/diesel dual-fuel (ADDF) engine has the significant drawback of low combustion efficiency and poor combustion stability at low loads, which seriously hinders its development. Therefore, this study explored the potential of pre-injection strategy in improving the performance of ADDF engine, comprehensively evaluating the effects of four pre-injection parameters (main injection timing (MIT), pre-injection timing (PIT), pre-injection quantity (PIQ), and diesel injection pressure (DIT)) on in-cylinder combustion process, incomplete combustion emissions, and energy efficiency. The results indicate that MIT has the most significant impact on combustion process. Advancing MIT can significantly increase in-cylinder combustion pressure and reduce CO, HC and unburned NH3 emissions. At MIT = -12 °CA ATDC, the highest brake thermal efficiency (BTE) of 32.3 % is obtained, while simultaneously generating high combustion noise. The advance of PIT has adverse effects on combustion stability. When PIT is advanced from −24 °CA ATDC to −36 °CA ATDC, COVIMEP increases from 2.61 % to 4.44 %, unburned NH3 emission increases by 23.5 %, and the BTE of ADDF engine decreases. Adjusting the PIQ and DIP can improve the reactivity distribution of fuel and reduce incomplete combustion emissions. However, the decrease in main injection quantity would increase the cycle fluctuation. At low loads, shortening the pre-main injection interval and increasing PIQ and DIP can enable the engine to achieve higher BTE while reducing pollutant emissions. This study elucidates in detail the influence of four pre-injection parameters on the operation characteristics of ADDF engine. It can offer insights and practical references for expanding the operating range of ammonia fuel engine