A comparative study on combustion and emission of ammonia/diesel dual direct injection engine in premixed and diffusion combustion modes

This study conducts a systematic experimental investigation on the combustion, performance, and emission characteristics of an ammonia/diesel dual direct injection engine, focusing on the effects of ammonia pre-injection ratio (APIR) and ammonia main injection timing (AMIT). Results demonstrate that ammonia diffusion combustion presents a stable dual peak heat release rate (HRR) profile, shortens combustion duration, enhances cycle stability, and significantly suppresses the generation of N2O emission. By contrast, premixed combustion shows a single high amplitude HRR peak but produces more unburned NH3, NOx and N2O emissions. With the increase of APIR, the peak HRR and in-cylinder temperature rise continuously, and brake thermal efficiency (BTE) as well as power output increase consistently. Properly retarding AMIT to −25° CA ATDC optimizes the combustion phase near top dead center (TDC), shortens ignition delay, and improves thermal efficiency. Diffusion combustion reduces unburned NH3 and N2O emissions by confining ammonia within the high temperature diesel flame envelope, but slightly increases CO emissions due to local fuel rich regions. The trade-off between thermal efficiency and pollutant emissions can be balanced by synergistically optimizing APIR and AMIT. This research provides theoretical support and engineering calibration fundamentals for the efficient and low emission operation of ammonia/diesel DDI engines in marine and heavy-duty transportation applications.