Exploring Heat Release from Liquid Ammonia Injections Dual-Fueled with Heptane

Ammonia (NH3) has the potential to decarbonize combustion engines for deep-sea shipping as a noncarbonaceous fuel. The low reactivity of NH3 necessitates the use of a combustion-enhancing strategy. This study explores a dual-fuel approach using small heptane pilot injections to improve the ignition and combustion of liquid NH3 injections under typical compression ignition engine conditions. A novel constant volume combustion chamber with direct fuel injectors is used. Among other criteria, ignition delay time is defined and measured from heat release analysis. Spray interaction is controlled by various relative injection timings, relative injector rotation, and pilot umbrella spray angle. Seven- and Single-hole pilot injections are performed to reduce the consumption of pilot fuel, reaching ammonia energy shares of 45–96%. The fuel spray interaction, both spatially and temporally, is found to be crucial in controlling the heat release characteristics of NH3 combustion. To reduce pilot fuel consumption, it is essential to aim the combustion energy and hot combustion products at the locations of the NH3 spray. The results show that hot pilot combustion products should interact with (1) areas close to the nozzle, where strong NH3 evaporation cools the surrounding air, resulting in predominantly rich NH3/O2 mixtures that lack sufficient temperature to reach autoignition, or (2) regions with lean NH3 mixtures that fail to sustain a flame. In (1), small pilot injections are prone to extinction owing to the influence of ammonia evaporation. Because (2) requires continuous pilot interaction, it is recommended to avoid very lean zones in general and ignite NH3 in areas close to stoichiometry, thereby reducing the quenching potential near the chamber walls and crevices. The results were combined to propose an ideal multipoint injection strategy.