Experimental and simulation investigation of n-heptane/ammonia dual fuel on a light-duty compression ignition engine

Ammonia is a carbon-free, promising fuel for hydrogen carriers. It is a liquid fuel under low-pressure conditions (8.62 bars) and can be easily stored. Many researchers have tried to apply it in engines because there are no carbon dioxide emissions when combusting it. However, ammonia is a low cetane number fuel and is difficult to ignite. It can be used in a dual fuel mode in compression ignition (CI) engines, i.e., a fuel such as diesel, n-heptane, dimethyl ether, hydrogen etc., is used as a pilot fuel to ignite the ammonia. In this work, n-heptane was used as the pilot fuel and ammonia was used as the primary fuel in a light-duty CI engine. The engine's heat release rate (HRR), cylinder pressure, ignition delay and indicated efficiency were investigated experimentally at 80 %, 89 %, 95 % and 98 % ammonia fuel energy proportion. In order to investigate the ignition and combustion characteristics of the dual fuel engine, a multi-zone combustion model for n-heptane and a flame propagation model for the ammonia fuel were developed. In this work, it is found that the combustion process of n-heptane/ammonia can be simulated in most cases by a multi-zone evaporative combustion model combined with a flame propagation model. This work also found that the ignition performance of n-heptane is more affected by the long chemical ignition delay. Eventually, it was found that the more minor the discrepancy between the simulated and experimental ignition delays, the more insignificant the discrepancy between the simulated heat release rates and cylinder pressures and the experimental ones. The HRR, cylinder pressure, physical and chemical ignition delays and indicated efficiency of the engine were then simulated. The simulation showed good agreement with experimental results for the 89 %, 95 % and 98 % ammonia fuel energy proportion cases. The simulated physical ignition delay was 1.45–2.37 CA deg, the chemical ignition delay was 7.21–8.88 CA deg, and the indicated efficiency was 39.84%–42.80 % (for the three cases with 89 %, 95 % and 98 % ammonia fuel energy proportions). A detailed analysis of each parameter was conducted.