Evaluating the Impact of Displacing Injected Gasoline with Aspirated Ammonia in a Direct Injection Spark Ignition Engine

Replacing fossil fuels with renewable ammonia could provide a crucial step towards the decarbonisation of transport sectors. However, many challenges remain in utilising ammonia within combustion systems: the volumetric energy density of ammonia is significantly lower than that of gasoline, exposure to ammonia (including ammonia slip) can be detrimental to human health, and the production of emissions, including unregulated emissions (such as N2O), from ammonia combustion can be catastrophic for the environment if not treated appropriately. Therefore, there is a need to determine the efficacy of ammonia as a fuel for internal combustion engines and the impact on the efficiency of energy release and the resulting exhaust emissions. A modern spark ignition engine was modified such that ammonia was aspirated through the engine intake air to incrementally displace engine gasoline and maintain a constant work output. It was found that displacing the fuel energy supplied by direct injected gasoline with premixed ammonia by 10% to 40% on an engine work performed basis decreased the peak heat release rate and delayed combustion. Spark timing was also advanced to up to 20 crank angle degrees before top dead centre for fuel blends incorporating up to 40% ammonia to allow for optimal conversion of chemical energy to useful work. The corresponding exhaust emissions analysis showed a linear decrease in CO2, however, an exponential decrease in CO as the proportion of ammonia increased. Additionally observed was an initial increase in unburnt hydrocarbons followed by a decrease as peak heat release rate decreased. However, a clear effect of ammonia level on NOx emissions was not apparent.