Numerical Investigation of a Heavy Duty Compression Ignition Engine Converted to Ammonia Spark Ignition Operation

The need for global decarbonization in power generation requires the increased use of zero-carbon fuels. Compared to hydrogen, ammonia is easier to store, transport, and produce. In addition, the products of complete combustion of ammonia are water and nitrogen. As a result, ammonia is an ideal green fuel for generating power in internal combustion engines. The drawbacks in power generation relate to the high energy needed to ignite the combustible mixture and the low laminar flame speed of ammonia compared to conventional fuels. The three-dimensional numerical study presented here investigated the potential of converting existing diesel engines to ammonia spark ignition operation. The results indicated that the use of ammonia in a diesel-like combustion chamber produced a slower kernel inception process. But the speed of the fully developed turbulent flame was large enough to rapidly complete the bulk combustion process despite the lower laminar flame speed inside the ammonia-air mixture, and this was in the absence of any combustion promoters such as the addition of hydrogen. The problem with pure ammonia operation was the reduced combustion efficiency and the high level of unburned ammonia emissions, since the slow spark inception process can be compensated by a larger compression ratio. The results also suggested that emissions formation and subsequent oxidation were a more complex phenomena. For example, lean ammonia combustion produced higher nitrogen oxides (NO_x) concentrations due to the nitrogen in the fuel, despite the lower combustion temperature. Moreover, advancing spark timing reduced the NO_x emissions, which was opposite to the traditional spark ignition engines. Additionally, the ammonia in engine crevices that escaped the late oxidation process was the main source of nitrous oxide (N₂O) species in the exhaust gas that usually do not appear in traditional engines. Overall, all these results suggested that more fundamental research on ammonia combustion is needed to understand its use in efficient, decarbonized engines.