Global decarbonization requires the increased use of zero-carbon fuels. Compared to hydrogen, ammonia is easier to store, transport, and produce. In addition, products of complete combustion of ammonia are water and nitrogen. Therefore, ammonia is an ideal green fuel for internal combustion engines. Drawbacks relate to the high ignition energy and low laminar flame speed of ammonia. This three-dimensional numerical study investigated the potential of converting existing diesel engines to ammonia spark ignition operation. Results indicated a slower kernel inception process, but the speed of the fully developed turbulent flame was enough to complete the bulk combustion process despite the lower laminar flame speed. 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 phenomenon. For example, lean ammonia combustion produced higher nitrogen oxides (NOX) concentrations due to the nitrogen in the fuel, despite the lower combustion temperature. Moreover, advancing spark timing reduced the NOX 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.