This paper presents the results of experimental tests with a spark-ignition engine powered using ammonia and DME (dimethyl ether). The tests were carried out on a CFR (cooperative fuel research) engine with a compression ratio of 10 and a rotational speed of 600 rpm. DME was used as a fuel to facilitate the initiation and then accelerate the combustion in the SI (spark-ignition) engine. It turned out that only about 10% of the energy share of DME ensures the correct combustion process. DME has a positive effect on the combustion stages, reduces the ignition delay time, and shortens the combustion duration. Thanks to this, for 18% of the energy share of NH₃, the highest engine efficiency (29.8%), the highest value of the average indicated pressure IMEP (712 kPa), and the minimum value of specific energy consumption (12.1 MJ/kWh) were obtained. Even the smallest DME content ensured the high repeatability of IMEP, below the permissible limit of 5%.