Performance characteristics of a compression-ignition engine using direct-injection ammonia–DME mixtures

Combustion and emissions characteristics of a compression-ignition engine using mixtures of ammonia and dimethyl ether (DME) are investigated. Ammonia can be regarded as a carbon-free fuel and is one of the world’s most synthesized chemicals. Its infrastructure is well established and it can be produced from renewable resources, making it an attractive energy carrier. In this study, a high-pressure mixing system is developed for blending liquid ammonia with DME, the latter serving to initiate combustion. The present engine uses a modified injection system without fuel return to prevent fuel mixture from vaporizing. Results using different mixtures of ammonia and DME show that ammonia causes longer ignition delays and limits the engine load conditions due to its high autoignition temperature and low combustion rate (e.g., flame speed). The inclusion of ammonia in the fuel mixture also decreases combustion pressure and temperature, resulting in higher CO and HC emissions. NOx emissions also increase due to the formation of fuel NOx when ammonia is used. However, improvements for the same operating conditions can be achieved by increasing the injection pressure. Exhaust ammonia emissions is on the order of hundreds of ppm under most of the conditions studied. Soot emissions are extremely low for all cases studied. Double injection schemes are also employed and their effects on the exhaust emissions vary with operating conditions.