Review of Technical Developments in Direct Injection of Ammonia for Engines With Assessment of Spray, Combustion, and Emission Characteristics

The global transition from fossil fuels to carbon‐free energy has accelerated interest in renewable fuels such as hydrogen and ammonia (NH 3 ). Ammonia has emerged as a promising carbon‐free energy carrier because it can be liquefied at approximately 1 MPa under ambient conditions, has a high‐octane number of about 110, and contains 17.6 wt% hydrogen, making it attractive for large‐scale energy storage and transportation. This article focuses on earlier research and recent studies on technical developments in direct injection (DI) of NH 3 ‐assisted combustion. Focus on the combustion characteristics of NH 3 DI in ICEs, as well as the spray characteristics, safety, and handling of NH 3 . This review paper develops the fundamentals of the mechanism of NH 3 combustion technology, providing new insights into the application of NH 3 in ICEs. Although NH 3 possesses favorable storage characteristics, its application in ICEs is challenged by a high autoignition temperature ( ∼ 650 °C), low laminar flame speed ( ∼ 7 cm/s), and lower heating value (18.8 MJ/kg), which contribute to combustion instability, NO x formation, and NH 3 slip. The review also explains the fundamental combustion mechanisms of ammonia and identifies key research gaps and future directions for developing efficient, low‐emission ammonia‐fueled internal combustion engines, supporting the transition towards sustainable and carbon‐free transportation.