This work analyzes NO x reduction in a marine diesel engine using ammonia injection directly into the cylinder and compares this procedure with water injection. A numerical model based on the so-called inert species method was applied. It was verified that ammonia injection can provide almost 80% NO x reduction for the conditions analyzed. Furthermore, it was found that the effectiveness of the chemical effect using ammonia is extremely dependent on the injection timing. The optimum NO x reduction was obtained when ammonia is injected during the expansion stroke, while the optimum injection timing using water is near top dead center. Chemical, thermal, and dilution effects of both ammonia and water injection were compared. The chemical effect was dominant in the case of ammonia injection. On the other hand, water injection reduces NO x through dilution and, more significantly, through a thermal effect.that the main factors that influence NO x formation are the temperatures reached in the combustion process and the amount of time in which the combustion gases remain at high temperatures [5][6][7]. Based on this, primary measures focus on addressing these factors and reducing the concentrations of oxygen and nitrogen [8,9]. Well-known primary measures are exhaust gas recirculation (EGR), Miller timing, common rail, modification of injection and other parameters of the engine, and water addition. Water can be introduced as a fuel-water emulsion injected via the fuel valve, through separate nozzles or by humidifying the scavenge air. Despite the extensive research on primary measures along the recent years, a procedure to reduce NO x without decreasing emission of other pollutants and/or consumption has not effectively been developed. In this regard, secondary measures reduce NO x from the flue gas through downstream cleaning techniques. Many applications have been undertaken to reduce NO x by selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). The disadvantages of SCR are its price, poor durability of catalysts, and deposition of particulate on the catalyst. These disadvantages are not present in SNCR, but this procedure is limited to a narrow temperature range with optimal temperatures that are much higher than those characteristic of flue gas from diesel engines [10]. This limitation constitutes a drawback for practical applications in exhaust gases from diesel engines. As SCR reducing agents, ammonia (NH 3 ), urea, and cyanuric acid have been extensively employed. SNCR using ammonia, urea, and cyanuric acid are known as DeNO x [11,12], NO x OUT [13,14], and RAPRENO [15-17], respectively. Between these, this work focuses on NO x reduction using ammonia. The NO x reduction capabilities of ammonia were discovered in the seventies by Lyon [18], who found that ammonia selectively reduces NO x without a catalyst over the temperature range of 1100-1400 K. Typical exhaust gas temperatures from marine engines, around 300-450 • C [19], remain considerably lower than this optimal temperature ra...