**1. Introduction**

Nowadays, diesel engines rule the transportation sector and power most of the ships in the world. These engines are e fficient compared with other thermal machines but emit harmful species such as nitrogen oxides (NOx), soot, carbon dioxide (CO2), sulfur oxides (SOx), carbon monoxide (CO), and non-burnt hydrocarbons (HC). Between these, NOx is a harmful component that must be reduced since it produces acidification of rain, photochemical smog, greenhouse e ffects, ozone depletion, and respiratory diseases. Several international, national, and regional policies have been developed to limit NOx and other pollutants. In the marine field, the European Commission and the Environmental Protection Agency limit emissions in the European Union and the United States, respectively. On an international level, the International Maritime Organization (IMO) maintains a comprehensive regulatory framework for shipping. In 1973, the IMO adopted Marpol 73/78, the International Convention for the Prevention of Pollution from Ships, designed to reduce marine pollution. In particular, Marpol Annex VI limits NOx emissions for marine ships depending on the manufacturing data, engine speed, and working geographical area.

Due to these increasingly restrictive regulations, several NOx reduction methods have been developed in recent years. One of them is the utilization of alternative fuels. The main alternative marine fuels may be found in two forms: liquid fuels including ethanol, methanol, bio-liquid fuel, and biodiesel; and gaseous fuels, including propane, hydrogen, and natural gas [1–4].

Operating under diesel, there are two procedures to reduce NOx, which are primary and secondary measures. The former reduces the amount of NOx during combustion, while the latter focuses on removing NOx from the exhaust gases through downstream cleaning techniques. It is well known that the main factors that influence NOx formation are the temperatures reached in the combustion process and the amount of time in which the combustion gases remain at high temperatures [5–7]. 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 NOx without decreasing emission of other pollutants and/or consumption has not e ffectively been developed. In this regard, secondary measures reduce NOx from the flue gas through downstream cleaning techniques. Many applications have been undertaken to reduce NOx 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 (NH3), urea, and cyanuric acid have been extensively employed. SNCR using ammonia, urea, and cyanuric acid are known as DeNOx [11,12], NOxOUT [13,14], and RAPRENO [15–17], respectively. Between these, this work focuses on NOx reduction using ammonia. The NOx reduction capabilities of ammonia were discovered in the seventies by Lyon [18], who found that ammonia selectively reduces NOx 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 range for NOx reduction. Comprehensive investigations have been reported about SNCR analyzing parameters such as temperature, the molar ratio (NH3/NO) [20], residence time, oxygen level, initial NOx, combustibles, and so on [21,22], verifying that the most important factor for NOx reduction is the temperature. Based on this result, Miyamoto et al. [23] proposed to reduce NOx emissions by injecting ammonia or urea directly into the cylinder. They found an optimum NOx reduction at injection timing 90◦ CA ATDC (crankshaft angle after top dead center), i.e., during the expansion stroke, under temperatures between 1100–1600 K. Nam and Gibbs [24] analyzed direct injection of urea and ammonia using a flow reactor which simulates a single cylinder diesel engine, while Nam and Gibbs [25] analyzed the influence of injection temperature, the molar ratio NH3/NO, residence time, and combustion products, focusing on kinetic parameters. Larbi and Bessrour [26] developed an analytical model to analyze ammonia injection and concluded that the temperature and thus injection timing is critical. In fact, if ammonia is injected near TDC (top dead center), it performs as a fuel instead of as a NOx reducing agent, since ammonia can also be employed as a fuel [27,28].

These aforementioned studies delivered interesting knowledge about ammonia injection, but an experimental analysis cannot provide complete information about the governing e ffects. In this regard, Computational Fluid Dynamics (CFD) o ffers an alternative method to analyze the performance and emissions on engines. In the field of medium and large marine engines, CFD is especially useful because an experimental setup is extremely expensive and a downscale model sometimes is not accurate enough. In particular, the so-called artificial inert species method allows us to investigate several chemical and physical e ffects separately. This method was initiated by Guo [29], who used an artificial inert component with the same properties as hydrogen to analyze the chemical, dilution and thermal e ffects of hydrogen addition on a HCCI engine. Voshtani et al. [30] and Neshat et al. [31] analyzed these chemical, dilution, and thermal e ffects on a blended fuel of isooctane and n-heptane. Subsequently, they studied these e ffects on reformer gas addition [32] and water addition [33].

This work presents a CFD analysis to study NOx reduction in a commercial marine engine, the Wärtsila 6 L 46. The NOx reduction procedure is based on ammonia injection during the expansion stroke. The artificial inert species method was applied to characterize thermal, dilution and chemical effects of ammonia injection. In addition, ammonia injection was compared with water (H2O) injection.
