Experimental optimization toward high efficiency and low emissions in an ammonia-fueled RCCI engine with various reactivity enhancements

Ammonia (NH3) is a carbon-free and renewable fuel for conventional diesel engines to effectively reduce carbon emissions. However, the low combustion efficiency, as well as high nitrous oxide (N2O) and nitrogen oxides (NOx) emissions, are the primary obstacles for ammonia applications. In this study, various combustion enhancements are applied to an ammonia-fueled reactivity-controlled compression ignition (RCCI) engine to improve performance by optimizing the reactivity of ignition trigger fuel and the spatial reactivity of premixed fuel/air mixture, as well as elevating the ambient temperature. Compared to diesel, using polyoxymethylene dimethyl ether (PODE) as the ignition trigger fuel reduces unburned NH3 by 13 %. Increasing intake temperature allows the engine to operate at an earlier start of injection (SOI) and a higher ammonia energy ratio (AER), contributing to higher indicated thermal efficiency (ITE). Advancing SOI or adopting split injection of PODE can improve the spatial reactivity, resulting in a maximum reduction of 71 % in unburned NH3. By adopting the three strategies of enhancements, the trade-off relationship among ITE, unburned NH3, and greenhouse gas (GHG) can be defeated to achieve satisfactory engine performances. Finally, ITE is increased by 7.56 %, the unburned NH3 is declined by 79.3 %, and GHG emissions are decreased by 25.4 % simultaneously.