CFD-based investigation of ammonia combustion and slip behavior in an ammonia-diesel dual-fuel engine

The use of zero-carbon ammonia to partially replace diesel in existing compression ignition engines via the port fuel injection strategy can reduce carbon dioxide emissions but introduces the challenge of ammonia slip. Developing effective strategies to minimize ammonia emissions requires detailed knowledge of the in-cylinder spatial distribution of ammonia during the combustion process. This study aims to numerically investigate ammonia combustion and emissions in an ammonia–diesel dual-fuel engine using multi-dimensional computational fluid dynamics (CFD) simulations, including quantification of the ammonia mass fraction consumed during each combustion stage and visualization of its spatial distribution within the cylinder. The CFD results indicate that a small portion of the fumigated ammonia can be consumed through low-temperature oxidation near top dead center, but self-ignition is suppressed, resulting in knocking-free combustion. In addition, under the conditions investigated, the ammonia–air mixture is too lean to support the propagation of turbulent flames. Consequently, most of the ammonia is consumed through concurrent combustion with diesel fuel within the diesel spray plume. Furthermore, a portion of the unburned ammonia reacts with nitrogen oxides (NOx) in low-temperature regions, forming nitrous oxide emissions. Ammonia that escapes oxidation in high-temperature regions and de-NOx reactions in low-temperature regions eventually exits the engine as unburned ammonia. A key factor influencing ammonia consumption and residual ammonia is the ammonia-to-diesel substitution ratio, which affects diesel spray development and the associated bulk gas motion. A higher substitution ratio shortens the diesel injection duration and weakens the motion of hot bulk gas induced by the momentum of the directly injected pilot fuel, leading to larger regions not reached by the diesel plume and reduced interaction with the ammonia-containing bulk mixture, thereby reducing ammonia combustion efficiency and increasing unburned ammonia emissions to potentially unacceptable levels. Overall, strategies aimed at enhancing ammonia combustion and minimizing emissions in dual-fuel engines should focus on enabling turbulent flame propagation initiated by pilot diesel. One promising approach is blending hydrogen with ammonia, which can help achieve the lean flammability limit and improve flame propagation characteristics.