Simulations of ammonia spray evaporation, cooling and mixture formation in a direct injection compression ignition engine

In this study, we numerically investigate aspects of direct injection of liquid ammonia into a combustion chamber of a compression ignition engine. Ammonia has been considered a promising energy carrier and possible fuel for specific applications, especially in agriculture or marine transport sectors. Direct injection of liquid ammonia into a compression ignition engine is a relatively novel approach. Ammonia has a high latent heat of vaporization, and its direct liquid injection has a strong cooling effect on the mixture formation and its ignitability. Additionally, due to its high vapour pressures at low temperatures, a flash boiling phenomenon might occur, posing some challenges for numerical modelling. Such a phenomenon occurs when liquid injected at high pressure enters a low-pressure environment below its saturation pressure. The liquid is then superheated, and rapid atomization changes the spray behaviour. We have selected numerical models based on available literature on flashing and non-flashing spray simulations and the recent experimental study of ammonia spray characteristics with a gasoline direct injection (GDI) injector. A series of numerical 3D CFD simulations were performed using the Lagrangian approach for discrete phase modelling to replicate ammonia spray characteristics. The objectives of this study were to assess the ammonia evaporation process and its cooling effect and provide information about the mixture formation inside the cylinder for different operating conditions. The simulations cover the full engine cycle with multi-hole GDI ammonia spray and pilot n-heptane, as diesel surrogate, in equal proportions on an energy basis. Three different injection timings of ammonia were assessed, which resulted in various thermodynamic conditions in the combustion chamber during each spray formation. Therefore, different modelling parameters and initial conditions were required to simulate spray behaviour, and to address this, a relevant procedure to assign the pre-defined spray angles was proposed. The findings constituted further guidelines regarding expected combustion regimes and suggested which diesel injector nozzles should remain open to most reliably lead to the ignition promotion using pilot fuel injection