LES of pilot n-heptane ignition and its interaction with the lean premixed methane–air mixture in a dual-fuel combustion engine

Abstract

Large eddy simulations of pilot fuel ignited, lean premixed, natural gas engines are performed to study the pilot-ignition process and its subsequent interaction with the premixed charge. The injection pressure (Pinj) and injection duration (Δinj) are varied (i.e. 800 bar/760μs, 800 bar/500μs, and 400bar/760μs) to study the impact of the injection process on the subsequent combustion evolution. Open-cycle simulations considering the full engine geometry are used to predict the in-cylinder flows, while combustion is modeled using a finite-rate chemistry model. In-cylinder methane (CH4) is shown to delay the low-temperature ignition of the pilot fuel, regardless of the pilot injection setting, which subsequently prolongs the overall pilot fuel ignition delay. Moreover, all simulated cases show the occurrence of back-supported combustion (BSC), where the burning of CH4-air mixture is “back-supported” by pilot fuel radicals. Despite both the 800 bar/500μs and 400 bar/760μs cases having the same injected pilot fuel mass, the peak in-cylinder pressure and burning rate of the premixed CH4-air mixture in the former case are higher. Higher Pinj and shorter Δinj lead to better mixing between the pilot fuel and the premixed CH4-air charge. Subsequently, this forms a larger volume of regions with elevated equivalence ratio due to the presence of pilot fuel (ϕ>ϕCH4) which, consequently promotes the formation of BSC. The impact of in-cylinder flow fields on the dual-fuel combustion process is investigated by performing two closed-cycle 800 bar/500μs cases with one assuming solid-body rotation and another without solid-body rotation (i.e. zero velocity field). In-cylinder flow field is shown to have a visible impact on the transition stage between the pilot ignition stage and the premixed flame propagation stage, but have an insignificant effect on the pilot fuel ignition process. In the transition stage, slower flame propagation is observed in the zero-velocity case. The results show that this is not only due to the turbulence effect on premixed flame but also due to differences in the volume and distribution of pilot fuels that impacts BSC.