Combustion of a lean air-fuel mixture diluted with a large amount of air or Exhaust Gas Recirculation (EGR) gas is one of the important technologies that can reduce thermal NOx and improve gasoline engine fuel economy by reducing cooling loss. On the other hand, lean combustion increases unburned Hydro Carbon (HC) and unburned loss compared to stoichiometric combustion. This is because lean combustion reduces the burning rate of the air-fuel mixture and forms a thick quenching layer near the wall surface. In this study, the relationship between the thickness of the unburned HC and the excess air ratio is analyzed using Laser Induced Fluorescence (LIF) method and Computational Fluid Dynamic (CFD) of combustion.
The HC distribution near the engine liner when the excess air ratio is increased is investigated by LIF. As a result, it is found that the quenching distance of the flame in the cylinder is larger for lean conditions than the general single-wall quenching relationship.
Two turbulent models, RANS and LES are used for CFD. RANS using a normal wall function cannot express the distribution of unburned HC as in the experimental results, because detailed mesh makes [Y +] too small and the turbulent behavior near the wall is not analyzed correctly.
LES without using wall function can give near-wall behavior that is similar to the experimental results. By analyzing quantities of state calculated by LES, it is found that turbulence affects the thermal diffusion from the flame when the excess air ratio increases, and the unburned HC region expands. A revised relationship for a single-wall quenching considering the effect of turbulence on the thermal diffusion from the flame is proposed, and the increase of the quenching distance for lean conditions can be explained.