Validation of a Lagrangian Ignition Model in SI Engine Simulations

Ignition process plays a key role in flame kernel formation and heavily affects further combustion development. The paper aim is to present a 1D lagrangian ignition model and to validate it against real engine configurations. A lump model for the electrical circuit of the spark plug is used to compute breakdown and glow energy. At the end of shock wave and very first plasma expansion, a spherical kernel is deposited inside the gas flow at spark plug location. A simple model allows computing initial flame kernel radius and temperature based on physical mixture properties and spark plug characteristics. The sphere surface of the kernel is discretized by triangular elements which move radially according to a lagrangian approach. Expansion velocity is computed accounting for both heat conduction effect at the highest temperatures and thermodynamic energy balance at relatively lower temperatures. Turbulence effects and thermodynamic properties of the air-fuel mixture are accounted for. Restrikes are possible depending on gas flow velocity and mixture quality at spark location. CFD solver and 1D/lagrangian ignition model are closely coupled at each time step. The model proves to strongly reduce the grid sensitivity. The CFD model validation phase is crucial for a correct representation of both kernel formation and combustion development: the operation has been carried out by means of an accurate statistical analysis of experimental in-cylinder pressure data in real engine configurations.