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In this study a new model is proposed for turbulent premixed combustion in a spark-ignition engine. An independent transport equation is solved for the mean reaction progress variable in a propagation form in KIVA-3V. An expression for turbulent burning velocity was previously given as a product of turbulent diffusivity in unburned gas, laminar flame speed and maximum flame surface density. The model has similarity with the G equation approach, but originates from zone conditionally averaged formulation for unburned gas. A spark kernel grows initially as a laminar flame and becomes a fully developed turbulent flame brush according to a transition criterion in terms of the kernel size and the integral length scale. Simulation of a homogeneous charge pancake chamber engine showed good agreement with measured flame propagation and pressure trace. The model was also applied against experimental data of Hyundai θ-2.0L SI engine.

Recent findings on the characteristics of flame surface density are introduced for turbulent premixed combustion in typical operating conditions of SI engines. The maximum flame surface density tends to show linear dependence on the K -factor defined as a function of the integral length scale and . The flame surface density shows an asymmetric profile in the space with the peak location correlated in terms of the dimensionless parameter, NB, which represents the degree of gradient or counter-gradient diffusion by turbulence. The effects of the K -factor and NB are discussed in the wrinkled flamelet and corrugated flamelet regime respectively. The flame surface density increases at a higher ambient pressure due to decrease in the laminar flame speed and the length scales of flame wrinkling. Comments are made on the turbulent stretch and turbulent flux terms in the Σ -equation in modeling combustion of an SI engine.

The autoignition delay of a turbulent methane jet in a Diesel-like environment is calculated by the conditional moment closure(CMC) model. Methane is injected into hot air in a constant volume chamber under various temperatures and pressures. Detailed chemical reaction mechanisms are implemented with turbulence-chemistry interaction treated by the first order CMC. The CMC model solves the conditional mean species mass fraction and temperature equations with the source term given in terms of the conditional mean quantities. The flow and mixing field are calculated by the transient SIMPLE algorithm with the k -ε model and the assumed beta function pdf. The CMC equations are solved by the fractional step method which sequentially treats the transport and chemical reaction terms in each time step. The predictions in quiescent homogeneous mixture are presented to evaluate the effects of turbulence in jet ignition.