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.