Turbulent flame and mass burning rate in a spark ignition engine

Turbulent burning rates were measured in an spark-ignition engine with extensive optical access for a fairly wide range of operating conditions. Skip-fired operation was employed to eliminate the effects caused by trapped or recirculated exhaust gases. Three main stages of turbulent flame propagation in SI engines were discerned: an initial acceleration, propagation with approximately constant speed and final deceleration caused by the proximity of walls. The rate of the initial flame acceleration showed little sensitivity to the engine operating parameters if the burning rate was expressed in terms of dimension-less flame radius. Propagation at a constant burning rate occurred over most of the charge volume but only for a minor fraction of its mass; the burning rate in this regime was proportional to the turbulence root-mean-square velocity while the influence of all other relevant factors could be encapsulated in either Karlovitz stretch factor or Damkohler number. The final stage of combustion, where the burnt gas radius was within one integral length scale from the walls, was responsible for more than two-thirds of the charge mass consumption and occurred at decreasing burning rate and flame brush thickness.