An analytical and experimental investigation of the combustion process in a spark ignition engine has been performed. The analysis is based on a flame propagation model developed by Blizard and Keck (9) which assumes that turbulent eddies are engulfed at an entrainment speed ue and burn in a characteristic time τ, equal to the eddy radius ℓe, divided by the laminar flame speed uℓ. Measurements were made of burning intervals and ignition delays on a C.F.R. engine.
The model could be adjusted to give correct trends in burning intervals for variations in equivalence ratio, EGR, compression ratio, spark timing, and level of turbulence. However, the amount of variation was less than that observed in experiment. Predicted burning intervals increased with RPM contrary to experimental data. Predicted ignition delays were a factor of four less than experimentally observed.
The difficulties with the model are thought to be due to assumptions of constant τ and ue. It is recommended that future work on the ‘eddy-entrainment’ type of model be directed at removing this restriction and at understanding the behavior of τ and ue during the engine cycle in terms of measureable turbulence scales, and ultimately in terms of engine operating variables such as RPM, spark timing and equivalence ratio.