Pressure measurements synchronized with high-speed motion picture records of flame propagation have been made in a transparent piston engine. The data show that the initial expansion speed of the flame front is close to that of a laminar flame. As the flame expands, its speed rapidly accelerates to a quasi-steady value comparable with that of the turbulent velocity fluctuations in the unburned gas. During the quasi-steady propagation phase, a significant fraction of the gas behind the visible front is unburned. Final burnout of the charge may be approximated by an exponential decay in time.

The data have been analyzed in a model independent way to obtain a set of empirical equations for calculating mass burning rates in spark ignition engines. The burning equations contain three parameters: the laminar burning speed s_l, a characteristic speed u_T, and a characteristic length l_T. The laminar burning speed is known from laboratory measurements. Tentative correlations relating u_T and l_T to engine geometry and operating variables have been derived from the engine data.