Transient flame propagation, gas velocities and heat release are measured in a spark-ignited engine with optical access using high speed shadowgraph visualization, laser velocimetry and pressure trace analysis.
For a set of running conditions varied around a reference case, ensemble-averaged and cycle-resolved flame front evolutions, velocity fields and heat release are presented for lean mixtures of propane and air. The changed parameters include flow-field (swirl, tumble), rpm (600 and 1200), equivalence ratio (0.7 and 0.9), volumetric efficiency (0.5 and 0.9) and spark-timing. Given the usable field of view, the combustion can be observed during 50% of its total duration which corresponds to a 15% burnt fraction.
An analysis of the data shows that the flame expansion velocities are highly transient during the observable combustion phase. An acceleration yields to a 2 to 4 fold velocity increase. The flame velocities depend strongly on the operating conditions. Taking into account the chamber geometry and the gas expansion ratio through the flame, the transient turbulent flame speeds free of convection effects are determined. A correlation is found between the ratio of the turbulent flame speed to the laminar flame velocity and the turbulence intensity.