High Speed Schlieren Visualization of Flame Initiation in a Lean Operating S.I. Engine

High speed Schlieren cinematography along with flame contour analysis is used to study the early flame propagation, from spark to a 4 mm flame radius. This is done for lean propane–air mixtures, and up to 1500 RPM. For normal engine speeds, the flame is turbulent immediatly after initiation, with no laminar phase. The burnt kernel is first driven by the electric ignition source. After about 0.5 ms, the effect of engine speed (i.e. turbulence) is very strong, the kernel expansion rate increasing with engine speed. Increasing the equivalence ratio also increases the propagation speed. The rate of flame growth measured in the engine is shown to go through a minimum for a burnt kernel radius of about 2 mm which varies with the equivalence ratio. The minima of the flame velocity at this radius depend on mixture strength and turbulence level. The simultaneous recording of the pressure trace shows a correlation between the early flame behavior and the overall combustion speed.