The behavior of combustion in a realistic SI Four-Stroke engine with a transparent liner and piston top is studied using stereo gated image-intensified NTSC video cameras to record instantaneous flame position.Flame development is measured by double exposing each frame at different crank angles in a single cycle. The images are digitized and analysed with a PC-based image processing system. A complete cylinder pressure history of the propane fuelled combustion is combined with the flame images to study each cycle at different equivalence ratios, concentrating on lean combustion.Cycle-by-cycle variation is explored at 500 rpm, and a generic lean-limit combustion oscillation caused by changing residual gas is described and explained. The effect of engine swirl on the flame is shown, and the development of the early flame kernel relative to the spark plug. Flame shapes are polygonal in each cycle rather than spherical, with occasional large variations due to the flow. Lean fuelling cycles with no apparent heat release all have visible flames.