The spark plug is fixed at 18° before top dead centre (TDC) in an SI engine running at 1800 r.p.m. It takes 8° of rotation to start combustion and get into flame propagation mode. Flame termination occurs at 12° after TDC. Flame front can be approximated as a sphere moving out from the spark plug which is offset 8mm from the centre line of the cylinder whose bore diameter is 8.4 cm. Calculate the effective flame front speed during flame propagation. The engine speed is increased to 3000 r.p.m. And subsequently as a result of which the effective flame front speed increases at a rate such that it is directly proportional to 0.85 times of engine speed. Flame development. After spark plug firing still takes 8° of engine rotation. Calculate how much engine rotation must be advanced such that the flame termination again occurs at 12° after TDC.