Combustion of a spark ignition engine is often idealized by a constant volume process. However, due to piston motion, constant volume combustion can not be realized in an actual engine cycle, and this leads to a loss in thermal efficiency when compared to an ideal Otto cycle. In this paper, such a loss associated with piston motion, termed as time loss, is rigorously studied for spark ignition engines of different piston motion mechanisms. It is found that the time loss of a conventional slider crank engine amounts to maximum 9 % of the total loss depending upon operating conditions. Especially, in the limit of long crankshaft, where piston motion follows a perfect sinusoid, it is shown that the time loss decreases by 2 - 5 %. To determine the dominant factors affecting the time loss, also investigated are the effects of major performance parameters such as compression ratio, combustion start and duration.