Original sentence: In this stage, the mixture (in the case of an Otto engine) or air (in the case of a Diesel engine) is compressed to the top of the cylinder by the piston until it is either ignited by a spark plug in an Otto engine or, in the case of a Diesel engine (a glow plug), reaches the point at which the fuel which has been injected spontaneously combusts, forcing the piston back down
Edited sentence: In this stage, the mixture (in the case of an Otto engine) or air (in the case of a Diesel engine) is compressed to the top of the cylinder by the piston until it is either ignited by a spark plug in an Otto engine or, in the case of a Diesel engine, reaches the point at which the fuel which has been injected spontaneously combusts, forcing the piston back down