a fluid engine for use in pneumatically operated toys such as wheeled vehicles or airplanes includes an engine having a fluid input cavity which is in continuous fluid communication with a source of compressed air , a fluid delivery cavity which is in continuous communication with a piston cavity bounded by a moveable piston mounted in a cylinder member and which is separated from the fluid input cavity by a wall having a valve opening , and exhaust apertures which are separated from the fluid delivery cavity . a valve rod is movably housed to open the valve opening and close the exhaust apertures during the piston &# 39 ; s power stroke , and to close the valve opening and open the exhaust opening during the piston &# 39 ; s exhaust stroke . the valve rod is operatively connected to a piston to act in synchronism with it by the use of a cam integrally secured to a propeller power shaft .