can greatly contribute to the overall efficiency of the device fig5 shows simple basic means for stopping one inner rotor 65a to provide ratchet steps 128a 128b 129a 129b the pair of steps 129a 129b interact with forward pawls 131a 131b to prevent the inner rotor 65a from rotating counterclockwise during the time it takes for the center shaft 58 to rotate 90 degrees the counterclockwise direction of rotation is assumed to be the positive direction of rotation assuming steam entering the two chambers of the drum 60a during the interval in which rotor 65a is held fixed the pressure of the steam will urge the inner radial pistons to reverse their motion and move in the negative direction the ratchet steps 128a 128b of the inner rotor 65a will then interact with the pair of rear pawls 132a and 132b and prevent the rotor 65a from moving in the negative direction a cam 130 rigidly attached and rotating with the shaft 58 is shown in fig5 at the instant when it acts upon rollers 135a and 135b rotatably supported on the pawls 131a and 131b through shafts 134a and 134b for displacing the pawls radially outwardly and thereby disengage the inner rotor 65a from the forward pawls 131a and 131b the inner rotor 65a will then rotate.