6a of the torque lobe 6 so that the slipper 5 can slide round the rim as the torque lobe rotates but so that the slipper is constrained to it alternatively the slipper 5 could be attached to the outer race of a roller bearing provided at the peripheral edge of the torque lobe the slipper 5 is provided with a pair of roller bearings 7 which constitute cam followers the cam followers 7 are constrained to follow cam tracks 8a formed in plates 8 positioned on opposite sides of the torque lobe 6 the cam tracks 8a are generally elliptical the operation of the single cylinder engine of fig1 and 2 will now be described with reference to fig3 a to 3d the operating cycle is best understood by describing the operation starting with the piston 2 at tdc in this position see fig3 a the piston 2 has just completed its compression stroke ignition having taken place just before tdc the expanding gases formed by the ignition explosion force the piston 2 to descend rapidly within the cylinder 1 whereby the connecting rod 3 and the slipper 5 force the torque lobe 6 to rotate to the position shown in fig3 b this movement of the piston 2 is the power stroke of the engine during the power stroke the slipper 5 is constrained to follow a curved path defined by the elliptical cam tracks 8a this enhances the torque produced by the engine by increasing the moment of the descending piston 2 about the axis of rotation the axis of the shaft 4 in a manner described below with reference to fig7 and 8 if the cam followers 7 were not constrained to.