An internal combustion engine for translating substantially linear motion of a piston between a top dead centre position and a bottom dead centre position to rotary motion of a crank shaft, the internal combustion engine comprising: a crank arm associated and rotatable with said crank shaft, said crank arm including a rotatable wheel having an outer peripheral edge and an axis of rotation; and a yoke structure associated with the piston (16, 18, 20, 22) and moveable therewith, said yoke structure including an inner peripheral edge along which said wheel is caused to roll during rotation of the crank arm, the inner peripheral edge including a first linear surface that extends substantially perpendicularly to a direction of travel of the piston (16, 18, 20, 22) and which is encountered by said wheel as the piston (16, 18, 20, 22) approaches said top dead centre position, wherein when the piston (16, 18, 20, 22) reaches the top dead centre position a contact point between the yoke first linear surface and the wheel outer edge is substantially along an axis that extends parallel to the direction of travel of the piston (16, 18, 20, 22) and through the wheel's axis of rotation, said inner peripheral edge further including a shoulder that is encountered by said wheel from the commencement of each shaft revolution and deviates outwardly and in a downstroke direction from the first linear surface, the shoulder having an inner surface dimensioned to cause said contact point to shift outwardly along the outer peripheral edge of the wheel and thereby outwardly from said axis extending parallel to the direction of travel and through the wheel's axis of rotation, wherein said shoulder is dimensioned such that from approximately a 45 degree crank shaft rotation, the engagement becomes crank-related, and from approximately 45 degrees to 90 degree crank rotation said contact point continues to shift outwardly along the outer peripheral edge of the wheel and thereby outwardly from said axis extending parallel to the direction of travel and through the wheel's axis of rotation, and wherein said outer peripheral edge and yoke structure inner edge are dimensioned such that engagement there between is crank-related when the wheel is encountering the first linear surface, during which the crank shaft translates the linear motion of the linearly moveable member into rotating motion against the crank shaft, and is cam-related from when the wheel commences its encounter with the shoulder to approximately 45 degrees crank shaft rotation, during which the piston (16, 18, 20, 22) dwells. a yoke structure associated with the piston (16, 18, 20, 22) and moveable therewith, said yoke structure including an inner peripheral edge along which said wheel is caused to roll during rotation of the crank arm, the inner peripheral edge including a first linear surface that extends substantially perpendicularly to a direction of travel of the piston (16, 18, 20, 22) and which is encountered by said wheel as the piston (16, 18, 20, 22) approaches said top dead centre position, wherein when the piston (16, 18, 20, 22) reaches the top dead centre position a contact point between the yoke first linear surface and the wheel outer edge is substantially along an axis that extends parallel to the direction of travel of the piston (16, 18, 20, 22) and through the wheel's axis of rotation, said inner peripheral edge further including a shoulder that is encountered by said wheel from the commencement of each shaft revolution and deviates outwardly and in a downstroke direction from the first linear surface, the shoulder having an inner surface dimensioned to cause said contact point to shift outwardly along the outer peripheral edge of the wheel and thereby outwardly from said axis extending parallel to the direction of travel and through the wheel's axis of rotation, wherein said shoulder is dimensioned such that from approximately a 45 degree crank shaft rotation, the engagement becomes crank-related, and from approximately 45 degrees to 90 degree crank rotation said contact point continues to shift outwardly along the outer peripheral edge of the wheel and thereby outwardly from said axis extending parallel to the direction of travel and through the wheel's axis of rotation, and wherein said outer peripheral edge and yoke structure inner edge are dimensioned such that engagement there between is crank-related when the wheel is encountering the first linear surface, during which the crank shaft translates the linear motion of the linearly moveable member into rotating motion against the crank shaft, and is cam-related from when the wheel commences its encounter with the shoulder to approximately 45 degrees crank shaft rotation, during which the piston (16, 18, 20, 22) dwells.