A mechanical engine cylinder system, comprising: a cylinder including an internal space; an occupying structure disposed in the internal space of the cylinder; and a crankshaft piston; wherein the internal space of the cylinder is modified by the occupying structure such that combustion pressure applied to the crankshaft piston is applied to a smaller surface area of the crankshaft piston during an early part of an expansion stroke, and to a larger surface area of the crankshaft piston during a later part of the expansion stroke, and wherein the occupying structure is a movable structure relative to the cylinder, and wherein movement of the occupying structure during the later part of the expansion stroke is controlled by one or more forces applied by a force application mechanism, characterized in that: the occupying structure comprises a cavity to define a combustion space to generate the combustion pressure to be applied to the occupying structure and to the crankshaft piston, wherein the combustion space is surrounded by and located within walls of the occupying structure and has smaller diameter than the internal space of the cylinder, wherein the occupying structure is configured to be advanced in the direction of the crankshaft piston, and retracted away from the crankshaft piston, within the internal space of the cylinder, wherein the occupying structure is configured to be moved initially towards the crankshaft piston by the combustion forces generated within the combustion space surrounded by the walls of the occupying structure, during the early part of the expansion stroke, and wherein the occupying structure is configured to be accelerated in a retracting direction, away from the crankshaft piston, by the combustion pressure between the crankshaft piston 204 and the occupying structure 202, in the later part of expansion stroke, to absorb a part of combustion forces that would otherwise be applied to the crankshaft piston; and the force application mechanism is responsive to throttle position by way of throttle position sensors such that one or more forces applied to the occupying structure are dependent on throttle position. a cylinder including an internal space; an occupying structure disposed in the internal space of the cylinder; and a crankshaft piston; wherein the internal space of the cylinder is modified by the occupying structure such that combustion pressure applied to the crankshaft piston is applied to a smaller surface area of the crankshaft piston during an early part of an expansion stroke, and to a larger surface area of the crankshaft piston during a later part of the expansion stroke, and wherein the occupying structure is a movable structure relative to the cylinder, and wherein movement of the occupying structure during the later part of the expansion stroke is controlled by one or more forces applied by a force application mechanism, characterized in that: the occupying structure comprises a cavity to define a combustion space to generate the combustion pressure to be applied to the occupying structure and to the crankshaft piston, wherein the combustion space is surrounded by and located within walls of the occupying structure and has smaller diameter than the internal space of the cylinder, wherein the occupying structure is configured to be advanced in the direction of the crankshaft piston, and retracted away from the crankshaft piston, within the internal space of the cylinder, wherein the occupying structure is configured to be moved initially towards the crankshaft piston by the combustion forces generated within the combustion space surrounded by the walls of the occupying structure, during the early part of the expansion stroke, and wherein the occupying structure is configured to be accelerated in a retracting direction, away from the crankshaft piston, by the combustion pressure between the crankshaft piston 204 and the occupying structure 202, in the later part of expansion stroke, to absorb a part of combustion forces that would otherwise be applied to the crankshaft piston; and the force application mechanism is responsive to throttle position by way of throttle position sensors such that one or more forces applied to the occupying structure are dependent on throttle position. the occupying structure comprises a cavity to define a combustion space to generate the combustion pressure to be applied to the occupying structure and to the crankshaft piston, wherein the combustion space is surrounded by and located within walls of the occupying structure and has smaller diameter than the internal space of the cylinder, wherein the occupying structure is configured to be advanced in the direction of the crankshaft piston, and retracted away from the crankshaft piston, within the internal space of the cylinder, wherein the occupying structure is configured to be moved initially towards the crankshaft piston by the combustion forces generated within the combustion space surrounded by the walls of the occupying structure, during the early part of the expansion stroke, and wherein the occupying structure is configured to be accelerated in a retracting direction, away from the crankshaft piston, by the combustion pressure between the crankshaft piston 204 and the occupying structure 202, in the later part of expansion stroke, to absorb a part of combustion forces that would otherwise be applied to the crankshaft piston; and the force application mechanism is responsive to throttle position by way of throttle position sensors such that one or more forces applied to the occupying structure are dependent on throttle position.