Abstract: An improved engine comprises a primary piston and primary cylinder in which air and fuel is precombusted. The piston is reciprocated in the cylinder by an external prime mover. The exhaust of the precombusted air/fuel mixture is controlled by a floating exhaust valve piston which seats against the exhaust port and has its other end pressurized at a set pressure preferably by an inert gas. The precombusted air/fuel mixtures are exhausted to secondary combustion chambers where additional air, fuel and prime mover exhaust gases are added to increase the velocity and pressure of the gas and provide for more complete combustion of the fuel.