A closed-loop piston engine system including a recirculating gas system with supercritical gas as a working fluid, the closed-loop piston engine system comprising: a supercritical gas injector for injecting the supercritical gas into a cylinder or a first cylinder of a plurality of cylinders; a first valve of a cylinder head of the cylinder or of the first cylinder of the plurality of cylinders, the first valve being fluidly coupled with the recirculating gas system; a piston or a plurality of pistons moving in the cylinder and coupled with a crankshaft, the piston or the plurality of pistons being driven toward a centerline of the crankshaft during a power stroke using a connecting rod and causing the crankshaft to rotate thereby causing one power stroke per crankshaft rotation and thereby producing two power strokes for every single power stroke that a similar engine would produce if run as a hydrocarbon fuel powered internal combustion engine; and the recirculating gas system comprising: a valve system fluidly coupled with the first valve of the cylinder head or with the first valve of the cylinder head of the first cylinder of the plurality of cylinders a gas storage tank receiving the used gas via the valve system; a high-pressure gas booster fluidly coupling to the gas storage tank for pressurizing the used gas to a supercritical state producing supercritical gas; and a working fluid rail fluidly coupling the supercritical gas to the supercritical gas injector, characterised in that, the closed-loop piston engine system comprises a superheating nozzle region for imparting heat into the injected supercritical gas; a second valve of the cylinder head of the cylinder or of the cylinder head of the first cylinder of the plurality of cylinders, the second valve being fluidly coupled with the recirculating gas system and the valve system. a supercritical gas injector for injecting the supercritical gas into a cylinder or a first cylinder of a plurality of cylinders; a first valve of a cylinder head of the cylinder or of the first cylinder of the plurality of cylinders, the first valve being fluidly coupled with the recirculating gas system; a piston or a plurality of pistons moving in the cylinder and coupled with a crankshaft, the piston or the plurality of pistons being driven toward a centerline of the crankshaft during a power stroke using a connecting rod and causing the crankshaft to rotate thereby causing one power stroke per crankshaft rotation and thereby producing two power strokes for every single power stroke that a similar engine would produce if run as a hydrocarbon fuel powered internal combustion engine; and the recirculating gas system comprising: a valve system fluidly coupled with the first valve of the cylinder head or with the first valve of the cylinder head of the first cylinder of the plurality of cylinders a gas storage tank receiving the used gas via the valve system; a high-pressure gas booster fluidly coupling to the gas storage tank for pressurizing the used gas to a supercritical state producing supercritical gas; and a working fluid rail fluidly coupling the supercritical gas to the supercritical gas injector, characterised in that, the closed-loop piston engine system comprises a superheating nozzle region for imparting heat into the injected supercritical gas; a second valve of the cylinder head of the cylinder or of the cylinder head of the first cylinder of the plurality of cylinders, the second valve being fluidly coupled with the recirculating gas system and the valve system. a valve system fluidly coupled with the first valve of the cylinder head or with the first valve of the cylinder head of the first cylinder of the plurality of cylinders a gas storage tank receiving the used gas via the valve system; a high-pressure gas booster fluidly coupling to the gas storage tank for pressurizing the used gas to a supercritical state producing supercritical gas; and a working fluid rail fluidly coupling the supercritical gas to the supercritical gas injector, characterised in that, the closed-loop piston engine system comprises a superheating nozzle region for imparting heat into the injected supercritical gas; a second valve of the cylinder head of the cylinder or of the cylinder head of the first cylinder of the plurality of cylinders, the second valve being fluidly coupled with the recirculating gas system and the valve system.