In some embodiments the disclosure is directed to a closed-loop piston engine system using a recirculating carbon dioxide (CO2) system with supercritical carbon dioxide (scCO2) as a working fluid. The closed-loop piston engine system may include a scCO2 injector; a superheating nozzle region; a first valve; a second valve; a piston moving in the cylinder and coupled with a crankshaft, the piston 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. The recirculating CO2 system recirculates the used carbon dioxide and there are no carbon dioxide emissions from the system.