Abstract: An internal combustion engine employing two identical piston assembly units, each providing several piston faces of various diameters, oriented and reciprocating opposite to each other within housing which provides chambers to accommodate the several piston sizes. Heat lost through the walls and heads of combustion chambers in primary cylinders from combustion therein is absorbed by incoming combustion air pumped by compressor pistons into plenum chambers surrounding the primary cylinders, wherein elevated pressure is maintained to aid the heat transfer and to provide pressure for scavenging and recharging the primary cylinders. Combustion in the primary cylinders drives the piston assembly units inward, and combustion gases expand into the common secondary system to a pressure determined by the space provided, with plenum air then admitted to scavenge and recharge the primary cylinders, and equalize plenum, primary, and secondary chambers' pressure.