A combined internal combustion and hot gas engine employs a working space surrounding a cylinder in an engine block. A piston reciprocates within the cylinder between combustion and expansion ends. A working fluid fills the working space and is moved by a reciprocal displacer mounted within the working space between a hot end adjacent the combustion end of the cylinder to a cold end disposed in fluid flow communication with the expansion side of the piston. The working fluid when heated increases in pressure and acts on the piston to assist in driving the piston between expansion and compression ends of the cylinder. A drive cam reciprocates the displacer in a timed phase relationship with the piston. In one embodiment, the displacer is formed of a regenerative heat transfer material to absorb and give off heat from the working fluid as the working fluid moves between the ends of the working space. In another embodiment, an external regenerative heat exchanger includes a first set of spaced tubes receiving hot exhaust gasses from the cylinder and a second plurality of tubes interposed between the first set of tubes and connected at opposed ends between the hot and cold ends of the working space. An oil distribution system includes crank case chambers separately associated with each combustion cylinder and pressurized by an air pump. An orifice formed in each crank case communicates with a common oil sump. An oil pump distributes oil from the sump to the combustion cylinder in a timed relationship with movement of the piston.