Abstract: A prime source of mechanical power comprising an expansion chamber, an inlet into which expansible fluid is injected, and an outlet through which said fluid is exhausted after expansion in said chamber. Expansion of the fluid is achieved by application of a heat source directly to the expansion chamber, which expansion acts through a piston to create useful mechanical motion. The engine is preferably of a reciprocating type, while the heat source can be solar, gaseous, petroleum, nuclear, or electrical.