Abstract: A fluid driven reciprocating engine includes a main body having an inlet for receiving a primary fluid and an outlet for discharging the fluid. A drive piston is sealingly mounted in the main body for reciprocating movement in response to flow of the primary fluid through the body. The drive piston divides the body into first and second chambers. A first valve selectively transmits the primary fluid from the inlet to the first and second chambers and a second valve selectively transmits the primary fluid from the first and second chambers to the outlet. An overcenter linkage mechanism that operably interconnects the drive piston and the first and second valves.