Often mistaken for a Firebird turbine prototype, or simply forgotten by many, this is the 1956 General Motors XP-500 free-piston car. Get the fascinating story here, including a video of the prototype in action.  

Unveiled in May of 1956 just as the General Motors Technical Center in Warren, Michigan was officially opened, the GM XP-500 prototype has been sort of mislaid by history. That’s a bit of a shame, but it is understandable. The technology proved to be a dead end, at least for automotive use, and the vehicle itself has been confused with the Firebird II, GM’s gas turbine passenger car concept.

The XP-500’s resemblance to the Firebird turbine cars was apparently by design, as free-piston and gas turbine technology are somewhat related. As the name implies, a free-piston engine employs cylinders and pistons like a conventional automotive engine, which compress and combust an air-fuel mixture to produce power. However, since there is no crankshaft to harness the piston’s reciprocating motion and convert it to rotating force, the expanding combustion gas is sent to a turbine and gearbox to provide the vehicle’s propulsion. This is an extremely simplified description of free-piston engine operation. More info can be found in a host of sources, including the original Society of Automotive Engineers paper no. 570032 on the XP-500 published in 1957. A nice layman’s explanation with animation is available at Wikipedia. 

XP-500 chassis layout, with piston unit in front and turbine unit driving rear wheels. 

The XP-500’s Hyprex 4-4 free-piston unit (called a gasifier in free-piston lingo) was designed under contract by the Swiss engineer Robert Huber, generally regarded as the father of the modern free-piston engine, and constructed by GM’s machinists and technicians in Detroit. The double-cylinder, four-piston gasifier was mounted up front, while the turbine and gearbox were mounted in the rear of vehicle, as shown in the schematic drawing above. (By the way, the single prototype is reportedly still in the possession of GM, but is not in exhibit-quality condition.)

Nominally rated at 250 horsepower, the XP-500’s powerplant could run on virtually any fuel, including kerosene, bunker oil, and vegetable oil. With no crankshaft assembly, vibration was said to be almost non-existent, and thermal efficiency was claimed to be in the range of 32 to 36 percent. However, the engine’s starting, lubrication, and control issues were found to be insurmountable, evidently, and the experiment was abandoned after three years. But fortunately, we can revisit the world’s first free-piston automobile in the video below.