GM XP 500 Firebird
General Motors XP 500 "Firebird" Free Piston Gas Turbine
Legendary GM XP 500 "Firebird"
The only car ever built with a free-piston gas turbine propulsion.
It was never planned to develop this form of propulsion for cars. According to former GM manager Art Underwood, this car was built solely for propaganda purposes (same as the following "Firebird" jet engine cars). Advertisements that generated the same attention would have been much more expensive.
The 250 hp twin engine was drawn by Pescara chief engineer Robert Huber. The plans were rewritten by GM on US customary units, then the engine was built at GM. It is reported that the cast aluminum housing was the most complex of its time.
Open The Door! - The most spectacular highlight in an engineer's career
1956 - General Motors was a SIGMA licensee who wanted to use this technique in an automobile. Robert Huber should draw the plans of the engine. This project led to a situation he described as the most spectacular highlight of his career.
The engine was drawn in Paris and built in Detroit. The prototype was ready on the test bench in Detroit but could not be started. Robert was ordered to Detroit by telegraph. He travelled to Detroit on an airplane with sleeping cabins. As he arrived, he found indeed that the engine did not start:
"I sat in a corner and thought again through each operation in detail: The valve opens so and so quickly, the air flows in so and so many milliseconds in the starting cylinder, the graphs correspond with the expectations, the compression pressure is achieved and a first combustion occurs - but there is no second combustion, why is that???"
"I said to a worker, open the door! and gave the order to start the engine and ... it ran and ran - until shut off - without objection. The explanation was simple: At all test stations there was a slight underpressure, in oder to prevent escaping petrol fumes. But the exhaust pipe ended outdoors, thus the higher external pressure made it impossible to achieve the second combustion. Opening the door enabled pressure equalization and starting could proceed normally".
Two days later, Robert Huber was back home again.
The GMR 4-4 "HYPREX" Engine- A Concept Of The Free Piston Engine For Automotive Use (SAE publication, 1956)
by Art F. Underwood, Mechanical Development Dept, General Motors Staff
As mentioned above, it was never planned to develop this form of propulsion for cars. According to former GM manager Art Underwood (see picture), this car was built solely for propaganda purposes. Advertisements that generated the same attention would have been much more expensive.