General Motors XP-500

I don't know about you, but I wasn't in any of the meetings held about the XP-500, and so can't say for certain why it didn't survive. First shown in 1956 during the launch of the GM Technical Center, it must have seemed like a moon shot taken by one of the world's most powerful companies.

And what was the source of such bravado? Well, for one, it looks like a jet fighter for the road. You may have thought it was a Firebird concept, but it isn't one.

Because inside the swoopy skin, General Motors was announcing its latest innovation: a "free piston" engine. Actually, Wikipedia has a great description of it:

"A free-piston engine is a linear, 'crankless' internal combustion engine, in which the piston motion is not controlled by a crankshaft but determined by the interaction of forces from the combustion chamber gases, a rebound device (e.g., a piston in a closed cylinder) and a load device (e.g. a gas compressor or a linear alternator)."

In the case of the XP-500, General Motors called it the "4-4 Hyprex engine." It was two parallel cylinders, each with its own set of two horizontally-opposed pistons. The air/fuel charge would fire between the pistons, with the compressed air created used to power a turbine, with drive to the wheels from there. General Motors said it had an output of 250 horsepower…which sounds surprisingly good. Even so, the project was cancelled within three years—I wonder how these engines would work with modern computer controls and hybrid drives in place.

Apparently, this technology is used primarily with both stationary and marine engines, but there's little information on the subject—perhaps a #bcotd reader can share why you may want to use a free-piston engine in a car.