April 15: GM reveals the XP 500 free-piston concept on this date in 1956

By the mid-1950s, as the largest automaker in the world, General Motors had the resources to explore outlandish ideas about the future of transportation. One of its more interesting efforts produced this, the XP 500 concept, a two-seat car powered by a free piston engine, the first time such a motor had been mated to a car. The free piston engine uses cylinders moving in opposite directions to compress air, eliminating crankshafts and many other parts. The Hyprex 4-cylinder unit developed by GM seems like something of a marvel even today; capable of making 250 hp, it could run on almost any fuel, and powered the XP 500 by spinning a turbine at the rear of the car. But the noise, controls and inflexibility of the free-piston design made it unusable in cars — although GM built a massive, 6,000-hp version for use on a Liberty ship.