The old Stirling "air" engine (originally patented by Robert and James Stirling of Scotland in 1816) was born in the wrong century. Its principle (power from expanding hot air) was good, but the crude materials and engineering methods of the time made it too clumsy and inefficient to be widely used.
In the current issue of Scientific American, Leonard Engel tells how technological progress has revived the Stirling engines. Just before the war, an engineer of the big and smart Philips electrical company at Eindhoven, in The Netherlands, stumbled on one of the Stirlings. He and his colleagues decided that all it needed was redesigning with modern materials. During the German occupation, they worked quietly to get their mechanical sleeping beauty in shape for the postwar world.
The essentials of the air engine are extremely simple: a "hot space" heated by an external firebox, a "cold space" cooled by water or air, and two pistons. When one piston shifts cold air into the hot space, the air expands and pushes the second piston away in a power stroke. Then the first piston shunts the air back to the cold space, where it contracts and is ready to start another cycle. A regenerator made of crimped steel wire between the hot & cold spaces keeps heat from being wasted by the moving air.
The Dutch engineers re-created the old engine with stainless steel and aluminum bronze parts where they would do the most good. They improved the regenerator and increased the pressure of the "working" air to 50 atmospheres (735 Ibs. per square inch) at peak. They tinkered hardly at all with the original idea of the 19th Century Scotsmen. When they got through, they had engines that weigh only 10 to 20 Ibs. per horsepower and are about as efficient as diesels. Their simplicity makes them cheap to maintain, and they burn almost any fuel, from oil to coal or corncobs.
Since the air engine requires a warming-up period, it is not well adapted to automobiles. The Philips people hope to make it popular for boats, farm machinery and small power plants.