A sterling engine is a highly efficient, combustion-less, quiet engine that harnesses the energy produced when a gas (not gasoline) expands and contracts as its temperature changes. Invented by Robert Stirling in 1816 in Scotland, the Stirling, or sterling, engine uses simple gases and natural heat sources, such as sunlight, to regeneratively power the pistons of an engine. Sterling engines are appealing because they require no replenished source of fuel, run silently, and eject no emissions.
In the early 1800s, the primary kind of engines were steam engines on boats and trains. These used the reliable technology of heated water that turned to vapor to power transportation, but they occasionally exploded and endangered operators and passengers. Therefore, Stirling invented an engine that was a closed-system with no explosives, no fire, and no steam, for safety and efficiency. He hoped that the sterling engine would be engineered to be strong enough to eventually power ships and locomotives.
A sterling engine works on very basic concepts of physics that describe how hot air expands, and has a higher pressure, than cooler air. A regular diesel engine, like in your car, uses fuel to cause tiny explosions that move pistons up and down. But in a sterling engine, the piston is moved by displacement of different chambers of air with different degrees of pressure. The air on one side of the displacer piston is heated by an outside source such as sunlight or decaying plant matter. The increased pressure pushes the piston to the other side of the chamber, where the air remains cool. Then that chamber heats, and the process repeats. There are usually two pistons, at 90° to each other, that can be connected to a crankshaft to move a vehicle or a generator to make electricity.
Certain modern applications still rely upon a sterling engine when they require a power source that doesn't need refueling, doesn't vent emissions, and operates quietly. For example, some submarines house a sterling engine, as do some solar energy plants. Recent interest in popularizing the sterling engine has been inspired by people's desire to power transportation and make electricity without fossil fuels. Researchers are working on developing updated technology that puts sterling engines in electrical plants and commuter cars.