The pressure of gas in a container goes up when the gas is heated and goes down when the gas is cooled. This fact is used in the sterling engine. A specific amount of gas is sealed in the engine. It is alternatively heated and cooled. When it is heated its increasing pressure pushes a power piston down. When it is cooled, its lowering pressure pulls the power piston up.
It consists of a power piston that moves up and down between the power chamber and the buffer space. The displacer piston does not produce power but simply causes the air to move between the heater and the cooler section of the engine. The two pistons are linked by a rhombic drive to a pair of synchronizing gears, which in turn drive the output shaft.
As the working gas is heated, its pressure increases which forces the power piston to move down, which ultimately pushes the two connecting links against, the two gears. The gears therefore rotate and turn the engine output shaft. Meanwhile, the rotation of the gears causes the two links to the displacer piston to pull it down. This increases the hot space above the displacer piston so that some of the working gas can flow from the power chamber forward to this space. While moving the gas through the cooler on the way to this upper space, the gas is cooled and its pressure goes down.
the power piston is at B.D.C. At this position when the power piston is at B.D.C. the pressure below the piston in the buffer space is higher than that above the piston in the power chamber, which causes the piston to move up.
Meanwhile, the working gas that has flowed into the hot space becomes heated. The heating is done by the continuous combustion of kerosene or oil. The pressure of the working gas increases causing the power piston to move down. This up-and-down motion of the power piston is transmitted through the two links to the gears, rotating them around.
The heating and cooling of working gas take considerable time and result in an awkward, slow and inefficient engine. However, the recent experimental engines, using helium as the working gas, at an average pressure of 100 kg /cm2 have operated at 3,000 r.p.m. with an efficiency of 30%.
Sterling engines are easy to start, silent in operation, and simple in design. They are used in lawnmowers. They are also used for remote electric power plants and pumping stations. It is possible that they will be used in space satellites and space stations. In space, the heat source is only the sun. With a large reflector, the heat will be gathered from the sun; and with a large radiator, it will be radiated away. Thus, the sterling engine would operate in space without any fuel at all.