Stirling, Robert (1790–1878)

Scottish inventor of the first practicable hot-air engine in 1816. The Stirling engine has a high thermal efficiency and a large number of inherent advantages, such as flexibility in the choice of fuel, that could make it as important as the internal-combustion engine.

Early life Stirling was born in Cloag, Perthshire, and attended Glasgow and Edinburgh universities. He was ordained a Presbyterian minister in 1816, responsible for the parish of Galston, Ayrshire 1824–76. He also designed and made scientific instruments.

First engine The patent on the first air engine (and related patents until 1840) was taken out jointly with his younger brother James, a mechanical engineer. Their first engine appeared in 1818. It had a vertical cylinder about 60 cm/2 ft in diameter, produced about 1.5 kW/2 hp pumping water from a quarry, and ran for two years before the hot sections of the cylinder burned out.

The Stirling engine In 1824, the brothers started work on improved engines and in 1843 converted a steam engine at a Dundee factory to operate as a Stirling engine. It is said to have produced 28 kW/37 hp and to have used less coal per unit of power than the steam design it replaced. However, the hot parts burned out continually.

The Stirling-cycle engine differs from the internal-combustion engine in that the working fluid (in Stirling's case, air) remains in the working chambers. The heat is applied from an external source, so anything from wood to nuclear fuel can be used. It also means that combustion can be made to take place under the best conditions, making the control of emissions (pollution) considerably easier. The burning of the fuel is continuous, not intermittent as in an internal-combustion engine, so there is less noise and vibration.

Stirling engines use what is effectively two pistons to push the working fluid between two working spaces. One space is kept at a high temperature by the heat source and the other at a low temperature. Between these two spaces is a regenerator which alternately receives and gives up heat to the working fluid. The pistons are connected to a mechanism which keeps them out of phase (usually by 90°). It is this differential motion that moves the working fluid from one space to the other. On its way to the hot space, the fluid passes through the regenerator, gaining heat. In the hot space it gains more heat and expands, giving power. After the power stroke the fluid is pushed back through the regenerator, where it gives up its residual heat into the cold space and is ready to start the cycle again.