Robert Stirling was born on 25 October 1790 in Cloag Farm, Perthshire. He studied Devinity but also followed lectures in Engineering and Physics and was appointed Minister in the Church of Glaston in 1824. His brother James Stirling was born 10 years later; 20 July 1800. He studied mechanical engineering and became a Civil engineer.

It was Robert who initially discovered the Stirling principle and patented an engine based on it in 1816, but James' persevering ingenuity perfected the engine in such a way that it proved an economical mode of producing energy for the town foundry and spinning mill. The Stirling cryocooler is based on the reverse principle of the Stirling engine.

The animation below shows the relative positions of the opposed pistons (P1 and P2) in the compressor module and the displacer D in the cold finger, at four points during the Stirling cycle. The displacer contains a regenerative heat-exchanger or “regenerator” R.

The pistons are driven by the linear motors, producing gaspressure fluctuations which act on the spring-supported displacer.

• Phase 1:
  Virtually all the gas is in the compression space at ambient temperature and the displacer is in the tip of the cold finger. In this phase the pistons are driven inwards, compressing the gas. This proces is nearly isothermal, the heat output Qc being dissipated via heat sinks around the compressor and the base of the cold finger.
• Phase 2:
  The pistons have reached the end of the compression stroke, the gas in the compression space is at ambient temperature and the displacer has not yet moved. This is the situation at the start of Phase II. Throughout this phase the pistons remain stationary and hence the total volume of gas remains constant. The displacer moves downwards as its spring compresses and gas flows through the regenerator, giving up heat Qr in the proces. This heat is stored in the renegerator until later in the cycle.
• Phase 3:
  The pistons are driven outwards and the gas expands. This expansion process, too, is nearly isothermal, the heat input Qe being drawn from the surroundings of the expansion space. As a result refrigeration occurs at the tip of the cold finger.
• Phase 4:
  Throughout this phase the pistons remain stationary. The displacer, however, moves upwards because of the lower gas pressure in the expansion space. Gas from the expansion space therefor flows back through the regenerator, taking up the stored heat Qr in the process and re-entering the compression space at ambient temperature.

This “gas spring” system is tuned to provide the correct phase relationship between the displacer and the pistons movements.

Refrigeration occurs around the tip op the cold finger, which contains an “expansion space”. The displacer moves gas into and out of this space from a “compression space” consisting of the space between the pistons, the space in the split tube and the space below the warmer end of the displacer.

The working of the rotary coolers is according to the same Stirling principle. The main difference is that the compression piston is driven in a different way. In the rotary coolers the piston is driven by rotary motor and crankshaft while in the linear coolers they are driven by linear coil magnet motor.