20-09-2008, 11:03 PM
The Stirling engine, is a heat engine of the external combustion piston engine type whose heat-exchange process allows for near-ideal efficiency in conversion of heat into mechanical movement by following the Carnot cycle as closely as is practically possible with given materials.
Its invention is credited to the Scottish clergyman Rev. Robert Stirling in 1816 who made significant improvements to earlier designs and took out the first patent. He was later assisted in its development by his engineer brother James Stirling.
The inventors sought to create a safer alternative to the steam engines of the time, whose boilers often exploded due to the high pressure of the steam and the inadequate materials. Stirling engines will convert any temperature difference directly into movement.
The Stirling engine works by the repeated heating and cooling of a usually sealed amount of working gas, usually air or other gases such as hydrogen or helium. This is accomplished by moving the gas between hot and cold heat exchangers, the hot heat exchanger being a chamber in thermal contact with an external heat source, e.g. a fuel burner, and the cold heat exchanger being a chamber in thermal contact with an external heat sink, e.g. air fins.
The gas follows the behaviour described by the gas laws which describe how a gas' pressure, temperature and volume are related. When the gas is heated, because it is in a sealed chamber, the pressure rises and this then acts on the power piston to produce a power stroke.
When the gas is cooled the pressure drops and this means that less work needs to be done by the piston to recompress the gas on the return stroke, giving a net gain in power available on the shaft. The working gas flows cyclically between the hot and cold heat exchangers.
The working gas is sealed within the piston cylinders, so there is no exhaust gas (other than that incidental to heat production if combustion is used as the heat source). No valves are required, unlike other types of piston engines.
To summarize, the Stirling engine uses the potential energy difference between its hot end and cold end to establish a cycle of a fixed amount of gas expanding and contracting within the engine, thus converting a temperature difference across the machine into mechanical power.
The greater the temperature difference between the heat source and cold source, the easier it is for the Stirling engine to operate and the less efficient the design has to be for the engine to run.
But small demonstration engines have been built which will run on a temperature difference of around 15 degrees C, e.g. between the palm of a hand and the surrounding air, or between room temperature and melting water ice.
