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Stirling engine

The Stirling engine, also known as the hot air engine, is a heat engine of the external combustion piston engine type. 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.

Table of contents

General description

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 sealed amount of working gas, usually air or other gases such as hydrogen or helium. The gas follows the behaviour described by the gas laws which describe how a gases’ 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 exhangers.

The working gas is sealed within the piston cylinders, so there is no exhaust gas other than that incidental to heat production. No valves are required, unlike other types of piston engines.

Some Stirling engines use a separate displacer piston to move the working gas back and forth between cold and hot ends. Others rely on interconnecting the power pistons of multiple cylinders to move the working gas.

In true Stirling engines a regenerator, typically a mass of wire, is located between the reservoirs. As the gas cycles between the hot and cold sides, its heat is transferred to and from the regenerator. In some designs, the displacer piston is itself the regenerator. This regenerator contributes to the efficiency of the Stirling cycle.

The ideal Stirling engine cycle has the same theoretical efficiency as a Carnot heat engine for the same input and output temperatures. The thermodynamic efficiency is higher than steam engines (or even some modern internal combustion and Diesel engines).

Stirling engines will also work in reverse: when applying motion, a temperature differential appears between the reservoirs. One of their modern uses is in cryogenics.

Stirling engine types

Engineers classify Stirling engines into three distinct types:

  • An alpha Stirling contains two separate power pistons in separate cylinders, one "hot" piston and one "cold" piston. The hot piston cylinder is situated inside the higher temperature heat exchanger and the cold piston cylinder is situated inside the low temperature heat exchanger. This type of engine has a very high power-to-volume ratio but has technical problems due to the usually high temperature of the "hot" piston and its seals. (See animation here [1])
  • A beta Stirling has a single power piston arranged within the same cylinder on the same shaft as a displacer piston. The displacer piston is a loose fit and does not extract any power from the expanding gas but only serves to shuttle the working gas from the hot heat exchanger to the cold heat exchanger. When the working gas is pushed to the hot end of the cylinder it expands and pushes the power piston. When it is pushed to the cold end of the cylinder it contracts and the momentum of the machine , usually enhanced by a flywheel pushes the power piston the other way to recompress the gas. This engine does not require moving seals in the hot portion of the engine and so can achieve higher compression ratios. (See animation here [2])
  • A gamma Stirling is simply a beta Stirling in which the power piston is mounted in a separate cylinder alongside the displacer piston cylinder, but is still connected to the same shaft. The gas in the two cylinders can flow freely between them and remains a single body. This configuration produces a lower compression ratio but is mechanically simpler and often used in multi-cylinder Stirling engines. (See animation here [3])
A Stirling engine and generator set 55 kW electrical output, for combined heat and power applications

Heat sources

Any heat source will power a Stirling engine and the term "external combustion engine" often applied to it is misleading. The heat source may be the result of combustion but can also be solar, geothermal , or nuclear.

Because a heat exchanger separates the working gas from the heat source, a wide range of combustion fuels can be used, or the engine can be adapted to run on waste heat from some other process. Since the combustion products do not contact the internal moving parts of the engine, a Stirling engine can run on landfill gas containing siloxanes without the accumulation of silica that damages internal combustion engines running on this fuel. The life of lubricating oil is longer than for internal-combustion engines.

Strengths of Stirling engines

  • The heat is external and the burning of a fuel air mixture can be more accurately controlled.
  • A continuous combustion process can be used to supply heat, so emission of unburned fuel can be eliminated.
  • Most types of Stirling engines have the bearing and seals on the cool side consequently they require less lubricant and last a very significant longer period of time between overhauls than other reciprocating engine types.
  • The engine as a whole is much less complex than other reciprocating engine types. No valves are needed. Fuel and intake systems are very simple.
  • They operate at relatively low pressure and won't blow up like steam engines.

Problems with Stirling engines

  • Stirling engines require both input and output heat exchangers which must contain the pressure of the working fluid, and which must resist any corrosive effects due to the heat source. These increase the cost of the engine.
  • Stirling engines, especially the type that run on small temperature differentials, are quite large for the amount of power that they produce, due to the heat exchangers.
  • A "pure" Stirling engine cannot start instantly; it literally needs to "warm up". This is also the case for internal combustion engines, but the warm up time may be shorter than for Stirlings.
  • Power output of a Stirling is constant and hard to change rapidly from one level to another.

History and development

Devices called air engines have been recorded from as early as 1699 around the time when the laws of gasses were first set out. The English inventor Sir George Caley is known to have devised air engines c. 1807. Robert Stirling's innovative contribution of 1816 was what he called the 'Economiser' now known as the regenerator which acts to retain heat in the hot portion of the engine as the air passes to the cold part and thus improve the efficiency.

During the nineteenth century the Stirling engine found applications anywhere a source of low to medium power was required , a role that was eventually usurped by the electric motor at the century's end.

It was also employed in reverse as a heat pump to produce early refrigeration.

External links

How it works

History

Academic and technical studies

Societies and conferences

Hobbyists and enthusiasts

Melbourne Society of Model & Experimental Engineers Journal A novel Stirling cycle hot air engine to build

Professional manufacturers


Directories & indexes

  • Stirling Engine Directory – List of resources including "how it works", demo units, kits, applications, project sites. (From FreeEnergyNews.com)

References

Gordon J. Van Wylan and Richard F. Sontag, "Fundamentals of Classical Thermodynamics SI Version 2nd Ed.", John Wiley and Sons, New York, 1976, ISBN 0471041882








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