A Stirling engine and generator set with 55 kW electrical output, for combined heat and power applications. Click image for detailed description.

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. Image File history File links Download high resolution version (1600x1200, 524 KB)A Stirling engine and induction generator set. ... Image File history File links Download high resolution version (1600x1200, 524 KB)A Stirling engine and induction generator set. ... A heat engine performs the conversion of heat energy to work by exploiting the temperature gradient between a hot source and a cold sink. Heat is transferred to the sink from the source, and in this process some of the heat is converted into work. ... An external combustion engine is an engine which burns its fuel to heat a separate working fluid which then in turn performs work. ... Royal motto: Nemo me impune lacessit (Latin: No one provokes me with impunity) Scotlands location within the UK Languages with Official Status1 English Scottish Gaelic Capital Edinburgh Largest city Glasgow First Minister Jack McConnell Area - Total - % water Ranked 2nd UK 78,782 km² 1. ... Reverend Dr Robert Stirling The Reverend Dr Robert Stirling (October 25, 1790 - June 6, 1878) was a Scottish clergyman, and inventor of a highly efficient heat engine. ... 1816 was a leap year starting on Monday (see link for calendar). ...

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. A steam engine is a heat engine that makes use of the thermal energy that exists in steam, converting it to mechanical work. ... A boiler is a closed vessel in which water or other fluid is heated under pressure. ...

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 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. Air is a name for the mixture of gases present in the Earths atmosphere. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... The gas laws are a set of laws that describe the relationship between absolute temperature (T), pressure (P) and volume (V) of gases. ... Pressure is the application of force to a surface, and the concentration of that force in a given area. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Volume, also called capacity, is a quantification of how much space an object occupies. ... piston + connecting rod In general, a piston is a sliding plug that fits closely inside the bore of a cylinder. ...

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.

Engine

Some Stirling engines use a separate displacer piston to move the working gas back and forth between cold and hot reservoirs. Others rely on interconnecting the power pistons of multiple cylinders to move the working gas, with the cylinders held at different temperatures.

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). The Carnot heat engine uses a particular thermodynamic cycle studied by Nicolas Léonard Sadi Carnot in the 1820s and expanded upon by Benoit Paul Émile Clapeyron in the 1830s and 40s. ... Thermodynamic efficiency (e) is defined as: where W is the absolute value of the work done in one thermodynamic cycle Q is the absolute value of the change in heat done in one cycle For heat engines, Sadi Carnot derived efficiency as a function of the temperature of its hot... An internal combustion engine is an engine that is powered by the expansion of hot combustion products of fuel directly acting within an engine. ... The diesel engine is a type of internal combustion engine; more specifically, it is a compression ignition engine, in which the fuel is ignited by being suddenly exposed to the high temperature and pressure of a compressed gas containing oxygen (usually atmospheric air), rather than a separate source of ignition...

Stirling engines will also work in reverse: when applying motion to the shaft, a temperature difference appears between the reservoirs. One of their modern uses is in refrigeration and cryogenics. Refrigeration (from the Latin frigus, frost) is generally the cooling of a body by the transfer of a portion of its heat away from it. ... Cryogenics is the study of very low temperatures or the production of the same, and is often confused with cryobiology, the study of the effect of low temperatures on organisms, or the study of cryopreservation. ...

Configurations

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])

Changes to the configuration of mechanical Stirling engine continues to interest engineers and inventors alike. Notably, some are in hot pursuit of the rotary Stirling engine. The goal is to convert power from the Stirling cycle directly into torque. A similar goal to that of the design rotary combustion engine. A heat exchanger is a device for transferring heat from one fluid to another, where the fluids are separated by a solid wall so that they never mix. ... Spoked flywheel A flywheel is a heavy rotating disk used as a repository for angular momentum. ... Many types of rotary combustion engine, like the Quasiturbine or the Wankel engine, have been devised [1], all having the same basic concept; to avoid the reciprocating motion of the piston with its inherent vibration and rotational-speed-related mechanical stress. ...

There is also a field of "Free piston" stirling cycles engines, including those with liquid pistons and those with diaphrams as pistons. [See the book "Free Piston Stirling Cycle Engines" by G. Walker]

An alternative to the mechanical Stirling engine is the fluidyne, which uses the Stirling cycle via a hydraulic piston. In its most basic form it contains a working gas, a liquid and two check valves for moving parts.

Heat sources

Any temperature difference will power a Stirling engine and the term "external combustion engine" often applied to it is misleading. A heat source may be the result of combustion but can also be solar, geothermal , or nuclear. Likewise a "cold source" below the ambient temperature can be used as the temperature difference. A cold source may be the result of a cryogenic fluid or iced water. Since small differential temperatures require large mass flows, parasitic losses in pumping the heating or cooling fluids rise and tend to reduce the efficiency of the cycle. Combustion or burning is an exothermic reaction between a substance (the fuel) and a gas (the oxidizer), usually O2, to release heat. ... Solar power describes a number of methods of harnessing energy from the light of the sun. ... Geothermal power is electricity generated by utilizing naturally occurring geological heat sources. ... This article is about power derived from nuclear reactions. ... Cryogenics is the study of very low temperatures or the production of the same, and is often confused with cryobiology, the study of the effect of low temperatures on organisms, or the study of cryopreservation. ...

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. Natural gas rig Natural gas (commonly refered to as gas in many countries) is a gaseous fossil fuel consisting primarily of methane. ... Siloxanes are a class of both organic and inorganic chemical compounds which consist entirely of silicon, oxygen, and an alkyl group. ... The chemical compound silicon dioxide, also known as silica, is the oxide of silicon, chemical formula SiO2. ...

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 greatly reduced.
• Most types of Stirling engines have the bearing and seals on the cool side, consequently they require less lubricant and last significantly longer 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.
• Low operating pressure means less robust cylinders and less weight.
• They can be built to run very quietly and without air, for use in submarines.

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 especially when they are designed to the high level of "effectiveness" (heat exchanger efficiency) needed for optimizing fuel economy.
• 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.
• Dissipation of waste heat is especially complicated because the coolant temperature is kept as low as possible to maximize thermal efficiency. This drives up the size of the radiators markedly which can make packaging difficult. This has been one of the factors limiting the adoption of Stirling engines as automotive prime movers.
• 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. Typically this is done by varying the displacement of the engine (often through use of a swashplate crankshaft arrangement) or by changing the mass of entrained working fluid (generally helium or hydrogen).
• Hydrogen's lowest molecular weight makes it the best working fluid to use in a Stirling engine, but as a tiny molecule, it is very hard to keep it inside the engine and auxiliary systems need to be typically added to maintain the proper quantity of working fluid. These systems can be as simple as a gas storage bottle or more complicated such as a gas generator. In any event, they add weight, increase cost, and introduce some undesirable complications.

A heat exchanger is a device for transferring heat from one fluid to another, where the fluids are separated by a solid wall so that they never mix. ...

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. Events January 26 - Treaty of Karlowitz signed March 30 - the tenth Sikh Master, Guru Gobind Singh created the Khalsa. ... Sir George Cayley (27 December 1773 - 15 December 1857) was an exuberant polymath from Brompton-by-Sawdon, near Scarborough in Yorkshire. ... 1807 was a common year starting on Thursday (see link for calendar). ... 1816 was a leap year starting on Monday (see link for calendar). ...

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. A heat pump is a machine which moves heat from a low temperature reservoir to a higher temperature reservoir under supply of work. ... Refrigeration (from the Latin frigus, frost) is generally the cooling of a body by the transfer of a portion of its heat away from it. ...

Kockums[4], the Swedish shipbuilder, had built at least 10 commercially successful Stirling powered submarines during the 80's. They are almost as undetectable as a nuclear submarine, but lack the endurance since they need oxygen to burn fuel. As of 2005 they have started to bring compressed oxygen with them. (No endurance stated.) Kockums in Malmö, 1970 Foto: Pål-Nils Nilsson. ...

On August 11, 2005, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems[5] over a twenty year period and in quantity (20,000 units) sufficient to generate 500 megawatts of electricity. These systems - to be installed on a 4,500 acre (19 km²) solar farm - will use mirrors to direct and concentrate sunlight onto the engines which will in turn drive generators. Southern California Edison, the largest subisdiary of Edison Internationl (NYSE: EIX), is the primary electicity supply company for much of the lower half of the State of California, the region known as Southern California. ... Solar power describes a number of methods of harnessing energy from the light of the sun. ... http://www. ...

External links

How it works

• How Stuff Works: Stirling-engine A good description with animated diagrams.

• How it works by Amitabha Mukerjee
• Animations:
  • Alpha type machine,
  • Alpha type machine with Ross yoke,
  • Beta type machine,
  • Gamma type machine
  • Stirling fly motor animation

History

• Amitabha Mukerjee: Stirling Engine History,

Academic and technical studies

• David Haywood University of Canterbury NZ: "Introduction to Stirling-Cycle Analysis" (PDF)
• Stirling-Cycle Research Group, University of Canterbury NZ
• University of Ohio
• Israel Urieli: Stirling Engine Simple Analysis,
  • Alpha Stirlings,
  • Beta Stirlings,
  • Gamma Stirlings
• Peter Fette: Stirling Engine Researcher, mirror
  • Animation,
  • Counterblow heat exchanger description
  • Stirling Engine with 8 cylinders, twice double acting
• Argument on why the Stirling engine can be applied in aviation, mirror

Societies and conferences

• The Stirling Engine Society
• Stirling News - newsletter published quarterly in the UK by The Stirling Engine Society
• Programme of the 10th Stirling Engine Conference 2001

Hobbyists and enthusiasts

• Webring for Sterling engine Hobbyists
• Simple Do-It-Yourself Stirling engine This only requires a temperature difference of 8°C to run. A hot hand and/or an ice cube is enough to keep it running.
• Japanese hobbyist's Stirling engine home page
• How to build a Stirling engine from a test-tube
• Melbourne Society of Model & Experimental Engineers Journal A novel Stirling cycle hot air engine to build
• Will Rausch - Stirling engine enthusiast Many links
• Robert Sier - Stirling engine enthusiast Many links
• Rotary Stirling Engines enthusiast website

Professional manufacturers

• Whispertech home-scale generators, Micro Combined Heat and Power MCHP
• American Stirling Company - Power Producing Engines
• Stirling Technology, Inc. Energy recovery ventilator technology
• Stirling Cryogenics & Refrigeration BV,
  • SPC: Stirling Process Cryogenerator,
  • SGL: The Stirling gas liquefier, **StirLIN: Stirling liquid nitrogen production plants,
  • StirLOX: Stirling liquid oxygen production plants,
  • Power Coolers
• STM Power Combined heat and power Stirling engine-generator sets
• QRMC Sterling engine manufacturer
• Thales Cryogenics

Directories & indexes

• Stirling Engines page on the Open Directory Project.

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

The Open Directory Project (ODP), also known as DMoz (for Directory. ...

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

G. Walker, "Free Piston Stirling Cycle Engines", Springer-Verlag (1985), ISBN: 0387154957