Stirling engine

The Stirling engine is a heat engine of the external combustion piston engine type. It was invented and developed by Reverend Dr Robert Stirling in 1816.

A well-designed Stirling engine can achieve 50% to 80% of the ideal efficiency in the conversion of heat into mechanical work, limited only by friction and material properties. The engines can theoretically run on any heat source of sufficient temperature, including solar energy, chemical and nuclear fuels.

While the Stirling engine is more expensive and larger than an internal combustion engine of the same power rating, its many unique advantages make it preferred for a variety of niche applications. Compared to internal combustion engines, Stirling engines can be made very energy efficient, quiet, reliable, long-lasting and low-maintenance. In recent years, these advantages have become increasingly significant given the general rise in energy costs and the environmental concerns of climate change. This growing interest in Stirling technology has led to the ongoing development of Stirling devices for many applications, including renewable power generation and Astronautics.

Functional Description


The engine cycle

Since the Stirling engine is a closed cycle, it contains a fixed quantity of gas called a "working fluid", most commonly air, hydrogen or helium. In normal operation, the engine is sealed and no gas enters or leaves the engine. No valves are required, unlike other types of piston engines. The Stirling engine, like most heat-engines, cycles through four main processes: cooling, compression, heating and expansion. This is accomplished by moving the gas back and forth between hot and cold heat exchangers. The hot heat exchanger is in thermal contact with an external heat source, e.g. a fuel burner, and the cold heat exchanger being in thermal contact with an external heat sink, e.g. air fins. A change in gas temperature will cause a corresponding change in gas pressure, while the motion of the piston causes the gas to be alternately expanded and compressed.

The gas follows the behavior described by the gas laws which describe how a gas's 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 compress the gas on the return stroke, thus yielding a net power output.

When one side of the piston is open to the atmosphere, the operation of the cold cycle is slightly different. As the sealed volume of working gas comes in contact with the hot side, it expands, doing work on both the piston and on the atmosphere. When the working gas contacts the cold side, the atmosphere does work on the gas and "compresses" it. Atmospheric pressure, which is greater than the cooled working gas, pushes on the piston.

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 hot and cold sources, the greater the power produced, and thus, the lower the efficiency required for the engine to run.

Small demonstration engines have been built which will run on a temperature difference of around 15 °C, e.g. between the palm of a hand and the surrounding air, or between room temperature and melting water ice.



The Regenerator

In true Stirling engines a regenerator, typically a mass of metal wire, is located in the path of the gas between the hot and cold heat exchangers. As the gas cycles between the hot and cold sides, its heat is temporarily transferred to and from the regenerator. In some designs, there is a displacer piston but no regenerator. The displacer piston does not have a seal, and with loose fit tolerances a small air gap between the piston and the cylinder allows the gas to flow around the displacer as it is displaced to the other end of the cylinder. In some designs, the surfaces of the displacer and cylinder alone can provide some regeneration. The regenerator contributes greatly to the overall efficiency and power produced by the Stirling engine. The regenerator was the key feature invented by Robert Stirling in 1816 which greatly improved his machine and distinguished it from other "hot air engines".

The regenerator is a reverse flow heat exchanger, which tends to improve thermal efficiency wherever it is found in technology or in nature.



Engine configurations

The Beta and Gamma type Stirling engines use a displacer piston to move the working gas back and forth between hot and cold heat exchangers. The alpha type engine relies on interconnecting the power pistons of multiple cylinders to move the working gas, with the cylinders held at different temperatures.

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 varies, but can be higher than steam engines and many modern internal combustion engines (Diesel or Gasoline ).


Engineers classify Stirling engines into three distinct types:


Alpha Stirling

* 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 the durability of its seals.


Beta Stirling

* 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 compress the gas. Unlike the alpha type, the beta type avoids the technical problems of hot moving seals.



Gamma Stirling

* 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 flywheel. The gas in the two cylinders can flow freely between them but remains a single body. This configuration produces a lower compression ratio but is mechanically simpler and often used in multi-cylinder Stirling engines.



Other types

Changes to the configuration of mechanical Stirling engines continue to interest engineers and inventors. Notably, some are in pursuit of the rotary Stirling engine; the goal here is to convert power from the Stirling cycle directly into torque, a similar goal to that which led to the design of the rotary combustion engine. No practical engine has yet been built but a number of concepts, models and patents have been produced.

There is also a field of "free piston" Stirling cycles engines, including those with liquid pistons and those with diaphragms as pistons.

An alternative to the mechanical Stirling engine is the fluidyne pump, which uses the Stirling cycle via a hydraulic piston. In its most basic form it contains a working gas, a liquid and two non-return valves. The work produced by the fluidyne goes into pumping the liquid.



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 or even biological. 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.

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.

The U.S. Department of Energy in Washington, NASA Glenn Research Center in Cleveland, and Stirling Technology Co. of Kennewick, Wash., are developing a free-piston Stirling converter for a Stirling Radioisotope Generator. This device would use a plutonium source to supply heat.



History and development

Invention of the Stirling engine is credited to the Scottish clergyman Rev. Robert Stirling who, in 1816, 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.

Devices called air engines have been recorded from as early as 1699 around the time when the laws of gases were first set out. The English inventor Sir George Cayley 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, it stored heat from the hot portion of the engine as the air passed to the cold side, and released heat to the cooled air as it returned to the hot side. This innovation improved the efficiency of Stirling's engine enough to make it commercially successful in particular applications, and has since been a component of every air engine that is called a Stirling engine.

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.

In the late 1940s, the Philips Electronics company in The Netherlands was searching for a versatile electricity generator to enable worldwide expansion of sales of its electronic devices in areas with no reliable electricity infrastructure. The company put a huge R&D research effort into Stirling engines building on research it had started in the 1930s and which lasted until the 1970s. The only lasting commercial product for Philips was its reversed Stirling engine: the Stirling cryocooler.

Los Alamos National Laboratory has developed an "Acoustic Stirling Heat Engine" with no moving parts. It converts heat into intense acoustic power which (quoted from given source) "can be used directly in acoustic refrigerators or pulse-tube refrigerators to provide heat-driven refrigeration with no moving parts, or ... to generate electricity via a linear alternator or other electroacoustic power transducer".



The Stirling Cycle

The ideal stirling cycle consists of four thermodynamic processes acting on the working fluid:

* 1. Isothermal Compression
* 2. Constant-Volume (or isometric) heat-addition
* 3. Isothermal Expansion
* 4. Constant-Volume (or isometric) heat-removal