Stirling Engine History more
Stirling Engine Name
Though it had been suggested as early as 1884 that all closed cycle air engine should be generically called Stirling engines after the inventor of the first practical example (see below), the idea found little favour and the various types on the market continued to be known by the name of their individual designer or manufacturer. Then, in the 1940s, the Philips company was searching for a suitable name for its version of the 'air' engine which by that time had already been tested with other gases. Rejecting many suggestions, including 'hot gas engine' ('gas engine' was already in general use for internal combustion engines running on gaseous fuels) and 'external combustion engine' (did not differentiate between open and closed cycles), Philips eventually settled on 'Stirling engine' in April 1945, though general acceptance of the term seems to have lagged a few years behind.
Stirling Engine Early years
The Stirling engine (or Stirling's air engine as it is was known at the time) was invented by Reverend Dr. Robert Stirling and patented by him in 1816. It followed earlier attempts at making an air engine but was probably the first to be put to practical use when in 1818 an engine built by Stirling was employed pumping water in a quarry. The main subject of Stirling's original patent was a heat exchanger which he called an "economiser" for its enhancement of fuel economy in a variety of applications. The patent also described in detail the employment of one form of the economiser in his unique closed-cycle air engine design in which application it is now generally known as a 'regenerator'. Subsequent development by Robert Stirling and his brother James, an engineer, resulted in patents for various improved configurations of the original engine, including pressurisation which by 1843 had sufficiently increased the power output for it to drive all the machinery at a Dundee iron foundry.
As well as saving fuel, the inventors were motivated to create a safer alternative to the steam engines of the time, whose boilers frequently exploded causing many injuries and fatalities. The need for Stirling engines to run at very high temperatures to maximize power and efficiency exposed limitations in the materials of the day and the few engines that were built in those early years suffered unacceptably frequent failures (albeit with far less disastrous consequences than a boiler explosion) - for example, the Dundee foundry engine was replaced by a steam engine after three hot cylinder failures in four years.
Stirling Engine Later nineteenth century developments
Steam boilers were eventually made much safer and development increased the efficiency of the engines themselves, but, though the Stirling engine did not succeed as a competitor to the steam engine as an industrial scale prime mover, during the latter nineteenth and early twentieth centuries smaller engines of the Stirling/hot air type were produced in substantial numbers, finding application wherever a reliable source of low to medium power was required, such as raising water or providing air for church organs. These generally operated at lower temperatures so as not to tax available materials and thus tended to be rather inefficient, their major selling point being that unlike a steam engine they could be operated safely by anybody capable of managing a fire. Several types remained in production beyond the end of the century but apart from a few minor mechanical improvements the design of the stirling engine in general stagnated during this period.
Stirling Engine Twentieth century revival
During the early part of the twentieth century the role of the Stirling engine as a 'domestic motor' was gradually usurped by the electric motor and small internal combustion engines until by the late 1930s it was largely forgotten, only produced for toys and a few small ventilating fans. At this time Philips was seeking to expand sales of its radios into areas where mains electricity was unavailable and the supply of batteries uncertain. Philips’ management decided that offering a low-power portable generator would facilitate such sales and tasked a group of engineers at the company research lab (the Nat. Lab) in Eindhoven to evaluate the situation. After a systematic comparison of various prime movers the Stirling engine was considered to have real possibilities as it was among other things, inherently quiet (both audibly and in terms of radio interference) and capable of running from any heat source (common lamp oil was favored). They were also aware that, unlike steam and internal combustion engines, virtually no serious development work had been carried out on the Stirling engine for many years and felt that with the application of modern materials and know-how great improvements should be possible.
Encouraged by their first experimental engine, which produced 16 watts of shaft power from a bore and stroke of 30x25mm, a development program was begun. This work continued throughout World War II and by the late 1940s they had an engine – the Type 10 – which was sufficiently developed to be handed over to Philips’ subsidiary Johan de Witt in Dordrecht to be ‘productionised’ and incorporated into a generator set as originally intended. The result, rated at 200 watts electrical output from a bore and stroke of 55x27 mm, was designated MP1002CA (known as the 'Bungalow set'). Production of an initial batch of 250 began in 1951, but it became clear that they could not be made at a price that the market would support and the advent of transistor radios with their much lower power requirements meant that the original raison d'ętre for the set was disappearing. Only around 150 of these sets were eventually produced, some of which found their way into university and college engineering departments around the world giving generations of students a valuable introduction to the Stirling engine.
Philips went on to develop experimental Stirling engines for a wide variety of applications and continued to work in the field until the late 1970s, but only achieved any commercial success with the 'reversed Stirling engine' cryocooler. They did however take out a large number of patents and amass a wealth of information relating to Stirling engine technology which was subsequently licensed to other companies forming the basis of much of the development work in the modern era.
Stirling Engine Free-piston engines
Various Free-Piston Stirling ConfigurationsIn the early 1960s Professor W. T. Beale while at Ohio University, invented a free-piston version of the Stirling engine in order to overcome the intractable difficulty of effectively lubricating the crank mechanism of typical Stirling engines . While the invention of the basic free-piston Stirling engine is generally attributed to Beale, independent inventions of similar types of engines were made by E H Cooke-Yarborough and C West at the Harwell Laboratories of the UKAERE . G M Benson has also made important early contributions and has patented many novel free-piston configurations .
What appears to be the first mention of a Stirling cycle machine using freely moving components is a British patent disclosure in 1876 . This machine was envisaged as a refrigerator (i.e., the so-called reversed Stirling cycle) and the piston was therefore driven externally. The very first consumer product to utilize a free-piston Stirling device was a portable refrigerator manufactured by Twinbird Corporation of Japan and offered in the US by Coleman in 2004.
Stirling Engine Functional description
Stirling Engine Engine operation
Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the "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, often with a regenerator between the heater and cooler. The hot heat exchanger is in thermal contact with an external heat source, such as a fuel burner, and the cold heat exchanger being in thermal contact with an external heat sink, such as 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 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, its pressure drops below atmospheric pressure and the atmosphere pushes on the piston and does work on the gas.
To summarize, the Stirling engine uses the temperature difference between its hot end and cold end to establish a cycle of a fixed mass of gas, heated and expanded, and cooled and compressed, thus converting thermal energy into mechanical energy. The greater the temperature difference between the hot and cold sources, the greater the thermal efficiency. The maximum theoretical efficiency is equivalent to the Carnot cycle, however the efficiency of real engines is only a fraction of this value, even in highly optimized engines.
Video showing the compressor and displacer of a very small Stirling Engine in actionVery low-power engines have been built which will run on a temperature difference of as little as 7 °C, for example between the palm of a hand and the surrounding air, or between room temperature and melting water ice.
Stirling Engine Pressurization
In most high power Stirling engines, both the minimum pressure and mean pressure of the working fluid are above atmospheric pressure. This initial engine pressurization can be realized by a pump, or by filling the engine from a compressed gas tank, or even just by sealing the engine when the mean temperature is lower than the mean operating temperature. All of these methods increase the mass of working fluid in the thermodynamic cycle. All of the heat exchangers must be sized appropriately to supply the necessary heat transfer rates. If the heat exchangers are well designed and can supply the heat flux needed for convective heat transfer, then the engine will produce power in proportion to the mean pressure, as predicted by the West number, and Beale number.  In practice, the maximum pressure is also limited to the safe pressure of the pressure vessel. Like most aspects of Stirling engine design, optimization is multivariate, and often has conflicting requirements. 
Stirling Engine Lubricants and Friction
At high temperatures and pressures, the oxygen in air-pressurized crankcases, or in the working gas of hot air engines, tends to combine with any lubricating oil that may exist in the engine, resulting in a very serious explosion hazard. (At least one person has been killed this way.)
Lubricants also cause problems with clogging the heat exchangers, especially the regenerator. For these reasons, to minimize mechanical power losses and wear on sliding surfaces, preferred designs use non-lubricated, low-coefficient of friction materials (such as Rulon (plastic) or graphite), with low normal-forces on the moving parts, especially for sliding seals. Alternatively, sliding surfaces can be avoided altogether by using diaphragms for sealed pistons. These are some of the factors that allow Stirling engines to often have lower maintenance requirements and longer life than internal-combustion engines.
A Stirling engine and generator set with 55 kW electrical output, for combined heat and power applications. Click image for detailed description.
Stirling Engine The Stirling cycle
For a detailed description see the Stirling cycle thermodynamics section below
The idealized or "text book" Stirling cycle is a thermodynamic cycle with two isochores (constant volume) and two isotherms (constant temperature). It is the most efficient thermodynamic cycle capable of practical implementation in an engine - its theoretical efficiency equaling that of the hypothetical Carnot cycle. However real-world issues reduce the efficiency of actual engines, due to limits of convective heat transfer, and viscous flow (friction). There are also practical mechanical considerations, for instance a simple kinematic linkage may be favored over a more complex mechanism needed to replicate the idealized cycle. See also Stirling cycle
Stirling Engine The regenerator
Main article: Regenerative heat exchanger
In a Stirling engine, the regenerator is an internal heat exchanger and temporary store placed between the hot and cold spaces such that the working fluid passes through it first in one direction then the other. Its function is to retain within the system that heat which would otherwise be exchanged with the environment at temperatures intermediate to the maximum and minimum cycle temperatures, thus enabling the thermal efficiency of the cycle to approach the limiting Carnot efficiency defined by those maxima and minima.
The primary effect of regeneration in a Stirling engine is to greatly increase the thermal efficiency by 'recycling' internally heat which would otherwise pass through the engine irreversibly. As a secondary effect, increased thermal efficiency promises a higher power output from a given set of hot and cold end heat exchangers (since it is these which usually limit the engine's heat throughput), though, in practice this additional power may not be fully realized as the additional "dead space" (unswept volume) and pumping loss inherent in practical regenerators tends to have the opposite effect.
A regenerator is challenging to design. The ideal regenerator would be: a perfect insulator in one direction, a perfect conductor in another, have no internal volume yet infinite flow area and infinite surface area. As with the hot and cold exchangers, achieving a successful regenerator is a delicate balancing act between high heat transfer with low viscous pumping losses and low dead space. These inherent design conflicts are one of many factors which limit the efficiency of practical Stirling engines. A typical embodiment might consist of a stack of fine metal wire meshes, with low porosity to reduce dead space, and with the wire axes perpendicular to the gas flow to reduce conduction in that direction and to maximize convective heat transfer.
The regenerator is the key component invented by Robert Stirling and its presence distinguishes a true Stirling engine from any other closed cycle hot air engine. However, many engines with no apparent regenerator may still be correctly described as Stirling engines as, in the simple beta and gamma configurations with a 'loose fitting' displacer, the surfaces of the displacer and its cylinder will cyclically exchange heat with the working fluid providing a significant regenerative effect particularly in small, low-pressure engines.
Also see: Economiser
Stirling Engine Engine configurations
Engineers classify Stirling engines into three distinct types. 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 Beta and Gamma type Stirling engines use a displacer piston to move the working gas back and forth between hot and cold heat exchangers in the same cylinder.
Stirling Engine 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. (See animation here)
Stirling Engine Action of an alpha type Stirling engine
The following diagrams do not show internal heat exchangers in the compression and expansion spaces, which are needed to produce power. A regenerator would be placed in the pipe connecting the two cylinders. The crankshaft has also been omitted.