Stirling engine

[citation needed] A 2-horsepower (1.5 kW) engine, built in 1818 for pumping water at an Ayrshire quarry, continued to work for some time until a careless attendant allowed the heater to overheat.

This engine moved all the machinery at the Dundee Foundry Company's works for eight or ten months, and was previously found capable of raising 320,000 kg (700,000 lbs) 60 cm (2 ft) in a minute, a power of approximately 16 kilowatts (21 horsepower).

When this engine had been in continuous operation for over two years it had not only performed the work of the foundry in the most satisfactory manner but had been tested (by a friction brake on a third mover) to the extent of lifting nearly 687 tonnes (1,500,000 pounds), approximately 34 kilowatts (45 horsepower).

[20] 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[21] in which application it is now generally known as a "regenerator".

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.

[33] Around that time, Philips was seeking to expand sales of its radios into parts of the world where grid electricity and batteries were not consistently available.

Philips' management decided that offering a low-power portable generator would facilitate such sales and asked a group of engineers at the company's research lab in Eindhoven to evaluate alternative ways of achieving this aim.

After a systematic comparison of various prime movers, the team decided to go forward with the Stirling engine, citing its quiet operation (both audibly and in terms of radio interference) and ability to run on a variety of heat sources (common lamp oil – "cheap and available everywhere" – was favored).

Additionally, the advent of transistor radios and their much lower power requirements meant that the original reason for the set was disappearing.

[36] Some found their way into university and college engineering departments around the world, giving generations of students a valuable introduction to the Stirling engine; a letter dated March 1961 from Research and Control Instruments Ltd. London WC1 to North Devon Technical College, offering "remaining stocks... to institutions such as yourselves... at a special price of £75 net".

They filed a large number of patents and amassed a wealth of information which they licensed to other companies and which formed the basis of much of the development work in the modern era.

On the surface, these boats are propelled by marine diesel engines; however, when submerged they use a Stirling-driven generator developed by Swedish shipbuilder Kockums to recharge batteries and provide electrical power for propulsion.

[citation needed] 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.

As a secondary effect, increased thermal efficiency yields a higher power output from a given set of hot and cold end heat exchangers.

[citation needed] The design challenge for a Stirling engine regenerator is to provide sufficient heat transfer capacity without introducing too much additional internal volume ('dead space') or flow resistance.

[citation needed] The displacer is a special-purpose piston, used in Beta and Gamma type Stirling engines, to move the working gas back and forth between the hot and cold heat exchangers.

The compression ratio is a bit smaller than in the original Alpha type engines, but the stress factor is pretty low on the sealed parts.

[52] In practice, this piston usually carries a large insulating head to move the seals away from the hot zone at the expense of some additional dead space.

[citation needed] Four basic steps in the cycle of a free-piston Stirling engine are:[citation needed] In the early 1960s, William T. Beale of Ohio University located in Athens, Ohio, invented a free piston version of the Stirling engine to overcome the difficulty of lubricating the crank mechanism.

The 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.

[citation needed] The ratio of the area of the heat exchangers to the volume of the machine increases by the implementation of a flat design.

[citation needed] Flat design of the working cylinder approximates thermal process of the expansion and compression closer to the isothermal one.

[citation needed] NASA has considered nuclear-decay heated Stirling Engines for extended missions to the outer solar system.

[70] The power piston in the displacer-type Stirling engine is tightly sealed and is controlled to move up and down as the gas inside expands.

This property is less of a drawback in hybrid electric propulsion or "base load" utility generation where constant power output is actually desirable.

For these reasons, designers prefer non-lubricated, low-coefficient of friction materials (such as rulon or graphite), with low normal forces on the moving parts, especially for sliding seals.

Other real-world issues reduce the efficiency of actual engines, due to the limits of convective heat transfer and viscous flow (friction).

There are also practical, mechanical considerations: for instance, a simple kinematic linkage may be favoured over a more complex mechanism needed to replicate the idealized cycle, and limitations imposed by available materials such as non-ideal properties of the working gas, thermal conductivity, tensile strength, creep, rupture strength, and melting point.

A further conclusion of this ideal cycle is that maximum efficiencies are found at lower compression ratios, a characteristic observed in real machines.