LTD ENGINE (Machine)

What is Stirling Engine: 

A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work.

The engine is like a steam engine in that all of the engine's heat flows in and out through the engine wall. This is traditionally known as an external combustion engine in contrast to an internal combustion engine where the heat input is by combustion of a fuel within the body of the working fluid. Unlike the steam engine's use of water in both its liquid and gaseous phases as the working fluid, the Stirling engine encloses a fixed quantity of permanently gaseous fluid such as air or helium. As in all heat engines, the general cycle consists of compressing cool gas, heating the gas, expanding the hot gas, and finally cooling the gas before repeating the cycle.

History

 The Stirling engine (or Stirling's air engine as it was known at the time) was invented and patented by Robert Stirling 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 pressurization which had by 1843 sufficiently increased power output to drive all the machinery at a Dundee iron foundry.

Though it has been disputed it is widely supposed that 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.

 Types of Stirling Engines

There are two major types of Stirling engines that are distinguished by the way they move the air between the hot and cold sides of the cylinder:

1. The two piston alpha type design has pistons in independent cylinders, and gas is driven between the hot and cold spaces.

2.The displacement type Stirling engines, known as beta and gamma types, use an insulated mechanical displacer to push the working gas between the hot and cold sides of the cylinder. The displacer is large enough to thermally insulate the hot and cold sides of the cylinder and displace a large quantity of gas. It must have enough of a gap between the displacer and the cylinder wall to allow gas to easily flow around the displacer.

  Alpha Stirling

An alpha Stirling contains two power pistons in separate cylinders, one hot and one cold. The hot cylinder is situated inside the high temperature heat exchanger and the cold cylinder is situated inside the low temperature heat exchanger. This type of engine has a 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. 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

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 and remains a single body. This configuration produces a lower compression ratio but is mechanically simpler and often used in multi-cylinder Stirling engines.

 The Development of Low Temperature Differential Stirling Engines

In the spring of 1983, prof. Ivo Kolin of university of Zagreb in Croatia pleasantly started the stirling engine world by publicity exhibiting an engine running on heat of boiling hot water. The setting was at a short course on stirling engines taught by Prof. Kolin, Prof. Walker, and myself at the inter university center in the historic coasts city of Dubrovnik. While Prof. Kolin described to the audience the engine that he had been developing about three years, his wife, Vlasta, devotedly poured boiling hot water into one compartment of he engine and cold water into another.

At the time, 100°C in itself was an incredibly low temperature difference for a stirling engine run on. It was all the more astonishing that the engine continued to run for a long time at lower and lower temperature differentials between the water reservoirs. Running slower and slower as the hot water cooled down and the cool water warmed up, the engine finally came to a complete stop when the temperature differential dropped below about 20°C (36°F).

Prof. Kolin’s engine was built entirely with hand tools. It featured a square displacer chamber and a rubber diaphragm in place of a piston and cylinder. The Styrofoam displacer was 20 cm (7-7/8 in) square.  A unique feather of this engine was a “slip link” drive for the displacer which gave it an intermittent motion; this type if motion is thermally beneficial in slow moving engines. A speed of 50 rpm was typical for his engine with a temperature difference of 50°C in its reservoirs. This first LTD engine is completely described in Ivo koflin’s book isothermal stirling cycle engine which even includes fully dimensioned drawings (in metric units) so that anyone interested can make an exact replica.

In the fall of 1983 the first ringbom type of LTD Stirling engine was built at Argonne National Laboratory. This engine design it introduce a round horizontally oriented displacer chamber which could be placed over a container of hot water for the heat source. The displacer of the engine was about 8.5” in diameter and was driven by a small piston and cylinder unit to grove an intermittent motion with a phasing the varied with the engine speed. The main piston drives features a rocking lever which the freed piston of virtually all side loading for low friction and wear.

The Argonne engine proved to be a good demonstrator of stirling engine operation, with about to cups of near boiling hot water poured into the lower insulated reservoir and like amount of ice water placed on the top of the displacer chamber, the engine starts after few minutes and run about 15 minutes. When set to a low compression ratio the engine will run at a temperature difference of 7°C. The engine resides at the laboratory under the watchful care of senior scientist Dr. Paul Roach where it is favorite attraction for visitors.

From these first two engine prof. Kolin and the James R. senft work parallel over the next decayed each developing a series of LTD engine.

There are three different type of engine were introduced L-27 solar rinbom engine, P-19 ultra low temperature differential engine, N-92 NASA demonstration engine whose construction is descried in this report.

Source : Report, Collection form Web, books, etc.