|
|
|
|
|
Stirling EngineThe Stirling engine is a type of external combustion piston engine, invented in 1816 by the Rev. Robert Stirling and Engineer 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 primitive materials. Stirling engines convert any temperature differential directly to movement: they use a displacer piston to move enclosed air(or other gas) back and forth between cold and hot reservoirs. At the hot reservoir, the gas expands and pushes a power piston, producing work and displacing the air to the cold reservoir. There the gas contracts and pulls the power piston, closing the cycle. The working fluid in a Stirling engine can be air, or other gases such as hydrogen or helium. In Stirling engines a "regenerator," typically a mesh 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 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). Because a heat exchanger separates the working fluid from the heat source, a wide range of fuels can be used, or the engine can be adapted to run on waste heat from other processes. 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. Stirling engines can also work in reverse: when applying motion, a temperature differential appears between the reservoirs. Incidentally, one of their modern uses is in cryogenics. 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.
- 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.
- A "pure" Stirling engine cannot start instantly; it literally needs to "warm up". So do most internal combustion engines. 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. However, increased complexity can change even that.
Strengths with Stirling engines - Because the heat is external the burning of the fuel air mixture can be more accurately controlled.
- A continous combustion process can be used to supply heat, so emision of unburned fuel can be eliminated.
- Because most types of Stirling engines have the bearing and seals on the cool side they require less lubricant and last a very significant longer period of time between overhalls.
- The engine as a whole is much less complex. No valves are needed. Fuel and intake systems are very simple.
- They won't blow up as steam engines.
Stirling engine types Stirling Engines come in three distinct types: - An alpha Stirling contains two separate power pistons, one "hot" piston and one "cold" piston. The hot piston is situated after the higher temperature heat exchanger and the cold piston is situated after 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.
- A beta Stirling has a single power piston arranged coaxially with a displacer piston. The displacer piston 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. This engine does not require moving seals in the hot portion of the engine and can achieve high compression ratios due to pistons being able to overlap in their motions.
- A gamma Stirling is simply a beta Stirling in which the power piston is not mounted coaxially to its displacer piston. This configuration produces a lower compression ratio but is often mechanically simpler and often used in multi-cylinder Stirling engines.
External links Indexes How it works - How Stuff Works: Stirling-engine
- David Haywood University of Canterbury: "Introduction to Stirling-Cycle Analysis" (PDF), Stirling-Cycle Research Group
- About The Stirling Heat Engine
- (Good information for builders) Why Aviation Needs the Stirling Engine by Darryl Phillips, main address, mirror Quote: "...This 4-part series appeared in the March 1993 through March 1994 issues of Stirling Machine World...Common four cylinder engines such as Lycoming and Continental show torque that varies from a negative 100% to a positive 350% of the nominal torque...A Stirling with the same number of cylinders and identical power has a torque variation of +/- 5%!..."
- Stirling fly motor animation
- Animations: Ross type, Beta type, Alpha type, Flash Animation
- Israel Urieli: Stirling Engine Simple Analysis, main address, Alpha Stirlings, Beta Stirlings, Gamma Stirlings
- Peter Fette: Stirling Engine Research and Computer (simulation) Program Development, animation, prozess, mirror
- Quote: "...One possibility of equalizing the regenerators loss in double acting engines is to design it as a counterblow heat exchanger as described in [1..."]
- Quote: "...This Stirling Engine with 8 cylinders is twice double acting. Its special highlight is the facility for the heat transfer from a liquid water to the working fluid air which results in extremely low temperature losses....Because of the nearly isothermalized heat transfer the efficiency is near Carnot's ..."
- Amitabha Mukerjee: Stirling Engine, usage, How does it work? Quote: "...As a final note a solar powered Stirling engine coupled with a generator achieved a record solar-to-electric efficiency of 30%!"
- http://www.keveney.com/Engines.html A fine collection of animations of how different engines work.
Information media Do-It-Yourself model Stirling/Hot-Air machines Applications 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
|
 |
| |
|
|