Q: How do Stirling Engines work?A: Stirling engines can be hard to understand. Here are the key points. EveryStirling engine has a sealed cylinder with one part hot and the other cold. Theworking gas inside the engine (which is often air, helium, or hydrogen) is movedby a mechanism from the hot side to the cold side. When the gas is on the hot sideit expands and pushes up on a piston. When it moves back to the cold side itcontracts. Properly designed Stirling engines have two power pulses perrevolution, which can make them very smooth running. Two of the more commontypes are two piston Stirling engines and displacer-type Stirling engines. The twopiston type Stirling engine has two power pistons. The displacer type Stirlingengine has one power piston and a displacer piston.

Displacer Type:The displacer type Stirling engine is shown here. The space below the displacerpiston is continuously heated by a heat source. The space above the displacerpiston is continuously cooled. The displacer piston moves the air (displaces theair) from the hot side to the cold side.

Displacer Engine Detail:

Click here for animation...

Gas expands when heated, and contracts when cooled. Stirling engines move the gas from thehot side of the engine, where it expands, to the cold side, where it contracts.

DISPLACER PISTONWhen there is a temperature difference between upper displacer space and lower displacerspace, the engine pressure is changed by the movement of the displacer. The pressure increaseswhen the displacer is located in the upper part of the cylinder (and most of the air is on the hotlower side). The pressure decreases when the displacer is moved to the lower part of the cylinder.The displacer only moves the air back and forth from the hot side to the cold side. It does notoperate the crankshaft and the engine. In other words, the connecting rod to the displacer couldbe a string in this engine and it would still work.

POWER PISTONWhen the engine pressure reaches its maximum because of the motion of the displacer, a powerpiston is pushed by the expanding gas adding energy to the crankshaft. The power piston shouldideally be 90 degrees out of phase with the displacer piston. The displacer type Stirling engine isoperated by the power of the power piston.

A special thanks to Koichi Hirata for the excellent illustrations!

http://www.stirlingengine.com/Displacer-Anim.adphttp://www.bekkoame.ne.jp/~khirata/

Two Piston Type:The two piston type Stirling engine is shown here. The space above the hot piston iscontinuously heated by a heat source. The space above the cold piston is continuouslycooled.

Two-Piston Engine Detail:

Click here for animation...

HEATINGLet's start from top dead center of the hot piston. The hot piston moves to the upper part of thecylinder and the cold piston moves to the lower part of the cylinder during the first 90 degrees ofrevolution. The working air is moved from the cold space to the hot space. And the pressure in theengine is increased.

http://www.stirlingengine.com/TwoPiston-Anim.adp

EXPANSIONDuring the next 90 degrees of revolution, the two pistons both move the lower part accepting theair pressure. The engine gets its power during this portion of its cycle.

COOLINGThe crankshaft revolves by power stored in the flywheel for the next 90 degrees. The hot pistonmoves to the lower part and the cold piston moves to the upper part. The air is moved from thehot space to the cold space. And the pressure in the engine is decreased.

CONTRACTIONThe two pistons are moved to upper part by the contraction of the air during the next 90 degrees.The engine also gets power during this portion of its cycle. The two piston type Stirling engine thenrepeats this cycle.

A special thanks to Koichi Hirata for the excellent illustrations!

1. Q: Are Stirling engines really the most efficient engines possible?

A: In the mid 1800's a very bright Frenchman named Sadi Carnot figured out themaximum efficiency possible with any heat engine. It is a formula like this(Temperature of the hot side - Temperature of the cold side)/Temp of hot side x100 equals the max theoretical efficiency. Of course the temperatures must bemeasured in degrees Kelvin or Rankine. Stirling engines (with perfectregeneration) match this cycle. Real Stirling engines can reach 50 percent of themaximum theoretical value. That is an incredibly high percentage!

1. Q: If Stirling engines are so efficient, why don't I have one in my car?

A: The best answer for that is to pick the MM-1 engine up after it gets up tospeed. Notice that it keeps running for a minute or so. While it's very easy to builda Stirling engine that will stop instantly, there is not one thing in the world anyonecan do to make one start instantly. When I get in my car I want it to startimmediately (if not sooner) and be able to burn rubber off the tires as I leave theparking lot! Stirling engines can't do that. In spite of these limitations, Ford, GM,and American Motors Corp. spent millions of dollars developing Stirling enginesfor cars, back in the 1970's. Ford even built a Stirling that could drive away fromthe curb (with relatively low power) twenty seconds after you turned the start key!Many prototypes were built and tested. Then oil prices came down in the 1980's,and people started to buy bigger cars. Suddenly there was no compelling reason tobuild an engine that was substantially more efficient than internal combustionengines, but wouldn't start instantly. Here is a picture of a 1979 AMC Spirit. Itwas equipped with an experimental Stirling engine powerplant called the "P-40".The Spirit was capable of burning gasoline, diesel, or gasohol. The P-40 Stirlingengine promised less pollution, 30% better mileage, and the same level ofperformance as the car's standard internal combustion engine. [From "AnIntroduction to Stirling Engines"] The French Research Sub Saga is Stirlingengine powered. Stirling engines also work exceptionally well as auxiliary powergenerators/heaters on yachts (see Victron Energy.), where their silence is valued

http://www.bekkoame.ne.jp/~khirata/http://www.victronenergy.com/Products/whispergen/whispergen.htm

and good cooling water is available. They would also work very well in airplaneswhere the air gets colder as the plane climbs to altitude. There is no aircraft powerplant (jets included) that gets any improvement in any operating conditions fromclimbing. Stirling engines won't lose as much power as they climb as do eitherpiston engines or jets. Also wouldn't you like to have silent airplanes with veryefficient engines that also have exceedingly low vibration levels?

1. Q: What are Stirling engines being used for today?

A: The modern uses of Stirling engines are invisible to almost everyone. Therehave been many research engines built in recent years but there are only threeareas where Stirling engines have made a dramatic impact. There are Stirlingengines in Submarines, stirling machines used as cryocoolers, and Stirling enginesin classrooms. Cryogenics is the science of things that are exceedingly cold andStirling engines are one tool that can be used to make things exceedingly cold. It'snot obvious but a Stirling engine is a reversible device. If you heat one end andcool the other, you get mechanical work out, but if you put mechanical work in,by connecting an electric motor, one end will get hot and the other end will getcold. If you design the machine correctly, the cold end will get extremely cold. Infact, Stirling coolers have been made that will cool below 10 degrees Kelvin.Micro Stirling coolers have been produced in large numbers for cooling infraredchips down to 80 degrees Kelvin for use in night vision devices.

1. A good general guideline is that if the hot side of the engine is not at least 500deg. F. (260 deg. C) the engine will be too bulky for the amount of power it putsout.

1. I don't think there is a theoretical upper limit on power in a Stirling engine. 2. Thebigger the temperature difference the easier it is to get power out of a smallengine. In other words to put out any significant amount of power an enginerunning on small temperature differences would have to be physically very large.3. The place where metals are critical is in the hot side of the engine. If you aregoing to build an engine that puts out a significant amount of power you willprobably want to build the heater head out of at least a good grade of stainlesssteel and perhaps a more exotic metal like Inconnel or Hasteloy.

Modern Stirling Engine Development

Today, there are many companies developing Stirling devices for niche markets, such as cogenerationunits and power generation using alternative fuels. Stirling engines have come a long way from the large andheavy engines of the 19th century, thanks to advancements in materials, manufacturing processes, theory andanalysis methods. This page contains a handful of links to some of these companies. Click on the images to learn moreabout these organizations and the engines they produce.

All images and information related to these devices are property of and are assumed to be copyrighted bytheir respective owners.

STM Corporation

SOLO Kleinmotoren GmbH

Stirling Energy Systems, Inc.

Kockums Sweden.

Sunpower, Inc.

Infinia Corporation

Tamin Enterprises

NASA Glenn Research Center

The Stirling Engine's most basic configuration consists of two pistons each in itsown cylinder. (Sometimes it is easier to envision these two cylinders as one longtube with the piston heads facing each other inside the tube (see the figurebelow)). Note that between these two pistons heads are the heater, cooler andregenerator. The regene