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Are Stirling engines really the most efficient  (heat) engines possible?

In the mid 1800's a very bright Frenchman named Sadi Carnot figured out the maximum 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 x 100 equals the max theoretical efficiency. Of course the temperatures must be measured in degrees Kelvin or Rankine. Stirling engines (with perfect regeneration) match this cycle. Real Stirling engines can reach 50  percent of the maximum theoretical value. That is an incredibly high percentage!

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

The best answer for that is to pick up the MM-1 engine up after it gets up to speed. Notice that it keeps running for a minute or so. While it's very easy to build a Stirling engine that will stop instantly, there is not one thing in the world anyone can do to make one start instantly.

When I get in my car I want it to start immediately (if not sooner) and be able to burn rubber off the tires as I leave the parking lot! Stirling engines can't do that. In spite of these limitations, Ford, GM, and American Motors Corp. spent millions of dollars developing Stirling engines for cars, back in the 1970's. Ford even built a Stirling that could drive away from the 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 to build an engine that was substantially more efficient than internal combustion engines, but wouldn't start instantly.

Car PhotoHere is a picture of a 1979 AMC Spirit. It was equipped with an experimental Stirling engine powerplant called the "P-40". The Spirit was capable of burning gasoline, diesel, or gasohol. The P-40 Stirling engine promised less pollution, 30% better mileage, and the same level of performance as the car's standard internal combustion engine. 

Yellow SubThe French Research Sub Saga is Stirling engine powered...

Stirling engines would work exceptionally well in places like auxiliary power generation on pleasure boats  , where their silence and efficiency would be valued and good cooling water is available. They would also work very well in airplanes where the air gets colder as the plane climbs to altitude. There is no aircraft power plant (jets included) that gets any improvement in any operating conditions from climbing. Stirling engines won't lose as much power as they climb as do either piston engines or jets. Also wouldn't you like to have silent airplanes with very efficient engines that also have exceedingly low vibration levels?

 Who was the Rev. Robert Stirling anyway?

Robert Stirling was a minister of the Church of Scotland who was interested in the health of his parishioners bodies in addition to the well being of their souls. He invented the Stirling engine (he called it an "air engine") because steam engines of his day would often explode killing and maiming those who were unlucky enough to be standing close by.

Robert Stirling's engines couldn't explode and produced more power than the steam engines then in use. In 1816 he received his first patent for a new type of "air engine".

The engines he built and those that followed eventually became known as "hot air engines" and continued to be known called hot air engines until the 1940's when other gasses such as helium and hydrogen were used as the working fluid.

Robert Stirling was an active minister and inventor all his life. Perhaps his most important invention was the "regenerator" or "economizer" as he called it. This is used today in Stirling engines and many other industrial processes to save heat and make industry more efficient.

What are Stirling engines being used for today?

The modern uses of Stirling engines are invisible to almost everyone. There have been many research engines built in recent years but there are only three areas where Stirling engines have made a dramatic impact. There are Stirling engines in Submarines, stirling machines used as cryocoolers, and Stirling engines in classrooms. 

Cryogenics is the science of things that are exceedingly cold and Stirling engines are one tool that can be used to make things exceedingly cold. It's not obvious but a Stirling engine is a reversible device. If you heat one end and cool 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 get cold. If you design the machine correctly, the cold end will get extremely cold. In fact, Stirling coolers have been made that will cool below 10 degrees Kelvin. Micro Stirling coolers have been produced in large numbers for cooling infrared chips down to 80 degrees Kelvin for use in night vision devices.


How Stirling Engines Work

Stirling engines can be hard to understand. Here are the key points. Every Stirling engine has a sealed cylinder with one part hot and the other cold. The working gas inside the engine (which is often air, helium, or hydrogen) is moved by a mechanism from the hot side to the cold side. When the gas is on the hot side it expands and pushes up on a piston.

When it moves back to the cold side it contracts. Properly designed Stirling engines have two power pulses per revolution, which can make them very smooth running.

Two of the more common types are displacer type  Stirling engines and two piston Stirling engines.
The displacer type Stirling engine has one power piston and a displacer piston.  The two piston type Stirling engine has two power pistons. 

Displacer Type Stirling Engine

The displacer type Stirling engine is shown here. The space below the displacer piston is continuously heated by a heat source. The space above the displacer piston is continuously cooled. The displacer piston moves the air (displaces the air) from the hot side to the cold side. Gas expands when heated, and contracts when cooled. Stirling engines move the gas from the hot side of the engine, where it expands, to the cold side, where it contracts.

Displacer Technical Illustration

Displacer Animation

When there is a temperature difference between upper displacer space and lower displacer space, the engine pressure is changed by the movement of the displacer. The pressure increases when the displacer is located in the upper part of the cylinder (and most of the air is on the hot lower 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 not operate the crankshaft and the engine. In other words, the connecting rod to the displacer could be a string in this engine and it would still work.

When the engine pressure reaches its maximum because of the motion of the displacer, a power piston is pushed by the expanding gas adding energy to the crankshaft. The power piston should ideally be 90 degrees out of phase with the displacer piston. The displacer type Stirling engine is operated by the power of the power piston.


Two-Piston Type Stirling Engine
The two piston type Stirling engine is shown here. The space above the hot piston is continuously heated by a heat source. The space above the cold piston is continuously cooled.

2 Piston Technical Illustration

Two Piston Animation

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

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

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

The 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 then repeats this cycle.

A special thanks to Koichi Hirata for the excellent illustrations!