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What is a Stirling engine?

A Stirling engine is a closed-cycle, regenerative heat engine. That means the working gas (usually air in a model, hydrogen or helium in a modern engine) never leaves the engine. Instead of exploding fuel inside a cylinder, a Stirling engine is driven entirely by the difference in temperature between a hot end and a cold end. Any source of heat will power it: a candle, concentrated sunlight, a wood stove, even warm hands or liquid nitrogen.

It was invented and patented by the Reverend Robert Stirling in 1816, with help from his engineer brother James. Stirling was a Scottish minister who wanted a safer alternative to the steam engines of his day, which had an alarming habit of exploding. His original patent was for a "heat economizer" (the regenerator), the key idea that makes the engine so efficient.

How the Stirling cycle works

The engine works by shuttling a fixed amount of gas back and forth between the hot and cold ends. Each cycle has four phases:

Expansion. The gas at the hot end heats up, expands, and pushes the power piston, doing work.
Transfer to the cold end. A displacer pushes the hot gas through the regenerator toward the cold end, where it deposits its waste heat.
Compression. The cooled gas contracts and the power piston compresses it, using only a little of the work gained in step 1.
Transfer back to the hot end. The displacer pushes the gas back through the regenerator, which returns the stored heat, reheating the gas before it expands again.

Because the heat is recycled again and again inside the engine instead of being dumped out an exhaust pipe once, a well-built Stirling engine can reach the maximum theoretical thermodynamic efficiency permitted by the Carnot limit.

Three common configurations

Alpha

Two separate cylinders, one hot and one cold, connected by a regenerator. High power density, used in many modern designs.

Beta

One cylinder containing both a displacer and a power piston in line. This is an efficient engine design.

Gamma

A displacer in its own separate cylinder connected to the power cylinder. This is the layout of the HK-1.

The regenerator is the secret

Sandwiched between the hot and cold ends sits the regenerator: a porous matrix of fine wire mesh or steel wool that soaks up heat as the gas passes one way and gives it back as the gas returns. Without it the engine still runs, but it wastes most of its heat and produces almost no useful power. One builder used a green "S.O.S" pad (the kind without the sponge) as his regenerator and reported it survived many sessions in pristine condition. Cheap, simple, and effective.

Why did they disappear, and why are they back?

In the early 1900s the internal combustion engine won the automotive race on power-to-weight ratio and instant start-up. Stirling engines take time to warm up, need careful sealing when pressurized, and were expensive to build with the materials available then.

Today, with modern materials, the Stirling engine is enjoying a comeback:

Solar power. Dish-Stirling plants concentrate sunlight onto a receiver and convert it to grid electricity at record efficiencies.
Space. NASA's radioisotope generators use Stirling convertors to turn the heat of plutonium into electricity for deep-space probes, and Stirling cryocoolers keep satellite sensors at cryogenic temperatures.
Submarines. The Swedish Gotland-class submarines run air-independent Stirling engines, letting them stay submerged for weeks.
Home energy. Combined heat and power (CHP) units burn natural gas or biomass to run a Stirling engine and use the "waste" heat to warm buildings.
Cooling in reverse. Run a Stirling engine backwards and it becomes a refrigerator. Stirling cryocoolers cool infrared cameras, superconductors, and liquefy gases.
Transportation. Quiet, clean Stirling-generator hybrids (Stirling HEVs) have long been proposed as a possible vehicle of the future.

What I like about Stirling engines

Double-action piston: It is a true one-stroke engine, since the piston both pushes and pulls, delivering smooth power on every stroke. A car engine, on the other hand, is a 4-stroke (or, in rare cases, a 2-stroke) engine. It only delivers rough, explosive power on every 4th stroke. As a result, car engines wear out much quicker and produce more noise.

Simple design: There are no spark plugs, coils, wire, points, injectors, valves, camshafts, oil sump, pumps, filters, carburetor, mufflers or timing belts/gears to wear out.

Efficiency: With modern designs and materials up to the maximum theoretical thermodynamic efficiency is possible because of the way heat is recycled many times inside the engine instead of used once and thrown away as in conventional engines.

They will run on almost any fuel: Power may be the heat of a candle or concentrated sunlight. Some carefully-built models can even run on the heat from warm hands. They will also run on chemical processes that produce cold instead of heat. I was able to run the HK-1 at great speed on liquid nitrogen until the oil became hard and brittle (next time I'll use graphite).

Quiet running: At maximum power all that can be heard is slight whirring or clanking and only then if the bearings are loose. I've witnessed engines that run totally silent and are even compensated for vibration.

Safety: Stirling engines are probably the safest engine to work with. They can't explode like steam engines and the models on this page don't really put out enough power to injure anyone. I suppose sunburn is possible (when setting up a solar engine) and soldering irons are hot. Use adult supervision!

Innovative potential: Old-style Stirling engines did not perform nearly as well as their internal combustion cousins. Their low power-to-weight and power-to-size ratios made them unreasonable candidates for automotive power plants. There were a few attempts at powering automobiles with Stirling engines but consumers wanted more 'pep'. Stirling engines can often take several minutes to warm up & deliver useful power. With space-age materials, however, the Stirling engine is freed from these limitations.

Build one today

Stirling is the correct spelling, by the way. I used to think they were spelled "sterling engines" (I could probably get away with calling my engines sterling engines because I lived near the town of Sterling, Alaska). Nevertheless, Stirling engines are named after their inventor, Robert Stirling. Some information sources confuse ordinary hot-air engines with Stirling engines; the regenerator is what makes the real thing special.

engine Feel free to browse the illustrations, but the best way to learn how Stirling engines work is to build one and watch the animated diagram! The detailed 12-page illustrated plans took 10 times longer to write than the actual engine took to build. You can build this amazing engine for less than $20 (including plans). It really works!

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Comments

"My compliments to the inventor of this sublime device!"

"Thank you so much for the plans. This is wonderful!"

"You have gone out of your way this time. These plans are great!"

"I like the new plans page. It prints out well now."

"I am going to have fun with this."

"Thank you so very much. You have helped me to create my own Stirling engine and get first place in my science fair. Not only did it give me first place but some extra credit points too!! Thanks again. You've been a big help!" -- Jason K.

"Thank you for sending me the address: After some more working around I finally got PayPal to do its thing. Received the plans and built the engine. Works OK. I used an 'S.O.S' pad as the semi porous differentiator. It is green and did not have the sponge part. I attempted to burn the pad and got no bad fumes, it resisted flame and did not melt into a ball. After many sessions I opened the can and my pad was still in pristine condition. I used a 'Top-Flight' 9" prop instead of the tin can lid, replacing the solder blob with a penny 3/4 of the way out on the opposing blade. It really takes a well balanced engine to work on the larger size. Additionally I used a 1/2" plastic rod instead of using either aluminum or steel for the power piston, and I added height to the unit to accommodate the longer swing of the prop blade. J.B. 4 minute weld will take the heat up to 300 degrees and sets up fast. If I were trying it again I would try a much smaller prop and weight. Greater stress on making sure the center of the piston and the center of the crank shaft are aligned would be nice in the documentation." -- J.P.

"I have two pairs of students (gifted 10th grade) using the plans to try to make engines. We got the plans and we have most if not all of the materials. It is nice the way the 1/2" aluminum rod fits into the brass tubing. I really appreciate that we could get a design that does not require machining." -- D.C.

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