Stirling Engines are Cool and Efficient
So, Why Aren’t They More Popular?
Stirling engines are efficient, environmentally-friendly, and clean running. Well, if they’re so great, why aren’t they used in cars, boats, and airplanes?
You’ve probably heard that Stirling engines can be very efficient.
In theory, Stirling engines can match the maximum theoretical efficiency of the Carnot cycle. In actual fact, some Stirling engines are very efficient¹.
Stirling engines can also be clean running and environmentally friendly.
So, why aren’t Stirling engines used more often?
Here’s the Short Answer:
Stirling engines are not good for applications that need to change their power output levels quickly, like cars for example.
Stirling engines like to change their power output levels slowly.
Plus, they tend to be heavier (and more expensive) than gasoline or diesel engines of a similar power output.
Summary of the Full Article Below
The article below will tell you the history of the Stirling engine, how they were forgotten, then revived, then forgotten again, and give you a more detailed answer about why you probably don’t have a power producing Stirling engine in your house today.
Click on any of the links below to jump to that section:
- The Original Engines
- Phillips Revival to Power Tube Type Radios
- The Wrong Research
- The Wrong Marketing
- Misguided Philanthropy
- The Curse of Carnot
- Dollars Per Watt
- Difficult Design
- What You CAN Buy Today
The Original Stirling Engine
Robert Stirling invented his first engine in 1816 and, at that time, it did compete well with the existing steam engines of the day in terms of power density.
However, the first Stirling engines had a problem compared to the first Steam engines. The material that was available for building any kind of engines in the early 1800s was cast iron.
Cast Iron Hates Being Hot
Remember that both steam engines and Stirling engines are external combustion engines where the fire does not happen inside the cylinder of the engine.
But in a Stirling engine, the hot side of the engine needs get to the average temperature at which you are trying to run the engine, and stay there.
Cast iron is NOT a long-lasting material when it is glowing red hot or close to glowing red hot. Modern stainless steels can last indefinitely at these temperatures but cast iron can’t.
How Steam Engines Competed
The engine part of a steam engine is a much lower temperature device than the hot side of a Stirling engine. You can build steam engines just fine out of cast iron and they last essentially forever.
But a steam engine by itself doesn’t make any power, it needs a boiler to make high-pressure steam. These boilers can and did fail with often fatal results for the people working nearby.
That’s why Robert Stirling invented his engine.
Steam Engine Boilers are Only Partly Red Hot
Boilers in steam engines only get exposed to the average temperature of the flame only on the hot side.
The inside of a steam boiler is operating at the temperature of the water under pressure inside the boiler.
That’s a much lower temperature than on the flame side.
Steam boiler designers could partly compensate for the problems of cast iron, by making it a little bit thicker for example.
A Problem of Materials
So, the early steam engines beat the early Stirling engines in high power applications, mostly because cast iron does not last a long time at high temperatures.
If the stainless steels that are easy to buy today had been available in 1816, the next 100 years might have been the age of Stirling engines, not the age of steam engines.
Where the Early Stirling Engines Won
Not everyone who needs a source of power is trying to run something big like a train.
Some people need to do things like
- Power small factories
- Run fans
- Grind small amounts of grains
These people needed something that was easy to start, run, and maintain and that lasted a long time.
The early Stirling engines competed very well in these niches and they eventually came to be called “hot air engines.”
Hot Air Engines
These engines were safe to operate, they ran at low power densities, and they lasted a long time.
How long did they last? They lasted so long that many of them are still available on the antique market today.
Antique Stirling vs. Antique Gasoline or Diesel
In 1870, John D. Rockefeller founded the Standard Oil Company and, by the time he retired in 1902, the age of petroleum was in full swing.
The power output from a gasoline engine that you could buy in 1870 pulled ahead of the power output from a Stirling engine available in 1870 and, every year, the power output from gasoline engines went up faster than the power output from Stirling engines.
The Rush to Electrify
In 1893, the World’s Colombian Exposition in Chicago was lit with and powered by electricity. While this was certainly not the first use of electricity, it marked a turning point in the rush to electrify the world
While small Stirling engines were competitive with small steam engines, both in their initial purchase price and ease of use, small antique Stirling engines were NOT competitive at all with small electric motors.
Phillips Revival to Power Radios
Shortly after World War II, the electronics giant Phillips from Holland needed a quiet, reliable emergency power source to for their tube type radios.
For those of you who are not familiar with electronics-based on tubes, you should know they glow hot and take a significant amount of power.
One engineer working at Philips electronics looked at some old hot air engines and realized he could design a quiet and reliable power source for their radios by improving them, and soon Philips had brought Stirling engines into the modern age.
Bad Market Timing Beats Good Engineering
Philips finally developed and perfected their power producing Stirling engines about the time that transistors took over the way that electronics were built.
The transistor radios used so much less power that when people needed to use radios in emergency situations, they could easily and cheaply run them on batteries.
Philips Wasn’t an Engine Company
Philips wasn’t interested in selling engines if those engines weren’t being used to power their electronics.
They weren’t interested in making engines for any uses other than electronics.
Philips immediately canceled their Stirling engine operation after it became obvious to them that the world of making electronics was all switching to transistors.
The Wrong Research
The work of Phillips was well published in engineering journals and it attracted academics who were able to prove that Stirling engines had a very high theoretical efficiency.
This proof of high efficiency attracted a lot of attention and a lot of research money.
Everyone needs efficient engines, right? Well, there are other things that are also important like initial cost and using engines that can change power levels quickly and easily.
Research to Put Stirling Engines in Cars
The United States Government funded a research and development program to put Stirling engines in cars, an application for which they were never well-suited.
The renewed interest in Stirling engines helped launch the following research programs:
- General Motors developed a silent Stirling engine power generator prototype for the military.
- NASA developed a number of Stirling engines, putting one in a pickup truck and developing others to power deep space probes.
- Private individuals attempted to develop Stirling engines to power airplanes.
The Wrong Marketing
Engineers love to engineer. But, they don’t really love to market their inventions once the engineering is done.
So they try to find an easy way out by finding someone (or worse yet another company) to do their marketing for them.
Instead of making a reasonable choice, like setting up a new company and going into production to serve profitable niche markets, what they often do is try to partner with a big utility to market their engines.
The big utility likes the project, because it makes them sound green and environmentally friendly, but, at the end of the day, they really don’t care about the technology.
Consider the WhisperGen Story
WhisperGen of New Zealand developed an excellent combined heating power Stirling engine for homes, yachts, or what have you.
Originally, their engines were marketed to yachts and could have been a great idea had they stayed with profitable niche markets.
The Wrong Partners
Instead, they attempted to find big utility marketing partners in who promised to spread their clean and green technology over Europe.
In the end, the utility companies never really cared about the technology and the utilities easily let their bean counters talk them into canceling the project.
Unfortunately, by the time the idea of putting a Stirling engine in every home in Europe was canceled, the staff had grown so large that there was no way of scaling back to profitably serve niche markets.
Many misguided philanthropists and Stirling technology enthusiasts have tried to promote the use of Stirling engines in the third world.
They failed to realize that the third world has zero interest in what might be optimal for them, especially if it costs one dollar more than the cheapest product they can buy today which will also do the job.
How to Get the Third World Interested
The third world becomes interested in new products when they see them on American television. Right now they are NOT seeing Stirling engines on American television, so they’re not interested.
A Great Product for Somebody Else
When you see a product and think it’s good for somebody else, it’s just a polite way of saying “I don’t want that.”
How to Find the Right Markets
If you want to promote Stirling engines, do it by making a product that YOU want to use in your own life, right where you live today.
Don’t try to make one for somebody halfway around the world whose needs you can’t possibly understand.
How to Develop Third World Markets
Technology enthusiasts who want to see Stirling engines used in the third world need to get them used widely in the first world first and then get them written into American television shows.
The Curse of Carnot
When I was taking my first physics class in Joliet, Illinois, I came across a sentence in my physics text book that said:
Stirling engines had the same theoretical efficiency as the Carnot cycle and the Carnot cycle has the maximum possible theoretical efficiency.
I thought to myself, “if these are so great, why are they not being used?”
It’s probably the same question that brought you to this page.
Other Measures of Goodness
But efficiency is just one way to measure the goodness of a product. There are many other ways to measure the goodness of designs, things like a low purchase price and ease of use.
A hell-bent focus on efficiency in the design of any engine will lead to a lopsided result that no one will want or be able to buy.
Technically, It’s True!
Indeed, extremely efficient Stirling engines can be built.
And if you tell engineers to build a Stirling engine with the maximum possible efficiency, they will do exactly that.
And when they are done, they will come back and drop the prototype on your desk, then they will prove the efficiency with their testing numbers, and say, “Give me my bonus. It’s very efficient.”
How Engineering Tradeoffs Work
Everything in engineering is a design tradeoff. When you improve one portion of your design, others will suffer. If you don’t like that, I don’t care.
I’m an engineer. I went to the University of Illinois and that’s the way nature works. It doesn’t give a shit what you think.
If you are an engineer, your job is to understand how nature works and build products that make things around you better.
Efficiency comes at a price in engineering. Your first efficiency improvements will be cheap and yield big results and you should do them.
Your second efficiency improvements will be more expensive and maybe you shouldn’t do all of them.
The Price of the Last 3%
The Best Figure of Merit – Dollars Per Watt
Customers are interested in one thing: does this product do the job I want it to do, in the way I want it done, at a price I can afford?
In my opinion, Stirling engines should be designed by always keeping in mind the cost per watt that the customer will have to pay to have it delivered to his doorstep and installed.
Stirling Engines are Hard to Design
I’ll put this bluntly: Stirling engines are a bitch to design.
The basic rules for designing a gasoline or diesel engine are simple: suck, squeeze, bang, blow. It’s called the four-stroke cycle.
Every student in auto shop class, or engineering school can easily get a feel for how to design a powerful gasoline or diesel engine.
Stirling Engines Have Different Design Rules
Unfortunately, the same rules of design don’t apply to Stirling engines.
In a gasoline engine, if you want to get more power, you make the cylinders bigger, put more air and fuel in your engine, and repeat the cycle faster.
The rules of Stirling engine design are not nearly that simple or obvious.
If you try to do the same things to a Stirling engine that you would do to a gasoline engine to improve it, you will either get an engine that runs worse or an engine that doesn’t run at all.
Think About Ratios
Stirling engine designs are all about ratios and everything in the engine is interrelated. Change one thing, and it effects everything else.
For example, If you improve anything in the Stirling engine, you’ll need to change the compression ratio to take advantage of that improvement.
NASA Stirling Engine Design Guide
I’m pretty sure that there are some useful things about Stirling engine design in that book, but there is no simple explanation that you can teach in auto shop class.
It’s a typical, incomprehensible government document.
Stirling Engines Are Unforgiving of Design Mistakes
If you build a gasoline engine and get one or two things wrong in your design, the engine will still run even if the performance not as good as you would have liked.
With the Stirling engines, if you get just one thing wrong, your engine might not run at all.
Good Design is Possible
It certainly is possible to learn how to design Stirling engines well.
I suggest aspiring Stirling engine designers build a lot of Stirling engine models to get a feel for all of the variables.
What You CAN Buy Today
Fans for wood stoves are probably the most widely available practical Stirling engine power uses today.
They haven’t become a commodity item at home depot, but if you have a cabin with no electricity and want a fan to move the air around your wood stove, they’re a good option.
Check this one out if you like the idea of a Stirling engine fan.
Combined Heating Power Engines
These engines have come to the market fairly reasonably and are perfect matches for companies that utilize a lot of electricity and hot water.
They’re good for use as heating for buildings or for things like commercial laundry.
Really Cool Models
Many companies, including mine, make a wide range of Stirling engine models for entertainment and education.
This is not a market that should be depreciated.
These are valuable teaching tools for physics and engineering students as well as technology enthusiasts.
The low-temperature difference models can be run indoors without filling up the room with carbon monoxide.
These models help all students understand the fundamental physics of all types of engines, not just Stirling engines
Very Deadly Submarines
The 2005, the United States Navy leased a Stirling engine powered Swedish Gotland-class submarine to play the bad guy in war games.
The result was that the super silent Swedish sub evaded the carriers anti-submarine defenses and in simulation sent the carrier to the bottom with 5 torpedoes before making a silent getaway².
So far, the United States navy is not interested even though it seems like these super silent submarines are the greatest invest to the current United States naval strategy.
Sun Pulse Engines
There are some low production, low-temperature difference power producing Stirling engines being made by a company called Sun Orbit in Germany.
They are probably available online today, although they do not yet have prices on their website.
More Uses of Stirling Engines
If you have enjoyed this page so far, you probably will like our page on Stirling engine uses today.
Please Check Out Our Stirling Store
If you take a minute to check out our Stirling engine store you would help us a lot even if you don’t buy anything. Please check it out here.
Leave us Your Comments
If you liked this article please comment and feel free to leave a link to any other Stirling engine sites in your comments.
All the comments are moderated, so it may take us a few days to get your comment approved. Thank you very much.