What is a Coffee Cup Stirling Engine?
How it Works and How to Make One
Engines that are powered by a fuel you can drink sound interesting, don’t they? They certainly are to me.
So What Are Coffee Cup Stirling Engines?
A coffee cup Stirling engine is a low-temperature difference Stirling engine with a round, horizontal displacer that is slightly larger than the coffee mug it will rest on.
These engines have a vertical flywheel that is supported on a stand above the displacer.
In the MM-1 Coffee Cup Stirling engine shown above, the flywheel is a model airplane propeller.
Power From Hot Coffee
Coffee cup Stirling engines get their energy to run from the steam produced from the coffee (or hot tea, if you prefer) which rises and hits the bottom of the engine.
They reject their waste heat to the room from the top side of the engine after turning some of the heat into mechanical work.
Summary of the Article Below
This article will explain how coffee cup Stirling engines work and, if you’re good at building things, show you how to build your own version using cheap things you can easily find.
Jump to any of the sections by clicking the links below:
How I Built the First Coffee Cup Stirling Engine
My First Attempt
I saw my first Stirling engine when I was at a trade show in Wisconsin.
It was a low-temperature difference Stirling engine owned by a man named Darryl Phillips. He had purchased it from an ad in a magazine for $165.
I thought to myself, “My, that’s expensive. I can certainly build one for less money than that!”
I was wrong.
By the time I had finished my first engine and had it looking as good as the one that I could’ve bought for $165, I had spent thousands of dollars.
I Did Make Cheap Stirling Engines
Actually, I did make cheap and ugly prototypes and you can, too, if you are handy.
It was making beautiful engines that turned out to be expensive.
But, after spending about $6,000, I finally had my first batch of beautiful engines.
To the best of my knowledge, I was the first person to market a coffee cup Stirling engine. You can still buy my original MM-1 Coffee Cup Stirling engine here.
I Asked for Advice
Today, people can use a search engine to find information quickly and easily but, before Google, you had to find someone who knew something about the subject and ask.
So, I asked Darryl how I could build a Stirling engine that would be simpler than the one he had shown me.
He said, “I have some ideas for you. I’ll draw up a sketch and send it to you.”
A few days later, I got the sketch by fax from Darryl. It was very similar to the one above, except his sketch didn’t have any numbers or explanations on it.
His notes simply said, “Something like this should work.”
Save This Drawing
If you want to build an engine like this, right click on the drawing above, save it to your computer, and print it out before you go through the instructions below. It will help you when you’re ready to build.
My First Engine Didn’t Run
After making a few changes, I was able to get the engine running – although it was running very slowly.
The concept Darryl sent me was a good one, but I had to learn on my own that engines like this must not have any leaks or have too much friction.
Differences Between the First LTD Engines and Coffee Cup Stirling Engines
Dr. James R. Senft, a professor of physics at the University Of Wisconsin River Falls, developed the first ultra-low temperature difference Stirling (LTD) engines.
He used a displacer about 6 or 7 inches in diameter and a vertical shaft with precise ball bearings connected to a flywheel at the top of the engine.
His engine also used an expensive graphite piston in a glass cylinder.
It was, and still is, an elegant design. It’s very similar to this commercially made heat of your hand Stirling engine.
Improving the State of the Art
Darryl’s innovation to that design, and what I followed through on by doing all the detail design and engineering work, was to make the engine smaller and simpler.
This way, it could fit on top of a coffee cup and run without expensive ball bearings or graphite pistons.
It was a cost and size innovation, not a performance innovation.
Other Low-Temperature Difference Configurations
If you’re ever looking at Stirling engine, coffee cup engine, or heat of your hand engine videos on YouTube, it may not be immediately obvious that there are many other possible configurations of low-temperature difference Stirling engines.
The coffee cup Stirling engine is just one example of a conveniently-sized engine. It has an easy-to-build mechanism on the top of the engine and it’s a very good configuration.
It’s a simple and easy-to-understand design and, fortunately, many people have been able to copy it. Hopefully, so will you!
Eye Candy Engines
In every society, people draw on the work of those who have done similar things in the past. Then, they expand on them with their own new ideas and inventions.
My particular talent is to make simple engines that work well.
But, there are some industrial designers who are true artists.
They looked at the Stirling engine designs on the market and decided, “Wow, that’s great, but I can make it beautiful, too!”
And indeed they have.
Feel like making one on your own?
I’m going to give you detailed instructions that will walk you through the drawing I showed you earlier in the article.
But first, here are the most important design guidelines to keep in mind when building any Stirling engine:
- No leaks
- Good heating and cooling exchange
- Low friction
- Proper contraction and expansion ratio
- Basically a sound design.
Build an Engine Like This
The DIY instructions below will show you how to build an engine very similar to our well-proven MM-5 Coffee Cup Stirling engine kit.
Keep your design very close to this one and it probably will work well.
Fortunately, we have very good online instructions for building that kit. I suggest you look at those instructions before building the engine below.
Keep in mind, you can definitely build an engine that works for much less than the price of our kit.
Plus, building them out of things you can find in your own kitchen and local stores is a lot of fun!
Design Guidelines Explained
Your engine can have only a tiny amount of air leaking around the shaft that moves the displacerup and down. The displace is #10 on the drawing above.
If too much air leaks here, or anywhere else, your engine will not run.
Good heating and cooling exchange:
Your engine must also have good heat acceptance on the hot side and heat rejection on the cold side.
If you build this engine as described, it has good heat transfer.
Friction in a Stirling engine comes from two places: the working gas (in this case, air) flowing inside the engine and the external friction of the mechanism used to make the engine run.
This engine can easily be built with low enough friction to work well. However, it’s also easy to get too much friction.
Proper contraction and expansion ratio:
The gas has to expand and push the piston up and then contract and pull the piston down. It’s fairly easy to see how you might get this wrong by setting up a crankshaft throw that has either too much or not enough travel.
This design should get you quite close to a good ratio if you are running your engine on ice or hot water.
Basically a sound design:
Thousands of Stirling engines similar to this one have been built to teach, amaze, and amuse people. So we know the design is basically good.
Read our Coffee Cup Engine Kit Instructions First
Before you start building an engine like this one, read our instructions for our coffee cup Stirling engine kit.
Our kit instructions are excellent and will help you build your engine. Plus, this will help you understand how you can do with cheap parts, what we have done with expensive parts.
What the Numbers Mean
I’m going to go through exactly what the numbers in the drawing above mean and tell you how you can find cheaper, more readily available parts that will work just as well as the expensive, beautiful parts we show in our kit instructions.
Remember, your job is to accomplish the same thing the kit does. The difference is you will only use things you can buy from the supermarket and your local hardware store.
- Flywheel. This design requires a flywheel. It should be something like a thin, round aluminum disk, model airplane propeller, or the top of a food can.
- Bracket. This is a U-shaped aluminum or steel bracket that holds up the crankshaft in this engine. I’d suggest making it from .050″ aluminum of any alloy that you can easily find. Almost anything rigid here will work.
- Crankshaft. Make this out of 1/16″ diameter steel rod. I’ve made these out of coat hanger wire, too. Note that the crankshaft must have two different offsets that are 90 degrees out of phase. Look at how we build these for our kit before you make yours.
- Gland in the center of the engine. This is anything that you use as the low friction linear bearing in the center of the engine. A tiny bit of air has to leak here, but not too much.
- You will need to drill this out to be a couple thousands of an inch larger than the wire you are using to raise and lower the displacer. If you can get it, brass tubing from K&S Engineering works well. They sell the right size steel wire, too. This is very easy to purchase at hobby shops in the United States.
- Connecting rod for the displacer. This can be anything from thread to silicone model airplane fuel tubing. This does NOT have to be rigid.
- Connecting rod for the piston. This DOES have to be rigid. I suggest 1/16″ wire from K&S engineering (buy it at hobby shops) or coat hanger wire. You will need some kind of adjustment linkage in the middle of this. Consider things like overlapping two wires and then rubber banding them together. Or, see how we do it in our kit.
- Power piston. Make this out of a latex (rubber glove) if you can get it. Latex is the easiest piston material to adjust correctly. Vinyl (also from a vinyl glove) or silicone rubber (a specialty industrial part) will also work. Latex is the easiest piston material you can use to get an engine running, but it goes bad after a couple of weeks of being exposed to light. Vinyl from a vinyl glove is harder to adjust (it doesn’t stretch) and it also doesn’t last. Silicone also has to be adjusted perfectly but it has the advantage of lasting a long time. We sell a spare diaphragm assembly that some people have used to build their own engines.
- Washers for your piston. You will need some method of making a plunger part (the central part) of your piston. I cut all the ones for my prototypes by using a set of steel punches and cutting them out of thin plastics. Punch the inner diameter to be 1/16″.
- Displacer piston material. This has to be light, but a lot of different materials will work. I like black window air conditioning fan material. Foam core board works well as do new Scotch Brite pads, balsa wood, and Styrofoam cut to shape. Make these about 1/4″ thick. Make the clearance between the wall and the cylinder 1/16″
- Rod for displacer piston. This has to be a rigid rod of some type to attach the displacer to. I like 1/16″ steel wire from K&S Engineering (hobby shops).
- Displacer cylinder material. I’ve used plastic pickle jars, water bottles, PVC pipe, and lots of other materials. Don’t use a metal, use some kind of plastic. I recommend using something clear as it is easier to adjust and engine when you can see what is happening inside it. Cut your cylinder to be 6/10″ tall. *** Note*** That’s shorter than in this drawing.
- Bearings. In our kit and ready to run engines, we make these bearings out of Teflon tubing stock that we machine. We buy the tubing material from McMaster Carr Supply. High-density polyethylene (the plastic that milk jugs are made of) is almost as slick as Teflon and is a lot easier to find, so you might want to figure out how to carve bearings similar in function to our from some milk bottles you have lying around.
- Top and bottom plates. I like making these out of .050″ aluminum in any alloy. They could be made out of steel (which should be thinner) and work about as well. If you try to use plastics, that might or might not work, but aluminum is the easiest to get good heat transfer with and copper would be great too if you have some around. Silver would be a wonderful material to use for the top and bottom plates.
- Bearings in the aluminum bracket. In our kits, we use a fancy, pressed-in plastic bearing with Teflon in it. For a DIY engine, I would use washers punched out of high-density polyethylene milk jugs and then taped it to the aluminum bracket with duct tape.
- Optional bearing. This is a bearing for the center displacer piston. If you want to make this easier, use a thread and loop it over the crankshaft. You will have to use a bearing and some kind of flexible tubing like model airplane fuel tubing here if you want your engine to run upside down like the MM-1 and MM-5 will. But, do you really need that? (smile)
- Hold-down ring. In this drawing, you can only see the cross-sectional edges of a hold-down ring to the left and the right of the piston. Make the holes for the piston about 8/10″ in diameter and, obviously, your rubber piston has to be larger than that. You can attach the piston to the top of the engine with hot glue when it is adjusted correctly.
Leak Check Your Engine
We have two short video clips in our kit instructions that show what a failing and a passing leak test should look like.
Be sure to watch these as you read through the kit.
Take a Look at Our Kit
Building an engine from scratch is a LOT of fun, but it takes more work than building one from a kit and it is less likely to work on the first try.
If you think you’re better off purchasing a kit to build first, look at our Coffee Cup Stirling Engine kit to see if you like it.
You Can Do It!
Between the instructions above and the instructions for our kit, you should be able to get a DIY version of this engine running.
When you get your engine running, make a video and send it to us!
Please Comment Below
Do you have other tips or questions? Leave them in the comments below!
Also please if you build an engine using these instructions, please come back and leave a comment below.