The 100 horsepower 80 pound Stirling engine for a reasonable price???   Back Home

Home built drawing of Solar Miller Layout Stirling engine

Solar-Miller (tm) configuration of Stirling engine.  The regenerators (inside between displacer and crankshaft) and  radiator ( on top of the two displacers drawn in red) have been removed for clarity.
April 2011  Welcome Indigogo

In support of the effort to raise funds sufficient to build an optimum model of this engine, here are some additional details and information.

To build a working model engineering help is needed.  If you want to contribute to the effort you can donate directly at and I am going to retry raising funds on Indigogo.  

Note how all moving parts are enclosed and protected from dust, dirt, combustion products, heat, mold and mildew.
Certain details that allow this design to function have been withheld
The most efficient working fluid is hydrogen gas.  Most any gas will work.  Avoid using air with oxygen, because it combines with the lubricants and then burns explosively. 
Note also, how this engine was designed as a pressure vessel to work at high power densities.


How It All Started

A few years back, I was reading about how the wells in Bangladesh naturally contain high levels of arsenic.  It seems that someone wanted to provide clean drinking water for the people.   They

developed a program to dig wells so that the people wouldn't have to drink the dirty water from the local river.  Good idea, good plan and clean water for the people to drink.  Eventually though, somebody

tested the wells and found the arsenic.  Sometimes doing good is harder than expected.

I wondered if a quick and cheap means of purifying the water could be found….one that would not require engineers to run and maintain it. So, I thought about ways to remove the arsenic.   Manually removing

the arsenic requires filters that the people can't make, so they would be dependent upon a large corporation for their water.  Chemical methods would require salt that is also expensive.  Distillation is  a

possibility and there are many good designs that might work.  Further,  distillation is a simple method that an average homeowner could maintain, and which would not require frequent expenditures to

operate.  So I began to imagine ways in which I could improve on an existing technology to make it fit their existing resources.  Maybe I could help.

For distillation to occur, the only thing that needs to happen is a phase change from liquid to gas (boils) and back again (condenses.)  The solids (like arsenic) can't gasify so they get left behind.  And what causes a phase change? Temperature and pressure together do.  So how much energy would you need?   How much less, if your design didn't waste any?  And how would you move the energy from where it is to where you need it to go?  How would you take the heat from the steam, to condense it into clean water, and put it into the dirty water to make it boil and produce steam?  And the answer is with a Stirling engine.

Heat is a form of energy.  When operated as a pump, Stirling engines move heat in a predictable way. Just as electric motors can either be turned to generate electricity or can use electricity to run, Stirling engines can be turned to pump heat or heat can be used to run them.  

The most important point about Stirling engines is that THEY WORK. The current models aren't real powerful, but they still WORK more efficiently than electric motors.  They are hard to understand, but THEY WORK more reliably than your car’s internal combustion engine.  

I began my research into Stirling engines.  In my studies, I found reference to the opinion that it is impossible to make a rotary Stirling engine. The word impossible is offensive when used to justify

an unwillingness to work toward a solution.  So, prepared to accept that the naysayers might be proven to be correct, I invested a few hundred hours and ultimately designed two practical rotary Stirling engines.

There was no other choice; I needed a specific energy path that only a Sterling engine could provide. This particular layout was made to accommodate the distilation application.  A large heat transfer area was needed and then higher throughput was needed which means either a larger engine or more engine in the same size so I raised the pressure.  With higher pressure you need a larger flywheel or you need to counterbalance it with a chamber on the other side of the  piston.  And putting another displacer over there makes the engine twice as powerful. Anyway that is how it all started.

With this Stirling engine, it will be possible to power a distillation process that is cheap to buy and operate, is easily maintained, reliable and user-friendly to  the most non-technology-savy people in Bangladesh and maybe even here at home.

Then I noticed that this design solved a lot of problems for the Stirling engine.  It eliminates the seals and reduces the weight. Power density and heat flow are completely controllable within the theoretical behavior of gasses.  What are the possibilities of a Stirling engine that has a better power density than a regular gas or diesel engine?  Hmmmm.


This is my interpretation of history.  If you want the popular history, go to Wikipedia.

The first piston engines were compressionless. They worked by internal combustion heating the air to expand it and then a valve closed and as the air cooled, ambient pressure pushed the piston  up.  There were two logical variations to achieve improved perfomance : internal combustion and external combustion.  

Robert Stirling invented the external combustion method of tranferring heat into the piston chamber to have the air expand and push on the piston and then removing the heat source allowing the air to contract and pull on the piston.

Edward Butler figured out how to create a fire inside the piston chamber which heated the compressed air causing it to expand and press against the piston.

Edward Butler's version had the advantage of compressing the air which gave his internal combustion engine a vast increase in power.  Robert Stirling never figured out how to increase the working pressure of his engine.  The rotary design gives the Stirling engine a greater increase in power than Edward Butler obtained because the limit of pressure is determined by gas laws in the Stirling and by fuel octane in the internal combustion engine.

Technology is driven by businessmen and not engineers.  When presented with the remarkable increase in the power of the internal combustion engine, businessmen only invested in the obviously superior power source.  So the internal combustion engine has had 200 years for its problems to be solved and few people have paid any attention to the Stirling.

Imagine the probems of building a fire inside a piston cylinder.  You need to build a fire let it burn out and then replace everything and do it again.  It sounds ludicrous.  But this was the only known method to obtain the power needed.


Stirling Engines Could Save a Lot of Natural Resources

The internal combustion engine has been accused of being an environmental nightmare and the cause of much pollution and fueling it the cause of much pollution and even wars. Many have advocated getting

rid of it, but it does something valuable to our society that we can't afford to get rid of.

Electric engines have always had a battery problem. And the batteries have to be charged by something.  But what if we could start on a path to something better?  Not a miracle cure for the world's ills but a small step in the direction of less pollution, noise and waste.without upsetting the status quo.

Progress is an evolutionary process.Something is invented and then people start using it and in the using and building of the item we find ways to improve it. The longer something is used, the cheaper it

becomes to make and the better it performs.

The Stirling engine has languished in obscurity because when it was invented it was impractical for most applications based upon power output compared to its weight and size. It was improved upon until the Internal Combustion Engine was invented with its much more powerful cycle. At which point industry abandoned the Stirling engine and it stopped evolving.

Before people knew about pollution, natural resources depletion, global warming and imported oil, the only performance measure that mattered was power to weight ratio. The internal combustion engine,

diesel or gas powered, got so far ahead in that race early on that it was the obvious choice to run our economy.

Today, in 2015, the internal combustion engine has been improving for over 100 years and is a wonder of technical evolution because it originally had better power to weight ratio or power density.  And the

Stirling is still a curiosity.


Pollution is waste and it doesn't pay.  It costs money to make a mess and then costs more to clean up. So avoiding the mess is worth the cost of the wasted materials plus the cost of materials and labor to

clean it up, assuming that you can clean up back to pristine condition.

The amount of design and parts used to remove the pollution and noise from the internal combustion engine is a significant portion of its costs, the fuel that is wasted exceeds the fuel that is used.


If you make noise, you need to muffle it. So if you make a noisy engine, you need a muffler and exhaust system. Stirlings don't make noise and so don't need mufflers.

If you burn fuel incompletely under varying pressure you need to treat the exhaust to remove the unburned fuel and undesirable pollutants that are produced. Stirlings burn fuel at low pressure under

consistent controllable conditions that allow for prevention of the formation of undesirable pollutant exhaust gasses.  The Catalytic converter is built into the combustion area so very little fuel is


Stirlings use the heat produced from the fuel, so it is possible to run them on any heat source including Solar, Geothermal, gasoline, diesel, natural gas, propane, wood pellets, corn or anything that produces heat.

If you start and stop the combustion 2,000 times a minute, you need to use energy to run a complicated ignition, timing system, that will inject fuel, compress air and then light it at the right time.  Stirlings use a constant heat source that does not need constant attention and control. You just start the burner and let it burn until it is time to turn it off. Stilings don't need carburetors, fuel injection, intake manifolds camshafts, EGR valves, PCV valves, Timing circuits, distributors or sensors on the exhaust to name a few things that waste money to buy and fuel to run.

Internal Combustions Engines (ICEs) take in air, compress it, heat it until it expands, use some of the force of the expansion to produce horsepower, then dump most of the heat and pressure out of the exhaust manifold while the rest wanders around and gets into the bearings and other working parts of the engine.  Stirlings have the heat sensitive parts separated and protected from where the heat is used and the fuel is burned so they don't need the mechanical parts cooled and the compression is never released.

If exhaust gasses are exposed to lubricants then the lubricants become polluted and need to be changed.  Stirlings have hermetically sealed bearings that are never exposed to outside air or exhaust so they

don't need oil changes and the bearings operate at ambient temperatures so they last a long time.

Internal combustion engines have pistons that go up, down, up down (four strokes) but which only produce power on the least efficient part of one of the down strokes.  The Stirling produces power at the most efficient part of each and every up or downward motion of the piston. So one Stirling piston does the work of 8  Internal Combustion Engine pistons. Fewer parts doing more work.


So how much cheaper would an engine be if it didn't need all the things we pointed out, like catalytic converter, ignition system, fuel injection system, camshafts, valves, water pump or oil pump?  How many

natural resources are used to build all those parts?  It is time to find out. Because, if you search Stirling Engines on the web, the one thing you will notice, is that THEY WORK,  but the design invented in 1816  just isn’t very powerful.

Engines have one basic function, they convert one type of energy into another.  That is it. The internal combustion engine takes gasoline or diesel and turns it into mechanical motion. The Stirling engine does

the same thing more simply.

The Stirling engine got stuck in an evolutionary backwater bereft of attention after the internal combustion engine was invented.  The higher power density of the internal combustion engine made it seem

the better choice.

Nobody was concerned about the complexity of the design or the inefficiency and pollution of the product because it performed a needed function as an engine. Bandaid solutions were hurriedly invented and applied to the Internal Combustion Engine to overcome its inherently unreliable, noisy and dirty nature.

It is as if someone invented Windows 1 and got to version 5 before people realized that GNU/Linux could be used and they were too invested to switch.

The Stirling engine doesn't have the problems that the internal combustion engines do but nobody could figure out how to make them as powerful.  The Stirling seemed hopelessly under powered but otherwise

very promising. Engines large enough to power ships were made. But the lower power density made them uncompetitive with the noisier smoke belching steam and then gas and diesel engines. Because time is money and fast is better than slow.

Stirlings have been made and operated in niche markets since their invention while the internal combustion engine has been the engine of choice,  polluting the planet and consuming vast resources as it has driven our economy.

Industrial evolution has addressed the major flaws of the Internal Combustion Engine.  Engineers design and build models that last longer,  invented components that fix its drawbacks and allow it to

produce more power and work more reliably. The engine still has the problems, we just don't notice them anymore. It would be much better for the planet if we didn't spend all the resources necessary to cover

up the problems of the Internal Combustion Engine.  It will never be more efficient to use four strokes to do the work of one proper stroke.

One curious property of the Stirling is that when you run it like a pump, it pumps heat from one side to the other.  Stirlings are efficient at liquefying air when used in this matter.  But with low power density they haven't been used for things like your refrigerator or your air-conditioner, where they would not use Ozone depleting gasses.

The Stirling is reliable; with only about 4 moving parts there is very little to break or go wrong.  If only it had power density comparable to the internal combustion engine it would save a massive amount of

resources and fuel.

At Last

Somebody finally did something with the Stirling design. Unfortunately it was the U.S. government.  They made it more powerful and so expensive that it can't compete with the lower costs modern

internal combustion engine. So Stirlings are the engine of choice in spacecraft and submarines.  I haven't been on either.

But, now there is a new design, that in addition to being powerful is also cheap and uses existing manufacturing infrastructure to make its parts.  But to build and promote it so that business will consider it

when they could just use the internal combustion designs that everyone has been using forever will cost some money.

I propose that this Stirling design needs to be built and used so that businesses will look at how much they can save by using it and the natural forces of the market can begin to cause its evolution into the

power source of choice and perhaps make a small dent in the dominance of the internal combustion engine.

You can research Stirlings and see that many different types have been tried and they do actually work. But every one of them suffers from a limitation in the working pressure and heat transfer area  which is

what governs the power output.  This design fixes those issues.

This design is just a modification in the layout that puts two Stirling engines back to back incorporating the Miller cycle so that the working fluid can be used at high pressure.  Just as the internal combustion engine obtains its power density from  operating with high pressure so can this Stirling.  The power density can be adjusted with the operating pressure. Theoretically making this Stirling engine more powerful than the Internal Combustion Engine.

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Copyright (c) 2011
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We use this spreadsheet to help us design the power output.  As we experiment, it gets more and more complex.
Last Updated on 4/25/2011
By fractalogic @