Micro Stirling Engine
Please note: As of late November 2016 we have modified this engine to be almost completely transparent, it is slightly different to the engine shown in the videos. Images and more information regarding this new engine can be seen at the very bottom of the page.
The Stirling Engine – Past, Present & Future
There are many types of Stirling engine available for sale at the very bottom of this page, however, if you would like to learn about our little engine, continue reading –
Before learning about our Micro Stirling Engine, i would first like to go through some history of the stirling engine and its possible development into the future, if you want to skip this section and learn more about our engine simply go further down the page to the section titled “Micro LTD Stirling Engine”.. The Stirling Engine is undoubtedly one of the most fascinating engines ever invented. It is an external combustion engine as opposed the internal combustion engines that we are all familiar with – 4 stroke, 2 stroke and diesel engines. The engine was invented in 1816 by Reverend Robert Stirling, hence its name. It’s invention was very nescessary – occasionally badly manufactured rivets would shear or snap while trying to hold steam engine boilers together under high pressure, the resulting explosion killed and maimed many people, usually the engines engineer and an innocent bystander or two. Robert Stirling not only invented this engine for his own financial reward, he also seen his engine as being a safer, lower pressure alternative to the steam engine as well as being able to run on any fuel source available. The engines had quite a bit of success after their invention and were employed widely as a stationary engine in coal mines pumping water, lifting coal and hauling the coal trolleys. Unfortunately for Mr Stirling, the 2 stroke engine, 4 stroke engine and electric motor were invented shortly after his own engine, these engines were smaller, more powerful and ran on seemingly limitless fuel supplies – electricity and gasoline. These new engines and the elctric motor were taken up by industry immediately and the stirling engine looked like it was to be left behind in the annals of history. However, as we know, today is a lot different to back then, – global warming, fuel poverty and the ever increasing cost of electricity and gasoline is helping the stirling engine to make a comeback. As stirling engines are external combustion engines, they can run on almost any fuel source – solar, geothermal, wood, coal ,gas and anything else you can think of that can make heat or a flame. In fact, some stirling engines appear as though can run on cold even though they are still running on heat – all these engines require is a temperature difference to exist between the hot and cold cylinders. There are several companies currently developing stirling engines as combined heat and power (CHP) systems to be used within homes & for off-the-grid applications, these engines can produce well in excess of 1000 watts. An example of a domestic CHP system would be the following – imagine a typical home with a wood burning stove in it – currently the stove is creating heat for the house and allowing the vast majority (over 70%) of the energy to travel up the flue and escape out the chimney into the atmosphere, why not mount a stirling engine generator to the stove or within the flue? The electricity produced can then be used within the home or sold back to the power companies. There are also several car makers working on stirling engines that can recover some of the energy wasted by the 4 stroke and diesel engines within hybrid electic motor/engine cars – a stirling engine is placed along the exhaust system, it runs off this heat, generates some electricity and sends it to a battery bank, this electricity can then be used to power the electric motor within the hybrid car when applicable.
Types of Stirling Engine
There are 3 general types of Stirling Engine, the Alpha, Beta & Gamma. Another type of stirling engine is the lamina flow engine, YTEngineer will soon be making a video about these, but for the time being we will stick to the standard stirling engine types –
- The Alpha type requires high temperatures and produces a lot of power as it has two power pistons, these are typically in a V-formation with the pistons joined at the same point on a crankshaft.
- The Beta type requires slightly less heat and has the power piston & displacer within the same cylinder, it looks quite strange but also very cool.
- The Gamma stirling engine has a displacer and a psiton. The job of the displacer is simply to shuttle air between the hot and cold sides of the engine, these changes in pressure affect the airtight power piston and allow the engine to run. Gamma Stirling Engines, which are also known as Low Temperature Differential stirling engines, can run on very low temperatures and is seen by many as an alternative to solar panels/photovoltaic cells – gamma stirling engines convert sunlight into power at much higher efficiencies than photovoltaic cells. Our Micro Stirling Engine is a variant of the gamma type stirling engine – we use magnets rather than rods, this lowers friction, increases efficiency and minimises the chances of air leakage – air leakage and friction are the arch enemies of low temperature stirling engines.
If you would like to learn how the Alpha, Beta & Gamma engine work in exquisite detail, i recommend watching YTEngineer’s video here – How Stirling Engines Work.
Micro LTD Stirling Engine
Our micro stirling engine is a gamma type – It can run on low temperatures and at high speeds, all that is required is a cup of boiling water and it will happily tick away. As far as we can tell, the EpicPhysics.com Micro Stirling Engine is easily the smallest commercially available low temperature differential stirling engine in the world – most other LTDs flywheels are bigger than our engine. Small low temperature differential stirling engines are more difficult to get to manufacture and run successfully than larger models – the amount of working gas within the engine is much lower and any friction AT ALL will stop a small engine whereas a larger engine might be able to overcome a small amount of friction. Our engine measurements in both metric and imperial are as follows –
- Displacer chamber: just over 25mm (1 inch) outer diameter with a 22 mm (.87 inch) inner diameter
- Overall engine height 60mm. (2.3 inches)
- Power cylinder height is 18mm (.7 inch) tall with an outer diameter of 8mm (.3 inch) & an inner diameter of 5mm (.2 inch)
- The power piston is 8 mm (.3 inch) in length and 5mm (.2 inch) in diameter.
- The top and bottom plates are made of aluminium and are both of 25mm (1 inch) in diameter.
- The displacer/regenerator stacks are of 21.5mm (.85 inch) in diameter and have a depth of 1mm (.04 inch) each. The individual sections have a 2mm (.08 inch) diameter x 1mm (.04 inch) deep neodynium magnet at their centerpoints, these are crucial to the functioning of the engine. More about the regenerator stack further down the page.
- Flywheel: 50mm (2 inches) in diameter
In the image below you can see all of the parts before assembly. Starting from left to right we have – the crank, conrod with 2 micro bearings, 3mm screw, power piston, power cylinder, power cylinder housing, aluminium top plate, top plate housing with screws. In the middle level – regenerator sections 1,2 & 3, aluminium bottom plate & bottom plate housing with screws. On the lowest level – displacer chamber, flywheel and power piston support with 2 micro bearings, 1mm main shaft & the flywheel.
The bearings within the connecting rod and flywheel assemblies are ridiculously small, you could easily fit ten of them on your thumb. Each bearing is rated up to 100,000 rpm, although they will never reach that speed on this engine,we have reached an astonishing 80,000 rpm with our Tesla Turbine Kit. The bearings will never need to be oiled, we pour ultra fine graphite dust into each bearing during assembly, this enables them to become self-lubricating.
The piston and cylinder are lapped together for several hours ensuring a smooth and precise fit. No other piston or cylinder combination will be as good as the two that have been lapped together, most pistons will not even slide freely within a “strange” cylinder. The piston is made from ultra low friction graphite and never needs lubricating, in fact, lubricating the piston and cylinder with even the lightest oil would cause a phenomenon known as Stiction – sticky friction, which would cause the engine to cease running. You will also note the neodynium magnets on the bottom of the piston,the micro bearing within the conrod and the 3mm screw holding a second micro bearing in position.
Once again, as far as we have researched, our engine is the only LTD on the market that contains a regenerator within the displacer. Most other LTDs have a simple foam displacer, no doubt this is to save cost and makes the engine easier to manufacture for the producers of those engines. We could not afford to be so complacent – due to the small size of our engine it most likely would not run without the regenerator stack. The regenerator was one of Robert Stirlings greatest innovations, it vastly increases both the power output and the efficiency of a stirling engine. There are several videos on Youtube showing LTDs running with and without a regenerator, the difference in performance is remarkable. Our regenerator consists of three seperate stacks, each one has a neodynium magnet at its center to allow for the interaction of the engine with the power piston. There are also five 7mm holes around each individual stack, these contain a micro mesh of copper wire, this aids in the cooling and heating cycles of the engine. The three individual stacks slot together to form one single regenerator stack. The image below shows all three individual stack assembled and sitting on my hand. This regenerator had to be redesigned many times – you need to find a happy medium whereby the air is passing through a substantial amount of mesh, yet not so much mesh that it begins to impede the airflow, otherwise the air will try to follow the path of least resistance and squeeze around the edges of the displacer, in that case the regenerating system becomes pointless and may as well be replaced by a bog-standard displacer.
If you have not yet seen the video created by YTEngineer showing our engine in action plus how it works, please watch the video below. The engine can be purchased through the Paypal links below the video.
Alternatively, there are a vast array of Stirling engines and kits available further down the page.
The new micro stirling engine
Our new engine is almost completely transparent. It is made using 3mm acrylic and it’s parts are cut on a high precision laser cutting machine. The engine has a slightly lower performance than the previous engine as it is missing a top plate, however, we feel that the transparency of the engine makes up for this minor shortcoming. Images of the new engine can be seen below –