Stirling Engine Design

[nick.gilling]

Hi all,

I have mentioned in a couple of posts that I am working on a design for a simple stirling engine.

I have looked at many stirling engines on the internet over the past couple of years but am just really getting back into model engineering.

In the past I've only made oscillating (wobbler) steam engines so I wanted to keep the design simple for my first stirling engine as I am aware that certain fits need to be much better than for steam engines if the engine is to work.

I have done some 3D modelling on a free downloaded version of Alibre design which is very good. The main problem is that it only lets you have a few components in an assembly, then it disables the save function! So I had to assemble my engine, take screen dumps then just quit and watch the assembly work go down the drain! I just assembled all the components to make sure they fit together ok and the engine looks ok. I am quite happy with the outcome, I have put a few cosmetic features in there to make the engine look a bit nicer which could have been left out to make it more simple. I figured I might as well make it look nice so that it's something worth looking at rather than a bodged engine that works.

Anyway, enough of that, I attach a few views of the assembled engine. I have left bolts and pins out because basically I could be bothered to model them!

I still have to create the working drawings for all the parts yet but that is fairly easy now the 3D models are there.

A bit about the engine:

The flywheel is steel as is the crankshaft. The crankshaft is a 1 piece design however I may make this up of three components, shaft, disc and pin. Depends what I feel like, I want to try one from solid.

This will run on ball bearings in the aluminium pedestal. The con rods are aluminium but will also have ball races in their big ends. The small ends will have silver steel pins through bearing in the brass of the power piston and displacer small end bush.

The power cylinder and piston are brass (I know there are better materials but this should work as long as the fit and surface finish is good.)

The displacer cylinder is finned aluminium (i know the fins could be deeper and more of them but I want to keep this simple, this may limit the running time of the engine as not efficient cooling).

The hot end of the displacer cylinder will be stainless steel if I can get hold of some.

The displacer itself is aluminium and it's rod will probably be bronze brazing rod or steel.

The power cylinder has a bore of 10mm and a stroke of 10mm. The displacer is 14mm diameter and is run from the same crank as the power piston with 90 phase angle.

The ratio of swept volumes is therefore 2.0. I designed it to this slightly high figure as the temperature differential won't be that large in this engine, I want to try to run it from a tea light. (one of those flat little candles)

If you could all let me know what you think I and if you can see any glaring errors that would cause the engine not to run I would be grateful.

All I am after is an engine that will spin over happily for a while as it's my first attempt!

Thanks,

Nick

[epineh]

Looks interesting Nick, unfortunately I don't know enough about the engine to assist you with the design, I am interested in the concept, mainly for power generation. We were recently hit by a category 5 cyclone, and one interesting thing I noticed, Although a lot of people had portable generators, getting enough fuel to run them was another thing, with all supply lines cut, we couldn't get fuel for love nor money. Something like this, could run on any kind of heat source, gas, coal, timber, whatever, and might be easier to run in emergency situations. Of course to be useful, it would have to supply at least 1KW or more, I wonder what physical size engine would be required for this, keeping in mind from a DIY viewpoint, I guess you could buy something like this, but where is the fun in that?

Russell.

[pminmo]

I too am interested for power generation. I'd like to make a practical engine that has more power than a toy (maybe 300watts) , and also has a fair amount of basis in off the shelf parts.

[epineh]

I guess if you made a modular block type design, you could add smaller units together to make a larger power output, possibly generate electricity via standard car alternators to a DC bus and have an inverter output stage like the newer generators, no frequency problems and the machine can throttle back under reduced loading.

Russell.

[pminmo]

Permanent Magnet generator plans are available on the web. Not a big problem to construct.

[nick.gilling]

Hello,

I am not sure on the specific outputs of these engines.

I think to get 1kW the physical size would be fairly large.

I think there would be a lot more things to take into consideration, I have basically just bodged this design together and hope that it will run!

A couple of things to consider would be the optimum ratio of swept volumes (power cyl / displacer cyl). This is believed to be about 1:1.5. The dead space would have to be kept to a minimum so the compression ratio would be relatively high.

You would also need a good design of firebox to keep the heat in the right place and minimise losses from the source.

Another essential thing would be to have an efficient cooling system to keep the cold end as cold as possible. This would be water cooled with a pump and a radiator and fan. (this would sap some power but think it would be worthwhile)

The modern designs have two good fitting pistons (displacer and power) and a regenerator stack between that holds onto and passes on some of the heat during the cycle.

http://www.keveney.com/Vstirling.html

This would probably be the way to go if you're looking for power. I'm sure it could be done but I think you'd end up with quite a large engine! Probably no bigger than a generator though?

Nick

[40fordcoupe]

Hello Nice design! I would however make one small suggestion. That is to use plain or needle bearings on the rods. IMHO If the engine is at all robust the ball bearings will quickly destroy themselves. Good luck!

[epineh]

So long as it fitted in the chook shed (6 by 9 m), SWMBO shouldn't have a prob with it, don't know how the chooks will feel, I know these engines are quiet, but they gotta make a little noise.

[nick.gilling]

Why do you think the ball races will destroy themselves 40fordcoupe? Bearing (no pun) in mind that this engine will have very little power. It is only a 10mm bore / stroke. I know needle roller bearings would be more suitable but I just happen to have these little ball races.

I've never used bearings like this in any engines I've built. If I were to put plain ones in, I would probably redesign the rods and just make them from brass. Or i could just put phosphor bronze inserts in the aluminium rods. I was just trying to think of ways of reducing the friction, as it's so small friction could play a major factor.

epineh the engines are very quite compared to any other types of engine as it's a sealed system. They obviously will make some sort of mechanical noise though.

Thanks

Nick

[40fordcoupe]

If the engine is lightly loaded the ball bearing will be fine. If the load is significantly more the very small contact area will overstress the balls on the side where the load is. The bearings will self destruct in short order. Check out some of the better quality 2-strokes. Also check out the book Making Stirling Engines by Andy Ross. Just a suggestion.

[tim-tbl]

Just Out of curiosity, doesn't a steam engine like that need some way of retarding it, my understanding is that they just run faster and faster until the go into self destruct mode,

Cheers

Tim

[nick.gilling]

There won't be any load on this engine other than spinning the flywheel over so the bearings should be ok for a long time. Thanks for suggestions.

Tim, no, the engine has no throttle as such. You can vary the rpm to some extent by having a smaller or larger flame, i.e. more or less heat onto the hot cap.

The engine revs will peak when the temperature differential is at it's greatest. Gradually the temperature differential will decrease as the cold end warms up, therefore the engine will slow down. With a constant flame the engine will find it's own natural speed and gradually decrease due to inefficient cooling. I am not expecting the engine I've designed to run for a particularly long time, I don't think it will as the cooling probably isn't very good at all, there are only a few fins to increase the surface area a little but nowhere near enough to be very effective really.