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Old 07-05-2008, 09:24 PM   #1
NickG
 
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Default My first Hot Air (Stirling) Engine

Hello all,

I am pretty new to this board and find it a really interesting and useful forum.

I have been working on my hot air engine very sporadically since the new year, well, before that if you include the design stage. I have just finished the last parts and assembled the engine. Although there are a few mistakes, the engine has gone together and seems to turn over quite nicely. The parts (I think!) were all quite a good fit and I am quite proud of my achievement as it involved lots of new techniques for me.

However, tonight I am quite disappointed as I tried to run the engine for the first time ... not only will it not run with a tea light candle (as I designed it for), but it won't even run with a butane pen torch on the hot cap with it glowing red!

I think I know the problems with it but would be interested to hear anybody else's views also! I think there are a few things:

1. The ratio of swept volumes between the displacer and power piston isn't large enough, so not a large enough volume of air is being heated and displaced. I designed it as 1:1 i.e. power and displacer pistons are about the same diameter, but with hindsight that was silly, I think that would require a massive temperature differential to get it to run, especially on this small scale.

2. The cooling on the cold end of the displacer cylinder is not good enough.

3. There is too much dead space in the tubing between the displacer cylinder and power cylinder, I've not worked this out but now I think about it, it's probably quite large compared with the actual cylinder volumes which can't be good. At the time I remember thinking I didn't want to drill the tubing too small as I thought it might choke the engine somewhat.

4. Friction - I think it's ok, it's got ball bearings for mains and big ends and seems to turn over quite well, however, I have nothing to compare against. It spins over for 4 - 5 secs with the tube disconnected between cylinders.

Does anyone have any thoughts on the above?!

My first thought is to try making a small bore copper tube as originally intended to connect cylinders and reduce dead space, don't think this will solve it though.

Then I was thinking of a larger displacer cylinder, piston, hot cap etc. bigger diameter giving a greater volume of air to heat and displace. I think at the time I designed it to suit the only stainless steel I had for the hot cap ... silly decision!

Either that or just put it down to experience and design and build another one! I changed my lathe for a much better one and sold my milling machine about half way through this project. I am now on the look out for a decent vertical miller so it would be nice to start something from scratch on this far superior equipment. Maybe this time I should use a recognised design, however, I am semi confindent that I could design one of these and get it to work as it did have some small glimmer of life, it nearly picked up when the hot cap was glowing cherry red, but obviously this is a long way from what I envisaged originally!

How people sucessfully get the LTD Stirling Engines to work is now beyond me ... hats off to you guys!

Nick















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Old 07-05-2008, 09:38 PM   #2
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Default Re: My first Hot Air (Stirling) Engine

Nick,

I am no expert on these engines, but just looking at it, you might be having a friction problem.

All those ball races need to have the shields removed, and all traces of lubricant washed out of them. They should be run dry to produce the least running resistance.

I am sure one of our other members can update you on your data and tell you if it is correct or not.

John


 
 
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Old 07-06-2008, 01:18 AM   #3
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Default Re: My first Hot Air (Stirling) Engine

First try a large, diffuse heat source. A can of Sterno would be perfect. Lacking that, a cup of alcohol with a large wick. You want lots of heat (as opposed to a high temperature).

Judging from the pictures, the hot cap looks way too small. That should be the first change if the above doesn't work. Try making your next hot cap from ordinary steel instead of stainless. It will transfer the heat to the air a bit better. Stainless will stay shiny and blingy but when it interferes with function...

Check for air leakage at the displacer gland. Apply some oil and look for bubbles. If you see any, apply some heavy gear oil as a temporary stopgap.

Stirling proportions are important. Build from a working design until you get a feel for things. (I expect you know this now.)

Although I doubt it's your problem, John's advice of getting the oil (or worse grease) out of the bearings is on the mark. Also, ensure that the displacer piston isn't dragging and has sufficient clearance so the air can flow around it.

Try the Sterno and get back to us with your results. We're interested in helping.

I built an engine to the same design but I scaled it from a larger design in ME. It didn't work until I built a bigger displacer chamber.
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Old 07-06-2008, 07:54 AM   #4
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Default Re: My first Hot Air (Stirling) Engine

Thanks for the advice guys. I cleaned the main bearings out so they are running dry, they have no covers. May try taking the big end covers off and cleaning, although as I said, it does seem to turn over pretty well. The power piston bore is only 3/8" so I may be fighting a losing battle.

I am thinking of trying a 3/4" displacer cylinder / piston / hot cap and if that doesn't work, maybe bore the power cylinder out to 1/2" I was reasonably happy with the power piston fit and the gland, however there may be slight leaks there so might try getting some ptfe and making a new gland and power piston.

I noticed when I'd first assembled it that there may be some slight drag in the displacer piston. I put my tongue over the port and spun the flywheel and I could feel it pumping and sucking a slight amount of air, despite there being around a 1/32" clearance all around it. Is this big enough? Thought it would be on one this small.

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Old 07-06-2008, 01:47 PM   #5
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Default Re: My first Hot Air (Stirling) Engine

I agree that the ratio of diameters is an important factor though I don't have any guidelines to suggest. Somewhere I have seen that though and will try to find it. The only other thought i had is that I assume (from the pictures) that the connection from the displacer cylinder to the power cylinder somehow runs down and across through the base. Could it be that due to the mass of metal and the length of the run, that the expansion taking place in the displacer is being counteracted by cooling before it has a chance to push up on the power piston? Just a thought.
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Old 07-06-2008, 03:11 PM   #6
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Default Re: My first Hot Air (Stirling) Engine

I don't think cooling of the air in the conduit between displacer and power piston is that much of an issue.

To first order, the air in that conduit acts as a hydraulic fluid to transmit the force of the air expanding in the displacer chamber to the power piston. The air expansion occurs in the displacer chamber and doesn't (and shouldn't) mix much with the air in the conduit.

By the same argument, the exact diameter of the conduit (assuming it's reasonable) doesn't seem very important.

Once in the hot end, the air has to absorb heat and expand rather rapidly - the air doesn't spend much time in the hot end. This means that the hot end has to be made of a material that can quickly lose heat to the air and it also means that it should be hot over its entire surface - heating a small portion of the hot end to a high temperature isn't nearly as efficient as heating the entirety to a somewhat lesser temperature.

Another "quick fix" you should try before reworking the hot end is to cool the cold end more - just to see if you can get it running at all. Try dripping some cool water on it while heating the hot end with a diffuse heat source.

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Old 07-06-2008, 08:46 PM   #7
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Default Re: My first Hot Air (Stirling) Engine

Hi Troops

The 'Stirling Engine Manual' [ Vol. 1 ] by James Rizzo suggests a ratio of 1.5 : 1, Swept volume of Displacer to Power Cylinder respectively.

This, together with Vol 2, are a gold-mine of designs and practical info. on Stirling Engines.

Got mine from Camden who are the publishers.

Far from cheap, but excellent value.

I would recommend for anyone interested in Stirling Cycle Engines

Also another good one from the same source is by Roy Darlington.



Bluechip.

edit ...in case you are not familiar with Camden www.camdenmin.co.uk
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Old 07-06-2008, 09:34 PM   #8
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Default Re: My first Hot Air (Stirling) Engine

Thanks for the info people. I gave it another go and it was nearly there but not quite. There are definitely signs of life and I am going to try as you suggester mklotz, if I get a large alcohol flame on the hot cap and try to drip some icy water over the cold end I think it might just turn over.

Also did some calculations earlier, currently the bore is 3/8" and stroke 1/2". Since the stroke is fixed due to using the same crank, I worked out I want a displacer diameter of about 9/16" to give me a ratio of volumes of 2:1 displacer to power cylinder. I think I can bore out the existing displacer cyl but will obviously need a new hot cap, maybe with a flange rather than thread as don't think I have a fine 5/8 die! Suppose I have no excuses now my lathe has a full screwcutting gearbox though!

For the record, the port comes down the displacer cylinder support through a rubber tube into the bottom of the power cylinder. It was one of my thoughts that this volume may be quite large in proportion to the volume displaced but as mklotz said, I don't see why it matters now, although I have seen other articles saying to keep dead space to a minimum. Probably something to do with volumetric efficiency for competition engines or something!

Will keep you posted!

Nick

ps Have always wanted to buy that book, might prove invaluable!
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Old 07-06-2008, 09:56 PM   #9
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Default Re: My first Hot Air (Stirling) Engine

A flange is probably better anyway since it gives you the opportunity to interpose a disk of some insulating material between the hot cap and the cold end. (Nevertheless, you should still learn to single-point threads. )

I think the dead space that you want to minimize is the dead space *in the displacer chamber*. IOW, the displacer should come near to hitting each end of the chamber at the extremes of its movement.

What material did you use for the displacer? In these engines, the displacer acts as a crude regenerator so it should be made from a material that can readily absorb and give up heat. Aluminum is a good choice.

Is Sterno or its equivalent sold in the UK? Here in the colonies its available in better hardware stores and cooking supply places because its used to heat chafing dishes in buffet lines. I especially like it for outdoor displays because slight breezes don't disturb the flame nearly as much as is the case with an alcohol lamp.
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Old 07-07-2008, 11:51 PM   #10
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Default Re: My first Hot Air (Stirling) Engine

I think I might try some screw cutting!

The displacer is aluminium, hollowed out to about 1/16" thick wall. I have seen displacers stuffed with steel wool in the past to enhance regeneration qualities, does this work?

I haven't heard of sterno to be honest! I did a quick search on google and it seems it is some sort of gel? Don't know what an equivalent would be.

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