Hot Air Engines

[Cornish JackParticipant @cornishjack]

[Cornish JackParticipant @cornishjack]

Much to my surprise, I have, at last, finished the making of the Sieg Hot Air engine. Not to my surprise, it doesn’t work! About the only thing I know about these beasts is that one applies heat to the hot end and, if the Displacer leads the Driver by 90 degs there should be rotation – but there isn’t!!

Any hints, tips or helpful pointers would be appreciated. Hand powered rotation is fairly free and the meths burner was left in situ for about two minutes – the hot end fins became too hot to handle.

TIA

Rgds and Happy New Year

Bill

[Ramon WilsonParticipant @ramonwilson3]

Can’t help at all here Bill I’m afraid, my knowledge of hot air engines is absolute zero but I can wish you a Happy New Year, commiserate on your disappointment and hope you are able to get it running eventually.

 

All the best for the New Year – Ramon

[Sub MandrelParticipant @submandrel]

Hi Bill,

 

As I’m not the only one here trying to ignore the drunken New Year conversations going on behind me all I can do is offer a few ideas…

 

A leak!

A blocked pipe or passage.

More friction than you realise – can you ‘run it in’ using the lathe or similar?

Trying to run it backwards…

 

Oh well,

 

happy New Year for 42 minutes time!

 

Neil

[Ian S CParticipant @iansc]

Hi Bill, you,v joined the mass group of frustrated stirling engine builders, never mind. How free is the motor when you turn it over. The piston must slide easily in the cylinder, are all the joints free, and does the displacer slide freely without leaks.

Some times its a good idea to turn the motor over by another motor for a while, I,v done it with an ammeter on the electric motor, and as the current dropped the stirling engine was able to be started. James G. Rizzo in his book suggests running the motor in with an electric motor. Ian S C

[SpringbokParticipant @springbok]

Hi Bill

The Stirling hot air engine society has some excellent books on this subject and also they usualy exhibit at most shows in the club stands area.

Will dig out the book and title etc for you.

 

Bob

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Too high compression? In case too high compression who caused difficult to run hot air engine, set the phase angle at 120 instead 90 degree. The engine must be in low friction and leakfree. Use all ball bearing if possible to replace from plain bearing to ball bearing in your engine.

 

You wrote:

“the hot end fins became too hot to handle.”  Wrong material in displacer piston can make trouble, use stainless steel (is a bad heat conductuctive material) tube with thin wall as displacer due to prevent to lead heat into the cold camber in displacer cylinder and the length must be longer (3-4 times longer in length/bore ratio) to make more difference in temperature in hot and cold ends in the displacer cylinder. In hot end use stainless steel with thin wall to lead the hot into cylinder in short time without loss of heat and use aluminium with thin fins in cold end of displacer cylinder to lead the hot out of cylinder. Water jacket is good too..

Edited By Jens Eirik Skogstad on 01/01/2012 07:47:06

[Cornish JackParticipant @cornishjack]

Thank you all and best wishes for the New Year.

Friction? Difficult to judge – if I ‘flick’ the flywheel, it rotates for about one and a half revs with no obvious hard spots. Ball bearings would need to be quite small to suit shaft dimensions.

Cylinders? These were provided, ‘ready to go’ and, presumably, correctly ‘specced’ , certainly better than I could have made them!!

Leaks? Quite possible but a) how would one best check for same? and b) what would be recommended to eliminate them?

Apologies for constant questions but all this is new territory

TIA

Rgds

Bill

[Ian S CParticipant @iansc]

Bill, If when disassembledyou put the power piston in the cylinder, does it slide through under its own weight, and if you block one end does the piston stop moving? If not try toothpaste to lap the piston/cylinder, then wash it out, or if the piston is graphite, polish it with a peice of plain paper. Ian S C

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

When i builded the first stirling engine, i used the piston sealant made of leather lubricated with oil and the packing box for displacerpiston was made of lamp wick with grease. It worked very well. In case your engine is without piston sealant, the tolerance must be correct to keep tight and less friction, be careful with oil since displacer cylinder must not be dirty by oil.

[Cornish JackParticipant @cornishjack]

Ian and Jens, thank you.

Difficult to do the piston drop test ‘cos the drive cylinder crank pin is a steel bolt which tightened itself rotating one way and loosened t’other … so I Loctite’d it in position so no longer easy to dismantle From memory, there was noticeable ‘stiction’ before it was assembled so improvement possible there.

The displacer piston may well have some oil on it as it’s vertical and the pin linking the piston to the crank passes through a brass guide which I lubricated so there may well have been some seepage into the cylinder. Have given it a few minutes running driven by a Mini Drill – it didn’t seize but there was some speed variation.

Presumably the ‘normal’ household sealants are no good for this sort of application?

Rgds

Bill

[Ian S CParticipant @iansc]

For the displacer gland I, v used a number of methods, starting with carbon impregnated teflon bushes, brass bush with teflon tape, plain brass, bronze, or cast iron. All on stainless shafts, and without lubrication. If made properly there seems little to choose between them.

Would not worry about any small amount of oil thats run into the displacer.

As long as there is no great leaks around the displacer gland, I think it just needs running in, You can use a crank with a large pully to a small pully on the motor if you don;t have a suitable electric motor. I would not use a mini-drill, or other portable drill, although the motor from an old battery drill would be a good start, perhaps with a 6V or so power supply, if you use a plug pack supply make sure its up to it. Ian S C

[Ian S CParticipant @iansc]

Bill, did you get the crankshaft assembled spot on? Ian S C

[Gordon WParticipant @gordonw]

I’ve not been very successful with hot -air engines either, but the above tips are all good. On a small engine friction is a killer, would definitely go for running in via a motor, maybe using the lathe. I built a low temp. motor that would not run, finally put the thing in a bowl of hot water to test for leaks, amazed to find where the leaks were. Not all motors will respond well to dumping in water! Some household sealants are OK, depends on temp .range etc. Still fascinating things tho’.

[Cornish JackParticipant @cornishjack]

Thank you Ian and Gordon.

Will continue the running-in and keep fingers crossed!

Re. the crankshaft – it’s a built-up job with the webs drilled as a pair (so, hopefully, exactly matching) and the shaft a press-fit as was the crank pin. Not too confident of the press fits so drilled and pinned them as well … an ‘interesting’ exercise with 4mm shaft and 3mm wide webs!! The crank bearings are (relatively) wide brass drilled and reamed as one before fitting into the standards as one piece and then separating – should maintain the line-up. All this is, by my standards, high precision but, no doubt, everyday ‘bread and butter’ stuff to most contributors here.

Rgds

Bill

[Ian S CParticipant @iansc]

My latest motor(no 17) is a little beam type power cylinder 3/8″ diax 1/2″ stroke, the displacer cylinder is made from the steel case of an AA nicad battery. the crank is fabricated as yours is, and held together with Loktite, it normally runs on a very small meths burner, but every now and then I put a larger one under it, or put the gas torch on it, havn’t got a tacho, but I would estimate, well over 2000rpm, maybe over 3000rpm, I,m trying to do in the crankshaft, using the ideal that if you don’t break it, your not trying hard enough! Ian S C

[Sub MandrelParticipant @submandrel]

Two thoughts:

 

1 “if I ‘flick’ the flywheel, it rotates for about one and a half revs with no obvious hard spots.”

 

I find steam engines reluctant to tick over on low pressure air unless they will do several turns on a ‘flick’, you might have more friction than you want, so try running in.

 

2 If you drive the motor externally it should ‘run in reverse’ acting as a refrigerator and cooling the hot cap – this offers a way to do a basic test on whetehr its works or not.

 

Neil.

[Ian S CParticipant @iansc]

Neil, you are quite right about running in reverse, but if the motor is run in normal direction, the cooling system is utilised sa it radiates the heat.

To measure the temperature, I use a indoor/ outdoor electronic thermometer, with the outdoor probe on the hot cap, with the Ross Yoke motor (in my album) driven by another hot air motor I’v had it down to -20*c. Ian S C

[Cornish JackParticipant @cornishjack]

Happy New Year to all and many thanks to all who offered help, advice and encouragement … IT RUNS!!! – not for very long but it DOES run, unaided!!

On the basis of the various suggestions, I motored it for an hour and a bit, tried it – no joy motored it again with a smear of Brasso on the driver piston, tried it – no joy Same again with toothpaste (Sensodyne – so, arguably, the wrong stuff REDUCING heat sensitivity!!!) – no joy. Sooooooo … retire to hot bath and contemplate – I had a ‘Eureka’ moment (or should that be a Diogenes moment?) When I made the driver conn rod, I reckoned that it was about 10 mm too long, so I shortened it.

Anyone care to guess what that ‘Eureka’ moment was?

All will be revealed tomorrow!!

Rgds

Bill

[Jim GreetheadParticipant @jimgreethead]

I’m hangin’ in there waiting for the answer. And for photos, videos (of the engine, not of you bounding down the street wrapped in a towel).

 

Jim

 

[Cornish JackParticipant @cornishjack]

Jim – no chance of me bounding ANYWHERE – knackered knee and back and wrapped in a towel would frighten the horses!.

Ah! – the denouement … what about installing the driver piston upside-down!! Yes, indeed. Confession, they say, is good for the soul, so maybe there is merit in it – and a warning to other similarly ‘unsharp tools’ that it’s worth investigating WHY plan measurements appear to be wrong BEFORE chopping bits off!! In my defence, I can point out that there were NO written instructions with this ‘kit’ and the only pre-made parts were the displacer cylinder and piston and the driver similar. Both had been tapped 3 mm and, having installed the displacer, I just repeated the process for the driver – it was, of course, the other way round.

The fix was to turn up a thin ali disc to fit over the open end of the driver piston and Loctite it in place. However, still no joy I have only found one other example of this particular build – a Youtube video, so examine it carefully. Towards the end of the video, there is a section showing it set up and running CCW (from the flywheel end) so I moved the driver pin to suit, gave it a bit of motoring and tried again … success! Only for a couple of minutes and then it slowed and came to a halt with wisps of smoke coming from the driver cylinder. Repeated the exercise later for SWMBO’s benefit (VERY impressed!). I am now thinking ‘remake driver piston and run-in again’ – any comments?

Rgds

Bill

[Ian S CParticipant @iansc]

Been looking at the drawings, don’t see were the problem might be. I would check that the displacer (pt 32) with its rod (pt19) do not leak, take the assembly and stick it in a pot of hot water, and see if there are any bubbles of air getting out where the rod screws in, if there are, seal it.look around for other leaks. Make sure the displacer does not touch the cylinder wall. You did get the right con rod in the right place?

Don’t worry, this is what Stirling Engines are about, some times your lucky, and they go perfectly first flip of the flywheel, Other times you try and try, give up, then come back a few weeks later, light up, give it a flip, and away it goes, don’t know why! Ian S C

[Ian S CParticipant @iansc]

Cornish Jack, did you get the motor going? If so what did it require to get it moving, apart from a sore wrist, flicking the flywheel. Ian S C

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