Tiny Stirling Engine

[madjackghengis]

Well Nick, that's what you get for posting a u-tube of tiny stirlings on your thread   I spent almost an hour yesterday just looking at all the different tiny engines and wondering why I am planning on getting rid of any scrap at all.  With all your cylinders showing no compression you either have a problem with the gauge and or its connection with the cylinders, which should be audible when cranking over, or you probably have a blown head gasket, which, with all cylinders affected, should also be audible when cranking.  Unless you've got a very new vehicle which has valve controls I've never seen, even a timing belt or chain would leave at least one cylinder with the valves closed, and thus get compression.  Kind of wish you were in my neighborhood, at least I could confirm your diagnosis.  Hope things work out better for you, and know the level of inspiration you have raised with that tiny stirling, and the not quite as tiny flame sucker, which is absolutely stupendous.  I'm also very taken with the tiny stirling set on the oak thumbnail, which runs so well and looks so perfect in all its glory, even when not running.   It might be time to work on a stirling powered vehicle, for reliability.  mad jack

[NickG]

Madjack,

you make a good point there about scrap! Until I made my flame gulper, I'd only ever used scrap bits of metal to make my engines. For the flame gulper though I was fed up of hunting around for bits and I knew I didn't have any cast iron so just took the opportunity to buy all the bits I needed whilst there. It was a bit of a revelation for me as I found rather than struggling on, all the material I bought machined beautifully! So horses for courses I guess. For the tiny stirling I will be using scrap again!

The engine on the Renault Clio is the 2.0 16v 172bhp - it has variable valve timing which I'm not that familiar with, apparently it could be something to do with that vvt pulley. I'm going to borrow another compression tester but I fear the worst, I put my hand over one of the spark plug holes and cranked - there was nothing! I'm at a loss trying to explain what's happened at the moment. I don't think it's head gasket as there is literally no compression and there is no oil in the water or vice versa. As you say, it should be audible when cranking but again, nothing! I'm with you on the cam belt - at least one of the cylinders would have both valves closed at some point and hence get compression. The only explanation I can come up with is that the at least 1 valve on each cylinder has stuck open - possibly being bent but then you would have thought I'd hear something metallic. I won't be able to confirm this until I get at least the rocker cover off though. Will see what the proper compression tester reveals and go from there!

  I actually hate cars at the moment!

Nick

[picclock]

Hi
in your first post you stated :

'I did calculations to ensure the ratio of swept volumes between displacer piston and power piston are almost the same - this is quite important as it largely governs the temperature difference the engine can run on, of course there are a lot of other factors but I think this engine should work'

If my understanding is right, this will result in a non running engine (its quite possible that I am incorrect as I have only recently started researching these). So please don't take offence.

These engines work on the expansion of air caused by increase of temperature. If the volume is fixed that's about 1 psi for every 5C. To return to normal atmospheric pressure the air will expand 1 / 273rd of its volume, so for 5C that would be around 2%.

For a coffee cup or LTD a reasonable differential would be 30C, with an expansion of 11% to normal atmospheric pressure. This would mean that the displacer volume / power piston ratio should be 89 - 11 or around 8 to 1.

If the displacer volume is larger, heat will be lost without doing work (no big deal), however if the ratio is the smaller then the energy produced will be lost as the piston works against the atmospheric pressure, overexpanding the gas.

As I said earlier, I am still coming to an understanding of these engines so feel free to tell me where I am wrong, I certainly will not be offended.

Best Regards

picclock

[BillTodd]

These engines work on the expansion of air caused by increase of temperature. If the volume is fixed that's about 1 psi for every 5C. To return to normal atmospheric pressure the air will expand 1 / 273rd of its volume, so for 5C that would be around 2%.

For a coffee cup or LTD a reasonable differential would be 30C, with an expansion of 11% to normal atmospheric pressure. This would mean that the displacer volume / power piston ratio should be 89 - 11 or around 8 to 1.

Interesting stuff 

Anyone know if using a scotch yoke on the displacer would help? My thinking here is that the yoke could provide additional dwell at the top/bottom centre of the displacer movement.

Bill

[picclock]

Hi Bill

Thats what my post in the design section, vertical shaft LTD was about.

Its not as elegent as a scotch yoke, but would have lower friction and less load.

Regards

picclock

[NickG]

Hi
in your first post you stated :

'I did calculations to ensure the ratio of swept volumes between displacer piston and power piston are almost the same - this is quite important as it largely governs the temperature difference the engine can run on, of course there are a lot of other factors but I think this engine should work'

If my understanding is right, this will result in a non running engine (its quite possible that I am incorrect as I have only recently started researching these). So please don't take offence.

Hi Picclock,

Thanks for the interest and your calculations. I didn't know that method for calculating an ideal ratio for efficiency, I did look into the maths behind it many years ago but can't remember any of it now so thanks for that. What I did know is generally, the lower the temperature differential, the larger the required ratio.

What are you saying would result in a non running engine though?

As I said, I calculated the ratio of Jan's engine and designed mine to pretty much match that. The ratio, if I recall correctly was 22:1.

Nick

[picclock]

OK I see where I have gone wrong. In your first post where you say

'the ratio of swept volumes between displacer piston and power piston are almost the same'

you are referring to the same ratio as in Jan's plans, not the same displacement.

From my figures optimum ratio for 30C differential is 8:1.    22:1 sounds a bit severe, needing 12.5C differential for optimum efficiency.  (edited due to bad math - See later post for a better understanding of this issue).

To me this gives the piston a higher average pressure over a much smaller diameter. Decreasing the ratio to nearer optimum will allow a more efficient extraction of the energy available. AFAIK, the closer to the optimim ratio, the better use is made of the available energy.

I'm quite new to this so my calculations may be in error, but on Jan's most recent coffee cup design he has a 100mm displacer and a 14mm power piston both being moved 5 mm. This gives an expansion ratio of 51:1 on my calculations (displacer=39250, power=769.3).

However, as I said before, I am still at the trying to understand   all aspects of this phase, so there may be reasons unknown to me why my calculations are incorrect  .

Just bought a couple of plastic tubs at Wilko's for experimenting with. When I get enough bits I will start a project log and try to figure it all out.

Best Regards

picclock

[madjackghengis]

Hi Nick, I was going to comment about the calculations but it seems that picclock has a fine handle on the important parts and what he says essentially covers everything except the friction parts, which are more easily dealt with by known materials, and experienced building, than calculations at this small size.  I am largely unaware of modern versions of car engines, as I gave up that career years ago, and don't want it back.  The VVT alters things as far as compression testing, but you still should have compression.  As far as a valve in each cylinder being off its seat, it's either an automatic compression release, for ease of starting, in which case it is normal, or the strangest of coincidences, which I don't believe in, and am supported by the likelihood of piston meeting with said open valve in at least one of the cylinders, making loud funny noise.  I know automatic compression release is becoming common in small engines and in some motorcycle engines, because they have to make so much power out of small volumes, and such engines rely on strong ignition spark because they lack on compression, for starting.  I would start with checking to see if your engine does have compression release, and I'm assuming it cranks over relatively quickly as it ought to.  I gave up on newer engines because I've spent all the money I want to spend on tools, and every new model needs new tooling, and new electronics to divine its problems.  I get enough work from people who have engines most mechanics never even heard of before, because they weren't born yet, when the engines started the first time.  No one teaches the old technology in schools.  Can I suggest you buy an old Mercedes?  I know this is  , but my new to me 81 240D is running great and has no electronics, only electrics, which are like plumbing, to someone who once worked on radar.   it gets good mileage too, and only cost me $1000 after my Buick got water logged with our recent flood.  That's less than the difference between what our son gave us for the Buick, and what he still owes, since his car got flooded far worse, one of those low sitting Mustangs with water all in the wiring and the dash.  If you've got one, put a timing light on one of the plug wires, and see if you've got spark at anything like close to the timing mark if it has one.  If it were a head gasket, you'd hear the huffing and chuffing, and I expect the same if it were a problem with stuck valves.  I hope you get this straightened out. mad jack

[picclock]

Hi Nick

I think I've got a bit more figured out now. First a nice simple optimum ratio equation:-

optimum temperature differential = 273 / (power cylinder volume/displacer volume)

In Jan's design with a ratio of 51-1 the temperature change for optimum work is 273 / 51  or only 5.4C.   

So why do Jan and others use such a high ratio, and optimise for such a low temperature differential ?. The answer is that during the time that the displacer is in a heating or cooling position, the maximum change of temperature that can be made to the working fluid is limited. These limitations are caused by the low thermal conductivity of the working fluid, and the small surface area available to heat/cool the fluid.

Which brings us back somewhat to my post in design ideas, where I suggested that if the displacer had a layer of pie tin foil (mmm Pies    ) on either side this would increase the area available to heat cool the fluid, allowing a lower ratio and making more power available.

Hope this helps to clear the waters I've muddied.

Good luck with the LTD and the car - seems like your due for some.

Best Regards

picclock

[NickG]

Hi Madjack,

I'll quickly get the car issue out of the way - it's grown but still no further forward! There is indeed zero compression on all cylinders but the cam belt wasn't snapped - quite puzzling. It has something called a dephaser pulley on the inlet cam which moves relative to the shaft hence changing the timing according to throttle position, rpm and inlet pressure! I think the only viable explanation is that it's slipped around  due to a faulty tensioner or water pump bearing and got drastically out of timing, pistons clattered valves and bent, sticking at least 1 valve open on each cylinder hence no compression. I can't confirm it yet as spend 1h30min trying to get rocker cover off, only to realise you have to take off wheel, inner wheel arch, crank pulley, timing belt cover, both cam pulleys then rocker cover! Anyway, it's a very sore point but the upshot is, the most cost effective and easiest way is going to be to get a 2nd hand engine (£600ish) and swap it.

Now back to the project - I really want to start this but with the car in the garage, off the road that may become a more pressing matter! Another thought is, to save money - get on with this engine and it can be my dad's christmas present - I can't afford to buy anything more with the car fiasco!

Thanks for the further analysis picclock. Firstly, remember a lot of people designing / making LTD stirlings are trying to achieve an engine that runs from the temperature of your hand hence the large swept volume ratio and as you say, the large surface area to take on as much heat as possible. A lot of people make the cold end finned to transfer heat to the air but yours is a good idea of effectively creating a larger surface area on the inside of the hot end to transfer the heat to the working fluid.

Remember mine is based on Jan's new micro stirling, I guess there's another limiting factor hear in that you want to keep it tiny - I don't think Jan has thought about this in detail to be honesst but it seems his engine runs on something with a temperature of around 70 C so about 40C differential? so I thought if I maintain his ratio, mine should run as it is also larger.

It would be interesting to construct something with what the calculations show to be the most efficient ratio and see if it works on the designed temperature differential - I'm not sure I've got my head around the calculations properly. Surely a massive displacer with a large swept volume and surface area (assuming it's kept light) to take on heat would be able to drive a small power piston more easily than something with a smaller volume and area?

Jan has made an LTD and done some experiments - I roughly worked out it has a ratio of swept volumes of 52 and he says runs down to 8deg temp differential so they could be right. It's strange his smaller one with the 22:1 ratio needs a temp of around 70 degrees to run though or a difference about 40 deg. But is this due to something madjack illuded to  - friction, which is probably much higher on the small engine in proportion to its power output - so it's something that can't be ignored in these calculations at least for small engines.

I see Jan on his website has done a 2nd improved version now, incorporating some of the same ideas as mine (I haven't shown him mine yet so he must have thought the same about the original). He has made his even smaller though instead of making it larger like mine!

Nick

[NickG]

Well, I finally got a little bit of motivation and got in the garage to cut some metal tonight!

A slightly frustrating night but it has yielded 1 component - I started off collecting some materials together but then got bored of that so started turning the bottom displacer cover or hot plate as I've called it on my drawings. I quickly realised that because of the thin register I need to turn that and part it off a longer piece of stock. I have some cast gunmetal that I'm using. The slither that I had planned to use has now ended up as the flywheel! This means I've already deviated from my drawings, when I felt the weight of the aluminium I'd planned to use for the flywheel I thought it was too light, so I thought I'd use the cast gunmetal and thicken it up by 1/16" to 1/4" thick. The diameter has ended up as 1.22" or something rather than the 1.5" I had originally planned. Actually, this now seems a bit heavier than I thought so may have to do the ally  one after all but maybe keep the 1/4" thickness, then it might be heavy enough.

Anyway, here it is:



The cake is 2nd hand, my 2 year old son licks the icing off but he didn't do a very good job on this one so waste not want not!



Feel a bit disappointed that that's all I managed in a night but at least it's done. As I said, it deviates from the drawing in size and material. I couldn't be bothered to find  / change tools etc either so the recess isn't quite right. I think I will modify the drawing just to widen the alloy flywheel. Unless this one works then I'll change it to brass. The plan is just to loctite it to the crankshaft - just easier for this little engine, it'll have no power so won't come loose.

This is probably the 2nd most complex part on the engine so at least it's a start.

Nick

[Stilldrillin]

That looks good Nick.Yer off to a good start. Despite the material decisions/ recycling.  

You are tougher than me..... I've managed 3off, 20 minute sessions this past week. Before the fingers gave up.  

Angle grinding yesterday. REALLY didn't need the extra cooling draught it produced......  

David D

[NickG]

Thanks David,

For some reason I didn't feel the cold actually - it must have been the coldest day for a long time yesterday, mind you, had long johns and thermal vest, then a fleece, jeans and overalls on so guessing that helped! Got a bit of carpet in front of the lathe too which helps. What doesn't help is the car sitting behind me with a half dismantled engine and bits everywhere to trip over!

Nick

[NickG]

Did some more in the garage today - again, not as much as I'd have liked. More messing around sorting the parting tool out - it had remained in snapped form since I tried to part off some tough steel but I need it for this. I always struggle turning thin things like cylinder covers for some reason!

Not much progress to show, below is the hot plate or bottom displacer plate, the other side doesn't look as good as the parting tool was not on dead centre height so it left a bigger pip that it should of, then I had to file it down - no matter though as it's the underneath of the engine.



I have almost finished the top plate which is probably the most complex part - touch wood it's gone ok so far, it had to stay in the milling machine though as just got to drill and tap the hole for the upright then it's finished. I got a good fit with the register on the plastic tube so am confident it will seal well. just need to work out what sort of glue to use. Maybe some quick ish drying araldite?

Nick

[cidrontmg]

Lo Nick,
that sure is a small engine! If you need a heavier flywheel, you can solder three twopenny pieces together, and done... I´d be likely to lose most of my ready made parts before I could assemble them together. Even if I kept them in a box, then I´d probably lose even the box...
Gluing with Araldite will certainly keep the bottom and the plastic tube together. Even if you later on would like to get them separated  
One possible alternative might be "Marston universal jointing compound", that you can get for instance, from
 http://www.bengs-modellbau.de/werkzeug/werkstattbedarf/marston.php
It´s not a silicone but polyurethane, and it never really hardens. It´s easy to take apart if needed, and absolutely tight when in place. The page says it´s blue, but mine is definitely red... Good up to 250C°, at least. The excess (and the stuff itself when disassembling) comes off wiping with a rag with some spirit (ethanol, methanol). I´ve used it in several engines, for gaskets. Despite its English-sounding name, it is made in Germany (http://www.marston-domsel.de/index.php?id=173&L=2&L=2). You don´t need much of it. I bought some years ago a 20 ml tube, and have used it for many engines, when I´ve stripped and put them together again, many many times, during the years. And I still have some 3/4 of it left. It certainly beats silicone gasket compounds (or paper or Klingerite, etc.) in model engines, hands down. And the usual: No connection with Bengs MB, except a satisfied customer.
 

[NickG]

Hi Olli, I've just done some more and am now only realising the true size of this thing myself - what have I taken on here?! 

Thanks for the advice on sealant, I will have to look into it, it would certainly be an advantage to have something that'll withstand temperature, although I'm not sure what the plastic displacer cylinder will go to! 

Got a bit more done today - I bet people can't believe this ... I've been in the garage a few days running!

Started off by finishing the top plate which is combined power cylinder and displacer rod guide on my version. All I needed to do was drill and tap the hole in the right position 4ba - how hard could it be?



Well, for me very hard apparently - I tried to be clever and tap it in the milling machine under power-  normally I do it loose enough so the tap will spin in the chuck - not this time  it stripped the thread straight out! At one point I was just considering loctiting the bolt in when it comes to assembly but then I realised the other bolt option I had got out was slightly larger-  4mm alen key screw. Won't look as good but it'll do the trick.



Next I cut the plastic tube to length, I gripped it in the lathe with a bit of nylon I found inside for support and parted it - worked fairly well, just needed tidy it up a bit with stanley knife, take little burrs off.



Unfortunately it's probably not as clear as it appears in the photo, it also has picked up a scratch on 1 side. On the plus side, it is a really good fit on the little registers on top and bottom plate and is actually just about air tight with no sealant at all, but think I'll use something at least on the hot end.

Next I decided to have a look around for some material for the con rods - I couldn't really find much, I had designed it 1/8" wide x 1/32" thick. I found some 1/8" x 1/16" brass strip and thought this would do. The little ends have a slot in them 1/16" wide so there won't be any clearance there - will just have to make sure there is a little. The big end journals are 1/8" wide so plenty of clearance there.

This was the point when I had to double and triple check my dimensions as it all seemed stupidly small!



That is the shorter displacer con rod, I have made the other too (see last photo). The only problem I see with these is whether I'll be able to get them over the crank or not - I was planning on turning that from solid. Hopefully they'll bend out enough to slip over then nip back with pliers. I annealed them before I made them to straighten them but may do again to be on safe side.

Here are all the bits so far:



Still got uprights, all rods and pins (they are just cutting to length though), crank, displacer and pistons to do.

Nick

[madjackghengis]

Hi Nick, you let other people play with numbers and glues and such, you're doing a bang up job on a tiny little engine and I can only wish I were ready to do the same.  I'm really looking forward to seeing it run, now that I have it in proper perspective, that is quite a little beastie, and should be beautiful running. mad jack

[NickG]

Thanks Madjack,

Forgot to mention in the text, I did use some thin aluminium to protect it while it was offset in the 4 jaw to drill / ream the power cylinder hole but I must have gone a bit far - didn't want it to come loose so it still marked the gunmetal through it. 

Never mind!

[raynerd]

Hi Nick, sorry, not clicked on this one for a while. It is looking a real stonker!! I`m really liking this and the size is crazy. I hope it runs for you - I`m sure it will!

Chris

[NickG]

Thanks Chris,

I'm doubtful as to whether it'll run or not, I should have tried a much larger jobbie for an LTD as these look tricky at the best of times, but I fell in love with it when saw it and thought I could make some improvements to the design. Have you revisited yours yet or are you concentrating on the Stuart? I know you've been making those great puzzles too.

I wish I had made it from materials that would take heat - suppose I could always replace the plastic for some glass and some sort of high temp adhesive though if I need to. It's going to be a present for my dad so even if it doesn't run it should look ok - bit choddy giving your dad something that doesn't work though 

Anyway, here is the latest instalment - no machining to show, all I did was cut / file / grind some pins to length and turn the displacer - I didn't have any balsa wood or really any wood I could easily turn so I thought i'd make it from nylon - had some 5 thou under the diameter I had planned so don't think that'll matter, I have no idea whether the material will work though! 

[Bogstandard]

Nick,

Sorry, I replied to your post over on HMEM.

I gave a few pointers that might help.


John

[sbwhart]

Looking good Nick

Stew

[Stilldrillin]

I have to smile, every time I look at this build Nick.  

Fiddly diddly parts/ my fumbly fingers....  

I'd have enough remade parts, stored under the duckboards/ in corners/ disappeared entirely, to make a batch quantity!  

Blummin well done! Of course it will run!  

David D

[madjackghengis]

Hi Nick, I'm keeping an eye on this one, the tiny ones are the hardest to get running, as the friction and power come closer to each other from opposite directions.  With regard to the nylon, your biggest issue is mass, and if you can counter balance the mass of that displacer, the power piston only has to make it change direction, not lift it, so it's a matter of inertia rather than sheer power.  If it's too heavy, you can drill many holes in it, and cover them with plastic film glued on.  Moving the displacer is necessary, but it is the big energy drain the power piston has to provide for.  I keep thinking of a well with two buckets and a rope so you only lift the water and not the weight of the bucket, and thinking about two tiny engines, with their displacers out 180 degrees and with the cranks connected, so all the parts counter balance each other.  I'm really hoping you get this little engine running, it ought to, for your dad.    by the way, what is "cold" there?  I'm feeling it here, and it's 25 degrees F.     On the car, I hope the engine swap goes well for you, and you find a warm day somewheres along the way, I know I don't look forward to engine swaps any more.  That engine sounds like it has too many parts for its own good, all in all.  mad jack

[NickG]

Thanks for the advice John / Madjack - I will have another look around for more suitable displacer material. I think I might have some balsa wood round at dads - can't imagine it machining well!

John mentioned exactly the same over on HMEM. I was planning on putting a counter balance on the flywheel but as you say, this could make the moment of inertia too high - funnily enough I had just sketched the bucket scenario, you also have the power piston which is 90 deg out but that is pretty light. If I can't find anything else easily, skimming a bit off, drilling a load of holes and sealing with some film seems a good idea though.

It is pretty cold here at the moment Madjack, on the odd couple of nights it's been under minus 10 Deg C, apparently -14 Deg C one night! I put the thermometer in my garage the other night and it said 7 though so I doubt if it's been much under 5 in there.

Cheers Stew!

David, surprisingly in this cold, I've managed not to lose anything yet - I put the 2nd little pin in a pin chuck to de-burr after the first nearly went missing though! I cleverly (or so I thought) put all the bits inside the displacer cylinder with the top on so as not to lose them - today I thought I'd take the lid off to admire my work and the bits went everywhere!    Was almost a disaster but managed to find them all - phew! I really hope I can get it running.

offtopic: On the car engine, that thing just gets stupider by the minute - apparently the problem could have been caused by a slipping crank pulley that drives the camshafts - to which I said, surely there must be a keyway - but no, there are no key ways on the engine, everything is controlled by friction and stretch bolts! So if somebody has torqued the crank pulley bolt wrongly or the bolt is passed it, the whole thing slips, pistons into valves and good by cylinder head! I can get an engine for around £650 but as somebody said, it'd be kind of daft not to have the cambelt done before putting a 2nd hand one in - but that costs about £400 due to 4 specialist locking tools you need - then there's the auxillary belt that drives aircon, alternator, power steering pump, water pump, tensioner etc - it's really just a nightmare. Somebody did point out that because of the model it is, it's prob worth about £500-£700 without an engine so that is an option I'll have to weigh up, just getting rid of it:offtopic: