Tiny Stirling Engine

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NickG:

--- Quote from: picclock on December 01, 2010, 05:32:53 AM ---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.

--- End quote ---

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 :hammer: all aspects of this phase, so there may be reasons unknown to me why my calculations are incorrect :scratch:.

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 :offtopic:, 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. :lol: 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

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