Comments on: Simple Stirling 1 performance with and without regenerator

By: Derek R

Derek R — Tue, 18 Oct 2011 11:59:57 +0000

Hi Admin,
Thanks for your advice. I do appreciate your help and will consider it carefully.
When/if my engine runs I will keep you posted.
Derek R

By: admin

admin — Mon, 19 Sep 2011 20:46:04 +0000

The swept volume ratio between the Power cylinder and the displacer is not fixed a fixed value. It depends primarily on the temperature ratios you have for the hot gas and cold gas, and secondarily is influenced by the dead volume. If your engine has a large dead volume the power cylinder would be a little larger. Probably the best way is to use the Stirling engine simulator http://www.solarheatengines.com/stirling-engine-simulator/ and size the power cylinder and displacer so that the isentropic compression heating is around 25%. If it is a lot higher than that (like 35%) then the engine will have too much compression. If it’s low then the engine will have less power than it would with higher compression. To give you some examples here are the ratios for some of my engines. Th = 450 deg F, Tc = 80 F (measured at the external surfaces, the gas will be have a smaller temperature spread) the displacer has a 1.875″ diameter and the power piston is 1.25″ diameter. Both have the same stroke so the volume ratio for displacer to power piston is 1.96. This engine doesn’t get much power at Th = 400F, 500-600F provides good power. The solar-powered engine I am currently working with typically operates between Th=125F and Tc = 80F. The ratio on this engine is 7.2. It starts running with a Th/Tc (absolute temperature) of 1.05 and by Th/Tc of 1.1 it is running unloaded at 140 rpm. If there is any question that your engine doesn’t hold compression well then favor the larger displacer ratios.
The diameter of the power piston really doesn’t matter, it’s just the volume that matters.
I built an engine with very short displacer that was essentially very fine steel wool very lightly packed between some end plates with lots of holes. The trick is getting small clearance around the circumference so that the gas is forced through the steel wool and doesn’t just flow around the edges. It’s interesting that this engine would turn at higher RPM (because the regenerator was effective) but did not have as much torque as the same engine with a solid displacer and only a few holes filled very lightly with steel wool. My advice is to use steel wool with the finest wires (#0000 is what I used) although very fine aluminum should work well if you can find it fine enough.

By: Derek R

Derek R — Mon, 19 Sep 2011 15:19:19 +0000

Hi,
Just found this website and I like it. I have an enduring interest in LTD Sterling engines and have built a successful engine that runs well on a 5deg “C” temp’ diff’. My next effort was directed at a Rotary Displacer/pressurised engine designe to run on Solar hot water panels and/or Compost derived heat source. I am just not a clever enough engineer to make the bits!
My current project is a more conventional LPG flame heated engine with linked , twin displacers and power pistons.
Before I spoil alot of painstaking work I need to know 3 things.
1. What ratio of swept volume is appropriate between Power cylinder and Displacer?
2. Is the diameter/area of the face of the Power piston critical in proportion to the working Gas volume? And what is that, expressed as a ratio?
3. My displacer is virtually the same area as the swept volume. However , while it is possible to fill it with a regenerative matrix (Can I suggest aluminium swarf from a lathe?); surely there must be an appropriate sized hole In proportion to the swept volume, bearing in mind that the potential dead/none working gas volume are almost the same? Otherwise, taken to extremes: Then removing the shell of the displacer while retaining the body of the regenerating matrix would be the best available solution. Clearly this is not the case but the solution as to the best size of entry and exit hole through the displacer eludes me. Can anyone help?
Derek R

By: rick s.

rick s. — Fri, 24 Jun 2011 10:23:44 +0000

I’m not an engineer nor do I own a sterling yet but after viewing several internet videos on the subject I’ve often wondered how Peltier
devices may be used in conjunction with sterlings since the device heats,cools and changes heat to electric, as a heat sink add-on,regenerator, heat or cooling source, heat reclaimation in order to make sterlings more efficient. If waste heat is a by-product of a sterling depending on the source temperature, and cooling is required, either on the working cylinder or on the other side of a regenerator, how about a Peltier device used to generate electricity (google: caframo ecofan) and use that current to power a peltier cooling the cool side of a regenerator? Would that make a more efficent Sterling? Automobile catalytic converter material used as regenerator cores? Also, since some sterlings are dubbed as thermo-acoustic devices whice operate at an audio rate or generate powerful sound as a by-product, have the sound impact a diaphram and the diaphram impact a peizo-electric crystal to generate electricity? Or a combination of all or part of the above to improve sterling efficiency?

By: admin

admin — Wed, 24 Nov 2010 18:08:09 +0000

As far as the regeneration function, it does not fundamentally matter whether the regenerator is inside the displacer or external. In both cases you need to be concerned about thermal isolation from its surroundings. Placing the regenerator inside the displacer has several advantages, the path length is short (it can be too short in the case of a thin displacer), the added dead volume can be less because the path is shorter, and my primary reason is simplicity. There are also some disadvantages: As you point out, you usually pay a weight penalty. I think the biggest disadvantage is that having an external regenerator lets you route the gas through a better heat exchanger after the regenerator, for both heating and cooling. With the internal regenerator you typically just dump the gas into the hot and cold chambers which provides less efficient heat transfer with the walls. This can be improved somewhat by using exit nozzles as the gas leaves the displacer to direct the gas against the hot and cold plates for increased turbulence and improved heat transfer. I have measured performance improvements with this technique although I have not optimized it. I have also used displacers that are essentially all regenerator which also seem to work quite well, even though they add additional dead volume. It all depends on careful attention to thermal isolation, heat transfer, and gas flow friction. My few experiments using polyester material for regeneration were unsuccessful. I believe the low thermal diffusivity of the material makes it a poor choice compared with metallic material.

By: Philippe

Philippe — Wed, 24 Nov 2010 10:59:02 +0000

Hi folks, i hereunder precise some more specific information about displacer body configuration since i’ve been studying a lot this sharp subject. First of all, one may consider that we are curently concerned with gamma ltd engine working with a low delta temp. In this configuration please notice the so called regenerator(s) is inside the chamber and not outside, which is a very important point to point out because its furtherless important to precise what is the real function of it. There is actually two different aspects one may considers : In the the gamma engine holes or stuffed area with polyester or polyuretnane foam are placed to help the homogeneity and the velocity of the fluid transfer from one area to the other of the chamber; the subject is then enhancing of the fluid transfer. Second point is the thermal aspect and here is the mater relative to the gamma architecture since regenerators are again located within the chamber; one may considers that basically the hot area may stay as hot as possible and the colod one as cold as possible so to improove delta temp and efficiency of the engine. What will really occurs if you include a thermic regenerative material inside the displacer body (ie stealwood or metallic mesh)in which fluid will go trough ? You may risk to achieve the oposite result first attempted whic is to comunicate heat from one side to the other area of the chamber which result will occurs in lowering the delta temp of the concerned areas. Its’ very important to notice that regenerators used as heat exchangers on others architectures are always placed OUTSIDE of the chamber in a way to emphasize the delta temp.This is why i’m not so long persuaded that internal thermic effect material used inside the gamma chamber are efficient, may be worse since classic design and material used for the displacer body is an insulated one (ie polystyrene)to prevent any thermic bridge.
For sure it’s steal interesting and may be an obssesion to once again recover part of the heat energy which is obviously lost but one should absolutely consider that further thermal effect goal shouldnt be at the opposite of the original concept which point out performant isolation of the two areas.
The main idea is to keep the hot hot and the cold cold areas with high transfer fluid velocity. Second idea would be to help both areas to keep hot and cold so long with a storage material within the displacer but without ant fluid transfer through it to my sense. Third is to take into account the weigh of the material used to perfom the regenerators since scotch brite fiber is really heavy and may occurs unperformance. As an example for you to compare with, the full mass of the displacer body of my engine is 0,9 g (165 mm diameter)and is approx 2g once reinforced and equiped with its rod and attachment. If used,each 30 mm dia scoth brite fpolyster fiber regen weight is about 0,4 which is * 6 equal to 2,4 g, ie more than 100 % more !!! I ve been testing a lot of different material weigh and the best micro tubular or porosity one still polyurethane or nothing!!! (just simply smaller holes to help fluid transfer whic dia are smaller in a way to compensate the lost dead volume) Polyurethane ones 30 mm dia is approx 0,1 g each so 400 % less !!! Radial clearence may be adapted to the type and size of the regenerators architecture and dimension you may choose. To achieve this subject about weigh, please consider that every additional mass you may introduce in a mooving part will create a negative performance in terms of additional friction on the axis.
At last and about new perspectives considering the heat efficiency of the displacer and regenerators body material and once considered all said upper please let me know your experience or ideas which are welcome whatever they may feel crazy or not.

By: Peter Gross

Peter Gross — Sat, 20 Nov 2010 07:04:35 +0000

I ran the engine on solar heat for about 40 minutes and experienced the moisture problem you mentioned. It was going great until then – doing up to 200 rpm. Hopefully now the balsa has been dried out it won’t be so much of a problem but it will probably depend on the humidity of the air. I might try sealing it with paint.
You get quite high temperatures from the Fresnel lens. I tried putting some clear acrylic sheet over the top of the cylinder to improve heat collection but things got a little warm and it started to melt! I might try again with some glass but I’ll need to be careful with the plastic cylinder.

By: admin

admin — Mon, 15 Nov 2010 17:07:18 +0000

I like the idea of using a CD added to the flywheel so you can add a balance weight. That’s a simple solution.

I’ve used balsa and other wood for the displacer and had problems with the moisture coming out of the wood at high temperatures and condensing on the power piston and cylinder. The water causes a lot of friction between the piston and cylinder and I have to dry it out before the engine will run properly. I can’t remember if it was a problem at 80-90C or only when I went over 100C on engines with a metal displacer cylinder. I’m wondering if this could be your start-up difficulty.

By: Peter Gross

Peter Gross — Mon, 15 Nov 2010 00:50:39 +0000

I added a CD to the flywheel (drilled and bolted under the fender washers) and it helped a lot. I’ve used the CD to mount washers at 90 degrees which smoothed out the running. I can get about 8 mins from boiled water (80-90C), but it doesn’t always start nicely. One difference from the plans is that I used a PVC pipe coupling (ABS is not readily available here). As this is longer I made the displacer deeper by 10mm to compensate. This made it a heavier than yours (40g) as it is balsa, but I am now working on one with a regenerator and expecting further improvement. I’ve made a couple of small refinements to the running gear which may be of interest. If you email me your address I can send pix. I like your design because of its flexibility and the easy access to the displacer, unlike the tin can engines.

By: admin

admin — Fri, 12 Nov 2010 18:25:29 +0000

I’m glad to hear it’s working. A balance weight for the displacer will help at low RPM. The club-type flywheel with weights at the end is efficient from the standpoint of getting most of the mass out as far as possible from the axis of rotation, but doesn’t give you a place to attach a balance weight for the displacer. A more traditional flywheel or a cross-type would give you a place to add the counterweight for the displacer. At higher RPM a displacer balance weight on the flywheel will give you problems with an out-of-balance flywheel. When you get a regenerator working the RPM should increase enough that balancing the displacer won’t be such an issue. I’d work on that first. Just for reference, the club-type flywheel is simple to build but isn’t safe when you go to larger masses or higher RPM. A round flywheel is much safer because it just rubs against body parts that get in the way rather than clubbing them.

I don’t know what the exact displacer shaft misalignment issue is. I assume it isn’t bent. You might be able to drill the hole in the rod end slightly larger to allow for the misalignment. A few thousands of an inch might reduce the friction. It may cause a little extra noise at high RPM.