Stirlingengineforum.com

Ian

Thank you for the suggested reading, I have not read any of Rizzo's work so I may have a look.

One thing I must clearup is the misunderstanding on my boat propeller, although designed for a model aircraft, I plan to run it submerged. They work well in this application but have a propensity to collect weed so they must be very accessible for cleaning.

Geoff V

Ah, thanks for clearing up the bit about props, You know a bit about the power absorbsion from your test rig. My only boat is about 2ft long, and built of tin, its go a little GAMMA motor. The boat has single channel radio control for the rudder, the boat does move, but not very fast, on the pond at the local domain it sails at a slow walking pace. Origionally the boat had a V twin oscillator type steam engine, that used to run for 15 min till the boiler needed filling, now as long as there is meths in the tank, and the radio battery is OK, it will go all day if need be. It was built for a model boat reggata last year, but because for earthquake activity in Christchurch (40 K away) the event was canceled. The idea I had was to show off hot air engines as possible power units for model boats. Ian S C

by Geoff V » Fri Apr 20, 2012 9:36 am
"but they seem lost amonst the hundreds of Hot Air engines, Tin can engines and Thermo Lag engines (often mistakenly refered to as Laminar flow engines) though the last thing you'll find in one is laminar flow."

Geoff, In the thermal lag engines that have nozzles between the piston and the heater, I always thought the reason was to delay heating until the piston reaches near the top of the stroke - by directing the incoming air through the center and away from the wall of the heated chamber until the air velocity falls sharply enough to break the column and a mixing pattern follows. The wire mesh commonly used confuses the matter, but yes, laminar at least in principle. That's my intuition but you seem fairly certain and it might help somewhere down the line if you could tell me where I'm wrong. Thanks.

Just a little note; James G. Rizzo (from Malta), is the chairman of the British Stirling Engine Society. He has been building small Stirling, and hot air engines for over 30 years, and has published articals in Model Engineer, not sure of other magazines, and a number of books. Ian S C

Thank you Ian, I am aware who he is, I gave a presentation at the SES AGM in Oxford two weeks ago and spent some time discussing burner designs with James.

Geoff V

Dear Bumpkin
I would not dare to say your theory is wrong, I'm not clear as to why they function at all. There must be a thermal delay to emulate a phase shift, whether this is produced by the restrictor I don't know as both of my TL engines have a cooler rather than a restrictor, although the cooler may behave as a restriction, but they discharge directly at the heater, not down the centre of the pipe.
One thing I am clear about, it is very difficult to achieve Laminar flow even with steady flow on the discharge side of a restriction, with smooth surfaces, a diffusion angle less than 14deg (inclusive) one might achieve laminar flow but with reciprocating flow, the chances are probably nil.
For these reasons I tend to agree with the academics, thermal lag is for sure, laminar flow is very unlikely.
I will make time to post some pictures of my pulse tubes as they are counter intuitive yet I have recorded 2900rpm all beit with a very sloppy little end bush.

Geoff V

Dear Bumpkin

I have posted some pictures, as promised, in the Gallery section under 'Bits and Pieces', hope they are of interest

Geoff V

I'm working on my 4th prototype of a Walking Beam Stirling Engine. I keep running into problems with the timing. Eventually I will get one to work, meanwhile it is enjoyable to try different approaches. Right now I'm preparing to try out an aluminum can displacer.

Geoff, I don't think a phase shift is needed to make a thermal lag engine run. I believe the original patent only relied on the natural slowing of the piston resulting in longer duration at either end of its travel, raising or lowering the average temperature for the following stroke. It's a very minimal energy difference though, which begs trickery to get more useful power.

Whether we call it laminar or not, I've seen exhaust pulses from a muffled single cylinder gasoline engine make small circles in the dirt quite a few feet away - but I'm not an academic. I think in SOME engines this is used to separate flow and delay mixing until the column breaks. Many such engines run poorly or not at all if the piston t.d.c. is too far below the restrictor/nozzle. This might be because of an air-spring effect that cushions the column to a stop instead of breaking it off. No worries about questioning validity of theories, I'm just trying to follow possibilities. I'll check out the pictures. Sorry for taking your thread off topic. Bumpkin

Butchwright, timing on a walking beam motor should be no problem at all, if built properly the timing is automaticly set with the design. Are you sure you have no unnessesary leaks, or excess friction, I think you may find thats were your troubles are. Ian S C

Ian,
I will go through the assembly once more to check for leaks. I just cut 2 8.4 0z. aluminum cans in half and rejoined the bottoms to form a displacer that is about the heighth of a small tomato paste can. The joint is bonded with JB Epoxy. I think it should help. Thanx for the advise.

Check that the hole where the rod (or what ever) passes out of the cylinder is not too big, and the piston fits the cylinder well. Ian S C

very nice engine .

I am very interested in the effects of altering volume ratios and phase on engine performance. What volume ratio are you using for your engine? and have you had any experience using a different value? If you have, what effect did it have on engine performance?

What material and type of seal are you using for the piston and displacer seals? Also what material are you using for displacer and piston cylinders?

In the photo of you engine it looks like you have transfer pipes from the hot to the cold end. Have you experienced any problems with choking /heating up, or do they have enough cross-sectional area to stop this?

Keep up the good work.

At last somebody who is building proper stirling engine that actually does produce some useful power.

Hello Ratty2e

At last, someone who is interested in watts!

I have spent quite a lot of time recently working with a friend who has an engine (Beta) in which he can change the phase angle and or the stroke between runs. In broad brush terms the phase angle changes the characterisitcs but has only a small effect on power output but the maximum volume ratio has a large effect on the temperature at which it runs, clearly there is much more to this than a short reply can offer, quality of heat exchangers, regenerator etc. For this engine with a very non sinusoidal motion, a maximium volume ratio around 0.75 seems the best compromise with a kinematic phase angle of 51deg.
My Alpha on the other hand has a fixed phase angle (kinematic) of 90deg but a maximum volume ratio of 0.64. So that there is no misunderstanding I am refering to the ratio of the maximum volume swept by the displacer and the maximum volume achieved between the top of the piston and the bottom of the displacer. On an Alpha this is the same a the swept volume ratio but on a Beta it varies as the phase angle is changed.
Currently my Alpha has a cast iron compression piston with a Rulon LR lip seal and the displacer piston which is aluminium has a three ring system which is preloaded with a small leaf spring. Compression cylinder is hard anodised aluminium and the displacer cylinder is cast iron.
Having plotted the internal pressures with an oscilloscope and two transducers I have added two more transfer pipes and realised about a 20% increase in power even after the increased dead volume.
Geoff V

Geoff, you mention a bit of a worry about possible ill effect of dead space when you added the extra tubes, I would not worry too much,If you can get a copy of Model Engineer for 18 Feb 1977, there is some research done by W. D. Urick on Stirling Engines. One of the things he tried was with a unpressurised V engine, he replaced tghe transfer tube with a 6 ft rubber hose 3/4" dia, there was little difference in power, and that could be put down to the pulsing of the rubber tube. Ian S C