My first LTD Stirling Engine - a few questions

atwooddon · Post by **atwooddon** » Mon Feb 04, 2008 5:56 pm

Hello All, a new member here, have been lurking for a few weeks and just recently registered. Discovered LTD Stirling engines on Youtube and had to have or build one. After watching videos until my eyes were blurry, I decided to try and build one from scratch. I am reasonable handy and have the luxury of a small lathe so I began accumulating 'stuff' to assembly the engine. I had no plans and just built what 'felt right'. Following is a description of what I used and then some questions..... and yes, it does run if you fiddle with it long enough. ;-)

Top and Bottom plates = Hard Drive platters approx 3.75" dia probably stainless steel
Flywheel = another of the Hard Drive platters also 3.75"
Displacer = 1/4" balsa
Vertical Cylinder = a slice from an old acrylic vase approx 3" id x .75" H
Power Cylinder = copper nipple smooth bore 5/8" ID
Power Piston = JB Weld molded in the copper nipple then sides turned down slightly leaving 2 sealing 'rings'
Stroke of displacer = .5"
Stroke of Power Piston = .375"
Crankshaft supported with ball bearings
Gland = brass fitting turned down with vertical hole 5/8" high
Displacer rod = chrome slider rod from old CD drive

System turns pretty freely, power piston action is smooth and pretty good compression. Had to fiddle with the displacer rod to get it run smoothly.

Initial tests didn't work at all. No sign of any attempt to run using a single tea candle. So, I slept on it overnight and thought thru the system. Decided to increase the displacer stroke to maximum (.5"). Still nothing so I added more heat and put some ice on the top and 'Eureka', it started to run. Slowly but running. It would run until the ice melted then stop.

So, a few questions. I assume the use of the HD platters was not the most efficient. I think they are stainless steel with an oxide coating and aren't transferring heat well and that is partly why it takes so much heat to run? Swept volume ratios? Stroke differences, do I need more power stroke?

Sorry for so many questions being a new guy but I am excited I actually go this first one to run I want to build another BETTER one. Not too interested in kits, would rather do it myself, but I am not reluctant to take accept some knowledgeable help along the way.

I live in Nothern California so if any of you are near Sacramento, let me know. Would love to share experiences.

Don

Cartech · Post by **Cartech** » Sat Feb 09, 2008 10:54 pm

Glad to hear you got it to run, neat huh? I'd say your top and bottom plates would do better with aluminum. It also sounds like maybe your displacer isn't fully bottoming out at each end of the stroke? You mention increasing the stroke there, it should pretty much touch at each end without causing friction. I would also think your power piston has considerable diameter with a short stroke, this would be more of a high rpm design and not good for LTD in my opinion. I've read that the displacer should fill about 2/3rds of the chamber meaning yours is too thin for the amount of air volume inside. I think the displacer should be half to 2/3rds and you should get better results. I have also heard that sweep volume needs to be at least ten time greater in the displacer than the power cylinder for a typical LTD. I also recommend that the stroke and piston diameter are the same i.e. 1/2 bore, 1/2 stroke etc. Figure your sweep volumes using that as a gide line. You can also play with timing if it's not too hard depending on your crank setup. Ninty degrees is base but some run better with more or less. Here is a link to the ones I have made if your interested http://nevadasteve.tripod.com/whatsstev ... s/id1.html

atwooddon · Post by **atwooddon** » Mon Feb 11, 2008 8:24 am

Thanks Cartech, I will try your suggestions. It will be easiest to adjust the displacer size first. When you say 'swept volume', does that mean the volume created by the area of the displacer (one side) times the stroke? Don

Cartech · Post by **Cartech** » Mon Feb 11, 2008 1:44 pm

The sweep volume (to the best of my knowlege) is the area (volume) of air space in your displacer after the displacer is installed. This will remain constant as it is a sealed system and it is the "working" fluid of your engine. Your displacer chamber must effectively heat and cool this air each cycle or you have no engine. If the chamber has too much air to heat and cool rapidly then I would start by increasing the thickness of the displacer and/or thining the height of the chamber itself. It seems most LTD's have about 50% of the chamber filled with the displacer. You must of course allow space for the air to get around the displacer too since it's not really a piston/cylinder relationship in the normal manner. If it all but touches each plate then it will be "displacing" the maximum amount of air to the hot and cold side respectively. It's the surface area of each plate that must transfer the heat to the air. Sorry if this rants on, you likey know most of this. I have assumed by your description of your engine that the plates are about 3 inches in diameter and that your piston is 5/8ths accross. This sounds to me like a large piston even with a short stroke and it will have more friction than a smaller piston with a longer stroke. Also the area of the plates seems small and this would be why it takes a large differential temp to run it. My LTD won't run off your hand but runs on about 60 degrees differential. I have made it run with just dry ice on the top too. It does about 800 rpm.

atwooddon · Post by **atwooddon** » Sat Feb 16, 2008 6:39 pm

Thanks CarTech, I appreciate your suggestions.

I decided to put the original engine on the shelf and built a bigger one (50%) that had adjustable stroke on both displacer and power piston, increased the relative diameter of the displacer and used your suggestions. It ran ok on the first try and gradually got better as I adjusted it. It would run for about 30 minutes on a bowl of very hot water. I noticed it would run better with the power piston slightly more than 90 degrees following the displacer stroke and needed a fairly short stroke so I assumed the power piston was too large a diameter. I made a new sleeve and piston from brass tubes and reduced the diameter about 25%. It runs very well now and I can get over an hour on a bowl of very hot water.

I have begun reducing the displacer height and displacer chamber volume in 1mm steps and it is continuing to get gradually better. I think I will leave this one as is and go for a 'real' engine (meaning better quality) and make a couple for the grandsons. They love mechanical stuff!!! :-)

Cartech · Post by **Cartech** » Sat Feb 16, 2008 7:02 pm

I am pleased to hear I was of some help and I think your idea to build a new engine with adjustable stroke and timing was good. Both the engines I made had adjustable timing but not stroke. I made timing easier to change on the second one. I also found my second engine performed best with the displacer leading the power cylinder by about 80 degrees but oddly, my first needs to be 90? I'm hoping to have time soon to start a new design I've been pondering for about a year.

atwooddon · Post by **atwooddon** » Thu Feb 21, 2008 10:15 am

I did post a video of my second LTD on YouTube. This video is the 'original' version. I have modified the engine by shortening the displacer and displacer chamber so the displacer is only about 1/4" thick. Runs an hour on a bowl of steaming hot water now. It still likes the timing delayed from 90 degrees but I think that was a piston stroke and displacement issue which I solved by making a smaller piston and sleeve from brass tubing. Thanks again for all the help. Will make a 'pretty' one now. Here is the link to the original version running.

Don

http://www.youtube.com/watch?v=QmMApl2xpH0