Here are a few photos and videos of a hot plate i built to display a cheap LTD Stirling engine I bought on eBay.
I was browsing eBay on my phone one sleepless night and spotted a cheap LTD Stirling engine being sold out of Singapore and purchased, 9 days later it arrived in the post. eBay mobile makes you feel like Wylie Coyote as you order and then just wait for the postman.
Out of the box the little engine would run on a cup of boiling water but would barely tick over on the router and just about ran on the PSU of my laptop.
I decided to tweak the engine here and there to see how fast i could get it to go. I bodged together a simple stand and used a small stub of candle from about 3-4 inches under the base plate. The engine ran much faster but would quickly over heat after only a few minutes.
I picked up a stainless steel spirit burner on eBay for £8 and decided to make a wooden display stand/hotplate that could also be used with other engines or for keeping a pot warm.
The base was turned out of mahogany and is recessed to take the spirit burner. the uprights re turned from seriously hard boom handles, I have no idea what the wood is but its so hard even the sharpest of tools seem blunt. The hot plate was made from a disc of aluminium i had planed to use for a sterling engine i was gathering parts for. the hot plate was drilled and countersunk so the screws would be flush with the plate.
The following tweaks to the engine have greatly improved its performance, and after a few hours flat out at approx 300 rpm it seems to be running in well and will run on low temperature object much better and even ran on a friends hand heat but he has better circulation than I do.
The linkage joints all where very loose fitting and had serious lateral play so i used needle nose pliers to squeeze them in for a more snug fit. this was a good improvement as the power piston would twist about 10-15* during each rotation and this is now almost unnoticeable.
I removed the power piston from the tube and used the graphite core from a builders pencil to colour in the piston so it had a uniform coating of graphite to lubricate the piston.
On examining the top of the glass power piston tube i noticed that it had sharp edges and there was a audible catch, so i used a round 600 grit wet diamond file to bevel the inner edge and remove and sharp edges. I made sure to wash away any traces of the powdered glass from the inside of the tube so they would not bind up the power piston.
The instructions said not to use oil on the model but to use pencil graphite, i tried this and it made no noticeable difference, so the inner engineer rebelled and decided to use a very light weight oil in a lubricating pen to lube the joints and the tips of the axles and the cups they spin in, this was a noticeable improvement over graphite, the oil was sparingly applied on the end of a piece of fine.
The displacer was fixed to the displacer rod with CA glue, it was also adjusted so there was about .5-1mm of a gap between the bottom of the displacer and the base plate at the bottom of the stroke.
Because the base of the engine was designed to sit on a mug of hot water it will not sit flat on the hot plate and needs a riser, i used copper 2p coins to make a riser, this required a stack of 2 coins to clear the base so i reversed the clamping bolts so the nut was on top and this allows for a single coin and gives better thermal transfer but coins are not perfect and still heat the base plate in hot spots rather than a uniform heat over the full plate, i will need to cut a 3mm disc of aluminium to use as a riser to improve the thermal transfer.
To stop the top plate overheating due to thermal transfer up the clamping bolts i folded up some TP into pads that fit the pot plate then wet them so the keep the top plate cool. they require regular wetting as they heat up and dry out if the model is left running for a long time. To reduce the thermal flow through the blots i plan to make either wood or plastic pillars with counter sunk screws to clamp the plates together, this should cut down the thermal transfer from the hot to cold plates.
I have a few more ideas as how to improve the performance of this little engine, this is a fun little toy to tinker at on the wet and rainy day like today when my pains play up and I tend to stay indoors so i plan to keep tinkering at this.
The engine will now run at approx 300 rpm and is happy to do so for hours at a time a long as few drops of water are added to the pads every now and then. the more it runs in at high speed the better it seems to be at running at low temperatures.
Thanks for looking and i hope the Stirling enthusiasts enjoyed this,i will leave you with a video of the engine running at approx 300rpm, I will post any future additions or improvements as they happen.
PS the hot plate cooked the crap out of the displacer tube and I had to redesign things check
this Ible for the details
