I could see a bit of warping in the acrylic due to the pressure of the clamping bolts because the acrylic was becoming soft from to much heat getting through to the base plate. I then knew I had a design problem with my hot plate and needed to make some changes to remedy the situation before i destroyed the engine.
This Ible shows the improvements to the hot plate that raised the bar to far and reached temperatures that brought the acrylic tube to the edge of melting point and how i resolved the problems of overheating.
I have to say at this point that these little engines are the AK47's of the sterling world, may not be all that well built but they can take some punishment and still work with a bit of a clean and some oil. I’m seriously impressed with this now after the abuse its been put through in the name of research and plain and simple messing.
--------------The cause of the problem ------------
I discovered that a hard disc platter is an almost perfect fit for the base of this engine. It required a little bit of a file at the 6 points where the screws are. the hole in the center is useful as it leaves the hot spot a bit cooler so
hopefully the displacer rod will not over heat.
The disc platter as a riser worked far to well as its an a perfect conductor, because it is perfectly flat it has a large surface area rather than the uneven copper coins that would only be in contact with the base plate at a few point on each coin. I started to notice a ripple in the acrylic tube as it had reached a high enough temperature to soften the acrylic and the pressure from the clamping bolts had squeezed the soft acrylic.
I also replaced the unsightly wet TP with mats of cooker hood filter as it does not fall to bits as soon as you wet it. this worked even better then the TP as I could tailor it to the shape of the top plate. I found that there was allot of heat rolling up around the hot plate and causing the water on the wet pad to evaporate quite quickly calling for more water to be added ever 10 mins or so.
----------------- the fix --------------------
So time to dial it back a bit and get the temperature down and more controllable.
I found a nice blue CPU heat-sink in my pc junk box and fixed it to the underside of the hot plate with a countersunk self tapping screw. The downward facing fins trap the heat and reduce the hot air flow up around the hotplate that seemed to dry out the wet pad to quickly, unfortunately it heats the hot plate even faster and to over 110*c the upper limit of my digital probe. The flame from the spirit burner was just to big no matter how low you turned the wick down, I would have to reduce the flame some how.
The heat-sink does greatly reduce the amount of heat that flows up around the edges of the hot plate, this helps reduce the rate of evaporation from the wet pad.
I fabricated a flame suppressor out of the bottom of a aluminium drinks can (deodorant cans would also be good) and some garden wire, I used a modified toffee hammer (just ground to a proper striking surface) to beat the aluminium around a strip of flat bar to form the shape. Think paper folding but with aluminium sheet.
Its basically cap that exposes just about 1/3 of the wick and reduces the size of the flame. I recalled a bit of sheet metal and wire work I learned at the technical college I trained at. I couldn’t even start to explain it as its just to small and fiddly to explain easily. This one is the second attempt to make one, the first was way to tall and narrow and spread the flame rather than suppress it, still needs some more experimenting with the shape when I find the ideal design I will post a detailed Ible of how to make it.
The reduced flame size dramatically increases the fuel efficiency and a tank full of meth now burns for a staggering 12:50 at an average of 60*c before the flame goes out and the engine will then run for a further 17 minutes before the hotplate drops to about 32*c and the engine stops.
I got sick of getting burned holding the probe on the hot plate so used a scrap of can bottom to fabricate a clip to hold the probe to the edge of the hot plate, I think there is a higher temperature at the center of the hot plate where the heat is concentrated I’m guessing its out by about 15-20*c (I could be wrong, I can get this checked)
I had a chance to get it checked while visiting a friend. Now here is the surprising bit the temperature difference is almost nominal about 2-3*c which took us totally by surprise so much that we used a laser thermometer and 2 thermal probes one of which was an ultra accurate expensive one and they all read the same.
---------------------------------- The Finishing Touches -----------------------------------------
At this point the hot plate was working but looked very untidy with loose wires and a the digital display unit is just plain ugly when not mounted in a panel or box
I decided to make a wooden surround for the digital thermometer, I have posted a Ible on its construction. I made the box slightly deeper than the body of the thermometer so the excess cable can be stored in it so it can be unclipped if i need to use it for something else in the future. rather than have a clip holding the probe to the hot plate I cut a slot in the top of one of the uprights so the plate its self would become the clip.
With the heat-sink in place a better conductive riser, better mats and a digital temperature readout I now can have this little trooper of and engine chugging away for hours on a tank full of meths without the fear of it self destructing. The engine is now well run in and will now run on my heat from my hand all be it very slowly.
There is a bit of lag with throttle response on the flame adjustment to when the engine speeds up or slows down, quite a bit actually it can lake a few minutes from when you adjust the flame for the temperature to creep up or down. I try and keep the hot plate at about 60*c with an upper limit of 70*c at which point if i can't throttle it back i will usually put the flame out.
The next stage...... well its obvious, isn’t it?..... I now have to water cool the top plate. This Hot plate is actually to display and run this and other future Stirling Engines, I’m going to build a water cooling module for use with other Stirling engines. The water pump has been ordered and I hope to have some sort of cooling system designed and made by the time the pump gets here from Hong Kong.
Stay tuned as I'm having so much fun with this that I have started gathering the bits for a home build sterling. I have already machined an old VCR head into a flywheel and crank assembly
I have posted a video of the new improvements, if you check out my You-Tube channel DrQuiMobile I will be posting some related videos that may not get posted in my Ible's.
Thanks for looking. I hope you like the new and improved design.