Melvin Engine

[timsefton]

Good Morning,

We are making a stirling engine in Michigan, called The Melivin, to produce electricity.

Always interested in getting feedback and I know there is a lot of stirling knowledge here.

Our goal was to make the least expensive, robust, simple to operate engine that can produce a useable amount of electricity.

The engine is about 8' long and weights about 200 lbs. It is made mostly from sheet metal (aluminum and steel) with a 3D printed carbon fiber piston.

The engine is designed for low end use, without pressurizing the crankcase and high end use with a pressurized crank case.

Our design target is 1 KW output at the high end, with about 200 Watts output or so at the low end.

We feel pretty good with the low end performance and are working to tune the high end pressurized configuration.

I've included a picture of it and we have developed a number of videos that show it in various stages of operation.

Look forward to hear any thoughts, suggestions or questions you might have.

Tim

Engine Update:
https://youtu.be/ZsYGZzr-IDw
Engine Animation:
https://youtu.be/N4ujQkQLMe0
Melvin Goes Camping:
https://youtu.be/lG0Du7q5PC8

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[Ian S C]

Looks interesting. I would change the hot end for stainless steel, mild steel after a while gets corroded inside, and after a while will droop possibly fouling the displacer. Stainless is the best for the displacer too as it has a higher resistance to heat conduction, and can generally be made lighter than mild steel.
Ian S C

[timsefton]

Hi Ian,
Thanks for the note - were using Aluminized Steel for the hot end chamber and hot end of the displacer. Its regular steel that is hot dipped into aluminum- gives us the corrosion protection of aluminum and higher temperature limits of steel. We've had it up pretty high in temperature and have not seen much degradation in the aluminum coating - the exterior of hot end of the chamber is painted with a high temperature paint. Tried to keep the stainless down to a minimum to keep costs down (about $200 for sheet of 26Ga stainless, vs $90 for sheet of 20 GA steel) - our goal is to get under $1/ watt cost for the system, either by upping the output or further reducing the costs (although there is not much left on that side). Might make sense for the hot end of displacer - will have to see how thin we could go. Displacer is at about 11 lbs with the steel hot end and like to reduce its weight significantly - maybe stainless could get us there. Anyway, thanks for your thoughts. Tim

[Ian S C]

I don't know where you are ? USA, have a look for a stainless steel kitchen bench, maybe some dumpster diving needed here, or some recycling place, another source would be a stainless fridge, or other kitchen ware like a microwave oven. You would of course need a bit of welding with a TIG welder.
My own engines are much smaller with a maximum output of 5 Watts, but I have done a fair bit in studying the materials used in building these motors to get the maximum power for minimum cost. Also I design for maximum time between overhaul, one motor nearing 1500 hrs, about 300 hours since overhaul, and with the up grade should exceed 2000 hours befor the next rebuild, even though the stainless displacer hot cap is showing a little distress. This motor runs at red heat and is water cooled with a fan on the radiator.

Ian S C

[timsefton]

Will do the stainless hunt - There is a great metal scrap yard here in Detroit (actually several) but the one I go to gets the most amazing stuff - at times brand new engines show up from some of the auto manufacture / suppliers that are being scraped out - I'll keep my eye out in their stainless area.

The reason that I'm going with the larger engine is there seems to be a void in most of the current work that I've seen out there. And good reason for that, as one of the keys in these engines is heat transfer, and that is somewhat dictated by heating / cooling surface area. As the displacement volume grows as cube, surface area only grows as a square so after not to long, it becomes difficult to get efficiency in the heating and cooling cycle with the large volumes. We've gotten the Melvin to run between 400 and 500F at the hot end without a water jacket (although typically we operate closer to 600F) - designed the displacer to have double the stroke (10") of the piston (5") to get the operating temperature a bit lower.

We've got an element we add to the engine to augment the available heating and cooling surface area. Think of like the planar springs in free piston engine design but instead of planar it spiral. It connects between the hot end and the displacer. So for each cycle - the displacer pulls the heating element out into the working fluid and it acts as a immersion heater / cooler, then on other side of cycle it become flat against the hot end and recharges.

Anyway getting off on a tangent, thanks for note- interesting hours for overhaul - what do you estimate the hours for overhaul are on the free piston engines that are being sold - read somewhere like 60,000 hours but that seems somewhat hopeful. From my experience, everything fails and everything leaks. Not sure how one gets that engine fixed when it does fail ?

Tim

[Ian S C]

60,000 hrs is a long time, I would imagine that at the end of that, it would end up in the scrap metal yard, by that time I would hope for an upgrade to an improved motor. One of the most important parts of a Stirling Engine is the cooling system, that was one thing about automotive use, the big radiator that was needed to cool the motor. For demonstration motors you can just connect to a tap, and run the water to a drain, but your motor will need to be self contained. I'd love to get to your scrap yard, I'm a bit limited here, that's the main reason I keep to fairly small motors. This is my biggest, a BETA motor with a 2 1/4" bore and a power stroke of 1 1/4"
Ian S C

[timsefton]

Looks cool, What did you make the piston & rings out of ?

Is it a direct crank for the displacer ? We tried to get rid of the linkage between the piston crank and the displacer on the melvin but had too much side force on the displacer shaft due to size constraints.

[Ian S C]

The piston is cast iron with no rings, piston rings are the death of Stirling Engines. The cylinder is made of mild steel(a bit of scrap), the hot end is made from a stainless steel kitchen container. The crankshaft has two throws, the long one for the displacer is 7/8", and the power crank 5/8" ( I think that gives a ratio of about 1.5 : 1). The flywheels are around 6 1/2" diameter. The design started with the legs, they came from a 1920 vintage electric stove. It is an upscaled version of James G. Rizzo's Dyna, another motor I,v built.
A light piston can be made of cast iron as it doesn't need the strength of the piston in an IC motor, and cast iron to a degree is self lubricating, and about the only material that can be run as a bearing surface on the same metal. I have wondered from time to time about plastic pistons with graphite included, I use PTFE/Teflon with graphite for the displacer rod bush through the piston.
Ian S C

[timsefton]

Interesting, its funny how one constraint (ie your legs) can drive the design - we had some issues with the piston / ring setup on the Melvin but have got a good solution at this point. The 3D printed piston is pretty cool due to the internal structure of infill (amount of which you choose) which is a triangle matrix and quite strong - we paired that up with PTFE rings in a aluminum cylinder, from a strength perspective everything worked but once it got hot the piston would get too tight in the cylinder due to the different expansion properties. So we went with the carbon fiber PETG material on the piston which reduces the expansion somewhat and allows for some heat flow from the piston and then we put a piece of metal shim stock (0.008"), with a bead down the middle of it, behind the ring - so it becomes an active force on the rings against the cylinder. The friction is low and the seal is pretty good - the nice thing with the PTFE is that it has no memory so if the cylinder is egg shaped the rings adapt to that over time. Like to get a solar front end set up for the engine but have not yet gotten to that piece of the puzzle - Enjoy, Tim

[Ian S C]

I'm still a bit worried about the aluminium power cylinder, I would like it better with a steel liner, aluminium is horrible for galling regardless of what it is running against, anodized, yes it's OK, hard chromed OK.
Ian S C

[cbstirling2]

How many hours are on the Melvin?

I think it needs many hours of running to see what is wearing...

[Ian S C]

True, I could only find information for PTFE on steel and PTFE on PTFE, both 0.04, no mention of aluminium. Andy Ross in his book "Making Stirling Engines",(free down load), uses a Teflon resin paint called Xylan on aluminium alloy pistons, this stuff is applied professionally and baked on. He used that with hard anodized aluminium cylinders, if you could get the cylinder hard anodized with your Teflon ring this would bring friction to a minimum. These motors are a good place to study metallurgy, and other materials, along with thermodynamics, and that is just to make the best of the stuff you have on hand.

Ian S C

[timsefton]

How many hours are on the Melvin?

I think it needs many hours of running to see what is wearing...

Hi cbstirling,

Not many hours yet, we are just starting to get into some more formal testing - took us a long time just to get it built and running !
We are seeing some wear in the hot end of the displacer - we moved to steel (from alum) and moved the weld joints off of the bend seams to reduce fatigue but still seeing some, what appears to be, fatiguing.

Tim

[timsefton]

True, I could only find information for PTFE on steel and PTFE on PTFE, both 0.04, no mention of aluminium. Andy Ross in his book "Making Stirling Engines",(free down load), uses a Teflon resin paint called Xylan on aluminium alloy pistons, this stuff is applied professionally and baked on. He used that with hard anodized aluminium cylinders, if you could get the cylinder hard anodized with your Teflon ring this would bring friction to a minimum. These motors are a good place to study metallurgy, and other materials, along with thermodynamics, and that is just to make the best of the stuff you have on hand.

Ian S C

Hi Ian,

I like the anodized idea, the aluminum cylinder is made from sheet metal ! - We have a local company roll and weld it for us but then we plant it in plaster of paris cast prior to boring it out on an old bridgeport - get a pretty good round cylinder but the machining can still be improved, alot, the finish is way too rough - we have a honing tool but think we can do better. We use a 0.093" thick PTFE piston rings - before installing them - we take a grinder and cut two longitudinal groves down the face of the ring. It helps with the sealing - as the air hits the grooves it gets turbulent and acts like a mini seal.

Tim

[Ian S C]

What is the bore of the power cylinder? I was wondering If a friendly scrap yard, or engine rebuilder might have a worn cylinder sleave the right size, I think you might find the bored aluminium cylinder difficult to get smooth enough, it really wants to be well polished. Even a bit of thick walled mild steel tube/large steel water pipe what ever. A rough finish will soon damage the Teflon rings.

Ian S C