Head cold from Hell and maybe a Stirling engine.

[Brian Rupnow]

I'm sick.--Sicker than a friggin' dog, suffering from a head cold!! Small wonder though--All my kids have had it, my grandkids have had it, my wifes had it, and the guys at work have had it. I thought I was going to be lucky and have it pass me by, but no such luck. My head feels like a giant bucket of snot!!  Now that I have shared that disgusting fact with you----I'm thinking of maybe building a Stirling cycle engine. I've always stayed away from Stirling engines, because they barely have enough power to get out of their own way. Today I watched a couple of YouTube videos by Tubal Cain, where he does a reasonably good job of explaining the operating principle of these engines. I would prefer to build a simple all metal engine with a power piston and a displacer piston, similar to the one in tubal Cain's video. I'm not really after the polished brass and glass test tube variety, which are works of art.--Nor am I after the "two tin cans and a piece of wire" variety. The one in Tubals video looks about right to me---Plain Jane, but all metal. I will have to see where I can get plans of that.---Brian
https://www.youtube.com/watch?time_continue=1&v=DkfXd8634WY

[Brian Rupnow]

I just spent a good portion of my afternoon watching all four videos of Tubal Cain building a Stirling engine from scratch. He makes it look easy, but he is quite emphatic about warning that there are no plans, no dimensions, and no blueprints available. All I was able to glean dimension wise, was that the bore of both power piston and diffuser piston was about 3/4", the crankshaft  and diffuser piston rods were  3/16" diameter, and the connecting rods were 1/16" x 1/4" brass bar, the piston pin was either 1/16" or 1/8" diameter, and the flywheel was 3 1/4" o.d. the stroke on the diffuser side looked to be about 1/4" greater than the stroke of the power piston. The stroke of the power piston looked to be about 1 1/4". the diffuser piston was a loose fit into the diffuser cylinder to let hot air to bypass the piston and the power piston had to be a very very good fit into the power cylinder. Although his engines used castings for the two main components, I see no reason why they couldn't be whittled from aluminum barstock.
https://www.youtube.com/watch?time_continue=1&v=DkfXd8634WY

[Brian Rupnow]

The displacer piston has about 1/16" radial clearance from the cylinder walls and the displacer cylinder is connected to the power cylinder by a 1/4" diameter passage. It seems only logical to me that the power piston at top dead center must not travel past this 1/4" passage.

[Brian Rupnow]

After carefully watching and listening to all of the associated videos, I have learned that the displacer cylinder may in fact be 7/8" diameter. Apparently its not critical, as long as it is about 1/8" larger than the displacer piston. The displacer piston is actually the outer aluminum shell of an industrial paint or ink marker with the felt removed from the inside and a custom aluminum "bung"  Loctited into the open end. At top dead center and at bottom dead center, the displacer piston should be about 1/16" clear of the inside ends of the cylinder. The power piston is machined from mild steel. The angular offset between the crankshaft throw on the power piston and the displacer piston is 90 degrees.  It seems that "displacer" and "diffusor" are basically the same thing.

[Pete49]

That's what happens when an engineer is sent to hell by mistake and installs A/C. Big trouble now and as there are no lawyers in heaven god can't sue for his return. 

[Brian Rupnow]

I dosed myself up with antihistamines and managed to stagger in to my office across town today, and at the end of the day watched all four of Tubal Cain's Stirling engine  videos again. No offence to Tubal Cain, but the work in those videos seemed--Ahem---Pretty damned crude. At least "crude" in comparison to a lot of the work I've done on my own engines. However, the engines ran.--and they ran pretty darned good. It's a shame that a few more reference dimensions weren't given, but with what information was given it seems like it won't be difficult to extrapolate all of the unknowns. Of course if I try and build my own version of what Tubal has done, I won't be casting the parts. They will be cut from bar stock. The only thing I won't have absolute control over is the displacer piston, as it has to be made from a purchased aluminum bodied ink marker. Even if I don't make this engine, it will be a nice little design exercise. I probably will make it, if only because I have never made a Stirling before, it doesn't have a lot of complex parts, and it seems to be a fairly "forgiving" design.

[Ian S C]

Brian, I would not recommend aluminium for a displacer, a couple of reasons, (1) aluminium is too good at conducting heat, from hot end to cold, (2) it wont take a great amount of heat before failure. As an alternative I have used thin walled stainless tube with an end TIG welded on, or depending on the size required, the steel(?) case of a suitable sized Nicad battery.
The top photo,aluminium aerosol containers used in the same free piston engine (I'm a slow learner), replaced with a stainless steel unit.
Bottom photo, some Nicad s (AA size) and an alkaline battery case, note the + tit is on the case on these ones.
In front of the standing batteries is a hot cap made from a AA size battery case for a little beam type Stirling Engine that has a 3/8" bore and 1/2" stroke of the power piston.
I have used a aluminium felt pen case on a motor, but this was replaced by a AAA Nicad case.
Ian S C
           
           

[Brian Rupnow]

Tubal mentioned that his flywheel was made from lead and was 3 1/4" diameter with a 3/16" center-hole. One of his engines had a spoked flywheel and one on a similar Stirling had the more simplified flywheel I have shown. No width is mentioned, but best guess is about 5/8" overall thickness. My flywheel will be 3 1/4" diameter x 5/8" wide made from cheap old mild steel.

[Brian Rupnow]

Tubal had the advantage of casting his crankshaft supports as part of the main base. I don't have that option, as I am building from barstock, so my bearing stands will be 3/8" thick aluminum (maybe brass for some bling) bolted in place with #10 shcs from under the base. I will also counterbore my bearing stands for two 3/16" ball bearings. These ball bearings will have the seals removed and all traces of factory grease washed out with laquer thinners to give the absolute minimum of rolling friction. That 1" wide x 3/16" slot in the other end of the base is going to be for an alignment key for components which will be built up from the base..

[Brian Rupnow]

And---Oh My!!!--What if we were to use the top end with the diffuser and power cylinder and mounting plate from the Moriya fan which is proven to work well---. The finned "cold end" will dissipate heat better than the solid casting used by Tubal Cain, and gets around the issue of me not being able to cast the displacer cylinder into the body like he did.--Also sets up the possibility of a cooling fan.---The power piston is the same diameter on both the Tubal Cain engine and the Moriya fan, but the Moriya displacer piston is slightly larger in diameter than the tubal Cain engine. (0.91" as opposed to about .780" dia.)

[b.lindsey]

It's starting to take shape Brian!! That last rendering looks like a nice design to me.  How's the cold??

Bill

[Brian Rupnow]

My cold is getting better. It's still there, but its going. I will try my favourite medicine soon---Gargle with whiskey for three minutes, then swallow. Repeat as necessary. I have worked most of today on this design, and although it is slowly taking on a bit more machining content than I initially wanted, I like the design. This is a very nice "middle of the road" example of an engine. It is not all polished brass and varnished base, but it's a step up from what I initially had in mind. I'm in no rush to start machining, so will keep working away at this 3D model until I have everything as I want it.

[Brian Rupnow]

Makes a nice size package!!

[Hugh Currin]

I think the heat transfer between the hot and cold ends of the displacer should be minimized. Any heat transfer to be done though the air moved rather than through any conduction path. Would it be worthwhile to either 1) isolate the hot side of the displacer by using a non-conductive flange, or 2) make the plate holding the power cylinder and displacer from low conductive material, like plastic? But Trubal Cain's original didn't do this looking to be aluminum between hot and cold displacer ends. So I don't know how important this is but would be easy to avoid.

Just thinking.

Hugh

[Ian S C]

Looks good, although it's a design of motor I don't like as there is no provision for a shaft to put pulleys on. The nearest I'v come to that is a parallel motor with the displacer operated by a bell crank, allowing the use of a single crankpin. This motor is based on one that appeared in Model Engineer a number of years ago, and can be built as a GAMMA, or BETA motor, the former being more simple to build.
Ian S C