Making second stirling engine

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millman

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Hello to all, this is my attempt to build my second stirling engine.
 

millman

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I had a thick bolt of plastic laying around, so I made a fixture for a roughly cut piece of ordinary 6 mm window glass. The fixture was made so that I could grind both the outside and inside diameter without having to reposition the glass wheel between the two operations. To keep the glass from cracking, a shallow recess were milled in the fixture to hold some cooling liquid. The pictures show the outside, then the inside grinding process.

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millman

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Time to plan the engine baseplate! The long red sections you see on the screen serves as “tubes” for the pressure difference generated by the displacer and for leading alcohol from the tank to the wick. These tunnels are 3 mm in diameter, drilled in the 9 mm baseplate.

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Time for some action! Baseplate cutting in 7075 aluminum, then facing it to correct thickness.

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millman

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Drilling tunnels. I brazed a longer shaft to the drill bit using the lathe chuck and tailstock as holders. Sorry for the blurry picture. The drillbit ended up about 120 mm long.

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millman

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Cutting a piece for the cooler section using the bandsaw followed by facing.

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millman

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The final engine. The second picture shows it together with my first LTD. The engine runs at around 900 RPM for 20 minutes on a tank. Thanks for Reading this long story:)

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millman

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Thanks Longboy – I really took a risk in making such a long path for the pressure variations. I estimate the combined length of the drilled “tube” between power piston and the displacer chamber to be at least 150mm long, and this length almost kept me awake some nights! I was quite worried that this critical part would result in a non-running engine. The long path would be very difficult to shorten, given the layout of the engine. However, my luck was larger than my knowledge because the engine runs just fine. It takes however at least two minutes before it starts and maybe the long path has something to do with that. This is my first attempt on an engine with a horizontal displacer shaft and I (think I) have learned that friction in such an arrangement is significant compared to a vertical sliding arrangement. I tried graphite in the bushing, but ended up with brass. I like your “Easy Tee” BTW!
 

Longboy

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The risk was just unfamiliar territory and your engine ran anyway! A score for Stirling;).....I find that Stirling is very tolerable in allowing for clearance variations from plan or book builds. Usually surrounding the displacer to cyl wall. As long as the internal air can be pushed and you get a pressure/temperature differential they will show signs of running on their own following up on fine tuning covering leaks and friction. Of all the Stirlings I have built, I find it inconclusive that the length of the air passage between hot and power cylinders plays a big part in prewarming til engine runs on its own. I would guess that time to prewarm revolves more around engine mass.....is there alot of aluminum to heat up before you get that temp differential between cylinders. I can answer with a maybe! Just part of the facinations around why these engines work. Note: a drop of WD40 on displacer pushrod helps.
 
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millman

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Allthough I am sure that a long tube between displacer chamber and power piston somehow affects engine performace, I agree that the most resonable explanation for the two minute prewarm time is –as you say- related to engine mass and probably friction as well. In my case, the displacer itself is aluminium and even though I made the walls for the displacer as thin as I dared (I ruined the first one I made) there is still some mass at the rounded end of the displacer. I don’t think there are any significant leaks, however I think friction in the horizontal displacer shaft contributes to the two minute startup time. It slides quite nicely, but not nearly as easy as the vertical displacer shaft in my LTD. With a non-contact digital instrument I measured the temprature difference to be about 200 degrees Celcius at 900 RPM. After 20 minutes run time, det cooling section gets quite hot and consequently the engine stops due to low temprature difference.
 

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