Engines > Your Own Design
Another Simple Hot Air Engine
PaulR:
Off we go again! This time I'm trying to make a hot air engine loosely fashioned after a Robinson type but a lot smaller, simpler and made from bar stock so basically nothing like one of those :lolb:
Once again I've started making parts before the plan is barely begun so there will probably be some deviations. I made the displacer from 1/2" tube and silver soldered in some enc caps. As it's going to be vertical I decided to reduce the weight by having the rod screw into an internal boss at the top only. All looked good until I put it into some hot water to check for leaks; there was just a tiny bubble or two at the bottom end but as I agitated it, it became worse and finally the bottom fell off :ROFL: That's the difficulty of soldering from the inside - I could see the solder had run but not where, turns out it was not where needed! Oddly, the top end which was done completely blind, was completely sealed. So I re-did the bottom and now it's fine. I put it in the lathe held by the rod and found there was some visible run out at the far end. I think I should have just drilled a small hole prior to soldering then left threading the hole until last and put the sealed displacer in the lathe to do it. Never mind, I doubt that it's bad enough to foul the cylinder.
I made the displacer cylinder from 5/8" solid, turning down the outside to 15.5mm and boring the inside to 14.25mm. I had to do each half separately which is why there's a bit of a ridge halfway down on the outside... can probably smooth that away though ;D
Dave Otto:
Off to a good start!
What I usually do on my displacer cylinders that are made from solid is to bore the inside to size, then make a mandrel that is a snug fit in the bore. slide the cylinder onto the mandrel and hold it in place with the live center and a pressure pad.
This way it is relatively easy to create very thin walls which are desirable for this part.
Dave
PaulR:
--- Quote from: Dave Otto on August 29, 2026, 04:43:27 pm ---Off to a good start!
What I usually do on my displacer cylinders that are made from solid is to bore the inside to size, then make a mandrel that is a snug fit in the bore. slide the cylinder onto the mandrel and hold it in place with the live center and a pressure pad.
This way it is relatively easy to create very thin walls which are desirable for this part.
Dave
--- End quote ---
Thanks Dave. I've got a wood mandrel I made for some of the displacer material (used for cylinders on the last rotary valve engine) so I used that but should have done the same for the displacer cylinder. What do you use for the pressure pad and how do you hold it?
Dave Otto:
Hi Paul
Nothing fancy, I just use a small piece of aluminum round stock, face both ends and center drill one. It only needs to be long enough to accommodate the center drilled hole. I suppose you could use wood for that too if you wanted.
The job that you are turning will dictate what diameter and length you use.
Dave
PaulR:
--- Quote from: Dave Otto on August 29, 2026, 06:34:11 pm ---Nothing fancy, I just use a small piece of aluminum round stock, face both ends and center drill one. It only needs to be long enough to accommodate the center drilled hole. I suppose you could use wood for that too if you wanted.
--- End quote ---
Sorry Dave, I don't quite follow. Is the tube you've bored longer than required so you can hold it and the mandrel in the chuck with the pad at the other end then turn down everything (including the pad) to the required length like this?
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