Stirling Hot Air Engine

[Mr Bowcat]

Morning all.

I've been back in the workshop over the last couple of evenings. I decided to get the base and cylinder mounting block made, that way I can add new parts to them as they are made and keep everything together.

The base is just a piece of 60x95x6mm ali plate, with holes drilled for mounting parts.



Then on to the block. This is mild steel, 115x32x11mm. It needed a 5 mm hole all the way through the long section, which is the air passage. Drilling long holes is difficult for several reasons, chiefly keeping the hole straight (twist drills have a tendency to wander) as well as the height limitations of smaller mills. To get around this the hole was drilled from either end and met in the middle, and I am very pleased that it met perfectly so I got something right. To finish the hole each end was tapped M6 for blanking grub screws.



Then it was on to drilling the ports and mounting holes for the displacer and power cylinders, one on each side. The displacer side also needed a brass gland for the displacer rod. This was turned on the lathe to a press fit as again this is the hot side so adhesives won't work. There are also two M4 tapped holes in the bottom of the block for mounting to the base.






With the block complete it just needed two stand offs which were made from 11mm hex bar. Simply drilled through and parted off to length in the lathe. And this is what it looks like so far all bolted together.




You can start to see how it will work now. The heated air in the displacer tube will expand and flow around the displacer rod and through the port, along the central hole, and then out the other side to the power cylinder port.

[Mr Bowcat]

Good morning all.

A very enjoyable weekend was spent in the workshop with several parts completed.

I decided to get the crank assembly done first, this started with the crankshaft supports. They could just be left square and would function perfectly fine, but the drawing called for aesthetic tapered sides and a rounded top, so that's what I did.



Next up was the crank, crank bushings, the power and displacer cranks and their pins.





Then I decided to make the power cylinder. As before this is just a length of seamless tube silver soldered to a flange. This still needs cleaning up and the inside bore polishing before the piston can be made.



Last (and the part which took the most machining) was the flywheel. This started life as a 76x30mm puck of cast iron. It was faced off, then the first 8mm turned down to 20mm to form the boss (that later had a cross drilled M3 tapped hole in order to secure it to the crank). Then the rebate was cut (this is mostly decorative but also partly to reduce the weight) and a 5mm drilled and reamed hole put through the middle. With that side complete a mandrel was turned and tapped in order to re-mount the flywheel the other way around, then it was just a case of removing the excess material (around 8mm) and forming the matching rebate in the other side.






I really enjoyed turning the cast iron, because it contains around 6-10% graphite it self lubricates and cuts well. However, the mess left behind is biblical! It took a good half hour to clean up the lathe.






So, this is what it currently looks like. Only a few more parts to make now.

[Mr Bowcat]

Good morning all.

Getting close to the finish line with this engine. Next part to be made was the piston and the two linkages (the other one is for the displacer). The piston was turned to a close fit with the power cylinder, then the two parts were lapped with some metal polish to a nice sliding fit. Should hopefully get some good compression, and the piston will continue to bed in when the engine runs.

The linkages are just 1.5mm mild steel. Holes were drilled at their respective positions, then the parts were rough milled to close dimension and finished with a file.



So this is how it looks now.




Essentially it would run like this (and does turn over very nicely by hand), but the last part to be made is the cooling fin. This started with some 1.5mm, ali, rough cut and then squared up in the mill to 75mm. 4 x 3mm holes were drilled at 14mm from centre for the securing bolts, and a 5mm hole drilled in the centre. This will be opened up to 10mm, then mounted in the lathe on a mandrel to turn the outside round.

[Jim R]

Brilliant. Really enjoying seeing this come together. The piston/cylinder fit must have to be spot on so well done getting that level of accuracy.

[Mr Bowcat]

Cheers Jim.

[HAWKERHUNTER]

Excellent work Bob.

[Mr Bowcat]

Cheers Steve.

[Tim Marlow]

Fascinating little project. This is building up very nicely indeed. Not a technology I know very much about so I had a read up on it. It seems this two hundred plus year old design is just coming into its own in the twenty first century, being used in some of the most advanced technology of the age. Quite incredible for something invented a year after the battle of Waterloo. Great stuff ?

[Tworrs]

Quite inspirational Bob, also very interesting.

[Airborne01]

Loved watching this - very envious of your skill-set here! (you couldn't knock me up a couple of dozen brass buttons for a potential 54mm Napoleonic build could you ... ?) ?
Steve

[Tim Marlow]

Loved watching this - very envious of your skill-set here! (you couldn't knock me up a couple of dozen brass buttons for a potential 54mm Napoleonic build could you ... ?) ?
Steve

Look up lace pins, they come in a large variety of sizes so should fit the bill nicely……

[Airborne01]

Don't believe you've chickened out of this little challenge LOL!
Steve

[Tim Marlow]

Er, what????

[Mr Bowcat]

Cheers guys, glad you are enjoying this project.

Steve, happy to give it a go if you supply the material, brass is spendy.

I've been busy this weekend with Christmas stuff, but did manage to get some workshop time to make the last part, the cooling fin. If you remember from the last post, this had been started and looked like this.



The bolted together fins were then mounted on a mandrel in the lathe and turned round, then once done removed from the mandrel, re-chucked in the lathe using the outside jaws and the central hole bored out to around 20mm (it was bored until it was a sliding fit on the displacer cylinder).




And the final parts were the brass spacers. This started as a length of 25mm brass rod, turned down to 24mm OD and bored to 20mm ID, then each ring parted off at 5mm.





And this is the completed assembly.




Unfortunately time ran out after this. I had to get the workshop cleared of swarf and oil as it is becoming a storage shed over the festive period, so I won't get to run the engine probably until the new year now.

[Airborne01]

Steve, happy to give it a go if you supply the material, brass is spendy

Was only joking mate - a vote of confidence in your abilities! I'm sure you would have made a pukka job of it!
Steve

[colin m]

This is brilliant. I seem to be looking at a mix of precision engineering and art.