Robinson Hot-Air Engine Model x2

[Cheshire Steve]

Have done a quick search here and have not found any reference to making models of the Robinson Hot-Air engine (Polly models do castings). I have only built an ST oscillating V-twin, and I am in the middle of building a Stothert & Pitt beam engine - so I am a newbie compared to some of the awesome work displayed here, but my father who is mid-80s has discovered a liking for hot-air engines, so I decided to build a pair of these - one for him and one for me - and have put the beam engine to one side for now.

Its been a couple of months in the making so far and I have had big challenges due to chilled castings, and disappointment due to both the flywheels having either side of the casting out of register by 1/8" - though this is all part of modelling I suppose. While I can handle some chill using carbide tools, I don't have the same for my milling machine and have blunted some cutters. In future I will heat treat all castings, as I have found you can get chill out by cooking up to about 900C.

Anyway - now for the tricky bit - and I am open for advice. I have a good machined finish on the cylinders, and have ordered some 1 1/2" ally to make laps to get them true and clean. The pistons are cast iron too, and are getting close to size. However I am a little unsure about getting them to exactly the right size without taking too much off - and how to construct an external lap to finish them to size. So I thought I would pitch up here for advice - as I know it is all too easy to take a fraction too much off and scrap the item - and I don't have any spare CI bar in 35mm - so would rather get it right first time (and second time - as I am building two).

I would be very happy to hear from anyone who has built one of these, as I am building one close to spec, but the other I am going to experiment with a little - maybe be fitting an electric heating element to run it by plugging in. I also think both could do with a slightly larger main shaft.

Steve

 

[b.lindsey]

Hi Steve,
What if you machined/polished the pistons to a specific size (just slightly over the diameter of the cylinder) and then used a hone to true up and polish the cylinder ID until the piston just fits. I am assuming there are no piston rings since that would probably add too much drag for a hot air engine. Even if the two pistons finished out to a slight difference in OD, each could still be matched to its respective cylinder. Just my 2 cents worth. Others may have different ideas.

Bill

 

[Longboy]

I would agree with Bills approach with the cyl. hone. But I would guess your concerns over getting the proper fit between piston & cyl. may be overstated. A unfamiliar engine type ??? calls for a little Stirling primer no matter who's kit your making. In Stirling you have an air sealed engine with pistons shuttling the fluid back and forth. External heat source provides for the expansion of the air but the internal pressures generated are not comparable to an I/C or steam motor. So what is the proper fit cyl to a Stirling power piston? Approch it like a gas engine without rings. It is more likely Steve your fit will initially will be too tight and the engine won't run with your caution of going undersize. That's a good thing. That's what they tell us, too tight you can still work the material. Too loose....welcome to the scrap bin. Enlarging cyl. with a hone or sanding on a piston with #400 carbide your choice. Others on Stirling forums & web sites can provide info also and get you to a runner! Dave.

 

[Cheshire Steve]

Strange - now that you say use a hone to fit the bore to the piston, I am not sure why I had such a fixed idea that I should finish and lap the bore and then fit the piston to it. I guess I had always felt that external turning was easier than internal, so do the cylinder first. But then I saw advice that for a really good fit you needed to turn the piston a tiny bit larger than the bore and then use a lap on until until it fitted, and then for the very final touch, to use the piston as a lap in the bore with some fine metal polish or similar. I still favour that route - and plan on making the piston just a bit longer than necessary so I have a bit on the end to try the cylinder on. That way I can avoid that horrible last cut when you just take off a shaving and end up with a sloppy fit.

The pistons are supposed to have 3 grooves, though no details are given of shape, depth, or width. I am assuming say 10 thou wide and only a few thou deep, and square edges - but made before fitting, not after.

The entire success of failure of this engine apparently depends on getting the piston and bore parallel and a perfect fit - so I am taking this bit carefully. Unfortunately most of the mags I have just say that this is covered in previous mags, and I have got back to 1950 and still they say the same.

Steve