Piston/Cylinder Materials

I'm planning to make a small air powered piston engine. It's going to be 6mm bore and stroke. I'm not expecting it do any real work, just a demonstration project.

The cylinder will have an inner sleeve, plain, no ports or anything. I'm hoping to keep the walls thin, 7mm OD if possible.

The piston will be plain with no rings.

My question is, what are the best combination of materials for these?

I would like the piston to be silver steel as I can use 6mm stock and make a D-bit from the same to bore/ream the cylinder sleeve.

Should the sleeve be bronze or would brass do?

Would I be better off with an EN1A sleeve with the piston brass or bronze?

I take it that steel on steel is a no-no. I would rather not use cast but if it was advantageous I'd give it a try.

Has anyone built one on a similar scale? Or possible a Gasparin style Co2?

Cheers, Steve

Edited By Steve Crow on 20/08/2019 18:36:08

Bump really, as I'm no expert.

My feeling:

• No need for Bronze unless the engine is expected to work hard.
• Cast Iron is good for cylinders and pistons because it's self-lubricating. Ditto Graphite for an air engine. (Graphite is light-weight, but not satisfactory for IC engines because it burns in an oxidizing flame.)
• Usual to have one material softer than the other when they rub together. The piston is generally the softer metal, mainly I think because it's easier to replace a piston (or rings) than to fix a damaged bore. But either way round will work.
• With an air-driven piston no need to worry about temperature coefficient expansion of the metals used. For example an engine fitted with a pair where where the piston expanded much faster than the cylinder would tend to seize.

I think a Silver steel piston in a brass cylinder would work fine. Most engines I've built have used brass pistons in mild-steel cylinders. Also used graphite in brass and brass in aluminium. They all worked.

Dave

Totally agree either will work and that it is easier to replace a piston, but if I remember my tribology lectures from nearly 50 years ago correctly, it is the harder material that is expected to wear as particles of the harder material become embedded in the softer that then lap the harder one, wearing it more.

Edited By Nick Clarke 3 on 20/08/2019 21:19:07

Thank you Dave, I've never heard of graphite being used as a material in an engineering context. Is it a material that's readily available and used?

Nick I've heard that about wear and it make sense. That is the principal behind watchmakers black polishing where hardened steel is lapped on tin or zinc plates.

[This posting has been removed]

Following Nick’s remark about dissimilar materials, I’m certainly no tribologist, but … since Steve’s engine is ‘just a demonstration project’, does life expectancy really matter?

Clearly, it shouldn’t seize during operation, and as with ‘normal’ practice, the two materials should be dissimilar or at least dissimilar in (surface) hardness.

Not that this has been suggested, but for new comers, the very worst situation in my opinion is fitting an aluminium piston into an aluminium cylinder. Once it grabs, forget it!

I’d probably be correct in assuming that the topic addressing ‘dissimilar materials’ has arisen many times before, and that those ‘in the know’ will come to my aid or … ?

However, here are a couple of ten bobs worth …

When I removed the substantially worn and scored gudgeon pin from a small 2-stroke petrol engine years ago, the replacement pin was a near perfect fit in the small end (bronze?) bearing.

In a similar vein, tests on a lawn mower spur gear made from high density, polyethylene meshing with a similar steel gear resulted in the steel gear wearing out.

Also, we used custom-made expandable brass laps for opening up holes a smidgen.

Let us know how you go Steve.

Sam

Edited By Sam Stones on 21/08/2019 03:01:23

I have built a metric version of Phil Duclos' Huff and Puff breath powered engine with 6mm bore and 6.3mm stroke. The cylinder and piston are brass and the piston rod and piston valve are ground stainless steel. The build log is on the Model Engine Maker Forum but you need to be a member to see the pictures.

For my hot air motors I try to use cast iron on cast iron for pistons and cylinders, if I can't do that I go for cast iron piston and steel cylinder. Hot air motors/Stirling Engines friction is the important thing to minamise.

This little motor has a 3/8" bore and 1/2" stroke CI piston and CI cylinder liner in the finned, vertical aluminium cylinder.

Ian S C

Edited By Ian S C on 21/08/2019 12:13:37

Some great little engines there! The one I'm building is to my own design with two horizontally opposed cylinders. It's not a boxer, the two con-rods share a single crank journal.

I think I'm going to try brass cylinders with silver steel pistons.

Boring the cylinders with a d-bit from the piston stock? Is this a valid method for this application?

Or will it need boring slightly small then lapping out to fit?

I'm not sure on the kind of fit required for an air powered piston of this scale.

Any tips or advice would be much appreciated.

Cheers, Steve

A couple of thoughts:

Why silver steel for the piston? It tends to rust quite easily unless you have dry air and good lubrication this may be a problem. Precision ground stainless steel is available.

Piston fit? This will depend on the pressure you intend to run the engine at and the friction/load on the system. I reamed the bore 6mm and turnêd the piston to a close fit so without lubrication (other than residual cutting fluid) it would fall under it's own weight with an open cylinder and fall in about 10 seconds with a closed cylinder. With light out applied the piston just stayed where it was.

If this link works this is the piston sinking in a closed cylinder:

**LINK**

and this is the engine running:

**LINK**

Best regards

Roger

Graphite is fairly easy to buy - I got mine from Noggin End, other sources are available including ebay.

Many applications for it; good lubricating properties (unless it's mixed with something else); conducts electricity (motor brushes, electrodes and resistors); highly temperature resistant - it melts at about 3600°C, much better than steel at 1500°C, making it ideal for rocket venturi and crucibles. Also used to make high-strength exotics like carbon-fibre, carbon-nanotubes and aerogels (see 'Frozen Smoke' )

As a piston material, Graphite has the advantage of being about a third of the density of cast-iron. Light pistons are better than heavy ones in an engine. Downsides of graphite are it's brittle and burns in air. Being brittle means it doesn't scale up well for making bigger pistons, but it's an interesting alternative for small engines and Stirlings.

Dave

Edited By SillyOldDuffer on 22/08/2019 08:48:21

Dave, your frozen smoke link takes me to Noggin End.

There is/was a company in Newton Le Willows who made shock absorbers which were graphite pistons in glass cylinders. I always thought this would be a neat engine, you could see the piston whizzing up and down!

The Dutch engine builder Jan Ridders has produced several designs with glass cylinders and graphite pistons. This is a flame licker:

https://www.vth.de/maschinen-im-modellbau/unsere-beitraege/flammenfresser-mit-glaszylinder-mit-bauplan

And this is a 4 stroke spark ignition version ( the article did carry a warning to run it behind a protective shield):

https://www.vth.de/maschinen-im-modellbau/unsere-beitraege/viertaktmotor-mit-zylinder-aus-glas-mit-bauplan

Best regards

Roger