Piston coating question

Road-race engines and ancillaries - general discussion
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Piero

Piston coating question

Post by Piero »

Hi,
Has anyone had any experience with piston coatings of any kind?
pro's and cons please.
Does it wear out etc etc etc.
Also has anyone ever had their bores coated.
Again, pro's and cons.
Thanks guys
Guy Croft
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Post by Guy Croft »

Interesting question.

I take a long term view of things like this. It has unfortunately become one of the so called 'low-cost' add-ons that are attractive on the face of it, even though there is a paucity of quantifiable information published about how well it works. By quantifiable I mean independently verifiable 'before/after' dyno test. There are a number of 'add-ons' that I'd put in this bracket, they appear as topics on this site from time to time.

My direct experience of piston skirt coating is limited to using (now hard to get) lightweight Mahle cast oversize pistons from a late generation (Tempra I think) TC, modifed by me (crown, VRs) to fit the early non-reversed port head, where the piston skirts had a thin coating of Teflon-based polymer. Well, I mean all coatings are thin, I'm talking, from my experience of Xylan, which it probably is, where the thermosetting polymer compound is going to be at most 0.001" thick. These pistons exhibited a lower turning torque than any other OE piston I've ever used, but I have to add that they had thin section low outspring rings - and I know that ring friction is greater by far than skirt to bore friction especially under load when the rings are energised.
However - the engine was noticeably quieter in running than one with non-coated skirts.

What does the Teflon skirt coating do? Well, the compound is almost frictionless especially when oiled, so that must help. How easy is it to apply? Not very. It has to be sprayed on and the fine surface finish of the skirts doesn't help adhesion, and because it's a thermosetting polymer it has to be oven-baked to cure for, say 20 minutes at maybe as high as 240 deg C (Xylan 1010 does anyhow that I use on supercharger rotors, imagine the black compound here is done at a similar temp). You could roughen the skirts by blasting but my, you'd have to know what you're doing because bead or grit blasting can totally reshape the skirt profile.

You're getting the picture, I'm sure. Not easy to do. I would never undertake anything that required alteration to the designed skirt of a manufacturer's piston because I as the engine builder have to embrace the risk for this, er 'tribological enhancement'. No ho! If it goes wrong I'm to blame. And, yes, I do get asked to apply this or that, and I know more about tribology than most engine builders.

And once it's on, what does it do? Well, first point is yes, it does wear away. Maybe, like tests done years ago with chrome plated top rings, tracing using radioactive method, where the chrome particles lose adhesion with the ring and kind of 'semi-coat' the bore with chrome improving it's longevity (Chromium being very hard), perhaps the Teflon wears off the skirt and coats the bore with a light dispersion of Teflon. I don't know, but yes, it wears off, see photo. Where has it gone? Clogged up the graphite matrix of the cast-iron bore? Does it matter? Is Teflon stuck in the matrix of cast-iron/graphite worse/better than than pure oil retention? You tell me.
I have not read any eminent papers on long-term test on this and until I do I am non the wiser! At least the particular compound used by Mahle is black, so you know it's there, if it's colourless that is a different matter...

Now of course I know there are also coatings for the tops of pistons, to provide thermal barrier to prevent heat loss to the piston. That heat has to go somewhere ordinarily - we want to retain it of course, heat loss is wasted energy - and it goes into the oil around the crown underside, dripping back to the sump, it goes into the wrist-pin and down the rod, and of course it is transferred from the skirt and rings themselves to the bores. Either way, that heat conducted through the piston top ends up in the oil or the coolant.

I can see that a thermal barrier coating would inhibit this, yes. Will it enhance power per-se? I do not know - and again, I will only give credence to independently quantified dyno tests with that as the only modification and I have not read anything like that. Not yet, I'm not saying such data does not exist.

But, and this is a big but! The accelerative stress on the top of the piston is enormous and I think that expecting a coating to survive in a combustion chamber long-term is asking a lot of what is essentially, well, glue. My biggest fear is the engine running into detonation (I hear some saying that the engine will be less prone to detonation...!) and that the coating will fracture. You can picture the scene. I saw a lot of ceramic (non metallic) and other coating technologies when I worked as a Chief Engr with a turbocharger manufacturer. Some, on paper (in their ads and brochures) appeared absolutely dazzling, 'must have'. The tribology industry (coatings and treatments to reduce friction) is mainly aerospace/USA led and is very BIG business. I'm talking $$billions, 'developed by Nasa (the prime movers because of the Shuttle)', that kind of thing. But in reality some of the ones I selected worked only briefly in pre-adoption trials under the attack of sulphur, water, particulates at 500 deg C plus and high velocity. Some were noticeably more survivable than others but none was what I'd call 'long term' in a combustion environment and would ultimately require overhaul and recoat. They were all super sensitive to substrate condition and material, and frankly, it is easy to test and develop and launch a coating but HUGELY expensive to validate long term in a particular engine/turbine/turbo application. So the best coatings, er, are expensive and if you buy cheap you are sure to be wasting your money. As for coating cast-iron, forget about it, NOTHING sticks to cast-iron, except seized pistons.

If you're racing and you want to run the risk, that's fine and I know people who have successfully coated and raced with various types, though I don't know anyone who can directly attribute their bhp to the coatings rather than just a good build and prep. Big difference.

When you get a DIY pack of coatings for engines, dry film, bake on etc etc, you will tend to read things like 'scrub part with wire wool', 'clean with this, clean with that', 'if the coating cracks or flakes don't use', and 'all information given in good faith' (naturally) or 'user assumes all risk', etc etc. All pretty amateur stuff to me and redolent of 'snake oil'. You will read 'lowers underbonnet temperature' or 'keeps exhaust temperature high' blah blah. All the right noises without the cutting-edge fact.

If you fancy a go at any kind of coating my strong advice is to go a firm who do auto industry-standard accredited work (or aerospace) and ask them their advice, make sure they have proper ISO certification and processes in place and a published track record of achievement in this field. After all, you will certainly be required to embrace the risk of failure of their methods in your motor. If you find someone who does coatings for F1 (if they are allowed to say so) you are about as safe as anywhere.

Remember too, the places where the power is really lost . There the friction is highest and the loading is also (by default) highest and thus the coating itself most risk-prone. If you dyno and see unexpectedly low mechanical losses in the engine, you have done well, a good dyno will tell you this. One assumes that you tested it before in precisely the same trim. If you test it 3 months later and the power is down but the in-cylinder leak-down is the same, it's probably time to re-coat, if you follow my drift..

If anyone thinks they are going to post and say 'I think it works', or 'a friend of mine did use it etc', don't because I'll wipe it. It's not that kind of website. I want quantifiable fact or reasoned questions regarding use and performance and nothing else on this thread please.

GC
Attachments
Mahle teflon coated skirt as described above, used by GC in Fast Road 2 liter Fiat.
Mahle teflon coated skirt as described above, used by GC in Fast Road 2 liter Fiat.
2004_0227Image0021.JPG (100.63 KiB) Viewed 6453 times
Fiat Tipo 16v piston removed from factory built engine after about 70,000 miles running, most of the coating has vanished, from the piston at least..
Fiat Tipo 16v piston removed from factory built engine after about 70,000 miles running, most of the coating has vanished, from the piston at least..
piston skirt scuffed.JPG (12.64 KiB) Viewed 6447 times
Piero

Post by Piero »

Very interesting read/reply.
Thank you Guy.
I started this topic as I was quite keen to see how far the coating technology has gone, so far as the auto industry is concerned.
My interest first started with bearing shell coatings, which I feel I might well be employing in my own engine build.
My interst soon spread to piston and bore coatings,
this area has not had the same rewards for me.
But am still keen to learn more about bore coatings, especialy ceramic.
would you be kind enough to write a few lines specificaly on this?
pro's and cons.
many thanks
Piero
Guy Croft
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Post by Guy Croft »

"As for coating cast-iron, forget about it, NOTHING sticks to cast-iron, except seized pistons.."

Coatings can only be used on steel and aluminium liners.

GC
Acki

Post by Acki »

A ceramic coating?
Fahrell
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Joined: October 12th, 2006, 2:54 am
Location: Brasil
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Post by Fahrell »

Excellent post !!
Thanks Piero for questioning and Thanks Guy for this answer... It resumes all piece of poor and scattered information I've crossed around and puts it in a useful way.
Andr’‚© Farkatt
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