Engine / Exhaust Coatings 2008 -- A Review

[bobdobbs]

Here are my current thoughts on the coatings from reading a number of posts on the subject on VAF and performance auto applications.

Piston tops / Combustion chamber coatings
Uncoated pistons / Combustion chambers will build a layer of carbon that will prevent heat transfer, which is what the effect these coatings accomplish. It is thought that one of the factors causing new / new rebuilt engines to run hot during break in, is the lack of this carbon heat barrier. During this time the coatings will act as a heat barrier. Also if you clean the piston (additives, physically) this layer of carbon deposits will need to build up again and you are at a higher risk of thermal damage during this time. The coatings do not prevent build up of carbon. I believe that it is a good thing to reduce excess thermal transfer to the heads / valves / pistons during break in, but getting the "greatest coating ever" is not necessary since it will be shortly covered with carbon deposits serving the same purpose. NOTE: If you are going to run LOP, less carbon will build up, this could be a reason why the LOP/ROP discussions occurs, and the stories that LOP destroys engines.

Piston Side Skirts
This reduces friction "scuffing". Get it. It is reported that over time the coating will wear off. My thought is if you get a couple hundred hours of better, lower friction running, you'll have a better chance of reaching TBO.

Exhaust coating
Coating the outside only will cook (melt, embrittle) your pipes. It has been recommended on VAF, that if you coat, you coat the inside or both. Seems like a good idea to keep under cowl temps down.

Heat Emitting Coating
Thick black enamel is an insulator. NASA document 19730058055. Seems to recommend a very thin layer of no more than .002" of black paint will help move about 3% more heat out of the cylinder versus raw metal.


References:
http://x-jets.com/a_cooling_efficiency.html
Nasa document #19730058055



NOTE: This is a derived work. I'm just trying to collect the best info as best I can. Any comments appreciated. Also I reserve the right to be completely wrong.

[n468ac]

Piston tops / Combustion chamber coatings
Uncoated pistons / Combustion chambers will build a layer of carbon on them that will prevent heat transfer. This is thought to cause new engines during break in to run hot until this coating is built up. If you clean the piston (additives, physically) this layer will need to build up again and you will risk damage. Coating does not prevent build up. I believe that it is a good thing to reduce damage during break in, but getting the "greatest coating ever" is not necessary.

--> great for high power short runs - racing - not proven with current tech that the coatings will last over the long term.

Piston Side Skirts
This reduces friction "scuffing". Get it.

--> again really good for high power short runs - not proven to last over the long term.

Exhaust coating
Coating the outside only will cook (melt, embrittle) your pipes. It has been recommended on VAF, that if you coat, you coat the inside or both. Seems like a good idea to keep under cowl temps down.

--> in aircooled engines the exhaust pipes are part of the heat sync process ... they draw the heat out of the heads toward the cooler tips of the pipes. Not sure if you would see any effect in the real world, but i would like to hear from someone that coated their pipes after running with uncoated pipes.

[RV8RIVETER]

I am having my engine coated and have done a fair amount of research.

Comb chamber/piston: While comb deposits due decrease heat transfer (20% it would seem by your link) , cermaic coating is a more effecient insulator and keeping the heat out of the pistons or cyl head is better for combustion. All racing engine poeple I talked with agree that the comb chamber, piston is most bang for the buck in coating. Unfortunately my cyl's were overhauled before deciding to coat and I am not going to break them down again. I have looked at more than a few coated pistons (non A/C) and do not see any evidence of the coating wearing off. The molly disulfide/ teflon coating is spec'd at 350,000 psi abrasion and also "hold" oil so how much will wear off will have to be seen.

Had my cam, all gears, oil pump ect. coated so that should decrease friction losses. Also being low friction and holding a film of oil dry starts will be a non-issue. Lycomings are famous for cam rust, my cam is now completely coated and will not rust.

Had my crank and rod bearings coated as well. Again mainly for dry start protection. There is also the added safety margin of oil loss. Example, NASCAR racer had oil pump belt break during race, drove 4 laps with no oil pressure, came in for new belt and finished the race. After the race no engine damage found at teardown. Not saying I can run without oil, only saying it may buy a small added safety margin in very rare event of oil loss.

Exhaust. Most say to coat inside and out. Some say only outside and have dyno runs to prove difference, though I do not understand why it would be different. I have not decided yet which way to go. I do not buy the argument that the exhaust pipe pulls any significant amount of heat out of the cylinder head, would have to see some data. If the exhaust pipe is not radiating heat, then the air around the cycl head will be at a lower temp and increase cooling.

A local guy coated his 6 cyl Lycoming in a RV-7. First, when putting his engine together he thought something was wrong because it seemed way too "loose". Pulling it apart he mic'd it and was actualy on the tight end of the specs. He now has had a PW radial out ouf a T-28 coated and before he said it would require a man pushing hard to rotate the prop (plugs out of course), now a boy pushing with one hand can rotate. His example here. He runs 50 deg cooler CHT's, can idle on the taxiway for 30+ min with oil temp never getting over low 200's.

These coating have been around since the early 90s and they have a long history. We may be pioneers on Lycomings, but I don't any problems with using them. Time will tell of course.

[Jconard]

NASCAR racer had oil pump belt break during race, drove 4 laps with no oil pressure, came in for new belt and finished the race. After the race no engine damage found at teardown.

Say, if you actually believe that, I have not only a bridge to sell you, but a naval contract to award for blowing it up, and a supply contract to support its rebuilding...heck if you just give me a couple hundred thousand.....there are guaranteed millions.

No question that these coatings have been useful in racing. Seems like they may be a good idea in aircraft, but a thousandth layer of moly is not going to support a plain bearing operating without oil pressure, at full tilt.

[RV8RIVETER]

I just knew it would happen, so I added "Not saying I can run without oil, only saying it MAY buy a small added safety margin in very rare event of oil loss."

To clarify. I thought I would include a both told and published example. Did not say I believed 100% of it. Do I think a low rpm coated engine will continue to run longer than a non-coated one? Yes. How long, 2 seconds, 30 seconds, 2 minutes, 10 minutes? Who knows and I am certainly not about to test it and find out.

Unfortunately, there is not alot of experience here on this subject and I did not get much help when I asked about it years ago, so just trying to share what I have learned to date.

[szicree]

Originally Posted by RV8RIVETER

NASCAR racer had oil pump belt break during race, drove 4 laps with no oil pressure, came in for new belt and finished the race. After the race no engine damage found at teardown.

This is EXTREMELY hard to believe. If a coating company is using this story as a sales tool, I would have to take all their claims with a large grain of salt.

[gmcjetpilot]

You all know when you run your engine it gets a coating of carbon on it. The carbon becomes a thermal barrier and its free!

I'm with Steve, not saying there's no benefit, just that your magic stuff will be covered in soot, carbon and coke in 50 to 100 hours.

Piston scuffing? Really? The rings and piston and cylinder wall surface has been engineered to work properly. Besides Cylinder wall coatings, surfaces & rings have not been lacking in development. There are about 3 or 4 fancy cyl treatment/ring combos to choose from By Lyc, ECI and Superior. One of the best is a plan iron jugs for planes flown often. They last a long time and work well. No magic, just good basic design and metallurgy.

I am not sold. Now the space shuttle has thermal tile "coating" that does do something.

As far as exhaust, yes it can help lower under cowl heat. So what? Some think it will crack and warp pipes (may be?). I don't know there is little evidence but reading the car guy forms there is debate. You need heat under the cowl to keep the carb from icing. You say you have FI, OK fine. You also need a HOT pipe for cabin heat.

Applying coatings in a long bent pipe and getting GOOD coverage is a challenge. You will likely have bare spots. What happens when a coated part of the pipe is cooler than the adjacent HOT bare part? There will be thermal stress. Enough to cause a problem? Who knows. I think the pipes are a good place to start since they are an accessory, if you are willing to inspect them often, since you are doing something different. Clearly if turbo'ed its a good thing to coat pipes (to get more heat to the turbo). Most turbo pipes have limited limited life anyway, so coat away.


I don't like the idea of outside only coating of exhaust pipes. I think that will cook them like wrap will.

How long have we lived with out "special" coatings. They say it work. I believe you. I'd would like to see results quantified w/ controlled test. If you ask shops that do ceramic coatings, they will tell you its fantastic and say that will $659.59. I'm not impressed but stand-by to be dazzled. It's hard to do a A/B, before after test, so people might be mentally prone to believing its money well spent if they spend it.

Right now my opinion is for marginal, undocumented, unquantified gains the effort is not worth it, especially with some negatives if the coating is not applied properly. With that said I have new custom 4 into 1 pipes and might coat the inside, BUT as I said, if I just give it a few 100 hours of operation the inside of the pipes will be COATED with carbon, a thermal barrier!

[penguin]

Originally Posted by bobdobbs

Here are my current thoughts on the coatings from reading a number of posts on the subject on VAF and performance auto applications.

Exhaust coating
Coating the outside only will cook (melt, embrittle) your pipes. It has been recommended on VAF, that if you coat, you coat the inside or both. Seems like a good idea to keep under cowl temps down.

As Vetterman and Sky Dynamics both strongly recommend against coating your exhaust (SD to the extent of specifically stating their warranty is void if the exhaust is coated), why would it be an attractive thing to do? My exhaust was wrapped before I bought the airplane - some of the pipes were so badly heat damaged that they had to be replaced.

Pete

[gmcjetpilot]

Originally Posted by penguin

As Vetterman and Sky Dynamics both strongly recommend against coating your exhaust (SD to the extent of specifically stating their warranty is void if the exhaust is coated), why would it be an attractive thing to do? My exhaust was wrapped before I bought the airplane - some of the pipes were so badly heat damaged that they had to be replaced. Pete

Pete are you saying your pipes where coated and warped?

The pipe makers (some not all) say no to coating. That's cool (pun intended). It's their prerogative. Think of it from their stand point. Coating the pipe has nothing to do with helping the PIPE, it is suppose to lower engine compartment temp. The pipe wants to shed that heat not keep it inside. From a pipe manufacture stand point coatings are not something they benefit from as a business. Also coatings are bad for weld repair. You have to grind it off. How do you do that with a crack mid pipe? So when the PIPE guys say no there are other factors in that recommendation or abomination. They are right from their perspective but it does not mean kit plane builders will or whould not experiment. I'm still curious but the pipe manufactures concerns should be considered.

From my post above you can see I'm dubious about coating claims and benefit, especially against cost and POSSIBLE problems. I say possible because there is little data on it. However anecdotal comments like yours is a strike against.

I know some pipe makers say its OK to coat? I think if its done well and completely, it might be moot or not objectionable to the pipe? Still the jury is out.

The same debate swirls around "warps" people put around theor pipes. Some admittedly say NO, it will distroy your pipe. Others say they did it with no problem? I think most have gone away from wrap, especially those who had bad experience with it. May be some local wrap (short length) close to cowl might be OK?

Think about it, metal wants to be metal. When you put some coating on, there are questions about interaction and durability of the coating. As I said when you run your engine the insides get coated with carbon and this acts as a great thermal layer.

[bobdobbs]

Just a thought, if you run LOP, your cylinders will be cleaner. The valves / combustion chamber / piston top. The carbon buildup is the thermal barrier which reduces the CHT in a normal engine after break in.

Does anyone think that LOP operation damage being occasionally reported might be caused by this effect, and that piston / cylinder head coatings could be especially important in this case for long engine life?