PDA

View Full Version : Ceramic exhaust coating - fact or fiction?



JohnD
2012-Oct-15, 08:34 PM
Can Shuttle experience challenge a commercial hype in motorsport?

It is fashionable to have the tubes of the exhaust manifold of your race engine ceramic coated. This is said to insulate them, to prevent excessive heat loss into to the engine compartment and keep exhaust gas temperature high to promote expulsion. Such coating can cost from $350 for a V8 manifold, or the same in Ukús.

But, the temperature inside an exhaust manifold is of the same order as that outside a Space Shuttle, whose airframe had to be protected from a temperature of more than about 350C, else it could deform. So the intended insulation criteria are similar. But the Shuttle needed a layer of ceramic tiles about six inches thick, whereas the coating thickness is about a millimeter.

When compared with the Shuttle, how can such a coating confer any benefit at all?

John

Darrell
2012-Oct-15, 09:44 PM
I don't claim to know, or have any solid data to support my opinion, but I am doubtful that these types of coatings make any significant difference in performance. I have some experience with this kind of thing with respect to motorcycles, and it has been my experience that the ceramic coated headers get just as hot as any other. Perhaps they take a bit longer to heat up, which in turn perhaps indicates that the coating is a somewhat effective insulator, and maybe confers some minor advantage during that period. But even if that were so we are talking about an interval measured in seconds to single digit minutes at most.

Another concept for high performance headers is just the opposite. Thin walled stainless steel. They heat up very quickly, and even after hard extended use are cool enough to touch within minutes of shut down. That last part is why I like them.

Jerry
2012-Oct-15, 11:20 PM
It is amazing how small changes in heat transfer effect overall performance - sodium filled valve stems, for example move a significant amount of heat from an engine. Low density ceramics are excellent insulators. The temperatures in an engine do not come close to what a shuttle feels on reentry; but I would venture that if racers are adding the weight and cost of a ceramic coating; they have the engine block temperature data to support it.

pzkpfw
2012-Oct-15, 11:38 PM
A lot of the motorcyle customisers wrap their pipes in a kind of cloth for this reason (and others).

http://www.allaboutbikes.com/feature-articles/motorcycle-maintenance/6963-unwrapping-the-mystery-of-exhaust-wrap


Though external heat control is the primary benefit, there is also a potential performance benefit. Hotter exhaust gasses (as created by the insulation of the wrap) are less dense. Thus, the exhaust ôscavengingö can be more effective. In effect, intake and exhaust gasses are pulled more quickly through the engine which increases performance.

I've no idea myself if it "works".

Trebuchet
2012-Oct-16, 04:05 PM
The coating certainly provides some benefit to the guys who apply it....

I doubt if there's any performance/insulation benefit at all. It just looks cool and prevents corrosion. As long as it stays intact, that is.

profloater
2012-Oct-16, 04:21 PM
Ceramic (thin) coating an exhaust pipe would make very little difference to the stream of hot gases passing through. However it might reduce corrosion on the outside. Over insulating might actually be a bad idea, the pipes run red hot near the block and their strength will be reducing.

ShinAce
2012-Oct-16, 04:28 PM
There's always ceramic coated pistons:
http://tst.subscriptions.sae.org/content/910460/
http://www.google.ca/patents?hl=en&lr=&vid=USPAT4245611&id=rEUyAAAAEBAJ&oi=fnd&dq=insulated+engine+exhaust&printsec=abstract#v=onepage&q=insulated%20engine%20exhaust&f=false

ShinAce
2012-Oct-16, 04:29 PM
However it might reduce corrosion on the outside.

Yet it will likely lead to increased corrosion on the inside. Maybe it's sandwich time?

Darrell
2012-Oct-16, 08:21 PM
A lot of the motorcyle customisers wrap their pipes in a kind of cloth for this reason (and others).

http://www.allaboutbikes.com/feature-articles/motorcycle-maintenance/6963-unwrapping-the-mystery-of-exhaust-wrap



I've no idea myself if it "works".

This type of wrap is popular with race shops, customizing shops, and private owners customizing their own rides. But in every case that I have specific knowledge of, lots, the applications have been for aesthetics and / or to provide a heat shield when either the OEM heat shields are removed from the stock headers, or on custom headers that don't have heat shields in the first place.

As far as performance gains I think you would be better off finding a way to reduce the weight of the bike a couple of more pounds. And, come to think of it, if use of the heat wrap in place of more standard metallic heat shields allows you to shed a pound or two, that would certainly do it.

profloater
2012-Oct-16, 09:09 PM
There's always ceramic coated pistons:
http://tst.subscriptions.sae.org/content/910460/
http://www.google.ca/patents?hl=en&lr=&vid=USPAT4245611&id=rEUyAAAAEBAJ&oi=fnd&dq=insulated+engine+exhaust&printsec=abstract#v=onepage&q=insulated%20engine%20exhaust&f=false
The basic efficiency of a heat engine is the Carnot efficiency which means raising the temperature of the heat source if you want more. Metal engines cannot push the combustion temperature up much more but a ceramic engine or a ceramic lined engine could push it way up. Then the BMEP goes up (Brake mean effective pressure) and the parts must also be stronger. This however is not the same as the aesthetic coating of the exhaust pipes.

cjameshuff
2012-Oct-16, 09:39 PM
I don't see the physical rationale for insulating the exhaust in any way for improving performance. Insulation increases temperature, increasing pressure that the engine is expelling exhaust gases against and increasing the speed of the expelled gases flowing through the exhaust system, thus increasing drag losses.

It seems that if you didn't need to protect nearby things from the exhaust heat, you'd actually want to use heat exchangers to cool the exhaust as quickly as possible. The ideal thing to do would be to exhaust directly into the atmosphere and wear insulated fire-proof pants. (and earplugs)

Solfe
2012-Oct-17, 04:12 AM
Cooking grills use ceramic coated parts as a selling point because it cooks more evenly. Considering what a hot tail pipe will do to you... ick.

On a more serious note, I am not sure the ceramic would effect performance do to heat exchange directly. I believe that the coating would smooth out the heating of parts so that they remain clean, free of corrosion and snugly fitting for years to come. On a typical car, that is purely cosmetic or a maintenance issue. On a performance car, where every erg of power is squeezed out of the engine, having the best maintenance free, snugly fitting parts simply helps you tune the vehicle to your needs.

novaderrik
2012-Oct-17, 11:01 AM
believe it or not, keeping the exhaust gasses hotter in the headers after they leave the engine can be beneficial- especially in a turbocharged application where that heat energy can be used to spin the turbo more efficiently. my only experience with this is on my 84 Buick Regal T Type. the factory up pipe from the manifold to the tuboe had developed a crack, so i had to cut the factory insulation away and weld in a new chunk of pipe. in doing this, i also got rid of a pretty big restriction by eliminating the flex joint.. anyways, i ran it for a few days without any insulation on it, and it ran better than it did before. but i decided to wrap the newly modified pipe after burning my fingers on it and i noticed that just adding the insulating wrap made the engine build boost faster. i don't have numbers to quantify it, but i'm not the first person to notice this..

regarding ceramic coatings on headers: i've had a couple of sets in different cars. i can't say if the coating makes any more power, but i do know that the underhood temps are lower and spark plug wires won't melt when they lay against a Jet Hot coated header. also, it's a lot more pleasant pulling the spark plugs out of an engine with ceramic coated headers than non coated headers- they are still pretty hot, but not so hot that you can't work around them. i don't own one of those temp guns to get any real measurements, but i know from experience that the coatings do something. also, the teams that run the fastest race cars and have the manpower, time, and budget to test these things simply wouldn't bother with the hassle and expense of coating headers and other exhaust components if it didn't do something beneficial. same with ceramic coatings for internal engine parts- they make some that you put on bearings and piston skirts that lower friction and some that you put on piston heads and combustion chambers to keep the heat in the chamber where it can be put to use instead of in the water where it is wasted thru the radiator.

JohnD
2012-Oct-17, 10:31 PM
Thanks for all the opinions and experience - I had hoped for more academic replies.

To amplify my point:
The Space Shuttle had ceramic tiles to protect its aluminium airframe from 3000C on re-entry. The tiles are 1 to 5 INCHES thick, depending on site, to keep the heat inside down to less than 700C at which aluminium starts to soften.
The temperature inside your exhaust manifold is about 800C, and let's assume you want the temperature outside to be 100C.
On the Sapce Shuttle, limiting 3000 down to 700C is a 2300C difference and needs 5 inches.
In an engine 800 to 100, is a 700C difference.
So the latter would need Shuttle tiles or a coating of the same stuff that is 5 x 700/2300 = 1.5 INCHES thick.
How thick is a ceramic coating? I've seen 2-4 thousanths of an inch quoted. Lets make it easy and say 5 thou.

So that coating has to be at least three hunded times a better insulator than a Shuttle tile.
Has ceramic insulation come on that much since the Shuttle was designed?
Could the Shuttle have survived with a coating of exhaust ceramic only 16 thou thick?

John

ShinAce
2012-Oct-17, 10:56 PM
Those are different applications of ceramic. Protecting the structural integrity of a ship is not the same as trying to retain some more heat. You don't add weight to a car just because you want the exhaust pipes to be under 100C.

A better idea might be a ceramic engine:
http://www.sof.or.jp/en/activities/pdf/06_07.pdf

Or insulated exhaust for turbochargers(as mentioned easrlier):
http://www.hindawi.com/journals/ijrm/2012/625453/

And then there's the question of automotive exhaust gases in general, which manufacturers are well aware of.
http://pic.sagepub.com/content/211/1/1

If that ain't scholarly, then academics isn't worth noting.

cjameshuff
2012-Oct-17, 11:25 PM
The Space Shuttle had ceramic tiles to protect its aluminium airframe from 3000C on re-entry. The tiles are 1 to 5 INCHES thick, depending on site, to keep the heat inside down to less than 700C at which aluminium starts to soften.

All your numbers are way off. The reentry shockwave was 1650 C, not 3000 C. Aluminum melts at 660 C. The Shuttle structure had to be kept below 175 C to maintain integrity, with substantial safety margin. Finally, the Shuttle was a closed system immersed in a high temperature environment, not an exhaust manifold with the entire atmosphere as a heat sink. The thermal protection system had to insulate the craft and quickly re-radiate the heat rather than absorbing it and slowly overheating the vehicle full of human astronauts with even tighter temperature requirements.

JohnD
2012-Oct-18, 11:01 AM
r
All your numbers are way off. The reentry shockwave was 1650 C, not 3000 C. Aluminum melts at 660 C. The Shuttle structure had to be kept below 175 C to maintain integrity, with substantial safety margin. Finally, the Shuttle was a closed system immersed in a high temperature environment, not an exhaust manifold with the entire atmosphere as a heat sink. The thermal protection system had to insulate the craft and quickly re-radiate the heat rather than absorbing it and slowly overheating the vehicle full of human astronauts with even tighter temperature requirements.

Thank you James for giving better figures for the Shuttles situation. That so thick an insulation was needed in the face of a lesser thermal challenge indicates even more clearly that a coating a few thousanths thick cannot be effectual.

cjameshuff
2012-Oct-18, 03:16 PM
Thank you James for giving better figures for the Shuttles situation. That so thick an insulation was needed in the face of a lesser thermal challenge indicates even more clearly that a coating a few thousanths thick cannot be effectual.

This is...not even wrong, and you've completely missed my point. The Shuttle had vastly different requirements, the tiles needed to protect a vehicle full of humans traveling at hypersonic speeds inside a hot reentry shockwave. An exhaust manifold contains much slower, cooler, lower density gases and has the entire surroundings to cool the exterior...the two can not be compared as you are attempting to do. Your 100C exterior temperature for the exhaust manifold is completely arbitrarily chosen, and you haven't even considered the reason why such an equilibrium temperature exists...heat transfer to the atmosphere.

Carbon steel has a thermal conductivity of around 40 W/m/K. Calcium silicate, 0.45 W/m/K.
A 5 mil steel sheet with a 5 mil calcium silicate coating would have about 1/90th the thermal conductivity of a plain 5 mil steel sheet. This could be substantially reduced by foaming the coating, perlite has a thermal conductivity of 0.031 W/m/K. Assuming a bit worse due to the thin layer, that'd give you more like 1/1000th the thermal conductivity. That is, keeping the exterior at a certain temperature would require removing 1/1000th as much thermal power. If you had to remove a kilowatt of heat before, you now only have to handle a watt. That's not "ineffectual".

ShinAce
2012-Oct-18, 04:16 PM
r

Thank you James for giving better figures for the Shuttles situation. That so thick an insulation was needed in the face of a lesser thermal challenge indicates even more clearly that a coating a few thousanths thick cannot be effectual.

Tell me, why does it have to be a few thousandths? Why not 1/64?

LotusExcelle
2012-Oct-18, 04:33 PM
The thickest header coating I am aware of is .015". Basically anything under that is cosmetic coating and not intended or capable of substantial heat blocking. The general rule is, if its shiny it isn't meant to work. Locally a company called Swain Tech makes on of the best exhaust coatings I've ever come across. The numbers they claim are modest and at the very least I can tell you that the radiant heat blocking is substantial in my application.

Most consumers wouldn't notice an improvement but in a race application where a few degrees in either direction can be tested and verified for effect on a dyno... cyramic coatings are used. I would say, to sum up, that not all coatings are created equal and not all are used appropriately.

**so I won't edit my spelling but for some reason I keep using "cyramic"... maybe because I keep typing "cylinder" too.

LotusExcelle
2012-Oct-18, 04:37 PM
I don't see the physical rationale for insulating the exhaust in any way for improving performance. Insulation increases temperature, increasing pressure that the engine is expelling exhaust gases against and increasing the speed of the expelled gases flowing through the exhaust system, thus increasing drag losses.

It seems that if you didn't need to protect nearby things from the exhaust heat, you'd actually want to use heat exchangers to cool the exhaust as quickly as possible. The ideal thing to do would be to exhaust directly into the atmosphere and wear insulated fire-proof pants. (and earplugs)

You actually made a few statements that are right but the conclusion you came up with is wrong. Hotter gasses do mean higher speed. The hotter the exhaust, the faster, more or less. So the hotter you can keep the gasses INSIDE the header, the faster that header will flow. Good ceramic coating intends to keep the header from radiating heat away, keeping the gasses inside as hot as possible and thus speeding up exhaust gas flow. Since engines are pumps you can use the gas from one cylinder to help "suck" the gas from another cylinder. In certain applications the effect is dramatic (note: not a Harley engine... which has the most insane firing pattern on earth).

JohnD
2012-Oct-19, 09:27 PM
Now we are talking!
Thank you, James, Thank you Lotus!

James, that's the first time I've been shown actual numbers to describe the heat transfer properties of the various materials, even asking the Q. on various motorsport related sites. That is what I hoped for from from this site, some real world materials science. I don't think that any exhaust coating is foamed - they are all very thin, as Lotus says.

Lotus, you make the point I was trying to, that in the ultimate race application, even a few degrees may make a difference, but unlikely to for club racing, where every better-off jock has coated manifolds.

Anyway, I'm still content not to bother spending all that cash!
John