Exhaust Manifold Temperature And Turbo Efficiency

[djr81]

The only thing you have proven is that the particular coating that you tested was not effective, not ceramic coating in general.

If a ceramic coating is only rated to 500oC and your gas flow has a temp of 800oC then obviously you wont see any difference. Try a coating thats rated to 3000oC.... then you will see a difference, even if you apply it to half your manifold.

Checkout some results at http://www.carcraft.com/techarticles/80038

Enjoy

Thanks, I enjoyed that immensely.

:laughing-smiley-014: :laughing-smiley-014: :laughing-smiley-014: :laughing-smiley-014:

Infact I believe the expression is ROFLMAO or similar.

Have a search for infra red thermometers. Look at the instructions. You will find that they DO NOT work on anything other than matt black surfaces...

The coating was sold as heat insulation. Good insulators do not change their coefficient of thermal conductivity much with changes in temperature.

My contention is that the coating is not effective, not because of anyshortcomings in the characteristics of the ceramic, but simply that the applied thicknesses are insufficient to create any meaningful resistance to the conductivity of the pipe.

[Color_Of_Green]

You are saying that ALL ceramic coatings are useless because of the results of ONE test on ONE product at ONE thickness?

Well, geeeee, you've convinced me ... NOT.

[djr81]

You are saying that ALL ceramic coatings are useless because of the results of ONE test on ONE product at ONE thickness?

Well, geeeee, you've convinced me ... NOT.

What I am saying is that in the thickness typically applied the product does not demonstrate any sustantial heat insulating properties. I have explained the photo & reasons for my opinion.

However, in light of your eloquent reply I am at a loss to further explain anything.

[Color_Of_Green]

I just have a few issues with what you are saying:

1) you applied the coating to the inside of the manifold, but only half of it and along the direction of flow of the gas? Thermal transfer occurs between the gas and the uncoated segment of pipe and then directly up the pipe to what was once a cooler section so that the temperature converges to an equilibrium along the surface.

2) Im talking about coating the outside of the manifold.

3) Not all "ceramic coatings" are homogenous, some are layers of different heat resistent matrial and anti-oxidsing properties.

4) what's a typical thickness? I can only infer that you are limited by the thickness of the coat on the inside of the manifold as any gains from insulation may be negated by a reduction in cross sectional area of the pipe.

djr81, I do appreciate your feedback

[djr81]

1) you applied the coating to the inside of the manifold, but only half of it and along the direction of flow of the gas? Thermal transfer occurs between the gas and the uncoated segment of pipe and then directly up the pipe to what was once a cooler section so that the temperature converges to an equilibrium along the surface.

Coating was applied to the turbo dump pipe. Unsure what you mean about the direction of flow of the gas (right to left if that is what you are wondering) Have a look at the temperature of the flange metal. It is below the temperature of the RHS of the pipe, ie the dump pipe. Makes what you are saying impossible.

4) what's a typical thickness? I can only infer that you are limited by the thickness of the coat on the inside of the manifold as any gains from insulation may be negated by a reduction in cross sectional area of the pipe.

WARNING: Science content (oh shit)

For a two layered ie coating + metal system the steady state one dimensional heat equation looks like this:

Q =

(T0 - T2)

----------------------------

(delta X1/(k1A)) + (delta X2/k2A))

Where:

Q = heat flow

T0 is the internal temperature

T2 is the external temperature

Delta x1 & delta x2 is the thickness of the metal and the ceramic.

k1 is the thermal conductivity of the internal metal and/or coating (whichever way around you arrange it, measured in W/mK

k2 is the thermal conductivity of the external metal and/or coating (whichever way around you arrange it, measured in W/mK

A is the surface area.

For the anoraks this is simply an externion of Fourier's law.

Salient points are:

Doesn't matter if the coating is internal or external.

For an individual layer the two fundamentally important variables are the k value, ie the thermal conductivity of the material & the thickness of the layer delta X.

This is why I have made the point that with the thicknesses typically applied, the coating doesn't have any real chance of insulating the pipe. My old textbook (Janna) puts asbestos at 0.113W/mK and glass fibre at 0.035 which are two of the best insulators, although you can't use asbestos anymore for good reasons.

To anyone using the heat wrap tape, please note the following:

DON"T BREATH IT IN, PARTICULARLY AFTER IT HAS BEEN HEATED.

You have been warned.

Edited June 7, 2006 by djr81

[Color_Of_Green]

djr81: What's k for mild steel? 45 W/mK?

Edited June 7, 2006 by Color_Of_Green

[djr81]

Yeah, about that order. Depends on what kind of steel & its alloying content.

[Scooby]

interesting discussion guys but I'm convinced that the coating is a good thing. i intend to get GTR turbine housings, exh manifolds and stainless long-style Trust dumps coated. The thing I'm not sure about is what temp coating to go for. HPC have a low 800C and a high 1200C temp coating plus a variation to increase the temp resistance of the 800C coating. the 1200C coating is around 3 times the price of the 1200C coating, what do you recommend in terms of what to coat in what?

cheers

[Color_Of_Green]

djr81 I'm still not convinced that you would really see any surface temperature difference on such a short length of pipe, only half coated where conductivity occurs back up the pipe through the metal. Perhaps initially you would but after running for some time I doubt it. The reason the flange is different is probably due to a non-metallic gasket or possible slightly different metal ( and hence difference thermal conductivity + thickness).

However, I definately agree that the thickness and conductivity are the determining factors [ as can be inferred from the equations ]. Also, dont forget that there are other factors involved that effect the distribution of heat, turbulance + edge effects, etc.

One should be able to deduce mathematically the required thickness of ceramic to achieve a 30% reduction in Q [ and hence T].

Cheers

[AlexCim]

I thought this thread was pretty clear?

Dont bother with ceramic coating, buy some thermal wrapping and steel cable ties and thats the best way about it?

So... lets not even bother considering ceramic coating. I mean, whats the benefit over wrapping?

From my experiance, wrapping drops surface temps HEAPS. Enough to be able to tough for a second without burning your fingers off.

[Color_Of_Green]

I thought this thread was pretty clear?

Dont bother with ceramic coating, buy some thermal wrapping and steel cable ties and thats the best way about it?

So... lets not even bother considering ceramic coating. I mean, whats the benefit over wrapping?

From my experiance, wrapping drops surface temps HEAPS. Enough to be able to tough for a second without burning your fingers off.

Good luck wrapping your engine block or head ;0