testing exhaust headers
09-12-2005, 09:56 PM
#48
Burning Brakes
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yes, but what I was thinking of, is that the interior ceramic coating would affect the thermal resistivity of the metal. Is that correct?
09-12-2005, 10:06 PM
#49
or Sam O
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Doesnt that get thrown off a bit when ceramic coatings are also used on the inside of exhaust headers?
Good question.
It doesn't get thrown off, you just add another term. Simply put, heat must travel through three parts: 1. interior ceramic coating of some thickness, inside and outer diamter and k, 2. stock pipe thickness, inisde and outer diamter and k, 3. outside ceramic coating thickness, inside and outer diamter and k.
True, you are not increasing the surface area of the pipe by coating the inside, but you are decreasing the internal diameter. This means you are defeating your purpose by either decreasing the flow rate of air, or you are maintaing the same flow rate but by increassing the pressure(more rpms are needed).
My point was that you can't state a ceramic coating is better or worse than a wrap just because a company says so. The k value for ceramics is simply lower than those of fiberglass, in general. This is fact. But heat transfer is also dependent on how much of a material you have and how much is exposed to a temperature gradient.
Through theoreticcal calculations given all info is the best method. You can test wrap vs. cermaics but the combinations are endless: what material for the wrap, wrap thickness, how much to wrap and the same goes for ceramics.
Where is the hard data with the test parameters?
09-12-2005, 10:08 PM
#50
Three Wheelin'
I'm probably remembering my chemistry wrong but you can calculate it by calculating the Q of the inner layer of ceramic then calc the heat that gets past that then calc Q of the metal and then finally use the heat past that to calc Q of the outer layer of ceramic and then somehow add them up to produce a final value for the whole thing.
09-12-2005, 10:09 PM
#51
or Sam O
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Thermal resistivity or (k) doesn't change with more or less material. How much heat is transferred (q) through a material with a specefic k is determined by how much material.
09-13-2005, 12:56 PM
#53
Race Director
Originally Posted by Geo
I did notice something in the SAE link about F1 headers not being coated, but one also needs to remember they are using pretty high tech alloys for their headers. I believe they use Inconel. I admit I don't know the properties of Inconel, but I know it's used in very high heat, harsh environment applications. I don't know if F1 header temps are too high for coatings or if they materials make them unnecessary or if their testing shows wraps and coatings to be worthless. One would think if coatings would help they would be used in F1. I just don't know the details of that and have never seen it discussed in Racecar Engineering.
I am in the aerospace industry. Turbine engines actually. We use all sorts of inconel in our engines. The reason is for high temperatire strength, Steels are used in "cold" sections at 600-700F or so. For us high temps are 1600 to 2000F. Most of the time metal temps (nickel based Superalloys of which inconels are based on) over about 2000F need to have special coatings to allow them to survive in hot combuston gas enviroments. Now these coatings are not thermal barriers, but a protective coatings.
We also use thermal barriers of all types TBC as we call them. These are use as you think as thermal barriers to prevent heat from reaching all sorts of places where they should not be, but not to protect the metals. Not every turbine engine uses them. It all depends on the design needs. We typically to avoid them as the are extra cost and can flake depending on conditions.
Now what this means is that an Inco header does not need a TBC to survive high temps. They may need other coatings, but these can be considered like corrosion protection.
Now... you could put a TBC on the outside of a header. It will do a couple things. Firstly it will keep more heat inside header pipes and keep the temps up. It will also increate header metal temps. Depending on the alloy and actual temps this can reduce the life of the part.
Now I can't really say if you WANT to keep the exhaust gases hotter. Seems like you would on a turbo car. However that same heat has a negtaive impact on the components of the turbo. Now header wrap vs TBC vs Swain.
Not sure. Really I don't even know if our version of TBC (of which there are many) are at all like Swain. Plust you'd probalby want to put the TBC on the INSIDE of the header to control the heat the best. For stock header this does have issues related to a smaller pipe diameter.
So... I guess I really don't have an answer....
