Thermal coatings

[Bernd Felsche]

Arnaud Westenberg tapped away at the keyboard with:

> Craig Dotson wrote:
> 
> > The ceramic coatings I've seen on exhaust headers go on the OUTSIDE of 
> > the header pipe.  The inner diameter is not reduced.
> 
> He he, I'm sorry, I stated that rather poorly :-) I didn't meant to say 
> that the coating makes the pipe smaller but it changes it's behaviour 
> into something similar to a smaller pipe (higher velocity, more volume 
> through the same diameter wich equals (kinda) the original volume 
> through a smaller diameter).

That would be valid for a steady flow.

> > If there is less heat transfer away from the exhuast gases, the gas is 
> > hotter and therefore lower density.  Lower density does yield a lower 
> > Reynold's number with all other things being equal.
> 
> But other things are not equal! Lower density but higher velocity. I'm 
> not sure if these cancel each other completely, resulting in the same 
> Reynolds number, I would have to do some calculations for that.

The Reynolds number is relevant if you're looking at steady-state
flows.

Pulsating flows are another matter altogether as these tend to be
turbulent throughout for part of the pulse and laminar otherwise,
with flow reversals likely at a number of operating points. While a
Reynolds number may be useful as an indicator for overall flow, it's
far from correct to use it to calculate flow resistance with
unsteady flows.

Gas pressure dynamics dominate the flow behaviour to a far greater
degree than boundary layer effects; except in very constricted flows.

> > Speed of sound within the exhaust gas is also higher,
> > slightly raising the "tuned" RPM if it is a tuned manifold.
> 
> Agreed, but that could have been reached through different manifold 
> design as well.

Velocity of pulse propagation also changes; that shifts the resonant
frequencies.

> > Coated headers don't heat underhood air as much, allowing more of the 
> > heat to be passed through the exhaust system further downstream.
> 
> In cars I can see the benefits of this, however these advertisements 
> speak about motorcycles as well !?

The general idea (theory) is that higher gas velocities will persist
of the gas isn't cooled. This may assist in getting the gas out of
the tail pipe, but depends on the dynamic behaviour of the exhaust.

> > It probably isn't done primarily because the intake is heated by 
> > conduction through the block and heads.  The best way to thermally 
> > insulate the intake manifold is to use some kind of thermoplastic or 
> > (egads!) wood adapter between the block, heads, and intake.
> 
> Hmmm... maybe a combination of both would be interesting.

Carbon fibre is quite popular for intake manifolds if your budget
can cope. A heat radiation shield with venting around the intake
manifold is a cheap alternative...

> I'm still not convinced about the benefits of exhaust coatings
> (besides under hood temp)

There's a definite (but marginal) benefit to exhaust manifold
coatings between head and turbine on turbochargers.  

By a similar train of thought; they may also be attractive on
catalytic convertor installations where the time to heat the
convertor is critical in meeting emission requirements.
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