The discussion on ceramic coating of exhaust
manifolds drove me back to some reference books on high temperature performance
of stainless steels. I believe that aircraft exhaust manifolds are
generally made of type 321 austinetic stainless steel, chosen because of it
properties, weldability, strength, corrosion resistance, and formability
(ability to make bends in tubing). Characteristic of all steels, strength
plummets rapidly with increases in temperature. I attached two figures
from an ASM Source Book on Materials for Elevated-Temperature Applications
(1979). The first shows the general trends of various types of steels as
a function of temperature. The second shows specific data for various
stainless steels including type 321 which I have highlighted in red.
The exhaust manifold is heated by the flow of hot exhaust
gases which as we know ranges typically from 1450-1650F when mixture is leaned
at cruise conditions. The manifold is cooled primarily by thermal
radiation losses to the surroundings because at the operating temperatures
thermal radiation losses (which go as absolute temperature raised to the fourth
power) become huge compared to convective losses from passing cooling
air. It is the thermal radiation that cooks temperature sensitive stuff
in the vicinity that is not being cooled by the blast of air passing down from
the cylinders.
I don’t know the typical wall temperatures of
the exhaust manifold at cruise conditions, but I am willing to bet that they
are 100-200F below the gas temperatures because thermal radiation is so
effective at dumping heat in the temperatures above 1200-1300F. The emissivity
of oxidized (gray black) stainless steel is close to one maximizing the
radiation losses (that is, maximize exhaust manifold cooling).
If you wrap or coat the exhaust manifold, the wall
temperature is going to be raised, and the strength reduced. From the
figures, we an see that 100-200F can make huge differences in strength. The
worst thing to do is wrap the manifold with high temperature insulation of the
type frequently sold for automotive use. I once saw a plane with braided
stainless steel mesh over the manifold which is fairly bad. The ceramic
coating probably has the least effect in increasing manifold wall temperatures, but
there is still certain to be a negative effect.
All these treatments increase wall temperature in a temperature
range in which short term strength is dropping rapidly with temperature, long
term brittleness accelerates, and oxidation corrosion rates are also rapidly
accelerating. Don’t expect the exhaust manifold to last to TBO if
you run it hot.
Worst of all, with insulating wrap, your ability to
detect cracks is gone. The exhaust pipes and welds are failing much more
rapidly if covered with insulation, yet your ability to detect failure is
reduced to zero until the problem is not a crack, but a big hole. Not good.
If you want to manage under cowl exhaust manifold
heat, put reflective materials on heat sensitive regions. Brent Regan introduced me to Zetex
aluminized silica fabrics produced by Newtex www.newtex.com. The lightest weight
aluminized fabrics reflect 90% of the radiant heat, can tolerate 2000F short
term for fire protection, and can be attached to “soft targets”
such as composite cowls with high temperature silicone adhesive just like the
aluminized firewall blanket from Lancair.
The exhaust cross over pipe on my TR-182 passes in
front of the engine near the nose bowl of the cowl in a region remote from the
engine cooling air flow. It started to bake the fiberglass and burn the
external paint shortly after I bought the airplane new. I applied a strip
of Zetex fabric over the heat affected zone, and it stopped the problem with no evidence of
additional overheating 2000 hours later. It works.
Fred Moreno