"So, all those racers out there (motorcycles, quads, drag cars,
sand rails, drag boats, snowmobiles, etc.) with heat coatings on
top of their pistons are only using it to "feel good"?"
No.
You fail to understand the thermal problem. Ceramic coating
piston tops and cylinder head cavities has been shown to gain 1-2% more
horsepower at full power. I had my pistons and
cylinder heads coated for this reason, and because it made me feel
good.
But it works on pistons because the thermal event lasts through the
power stroke only, about 180 degrees of rotation, or about 12-15
milliseconds. The peak heating only occurs for
about half of this when the pressures and temperatures are
highest. This is when most of the power is made, and avoiding
a bit of heat loss for a few milliseconds is worthwhile.
Here is the difference between pistons and exhaust pipes. In
case of the piston, the thermal "wave" from the transient heating
only penetrates a few mils into the ceramic coating during the
event. Then the gas temperature falls dramatically, falls some
more during the exhaust stroke, and then there is a blast of cold air
and evaporating avgas taken in and compressed. The top few mills
of ceramic coating are cooled, a reduced amount of heat
reaches the piston or head metal, and the cycle continues.
Contrast this very rapid transient piston heating and cooling with
the exhaust pipe. Blast of hot gas, then slowing of the hot gas,
then some oscillations of hot gas back and forth in the pipe, and then
another blast of hot gas. A thermocouple buried in this rapidly
changing flow can not respond fast enough to report instantaneously, and
displays an "average" temperature which we interpret as the EGT.
So does the inside of the exhaust pipe.
With no cooling event occurring, the thermal coating inside the
pipe soaks up energy, temperature rises and stays high, and the
interface between ceramic and metal gets hot. Then the metal gets
hot and it stays hot. Everything gets red hot. This is not
aluminum piston country.
The ceramic coating inside the exhaust pipe reduces the heat flow
to the metal a bit, but only a small bit. The metal still gets
hot, but its temperature is lowered slightly. Even slight
lowering of temperature helps a little bit, so it is up to you to decide
if it is worth the cost and effort. If it feels good, then do
it.
Don't disparage things that feel good. That is why
we fly, build nice airplanes, and enjoy sex. Don't knock it.
Gary also wrote: "On a side note, I know it sure keeps my chrome
motorcycle pipes from turning brown or blue. Oh, and they have not
shown symptoms of failure after years of use. Read below."
Come on, Gary, to you seriously consider a motor cycle
spending most of its life at low power settings to be comparable to
an aircraft engine running 50F lean of peak at 65% power? If you
want to make it a good comparison, open the throttle on your motor
cycle, accelerate up to about 90% of top speed, back off the throttle to
hold this speed, and then continue for 1000 hours. After
that I would like a report on your chrome exhaust pipes.
Then we have something to compare and discuss objectively because only
then will the operating conditions and duration be approximately the
same.
...Don't do it. It will droop off due to
gravity having absolutely no creep strength, and will drop in a
heap in a few hours. And it will rust to dust in a few
cycles....
...Mild steel has no capability in this operating
regime.
Gary wrote further: "Why is it then that there are hundreds, no
thousands, of experimentals out there (several at this airport) with
steel [my emphasis added] exhaust systems that have
been flying for years, decades, with no problems or symptoms that
you describe?"
I am sorry if the context of my comment in italics above evaded
you. The question I was responding to was about using mild
steel for aircraft exhaust pipes. Pursuant to earlier
emails discussing I noted that even stainless steel had
low creep strength at the higher temperatures of interest.
Mild steel has virtually no creep resistance or
oxidation resistance at the same temperatures. Build mild
steel headers for your airplane and when they get hot, they
will get soft, creep, and deform. I will leave it to you to look
up the creep strength and oxidation rates of mild steel in the
temperature range of interest for this discussion, namely 1200-1500F.
As to your comment about experimental (aircraft ) operating
for years with no problems, let me again clarify since it appears
my earlier text was unclear in conveying its message. I was
referring to mild steel (considered to be 1020, 1030,
etc. ) while aircraft exhaust pipes are all made with a steel alloy
normally referred to as STAINLESS steel, usually
321 stainless, an austenitic alloy with a mix of properties making
it attractive for exhaust pipes on airplanes. Unlike mild
steel which is almost entirely iron (Fe) with a bit of
carbon and low quantities of other alloying agents,
321 stainless steel has the following alloying
agents:
Fe, <0.08% C, 17-19% Cr, 9-12% Ni, <2% Mn, <1% Si, 0.3-0.7%
Ti, <0.045% P, <0.03% S Translation: lots of nickel
and chromium and important additions of titanium. What this
means is summarized as follows:
"Grades 321 and 347 are the basic austenitic 18/8 steel (Grade 304)
stabilised by Titanium (321) or Niobium (347) additions. These grades
are used because they are not sensitive to
intergranular corrosion after heating within the carbide
precipitation range of 425-850°C. Grade 321 is the grade of
choice for applications in the temperature range of up to about 900°C,
combining high strength, resistance to scaling and phase
stability with resistance to subsequent aqueous
corrosion." See http://www.azom.com/article.aspx?ArticleID=967 for
more insights.
Mild steel it ain't.
I hope this helps your understanding.
Fred Moreno