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Dear Peter,

Coatings were originally developed as a means of prolonging the life of
the blades in the hot-section of turbines was metal spray using
plasma-arc, which meant the base metal had to withstand some
significant temperatures during the coating process.

Tech-Line's approach (and others) successfully bonds a
zirconium-ceramic alloy to a properly prepared aluminum surface at a
temperature of only 350F. Since this is clearly impossible I'm sure
most folks have discounted such coatings out of hand :-)

The thermal barrier coating material comes as a thick water-based
'paint' having unusual wetting qualities. After being sprayed or
brushed onto a properly prepared surface the stuff is allowed to dry.
The coated pistons & heads are then put into an oven, brought up to the
cited temperature and held there for a given amount of time. The
result is an apparently alloyed ceramic-metallic surface.

I don't know how it works but here are some guesses based on my
experiments.

The most critical factor appears to be the proper preparation of the
surface, which must be abraded with a sharp, relatively fine-grained
media, such as #120 aluminum-carbide. Examined under a 30x binocular
inspection scope your nicely machined surface has been converted to an
infinity of edges so fine that they refract light. (If you put an
abraded but un-coated sample into the oven for the required amount of
time, on examination you will see that the refraction vanishes; the
surface still appears abraided but is now smoother.)

Getting the coating material to 'wet' the abraided surface can be
difficult. The metal must be perfectly clean -- touching an abraided
surface with your bare hand is enough to cause the coating to fail (but
leaves a nifty metallized fingerprint :-)

The coating material appears to consist of a combination of finely
divided (ie, powdered) frits. During the heat-soak period -- after the
coated part is brought up to the required temperature -- the frits
appear to melt in a eutectic-like process, with those which melt at a
low temperature forming a solution which cascades the melting of those
having a higher melting temperature.

I can't say if the result is a true alloy or simply an exotic form of
hard-facing but the result is a durable, heat-resistant surface. You
can bend it or beat it with a hammer and it stays put. You can also
heat it with an O/A torch immediately adjacent to an untreated coupon
and see the latter melt (!) will the treated surface remains unchanged.

Obviously, impossible, right? :-)

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So what does all that mean?

According to Sir Harry Ricardo (and others) during the intake cycle the
residual heat causes the incoming charge to expand, effectively
reducing the engines volumetric efficiency. In a similar vein, the
instant combustion is initiated the surrounding structure begins
absorbing heat produced by the combustion process, so that by the time
the process ends the temperature within the combustion chamber -- and
the pressure resulting from it -- are reduced.

In the Otto cycle engine TBC's yield slightly higher torque for the
same fuel consumption. I've no idea how much of this may be attributed
to improved VE or increased BMEP -- and on a small engine, with
home-made sensors it's impossible to quantify those results -- but with
a test club turning the same rpm I've seen a reduction of fuel
consumption of 3% to 7%.

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As you've pointed out, there's no such thing as a free lunch. Thermal
barrier coatings cause more heat to appear in the exhaust port and
exhaust stack.

The need to coat the head & neck of the exhaust valve is clearly
indicated but even so, the stock VW valve is a rather dinky bit of
goods having a stem only 8mm in dia. On the advice of Tech-Line I've
treated the valve stems and guides with tungsten disulfide, a dry-film
lubricant that is merely burnished into the clean, unabraided metal
surface. I've also modified the VW's lubrication system so as to
increase the amount of oil reaching the rocker galleries by about 8x.
The valve gallery was abraided with coarse media (sand, in this case --
I was worried about media residue contaminating the oil) cleaned
ultrasonically and treated with a thermal dispursant. On the stock test
engine the oil temp was 8% to 10% higher, compared to an untreated
engine.

As for the exhaust stacks, while monel or stainless steel might serve,
my budget dictated plain carbon steel. This was treated with another
type of thermal barrier coating and held up quite well, assuming the
coating was properly applied. The tricky bit here was getting a
uniform coating on the interior of the tubes, which proved impossible
when the stack was made up of welded sections. Although unsuitable for
flight, I fell back on cheap, after-market headers and 'J-tubes.'
These are seamless, mandrel-bent tubes which were easy to prep and
coat.

(As a point of interest, the test engine had a unique 'bark' unlike
anything I'd heard before. To keep peace in the family I fitted
mufflers to the stacks.)

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Finally, as with most experiments I had more failures than successes.
But there were enough of the latter to convince me that, when properly
applied, coatings would do no harm and had the potential to provide
some improvement in the durability of an engine assembled from
after-market VW components.

-R.S.Hoover

Thanks,

Those are truly fascinating results, and I wish that your experiments could
continue. If a local EAA chapter in your area has 501c3 status, and if
enough of the lurkers are interested, anything is possible, although not
necessarily probable. That would at least allow the original goal of
determining durability to be met. Obviously the addition of the mufflers
during the experiment add a "fudge factor", but the circumstance you
described is interesting in that it suggests that the residual pressure from
combustion is higher in the treated engine at the time that the exhaust
valve opens.

All of this suggests, at least to me, that the claims made by both Tech-Line
and the maker of the MPG-CAPS applied through the fuel system are true.
BTW, I am currently trying MPG-CAPS in my car, which does appear to run
better and more smoothly at the lowpower levels at which cars normally
operate; however I doubt that I will ever be able to accurately quantify the
effect on fuel consumption. I am sorry that I don't have more training
and/or real world engine experience to contribute.

Peter