Message-ID: <39EDF330.DEC4F72C@home.com>
Date: Wed, 18 Oct 2000 12:00:40 -0700
From: David Bachman <dbachman1@home.com>
Organization: @Home Network
To: Lancair.list@olsusa.com
Subject: TSIO-550 Heat vs.Fuel Consumption Revisited
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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In recent years, past 10 or so, substantive progress has been made regarding
application of ceramic coatings (sometimes called thermal barrier coating) for
combustion chamber components, particularly in piston engines. Like most
emerging technologies, ceramic coatings have gone through a maturation process
which hopefully is not totally complete. But it is much more mature than has
been indicated in a previous contribution to the Lancair List. And while
educated skepticism is a healthy and welcome element in discussion of science
and technology, vehement cynicism leads one to believe some ulterior agenda
may be at work.

In response to specifically stated concerns I offer the following:

Detonation:		One of the ceramic materials used for a number of years with
decided results is a matrix comprised mainly of zirconium oxide, or
"zirconia," which has a thermal conductivity some 140 TIMES lower than the
aluminum alloys typical of engine construction (i.e. Ş1150 BTU in/hr-ft2 °F
for Al vs. 8 BTU in/hr-ft2  °F for ZrO2 ).  Zirconia’s extremely low rate of
thermal conductivity not only allows a very thin coating to effectively
insulate the aluminum substrate, but also gives up it’s heat very slowly to
whatever material in contact, including air. Therefore, in the relatively
short time that an incoming air/fuel charge is in contact with coated chamber
walls very little thermal transfer from the ceramic into the charge takes
place. Conversely, aluminum gives up it’s heat very readily and can account
for a measurable portion of the total heating of the charge during induction
and compression. Heat from any source will contribute to detonation. Under
some circumstances the margins for detonation can be very fine. In the case of
the TSIO-550, the margin is such that Continental has chosen a low (even for a
supercharged engine) compression ratio piston, i.e. 7.5 : 1. Such a low
compression ratio contributes measurably to higher EGT and higher fuel burn
(in addition to that used for cooling) due to lower thermal efficiency (a
resultant of the corresponding low expansion ratio). Air-cooled aluminum
constructed piston engines having high rates of boost and operated at
consistently high power levels can derive great advantage from modern thermal
barrier ceramic coatings in raising the detonation threshold without resorting
to unduly low compression ratios or excessively rich mixture. Note that higher
compression/expansion ratios yield higher thermal efficiency (hence, lower
SFC) and increased power. 
	The foregoing is founded in physics not faith in silver bullets (…or
whatever) as was so fervently suggested.

Emissions:		By far, the greater emission concern for air-cooled aluminum
constructed aircraft engines is the excessive hydrocarbon expulsion generated
from using overly rich mixture for cooling. While rich mixture will absorb
heat and contribute slightly to power, by definition, there lacks adequate
oxygen for complete combustion and is either expelled as polluting HC or
continues partial combustion once outside the cylinder, often in the exhaust
tubing, elevating EGT/TIT. Comparing such early attempts toward adiabatic in
monolithic ceramic, uncooled diesels to the highly cooled
aluminum-construction, air-cooled aircraft engine with ceramic coatings when
discussing NOx production (or any other parameter) is a highly overstated
comparison. Yes, a hotter chamber tends to create higher levels of NOx but at
the temperatures generated in ceramic coated air-cooled engines, the increase
is marginal. And as correctly stated, there are no present regulations
governing NOx levels in piston aircraft engines, and because of the exceeding
low population, it is doubtful that they ever will be. But, one never knows
the insanity political winds may stir. If the regulatory need arises, however,
the auto industry has used catalytic converters for years with reasonable effect.
 
Longevity:		When ceramic coatings were originally applied to piston engine
components (chiefly, the piston crown) the conventional wisdom was "the
thicker, the better." ‘Turned out that wisdom did not work well. The
relatively rapid thermal cycles of the intermittent combustion chamber caused
sufficient thermal stress in such "thick" (0.040 in. and greater) coating that
material cracked and ablated over time. Within the last ten years much thinner
coatings have proven very effective in retaining a large degree of thermal
barrier effectiveness, but far less subject to the thermal stress and material
loss of earlier thicker coatings. So too have significant advances been made
in matrix materials and application techniques … especially since the 80s.
	Actually one should ask, "How long should ceramic coating last to be
economically viable?" If the material will continue to adhere and be effective
for 1500 hours of operation, has it provided worthwhile service? In the case
of the TSIO-550 where real TBO can typically be around 1000 hours or less and
with top overhaul necessary anywhere from 500 to 800 hours, would the coating
not provide useful service only considering top overhaul costs if the interval
is extended to the TBO declared by the manufacturer? 
	Aircraft piston engine component longevity experience made available by
engineers testing coated parts indicate a 1500 to 1800 hour service life of
the coating. Erosion of material, principally on the exhaust valve, can render
reduced effectiveness of the coating along a small band on the outer edge
after about 1500 hours.  However, at worst case, better for the thermally
tolerant ceramic coating to slowly erode than your not-so-thermally-tolerant
exhaust valves or piston crowns in far less time and with more dramatic
results.  Extensive experience on material longevity in piston aircraft
engines is limited at present. As time builds on those treated engines in the
field, more longevity data will become available.  Ceramic coatings as
presently applied will not last thousands of hours in piston engines
consistently operating at high levels of power (75+%). But I would venture to
guess that many TSIO-550 operators would be more than pleased at the prospect
of having a true 1500 hour engine especially without extensive top overhaul
somewhere in the middle.  These numbers may be considered short-term by some,
but I think they are long enough to provide honest value. 
	Even though Richard Perry’s experience of operating his ceramic coated
TIO-360A is limited to about 300 hours, to date he has enjoyed approximately
29 % reduction in fuel consumption measured at the pumps. (Probably not
calibrated to the standards of some, but nonetheless to standards of the state
government under which jurisdiction they fall.)  That’s a significant number.
Is it accurate to the second decimal? I don’t know. How accurate is Mr.
Perry’s wallet?  If you care, ask him. I venture that he would welcome your
inquiry. His e-mail address is on the Lancair list.

David Bachman
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