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Making TBO (Time Between Overhaul)
Today we live with engines that should be
able to make TBOs of as little as 1500 to
2000 hours! That is the equivalent of 50,000 to 85,000 miles on
an automotive engine! Worse yet, few air-cooled
engines even make it 1500 to 2000 hours without some sort of
serious top end problems – often requiring a partial or even
full top overhaul prior to reaching TBO.
How many of us would tolerate such a poor service reliability
and short lifespan in our automotive engines? Today we expect
and often achieve 150,000 to 300,000 miles on our automobile
engines with hardly a problem along the way. Why is it then that
we can't expect the same out of our far more expensive aircraft
engines?
The answer is simple – too much heat!
Air-cooled piston aircraft engines endure
cylinder head operating temperatures of 350 to 450F that are
easily twice as high as any water cooled engine. Keeping
a high-powered air-cooled engine's temperature under control
is an almost impossible task.
As power increases per cubic inch of
displacement the heat dissipation requirements increase
substantially. That is why turbo charged air-cooled engines have
even shorter TBOs. The top end of the engine must endure even
higher continuous operating temperatures because there are only
so many square inches of surface area on an air-cooled engine in
which to dissipate the heat. That in a nutshell is the entire
problem with air-cooled engines. Their ability to dissipate heat
is limited to the surface area of the fins on the cylinder head
and walls and requires a monumental amount of airflow in order
to keep the top end of the engine from literally melting down.
General Aviation piston engines are
essentially the last of the air-cooled piston engines –
virtually all other internal combustion engines have converted
to water cooling? The vast majority of motorcycle engines and
even die hard air-cooled automotive engine manufacturers like
Porsche have gone to water cooling in the quest for more power
and reliability per cubic inch of displacement. In fact water
cooled automotive engines have in many instances more than doubled
the power output per cubic inch of displacement in the over the
last 10+ years. One of the principle reasons such advances have
been possible is because the operating temperature of the engine
is kept at a stable and efficient level allowing for higher compression
ratios and more precise fuel and engine management systems to
be developed.
Imagine TBOs of 3000 to 5000 hours
Sound hard to believe? Well it shouldn't be - it should be
what is expected of our aircraft engines today. The original
manufacturers haven't made any serious efforts to extend the
life of our aircraft engines. The main hurdle to overcome is the
continued use of air cooling rather than water cooling. Water cooling substantially reduces and stabilizes engine temperatures
and substantially extends the top end component life especially
the exhaust valve and seat.
Generally at time of TBO most of today's air-cooled engines
lower end parts are in good condition and still within
tolerances to go for at least another 1000 to 1500 hours which
for most engines would almost double the TBO. The main area for
wear and problems are the cylinders and top end valve components,
primarily the exhaust valve, seat and guide. With water cooling
we foresee TBOs reaching upwards of 3000 hours and expect to
ultimately achieve 5000 hours. Such an extension in TBO would
more than offset the increase in the cost of a water cooled
engine however, and more importantly we would expect no top end
problems during the service life of the engine. This contrasts
sharply to the fact that many air-cooled engines rarely go to
their TBO without some form of top end problems requiring either
a partial or complete top overhaul prior to reaching TBO.
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