AOPA Turbine Pilot Magazine - January 2023 - 93

Savvy Maintenance coverage
sponsored by AIRCRAFT SPRUCE
SAVVY MA INTENANCE / OPINION
Obsessed with EGT
Don't use exhaust gas temperature as a leaning reference
BY MIKE BUSCH
I RESPOND TO at least 100 queries from
aircraft owners and pilots each week. At
least a dozen of those are questions or
requests for advice about leaning, and
most of them relate to exhaust gas temperature.
A few common ones:
Q: My POH recommends leaning to 50
degrees Fahrenheit rich of peak EGT, but
your articles/webinars/books seem to say
I shouldn't do this. What's with that?
A: In the 1960s or 1970s when your
airplane's POH was written, engineers
believed this was a good way to lean the
engine. Since then, we've learned a lot
more about the effect of mixture on the
combustion event-largely through the
research efforts of George Braly at the
world's most advanced piston aircraft
engine test facility he created at General
Aviation Modifications Inc. (GAMI) in
Ada, Oklahoma-and we now know
that 50 degrees F rich of peak is very
nearly the worst possible mixture for
engine longevity. I recommend against
leaning to 50 degrees rich of peak
because I want your engine to live long
and prosper.
Q: When leaning my engine to X
degrees rich or lean of peak, should I use
the first cylinder to peak, the last cylinder
to peak, or split the difference? Since
my cylinders don't all reach peak EGT at
the same mixture control position, what
" peak EGT " mixture should I use as my
leaning reference?
A: I recommend against using EGT as
a leaning reference. Google " Mike Busch
leaning " to find lots of information about
what I recommend.
Q: How many degrees lean of peak do
you operate the engines in your Cessna
Turbo 310 in order to obtain the phenomenal
engine longevity you've achieved?
A: I have no clue. Since I never use
EGT as a leaning reference, I have no idea
how many degrees lean of peak I operate-
nor do I care.
Why EGT peaks
If you slowly pull back on the mixture control
while watching EGT, you'll see that it
goes up to a certain point, then starts coming
back down as you continue to lean.
The mixture at which EGT stops rising
and starts falling is called " peak EGT. "
Combustion is a chemical reaction
in which certain substances (reactants)
combine with one another to produce
other substances (products). For gasoline
engines, the reactants are mainly
(chemically perfect) ratio of air and fuel
that would theoretically result in no leftover
oxygen or octane after combustion
takes place.
Why does EGT peak at this stoichiometric
mixture? At richer mixtures,
there's excess fuel that can't be oxidized
and the evaporation of this excess fuel acts
as a refrigerant to reduce EGT. At leaner
mixtures, there's less fuel to combust, so
less energy is liberated which again lowers
EGT. That's a bit oversimplified, but
it's close enough.
Is peak EGT the place to be?
You might think that this chemically perfect
air-fuel ratio at which EGT peaks
You might think that this chemically perfect air-fuel
ratio at which EGT peaks would be where we should
always operate our engines, but you'd be wrong.
the fuel hydrocarbon octane (C8
the air component oxygen (O2
H18) and
), and the
resulting products of combustion are
carbon dioxide (CO2
) and water (H2
O)
plus a bunch of liberated energy (heat)
that our pistons, crankshafts, and propellers
convert into airspeed. A chemist
might describe the combustion of gasoline
using the formula:
2 C8
H18 + 25 O2
16 CO2
+ 18 H2
O
Note that it takes precisely 25 molecules
of oxygen to combust two molecules
of octane. We can achieve this ratio by
combining 14.7 pounds of air with 1 pound
of gasoline. This is the " stoichiometric "
would be where we should always operate
our engines, but you'd be wrong. Just
because it's chemically perfect doesn't
mean it's operationally useful. It's not the
mixture that produces best power, and it's
not the mixture that produces best economy.
So, it's not generally the optimal
mixture for flying.
For best power, you need an air-fuel
ratio of about 12.5-to-1, quite a bit richer
than stoichiometric, which occurs at about
100 to 125 degrees F rich of peak. This rich
mixture burns somewhat dirty (due to
unburned hydrocarbons), but it also burns
faster which yields a bit more power. For
AOPA PILOT / January 2023
93
AOPA Turbine Pilot Magazine - January 2023

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