><< We put a GEM-602 (4-cylinder engine monitor) on our Traveler several weeks
> ago and have been watching it ever since to get an idea of 'how things are'.
> I don't understand something we are seeing and before we start switching
> probes around I thought I would see if someone else with more experience
> with these sorts of things can lend me a clue.
>
> On climb out, the CHTs for all 4 cylinders is 'fairly' flat, but the EGTs
> are all over the chart. The EGTs for cylinders 3 and 4 seem to be close to
> the anticipated levels, but the EGTs for cylinders 1 and 2 are much lower.
> Once at cruise settings, the EGTs for 3 and 4 remain at expected levels and
> the EGTs for 1 and 2 rise to expected levels.
A friend of mine retired from GM after 'bout a bizzilion years as
an engine guru and went to work for McClarin (sp?) to help with
development of specialized engines including very high performance
engines for racing. I recall some conversations we had about 10
years ago when he was running his Lyc O-235 in a test cell
in anticipation of putting the engine in his Long-Ez fitted with
a special carburetor. I can share some things I learned from our
discussions.
Two considerations high on the list of priorities for induction
system design are FLOW and MIXURE. The two qualities are interdependent
to a degree that causes a lot of brick throwing between guys trying
to whip carburetor problems and those designing the plumbing that
hooks everything up.
CHTs are an indication of a balance between cooling air over the
cylinders and the heat energy thrown off by the fires and friction
within the cylinder . . . and may have very little significance
with respect to how well that cylinder is puttine energy into the
propeller. For example, if one cylinder is putting out half the
horsepower it was designed for, it may still show the same temperature
as the others if it suffers from restricted air flow for cooling.
EGTs are a stronger indicator for energy produced by the cylinder
but I undestand that the temperature can be influenced both by
flow (amount of fuel/air consumed each stroke and restrictions
in the ability of the exhaust system to move the spent gasses
out) and mixture (the most efficient use of fuel is indicated
by adjusting mixture for maximum EGT reading meaning that their
is neither an excess of fuel or oxygen). Mixture is the most
powerful infuluence of the two.
The ability of carbutetors to discharge a completly homogenous
concoction of fuel vapors and air is a dicy proposition. By-
in-large, carburetor designs on most certfied airplanes have
not been modified in things that affect performance since they
were liberated from the farm over 50 years ago. Updraft
carburetors for deep breathing engines were fairly common
on tractors back then . . . while cars were moving rapidly
away from the design. Given aviation's reverence for things
traditional, it's not hard to understand why cars run so
much better than our airplanes.
If one observes a large difference in EGT readings on an aircraft
engine, there's a relatively easy way to scope out the cause.
In straight and level flight, adjust the mixture over a range
as needed to observe the peak in each cylinder. Do this one
cylinder at a time returning to a "too rich" setting for a
minute or so between each reading. It's not sufficient to
do this test at a very low power setting where continuous
operation at peak EGT is allowed. The ability of
a carburetor to maintain the same mixture characteristics
over the full range of throttle travel is generally poor.
The differences you cited for various power settings
is an example of this phenomenon.
If this test shows that you can achieve the SAME peak reading
on every cylinder, then you know that the unballance is due
to mixture variations on a cyclinder by cylinder basis. If
all cylinders peak at the same time but show different
tempratures, then there is a difference in flow of gasses
on a cylinder by cylinder basis -or- there is a variation
in the calibration the probes and indicating system on a
cylinder-by-cylinder basis.
Institutionalized aviation has managed to push this design
flaw under the rug for decades by installing a single
EGT probe in the exhaust gas stream of the cylinder that
peaks first as the mixture is leaned. After enriching to
the value recommended for cruising flight one could be
assured that all other cylinders are operating at or below
the same temperature on the rich side of peak. It was
not apparent when one or more cylinders operated a
hundred degrees cooler than the hottest . . . not until
you folks started installing fancy instrumentation.
See what progress has done for us? After decades of
operation in the fat, dumb and happy mode, we now have
something new to worry about, as if good pilotage wasn't
enough to occupy our minds while airborne. My suspicions
are that wide variations in EGT readings are pretty common
in carbureted engines. Injected engines can be fine tuned
by adjusting the ports for each cylinder but you guys
with tractor carbs are pretty well stuck with what you've
got.
By the way, my friend was considering installation of
a more modern carburetor design on his Long-Ez and was
so disappoined in the test-cell results that he bolted
the tractor carb back onto the engine. His experience
suggests to me that carburetors should have been
eliminated from aircraft engines a LONG time ago. But
then, we're not nearly the influencial consumer group
as car buyers. Regulation has so discouraged competition
and new development; we must resign ourselves to living
with the unhappy information acquired by sticking that
new-fangled gage on your antique airplane!
Bob . . .
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