I’d like you to conduct a thought experiment: Imagine that you’re an EGT probe,
located in an exhaust riser between 2 and 4
inches from the exhaust port of a cylinder,
and think about what you would see.
As Figure 3 illustrates, you’d see nothing
much two-thirds of the time—during most
of the intake, compression, and power
strokes—because the exhaust valve is closed
and so no exhaust gas is flowing out of the
exhaust port and past the probe. During the
one-third of the time that the exhaust valve
is open, you’d see a constantly changing gas
temperature that starts out very hot when
the valve first opens but cools very rapidly as
the hot compressed gas escapes and
expands, and then ultimately is scavenged
by cold induction air during the valve overlap period (at the end of the exhaust stroke
and the beginning of the intake stroke) when
both intake and exhaust valves are open
simultaneously.
Now, all these gyrations are happening
about 20 times per second, and you (the
EGT probe) cannot possibly keep up with
them. You wind up stabilizing at some tem-
perature between the hottest and coolest gas
temperature you see, and you dutifully
report this rather arbitrary temperature to
the panel-mounted instrument, where it is
displayed to the pilot as a digital value accu-
rate to 1 degree. The temperature you report
to the pilot is not exhaust gas temperature
(which is gyrating crazily 20 times a sec-
ond), but rather exhaust probe temperature
(which is stable but related to actual exhaust
gas temperature in roughly the same fashion
as mean sea level is to high tide).
TURBINE INLET TEMPERATURE (TIT)
Turbocharged engines are often equipped with
a turbine inlet temperature (TIT) gauge or a TIT
probe connected to the aircraft’s digital engine
monitor. Unlike EGT, absolute values of TIT are
meaningful, because the TIT probe is mounted
far downstream in the exhaust system where
the gas flow past the probe is steady (not
pulsed) and has relatively steady temperature
(not constantly fluctuating). Observance of an
absolute TIT redline (typically 1,650 degrees
or 1,750 degrees) is appropriate and prudent
to obtain maximum useful life from the
turbocharger.—Mike Busch
other four cylinders. The exact same
phenomenon also occurs on the right
engine. This is not because those front
cylinders produce cooler exhaust gas than
their neighbors (they don’t), but because the
exhaust risers for those cylinders curve aft
