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G SERIES PILOT'S GUIDE
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A New Approach to Engine Management
The G3-4 is a sophisticated tool for engine management. Its microprocessor performs many tasks that used to be handled
by the pilot. One of the basic functions performed by the G3-4 is monitoring exhaust gas temperatures for all cylinders with
one degree resolution. What is important is the exhaust gas temperature of a particular cylinder in relation to its peak.
But peak EGT is not a constant; it changes with atmospheric conditions, altitude, power setting and engine condition and for
this reason absolute exhaust gas temperatures in degrees Fahrenheit are quite meaningless.
The real objective of mixture management is finding a mixture setting which represents the correct position on the EGT/Fuel
Flow Curve. As we will see later, this abstract task is easily accomplished by the G3-4's microprocessor which samples
EGT's for all cylinders many times a second and subjects this data to a complex mathematical analysis that can identify
peak.
This capability allows the pilot to operate his or her aircraft engine at the most economical mixture settings.
It is generally known that EGT can be a valuable source of information for engine diagnosis and troubleshooting, but there
is a great deal of confusion when it comes to interpreting this data.
One of the basic principles of EGT engine analysis is that engine temperatures (EGT and CHT) achieve equilibrium in an
engine operating at a constant power and mixture setting.
What is often overlooked is that this equilibrium cannot be defined as a single point but rather a range of temperatures.
Fundamentals of EGT
The basic ingredients of combustion are fuel, air (oxygen), compression, ignition, and timing.
The measurement of Exhaust Gas Temperature (EGT) is really an indication of the harmony of interaction of these ingre-
dients.
A slight change in any of these five factors will result in noticeable changes in EGT.
The measurement and dynamic analysis of these changes is a very valuable tool for engine management.
The use of exhaust gas temperature for mixture control depends on certain characteristics of combustion that are common
to all engines. It is generally known that the exhaust gases get hotter as the mixture is leaned.
This temperature rise is a sign of increased combustion efficiency as the optimum mixture setting is approached.
If the leaning progresses past a certain point, the temperature will begin to drop. This temperature drop is the result of re-
duced energy output from the diminished fuel flow.
For a variety of reasons, the best operating mixture for aircraft engines is in the vicinity of this peak.
Some high performance engines require slightly more fuel for cooling and run best on the rich side of peak while others are
designed for operation on the lean side of peak.
The shape and character of this curve is typical for all normally aspirated engines; it is, however, slightly affected by some
turbocharger installations.
Principles of EGT Measurement
Exhaust Gas Temperature is measured with a temperature-sensing probe that penetrates the exhaust stack a few inches
away from the cylinder. The sensing probe is made from a special alloy designed to provide long term protection for the
temperature sensing elements inside. The EGT probes are designed to have a small thermal mass for fastest possible re-
sponse, and the manufacturing procedures are tightly controlled to maintain probe calibration to within one degree.
In fact, the G3-4 will help you monitor mixture, timing, fuel distribution, compression, oil consumption, and many other subtle
engine phenomena. The G3-4 can actually resolve engine phenomena that occur in millionths of a second.
Principles of CHT
The G3-4 is designed to work with three different kinds of probes.
The gasket probe replaces one of the spark plug gaskets on a cylinder and is held in contact with the cylinder by the spark
plug. The spring-loaded probe screws into the temperature well in the cylinder and its tip is pressed against the cylinder by
spring pressure. The third kind of CHT probe is called an adapter probe. It screws into the temperature well, but unlike the
spring-loaded type, it allows the factory installed bayonet probe to remain in place.
INSIGHT
INSTRUMENT CORPORATION
BOX 122, FORT ERIE, ONTARIO, L2A 5M6
21
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