August 2005 Article of the Month
By Rod Machado
©2005 Rod Machado
The EGT Primer
Combustion is a chemical change accompanied by heat and light. Since I do a lot of
barbecuing, I am very intimate with the “light” part of this definition. With enough starter
fluid on those briquettes, you can sustain daylight an additional three, possibly four hours
beyond sunset in your neighborhood. This is why it's important to know how to regulate
the combustion process, especially in your airplane's engine.
Pilots have several tools that help them regulate combustion, one of which is the EGT
(Exhaust Gas Temperature) gauge shown in Figure 1. On many EGT gauges, there are
temperature calibrations but no total temperature scale. When adjusting the mixture with
the EGT, we're not usually concerned with the actual temperature of the expelled exhaust
gas (unless it's a turbocharged engine with a Turbine Inlet Temperature limitation). We're
more concerned with peak temperatures for a given set of conditions: throttle position, air
density, etc.
That's why most EGT gauges have a thin, red, manually rotated reference needle. We
lean the mixture until the white needle indicates peak EGT (in other words, the needle
won't move any farther to the right) as shown in figure 2A. Then we manually rotate the
red needle over the white one as sort of a temperature “bookmark” as shown in Figure
2B.
Now we'll enrich the mixture an amount appropriate for the desired engine performance.
How much is that? Sometimes, best economy operations are conducted at peak EGT as
shown in Figure 2B. Make sure, however, that you check the engine manufacturer's
recommendations and limitations for EGT. For instance, there may be a maximum
horsepower or time limit that accompanies peak EGT operations (some engine
manufacturers prohibit engine operation at or near peak EGT). Best power operations are
usually conducted 125 degree F on the rich side of peak as shown in Figure 2C. Enrich
the mixture until the EGT temperature needle indicates 125 degrees cooler from peak
(that's five increments on a scale of 25 degrees F per increment).
Now that we've reviewed EGT basics, we're almost ready to see how the EGT gauge
might warn us of impending engine problems. A few more basics on combustion are
necessary first.
During the engine's compression & power stroke, spark plug ignition takes place before
the piston reaches top dead center. At about 20 degrees of crankshaft rotation before the
top-dead-center position, both plugs fire and a flame sphere spreads outward as shown in
Figure 3A. These burning gases are further compressed by the ascending piston, with
peak combustion pressures occurring after the piston starts its downward stroke. The
burning fuel-air charge is converted into two different forms of energy: heat and a
relatively smooth force which pushes the piston down.
What happens if detonation occurs? Detonation causes the unburned portion of the fuelair charge to explode after the spark plugs fire. Instead of burning smoothly and evenly
within the cylinder, the remaining portion of the fuel-air charge explodes with a
suddenness likened to a shotgun blast. In our automobiles we hear this as the familiar
“pinging” associated with cheap gas (in the typical flight instructor car, no pinging means
the engine isn't running). Of course, we can't hear this pinging from within the airplane.
We need another means of detecting detonation. Here's where the EGT is useful.
Detonation causes the fuel-air charge to release heat all at once, rather than slowly, over
time as shown in Figure 3B. The heat is transferred to the cylinder and its components
and is reflected in a rising CHT (Cylinder Head Temperature) reading. But what happens
to the EGT reading? If you were monitoring the EGT, you'd see a decrease in temperature
as shown in Figure 4. Why? Exhaust gases are cooler after detonation occurs because the
fuel-air charge has already released much of its heat to the piston and cylinder.
Combustion took place early, so the exhaust gases are cooler.
Of course, exhaust gas temperatures may decrease for other reasons too. The incomplete
closure of a valve reduces compression and lowers peak combustion temperatures (a
burnt valve or a valve blockage can also restrict valve closure and lower peak combustion
temperatures). All of these may result in a drop of exhaust gas temperatures. But rising
CHTs, accompanied by a falling EGT, usually indicate detonation. Do something to cool
that engine now!
Suppose you're in cruise flight and have the EGT set for best power. Suddenly you notice
a rise in the EGT reading. What do you think? Consider the problem this way: If exhaust
gas temperatures are rising (with throttle position, mixture setting, air density, etc.
remaining the same), then the fuel-air charge takes longer to burn or is still burning as it
leaves the cylinder. What would cause this? How about a fouled spark plug or anything
that prevents a spark plug from firing properly? Without the benefit of two plugs firing in
each cylinder, the fuel-air charge won't burn as quickly as shown in Figure 5. Therefore,
exhaust gases are hotter when expelled from the cylinder, which is shown as an increase
in EGT. (This assumes the EGT probe is located so it can sample the suspected
cylinder(s). It's also a good reason to know where the EGT probe is located on the
engine, if it's a single-probe type.)
This also explains why EGT readings show an increase (up to a point) when leaning the
mixture. As a general rule, leaner mixtures burn slower than rich mixtures. As we lean
the mixture, the fuel-air charge burns slower and the exhaust gas is hotter as it's expelled
from the cylinder. Of course, if we continue to lean, EGT reading eventually drop
because there's less and less fuel to burn.
If you really want to learn more about engines obtain a copy of Aircraft Engine
Operating Guide by Kas Thomas. It's one of the best books I've seen on understanding
airplane engines. It's well worth the money. You'll be a better pilot for it.