EGT for mixture setting

[Robert Broberg]

<snip>

> How does the mixture affect EGT?  Do lean mixtures produce lower or higher 
> temps than rich ones?  

When I received my flight training in an aircraft with a manual
mixture control, I was taught to lean the mixture for cruise:
starting with "full rich", lean until the EGT reaches a peak and then
richen the mixture until the EGT drops ~50 F below peak.

The peak EGT is at some point slightly richer than stoichiometric, and
is the point of maximum power.  Some pilots recommend running somewhat
lean of peak EGT for maximum economy, ie. at stoichiometric.  The EGT
drops off rapidly when leaning past stoichiometric.  Because the
training aircraft only had one EGT probe (tied to one cylinder),
richening the mixture slightly after reaching peak EGT accounts for
cylinders that may run leaner (and hotter), and generally it's safer
to run on the cool side.

> Can you determine if you are at stoich with this method (with any accuracy)?
> Is there a linear relationship between EGT and AFR or is it a complex 
> function?

Unfortunately the EGT curve is rather flat near stoich, so it is hard
to pin point with high accuracy.  The relationship is not linear.

> Is this a practical/effective/reliable way to adjust mixture?  Is it only 
> useful at WOT, or does it maintain some validity at all throttle settings?

Piston aircraft have been using this method (manually) for years.  The
mixture adjustment is usually done at part throttle, except at high
altitude where the throttle is wide-open anyway.  "Full rich" mixture
is recommended for WOT at lower altitudes, to keep the CHT down.


Rob Broberg
broberg at asc.on.ca

AFR = air/fuel ratio
CHT = cylinder head temperature
EGT = exhaust gas temperature
stoichiometric = at chemically ideal AFR