AOPA Pilot Magazine - February 2018 - 87

THE OPTIMUM AIR/FUEL RATIOS for piston-

engine power and efficiency have been
known for decades-but aircraft engine
gauges don't display them. Pilots monitor exhaust gas temperatures, fuel flows,
and cylinder head temperatures-all indirect indications of the air/fuel ratio (AFR).
Things get even less precise when pilots
lack engine monitors and are forced to lean
the old-fashioned way: Pull the mixture
knob out until the engine runs rough, then
push it in until the engine runs smoothly
again.
There's a better way.
Many high-performance cars are
equipped with a lambda gauge that continuously displays AFR (and uses a sensor
in the exhaust manifold to measure it).
Aircraft engine manufacturers don't use
lambda gauges, but owners and builders of
Experimental-category aircraft are beginning to adopt them.
Klaus Savier, founder of Lightspeed
Engineering, a California firm that designs
and sells electronic ignition systems, says
lambda gauges take the guesswork out of
leaning. Pilots taking off from high-altitude airports can at a glance precisely set
the mixture for best power before beginning
their takeoff rolls, and pilots leveling off at
high altitude in cruise can easily and quickly
set the mixture for maximum efficiency.
"I thought about installing a lambda
gauge in my airplane many years ago,"
Savier said, "but I was told they wouldn't
work with leaded avgas, so I didn't question it, and I didn't try. Finally, I decided to
see for myself. I've been flying with lambda
gauges in my two airplanes now for a combined 800 hours.
"They're tremendously useful," he said.
"I hardly even look at [exhaust gas temperatures] anymore."
Engineers have long known that the optimum AFR for peak output is 14.7:1-and the
first gauge for measuring it in cars was developed in the late 1960s. In air-cooled piston
aircraft engines, full power at peak output
can create dangerously high temperatures, so
pilots are wise to avoid them. Piston engines
typically run at relatively rich mixture settings during takeoff and climb.
Savier says he takes off and climbs at a
best-power setting of 12.7:1. In cruise, he

typically leans for maximum efficiency at
a lean-of-peak setting of 16:1. The graphical engine monitor on his Rutan-designed
Long-EZ confirms exhaust gas and cylinder head temperatures are well within
limits.
Mike Filucci, an AOPA colleague,
installed a lambda gauge in his RV-4 and
says it's become his primary reference for
setting the mixture in his ECI Titan IO-370
engine. He documented the results on a
series of flights that showed remarkable
reductions in fuel consumption accompanied by relatively minor speed penalties.
At 9,500 feet (and a density altitude
of almost 11,000 feet), for example, a rich
AFR setting of 11 on the lambda gauge (best
power) resulted in a true airspeed of 178
knots (152 indicated) at a fuel flow of 12
gallons per hour.
Then he leaned the AFR to 16 on the
lambda gauge and airspeed dropped five
knots to 173 true (147 indicated) while fuel
flow plummeted to 7.1 gallons per hour.
That's a 3-percent airspeed reduction for
a 40-percent drop in fuel consumption.
Engine CHTs and EGTs stayed well
within normal ranges at both rich and
lean settings. (The engine was equipped
with dual P-Mag electronic ignitions and
automotive spark plugs and wiring harnesses, so it ran smoothly despite being
100 degrees Fahrenheit lean of peak EGT.)
Savier says pilots who avoid overly rich
mixture settings should expect meaningful improvements in engine efficiency and
reliability, too.
"Unburned fuel from excessively rich
mixture settings will dilute the oil and
cause ring and cylinder wear," he said.
Savier said the lead fouling caused his
first lambda probe to fail. But that problem hasn't recurred, primarily because the
information the gauge provides allows him
to run his engine far leaner-and that keeps
lead deposits from building up. Also, the
shift away in general aviation from leaded
fuels in the future may make lead fouling
a moot point.
"I only had one probe go bad," Savier
said, "and that happened early, when there
was a lot of rich operation."
AOPA
EMAIL dave.hirschman@aopa.org

www.aopa.org/pilot AOPA PILOT | 87

2018 AOPA

PILOT
GUIDES
Now Available!

ALL NEW
REDESIGN!
Buy at
flytheislands.com

http://www.flytheislands.com http://www.aopa.org/pilot

AOPA Pilot Magazine - February 2018

Table of Contents for the Digital Edition of AOPA Pilot Magazine - February 2018