Leaning Aircraft Engines, II
Aero engines usually have a mixture control, its the red knob. It is used to control the fuel air ratio in the carburettor and to stop the engine at the end of a flight. Some engines have an automatic mixture control unit in the form of a pressure sensor (Rotax/Bing) or a sophisticated FADEC engine controller (Aerosance). These engines normally do not have a manual mixture control and must be stopped by switching the ignition off.
In a carburettor fuel is metered on basis of a volume of air and fuel. As altitude is gained, the volume of air the engine intakes remains the same but its density decreases however.
This is the reason we need to use the mixture to lean the fuel as to keep the engine running smoothly. Fuel savings up to 20 - 25% can be reached by leaning the engine properly.
Leaning during cruise
This is where leaning can really save fuel. And we have a number of leaning methods available: best power, peak EGT, lean of peak (LOP) and with power recovery.
Best power (ROP)
Set the mixture so that the leanest cylinder runs at 100°F rich of peak EGT. This ensures that the exhaust valves stay cool. Maximum power is produced along with the highest airspeed, fuel consumption and CHTs.
This method is know to cause engine trouble over time, problems as sticky valves, fouled sparks plugs, carbon deposits to name a few. See mixture too rich in part I of engine leaning.
The way to achieve this without a EGT gauge is to lean the mixture until the engine start to run rough and then enrichen until it runs smooth again. Keep in mind to move the mixture knob slowly.
Peak EGT
Set the mixture so that the leanest cylinder runs at peak EGT. The EGTs will be 100°F hotter than the best power method and heat damage is possible to exhaust valves. The engine will produce some 4% less power and fuel consumption will be 14% lower than with best power. CHTs are approaching the highest values.
The engine is now run on a chemically correct mixture, stoichiometric, and the fuel burn is at it hottest. EGTs and CHT are peaking.
Lean of Peak (LOP)
Set the mixture so that the richest cylinder runs at 30-80°F lean of peak EGT. Now the CHTs will be at their coolest and the EGTs will be 20-70°F hotter than the 100°F rich of peak (ROP) method. We now have reasonable cool exhaust valves with the engine producing 7% less power and fuel consumption will be 20% lower than the best power method.
Recover power
If you want to "recover" the power, increase MAP with an inch or so and make sure that the hottest cylinder doesn't get hotter than 30°F rich of peak EGT, do maintain LOP operation.
Big radial engines like Pratt&Whitney Wasp / Wasp Major and the Wright Cyclones were all run at LOP, ensuring reliable long service life and without the problems caused by running rich of peak.
Descent
Maintain the same mixture level as during cruise, just reduce the throttle (reducing MAP) to maintain the same power levels. Before entering the circuit set the mixture to fully rich, just in case a go-around is needed.
From the above we can see that the Lean of Peak (LOP) method is the best way to reduce fuel consumption and be assured that the engine has a long and healty life.
