Conventional aero gas engines usually have a mixture control, its the red knob. It is used to control the fuel air ratio in the carburetor 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 et al).
These engines normally do not have a manual mixture control and must be stopped by switching the ignition to the off position. In a carburetor fuel is metered on basis of a certain volume of air and fuel. As altitude is gained during climb, 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.
The engine is normally operated with a slightly richer mixture to safeguard it against detonation, preignition and possible overheating of exhaust valves. This very small amount of extra fuel cools the cylinders by evaporation and a richer mixture burns also at lower temperatures. Leaning by the pilot is normally done below 75% of maximum power.
If the aircraft is equipped with exhaust gas temperature (EGT) and cylinder head temperature gauges (CHT), preferable for each cylinder, it then becomes possible to lean very precisely. Keep in mind that with a carburetted engine there always will be a difference in temperatures among the cylinders. With a balanced fuel injected engine (Gami) this will be minimized
Installing a FADEC is a real advantage to you, the system is making sure that the engine is properly leaned in each cylinder and is not running too rich or too hot, this will save fuel and money at the end of the day/flight.
The fuel / air ratio for best power is around 1:13 and for best specific fuel consumption (SFC) it is 1:17. Peak EGT occurs at the chemically correct ratio of 1:14.7 and peak CHT occurs at a slightly richer ratio.
Combined with high power settings this will result in high cylinder temperatures, possible detonation, very hot or burnt exhaust valves and maybe damage to the pistons resulting in a power failure and a possible off airport emergency landing.
This causes rough running, loss of power, fouled spark plugs, lead deposits and lower cylinder and exhaust temperatures. A good example is if the primer pump not locked properly (usually just after a cold engine start where you may have used the primer) which results in these symptoms accompanied with black puffy exhaust, rough running engine and possible backfiring.
Idling the engine during a long taxi and waiting for take-off with the mixture in the full rich position results in carbon deposits on the spark plugs and the engine will start to run rough when power is applied. It helps to lean the mixture and increase the combustion temperatures so the spark plugs will burn clean. Especially so with AVGAS 100, which has twice the amount of lead compared with AVGAS 100LL.
At low power settings during idling and taxi, exhaust temperatures, CHTs and detonation will not be a problem. You will do no damage to the engine by leaning. Notice, however, that the mixture control needs to be pulled out considerably before any leaning will occur.
Do not forget to set the mixture to fully rich just before take-off. Runup is a really good time to check this.
The mixture must be fully rich before take-off. EGT for most non turbo engines will range from 1100 - 1250°F and for most turbo engines from 1200 - 1350°F. You have to make sure to stay below 1350°F at all times.
If climbing is continued over 3000 ft start reducing the mixture control to keep the mixture from getting too rich. If the mixture gets too lean the RPM will start to drop and the engine is going to run a little rough. This rough running is caused by mixture differences among the cylinders (carburetted engines) and not by detonation as some may think. Enrichen the mixture gently until you get to the point where the engine runs smooth again. Advise: do keep an eye on the EGTs.
A reduction in air density will lead to a richer mixture. Leaning is therefore needed to keep the engine running smoothly during these take-offs and climbs.