Air-Fuel Ratio (AFR)
The air-fuel ratio is the mass of air per mass of fuel in combustion. The stoichiometric ratio for complete combustion of gasoline is approximately 15
(15 parts air to 1 part fuel).| AFR | Name | Use |
|---|---|---|
| 15 : 1 | Stoichiometric (theoretical optimum) — forms an ignitable mixture | Complete fuel combustion; basis for "ignitable mixture" |
| ~ 12.5 : 1 | Best power — max output | Climb / cruise at high power; slightly rich of stoichiometric (extra fuel cools) |
| > 15 : 1 (e.g. 16) | Best economy — max range | Cruise, leaner than peak EGT |
"Rich" = higher fuel share (AFR < 15
, e.g. 12), "Lean" = higher air share (AFR > 15, e.g. 18). Mixtures with high amounts of fuel are referred to as "rich".Best engine performance is NOT at full rich
The best engine performance is not reached with full rich mixture — reason:
- At full rich the mixture is much richer than stoichiometric (typically 10 or richer).
- The excess fuel does evaporate (which cools), but it does not burn completely — energy released per cycle is less than at optimal AFR.
- → RPM is lower and power reduced.
Best power is at AFR ~ 12.5
— slightly rich of stoichiometric (extra fuel cools combustion and prevents detonation), but not so rich that unburnt fuel is wasted.Mixture lever use
| Position | Effect | Use |
|---|---|---|
| FULL RICH (forward) | Richest mixture | Start, take-off, sea-level climb |
| Leaned (middle) | Leaner mixture | Cruise above AFM threshold (typically 3 000–5 000 ft DA — consult AFM) |
| ICO (idle cut-off, fully back) | Fuel shut off | Clean engine stop (prevents "run-on" / diesel effect) |
Engine shutdown by leaning — safety rationale
An Otto engine is shut down by fully leaning the mixture (idle cut-off) — not by switching off the ignition. Reason:
- With mixture at idle cut-off no more fuel enters the cylinder → the engine has nothing to burn and stops.
- With ignition switched off, unburnt fuel remains in the cylinder — risk of uncontrolled ignition (glow ignition in a hot engine) and backfire at next start.
→ Standard: mixture ICO to stop the engine, then ignition switch OFF.
Mixture change with altitude — automatically richer
With increasing flight altitude (decreasing air pressure) and unchanged mixture setting, the air-fuel mixture becomes automatically richer:
- Higher altitude → lower air density → less air mass per unit time through the carburettor.
- With the same fuel flow (mixture unchanged) the air/fuel ratio becomes smaller = richer.
- Consequence: with rising altitude the pilot must pull mixture back (lean) to keep the optimum.
Effects of mixture adjustment
Mixture adjustment affects:
- Air-fuel ratio (primary).
- Cylinder head temperature (CHT) — leaner = higher CHT (closer to stoichiometric, more heat).
- Oil temperature — follows CHT with delay.
- EGT (Exhaust Gas Temperature) — primary mixture indicator.
Mixture monitoring instruments
For correct mixture setting in cruise the following instruments are used:
| Instrument | What to watch |
|---|---|
| EGT (Exhaust Gas Temperature) | Primary indicator — leaning raises EGT to peak, then falls (too lean) |
| CHT (Cylinder Head Temperature) | Secondary — leaning raises CHT, never exceed AFM limit |
| Flowmeter (fuel flow) | Direct consumption in gph or l/h — drops as leaning |
Mixture check on the ground
Mixture control can be checked on the ground with the engine running at any desired RPM: the pilot slowly pulls the mixture back — RPM rises first (better mixture) and then drops (too lean). If the control works, mixture adjustment is effective.
An important pre-flight routine to verify mixture function.
Lean during taxi / long ground time
On long taxi routings or long ground waits it is advisable to lean the mixture during taxi:
- At idle / low load the standard mixture is too rich → unburnt fuel fouls spark plugs.
- Leaning makes combustion cleaner → plugs stay clean → better start, better performance.
Typical recommendation (Lycoming): in taxi/idle pull mixture back slightly until RPM peak.
Leaning procedure
For fixed-pitch propeller:
- At cruise altitude and stable cruise power, pull mixture back slowly.
- RPM rises first (better mixture), then drops (too lean).
- Best power: mixture at RPM peak.
- Best economy: further back, then slightly forward to ~50 RPM below peak.
For constant-speed with EGT:
- Slowly lean mixture, EGT rises.
- Peak EGT — leanest without detonation risk.
- LOP or ROP per engine manufacturer.
Why lean?
At altitude air density is lower → without leaning the mixture stays rich, too much fuel per air mass → unburnt fuel reduces power, fouls plugs, raises consumption and CO emissions.