Fuel-Air Mixture Control in Piston Engines

021-10-08Pablo Asensio Martínez2026-03-241 min

The fuel-air mixture must be correctly proportioned for the engine to run efficiently. The ratio is measured by mass (weight).

Mixture Ratios

  • Chemically Correct (Stoichiometric): 15:1 (15 parts air to 1 part fuel). Theoretically complete combustion.
  • Best Power: Around 12:1. Slightly richer than chemically correct. The extra fuel provides cooling and prevents detonation.
  • Best Economy: Around 16:1 (Lean of peak).

Effect of Altitude

As altitude increases, air density decreases.

  • If the fuel flow remains constant, the mass of air decreases, making the mixture richer.
  • To compensate, the pilot must lean the mixture (reduce fuel flow) using the mixture control lever (red).

Leaning Procedure

Leaning is often done by reference to the EGT (Exhaust Gas Temperature) gauge.

  1. From a rich setting, slowly lean the mixture.
  2. EGT rises as combustion becomes more efficient.
  3. Peak EGT is reached (approx. 15:1 ratio).
  4. Further leaning causes EGT to drop and the engine to run rough (starvation).
  5. For Best Power, enrich slightly from peak EGT (Rich of Peak).

Effects of Improper Mixture

  • Too Rich:
    • Loss of power.
    • High fuel consumption.
    • Spark plug fouling (carbon deposits), especially during prolonged idling or high-altitude operation without leaning.
    • Rough running.
  • Too Lean:
    • Overheating (High CHT - Cylinder Head Temperature).
    • Risk of detonation.
    • Loss of power.

Carburetor Heat Effect

Applying carburetor heat introduces warm air into the intake.

  • Warm air is less dense than cold air.
  • This results in an enriched mixture (ratio shifts towards more fuel relative to air mass).