# Fuel-Air Mixture Control in Piston Engines

> Mixture ratios, leaning procedure, altitude effects, and detonation prevention in piston engines.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-10 — Piston Engines
- **Topic code:** 021-10-08
- **Last reviewed:** 2026-03-24
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/piston-engines/mixture/

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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).

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.