# Piston Engine Fuel Properties and Management

> Fuel properties, detonation, pre-ignition, vapour lock, and fuel management.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-10 — Piston Engines
- **Topic code:** 021-10-02
- **Last reviewed:** 2026-03-24
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/piston-engines/fuel/

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## Aviation Fuel Properties

*   **Octane Rating**: Indicates the fuel's resistance to **detonation**.
    *   **Higher Octane**: Higher resistance to detonation (burns slower/smoother).
    *   **Lower Octane**: Lower resistance.
    *   **Usage**: Usually safe to use a **higher grade** than recommended (e.g., using 100LL instead of 80/87), but strictly forbidden to use a lower grade.
*   **Contamination**: Water is the most common contaminant.
    *   Can accumulate from condensation (empty tanks overnight) or leaks.
    *   **Prevention**: Fill tanks overnight to minimize air space for condensation.
    *   **Check**: Drain fuel sumps **before the first flight of the day** to remove water/sediment.

### Abnormal Combustion

#### Detonation ("Engine Knock")
*   **Definition**: Spontaneous **explosion** of the fuel/air mixture inside the cylinder (instead of a smooth burn).
*   **Effects**:
    *   Exerts excessive force/shock load on piston and cylinder head.
    *   Increased noise ("knocking") and vibration.
    *   **High Cylinder Head Temperature (CHT)**.
    *   Loss of power.
    *   Engine damage (piston/ring failure) if severe.
*   **Causes**:
    *   **Too low octane fuel**.
    *   High Manifold Pressure (MAP) with Low RPM.
    *   High Cylinder Head Temperature.
    *   Too lean mixture.
*   **Corrective Actions**:
    *   **Reduce Manifold Pressure** (Retard Throttle).
    *   **Enrich Mixture** (Fuel cools the cylinder).
    *   Increase Airspeed (Improve cooling).

#### Pre-Ignition
*   **Definition**: The mixture is ignited **prematurely** (before the spark plug fires) by a hot spot in the cylinder (e.g., carbon deposit, glowing spark plug tip).
*   **Effects**:
    *   Rough running.
    *   Power loss.
    *   Further overheating.
    *   Can cause the engine to continue running after ignition is switched off (*Dieseling*).
*   **Difference from Detonation**: Detonation is an explosion *after* ignition; Pre-ignition is an ignition *before* the spark.

### Vapour Lock
*   **Definition**: Fuel vaporizes in the feed lines, creating bubbles that block the flow of liquid fuel.
*   **Cause**: **High fuel temperatures**, low pressure, or high altitude.
*   **Prevention**: Booster pumps (low-pressure pumps) in the tanks pressurize the fuel lines to suppress vaporization.

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