# Fundamentals of Piston Engine Operation

> Basic principles of reciprocating engines, components, cycle differences, and power definitions.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-10 — Piston Engines
- **Topic code:** 021-10-01
- **Last reviewed:** 2026-03-24
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/piston-engines/general/

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## Basic Principles & Components

The **reciprocating engine** (piston engine) consists of:
*   **Cylinder**: Closed at one end (Cylinder Head) containing inlet/exhaust valves and a spark plug.
*   **Piston**: Slides up and down inside the cylinder.
*   **Connecting Rod**: Connects the piston to the crankshaft.
*   **Crankshaft**: Converts the **reciprocating (linear) movement** of the piston into **rotary movement**.
*   **Camshaft**: Operates the valves (driven at half crankshaft speed).

### Piston vs. Gas Turbine

Both engines use a cycle of induction, compression, combustion, and exhaust.

*   **Piston Engine**:
    *   Process is **intermittent**.
    *   Combustion occurs at **constant volume**.
    *   Reciprocating motion.
*   **Gas Turbine (Jet) Engine**:
    *   Process is **continuous**.
    *   Combustion occurs at **constant pressure**.
    *   Entirely rotational motion (smoother operation).

### Manifold Pressure (MAP)

*   **Definition**: Measures the **absolute pressure** in the intake manifold **just downstream (behind) of the throttle valve**.
*   **Indication**:
    *   Engine Stopped: Indicates atmospheric pressure.
    *   Engine Running (Idle/Cruise): Indicates lower than atmospheric (due to suction).
    *   **Gauge Leak**: If the gauge reads atmospheric pressure while the engine is running, it indicates a **leak in the pressure gauge line**.
*   **Significance**: Provides a reliable indication of power being generated.

### Engine Volumes and Ratios

*   **Swept Volume (Displacement)**: The volume displaced by the piston moving from Bottom Dead Centre (BDC) to Top Dead Centre (TDC).
    *   Calculation: $\text{Stroke Length} \times \text{Piston Crown Area}$.
    *   Total Displacement = Swept Volume $\times$ Number of Cylinders.
*   **Clearance Volume**: The volume remaining above the piston when it is at TDC (smallest volume).
*   **Total Volume**: Volume above the piston at BDC (largest volume).
*   **Compression Ratio**: The ratio of **Total Volume** to **Clearance Volume**.

### Power Definitions

*   **Indicated Horsepower (IHP)**: The total power developed **in the cylinder** (theoretical potential). Depends on cylinder pressure, swept volume, and RPM.
*   **Friction Horsepower (FHP)**: Power lost to friction and driving accessories.
*   **Brake Horsepower (BHP)**: The actual power available at the **crankshaft** (output power).
    *   $BHP = IHP - FHP$
    *   $Power = Torque \times RPM$
*   **Mechanical Efficiency**: $(BHP / IHP) \times 100\%$.

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