# Piston Engine Lubrication Systems: Wet-Sump and Dry-Sump

> Learn piston engine lubrication system fundamentals, including wet-sump and dry-sump architecture, oil properties, and pressure regulation.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-10 — Piston Engines
- **Topic code:** 021-10-06
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/piston-engines/lubrication-systems/

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The main purpose of lubrication in piston engines is to **reduce friction** between moving surfaces and **dissipate heat**. Oil also serves to clean the engine interior, accumulating deposits for removal via filters. Piston engines are highly dependent on lubrication; failure can lead to rapid overheating, damage, or fire.

## Types of Oil Systems

There are two main types of oil systems: **Wet-sump** and **Dry-sump**. The primary difference lies in where the oil is stored.

### Wet-Sump System
In a wet-sump system, the oil is stored in the **sump**, which is the bottom part of the engine crankcase where oil naturally drains.
*   **Components**: Integrated oil reservoir (the sump itself), one pressure pump.
*   **Pros**: Less complex, fewer components (no separate tank or scavenge pump), common in simpler engines.
*   **Cons**: Limited oil capacity, reduced cooling efficiency (oil sits in hot engine), risk of starvation during aerobatic manoeuvres.

### Dry-Sump System
In a dry-sump system, oil is pumped from the engine to a separate **external reservoir**.
*   **Operation**:
    1.  Oil enters the engine from the external tank via a **pressure pump**.
    2.  After lubricating, it drains to the sump.
    3.  A **scavenge pump** collects the oil from the sump and returns it to the tank.
*   **Scavenge Pump**: Must have a **larger capacity** than the pressure pump to handle the oil, which is often aerated (foamy) and less dense by the time it returns, preventing accumulation in the sump.
*   **Pros**: Better cooling control, suitable for aerobatics, higher oil capacity, less oil oxidation.
*   **Cons**: More complex and heavier due to extra components (scavenge pump, separate tank).

## Oil Properties

**Viscosity** is a fluid's resistance to flow (internal friction) and is **temperature-dependent**.
*   **Cold Oil**: High viscosity, flows slowly.
*   **Hot Oil**: Low viscosity. If too hot, the oil film can break down, leading to metal-to-metal contact and damage.

## System Monitoring and Regulation

*   **Monitoring**: Frequent checks of **oil temperature and pressure** are vital.
*   **Pressure Measurement**: Taken downstream of the pressure pump.
*   **Pressure Relief Valve (PRV)**: Regulates pressure.
    *   If stuck **closed**: Gauge shows **higher-than-normal** pressure.
    *   If stuck **open** (or not seated): Dumps too much oil, causing **lower-than-normal** pressure.

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