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

021-10-06Pablo Asensio Martínez2026-03-272 min

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.