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Cooling & Lubrication

Lubrication System

Engine oil keeps rotating metal parts from ever touching by holding them apart on a pressurised film. Learn how the pump, filter and galleries feed the bearings, what viscosity grades mean, and why oil pressure warnings are urgent.

Intermediate12 minute readFull lesson

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Inside the engine

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8 earlier steps are still open: The Four-Stroke Cycle, Pistons, Connecting Rods and the Crankshaft, Valves, Camshafts and Valve Timing, Spark Ignition and Combustion Timing, Fuel Injection, Engine Layouts: Inline, V and Flat, Diesel Engines, Engine Cooling System.

Skipped earlierThe Four-Stroke Cycle

The idea

Beginner explanation — switch to Technical for the engineering detail.

Inside an engine, metal parts slide and spin against each other thousands of times a minute. If they actually touched, they would wear out or weld together within minutes. Oil prevents that by forming a thin film between the surfaces so they glide on liquid instead of rubbing metal to metal.

The oil lives in the sump at the bottom of the engine. A pump draws it up through a strainer, pushes it through a filter that catches dirt, and sends it along drilled passages to the crankshaft bearings, the camshafts, the valve gear and often the turbocharger. It then drains back down to the sump to start again. Some oil is also sprayed or splashed under the pistons to cool them.

Oil gradually breaks down and collects soot, fuel and acids, so it is changed at regular intervals. If the oil pressure warning light comes on while driving, the film may no longer be protecting the bearings — stopping the engine promptly can be the difference between topping up and rebuilding.

What you will learn

  • Explain how an oil film separates a spinning journal from its bearing
  • Trace oil from the sump through the pump, filter and galleries
  • Interpret an SAE viscosity grade such as 5W-30
  • List the jobs oil does besides reducing friction

How it flows

Lubrication System: flow diagramSumpOil pumpFilterMain galleryCrank bearingsValve gear
Read the diagram as text
  • Sump → Oil pump
  • Oil pump → Filter
  • Filter → Main gallery
  • Main gallery → Crank bearings
  • Main gallery → Valve gear
  • Crank bearings → Sump (drain)
  • Sump: Oil reservoir
  • Oil pump: Pressure + relief valve
  • Filter: Full-flow with bypass
  • Main gallery: Drilled passages
  • Crank bearings: Hydrodynamic film
  • Valve gear: Cams, lifters, VVT

Step by step

Read each step in order.

  1. Step 1: Draw oil from the sump

    The pump sucks oil up from the sump through a mesh strainer that stops large debris.

  2. Step 2: Pressurise and filter

    The pump pushes oil through a filter that traps fine dirt and metal particles.

  3. Step 3: Feed the bearings

    Drilled passages carry the oil to the crankshaft and camshaft bearings, where it forms a protective film.

  4. Step 4: Cool, clean and seal

    Oil also carries heat away, holds dirt in suspension and helps the piston rings seal.

  5. Step 5: Drain back

    The oil runs back down to the sump and the cycle repeats many times a minute.

Go deeper

Optional. Open a panel for the engineering detail — or switch to Technical above.

The engineering explanation3

Crankshaft main and big-end bearings are plain (journal) bearings running with clearances of a few hundredths of a millimetre. As the journal rotates it drags viscous oil into the converging gap, generating a pressure wedge that lifts the journal off the shell: hydrodynamic lubrication. The film thickness depends on viscosity, speed and load. The Stribeck curve describes the regimes: at low speed or high load the film collapses into boundary and mixed lubrication where surface asperities touch, which is why starting and labouring at very low engine speed produce disproportionate wear. Anti-wear additives (for example zinc dialkyldithiophosphate) form protective layers for those moments.

The pump — gear, gerotor, or a variable-displacement vane type that reduces parasitic loss — supplies more flow than the bearings need; a relief valve caps pressure. A full-flow filter has a bypass valve so a clogged or cold, thick filter never starves the engine (unfiltered oil is better than none). Main galleries feed the main bearings, drillings through the crankshaft feed the big ends, and further passages supply cam bearings, hydraulic lifters, variable valve timing actuators and turbo bearings. Piston-cooling jets spray the underside of the crowns. Most cars use a wet sump; racing and some performance cars use a dry sump with scavenge pumps and a remote tank to avoid oil surge under high cornering loads.

SAE J300 grades describe viscosity: the 'W' number rates cold-cranking and pumping performance (lower flows better when cold), and the second number is the viscosity class at 100 °C. A 0W-20 thus pumps quickly on a cold start but is thinner when hot than a 5W-40. Oil also transfers heat away from pistons and bearings, carries detergents and dispersants that hold soot in suspension, neutralises acids, helps the piston rings seal, and protects against corrosion.

Key concepts4
Hydrodynamic lubrication
A rotating journal drags oil into a narrowing gap, generating pressure that holds the surfaces fully apart.
Viscosity
A fluid's resistance to flow. It falls sharply as oil warms, so grades specify behaviour both cold and hot.
SAE grade (e.g. 5W-30)
The W number rates cold performance; the second number is the viscosity class at 100 °C.
Oil film thickness (trend)
Thicker films come from higher viscosity and speed and lower load, which is why lugging at low rpm is hard on bearings.
h ∝ (η × ω) / load
Common misconceptions3

  • MythA thicker oil always protects better.

    RealityThick oil flows slowly when cold, when most wear happens, and adds friction. Modern engines with tight clearances are designed for the grade in the handbook.

  • MythThe oil pressure light means the oil level is a bit low.

    RealityIt means pressure has fallen below a safe threshold. That may be low level, but it can also be a pump or bearing problem. Stop safely and investigate before driving on.

  • MythIf the dipstick is full, the oil does not need changing.

    RealityOil degrades: additives deplete and contaminants build up even when the level is correct. Change it at the interval the manufacturer specifies.

In the real world3
  • Checking the dipstick on level ground with the engine off for a few minutes gives a true reading because the oil has drained back to the sump.
  • A brief flicker of the oil light during hard cornering with low oil shows surge uncovering the pickup.
  • Turbocharged engines are particularly sensitive to oil quality because the turbo shaft runs at very high speed on an oil-fed bearing.

Common questions

Short answers to the questions people most often ask about this topic.

What does engine oil actually do?

Its main job is to stop moving parts touching: in the bearings it forms a pressurised film that holds the metal surfaces apart. It also carries heat away, keeps dirt suspended until the filter catches it, protects against corrosion and helps the piston rings seal.

What do oil viscosity grades like 5W-30 mean?

The number before the W describes how easily the oil flows when cold, and a lower number flows better for cold starts. The second number describes its thickness at normal operating temperature. Always use the grade the vehicle's manufacturer specifies.

What does the oil pressure warning light mean?

It means oil pressure has fallen too low to protect the engine, often because the oil level is very low or the pump has a problem. Stop as soon as it is safe and switch the engine off, because running without oil pressure can destroy the bearings within minutes.

Check your understanding

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0 of 3 answered

Question 1 of 3

In a healthy crankshaft bearing running at speed, what separates the journal from the bearing shell?

Almost there

  • Overview — Read the introduction to the end.
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  • Go deeper (optional) — Optional: open a deeper panel or switch to Technical.
  • Check — Answer the quiz questions.

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Skipped earlierThe Four-Stroke CycleInside the engine · 0 of 9 done

Earlier in this route you skipped The Four-Stroke Cycle, Pistons, Connecting Rods and the Crankshaft, Valves, Camshafts and Valve Timing, Spark Ignition and Combustion Timing, Fuel Injection, Engine Layouts: Inline, V and Flat, Diesel Engines, Engine Cooling System.

Sources & attribution

Further reading

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AutoVerse original content

This lesson is original AutoVerse writing. The links above are suggestions for further reading, not the source of this text.