How Friction Works Inside an Engine
Every time an engine starts, thousands of metal components begin moving at high speed. The crankshaft rotates, pistons travel up and down, camshafts open the valves, and bearings absorb enormous loads. All of this is accompanied by friction.
Without an oil film, damage begins almost instantly: turbocharger bearings, where the film is only 6-9 micrometres thick at speeds exceeding 100,000 rpm, can fail within seconds of losing oil pressure. Crankshaft main and rod bearings are more resilient, but they too start suffering damage within 30-60 seconds, and irreversible wear sets in within a few minutes. This is exactly why the main job of engine oil is to create a strong oil film that separates the surfaces of components.
Engine oil doesn't just reduce friction. It creates conditions under which metal surfaces don't touch each other directly during most operating regimes.
Where friction comes from
Under a microscope, even perfectly machined metal is covered in microscopic irregularities, known as surface roughness. As components move, these microscopic peaks interact with each other, creating resistance to motion and generating friction. It is precisely the relationship between the thickness of the oil film and the height of these irregularities that determines how reliably the components are protected from each other.
Three lubrication regimes: the Stribeck curve
Tribologists (friction specialists) describe lubrication behaviour using the Stribeck curve, named after the German engineer Richard Stribeck, who studied the relationship between friction and the speed of moving parts in the early 20th century. The curve shows that, depending on speed, load, and oil viscosity, lubrication inside the engine operates in one of three regimes.
| Lubrication regime | What happens |
|---|---|
| Boundary lubrication | The oil film is thinner than the surface irregularities, and the components partly contact each other directly. Friction is at its highest. |
| Mixed lubrication | The load is carried partly by the oil film and partly by the surface irregularities themselves. A transitional state between the other two regimes. |
| Hydrodynamic lubrication | The oil film fully separates the surfaces, with a thickness many times greater than the height of the irregularities. Friction is at its lowest. |
In the hydrodynamic regime, the oil film thickness is usually between a few and around a hundred micrometres, enough to fully separate the surfaces at high speed. It's important to understand that all three lubrication regimes operate simultaneously inside a running engine, just in different components and at different moments of the cycle.
Where each regime operates in the engine
Crankshaft main and rod bearings spend most of their time in the hydrodynamic regime: the high rotational speed generates a stable oil film that fully separates the shaft and the bearing shell. Piston rings, by contrast, regularly find themselves under boundary lubrication conditions: at top and bottom dead centre, the piston momentarily stops, speed drops to almost zero, and the oil film doesn't have time to build up to its full thickness. The valve train (the contact between the camshaft lobe and the follower) also operates predominantly in the boundary or mixed regime, due to high point loads and low sliding speed.
Why metal doesn't weld together
If there's no oil film between components, direct metal-to-metal contact leads to rapid wear, localised overheating, and damage to the working surfaces, up to and including micro-welding at individual contact points. This is exactly why the boundary lubrication regime, where such contact is partly unavoidable, requires not so much oil viscosity as specialised anti-wear additives.
The classic anti-wear additive ZDDP (zinc dialkyldithiophosphate) works precisely in the boundary regime: under friction, it forms a protective film on the metal surface, usually 50-150 nanometres thick, which sacrifices itself in place of the base metal, gradually wearing away and reforming. This film forms only where it's needed, that is, specifically in zones of boundary lubrication, rather than evenly across the entire engine surface.
Common misconceptions
Friction inside the engine is always bad.
Not quite. The job of the oil isn't to eliminate friction entirely, but to keep it within a controlled and predictable regime that minimises wear, rather than physically preventing any interaction between surfaces.
A thicker oil film is always better, in any situation.
No. In the boundary regime (for example, at engine start-up or at the piston's dead centres), film thickness is physically limited by speed and load, and in these conditions protection comes not from film thickness but from the chemical composition of the anti-wear additives.
In a well-lubricated engine, metal never touches metal at all.
No. Brief contact between surface irregularities under boundary and mixed lubrication is a normal part of engine operation, especially in components like piston rings near dead centre and the valve train. Additives exist precisely to provide protection at these moments, not to eliminate them entirely.
Frequently asked questions
- Why does an engine wear the fastest during a cold start?
- At start-up, oil hasn't yet been pumped through the entire system and has a higher viscosity due to the low temperature, so many components briefly find themselves in the boundary lubrication regime with an increased risk of metal-to-metal contact.
- What is the Stribeck curve in simple terms?
- It's a graph showing how friction changes depending on the speed of moving parts, load, and oil viscosity, clearly dividing lubrication behaviour into boundary, mixed, and hydrodynamic regimes.
- Can the same engine component operate in different lubrication regimes at once?
- Yes. For example, a piston ring shifts from the boundary regime at the dead centres to the hydrodynamic regime in the middle of the piston stroke, where speed is at its maximum, all within a single crankshaft revolution.
- Does the lubrication regime depend on oil viscosity?
- Yes, viscosity is one of the three factors in the Stribeck curve alongside speed and load: a more viscous oil, all else being equal, shifts a component closer to the hydrodynamic regime, but it cannot fully eliminate the boundary regime where speed drops to zero.
Conclusion
Friction inside an engine isn't a single phenomenon, but a combination of three different physical regimes operating simultaneously in different components: from almost complete separation of surfaces in the crankshaft bearings to brief contact between surface irregularities at the piston's dead centres. Understanding this picture explains why engine oil isn't just a viscous liquid, but a complex system in which viscosity and additives handle different tasks under different conditions.