What are clearances in friction pairs

To start with, a rhetorical question: is the diameter of the piston (assembled with its rings) the same as the inner diameter of the cylinder? Of course not! For a piston to be able to move back and forth inside a cylinder hundreds of times a minute, its diameter simply has to be slightly smaller, otherwise friction would instantly heat both members of the friction pair we're examining to temperatures at which they'd be destroyed.

So there is a difference in diameters (a clearance) — the next question is how big this clearance is, what fills it, and what it affects. Given how an internal combustion engine (ICE) works, it's precisely this clearance that ultimately determines the engine's efficiency, because it's through this clearance that the driving force of the fuel mixture's combustion in the cylinder “leaks away.” So does that mean the smaller the clearance, the greater the power?

On the other hand, as already mentioned, a clearance (even a minimal one) is still necessary, and besides, like any other friction pair, our pair absolutely needs constant lubrication too. So the designers' main task is to make this clearance exactly match the oil film that the engine oil forms, an oil that has a property called viscosity. In that case, engine power will be at the maximum possible level (all else being equal) for its design.

And this is exactly where the problems start. Why? Because oil viscosity is a variable quantity that depends heavily, and inversely, on temperature. For instance, with a standard 5W-40 oil, as the engine warms up from, say, 40 to 100 °C, actual viscosity drops from around 90 to 14 mm²/s, meaning it falls more than sixfold! And viscosity doesn't drop all at once, but gradually, along a curve. And every oil has its own curve. Correspondingly, if the oil temperature is below 40, viscosity will be even higher; if it's above 100, viscosity will be even lower. Obviously, the thickness of the film on the friction pairs changes right along with the viscosity value.</content>