Engine oil and lubrication

We have a couple of cars that are beginning to age, and with that do things like breakdown occasionally and go through oil.  That an engine can survive 200 000 km quite happily is to a large extent down to the lubrication.  Just a few quick calculations can give the scale of the problem the oil has to solve.

How many revolutions of the engine does 200 000 km represent? Doing some very rough calculations, if I run the car at 60 km / h at 2500 rpm (60 km/h is probably a reasonable average and makes the maths nice and easy), that means it does one kilometre a minute, so 2500 revolutions per kilometre.  Multiply that by 200 thousand kilometres gives us 500 million revolutions of the engine in its life so far. Half a billion. That’s a lot of up-and-down in the pistons and opening and shutting of valves, amongst other things.  (Though to put it in perspective, compare it to the number of times someone’s heart will beat during an average lifetime – about 2 billion).

How much damage is done to the cylinder lining, cam shaft, valve seals, etc etc with each revolution? Well, let’s assume for starters that each revolution takes off one plane of atoms from our moving engine surfaces. In other words one atom’s thickness is removed from the inside surface of each cylinder each time the piston passes up and down. In metals, atomic spacings are a few Angstroms, and angstrom being ten to the power of minus ten metres. Let’s say 3 Angstroms for the sake of argument, though I’d have to look it up.  Doesn’t sound much.  But what’s 3 Angstroms times half a billion revolutions?  About 15 centimetres. 

Now, clearly my cam shaft hasn’t reduced in radius by 15 centimetres since the car was new. That means that my initial guess of one atom thickness per revolution is a large overestimate of the damage done. Possibly we are talking of more like one atom thickness removed  per hundred revolutions. I think that’s pretty impressive really, and it means some pretty decent lubrication.

And 200 000 km isn’t really that  large a distance for an engine to do, anyway.


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