Quote:
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Originally Posted by Brucelee
From the Car Bible
...So the adverts are nearly right - most of the engine wear does happen at startup, and it is because of a lack of oil, but it isn't because the oil isn't coating moving parts - it's [/B] because it's not transporting these acidic gases away...
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Welllllll .... ??
In reality, your crankshaft and camshafts actually never touch the bearings (which are properly called fluid film bearings), instead they float on a thin film of oil - as thin as 0.0001". This is made possible by the flow and pressure supplied to the oil by the oil pump as well as a very thin film of lubricant at a sufficiently-high pressure to match the applied load because of the relative motion between the crankshaft and the bearing. Without this pressure, the oil cannot withstand the weight of the crank alone.
But all this requires the engine to be running so the oil pump can pressurize the oil, and so the crankshaft rotations create the necessary hydrodynamic pressure. When you stop your engine, the oil pump and crank stop too and your oil pressure and hydrodynamic pressure goes away and the crank settles back in the bearing, which is OK because nothing's moving and so no wear takes place.
But, when you restart the engine, you now start turning the crank which is resting directly on the bearing (minus any residual oil film clinging to the parts) and continues to do so until the oil pump and the crank are operated at sufficient rpms to repressurize the oil and once again float the crank. 85% of engine wear occurs at startup, because you get parts moving against one another until the pressure rises enough for them to float.
This is why pre-lubers (or sometimes called pre-oilers) are popular. They are in essence externally operated pumps (usually electric, driven off the battery) which run for a few seconds to pressurize the oil, supply the bearings, and float the crank before the starter is engaged so there is no crank-to-bearing contact resulting in excess wear.
The acid theory is nice, and acids are produced as combustion by-products. But what does running the engine do? Where does this acid go? Through the pickup tube to the oil pump then out to the galleries, the top of the head only to recirculate back down to the sump? The oil still maintains it's acidity and is still in contact with the metal parts whether they're moving or still. Only exhaust gasses from the combustion chamber flow out the exhaust, except for whatever vapors the PVC valve passes from the sump to the intake or combustion chamber and out the exhaust. But, when the oil is cool, it's not releasing much vapor.
