Quote:
Originally Posted by SAJ
The hydrodynamic pressure in car engines needed to keep the bearing surfaces separated is far in excess of oil pump feed pressure. Anywhere from maybe 1000 to 6000 or more psi depending on engine speed, bearing load, clearances etc etc. Spark advance has a big effect on bearing load, the ideal maximum piston load from combustion being at about 17 deg ATDC for best power (usually quoted at 15 to 20 deg ATDC) for petrol engines. This maximum oil pressure occurs a bit after bottom dead centre in the bearing, since the journal runs a bit off centre from the bottom of the cap.
One can calculate the bearing load per square inch from engine torque and the angle at which it is maximised for a given engine. And hence the hydrodynamic oil pressure needed to keep the bearing surfaces seperated to avoid "boundary lubrication" such as occurs during engine start up before the high pressure hydrodynamic film has formed. But too involved to do here!
So an interesting question is how does oil from the pump at 45 psi manage to feed into an area where hydrodynamic pumping of the bearing is generating at least 600 psi and a lot more at higher revs?
I don't doubt that feed into the bottom high load area helps, since it has been proven many times practically. But i feel intuitively that feed pressure into the top of the bearing where gravity normally supplies it would work even better. Does any one feed oil pressure here?
SAJ in NZ
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SAJ, you are spot on with that post. I have seen pictures of modifications done to an A engine where plumbing is added tot he valve chamber to force oil down the original oil passages to the main bearings. That plumbing is not an easy task.