From a previous post:
I finally got to review an article:
Oil Flow in Dynamically Loaded Plain Bearings, April 2007, Ing E.J.M. Slaats:
I am still studying this bearing flow dynamics paper. But here are some initial observations:
They state that the oil hydrodynamic (HD) pressure is the sum of the oil feed pressure and the intra bearing HD pressure. Since the system feed pressure is so low compared to the HD pressure, the bearing Force is the same even with a system feed pressure of zero. This goes along with what I have said in the past. The oil pressure is not the determining factor for lubrication in a bearing. The effect is basically zero. The pressure only exists to move oil into the bearing as it is lost from the side of the bearing.
My comment - As long as the system pressure is 10 PSI for every 1,000 RPM then oil flow should be sufficient to keep the bearing wet in a properly working SI engine.
I cannot reproduce the equation here but see item 2.4.1. In order to keep the force, F, in the bearing a constant value while increasing the clearance, C, (more leakage of oil) you need to logarithmically increase the oil flow, Q. Note the multiple powers of the clearance, C, in the numerator. I also read this as a 1:1 ratio of force, F, to flow, Q. This is the best I can do to reproduce this equation:
Q Flow = nsides X FC/2u X (C/R)squared X 1/(B/D)squared X M
R is the bearing radius, B is bearing width, D is bearing diameter, M is the squeeze velocity, u the kinematic viscosity.
My point is that the oil pressure has nothing to do with lubrication or wear in a bearing.
Viscosity and film thickness are still important because of the spacing needed for dirt, soot and to keep the asperities separated. The paper did note that the combined roughness of the journal and bearing made oil film thickness a factor. But after the break in period, there was much less film thickness needed. They stated the roughness value for the bearing surface actually becomes zero so it is only the journal roughness that comes into play after this time period.
What goes against this is that many new cars are supplied with a thinner than specified oil grade than placed into the engine as OEM. Maybe they want a lesser film thickness than is needed during break in to help smooth the surfaces.
aehaas
More:
They infer that wear (of main bearings) does not occur under normal operating conditions. Wear occurs when the load and oil sump temperatures are high and even at high loads under low RPM situations. They repeat that wear is also a function of the start up period.
They state that when the hydrodynamic pressure increases under high engine loads the oil viscosity increases. This would of course contrast to the shear thinning of oil with bearing velocity. Oil viscosity is not only shear but also pressure dependent. They say film thickness increases with increassing peak pressures because of the Elasto-hydrodynamic behavior of the oil.
It seems they study only the effects of hydrodynamic lubrication. When the film thickness decreases, the additive packages of the finished oil lubricant come into play. They do not go into this aspect of lubrication and wear protection.
What is not explained is why there is start up wear. They state that as long as the film thickness is 3 times the roughness there is no wear. Yet the oil is very thick at start up (relative to operating oil temperatures) and there is the highest level of wear. This was not addressed in the paper.
aehaas
