Originally Posted By: SonofJoe
On a serious note, can I ask you a question? In another thread, you recently posted up a table of oil flow rates at two different rpm's for four different viscosity grades. Although the differences were small, the thickest oil gave the lowest flow. I've always assumed that a typical oil gear pump would push through a constant volume of oil for a given engine speed regardless of it's viscosity but your table said different. Any thoughts on why the differences in flow exist? Oil's not compressible so that can't be it. It could be that thick oil leaks more from the main bearings but I would have thought the flowrate would be measured directly after the pump so that's unlikely. I know some badly designed oil pumps can leak as the back plate flexes so that might be it. Or it might be something to do with the oil swelling in volume as it traverses the main bearing and increases in temperature.
Any thoughts?
LOL, wonder if I'll be stalked in this one as well.
Design of lubrication systems is that due to the bearing's rotation and the fact that hydrodynamic bearings go from a wide gap to a narrow gap, which provides the hydrodynamic "lift" that keeps the parts separated...this narrowing increases the local pressure, and oil can flow in the directions of rotation through the bearing circumference, OR out the side...this is called "side leakage"...the part that goes around does another circuit, the side leakage is lost to the sump, and has to be replenished.
The role of the oil delivery system is to replace that which is lost to side leakage.
When the pump supplies more pressure than the side leakage, "backpressure" is generated and you get oil pressure as an artifact.
Now, for when my stalker gets here...of course, this increase in pressure then forces MORE oil out the unloaded side of the bearing than the normal side leakage would otherwise be...you can use that to oversupply oil to bearings to control temperatures when the natural flow isn't enough to control temperatures.
That's not the normal case in engines, as you design things around their natural point.
So in the paper I pulled the bits out of, they used the "short bearing" approximation to calculate the "natural" side leakage from the bearings, and don't include the pressure aspect...it's how it's typically done.
Now things that reduce the side leakage are:
* Increasing the RPM (shifts the ratio of side leakage to circulation around the bearing by increasing the MOFT, and thus the change in pressure that the oil experiences pushing it out the side)
* Reducing the bearing projected load (does exactly the same as the above)
* Increasing Viscosity, which again increases the MOFT, reduces the delta P, and also increases the resistance for it to flow axially along the bearing.
* Increasing the bearing length
* reducing the bearing clearances.
So in the case of the normal side leakage used in the paper, changing the viscosity reduced the side leakage through the above mechanisms, requiring a lesser make-up flow to the bearing.
NOTE...and I repeat again, if the supply pressure is increased, this flow will obviously increase above natural flows, however if your temperatures are under control, then the natural side leakage is the design point, as anything above that is "safe" under any other circumstances of pump degradation etc.
Here's a "cherry picked example" of "someone else's work", which is my M.O. in providing "pseudo-scientific" "mumbo-jumbo"...IIRC, I think you once knew the authors ???
It was in the days when they were trying to work out HTHS and why the multigrades of the day didn't do so well in the bearings as their grade should have suggested, and they discovered the temprary shear thinning effects of high shear regimes on non newtonian oils.
They took an engine that they could access number 4 main via an external drilling, and applied oil to it at 40psi. Using a bunch of Newtonian (monogrades), they plotted the time that it took the bearing to swallow 250ml (1 cup for those not conversant with S.I.), so you can see how all else being equal, viscosity increase, flow rate decrease (at constant pressure).
Can also see how at the high shear rates, the apparent viscosity was significantly lower...the other chart shows how that apparent viscosity changed with RPM (shear rate) for a bunch of other oils.
Cheers
