Oil Pressure Does Not Equal Lubrication

[il_signore97]

Here's my take on this:

Assumptions:

-oil pump relief valve is closed
-engine RPM remains constant throughout example
-average oil temp is constant
-negligible pump losses (flow losses through pump) due to small absolute viscosity differences between grades at hot engine operating temps


Since the oil pump in an engine is a positive displacement pump, the flow rate will be roughly the same (volumetric output of pump is only dependent on RPM, assuming pressure limitations of pump are not exceeded -- since RPM is constant, volumetric flow is also constant).

Since flowrate is held constant, when the viscosity is changed, the pressure must change. The pumping power also changes, as it is a function of flowrate, viscosity, and pressure.

Thus, if one were to use a lower viscosity fluid, the flowrate would be the same, but the pressure and required pumping power would be reduced.

If one were to use a higher viscosity fluid, the pressure and pumping power would increase, but the flowrate would still remain relatively constant.

Now, take the case of relief. The flowrate through the pump is still constant, but since the relief valve is opened, the engine is not seeing the full flow of oil, since some is being dumped back into the pan. This is the only case in which the engine will see reduced flow due to higher pressure.

Now, this does not say anything about the distribution of oil within the engine itself. Just because the flowrate through the pump is equal at all times at a given RPM (assuming relief valve is closed), does not mean that all parts of the engine receive the same amount of oil. It just means that the engine as a whole receives the same amount of oil. However, without pressure/flow gauges installed at each oil outlet in the engine, one would be hard pressed to determine the flow of oil at each outlet and how it will differ with viscosity.

Interesting topic BTW

 

[1sttruck]

"Assumptions:

-oil pump relief valve is closed
-engine RPM remains constant throughout example
-average oil temp is constant
-negligible pump losses (flow losses through pump) due to small absolute viscosity differences between grades at hot engine operating temps"

If you vary the rpm on a warm engine from idle thru cruising speed the flow and the pressure increase. It'd be better to make a table, rows of engine cold / warm / very hot, and columns of idle, accelerating, cruising speeds, and note the pressure/flow insteractions at each combination.

 

[il_signore97]

Quote:

---

If you vary the rpm on a warm engine from idle thru cruising speed the flow and the pressure increase. It'd be better to make a table, rows of engine cold / warm / very hot, and columns of idle, accelerating, cruising speeds, and note the pressure/flow insteractions at each combination.

---

Yes, something like this would be great to have. However, it would entail knowing the inside diameter of the output pipe carrying oil away from the pump, the exact viscosity of the oil, and one would need to have a pressure gauge installed before the pump bypass valve to ensure that it is not opened during experimentation (you can tell by checking the pressure difference between the oil before the bypass and after the bypass at the stock oil pressure sender location -- the difference should not be greater than the anticipated oil filter pressure drop).

But definately something neat if one were to invest the time and resources to do it!

 

[Gary Allan]

Not necessarily all that complicated to determine if you're in relief or not. You just need to measure across your filter. If you're in relief (antenuated pressure) ..your downstream pressure will be reduced. To what degree will be variable depending on how much oil is being shunted through the relief.