piston guy wrote: ↑Tue May 19, 2020 11:49 am
David Redszus wrote: ↑Thu May 14, 2020 5:10 pm
Oil volume is much more important than pressure. Oil pressure is merely a surrogate for estimated volume through the bearings.
Given bearing dimensions, clearances, oil viscosity at operating temperature, and engine rpm, it is possible to calculate
oil flow volume. Given oil specific gravity, the oil mass flow can be determined which is necessary to calculate the heat
transfer from the bearings into the oil. The oil temperature will also determine the minimum oil film thickness.
An alternative method is to measure the bearing cap temperatures directly.
Dave ,
Do you think of "oil pressure" as "excess volume"? In other words if the "supply" or volume exactly matched the "need" , wouldn't oil pressure be "zero"? And if the volume was sufficient to cause pressure , why would more volume be advantageous? Oil pressure "alone" does not prevent metal to metal contact. To "accept" more volume , wouldn't the oil passages need to be increased in size ? We know bearing clearance will cause the pressure to drop because of less restriction , but the same amount of oil is flowing from the positive displacement oil pump.
Engine oil serves two purposes; to keep metal surfaces from touching and to carry heat away from the bearings.
The first is a function of oil film thickness which varies with viscosity and therefore with temperature.
The second is a function of oil mass flow. Mass flow is governed by: bearing journal diameter, sum of bearing clearance flow areas * Cd including oil viscosity * square root of (oil density * oil pressure)
Increased bearing flow area will allow a greater oil mass flow provided that viscosity, oil density and pressure remain the same. But oil pressure does not remain the same under all operating conditions, so oil mass flow will change with pressure. How much you ask?
press. flow
psi ... rate
10... 3.16
20... 4.47
40... 6.32
60... 7.75
80... 8.94
Note that oil flow increases with the square root of oil pressure. An increase in pressure from 20 psi to 80 psi
will double oil mass flow.
The oil mass flow required by the engine is determined by the heat generated in the bearing/journal interface.
At lower speeds and loads, the engine requires less oil mass flow.
So far...easy. But oil mass flow is not a constant, static flow; it pulsates much like a fuel injector nozzle.
And is subject to density and viscosity changes due to aeration.
The best way to come to slippery grips with it all, is to measure and log oil pressure (hopefully in the oil galley)
against engine rpm. The oil pressure should increase with rpm up until the pressure relief valve opens at which time the pressure curve will flatten. Logging oil temperature the same way is very useful.