At 08:45 PM 8/10/02 -0500, Bullwinkle wrote:
>I've heard it said that raising the oil pressure in a vehicle can actually
>decrease the oil flow to the bearings.
That would never be true with an MG
>I can see where this might be true if one is using a pressurized system
>which has only one exit. Decreasing the size of that exit to raise the
>pressure would decrease the flow exiting the single tap.
That might be true if the pump output was sensitive to pressure. The oil
pump in an MG is a positive displacement pump which will put out a fixed or
constant flow of oil with the volume only dependent on the speed of the
pump. Increasing the pressure does not decrease the flow (except for a
miniscule amount of internal leakage).
For any individual point of exit this is also true, that decreasing the
size of the exit (bearing clearance) will decrease flow.
>When that concept is applied to our MG engines, I can't visualize how
>raising the oil pressure could decrease the flow to the bearings.
It doesn't.
>The oil pressure is controlled by a dump valve which releases the excess
>oil rather than trying to squirt it through the bearings. So, by raising
>the oil pressure aren't you also increasing the amount of oil flow in the
>system rather than letting it be dumped back into the sump?
Not exactly. At low speed the oil pressure is determined by the flow rate
and bearing clearances. At higher speed, once the speed is sufficient to
achieve relief pressure, then increasing the pressure will increase the
amount of flow through the bearings (slightly) while allowing a little less
to be dumped back into the sump, but the total amount being pumped does not
change. The problem is that the pressure required to force oil through the
bearings increases exponentially with the flow rate. This means that it
takes a large increase in pressure to produce a small increase in flow.
Incidentally, increasing oil viscosity increases oil pressure at low speed,
but it also DECREASES flow through the bearings at ALL speeds. It is
important to have a reasonably good match between oil viscosity and the
size of the internal bearing clearances. Smaller bearing clearance wants
thinner (less viscous) oil, while larger bearing clearance wants thicker
oil. And if you run 20W50 oil in your MG at cool temperature (below 40dF)
you may notice significantly lower oil pressure on startup until the oil
warms up some.
The rest of this may be more than you were asking for. Delete now if
you're tired of reading.
The thickness of the oil film and the viscosity of the oil is used to
support the bearing load. When the engine is running at full throttle or
high speed (not necessarily both) the momentary working load on the bearing
journal will far exceed the oil pumping pressure. Oil flow through the
bearing is needed to carry away heat to prevent breakdown of the oil and/or
melting of the bearing material. The oil pump is used to maintain oil flow
through the bearings, while the pressure you see on the gauge has very
little to do with the load bearing capacity of the bearing.
The way to achieve a larger increase in flow is to increase the clearance
in the bearings, but that also makes for substantially lower oil presure at
low engine speed. This is often done in racing engines where the increased
flow is intended to improve cooling of the bearing journals. To prevent
low oil pressure at idle a high volume oil pump is (usually) used. This
leads to pumping more excess oil over the pressure relief valve at higher
speed, which generates more heat in the oil. This leads to the requirement
for using a large oil cooler to keep the oil temperature in a reasonable
range, which in turn makes for longer time for the oil to warm up to normal
operating temperature after startup. This in turn leads to some advantage
to install a thermostatic bypass valve in the oil cooler lines to bypass
the oil cooler until the oil is up to running temperature to reduce the
warm up time (because oil does not flow well at low temperature).
If you try real hard to increase oil flow through the bearings by using
excessive oil pressure (greater than 80 psi), this makes for excess flow
velocity through the bearing journals which can lead to erosion of the
bearing shells. The end result looks like little worm holes in the surface
of the soft bearing material on the inside of the bearing shells, which
effectively reduces the bearing surface area supporting the load. This is
not critical until there is some metal to metal contact, at which time the
bearings go away very quickly. In the meantime the worm holes increase the
clearance for oil flow through the journals, which has about the same
effect as increasing the bearing clearance, which reduces oil pressure
(especially at low speed). In addition to possibly damaging the bearing
shells, the higher oil pressure requires more power to drive the oil pump,
which leads to more wear on the drive gears, possible erosion of the
internal parts of the oil pump (similar to erosion of the bearings), and
more heating of the oil as the higher pressure excess supply escapes over
the relief valve.
Pumping oil does not produce any mechincal output at the flywheel, so all
of the energy used to pump the oil is ultimately truned into heat. The
higher pressure and higher volume you use, the more heat you will be
producing. This is why it is not adviseable to use a high volume oil pump
or excessively high pressure in a stock or near stock street engine. If
you have low oil pressure at idle (20 psi or less) it's time to replace the
crankshaft bearings. A high volume oil pump would improve idle oil
pressure, but that's just treating the symptom instead of the disease.
For a little more information on oil flow, check here:
http://www.ntsource.com/~barneymg/mgtech/engine/of102.htm
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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