Posted by Todd Wilson on Monday, October 15, 2001 at 0:13AM :
In Reply to: Re: Is is possible to have too much pressure? posted by Vaughn on Sunday, October 14, 2001 at 5:40PM :
{author unknown}
MYTH: The oil pressure gauge is a measure of oil flow in the engine
TRUTH: The oil pressure gauge is only a measure of oil pressure. Oil
pressure
is a measure of the oil's resistance to flow, not a measure of the actual
flow
rate. Oil pressure is really only a measure of whether or not the oil pump
is
pumping oil, not whether or not the oil is reaching the critical bearings,
or
even how much of it is being pumped.
SIMPLE EXPLANATION
Oil pressure is a measure of the oil's resistance to flow. This cannot
be
overemphasized, so it bears repeating. Oil pressure is a measure of the
oil's
resistance to flow.
What causes the oil to resist the force causing it to flow? Viscosity.
The
higher the viscosity, the higher the resistance to flow. Thus, using a
higher
weight oil in the engine will drive the needle on the gauge up to higher
numbers, but this is NOT an indication that more oil is being pumped
through
the engine. It is only an indication that the heavier oil is more
actively
resisting the oil pump's effort to push it through the engine. In fact,
the
oil flow rate for the higher weight oil may actually be lower than the
flow
rate for a lighter oil would be, and this will most surely be true if the
pump
efficiency with heavier oil does not go up enough to compensate for the
increased resistance to flow.
Ironically, the viscosity effect that causes the oil to resist flow is
the
same viscosity effect which makes oil such an effective hydrodynamic
lubricant
(see Myth #1: The Motor Oil Myth).
DEEPER EXPLANATION
Oil pressure is a measure of the oil's resistance to flow. The oil
pressure in
the engine oiling system is a function of three factors: oil weight, oil
temperature, and pumping effort.
Understanding these variables is best accomplished by holding any two of
them
constant, and examining the impact the third variable has upon oil
pressure.
Scenario #1 -- Effect of Oil Weight on Oil Pressure
Oil temperature is held constant.
Oil pumping effort is held constant.
As oil weight increases, oil pressure increases. As oil weight decreases,
oil
pressure decreases. Oil weight helps determine the viscosity, or innate
tendency of the fluid to resist flow. A higher weight oil will have
higher
viscosity at all temperatures, and a lighter weight oil will have lower
viscosity at the same temperatures. In other words, changing from a 30
weight
oil to a 40 weight oil, with the other two variables held constant, will
result in higher oil pressure. Does this mean a necessary increase in oil
flow
rate? Not necessarily. Since oil pressure is a measure of the oil's
resistance
to flow, it makes sense that the heavier oil shows higher pressure, but
since
it resists more, less flow rate. The flow rate will increase IF and only
if
the pump efficiency increases more than the increased resistance to
flow.
Scenario #2 - Effect of Oil Temperature on Oil Pressure
Oil weight is held constant.
Oil pumping effort is held constant.
Oil viscosity is very sensitive to temperature. As everybody knows, oil
gets
"thick" when it is cold, and "thin" when it gets hot. Physically what
happens
is that the viscosity of the oil decreases as the temperature of the oil
increases. Since viscosity is the oil's innate tendency to resist flow,
then
as temperatures increase, oil pressure seen in the oiling system
decreases,
since the oil is less able to resist flow. Conversely, as the oil
temperature
decreases, oil pressure in the oiling system increases, since the oil
offers
greater resistance to flow.
Herein lies a mind-bending paradox to the "common sense" interpreter of
automotive phenomena. Nearly everybody associates higher oil pressure
with
higher oil flow rate. But if the pumping effort and oil weight are held
constant, it is entirely possible that the oil flow rate actually
increases as
the oil temperature increases, even though the oil pressure gauge shows a
drop
in oil pressure! This is why it is so very important for those who live
in
cold climates to use the proper multi-viscosity oil. When the temperature
is
very cold, an oil which is too heavy may so actively resist flow that it
does
not properly flow to critical areas of the engine, and accelerated wear
will
result. Ironically, the oil pressure gauge will show the highest pressure
ever
at this time, even though oil flow rate may be dangerously low.
Scenario #3 - Effect of Oil Pumping Effort on Oil Pressure
Oil weight is held constant.
Oil temperature is held constant.
If oil weight and temperature are held constant, then there will be no
changes
in the viscosity of the oil. Thus, the only thing left to change is the
force
with which the oil is shoved through the oiling system. In general, oil
pressure increases with an increase in oil pumping effort, and oil
pressure
decreases with a decrease in oil pumping effort. In this case, the oil
pressure gauge shows a direct correlation to pumping effort, and oil
flow
rates will likely increase with an increase in oil pumping effort.
What constitutes and increase in oil pumping effort? Two things: (1) an
increase in the RPM of the oil pump; and (2) an increase in the overall
capacity of the pump. This is why, for a given weight of oil and constant
oil
temperature (as would happen under normal operating conditions), the oil
pressure is low at idle, and substantially higher at some RPM above idle.
At
idle, the pump is turning slowly, and is pushing very little oil; thus,
there
is little resistance to flow, because there isn't much flow. But once
the
engine RPM builds, the oil pump begins to shove more oil through the
system.
This increase in flow effort is met with an increase in resistance to
flow,
and since oil pressure is a measure of resistance to flow, the oil
pressure
increases.
Changing to a bigger oil pump will also increase the pumping effort at
all
RPM, for given conditions. This will also increase the oil pressure, since
the
bigger unit will pump more oil, and this increase in pumping effort will
be
met with an increased resistance to flow.
Now a myth corollary arises:
MYTH #2 COROLLARY 1: It is possible to use a low-pressure, high-volume
oil
pump.
TRUTH: There is no such thing as "low pressure, high volume" for a
closed
pumping circuit, and an increase in oil pumping volume WILL result in an
oil
pressure increase. If the oil weight is the same, the oil temperature is
the
same, and the oiling passages are unchanged, then the only way to shove
more
oil through the engine is to increase the pumping pressure at the pump.
This
particular point is made with some finality, since it has been revealed on
the
Fiero list that there are simply some who believe in the Jackalope-like
"low
pressure, high volume" oil pump, regardless of the most strenuous
engineering
explanations.
CONCLUSION
So, what IS the oil pressure gauge good for, if it is not a measure of
oil
flow rate? It is good for three things:
- To show that oil is being pumped. If the engine runs out of oil, or the
pump
fails, the oil pressure gauge will immediately show zero pressure.
- To indirectly keep tabs on oil condition, namely, temperature and
viscosity.
If the oil viscosity gets too low (due to high temperature or something
else),
the driver will note this because the oil pressure gauge will read an
abnormally low pressure. The key here is to understand that the gauge is
good
for setting a baseline, and then monitoring the gauge for deviations
from
baseline. If the gauge normally reads 50 psi at 3,000 RPM at normal
operating
temperature, but it begins to read 25 psi at 3,000 RPM after some
spirited
driving, this may be a good indication that the oil is much hotter than
normal, and that time to cool may be wise.
- To check for serious mechanical problems downstream of the pump in the
oiling system. For example, a failing main or rod bearing will often allow
so
much oil to flow freely through it that the oil pressure drops to zero
at
idle. The oil pressure may be well above zero while cruising, but an oil
pressure at or near zero at idle is often and indication of a problem
somewhere in the oiling system
Todd
|
Follow Ups:
This board is powered by the Mr. Fong Device from Cyberarmy.com |