Oil Pressure Does Not Equal Lubrication

AEHaas · Sep 23, 2006

In a typical gas engine journal bearing oil is under pressure on one side and the other generally has zero pressure. In fact no pressure is needed at all on any end. If oil was just dripped onto the side of the bearing it would be taken up into the space by capillary action. A bearing partly under an oil bath or oil splash will take up oil by itself. Many electric motors have a thick felt oil reservoir that merely holds oil next to a sleeve or ball bearing. There is lubrication but no pressure. If somehow you pressurized the bearing there would be no additional force of separation between the journal and bearing surface, none. The innate oil properties are solely responsible for the lubrication effects. The pressure is there to deliver oil and to create flow. This allows for the other function of oil in SI engines, that is to cool the bearing. Only flow can provide this function. Also, only the flow of oil increases the force of separation in a bearing. Pressure has no effect at all. Oil pressure is insignificant to the fluid pressure, the force of separation in a bearing. The fluid pressure may be several thousand PSI. An oil inlet pressure of 50 PSI increased to 90 PSI will have NO effect. Doubling the flow however will have numerous effects. The force of separation is directly related to the flow. Increased flow will cool the bearing and cooler parts wear less. If you increase the flow, even with a decrease in pressure, you will increase the force of separation in the bearing. If you cool the bearing then a thinner viscosity oil, thinned less than by a hotter bearing, will have the same oil film thickness. The biggest misconception in motor oil is that pressure equals lubrication. This is the opposite of the truth. aehaas

Rickey · Sep 23, 2006

OTOH I can think of several instances where no oil pressure equals no lubrication soon followed by smoke,knocking and other unpleasantness. Increased pressure does equal increased volume all things equal. Which will result in increased cooling resulting in increased film thickness at the bearing. If the situation were marginal to begin with increased pressure may allow an otherwise poorly designed or overstressed engine to live. This has led to the concept of increased oil pressure equals better lubrication in the past. Cheers Rickey.

oilyriser · Sep 23, 2006

Assuming the pressure at the outlet of the oil filter remains the same, how would different viscosities affect the available oil pressure at the furthest bearing? A high viscosity would leak out more slowly, so the pressure loss from leakage would be less. But, the pressure loss from viscous drag along the way would be greater. A low viscosity would bring more pressure to the furthest reaches of the system, but more would leak out along the way. Do both cases balance out perfectly? If so, then a low viscosity would be better, since it would give a higher flow, and better cooling.

Rickey · Sep 24, 2006

IMHO everything concerning lubrication in an ICE is a tradeoff. A whole lot of the particulars are influenced by a given engines design and state of repair over which we have little control compared to oil selection. It appears to me that there should be some middle ground concerning viscosity both in the pan and in the bearing. On one extreme with too little viscosity there is great flow while maybe starving the last oiled elements in the system and/or allowing metal to metal. With too much viscosity there is restricted flow and insufficent cooling. I hypothesize that the localized heating in the bearing with too thick oil would tend to thin the oil thus correcting partially for the insufficent flow with somewhat of a self regulating effect. This would not however be a substitute for having the correct viscosity in the first place. Rickey.

AEHaas · Sep 24, 2006

Take a V8 engine and run a 30 wt. oil for its entire life span. Let us say that over time the oil ways narrow from sludge and varnish or whatever. The oil pressure will slowly go up, yet the flow will slowly go down. Failures occur for just this reason. To make matters worse one may think that you should try a thicker oil, say a 40 or 50 wt. That would only make matters worse. This is why 20 wt oils are used in many new cars and they do not recommend thicker oils with age. They say to use the 20 wt. oil for the life of the engine. Dirty oil ways are like arteries in a human. A chunk of debris may become dislodged and go down stream. It may lodge at a narrowing to a bearing. Then no flow to the bearing may occur. The left over oil in there will not fall out during rotation. Bearing starvation occurs because the same oil sits there, heats up then boils off. NOW the bearing has no oil and over time fails. Many bearings are splash lubricated. The oil lands on the journal / bearing interface and is pulled in. The oil does not just fall out as one may initially think, it gets sucked in if needed. If you go from a 40 wt. to a 20 wt. oil in an engine the internal resistance and pressure will go down and the flow will go up. See the numbers in may later oil chapters in the Interesting Articles section (soon to be restored). aehaas

427Z06 · Sep 24, 2006

Right on, Doc. There is such a thing called a hydrostatic journal bearing as opposed to the hydrodynamic journal bearing you describe above, but I've only seen them used in places like large steel mill equipment or very large diesel engines.

Jay · Sep 24, 2006

Quote:
...Also, only the flow of oil increases the force of separation in a bearing. Pressure has no effect at all... ... If you increase the flow, even with a decrease in pressure, you will increase the force of separation in the bearing... aehaas

Flow increases the force of separation?

In plain bearings, parts are held apart not by the oil pressure of the oil pump, but by hydrodynamic forces that depend on the viscosity of the oil, and the speed that parts move over each other. The faster the parts move over each other, the more pressure can be applied to them before the film is punctured. The purpose of circulating oil is to keep a fresh supply of cool oil in the bearing.

bar1 · Sep 24, 2006

Great write but most old engines do not see oil pressure rising, it's dropping because of wear.. If you are running with synthetic oil and with conservative OCIs clogged oil ways is not going to be a problem. It's like the arteries in a human living on a "non fat diet"

G-MAN · Sep 24, 2006

Quote:
Increased pressure does equal increased volume all things equal.

How do you figure that?

Master ACiD · Sep 24, 2006

ive never seen an engine gain oil pressure at it ages. has aoyone?

Rickey · Sep 24, 2006

Quote:

Quote:
Increased pressure does equal increased volume all things equal.
How do you figure that?

look at the inverse: reducing the oil pressure say by half (softer oil pump pressure bypass spring) all other things being equal will reduce the volume of oil going to the bearings. Rickey.

G-MAN · Sep 24, 2006

Quote:

Quote:

Quote:
Increased pressure does equal increased volume all things equal.
How do you figure that?
look at the inverse: reducing the oil pressure say by half (softer oil pump pressure bypass spring) all other things being equal will reduce the volume of oil going to the bearings. Rickey.

Again I ask, how do you figure that? Pressure does not equal volume.

Rickey · Sep 24, 2006

If you have zero pressure then there can be no flow or volume.There has to be a pressure differential between source and destination to produce volume. Without pressure in a hydraulic system there's not going to be much flow going on. Volume and flow are directly (though not linearly) related to pressure. Rickey.

1sttruck · Sep 24, 2006

"Oil pressure is insignificant to the fluid pressure, the force of separation in a bearing..... Doubling the flow however will have numerous effects.... The biggest misconception in motor oil is that pressure equals lubrication. This is the opposite of the truth." In almost all (?) cases oil pressure is a 'one sided' indirect measure of oil flow, and is a fairly reliable primary indicator of of the lubrication system; keep driving down the road when the oil pressure goes from whatever is normal to zero and see what happens. Pressure will rise when attempting to pump more than can be flowed, and is high when the engine is cold or when using a heavier oil. On the other side low pressure indicates an inadequate flow due to excessive temps and/or too thin of an oil for the conditions. It's the same but different than what you said :^)

G-MAN · Sep 24, 2006

Quote:
If you have zero pressure then there can be no flow or volume.There has to be a pressure differential between source and destination to produce volume. Without pressure in a hydraulic system there's not going to be much flow going on. Volume and flow are directly (though not linearly) related to pressure. Rickey.

Volume and flow are directly related to pressure, but increased pressure does not de facto mean more flow (or volume).

427Z06 · Sep 24, 2006

Quote:
I hypothesize that the localized heating in the bearing with too thick oil would tend to thin the oil thus correcting partially for the insufficent flow with somewhat of a self regulating effect.

However, the "too thick" oil still has to get to the bearing. Worst case scenario is when at maximum RPM and the oil pump is in maximum relief. Big diesel operators can probanbly tell some cold weather stories.

Thermactor · Sep 24, 2006

Quote:
ive never seen an engine gain oil pressure at it ages. has aoyone?

No sir

Bryanccfshr · Sep 24, 2006

Quote:

Quote:
If you have zero pressure then there can be no flow or volume.There has to be a pressure differential between source and destination to produce volume. Without pressure in a hydraulic system there's not going to be much flow going on. Volume and flow are directly (though not linearly) related to pressure. Rickey.
Volume and flow are directly related to pressure, but increased pressure does not de facto mean more flow (or volume).

Exactly. The zero pressure scenario does not apply to this discussion(of course zero pressure would equal zero flow). The good Doc has provided us with good material describing how having pressure may not indicate having lubrication or adequate flow for the lubrication system. In many ways high oil pressures may give a false sense of security.

AEHaas · Sep 24, 2006

Bingo

Rickey · Sep 24, 2006

Quote:

Quote:

Quote:
If you have zero pressure then there can be no flow or volume.There has to be a pressure differential between source and destination to produce volume. Without pressure in a hydraulic system there's not going to be much flow going on. Volume and flow are directly (though not linearly) related to pressure. Rickey.
Volume and flow are directly related to pressure, but increased pressure does not de facto mean more flow (or volume).
Exactly. The zero pressure scenario does not apply to this discussion(of course zero pressure would equal zero flow). The good Doc has provided us with good material describing how having pressure may not indicate having lubrication or adequate flow for the lubrication system. In many ways high oil pressures may give a false sense of security.

So is it being asserted in this thread that with all other things being equal: increased oil pressure does not equal increased flow to the oiled engine components and does not equal increased lubrication??? Rickey.

Oil Pressure Does Not Equal Lubrication

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