Pressure vs flow

petrv · Mar 20, 2005

If I understand it correctly, the oil pressure ensures lubrication and avoids metal to metal contact, while oil flow cools the metal parts.

If I closed the "drain" of oil and keeped the oil pressurized, so there was no oil flow, will the oil lubricate and avoid the metal to metal contact? Just imagine this case, I know that it would overheat.

On the other hand, if there was too large drain, so the oil pump wasn't able to create pressure, so all oil pushed by the pump will easily flow through the bearings and out of the drain hole. All bearings etc. will be full of oil, but this oil without pressure will not lube, right?

williar · Mar 20, 2005

Right; you need both flow & pressure.

MolaKule · Mar 21, 2005

quote:

If I closed the "drain" of oil and keeped the oil pressurized, so there was no oil flow, will the oil lubricate and avoid the metal to metal contact? Just imagine this case, I know that it would overheat.

Primarily correct, but the oil will heat up and shear and the hydrodynamic film will break at a certain temperature.

For the main bearings, they would suck in the oil into the bearing/journal interface via the "wedge" effect anyway. In most new engines, there are squirters than force oil to the underside of the piston for cooling in high power density engines.

The upper engine parts would suffer with no oil flow/pressure since they need oil flow for cooling, lubrication, cleaning, and for hydraulic lifters, oil is needed to pump up the lifters and to reduce tappet clearances.

petrv · Mar 21, 2005

quote:

Originally posted by MolaKule:
Primarily correct, but the oil will heat up and shear and the hydrodynamic film will break at a certain temperature.

For the main bearings, they would suck in the oil into the bearing/journal interface via the "wedge" effect anyway. In most new engines, there are squirters than force oil to the underside of the piston for cooling in high power density engines.

The upper engine parts would suffer with no oil flow/pressure since they need oil flow for cooling, lubrication, cleaning, and for hydraulic lifters, oil is needed to pump up the lifters and to reduce tappet clearances.

Clear. If there was no pressure in the "closed oil", it wouldn't lube a bit?

With closed and pressurized, an oil with very high HTHS could lube in such conditions?

MolaKule · Mar 21, 2005

If the heat could not escape, the oil would continue to lubricate until the heat builds up to the point of severely degrading even the HTHS viscosity to the point of hydrodynamic film failure. Then you switch over to boundary lubrication until the Anti-Wear additives would take over up until THEIR heat capacity would then be exceeded. After that, heat would melt the bearings and then scoring of the journals would ensue.

Shannow · Mar 21, 2005

quote:

Originally posted by petrv:
Clear. If there was no pressure in the "closed oil", it wouldn't lube a bit?

It certainly would lunricate, as the difference in clearance between the unloaded and loaded sections is what creates the oil wedge which provides the separating force between the two.

As it gets hotter, the oil gets thinner, and the parts get closer together.

Keeping a fresh supply of clean cool oil prevents this.

(As an aside, in a recent overhaul, we replaced a badly wiped 16" white metal bearing which had 0.008" excess vertical clearance with a new bearing sized at the bottom of the allowable clearance, i.e we took about 0.012" out of the clearance. This reduction in clearance reduced the axial flow of oil on the unloaded portion. We saw nearly a 15 degree C increase in bearing oil temperature due to the lower oil flow)

Jay · Mar 21, 2005

petrv, it isn't oil pressure that separates metal parts in an engine, it's hydrodynamic pressure that separates the parts.

The oil pump can only supply 100psi of pressure, or so, but the pressure on bearings can exceed 10,000psi at high rpm.

As metal parts move over one another, they float apart on the oil's viscosity like a water skier floats on water. The faster the parts move over the oil film, the more pressure can be put on them before they puncture the film. Hydrodynamic lubrication relies on the oil's viscosity, and the speed of the moving parts to keep them separated.

mike142 · Mar 21, 2005

Jay you are correct, hydrodynamic film strenght is
what supports the metal parts. Pumps create flow
NOT pressure. This is created by the resistance to flow.

[ March 21, 2005, 09:10 PM: Message edited by: mike142 ]

TallPaul · Mar 21, 2005

quote:

Originally posted by Jay:
Hydrodynamic lubrication relies on the oil's viscosity, and the speed of the moving parts to keep them separated.

This supports the supposition that low rpm engines require thicker oil and high rpm engines can run thinner oil.

moribundman · Mar 21, 2005

quote:

This supports the supposition that low rpm engines require thicker oil and high rpm engines can run thinner oil.

I'd prefer to say whether a thin oil can protect sufficiently or whether a thicker oil is required depends more on load than RPM. At top-speed at high RPM, the load on the motor will be great, mostly due to having to overcome air resistance of the vehicle. The load on the motor will also be high if someone lugs the engine.

1sttruck · Mar 21, 2005

Don't most motorcycles, even the water cooled ones, run heavier oils ? 1 liter bikes these days can turn 10 grand and more. As I recall the plain bearing engines needed lower oil flow at higher pressures, while the roller bearing cranks needed more flow at lower pressure. The roller bearing cranks seemed to handle higher loads more reliably with fewer tweaks, as a robust plain bearing crank needed to be built that way initially.

petrv · Mar 21, 2005

Thanks for all replies

If I understand it, if the bearing would be inside some bottle of oil, it would work without any problems? Of course, the oil would be cooled. It means that the bearing is "full of oil", because oil is everywhere near the bearing, so the created hydrodynamic pressure would separate the parts.

Shannow · Mar 21, 2005

quote:

Originally posted by Jay:
petrv, it isn't oil pressure that separates metal parts in an engine, it's hydrodynamic pressure that separates the parts.

The oil pump can only supply 100psi of pressure, or so, but the pressure on bearings can exceed 10,000psi at high rpm.

Jay,
back to the turbine, the oil pressure at the bottom of the wedge is 1.2MPa (175psi), while the supply pressure is around 30 psi.

The lift oil that we use to separate the bearing for initial run-up is 2500psi.

BlazerLT · Mar 21, 2005

Pressure is the resistance to flow.

Example, thicker oil will resist flow more and will cause higher pressure.

petrv · Mar 21, 2005

quote:

Originally posted by BlazerLT:
Pressure is the resistance to flow.

Example, thicker oil will resist flow more and will cause higher pressure.

This is clear an similar to electricity, where flow = current and pressure = voltage. The oil pump is a current (flow) source and all narrow necks in the oil patha are resistors. The question is if the metal parts need voltage (oil pressure), current (oil flow) or both to stay separated?

mechtech · Mar 21, 2005

Roller bearings don't have that hydrodynamic wedge of oil like plain bearings do, and like thicker oils.

LarryL · Mar 21, 2005

What about each roller? There's a wedge there, too.

1sttruck · Mar 22, 2005

I'd use the 100% synthetic in the winter and a blend in the summer. I use a blend all year, 1 gal Mobil 1 T&S with 2 gal Delvac 1300, but would use 100% synthetic in a colder climate. I did start using a blend after trying some Exxon XD-3 Elite (?), a synthetic blend from what the barrel said, but started making my own after I couldn't find it anymore.

Besides lower oil pressure another concern with a thinner oil would be more wear for a given amount and size of particles in the oil. It should be slight with good air filters and/or bypass filters and/or shorter drains, but still a difference.

Ken2 · Mar 22, 2005

As long as the journal bearing has a supply of oil it will create the hydrodynamic wedge when it rotates. The oil supply can be a static reservoir that partly submerges a slow moving bearing or a pumped supply. The oil pressure does lift oscillating bearings that do not form the wedge.

Discussion whether a pump provides pressure or flow is similar to discussing how many angels can dance on the head of a pin. You need proper pressure and proper flow for the job.

Bigger bearings generate a bigger wedge, so may use a lower viscosity oil. Larger clearances allow the oil to leak away faster, so need a higher viscosity oil. A high speed, hot motorcycle engine may need a 20W-50 oil. A very large diesel engine producing 7000 hp per cylinder uses 30 wt. crankcase oil, while a smaller locomotive engine uses 40 wt. It depends on many factors.

Ken

Cyprs · Mar 22, 2005

Esso tech just told me that I can expect lower oil pressure with 0-40 syn in summer but not to worry, more oil runs throught bearings with less pressure.

He also told me I can blend 33/66 syn to dino in XD3.

I think this thread in here has much more detail and resource than the info I got today on the phone, good read here.

Cyprs

Pressure vs flow

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