Mercedes Power Unit Hardware & Software

[hardingfv32]

The journal bearings actually do not need much flow or pressure to maintain clearance. Flow is mainly for cooling. The journal inside the bearing does not stay center. On the side where the clearances are reduced the oil is pinched down creating a wedge (pressure) of oil that prevents contact between the two surfaces. This system/design creates its own oil pressure. Note: as long as there is movement/rotation a clearance is maintained… hydrodynamic regime.

The piston and rings are more complex issue. While they are moving they use the hydrodynamic model. The problem is at each end of their travels when they change direction and must by definition come to a stop. They fallout of the hydrodynamic regime and into what is called boundary regime. In the boundary regime you can have actual surface to surface contact and an associated increase in wear. This is where/why you must increase your wear protection.

In an engine a greater percentage of friction is found in the hydrodynamic regime as compared to the boundary regime. The best way to reduce hydrodynamic friction is thinner lubricants. Basically the engine developers are taking this approach and tasking the oil and additive companies to solve the boundary friction issues that this approach creates.

PM your Email and I will provide an article (PDF) on this subject.

Brian

[MrPotatoHead]

The journal bearings actually do not need much flow or pressure to maintain clearance. Flow is mainly for cooling. The journal inside the bearing does not stay center. On the side where the clearances are reduced the oil is pinched down creating a wedge (pressure) of oil that prevents contact between the two surfaces. This system/design creates its own oil pressure. Note: as long as there is movement/rotation a clearance is maintained… hydrodynamic regime.

The piston and rings are more complex issue. While they are moving they use the hydrodynamic model. The problem is at each end of their travels when they change direction and must by definition come to a stop. They fallout of the hydrodynamic regime and into what is called boundary regime. In the boundary regime you can have actual surface to surface contact and an associated increase in wear. This is where/why you must increase your wear protection.

In an engine a greater percentage of friction is found in the hydrodynamic regime as compared to the boundary regime. The best way to reduce hydrodynamic friction is thinner lubricants. Basically the engine developers are taking this approach and tasking the oil and additive companies to solve the boundary friction issues that this approach creates.

PM your Email and I will provide an article (PDF) on this subject.

Brian

Your description of journal bearings not needing much flow or pressure has me scratching my head.

[MrPotatoHead]

The journal bearings actually do not need much flow or pressure to maintain clearance. Flow is mainly for cooling. The journal inside the bearing does not stay center. On the side where the clearances are reduced the oil is pinched down creating a wedge (pressure) of oil that prevents contact between the two surfaces. This system/design creates its own oil pressure. Note: as long as there is movement/rotation a clearance is maintained… hydrodynamic regime.

The loading on one of the main bearings in these engines is between 2000 and 5000 psi. That's some serious load.
That load is supported by the oil pressure in the oil wedge between the bearing and the crank journal.
If you had no pressure the oil thickness would quickly be zero.

The rod bearing journals have a load of about double the main bearings. Once again the pressure in the oil wedge is what stops contact.

[PhillipM]

They don't in the scheme of things, technically they run on very little pressure bar what they self generate - the oil pressure is just to keep the flow there.

[gruntguru]

I agree with Harding. Compared to 2,000 psi oil pressure in the film, 100 psi in the galleries is "not much" pressure.

[MrPotatoHead]

I agree with Harding. Compared to 20,000 psi oil pressure in the film, 100 psi in the galleries is "not much" pressure.

He said they do not need much flow or pressure. Not just pressure.

[J.A.W.]

AFAIR, a significant purpose of the force pumped, positively high pressure/flow ratings required by plain bearings..
..is due to needful heat control, soft-ish bearing journals are certainly not forgiving of excessive heat/forge loads..

When Daimler-Benz substituted their rolling element crankshaft DB 601 with a plain bearing crankshaft for the DB 605..
..they had quite a few unanticipated problems to fix, & make the adapted design work.. to an acceptable standard..

[gruntguru]

I agree with Harding. Compared to 20,000 psi oil pressure in the film, 100 psi in the galleries is "not much" pressure.

He said they do not need much flow or pressure. Not just pressure.

If cooling is provided by other means and leakage is eliminated, the hydrodynamic bearing will operate with no flow (which is considerably less than "not much").

[MrPotatoHead]

AFAIR, a significant purpose of the force pumped, positively high pressure/flow ratings required by plain bearings..
..is due to needful heat control, soft-ish bearing journals are certainly not forgiving of excessive heat/forge loads..

When Daimler-Benz substituted their rolling element crankshaft DB 601 with a plain bearing crankshaft for the DB 605..
..they had quite a few unanticipated problems to fix, & make the adapted design work.. to an acceptable standard..

Of course the oil flow provides cooling through extraction of heat generated. That's a given. If the oil is not allowed to leak out of the bearings then the oil itself would break down from the heat generated.

Why increase pressure above idle engine speeds at all if the pressure does not serve a purpose?

[MrPotatoHead]

I agree with Harding. Compared to 20,000 psi oil pressure in the film, 100 psi in the galleries is "not much" pressure.

He said they do not need much flow or pressure. Not just pressure.

If cooling is provided by other means and leakage is eliminated, the hydrodynamic bearing will operate with no flow (which is considerably less than "not much").

Semantics. Such a race engine bearing does not exist.

[MrPotatoHead]

An interesting read in this topic if you have time.

http://www.substech.com/dokuwiki/doku.p ... al_bearing

[J.A.W.]

AFAIR, a significant purpose of the force pumped, positively high pressure/flow ratings required by plain bearings..
..is due to needful heat control, soft-ish bearing journals are certainly not forgiving of excessive heat/forge loads..

When Daimler-Benz substituted their rolling element crankshaft DB 601 with a plain bearing crankshaft for the DB 605..
..they had quite a few unanticipated problems to fix, & make the adapted design work.. to an acceptable standard..

Of course the oil flow provides cooling through extraction of heat generated. That's a given. If the oil is not allowed to leak out of the bearings then the oil itself would break down from the heat generated.

Why increase pressure above idle engine speeds at all if the pressure does not serve a purpose?

As a heat-exchange/cooling medium?
Viz:
Suzuki, notably, utilized low pressure rolling element crankshafts in its 1st generation of large 4 cylinder 4T motorcycles..
..but when they adopted plain-bearings for the GSX-R series they (oddly, since liquid cooled 2Ts had been their thing)
adopted the use of excess oil delivery as a defacto liquid coolant, with a large radiator - for that purpose..

Of course, it wasn't really a satisfactory substitute for primary heat control, & was duly replaced by a proper system..

[MrPotatoHead]

They don't in the scheme of things, technically they run on very little pressure bar what they self generate - the oil pressure is just to keep the flow there.

Correct. The flow is very important though.
No flow and you will have no oil in the bearing because of the leakage caused by the bearing clearance.
Tighten up the clearance too much to reduce leakage and you get too much heat and the oil will break down.
It's quite the vicious cycle.

[gruntguru]

He said they do not need much flow or pressure. Not just pressure.

If cooling is provided by other means and leakage is eliminated, the hydrodynamic bearing will operate with no flow (which is considerably less than "not much").

Semantics. Such a race engine bearing does not exist.

I think it is you who is nit-picking.
Brian said "journal bearings actually do not need much flow or pressure to maintain clearance".

That statement is absolutely correct.

[MrPotatoHead]

If cooling is provided by other means and leakage is eliminated, the hydrodynamic bearing will operate with no flow (which is considerably less than "not much").

Semantics. Such a race engine bearing does not exist.

I think it is you who is nit-picking.
Brian said "journal bearings actually do not need much flow or pressure to maintain clearance".

That statement is absolutely correct.

I probably am just nitpicking. But. Compared to other bearing types I still maintain that engine main and rod journal bearings need a lot of flow relatively speaking.

That's why there is a dry sump pump on my breakfast bar. The OEM pump on my street car could not flow enough volume at 200HP+ per cylinder with the main bearing clearances required to support the much power.