Engine oil

[riff_raff]

+1

Raptor22-

Thanks for the +1.

Just to make things fun, the ultimate engine lubricant would be one that provides a variable viscosity that increases proportionally with dynamic pressure, but at the same time maintains a low shear resistance. And to make it even better, the lubricant should have extremely high properties for specific heat and flash point.

[Raptor22]

hmmmmm lets think about this...

Viscosity index is the slope of the viscosity vs temperature graph so if that curve is flat then we have the best VI possible.
Indeed if the lubricant could be almost solid under high pressure, low shear conditions i.e. in the boundary then that would provide the EP function, so yes that would be most ideal.
In fact we can achieve some of this in dry sump applications by pressurising the oil. In the past it was thought addition of some solid lubricants would have this benefit but it proved to be problematic. SO naphthenic base oils exhibit some of the properties you mention but they have other undesirable properties like foaming gums when they decompose.

[riff_raff]

.....Indeed if the lubricant could be almost solid under high pressure, low shear conditions i.e. in the boundary then that would provide the EP function, so yes that would be most ideal.
In fact we can achieve some of this in dry sump applications by pressurizing the oil.

The lowest fluid film shear rate conditions (ie. frictions) occur within the narrow elastohydrodynamic regime, and not the boundary conditions. Boundary conditions would imply that there is some surface contact, which would result in much higher frictions and heat generation. I would agree that EP additives function by reducing frictions resulting from microwelding/adhesion effects produced during boundary contact conditions.

As for the impact of higher oil system feed pressures, it is of negligible effect on the dynamic fluid film pressures present in the journal bearing contact zone. These dynamic fluid film pressures can easily exceed 40 or 50 ksi.

[Raptor22]

I think we have missed each other. I was implying that if we could lower the shear in the boundary condition there would be more stability in the oil as a shear diagram would illustrate. This would imply lowering the friction between oil and component but that would mean less cohesion and you lose other desirable properties EP and Anti Wear brings. The altering of the shear gradient is why multigrades are so desirable. Then add EP and Anti wear to handle the boundary and mixed film regimes.

Also the High oil feed pressure is not intended for the journal bearing hydrodynamic lubrication zone but rather zones where boundary layer and mixed film lubrication occurs. The higher pressures also do allow hydrodynamic to take place in F1 engine journals since without the high pressure, G-force would alter the oil feed, often resulting in engine failures. (the last statement is to keep the topic relevant to F1)

[riff_raff]

Raptor22-

The oil flow rates required by journal bearings are mostly a function of the mass flow rate required for cooling, and not for maintaining dynamic oil films. In reality, the lube oil flow path from the main journals to the rod journals in an F1 engine is not that bad, due to the small journal diameters and short crank throws. The primary limit to rod journal oil flow is the clearance at the rod thrust face and crank cheek. With a race engine, even if it was beneficial to increase the lube oil circuit pressure to something like 200 psi, or even 500 psi, the parasitic pressure pump losses would only amount to an additional 3 or 4 hp.

Engine journal bearings should be designed such that they never operate in boundary conditions, except for during start-up. Indeed, most modern high-performance engines use crank journals that are ground with extremely precise 2D profiles to ensure consistent contact conditions under bending and torsional deflections, and precisely controlled variable bearing shell wall thicknesses to ensure optimum operating clearances.

[Ferraripilot]

From an engine builders perspective (I do quite a bit of Ferrari engine work, racing and stock), I often get rather particular regarding oils especially in a motor I have built where I know the rod and main bearing clearances. Ferrari rod and main journals are rather small and run between .0015-.002 bearing clearance yet these engines obviously rev on the high side, does it matter one way or another what weight an oil is as long as the boundary layer is strong enough to protect at high rpm and high heat? I'm guessing film strength and oil weight are hand in hand?

For instance, I typically like to see an oil with an HTHS of no less than 4, but kinetic viscocity at 100C needs to be lower than 19. These are often 40w and 50w oils and I seem to have to often compromise one thing over another and I'm not sure if I'm wasting my time doing this. Motul 300v is without a doubt the best stuff I've ever used for what it's worth though.

Kind thanks for posting regarding a topic you are obviously massively knowledgable.

[olefud]

From an engine builders perspective (I do quite a bit of Ferrari engine work, racing and stock), I often get rather particular regarding oils especially in a motor I have built where I know the rod and main bearing clearances. Ferrari rod and main journals are rather small and run between .0015-.002 bearing clearance yet these engines obviously rev on the high side, does it matter one way or another what weight an oil is as long as the boundary layer is strong enough to protect at high rpm and high heat? I'm guessing film strength and oil weight are hand in hand?

For instance, I typically like to see an oil with an HTHS of no less than 4, but kinetic viscocity at 100C needs to be lower than 19. These are often 40w and 50w oils and I seem to have to often compromise one thing over another and I'm not sure if I'm wasting my time doing this. Motul 300v is without a doubt the best stuff I've ever used for what it's worth though.

Kind thanks for posting regarding a topic you are obviously massively knowledgable.

Let me add to this question. My sense is that for rod and main bearings you’ll be in the hydrodynamic domain that is less sensitive to film strength and should be happy with lower viscosities with the advantage of lower drag and faster charging from start up. With roller cam followers and such the need for viscose boundary lube has been greatly diminished, though there is still startup.

Does this make sense?

[Alan97]

I was implying that if we could lower the shear in the boundary condition there would be more stability in the oil as a shear diagram would illustrate.

As part of my continued learning would you be so kind to explain this statement further please?

Many Thanks

Alan

[Raptor22]

in a nutshell, if the boundary layer were repelled by the metal surface instead of being cohesively attracted you could remove the shear in the boundary layer.
This implies the best lubricant is were surfaces are repelled by low shear fluids or no fluid at all (maglev)

[olefud]

in a nutshell, if the boundary layer were repelled by the metal surface instead of being cohesively attracted you could remove the shear in the boundary layer.
This implies the best lubricant is were surfaces are repelled by low shear fluids or no fluid at all (maglev)

Like an air bearing or the water film on ice. But if the solid/lube interface were truly phobic, the lube wouldn’t form a film

[riff_raff]

From an engine builders perspective (I do quite a bit of Ferrari engine work, racing and stock), I often get rather particular regarding oils especially in a motor I have built where I know the rod and main bearing clearances. Ferrari rod and main journals are rather small and run between .0015-.002 bearing clearance yet these engines obviously rev on the high side, does it matter one way or another what weight an oil is as long as the boundary layer is strong enough to protect at high rpm and high heat? I'm guessing film strength and oil weight are hand in hand?

For instance, I typically like to see an oil with an HTHS of no less than 4, but kinetic viscocity at 100C needs to be lower than 19. These are often 40w and 50w oils and I seem to have to often compromise one thing over another and I'm not sure if I'm wasting my time doing this. Motul 300v is without a doubt the best stuff I've ever used for what it's worth though.

Kind thanks for posting regarding a topic you are obviously massively knowledgable.

These are good questions.

First of all, we need to use the correct terminology. With journal bearings, it's the hydrodynamic fluid film that supports radial bearing loads. The term "boundary" implies certain conditions exclusive of hydrodynamic operation.

Secondly, while oil viscosity characteristics do have an effect on the fluid film contact conditions, it is only part of the equation. The variable that actually matters most is the local heat transfer occurring within the fluid film contact between the fluid and the bearing/journal surfaces. There is a local rapid temperature rise within the oil film that takes place at the contact zone, referred to as "flash temperature". The flash temp increase results in reduced viscosity within the oil film.

As for small variations in bearing clearances, they have little real effect on load capacity or efficiency. Bearing clearances are mostly based on ensuring there is enough oil mass flow through the bearing gap to ensure adequate cooling of the bearing and journal surfaces. The thickness of a bearing's hydrodynamic oil film is only a few micro-inches. Thus a variation of .0005 inch in installed bearing clearance, which is many times greater than the total operating oil film thickness, is relatively insignificant.

However, what matters far more than assembled bearing clearances are the structural deflections of the main and rod journals during operation. Once you understand that the hydrodynamic fluid film thickness in a journal bearing is only a few micro-inches, you'll appreciate that even tiny torsional/bending deflections produced in a crankshaft can cause excessive edge loading across a bearing journal face.

[olefud]

From an engine builders perspective (I do quite a bit of Ferrari engine work, racing and stock), I often get rather particular regarding oils especially in a motor I have built where I know the rod and main bearing clearances. Ferrari rod and main journals are rather small and run between .0015-.002 bearing clearance yet these engines obviously rev on the high side, does it matter one way or another what weight an oil is as long as the boundary layer is strong enough to protect at high rpm and high heat? I'm guessing film strength and oil weight are hand in hand?

For instance, I typically like to see an oil with an HTHS of no less than 4, but kinetic viscocity at 100C needs to be lower than 19. These are often 40w and 50w oils and I seem to have to often compromise one thing over another and I'm not sure if I'm wasting my time doing this. Motul 300v is without a doubt the best stuff I've ever used for what it's worth though.

Kind thanks for posting regarding a topic you are obviously massively knowledgable.

These are good questions.

First of all, we need to use the correct terminology. With journal bearings, it's the hydrodynamic fluid film that supports radial bearing loads. The term "boundary" implies certain conditions exclusive of hydrodynamic operation.

Secondly, while oil viscosity characteristics do have an effect on the fluid film contact conditions, it is only part of the equation. The variable that actually matters most is the local heat transfer occurring within the fluid film contact between the fluid and the bearing/journal surfaces. There is a local rapid temperature rise within the oil film that takes place at the contact zone, referred to as "flash temperature". The flash temp increase results in reduced viscosity within the oil film.

As for small variations in bearing clearances, they have little real effect on load capacity or efficiency. Bearing clearances are mostly based on ensuring there is enough oil mass flow through the bearing gap to ensure adequate cooling of the bearing and journal surfaces. The thickness of a bearing's hydrodynamic oil film is only a few micro-inches. Thus a variation of .0005 inch in installed bearing clearance, which is many times greater than the total operating oil film thickness, is relatively insignificant.

However, what matters far more than assembled bearing clearances are the structural deflections of the main and rod journals during operation. Once you understand that the hydrodynamic fluid film thickness in a journal bearing is only a few micro-inches, you'll appreciate that even tiny torsional/bending deflections produced in a crankshaft can cause excessive edge loading across a bearing journal face.

Very well put. However, an added caveat that the clearance/viscosity relationship, while more a cooling factor than influencing the hydrodynamic wedge, should not be varied from the engine designer/builder’s recommendations. It’s easy to get an advantage you recognize; but much more difficult to avoid accompanying adverse results.

[riff_raff]

olefud- It's hard to overstate how important heat transfer is with journal bearing performance. For example, consider the theoretical case where both the journal and bearing surfaces have no thermal conductivity. In this situation, 100% of the energy resulting from the fluid film contact viscous and shear losses would result in heat transfer to the very small mass of lubricant within the contact area. The large, instantaneous "flash temperature" rise would cause a big drop in the lubricant film's viscosity, and would likely result in a transition from hydrodynamic contact conditions to mixed or boundary contact conditions. Once this occurs, friction increases, heating increases, and the failure mode become self-perpetuating.

[olefud]

olefud- It's hard to overstate how important heat transfer is with journal bearing performance. For example, consider the theoretical case where both the journal and bearing surfaces have no thermal conductivity. In this situation, 100% of the energy resulting from the fluid film contact viscous and shear losses would result in heat transfer to the very small mass of lubricant within the contact area. The large, instantaneous "flash temperature" rise would cause a big drop in the lubricant film's viscosity, and would likely result in a transition from hydrodynamic contact conditions to mixed or boundary contact conditions. Once this occurs, friction increases, heating increases, and the failure mode become self-perpetuating.

Yes, dumping in 50 wt. to protect the bearings may not be a good idea unless the hydrodynamic wedge has failed and lube has degraded to boundary protection –which the 50 wt. may well cause.
The oil temp gauge will only tell there’s trouble after it’s happened.

[Raptor22]

its not the wt of the oil that matter buts its viscosity index under the conditions you are refering too.

what is oil wt anyway? we talkin universal saybolt second or centipoise.

50wt means absolutely nothing.

If you are refering to SAE J300 table classifcations then its worth understanding that there are 40W oils that perform similarly to 50W oils because those classifications refer to a range of kinematic viscosities and VI's of oils.