ILSAC thin oil & fuel economy -- half truths

[Gokhan]

Originally Posted By: Merkava_4
What is hydrodynamic lubrication ? If I set a crankshaft down into its main bearing journals and bolt the main caps on and give the crankshaft a spin by hand, is that hydrodynamic lubrication?

It's the same thing as hydroplaning of a car on water. There are three lubrication variables: viscosity , speed (v), and load pressure (P). When the load pressure (weight of the car divided by the total contact area of the tires or the load pressure on the crankshaft/connecting rod/piston) is small enough, and/or the speed (speed of the car or RPM of the engine) is high enough, and/or the viscosity (of the oil or water) is high enough, your tires lose contact with the road or there is no metal-to-metal contact in the bearing/piston/whatever is lubricated.

 

[Merkava_4]

Originally Posted By: Gokhan
It's the same thing as hydroplaning of a car on water.


That's gotta be what the crankshaft is doing because the bearings are just plain bearings and the crankshaft will spin for like 10 revolutions from a twist of the wrist. I never will forget the first time I tried that. It was an Oldsmobile Rocket 350 on the engine stand and I was Plasti-Gaging the main bearings with the main bearing caps torqued to spec. I was absolutely AMAZED by how easy it turned from just a few squirts of 30 weight oil between the block mains and the crankshaft mains. It felt like it had ball bearings supporting the crankshaft on each end, but they were just plain bearings. That crankshaft was hydroplaning on a film of oil!

 

[Gokhan]

Originally Posted By: Merkava_4
That crankshaft was hydroplaning on a film of oil!

Since the load pressure is very small in this case (whatever pressure being applied just by the weight of the shaft, as no power being generated by the engine), it will hydroplane (be hydrodynamically lubricated) even at very low RPM. It would be interesting to see at what RPM it starts to stop spinning (ceases to be hydrodynamically lubricated and starts entering boundary lubrication [starts metal-to-metal contact]).

 

[Shannow]

The big turbines have jacking oil on the plain bearings to lift the shaft off the bearing metal for run-up.

At 400rpm (normal speed is 3,000) the can generate their own hydrodynamic wedge.

On run down, the jacking oil comes on at about 800, but the shaft could safely run to a stop as long as it was from speed... Could never start it without jacking, going from lhs to rhs on the Stribeck

 

[Shannow]

Merk try here

http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3686791/all/Bearings...how_they_work

 

[Garak]

Originally Posted By: Rand
almost no 0w20 oils are near the thin end of the viscosity allowed.
Many other oils are near the midpoint in the allowed range.

Excess additives can have many bad side effects.. the most common excessive deposits.

Many times euro oils are also formulated for extremely long oil change intervals as well.. You cant just focus on one thing and call your opinion a factual conclusion.

Yes. Gokhan, you've made some pretty giant leaps of faith here, and glossed over plenty of stuff. There's no talk of what friction modifiers or treat rates, either. None of this does a darned thing to overturn the fact that reduced viscosity increases fuel economy, if only fairly marginally.

You keep showing Stribeck curves yet haven't demonstrated that you know the difference between friction modification and anti-wear yet.

 

[KrisZ]

Too me all this means is the erosion of the safety margins by the automakers. Traditionally we've always had hydrodynamic lubrication as the main goal, mixed lubrication and boundary lubrication as the safety net for the worst case, short lived events.
Now the manufacturers seem to be moving into the mixed hydrodynamic and boundary lubrication as the main mode, leaving only full boundary lubrication as the safety net.

IMO, the future engines will be far less tolerant to lapses in maintenance.

 

[wemay]

Originally Posted By: Merkava_4
What is hydrodynamic lubrication ? If I set a crankshaft down into its main bearing journals and bolt the main caps on and give the crankshaft a spin by hand, is that hydrodynamic lubrication ?



"Lightly Loaded" represents hydrodynamic.

 

[hatt]

Originally Posted By: fdcg27

The reality is that modern engine management systems will deliver pretty good fuel economy on any grade of oil and any grade of oil will also deliver 200K+ life with most engines.
The engine still outlasts the rest of the car even if a 0W-20 grade oil is used.

This "engine outlasting the body with xW-20" is constantly used around here but never qualified. In the great white North it may be true. Around here there are plenty of old vehicles running around. Engines need all the help they can get.

 

[Solarent]

Originally Posted By: Shannow
The only "zero wear" area is in full blown hydrodynamic lubrication...in times past, it was referred to as the zero wear area.

When that's gone, you are relying on additives and tribofilms, and while in that regime, you don't have "zero wear", there's always something happening. You can't develop an additive that provides an improvement over parts never coming into contact. (*)

It might be slow enough that you will never find the end point in 200,000 miles of vehicle ownership, and thus be a moot point. Like the mileage improvement that you can't prove as an end user, but in reality IS there

(*) just on that, if boundary is the new operating point, you need some BIG filtration improvements. there's no room for any particle between (say) shafts and bearings.


IN TIMES PAST.... I would agree with you as hydrodynamic being the only "zero wear" area. However as we discussed in the other ULV thread - I think that those days are now firmly behind us.

Insert my comments about a semi-plastic euctectic film which replaces the need for a viscosity driven oil film and you have a new "zero wear" regime in what has previously been the boundary/mixed/elastohydrodynamic regimes. The problem with looking at a pure stribeck curve (taking into account viscosity, speed and load) is that you are ignoring other key factory to the overall equation - including temperature (as a function of viscosity) and the effect of friction modifiers - which was mentioned at the end of Gokhan's post. Unfortunately we can't take a look at these in a vacuum and finding laboratory, repeatable testing that can quantify the results of all these factors is very difficult.

For oil enthusiasts, and those that waste away their lives trying to understand and develop products that meet these challenges - today is one of the most exciting times to be a part of the industry and seeing the cool technologies that are being developed - both in hardware and in lubrication engineering.

The average person doesn't care even in the slightest. Just ask my wife.

 

[Shannow]

Solarent,
point taken, and I wasn't intentionally overlooking it.

I see those as "additive based" solutions rather than hydrodynamic solutions.

And clearly it's where the industry is heading, they had to change test engines for GF-6 because they needs engines more representative of current operation with more operation in the bottom of the Stribeck curve...the one with friction modifers that doesn't show the huge leap at the left hand side.

 

[Merkava_4]

Originally Posted By: Shannow
Merk try here

http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3686791/all/Bearings...how_they_work


I think figured out what hydrodynamic lubrication is - and I can picture in my mind how it works, but I don't know how to explain it.

 

[Shannow]

Merk,
I'll try another tack...have you seen the kids in the surf on those skim boards ?

Their speed, the angle of the board, and the viscosity and density of the water keep them up, and "hydrodynamically lubricated", a wedge of water between the board and the sand.

There's an engineering parallel, a tilting pad thrust bearing.

Now make the two surfaces long, and wrap them around so that one surface a shaft, and another the bearing, and the "wedge" is produced by the shaft sitting off centre, and creating a wedge of oil, and a pressure distribution that resists the load.

As an aside, those tilting pad bearings are amazing, operating at low speeds still in full hydrodynamics, as the angle of attack of the pad changes.