Viscosity Wear Friction Modeling

[1sttruck]

One of my favorites is 'all models are wrong, but some are useful'. This model seems to explain why some might see better fuel mileage with a heavier oil.

http://www.et.anl.gov/sections/tribology/highlights/parasitic_engine_losses.html

Parasitic Engine Losses

In the collaborative effort with Ricardo, Inc., preliminary results from the computer simulations showed that hydrodynamic friction decreased with reductions in lubricant viscosity, while wear loads and asperity friction increased. This is mainly due to the reduction of churning losses associated with the shear rheology of high-viscosity oils. However, the decline in hydrodynamic friction is eventually offset by the increase in asperity friction. For each level of asperity friction reduction, there is a lubricant viscosity that provides the minimum overall fuel consumption (see figure). For an SAE 5 grade oil, simulations predict an overall fuel savings of greater than 4%, if a surface treatment is used to reduce boundary friction by 90%. Contact severity and wear loads are substantially increased in such low-viscosity environments, and they would need to be mitigated to avoid increased wear by using superhard coatings.

 

[GMorg]

Thanks 1sttruck,

So, with great boundary lubrication, thinner is better. Does that mean that if we could get "great enough" boundary lubrication, then the only purpose of the oil would become cooling.

 

[Bryanccfshr]

Boundary lubrication is less effecient (more drag) than hydrodynamic lubrication. If the viscosity is lowered to a point that the Hydrodynamic film fails then effeciency drops and wear increases.

 

[1sttruck]

You would also want better surface finish and form/geometry control, better control of particles, better engine design to minimize 'hot spots', localized low oil pressure, etc., as you'll lend up with less margin for those types of problems. Even then the model seems to suggest that it's still a tradeoff with respect to maximum engine life.

 

[bruce381]

Boundry lubrication is a WEAR regime bearing will wear out faster with metal to metal contact, Hydrodynamic is a LOW wear regime.
bruce

 

[Big O Dave]

Excellent observation, Bruce.

In fact, this thread really gets to the heart of what we are all interested in, with regard to lubricants.

Great find, 1sttruck!

 

[oilyriser]

I can imagine break-in as the asperities getting pruned down until their tops are just wide enough to support hydrodynamic lubrication. I'm wondering, if an engine is broken in with 5w30, then run on 5w20, would it adjust itself to 5w20 after a while, but suffer increased friction if you went back to 5w30?