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Lube-Tech


engine friction at high rpm. Both viscosity grades were formulated using the same additive package and had identical chemical limits. However, the effect gets progressively smaller when going to lower rpm. It is interesting that for the older Ford engine featuring conventional cast iron cylinder bores and a DAMB valvetrain, the lowest viscosity oil gives the highest friction in the low rpm end. Once again, this shows the hydrodynamic lubricant film collapse may be a real problem. For the newer Mercedes Benz engine featuring spray-coated bores and RFF valvetrain, the friction torque is nearly linearly dependent on engine speed. This shows the new design effectively avoids boundary friction.


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


No.133 page 7


Figure 7 shows how engine friction responds to the use of a FM in the lubricant formulation. One can see that the engine with a DAMB valvetrain and conventional cast iron cylinder bores gains more benefit from the use of FMs than the engine with an RFF valvetrain and thermally sprayed bores. This shows that the use of friction modifiers is only beneficial when there is a substantial contribution of boundary friction to the total energy loss.


It is important to understand that different FMs may compete with each other for vacant surface sites, and they may also compete with detergents – another important class of additives invariably present in crankcase lubricants. Therefore two different formulations with identical viscometrics may still have different fuel economy properties, although variations rarely exceed 1 percent.


Some insights regarding hybrid powertrains Hybrid powertrains bring new challenges for oil formulators: since the ICE is not permanently firing during the vehicle’s use, it may fail to reach working temperate. Oil viscosity changes significantly with temperature, resulting in cold engines having higher friction losses. Furthermore, low oil temperature creates conditions for water condensation on power cylinder walls resulting in water accumulation in the crankcase. Cold engines also experience increased fuel dilution in the sump. While dispersants help to solubilise water and drive it away from the crankcase, their effect is limited, and in extreme cases, oil may turn into a “mayonnaise” like substance failing to efficiently lubricate the engine. The only practical solution currently available is to program powertrain control electronics to engage the ICE at intervals to heat up the oil and evaporate excess water and fuel.


Figure 7: The effect of molybdenum friction modifier on engine friction: l.h.s. - Ford Duratec, r.h.s. – M.B. M133


Hybrids tend to use low SAE 0W-20 (Volvo, Mercedes) and ultralow SAE 0W-8 (Honda) viscosity lubricants. Ultralow viscosity lubricants depend heavily on friction


LUBE MAGAZINE NO.162 APRIL 2021 29


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