Basestocks vs AW/FM at startup?

[LM]

Many believe that the bulk of engine wear occurs at startup. Here's a link that suggests it's due combustion byproducts/acid, rather than metal-to-metal contact http://www.fernblatt.com/longhurst/additives.html Does this have merit? If not - if it's more metal-to-metal contact causing the wear - would the basestock play more of a protective role during the startup period, as the AW/FM won't be activated until moderate-to-higher engine temps are reached? Altho ZDDP (apparently activates at lower to moderate temp), moly (higher temps), boron (what temp does boron activate at?) may have gone thru an "uptake" process by the metal, will they not become "plastic" until heat activated? So would startup protection be subsequently relegated to the basestock properties?

 

[Drstressor]

He is formally correct. The type of wear he describes is known as corrosive-abrasive wear. The longer an engine sits between starts, the more this type of wear becomes a factor. As the oil drains off surfaces, the oxidation inhibitors in the thin film of remaining lubricant become themselves oxidized and can no longer protect the metal. The TBN of the oil in the sump is not completely relevant here since the oxidation takes place in microenvironments. However, an oil with high TBN should inhibit, but not completely prevent, corrosive abrasive wear. The initial level of ZDDP and other boundary lubrication agents should also not affect corrosive abrasive wear. As long as the levels of the active agents are not depleted, the same boundary layer should form during each cycle of operation. To say that 85 or 90% of wear occurs at startup is nonsense unless put into context. It depends on how the engine is used. If an OTR truck is started only once a day, most of the wear will occur under hydrodynamic lubrication conditions. Actually, a transient mixed boundary/hydrodynamic condition occurs every time the engine speed or load changes significantly. This is where both the quality of the base stock (to maintain viscosity under high temperature and shear conditions) and the AW/FM additives come into play. To say that most of wear in an engine in typical automotive service takes place at startup is probably right.

 

[MolaKule]

quote:

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Altho ZDDP (apparently activates at lower to moderate temp), moly (higher temps), boron (what temp does boron activate at?) may have gone thru an "uptake" process by the metal, will they not become "plastic" until heat activated?

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Boron activates just minutes after being put into solution. It does not depend on heat but electrostatic action. The heat being discussed is not the heat of the bulk oil, rather but the contact pressures causing the heating. It does not matter what the ouside or bulk oil temperature may be, Moly, Antimony, and ZDDP become plastic when the pressure (load/area) becomes high enough to cause localized heating, which in turn results in the "plastic" flow and reduced friction. At higher bulk oil temperatures, the above additives also provide Friction Modification. This is the mechanism for these types of AW/FM additives. So if any othese additives are present in the oil, they will provide protection at startup, when the hydrodynamic film is the thinnest.

 

[LM]

Thanks for the great concise answers

 

[rugerman1]

LM , Arn't these boys impressive? They could talk the paint off a barn! Just think,all this information for FREE!!! Mark