HDDO vs. PCMO low T viscosity

[drwebb]

In US most SAE 40 multigrades for passenger car motor oils are 10W while heavy duty diesel oils are 15W. Is this a consequence of the total base numbers of the passenger car oils being 6-7 while the diesel oils are 9-10, or is it something else?

 

[patprimmer]

Is this a trick question, as you normally show a lot of knowledge of oil formulations and this is a basic question., but to the best of my knowledge which is not expert in this field, and from my understanding of the question, it is something else.

TBN is a measure of the very mild alkalis or buffers being added to neutralise the acids in blow by.

Viscosity is dependant on the molecular weight of the base stock and added viscosity improvers which are chemicals that get thicker rather than thinner when heated and so offset the thinning of the base oil when heated. This allows light base oil to be used without it thinning out to much at higher temperatures.

If one additive does both jobs, it is a fortunate coincidence.



Regards

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[romke]

no, it is not. there is no relationship between viscosity and TBN. The TBN of diesel engine oils is usually higher then gasoline engine oils because they have to cope with more sulphur in the fuel. that sulphur, combined with water can form acids that have to be neutralized.

the different viscosity is chosen because the average bearing load in heavy duty diesels is higher then with gasoline engines, the higher viscosity is thought to give some more protection. whether thas is right is open to debate - the real viscosity in the bearings is much lower since oil temperature there can be in the region of about 180 deg C.

another reason can be that less viscosity index improver is used. that might be benificial in those areas in the engine where there is at best elasto-hydrodynamic lubrication, eg the valve train. there an oil with less tendency of "temporary shear" dut to the "stretching out" of the large VI-improver molecules may be an advantage.

a third reason might be conservatism: diesel engine builders tend to require a somewhat thicker oil then gasoline engine builders.

there is also a difference between the US and Europe: in Europe the best diesel engine oils are nowadays usually of the 10W-X variety and gasoline engine oils 5W-x or even 0W-x. Thas has something to do with closer tolerances in European engines compared with US-designs.