Found all these formulas you' re referencing in my Shigley's book. I played around with the Sommerfled number, and bearing characteristic number for a few different oil viscosities (input as temperature dependent vectors) for my engine. Pretty interesting stuff. Looks like a 0w40 or 5w40 appears to be the best all around from startup temp to operational temps. 0w30 or 10w30 falls below the minimum bearing characteristic number (1.7*10^-6) for thick film stable lubrication around 5500 rpm at high oil temps (over 100 C). 10w40 or 15w40 has a sommerfeld number way above the recommended high end of the range (~15) at startup temps if I tried to run the engine hard immediately (obviously not a good idea for a number of reasons). If I let it warm up, the higher winter number viscosity oils are fine.David Redszus wrote: ↑Tue Nov 20, 2018 11:38 pm Determining the correct oil viscosity for a given engine and operating condition is no simple matter.
Trial and error is often employed and often results in trial and error, error, error, error, etc.
Oil viscosity is a function of the oil itself and operating temperature; the units of actual viscosity are CentiStokes (cSt), which can be predicted at any temperature using cSt values at 40 and 100C.
A very useful equation used to predict required oil viscosity is given by the Sommerfeld Number. There are a number of
variables to be considered:
Journal diameter (in)
Bearing clearance (in)
Bearing length (in)
Bearing load (lbs)
Bearing speed (rpm)
Oil Temp (F)
Oil cSt @ 40C
Oil cSt @ 100C
Oil Specific Gravity
Oil pressure (psi)
Using the above inputs, a spreadsheet can be built that will give:
Absolute oil viscosity (cSt)
Dynamic viscosity (cP)
Bearing area (in2)
Bearing unit pressure (lbs/in2)
Bearing flow area (in2)
Now we can determine the Sommerfeld number inflection point; an oil viscosity that will yield the minimum friction under any set of operating conditions.
Oil that is too thin or too thick will both produce excessive bearing wear due to increased friction.
Oil that is too thin has the additional benefit of producing engine destruction.
Oil that approaches a too thin condition must rely on the oil anti-wear additive package to survive.
OE mfgs work closely with oil companies to design and specify the correct oil for intended operating conditions.
Then we over-rule them and race the engine beyond its intended limits.
Of course this is all making some assumptions to make the calcs simple. I can see how the rabbit hole can get really deep when one starts to consider all the relevant input variables. Cool sh*t.
