I see and read lots of people talk about how thinner oil is better for engine start up because it "gets to the bearings faster". I have not really put any thought effort into it, and generally considered it an acceptable argument. After some thought, I am not so certain this is a valid argument.
Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for
Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases
so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.
If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.
Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.
Looking forward to the discussion.
Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for
Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases
so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.
If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.
Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.
Looking forward to the discussion.