D
David G. Bell
Guest
On Wednesday, in article
<[email protected]>
[email protected] "Badger" wrote:
> "Huw" <hedydd[nospam]@tiscali.co.uk> wrote in message
> news:[email protected]...
> >
> >
> > As long as the oil pressure does not drop off the scale when idling and it
> > is not consumed at an accelerated rate then the difference in viscosity is
> > of no consequence.
> >
>
> WRONG! Pressure will be maintained at the pump relief valve (and gauge, if
> one is fitted) but this is not a true representation of what is happening
> within the rest of the engine. Every bearing surface is an orifice, the
> thinner the fluid is then the greater the flow rate across the orifice for a
> given pressure. As we are talking dynamic pressures not static, the pressure
> beyond that orifice will be lower than before it, this repeating through the
> engine until there is virtually no pressure (or flow) at the rear main
> bearing! The firther away you go from the pump outlet, the lower the oil
> pressure becomes at each successive bearing, amplified by the use of a
> thinner fluid.
It's a long time since I was taught anything about viscosity, but
there's something which feels odd about this...
[thinks]
I think that the critical aspect is the pipework between the orifices,
and the total possible flow. Pressure is a secondary effect here. The
pump doesn't shift any more volume with a thinner oil, while each
orifice can pass more oil. So there's not enough oil being pumped
through the system to meet all the demands, and the differences in the
pipework, distance and size and such, decide which bearing gets what
oil there is.
Is that right?
If you could pump more oil, it'd be different.
I expect the design of the oil passages in an engine is very important,
so that the more distant bearings still get oil.
--
David G. Bell -- SF Fan, Filker, and Punslinger.
"I am Number Two," said Penfold. "You are Number Six."
<[email protected]>
[email protected] "Badger" wrote:
> "Huw" <hedydd[nospam]@tiscali.co.uk> wrote in message
> news:[email protected]...
> >
> >
> > As long as the oil pressure does not drop off the scale when idling and it
> > is not consumed at an accelerated rate then the difference in viscosity is
> > of no consequence.
> >
>
> WRONG! Pressure will be maintained at the pump relief valve (and gauge, if
> one is fitted) but this is not a true representation of what is happening
> within the rest of the engine. Every bearing surface is an orifice, the
> thinner the fluid is then the greater the flow rate across the orifice for a
> given pressure. As we are talking dynamic pressures not static, the pressure
> beyond that orifice will be lower than before it, this repeating through the
> engine until there is virtually no pressure (or flow) at the rear main
> bearing! The firther away you go from the pump outlet, the lower the oil
> pressure becomes at each successive bearing, amplified by the use of a
> thinner fluid.
It's a long time since I was taught anything about viscosity, but
there's something which feels odd about this...
[thinks]
I think that the critical aspect is the pipework between the orifices,
and the total possible flow. Pressure is a secondary effect here. The
pump doesn't shift any more volume with a thinner oil, while each
orifice can pass more oil. So there's not enough oil being pumped
through the system to meet all the demands, and the differences in the
pipework, distance and size and such, decide which bearing gets what
oil there is.
Is that right?
If you could pump more oil, it'd be different.
I expect the design of the oil passages in an engine is very important,
so that the more distant bearings still get oil.
--
David G. Bell -- SF Fan, Filker, and Punslinger.
"I am Number Two," said Penfold. "You are Number Six."