[Diy_efi] Venturi effect crankcase breather

[clay0052]

I'm not sure I'm completely getting your statement or question. I would
have to agree with you, that reducing crankcase pressure would probably
help deaerate the oil to some degree. But I would have to say again, that
most of it is going to be viscosity dependant.

Also, with a dry sump, the reservoir is going to be doing most of the
deaerating, or at least should be. It's function is to store the oil, but
also, and most importantly, to deaerate the oil. It doesn't matter if low
crankcase pressure is going to help you deaerate because by the very nature
of a dry sump system, under transient g's and such, it is going to entrain
more air into the oil, as the oil is drawn through the pickup. It depends
on the system, scavenge ratio, line velocities, etc. 

With a wet sump, having very low crankcase pressure might help with
deaeration, but again I'm not absolutely sure. Viscosity and surface
tension will for sure, be important on deaerating the oil. Also with a wet
sump, reducing the absolute pressure in the crankcase (ie- more vacuum),
will reduce the volumetric efficiency of the pump. Enough vacuum and you
will have real problems. 

Mark


On 9 Jul 2003, Dave Dahlgren wrote:
> Has anyone ever seen something get vacuum packed?? The air is at a higher
> pressure than the crankcase so the bubbles come to the surface and out
the
> pipe
> through the scavenge stage of a dry sump pump perhaps?? If the oil is
> forcefully
> folded over and compressed slightly wouldn't the air in the oil be at a
> higher
> pressure than the crankcase?? If that were true and it seems logical then
the
> lower the pressure in the crankcase the faster the air comes out of the
oil..
> Dave
> 
> clay0052 wrote:
> <Major snip>
>  3)Entrained air. This is what most people are referring to
> > when they're are talking about aeration. This will effect the ability
of
> > the oil to support a load (along with cavitation, too). Typically
entrained
> > air is from oil being whipped around or thrown into itself. Take a
quart of
> > oil and pour it into the beaker and you will see some air being
entrained,
> > even as you pour it.
> > 
> > Dropping the viscosity and/or surface tension is the largest factor in
> > getting the air out of the oil. Lowering crankcase pressure will help
some,
> > but is not a big deal. There were some SAE papers on WW2 dry sump tanks
> > that I read (1945 I believe) and they were trying to keep the oil as
hot as
> > absolutely possible going into the tank to allow the bubbles to rise to
the
> > top and leave the oil, as quickly as possible.
> > 
> > So, I think if you tried raising the crankcase pressure you would end
up
> > dissolving more entrained air. Later on in the oil flow pathway, a drop
in
> > pressure would reduce the saturation point, and release that air back
into
> > the oil as entrained air.
> > 
> > Mark
> > 
> > On 9 Jul 2003, Ne14RoxCJ at aol.com wrote:
> > > In a message dated 7/8/03 11:56:41 PM Central Daylight Time,
> > > espresso_doppio at yahoo.com writes:
> > >
> > >
> > > > lowering the pressure will only egxacerbate this
> > > > problem.
> > >
> > > So how come the engineers have found otherwise?
> > > Doesn't make sense to me.  You're saying the exact
> > > opposite of the research I've seen.
> > >
> > >
> > >
> > > My visualization of this involves my coil-over Sway-Away shocks with
> > their
> > > remote reservoirs. To reduce oil areation, we apply 120-150 psi of
> > nitrogen
> > > to
> > > the piston of the reservoir. This places the oil under that same
> > pressure. If
> > >
> > > oil areation were solved by a vacuum, then all of those desert
racers,
> > > monster
> > > trucks, rock-crawlers, indy-cars, etc. have got it severely wrong.
I'm
> > not
> > > convinced that I should pull a vacuum on my shock reserviors (as if
it
> > were
> > > even
> > > possible). My classes in fluid dynamics taught me this, but it helps
to
> > have
> > > actual applications to SEE the relationship. Feel free to cut into
ANY
> > shock
> > > absorber to prove this, but use extreme care since there WILL be a
very
> > > high-pressure stream of oil squirting you in the eye. Hope this is
> > helpful.
> > >
> > > Beau
> > >
> > > _______________________________________________
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> > >
> > 
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> 
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