[Diy_efi] Venturi effect crankcase breather

[clay0052]

At the risk of being pedantic I will point out that the shocks are
pressurised to avoid cavitation due to high velocity of the oil moving
through the orifices in the piston/valve assembly. If a bubble were to form
in the shock it would be comprised of oil vapor. Raise the pressure high
enough and it would disappear. So increasing the pressure in the shock
allows higher oil flow velocities without cavitation. 

With an engine oiling system, pressure lines can have higher velocities
than suction lines, for the same reason --- cavitation. 5feet/sec is the
maximum velocity usually allowed on a suction line, as a rule of thumb.
With that said, there are three forms of vapor that can be in an engine oil
system (that I know of), 1) dissolved air in the oil (not entrained). This
is always in the oil, even when you get it right out of the bottle. This
does not effect the ability of the oil to support a load at the bearing.
2)Cavitation. This should be obvious. However, cavitation can release
dissolved air that might be over the saturation point, and thus create
entrained air. 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
> 
> _______________________________________________
> Diy_efi mailing list
> Diy_efi at diy-efi.org
> http://www.diy-efi.org/mailman/listinfo/diy_efi
> 



_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi