Date:         Mon, 6 Oct 2003 13:52:54 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Bentley - Page 24.24b -- Question -- vacuum oriface(?)
Comments: To: Damon Campbell <damoncampbellvw@YAHOO.COM>
In-Reply-To:  <20031006052317.96875.qmail@web13106.mail.yahoo.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 01:23 AM 10/6/2003, Damon Campbell wrote:
>Is the point of the oriface to reduce the amount of
>vacuum whatever is at the end of that hose sees?
>
>In a dynamic system, an oriface would be a pressure
>loss (or in this case, rise), but especially with
>highway driving, we should be dealing with more steady
>state manifold vacuum conditions, right?

Drop <g> -- the world gets a lot simpler -- to think about if not
necessarily to measure -- if we count up from zero pressure instead of up
from atmospheric pressure using one (linear) unit and down using a
different (proportional) one.  You can see how it happened historically,
but imagine what your friends would think if you suggested developing such
a convention...they're coming to take me away, ha ha, they're coming to
take me away, hee hee, to the funny farm where life is beautiful all the
time and I'll be happy to see those nice young men in their clean white
coats and they're coming to take me away...

>  In which
>case, the oriface would just slow the response time a
>little bit... almost like acting as more of a
>damper(?).

Right -- it appears to inhibit flapping of the charcoal-canister vacuum
valve under changing throttle/load conditions, and/or cause the valve to
operate more gently than otherwise.  I suppose it might also limit leak
rate if the diaphragm ruptured (instead of the tee breaking which ISTM
usually happens) -- if there's any conceivable way to pull a working fuel
mixture through that canister the orifice would stop any possibility of the
engine running away in that case.


>Can anyone explain to me why orifii and different
>sized lines are necessary for the vacuum sub-system?

Think of them as resistors -- same uses as in an electrical/electronic
system.  Time-constant changing, impedance buffering, catastrophic (or
otherwise) rate-limiting, energy extraction and so on.

d

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"