quattro digest, Vol 1 #582 msg #14

[John Larson]

Sorry, De Witt, your theory about the lower o-ring and the purpose of
the air shrouding is frightenly wrong. The injectors are "air shrouded"
as a means of cooling them. No air is introduced into the fuel and
atomization remains unaffected unless the minor drop in injector
temperature makes a difference. The air is pulled from the intake after
the air flow sensor and before the throttle body. I can't find a
diagram, but the design looks like it gets pulled into the intake
passage just above the valves. Kind of a controlled vacuum leak like the
PCV valve. HTH, John
> 
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> 
> Message: 14
> Date: Mon, 04 Dec 2000 14:29:06 -0700
> To: quattro at audifans.com
> From: DeWitt Harrison <de at aztek-eng.com>
> Subject: Re: Injector line routing
> 
> At 03:23 PM 12/4/00 -0500, Matthew Beaubien wrote:
>  > What about the o-rings?  What is the purpose of the black ones?  Do they
> just pilot the injector further down in the head?
> 
> Oh, yeah. Forgot. They do help locate the injector tip but,
> I believe, they also provide an air seal for the air "shrouding"
> pressure. Higher pressure air from before the throttle body
> is routed into an air passage in the head which feeds each
> injector position. I don't have one in front of me to verify this,
> but there must then be small air openings in the injector body
> between the two o-rings to allow this air to be forced through
> the injector. The purpose of the shrouding air jet is to further
> atomize the spray of fuel as it exits the injector tip.
> 
> This is still metered air so the a/f mixture control has no problem
> with this small amount of air that bypasses the throttle plate.
> 
> Cheers,
> 
> DeWitt
> 
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