Date: Fri, 24 Feb 2006 23:19:50 -0500
Reply-To: dhaynes@OPTONLINE.NET
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: Compression braking & Power Loss
In-Reply-To: <43FF18A3.245.176CEE6@Henry.Luhrs.Bruker-BioSpin.com>
Content-type: text/plain; charset=us-ascii
The fuel cut off is actually an emissions issue. At very high manifold vacuum, fuel will actually be pulled through the injectors. The fuel pressure regulator provides some compensation but ultimate fuel control is impossible as the vacuum is uncontrolled which also causes the fuel to immediately vaporize before getting into the cylinders causing back fires. In the early days of carberatours this was controlled with devices that would actually hold the throttle open. remember the "Dash-pot" or throttle positioner?
Dennis
----- Original Message -----
From: "Henry.Luhrs@Bruker-BioSpin.com" <Henry.Luhrs@BRUKER-BIOSPIN.COM>
Date: Friday, February 24, 2006 2:30 pm
Subject: Re: Compression braking & Power Loss
> What I read in the one of the Bosch Fuel Injection manuals is that
> the fuel is cut off if
> the idle switch is closed and the RPM's are above the idle speed.
> So as soon as you
> step on the clutch, the engine speed drops and the injection
> starts to provide fuel
> again as soon as it gets down to the idle speed. I guess this is
> to save on fuel.
>
> Henry
>
> On 24 Feb 2006 at 12:56, dooley wrote:
>
> > The idle switch can't shut off fuel flow while coasting or the
> motor would
> > die. Sure if the van was in gear and coasting the engine would
> still turn,
> > but try pressing in the clutch- the engine still idles, so fuel
> is being
> > injected.
> > More likely "idle" position on the throttle switch changes fuel
> curve to an
> > "idle" situation and puts lower emphasis on signal inputs from
> MAF and RPM.
> >
> > -----Original Message-----
> > From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On
> Behalf Of
> > Henry.Luhrs@Bruker-BioSpin.com
> > Sent: Friday, February 24, 2006 11:49 AM
> > To: vanagon@GERRY.VANAGON.COM
> > Subject: Re: Compression braking & Power Loss
> >
> > In answer to John's question, when you are coasting down a hill
> with your
> > foot off the
> > gas pedal, there is no fuel being injected (if your idle switch
> works), so
> > the engine is
> > just an air pump. The compression and power strokes sort of
> cancel each
> > other out,
> > but the intake stroke has to try to pull air into the cylinder
> past the
> > closed throttle
> > plate. This makes it a vacuum pump, and it takes some power to
> do that. The
> > exhaust stroke has to push the air that actually got into the
> cylinder out
> > past the cat
> > and muffler, which also takes a small bit of power. This power
> comes from
> > the
> > momentum of the van, so this is what slows the van down when you
> are in
> > gear,
> > along with the air friction and the tire rolling resistance.
> >
> > Henry
> > 87 syncro(stock with slightly leaking head o-ring)
> > 91 jetta GLI16V
> > 88 Scirocco16V
> >
> > ------------
> > On 24 Feb 2006 at 5:27, John Rodgers wrote:
> >
> > > John Bange wrote:
> > >
> > > >>For some reason,
> > > >>COMPRESSION BRAKING seems to throw something out of whack.
> > > >>
> > > >>Is this a clue? Or a red herring? If it's a real clue, where
> would you
> > > >>look?
> > > >>
> > > >>
> > > >>
> > > >
> > > >FWIW, compression braking is generally only found on diesel
> trucks.> Theres a
> > > >valve flap in the exhaust manifold they close and the pistons
> have to
> > > >compress air against that on the exhaust stroke. On gasoline
> engines it's
> > > >actually VACUUM braking, i.e. the pistons are pulling against
> a vacuum in
> > > >the intake tract. The big air valve in the throttle body
> provides the
> > seal
> > > >when pressure is taken off the gas pedal. Dunno what might be
> the problem
> > > >after pulling a big vacuum for a while, but it might be worth
> looking for
> > > >collapsed or rotten vacuum lines.
> > > >
> > > >--
> > > >John Bange
> > > >'90 Vanagon - "Geldsauger"
> > > >
> > > >
> > > >
> > > John, would you please expand on that process a bit. As I
> have always
> > > understood it, in gasoline engines, "compression braking"
> relied on
> > > the compression stroke to provided resistance to slow down the
> vehicle.> > Without fuel and ignition the engine just basically
> becomes a glorified
> > > air compressor, and the compression resistence will act to
> slow down the
> > > vehicle. Please enlighten us with more detail as to how
> "vacuum" plays
> > > a part on "braking" with a gasoline engine.
> > >
> > > Thanks,
> > >
> > > Regards,
> > >
> > > John Rodgers
> > > 88 GL Driver
> > >
> >
> > ====================================================
> > . o . .
> > . . . . Henry Luhrs
> > . . . . Henry.Luhrs@Bruker-BioSpin.com
> > B R U K E R
> > . . . . Phone 978-667-9580 Ext 5329
> > . . . . Fax 978-667-6406
> > . o . . Web http://www.Bruker-
> BioSpin.com> ====================================================
> >
>
| 
