Date: Fri, 24 Feb 2006 14:08:39 -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: <000001c6396b$af73e5d0$2f01a8c0@patco>
Content-type: text/plain; charset=us-ascii
Actually, when the throttle valve switch is closed, and the engine is being forced to overrun, Digijet turns the injectors completly off at ~1,300 rpm and Digifant turns them off at~1,600 rpm. Then, when the engine is no longer over run, the injectors turn back on and the O2 sensor signal is ignored for 25 crank revolutions. Van the engine down a hill long enough in winter and the engine can actually go cold. This is why the basic settings at idle are so important and why the Digijet engines are so prone to the idle surge. Add weak mechanical advance springs in the distributor and idle set becomes a nightmare.
Dennis
----- Original Message -----
From: dooley <pdooley@GTE.NET>
Date: Friday, February 24, 2006 12:56 pm
Subject: Re: Compression braking & Power Loss
> 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
> beinginjected.
> 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
> eachother 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====================================================
>
| 
