Date: Fri, 24 Feb 2006 11:48:31 -0500
Reply-To: "Henry.Luhrs@Bruker-BioSpin.com" <Henry.Luhrs@BRUKER-BIOSPIN.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "Henry.Luhrs@Bruker-BioSpin.com" <Henry.Luhrs@BRUKER-BIOSPIN.COM>
Subject: Re: Compression braking & Power Loss
In-Reply-To: <43FEED95.6010905@charter.net>
Content-type: text/plain; charset=US-ASCII
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
>
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. . . . Henry Luhrs
. . . . Henry.Luhrs@Bruker-BioSpin.com
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. . . . Phone 978-667-9580 Ext 5329
. . . . Fax 978-667-6406
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