Date: Fri, 24 Feb 2006 11:38:00 -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: <43FEED95.6010905@charter.net>
Content-type: text/plain; charset=us-ascii
On a throttle controlled engine, it is the manifild vacuum that helps to slow a vehicle when the the throttle is closed. At speed, the engine compression stroke will quickly be cancelled out by the de-compression after the piston passes top dead center. When turning an engine by hand, you feel a compression effect but that is because the compressed air has leaked past the rings and now you are pulling a vacuum on the down stroke. Diesel engines do not have throttles. Therefore, thier is no braking effect by lifting your foot off the pedal. Most diesels emply some type of rearder such as an exhaust brake, or leaving exhaust valves closed, or even using hydrualics in the transmission to slow vehicles down hills.
Dennis
----- Original Message -----
From: John Rodgers <inua@CHARTER.NET>
Date: Friday, February 24, 2006 6:27 am
Subject: Re: Compression braking & Power Loss
> 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"
> playsa part on "braking" with a gasoline engine.
>
> Thanks,
>
> Regards,
>
> John Rodgers
> 88 GL Driver
>
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