Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (February 2006, week 4)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Fri, 24 Feb 2006 12:56:48 -0500
Reply-To:     dooley <pdooley@GTE.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         dooley <pdooley@GTE.NET>
Subject:      Re: Compression braking & Power Loss
Comments: To: "Henry.Luhrs@Bruker-BioSpin.com" <Henry.Luhrs@BRUKER-BIOSPIN.COM>
In-Reply-To:  <43FEF28F.2626.E20FE3@Henry.Luhrs.Bruker-BioSpin.com>
Content-type: text/plain; charset=us-ascii

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 ====================================================


Back to: Top of message | Previous page | Main VANAGON page

Please note - During the past 17 years of operation, several gigabytes of Vanagon mail messages have been archived. Searching the entire collection will take up to five minutes to complete. Please be patient!


Return to the archives @ gerry.vanagon.com


The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.

Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.