Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (December 2010, week 1)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Tue, 7 Dec 2010 01:14:38 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: MAF Sensors and the Vanagon
Comments: To: Larry Alofs <lalofs@gmail.com>
In-Reply-To:  <AANLkTimc6EUrSX45ZFwhVbCo-wAx8OottFYLAVx7OnQ_@mail.gmail.c om>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 11:07 PM 12/6/2010, Larry Alofs wrote: >sluggish. It seems to me that the inertia of the vane would delay the >ECU learning that the throttle had opened.

It does, but the delay effect is less (I *think* a lot less) than the overswing, because -- I'm not saying this well, so if need be I'll do it more carefully tomorrow -- the vane sits at the equilibrium of two springs, the one inside the AFM and the springiness of the intake air. The "spring rate" of the air flow increases quickly if the vane is deflected from the equilibrium position, so the restoring forces increase more rapidly with increasing deflection. But the actual mechanical spring is a very long one, coiled up like a watch spring; and the effect is that its rate doesn't change significantly over the range of vane motion; the force changes at very nearly a linear rate with vane displacement. So a rapidly increased flow will shove the vane open with a high initial force that decreases as the vane comes into equilibrium; whereas the spring can only exert a force that changes linearly with vane position.

Another way to say it, maybe better: the force from the spring depends on the vane position only; the force from the air depends on how far away the vane is from the balanced position.

The vane extends in the other direction as well, away from the intake opening. This adds mass and increases the inertial effects, but it's there to operate as a dash pot, a damper that keeps the system from breaking into uncontrolled oscillation.

> This would result in a >temporarily lean condition since the air flow increases when the >throttle opens suddenly but the ECU takes a moment to realize this and >doesn't increase the injector pulse width as quickly as it should.

I've spent many years with a small (30 hp) inline four marine engine with a simple updraft carburetor that has no accelerator pump. When you jam the throttle open on that the engine simply quits running. I've only done it once which was plenty -- if I hadn't managed to jam it shut again before the engine quit turning I would have been spitted on the main boom of a great big schooner, jammed there sideways by a fast current.. My heart quit running at the same time as the engine, and started again as the engine fired at idle and I was able to increase the throttle at a more controlled rate. Fortunately for everybody I knew what had happened and why, and my hand shoved the lever back without consulting my brain.

Now I'm suddenly realizing that I've likely been talking through my...hat. The carburetted engine needs the pump to keep the mixture from leaning out and killing the motor. But does an injected engine (or any engine) actually need a richer mixture while accelerating rapidly? I think it does, but I don't know. I *do* know the vane overswings briefly -- I've watched it do. But as for the engine needing enrichment -- suddenly there's nothing beneath my feet where I thought there was solid ground. Someone who knows more about engines than I do will have to answer that. Dennis probably knows.

Yours, David


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.