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
In-Reply-To: <AANLkTimc6EUrSX45ZFwhVbCo-wAx8OottFYLAVx7OnQ_@mail.gmail.c om>
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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