Date: Sat, 5 Jul 2014 18:34:35 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
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From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: AFM Video
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At 05:06 PM 7/5/2014, Neil N wrote:
>idle can cause undue wear and tear on the AFM resistive strip? My
>understanding is that if the idle is good, the AFM wiper should be
>steady. If it is a possible issue, my guess is it would take many
The ones I've seen (not that many really, probably fewer than ten)
have all oscillated over a short arc whilst at idle. If that's what
you mean I have trouble seeing how it could be avoided without
altering mass or damping or both of the vane, as unless I'm wrong the
vane is quick enough to couple into the intake tract pulsations at idle speed.
In case it may be any help thinking about it, here are four scope
shots I took four years ago showing AFM output as I open and then
close the throttle as rapidly as I can.
https://picasaweb.google.com/117189706757545167023/VanagonAFMSuddenThrottleResponse?noredirect=1#5548079967836817826
The second shot seems to clearly show that when snapping the throttle
open the AFM vane will make a rapid large excursion that can reach
near wide open in around 25 milliseconds and then fall back almost as
rapidly until the rising volume of air demanded by the engine as it
speeds up is sufficient to support the vane, which then follows along
as rpm and airflow rise. And the third shows that whether or not
that's the maximum opening rate, the vane can close faster,
overshooting once beyond the point where the idling airflow can
support it. Seems to me that by definition that's underdamped in
either direction. And it seems clear that the vane can achieve a
substantial out-and-back deflection in 50 milliseconds or so if
encouraged, which happens to be about half of the 113 ms per intake
valve opening of a 4-cyl engine idling at 850 rpm
I'm not doing my best thinking right now 'cause I'm pretty tired but
if seems to me that a bouncy vane that can do a min-max-min that
would correspond to the rate of airflow changes as each cylinder
pulls in a charge without breaking a sweat makes a fairly strong case
the the vane should be expected to oscillate, or maybe more
accurately to follow along with the airflow deltas at less than its
resonant speed. If that's true I might expect the arc this covers on
the track to increase a bit as the engine is accelerated some little
bit, and then smoothly decrease to zero as rpm increases beyond that point.
Yrs,
d
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