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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