DIY_EFI question

[Ed Lansinger]

Bruce Bowling wrote:

>The pulsing effect is caused when the valve slams shut on the head, which
>produces a pressure pulse which bounces between the throttle plates and
>the valve, so in some sense valve timing IS taken into account.

Just to clarify this portion of his message, it is my understanding
that the movement of the valve does not cause the pulse per se.  The
rarefaction wave that progresses away from the valve up the intake runner
begins shortly after the valve opens. When it reaches the end of the runner
(let's say we're at WOT and the runner opens to the atmosphere), the wave
gets reflected back toward the valve as a compression wave.  If the timing
is right, that wave arrives at the valve during the valve overlap period
(four-stroke assumed here) to assist in scavenging.

It's really a question of impedance.  When the valve does close, it may
push a little air backwards on its own (although the air would prefer to
slide around the valve, it's not like it's trapped).  However, the air
column in the runner has momentum and rebounds against the closed
valve, which I believe is the more significant effect.  What happened is
that the impedance of the valve suddenly went from low to high, and you
had inductance in the runner before it, so you get a ringing condition.

You should find the same effect in an engine with, say, rotary valves that
do not move colinearly with air flow.

I'd love to see Bruce's WWW page, but regrettably I don't have access from
here.

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Ed Lansinger
General Motors Powertrain
Powertrain Control Center
Premium V Software & Calibration Group
Milford Proving Ground, Milford, MI
elansi01 at mpg.gmpt.gmeds.com  8-341-3049  (810) 684-3049
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