Accel enrichment

[Ludis Langens]

Raymond C Drouillard <cosmic.ray at juno.com> wrote:
> Well... after considerable thought, it's quite simple!  <:)
>
> When you suddenly open the throttle, a lot of air suddenly rushes into
> the manifold.  You need to inject enough fuel for that air.  How much
> will depend on the vacuum before and after, and the volume of the
> manifold.

Yes, of course.  But - GM appears to inject even more fuel above and
beyond what that air should need.  The normal speed-density calculations
should provide the matching fuel for that air.  However, when the ECM
sees a sudden opening of the TB, or a sudden increase in MAP, it starts
asynchronously injecting additional fuel by pulsing the injector(s) at
160 Hz.

The normal synchronous injection occurs when there is a reference pulse
(from the ignition module.)  If the ECM's sensor inputs change before
the next reference pulse, the ECM can change it's mind as to the
injector pulse width.

The asynchronous injection happens _right_now_.  The injector pulse
width is clipped to a duration of less than 1/160th of a second.  If the
called for duration was longer than this limit, the remainder is saved
for the next 160 Hz cycle.  Further, this async injection is cumulative
- as long as the ECM sees a rising TPS or MAP, it will keep adding to
the pending duration.  And if the TB suddenly snaps closed, any pending
async fuel will still be injected.

[The above applies to SD applications, but MAF should be similar.]

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