Crank Trigger Signal (was monitoring RPM)

[Matthew B. Watts]

Hey all,

We talked a little about how to monitor RPM.  What I'd like to
know is why do many EFI systems prefer multi-tooth crank
trigger wheels over a single tooth wheel.  What is the advantage
of receiving 60 pulses per revolution over a single pulse?

Does this simplify the hardware or software?

Can the engine change its RPM significantly within a single
revolution?

The work that I have done has been for a single tooth wheel
which explains why my solution to monitoring RPM is much
different than other folk's methods.

Anyone care to shed some light on the subject?


Back to using optical methods for a minute--a trick that
I used for crank pickup to overcome the harsh environment
is to mount a laser diode as the source and cut the gain way
down on the photo transistor.  Going this route, one has
a very strong signal that in most cases, cuts through
dirt and oil effectively.  Plus, the beam can be seen in
bright daylight for static checks and adjustment.  It's a
similar method that is used for barcode readers found in
the grocery store.  Certainly an eye catcher for the curious
minded, especially when you put a little dry ice under the
crank and watch the beam illuminate the fog in pulses.


Matt Watts
University of Utah
Formula SAE Racing
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