Crank Trigger Signal (was monitoring RPM)

[Michael Farrugia]

>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?
>
I think that for normal engines ( not formula 1 which have the least 
immaginable inertia and can change speed instantly) checking the speed 
once every turn is enough.  But the problem is how.  If you measure the 
length of time taken for one turn then you will end up with a 
measurement which is reciprocal to the RPM.  Therefore your measurement 
will be very accurate in low revs and very descrete in the higher revs.
The way I did it I put 32 teeth on the camshaft and check how many teeth 
are counted in a spesified time.  But maybe you have a better solution.

In my opinion also the teeth are not just for speed but also for 
position.


>
>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.
>
For me also the optical solution is the best, but I cannot afford 
lasers.

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