Wim E. van Bemmel wrote:

>  Hello,
>
> I intend to build such a device too,
> for my small yacht, that has a Sabb 8 hp Diesel engine.. no ignition by
> electric means, and mechanical fuel injection.

snip

> The engine drives an AC generator through a V-belt. The start motor is a Bosch
> Dynastart, attached by V-belt also.
> No more V-belts can be driven.
> Getting the RPM info is in my opinion the hardest part, interfacing it to the
> PIC is a challenge, the rest is FUN though a challenge to me, as a novice in
> PIC world!

On some Mercedes trucks the first sign of alternator brush failure is
erratic rpm readings

I've never had cause to delve deeper into this but this must be that
the RPM is measured via the Alternator

I never really looked hard at the system wiring but I believe they
only use two wires ie. 12v and ground

The ripple is probably large enough to measure easily
though frequency to rpm will not be a simple
divide by two/three/four etc, as for petrol engines

Alternators have twelve? poles three phases
12? x3= 36 pulses per alternator rev
reduction  2:1 ? =18 pulses per engine rev

700 to 4000 rpm = 210Hz to 1200Hz
easy to filter and time/count
any extra pulses from switches
will have little effect on the reading

--
If God so loved the World (and created it)
then why is it so imperfect
It's got more bugs than windoze 95

peter@cousens.her.forthnet.gr