Measuring engine RPM from alternator ripple?

[blapcb]

alternator current ripple

Hi guys. I understand it is possible to derive engine speed (RPM) from the AC ripple present on the DC line of the vehicle (the source is the alternator ripple). I have searched high and low but cannot find a circuit to condition this signal (which I understand to be very low) before being able to take it on the uC IO. Any help greatly appreciated. Thanks!

 

[Sidley1965]

measuring engine rpm

I'm assuming you would first have to know the pulley sizes as the rotation of the crankshaft pulley would not translate 1:1 (for giving RPM) if it was a different diameter to the alternator pulley. That is to say that RPM would be a function of;

1) Pulley sizes (ratios)

2) Number of poles in the alternator (and hence the ripple pulse count per revolution)

This is not an easy task, far simpler to take wrap a few turns of wire around the No.1 (or ANY of them come to think of it) spark plug lead and use the transformer effect to provide a signal. Don't forget to divide your answer by TWO for a four stroke as each cycle spark means TWO revolutions (induction, compression, ignition, exhaust) and ONE for a two stroke.