Re: Frequency counter as gas engine tach?

In message <vmrmp39fttfn46906ib9r0unc225tdpt78@xxxxxxx>, John O'Flaherty <quiasmox@xxxxxxxxx> writes
The frequency of an engine at highway speed, maybe 2500 rpm, is 41.66
Hz. The frequency of firing of a single cylinder is 1/2 of that
(four-cycle engine), or about 20.83 Hz.
Actually, it might not be. Depends on the ignition system in use, if it's a Distributorless Ignition System it could be using wasted spark where the coils are double ended and fire two cylinders (one firing, one exhaust stroke) at a time, hence it fires twice per revolution. It could also be multi-spark in which case you're screwed. Of course it might not even use an ignition coil if it's a lawn-mower engine or similar!
If you can pick up a signal
for all cylinders, as on the center of the distributor cap on an older
car, you would get 125 Hz for a six-cylinder engine.
On a DIS coil pack you can get a very usable signal from the cables back to the ECU from the coil-pack, I've used 12 turns of roughly 26 SWG ECW on a toroidal ferrite core using a couple of diodes and resistors to clamp the signal as 'Jamie' describes and had excellent results.

The best results I've had were using a hall effect device (get one from a dead DC brushless fan) on one of the injector bodies feeding a simple comparator (LM393). Of course this will also work on the body of an ignition coil if the car doesn't have fuel injection or you just can't get at the injectors (single point injection)
According to some web references, the frequency counter you have has a
lower frequency limit of 1 MHz, so it probably won't work.
Even if your frequency counter could see it, for such low frequencies
a period measurement might be more practical, if you invert and scale
it for rpm.
Period measurement with averaging and smoothing (you have to reach a compromise though if the engine can change speed quickly, think bike engine) is the best way to go, even if the frequency counter can measure that low the gate time will be a killer unless there's a PLL to lock in (then you have problems with jitter because internal combustion engines are rarely stable from one revolution to the next for a myriad of reasons)

If the OP is just trying to measure RPM it doesn't have to be a spark plug signal, there may well be a top dead centre signal from the camshaft or crankshaft or at least a crank position sensor that outputs some kind of pulse train (usually an inductive sensor and a pattern formed into the flywheel with a missing pulse to indicate some period before TDC)
--
Clint Sharp
.

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