Tachometer

In a four stroke engine, does the tachometer (or the ECU) measure and
report crankshaft rpm or engine rpm? Obviously, the crankshaft rotates
twice for every time the engine goes through a full cycle. So a
reading of 1750 rpm could mean the crankshaft is rotating 1750 times
per minute and the engine is going through 875 cycles per minute, or
that the engine is going through 1750 cycles per minute and the
crankshaft is rotating 3500 times per minute.

Since, I believe, the trigger comes from the ignition circuit (in a
spark engine) I'm inclined to think the reading is engine cycles per
minute. But various places I've looked on the web imply (but don't
specifically say) that it's crankshaft revolutions per minute.

Hopefully someone here can clear up my confusion.

Thanks,
Rob

wrote:
>
>Since, I believe, the trigger comes from the ignition circuit (in a
>spark engine) I'm inclined to think the reading is engine cycles per
>minute. But various places I've looked on the web imply (but don't
>specifically say) that it's crankshaft revolutions per minute.

Could be either, or something else again. Many EFI systems waste a
spark... ie the ignition triggers each revolution for each cylinder.

Either way the tacho merely counts pulses and corrects for the number
of pulses per revolution -- which could be anything from once per
cycle to the number of cylinders multiplied by two (in a system that
uses a single coil and wastes a spark).

--
John H

On Jun 4, 12:33 am, wrote:
> In a four stroke engine, does the tachometer (or the ECU) measure and
> report crankshaft rpm or engine rpm? Obviously, the crankshaft rotates
> twice for every time the engine goes through a full cycle. So a
> reading of 1750 rpm could mean the crankshaft is rotating 1750 times
> per minute and the engine is going through 875 cycles per minute, or
> that the engine is going through 1750 cycles per minute and the
> crankshaft is rotating 3500 times per minute.
>
> Since, I believe, the trigger comes from the ignition circuit (in a
> spark engine) I'm inclined to think the reading is engine cycles per
> minute. But various places I've looked on the web imply (but don't
> specifically say) that it's crankshaft revolutions per minute.
>
> Hopefully someone here can clear up my confusion.
>
> Thanks,
> Rob

The convention is crankshaft revolutions. Even if the tach drive is
taken from the camshaft, the device is then calibrated to crankshaft
revolutions.

wrote:
> In a four stroke engine, does the tachometer (or the ECU) measure and
> report crankshaft rpm or engine rpm? Obviously, the crankshaft rotates
> twice for every time the engine goes through a full cycle. So a
> reading of 1750 rpm could mean the crankshaft is rotating 1750 times
> per minute and the engine is going through 875 cycles per minute, or
> that the engine is going through 1750 cycles per minute and the
> crankshaft is rotating 3500 times per minute.
>
> Since, I believe, the trigger comes from the ignition circuit (in a
> spark engine) I'm inclined to think the reading is engine cycles per
> minute. But various places I've looked on the web imply (but don't
> specifically say) that it's crankshaft revolutions per minute.
>
> Hopefully someone here can clear up my confusion.
>
> Thanks,
> Rob
>

RPM means revolutions per minute.

There is only 'one' engine RPM ever reported and that is the crankshaft one.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
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