Makeshift tachometer

May 25, 2006 5 Replies
L

Hello -



I would like to measure the RPM of my type 4 engine, but I don't have a tachometer.



I do however have a digital multimeter which can measure frequency. In this mode it displays the frequency of an incoming signal in Hertz.



I am assuming that I can measure RPM with this meter as follows. I connect the frequency counter to the LT side of the ignition coil. When the engine is running, the frequency displayed is the frequency at which the coil is being energised. This is exactly four times the frequency at which the engine is completing a full revolution, since there are four spark plugs each firing once per engine revolution. To calculate the RPM I would therefore first divide the frequency by 4 and then multiply by 60 to give the rotations per minute of the engine.



Did I get that right?



Leo



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<

Leo, Logical thinking, but i am afraid not. Your meter is calibrated, and designed, to measure frequency of a sign wave signal. It is calculated to use the "RMS (root-mean-square)" value of the sign wave. Your ignition coil puts out an "RC (resistance-current)" signal that is more of a spike (sawtooth to be more exact) and therefore your reading would not be accurate or linear.

Sorry Joe

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A

Your engine fires all 4 cylinders every 2 revolutions so you're half right about this part anyway.

Tony

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H

comparatord like to measure the RPM of my type 4 engine, but I don't have a

Just build one with this chip.

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Or just use an op amp comparator setup to cleanup up waveform. The op amp will take care of the ossolatons by level triggering.

May have to use filter on input of the primary side of the coil to deal with big spike, (I would have to think about how to deal wit this spike).

Here is the waveform:

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J

This is all fine for conversation and theory development but is a tach so expensive or inadequate in design that a retail purchase is not favorable? This is an example of having too much time on your hands!

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L

Aha! Thank you for this critical and rather fundamental piece of information. I knew I'd learn something by trying this...

Leo

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