The Stovebolt Site Forums The Shop Area Engine Shop How Stuff Works

I asked a question yesterday and instead of just getting an answer, I was given a resource. Thank you for that! I will only speak for myself and say that I sometimes get lulled into allowing myself to be spoonfed by some or you answer guys. I just wanted to thank you for the answers and for pointing me in the right direction to find the answers for myself.

The howstuffworks.com web page is really cool. If you havent checked it out, do it.

Now to my new question....and yes I looked through Howstuffworks.com first, but I couldnt find what I needed.

....How does a tachometer measure time? ie. where does the time measure come from in Revs/Min?

Tom

I would guess it is calibrated to read a particular rpm. The minute is the contant while the rpm is the variable.

I don't think there is a little Timex in there.

That is my point. If you measure a single rotation of the drive shaft you have 1 rev. Now that Rev could have happened in any time frame. Until you have rev #2, you have no way to measure any time interval with or without a clock.

So lets say that you give a little to the engine after startin it. #1 might fire 3000 times, each of these fires being accounted for by the Tach wire hanging off of #1 cable. Without some time constant, how can you know that the 3000 fires happened in 1 min as apposed to 30 seconds?

I fully understand how to count firings, that is simple. I just want to understand how the second value of time is incorportated into an analog or gear driven type of device?

I have been trying to work out the mechanics/logic of this type of measure for a while now and finally decided I had to just ask. To me this is like trying to imagine infinity. Please help me with my mental stress.

Tom

Glad I don't have a "blackboard" brcause it would be scribbled on something terrible-LOL So Blue Bird, if you will try this link I think this guy does as good a job on explaining it as I can. Ken may come in here if he sees this but just pay him no never mind-LOL. Here is the link
Explanation on how a tach works

I think I get it. I never thought that I would need a EE degree to work on trucks but I may need to hit the books again.

Correct me if I am wrong but in this case the 12V or 6V pulses are in a syn type wave form and the wave is static in length, thus the time factor is a constant and the spark or rev is the dynamic factor?

That is sufficient to shut me up for now anyway.

Thanks for the input, it really does give me one less thing to clog my synapse.

Tom

Mechanical tachometers work exactly the same way as a speedometer does. There is a cable that runs from the camshaft or distributor to the gauge.

The old electric analog tachometers work the same way, but replace the cable with an electrical circuit. The way it works, in mathematical terms, is that first the waveform from the distributor is normalized for amplitude, then fed to a trigger circuit which creates pulses of a pre-defined width to normalize out the dwell factor of the distributor. Then that signal is fed into an integrator (same as a low-pass filter), which measures the power of the signal. Then the integrator feeds the analog meter movement, which deflects the needle linearly with applied power. Since power is proportional to RPM (frequency) with a constant amplitude, it all works. The tachometer is calibrated by either adjusting the R-C time constant of the integrator or the width of the pulses. An integrator is a circuit that performs the calculus function of integration by charging a capacitor (C) through a resistor (R).

Other tachs use the method given by Oly's site, which is really the same thing but it uses the meter movement itself as the integrator rather than a separate circuit.
The separate electrical integrator makes the meter movement design less critical and keeps it from being overly twitchy. It also allows a switch for switching between 4-6-8 cylinder configurations and allows use on engines with uneven firing orders (like Buick oddfire V6's)

The newer tachometers are actually digital and use a frequency counter circuit. The clock is provided by a crystal oscillator just like in a watch. The meter movement is driven by a pulse width modulator which displays the RPM. Some of the nicer tachometers have a microcontroller (computer) in them and can save the history of the RPM and run a shift light.
(Why yes, I am an engineer. )