Re: Audio Tachometer for 2 stroke engine

Ross Herbert <rherber1SPAMEX@xxxxxxxxxxxxxx> wrote:

>On 24 Apr 2005 09:59:04 -0700, "BobG" <bobgardner@xxxxxxx> wrote:
>
>>Oh you old guys are just full of crap. A 50 cent electret mic in a
>>plastic bag and an opamp preamp into an edge detect on a pic would work
>>just fine. Just measure the period of the zero crossings of the pops
>>and log the max values into eeprom every second or so. You near
>>Orlando? I'll help you with this project no prob.
>
>Age has nothing to do with the practical realisation of measuring
>physical phenomena using electronics. The experience gained over many
>years of realising what is practical and what isn't, is lost on the
>young would be's if they could be's as you obviously are.
>
>I'll bet you that you would not be able to prove absolutely that the
>proposed audio technique would accurately measure the real rpm of the
>motor while the boat is actally racing, using an electret mic as the
>transducer. How would you filter out ALL of the myriad noises of water
>slapping the hull or other vibrations (ie. "NOISE") etc, AND retain
>ONLY the engine combustion pops,

It likely doesn't need to filter out much at all. The rapid opening of a
two stroke exhaust port to release the results of a recent explosion
produces a huge pressure impulse. I would not be surprised to find it 10's
of db larger than any other source although I am assuming these high
performance models do not have silencers.

My biggest worries would be the risk of hearing reflections of itself in a
resonant exhaust system, and the dynamic range of the microphone, even
mechanical damage to the microphone.

My guess is some quite crude analog filtering and perhaps ALC would produce
a reliable digital once per rev pulse for counting.

The OP should record a sample from a running engine and see what it looks
like. He might also consider using a directional microphone from the shore.
.

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