Quote:
Originally Posted by whunter
No doubt you are correct.
I am not a mathmagician..
0.708 or 0.141176471 are both far beyond thermocouple accuracy.
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I agree with these figures at idle of 850 rpm you are getting 7.08 bangs per second
The time between each bang is the reciprocal of this number = 0.14 seconds.
If you were to (be stupid and) rev your OM617 to 6000 rpm - the camshaft would be rotating at 3000 rpm - which would mean as you get one bang per rotation of the camshaft - you would get 50 bangs per second.
Bangs per second is like saying cycles per second which is often described by the unit Hertz (Hz).
So we are working in a frequency range between 7Hz and 50Hz. For most frequency analysers, data loggers, and computers this is really slow - really low frequency stuff - as you'd expect from a mechanical machine.
Thermocouples are used to measure frequencies up to 1000Hz (1kHz). See this pdf for example:-
http://www.grc.nasa.gov/WWW/sensors/PhySen/docs/RSI%20Fralick.pdf
On the first page you will see this text:-
"For example, the measurement of fluctuating temper-
atures in the high speed exhaust of a gas turbine engine
combustor is required to characterize the local gas density
gradients or convective heat transfer.’ Although thermo-
couples are suitable for the measurement of high frequency
temperature fluctuations ( < 1 kHz) in a flowing gas or
liquid, the measured signal must be compensated since the
frequency of the time dependent fluid temperature is nor-
mally much higher than the natural frequency of the ther-
mocouple probe."
So it is saying here that thermocouples are used to measure high frequency temperature fluctuations but compensations need to be made when the changes in the fluid being measured are faster than the natural frequency of the thermocouple.
So I argue that thermocouples are used to measure high frequency temperature fluctuations and if you were to use an appropriate thermocouple as a transducer you could probably pick out the heat pulses created during ignition.
As for any transducer it needs to be suitable / appropriate - and a glow plug is not designed to act like a thermocouple. This is why I suggested that the first step of proving or disproving this myth would be to ascertain the behaviour of the glow plug when you are asking it to do the job of a thermocouple.