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Knock drum


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#1 NeilR

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Posted 26 March 2012 - 09:54

Has anyone seen a knock 'drum' (a copper rod with a foam cup on the end - I kid you not) in operation in an engine test cell?

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#2 cheapracer

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Posted 26 March 2012 - 14:05

No but I've seen a simple beam style torque wrench (flexible type with a long pointer to the numeral scale) with a piece of foam on the scale that gets moved by the pointer when you shut the throttle off and the engine shakes while the revs die and I don't know why.

Some sort of balance or frequency test?



#3 Greg Locock

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Posted 26 March 2012 - 22:03

Yes I have. Every engine test cell has one here. A 7oz coffee cup from a vending machine and a bit of copper pipe is the best knock detector invented. You can use $1000 of microphone and $2700 of measuring amp and another amp and a speaker, which all looks dead posh and in fairness lets you fft the signal, but then you'll cry when a hose bursts and wrecks the mic.



#4 gruntguru

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Posted 26 March 2012 - 22:37

I am fascinated. Can someone post photos or a diagram? I assume the cup is in the control room - how is that done?

#5 Greg Locock

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Posted 26 March 2012 - 23:14

I am fascinated. Can someone post photos or a diagram? I assume the cup is in the control room - how is that done?


Take a long piece of that flexible copper brake tube. poke it through the wall of the test cell. point the open end at the engine, say 6 inches from the cylinder head.

Buy a cup of something that almost but not quite bears no resemblance to tea from a machine. Throw the contents away. Spike the cup onto the control room end of the pipe. listen to the noise.

Calibrating knock (det) sensors is tricky, they are bandpassed, high frequency accelerometers and cost about 23c each so they aren't exactly high quality. As engine speed inceases they respond to the various clatters and bangs as well as knock, so the trick is to choose the right location for the sensor, the right bandpass characteristic, and the right threshhold vs rpm (there is quite a bit more to this). Your ear suffers from none of these deficiencies. However, there is inaudible det, which can kill engines as well.

#6 gruntguru

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Posted 27 March 2012 - 00:33

Thanks Greg. I was expecting the tube to be in contact with the engine and operate like a "string telephone". Interesting.

#7 NeilR

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Posted 27 March 2012 - 11:50

bit like a stethoscope then