Knock Sensor Spectrograms



MTG Technologies is currently capable of conducting the tests and analysis of Knock Sensors that are displayed below. Turn around time for testing is approximately 2 days with a summary email report of findings. Please email usCLICK FOR EMAIL to schedule your testing.

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Background Information: Here are a few SAE reference papers to get you thinking correctly about knock.

SAE 871911 "Individual Cylinder Knock Control by Detecting Cylinder Pressure" by Kunifumi Sawamoto, Yoshiaki Kawamura, Toru Kita and Kenjiro Matsush*ta of Nissan Motor Co. Ltd. This paper will show you the relationship of knock frequency with acoustic velocity in the combustion gasses, bore diameter and resonance mode. It will also characterize the frequency multiples of the modes, and it 'taint x2, x4 ,x8 etc. Paper will also show where in crank angle knock typically occurs.

SAE 890156 "Characterization of Knock in a Spark Ignited Engine" by Kwang Min Chun and John B. Heywood of MIT Fame. This paper has a couple of nice graphs and drawings that will show you why knock collapses the ring lands.

SAE 982477 "Knock in a S.I. Engines: A Comparison between Different Techniques for Detection and Control" by F. Millio and C. V Ferraro of the Politecnico di Torino, Departmento di Energetica... like in Italy. This work will show you that the resonances are Bessel wave functions ... also that predominant resonances are are engine speed dependent. Also has some really nice graphs that show you how valve chatter and piston slap overlay the knock signatures. That's sure to get you depressed if you are trying to measure and control knock!

Knock Tips:

The factories go to great efforts to find a good responsive place on the block to place the sensor, so if we are retrofitting a knock system we want to put the sensor where the factory put it on this or other similar models. If my memory is correct the MSD Knock Alert comes with a GM sensor with a 1/8 in NPT on the end. GM uses it to plug the lower block drain hole on their V8 engines. If you are fitting another type of sensor to this location, my first choice would be to retap the hole, but a close second would be if you can machine an adapter, a robust adapter out of say 1" dia steel with a male thread on the bottom and a female 1/8 NPT on the top. Make sure to surface the part of the adapter that will tighten against the block. I am assuming your stock sensor is the type with a bolt thru the middle that tightens against a machined surface on the block. You want this adapter to "look" like an extension of the block. Rigid. You don't want it to have a resonance of its own in the frequency of interest. 4KHz thru 14 KHz. It wouldn't hurt to turn the outside diameter to the diameter of the stock sensor that kisses the block.

Notice that the resonant sensors ... well resonate. They ring like a bell, so they do not have the best ability to time differentiate the signal. For example the GM style sensors are ringing for around 2 mS undamped. Now they sure were not designed for this application, but a V8 at 7000 RPM will have ignition events around 2.14 mS apart, the sensor ring is starting to carry over into the next ignition event, a bit. The flat response sensor does not resonate, therefore its a good transducer of what the block is really doing. It gives you a little edge on discerning knock at the higher RPM's as it does not smear the signals.

John and Shirley Pizutto make a great knock control device called Safeguard. You can see it and contact John at J&S Electronics.

I found a good discussion of practical knock sensing and management on the Holly EFI forums here. They even referenced this page in their dicussions for finding sensor resonant frequencies. I felt it would be gentlemanly to reference them also!

Links to Knock Sensor Spectrograms:

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