Knock Sensor Output
Knock Sensor Output
(OP)
Does anyone know what kind of signal a knock sensor puts out?
I see MSD makes a cheezy box that you wire up to a knock sensor; it has a couple LED's, a beeper, and a sensitivity adjustment on it. And they can still sell it for $150 because nobody knows how it works.
I am wondering how difficult it would be to modify the knock sensor output to control an electronic advance distributor. Has anyone done it?
I see MSD makes a cheezy box that you wire up to a knock sensor; it has a couple LED's, a beeper, and a sensitivity adjustment on it. And they can still sell it for $150 because nobody knows how it works.
I am wondering how difficult it would be to modify the knock sensor output to control an electronic advance distributor. Has anyone done it?





RE: Knock Sensor Output
GM for a long time in the mid 80's through early 90's made external knock sensor modules that took the raw signal from the knock sensor, filtered and conditioned it, then set the ECU what basically amounted to a digital signal for Knock/No Knock. You can get one from a junkyard for $10. The olny problem with this is that different knock sensors have different natural frequencies just as different engines will have different natural frequencies when they knock. So, either you would need to determine the natural frequency of your engine (I'm not sure how to do this other than strapping an accelerometer on it and tappping it with a hammer, but I'm sure there are more exact ways), or by finding a GM car that is a 4cyl with as small a bore as possible, as I've read that bores affect the frequency knock sensor that should be used
As you can tell, I'm not much of an EE, but if someone on your team is sharp with electronics, they could back-engineer from the GM module what kind of filtration is done. It looks to be almost all hardware filtered, however the ECU steps the ignition differently under different conditions (ie, if knock is sensed at the same time as rapid delta TPS, a stock ecu will be under the impression that you are doing a very hard acceleration and will be a little more dramatic to protect the engine, whereas if you are cruising and one or two knocks are sensed, timing adjustment will be a little more gradual. I think it also counts knocks between fueling calculations, and figures the more counts=more timing adjustment needed. All these aren't "filtering" though)
For the reasons mentioned aboe the MSD setup probably wouldn't work for you anyways, since they are probably using large V8 style knock sensors.
To do it right yourself, determine the natural frequency of your engine (use an accelerometer and either tap the block, or purposely induce knock (be carefull with that one!!)). then find either a true knock sensor or a rugged accelerometer with the same natural frequency as your engine, and find an EE guru to filter it accordingly. OR just try a GM setup from a 4 cyl and see if it works!!
Andy
RE: Knock Sensor Output
http://acc-electronics.com/cloud/images/page3_55.jpg
Note that the 5.8L is a 4" bore 3.5" stroke V8 wherein the 4.9L is a 4" bore 3.98" stroke I6. Also note that the frequencies are vastly different.
-=Whittey=-
RE: Knock Sensor Output
Engine configuration is obviously the number one factor. A V8 and straight 6 will have diffenrent values fore sure. My point was that, given two 4 cyl cars from which to choose a knock sensor, you should go with the one that has a bore closer to the engine you will be mounting this setup on. If imports use the same basic piezoelectric sensors, it may even be possible to mix and match (wire a knock sensor from smaller disp import engine to a GM ESC module) to get the best match possible. That will require a little more research and time to make sure it will work (for instance, the modules may only be able to filter certain frequencies, and so they may form "matched pairs" with the sensors that are used with. I have no idea about that though). Sorry for the confusion
Andy
RE: Knock Sensor Output
Quick maths: say temp is 600 K (on average). c~500 m/s
First mode in 100 mm cylinder has lambda=0.2m , so frequency is around 2500 Hz.
Which is close enough to 5 kHz that the idea may be valid, or at least part of the story.
However I do know that selecting the tune for a knock sensor for a production engine is done by a highly sophisticated procedure involving an engine on a dyno, a long piece of brake pipe, and a plastic cup, and a handful of different knock sensors.
Cheers
Greg Locock
RE: Knock Sensor Output
I built a box that works similar to the msd box, the led flashes and the buzzer sounds when knock in the motor makes the sensor produce enough voltage to pass a predetermined setting.(both times I used the motors original knock sensor.)
All this helps, is it tells you when the motor is about to hurt itself so that you can back off.
Connecting it to a distributer to controll advance I could not get to work.
From what I could figger out you would need a small micro processer and an a/d converter to do the job.
Note: That little box came in handy quite a few times.
RE: Knock Sensor Output
tell my the make and size and il ltell you if you can add knock sensors.
i hope its a small block chevy..
RE: Knock Sensor Output
RE: Knock Sensor Output
Maybe you could watch the output of the sensor on a scope and advance your timing till it pings and see what kind of output you have?
-Jon
RE: Knock Sensor Output
Maybe some do it like that, but none I've seen. (which is not saying much).
Our advance is calculated from a lookup table based on ECT, rpm and MAP, with some adders in for other things, roughly. WOT spark is open loop at high rpms but may be controlled by the knock sensor at lower speeds.
For a track car you could probably get away with intercepting the signal from the crank position sensor and delaying or advancing that, conceptually. That seems nearly as much work as buildinga your own system.
Cheers
Greg Locock
RE: Knock Sensor Output
The engine is going to have a lot of background noise which is going to change with engine RPM, and there will also be a variety of mechanical resonance points. The knock signal will have its own natural characteristics as well.
So what you might need to do is look at the output of the knock sensor on a spectrum analyzer, and try to find at what frequencies the knock signal is most visible above the background noise.
You would then experiment with different sensors in different locations in an attempt to maximize the knock signal amplitude with respect to the background noises.
Hopefully this has already been done if you are lucky enough to already have a factory knock sensor fitted.
When you have done that, you then apply some frequency selective filtering to further enhance the signal to noise ratio. You then accept any signal over a set threshold as genuine knock. That would be my approach to the problem.
But I can see it would be far from easy for the average person to do at home. You would also need to deliberately induce knock, which will have its dangers.
Because of all the variables, I cannot see a generic one size fits all black box knock sensor working very well.
RE: Knock Sensor Output
I agree that a general purpose knock sensor system would be unfeasible. In effect if one were available we'd use it and then clone the settings into a production unit. That is not how it is done.
Cheers
Greg Locock
RE: Knock Sensor Output
RE: Knock Sensor Output
I would emphasise that tuning a knock sensor is very much reliant on the engineer involved, both for positioning, tuning and calibrating. That is why I am suspicous of a 'universal' plug and play knock solution.
Cheers
Greg Locock
RE: Knock Sensor Output
I have long since binned them but I could redraw them. Tell me, can you or do you know anyone who could build it for you. You will need to make a board from scratch.
The way it works is (this could be wrong - but it works for me)
1) the knock sensor it tuned to the motor by the guys in the factory, placement of the sensor and the type of sensor is critical. there is a tuned weight in the sensor which only vibrates at a predetirmined level(the guys at the factory.)
The frequency of a knock and that of a bearing rumble is very different.
2) the output signal is very rough and very fast. The voltage produced by the sensor changes from a few mv to over 12 volts depending on how how hard the knock is.
3) The current output of the sensor is tiny ( less than 1 MA) So an op-amp is needed to do most of the work.
there is a lot of noise produced by the sensor, but it is fairly easy to filter out.
4) The problem is that some of the components might have to be replaced with components that are more suited to the sensor you are useing.
5) when setting the trigger threshold the motor must be made to detonate (most wellbuilt motors can take a small amount) If anyone knows of a better way to set it up please tell me!
tell me if you want the drawings.
cheers grogg
RE: Knock Sensor Output
RE: Knock Sensor Output
I am present having a problem to find an simple equipment to show if the engine is knocking or no. Could you send me your drawings to construct this knock meter.I would like to evaluate this idea.
My email is Luis.nagy@magnetimarelli.com.br
thanks in advance
nagy
RE: Knock Sensor Output
RE: Knock Sensor Output
Same thing as is used in Time Domain Reflectometry.
A wideband transducer connected to an audio spectrum analyzer will reveal the characteristic resonant frequencies. These will of course vary somewhat with temp, but the filter bandpass can be broadened a bit to cope.
Carter Shore
RE: Knock Sensor Output
RE: Knock Sensor Output
for schematics using a gm sensor
RE: Knock Sensor Output
For a background on knock sensors, read SAE Technical Paper Series 900488, Combustion Knock Sensing: Sensor Selection and Application Issues
Authored by Steven M. Dues, Joseph M. Adams, and George A. Shinkle: Delco Remy Div, General Motors Corp.
In the mid '90's, a GM engineer sent me a list of GM sensors, showing part numbers, resonant frequencies, parrallel load resistance, and mounting style.
At the time, they were producing sensors in these frequencies: 5.2kHz, 6.0kHz, and 7.0kHz.
A chart from the SAE paper shows that the fundamental frequency of a 75mm bore is about 7kHz, a 94mm bore is about 6.0 kHz, while that of a 110mm bore is about 5.2kHz. The chart shows a fairly linear relationship of frequency to bore diameter.
The paper terms the GM sensors to be "broadband resonant", with "bandwidths approaching one thoundsand hertz".
The detector in the J&S unit is in software, running on a 68HC11. The unit controls dwell and timing as well.
Software sets up a knock window, since knock is expected to occur around TDC. The window opens 32° after the ignition trigger, and remains open for 44°.
A load qualifier is used to disable the detector under light engine loads and decel, where piston slap can cause a large interfering signal.
The unit uses a proprietary algorithm to develop a threshold, against which the signal is compared.
jpizzuto@jandssafeguard.com
RE: Knock Sensor Output
An example of a non resonant sensor is the Bosch sensor, which requires a shielded cable.
RE: Knock Sensor Output
Check your e-mail in a few days. I wonder though, your e-mail looks like you work at magnetimarelli? You guys build ignition modules for ferrari and fiat ect... can't you get help from the people there?
Heuf, hi.
If I remember correctly the renault 5 turbo uses wet liners and a knock sensor.
Surfpac....how do I contact you, E-mail, post..?
The circuit at the turbo regal site does look like it could work on the bench but i cant see it working in the field. the output side of the circuit is ok, the trigger needs some work.
cheers grogg
RE: Knock Sensor Output
Remember: You are asking a circuit to detect knock that you can't hear, while ignoring engine noise that you can hear. And the input is essentially a microphone.
It takes signal processing.
RE: Knock Sensor Output
Cheers, D.
"If you are interested in a knock sensor monitor have a look at Knocksense http://www3.sympatico.ca/borism/Knock "
and
"Another approach would be to bolt a seperate knock sensor to the block. The electronics sits in a tiny box under the dash. It contains an active filter that amplifies the knock frequency signal and a comparator circuit that drives an LED which flashes on every knock event. The LED is separate from the box so that it can be mounted anywhere. There is a threshold pot in the box the you can set so that it triggers on the knock signal only.
The unit sells for $60CAD (Canadian Dollars) "
Enjoy
RE: Knock Sensor Output
http://home.netcom.com/~bsundahl/knock/sound/KnockSound...
And best of all to listen into the secret conversations being had:
http://home.netcom.com/~bsundahl/knock/listen/listen.ht...
Ed.
RE: Knock Sensor Output
For an engine, there will be a cylinder that has a knocking tendency more than the rest of the cylinder. This cylinder is generally the far from the entrance of water pump, has the poorest cooling, strongest intake mixture charge, etc.
During the development, this cylinder will be identified. Once it is identified, you can weld several M8 bosses in different locations to test for the best signal to noise ratio.
Once the best sensor location is identified, it will serve as the focal point for the knock sensing and ignition timing tuning.
Using only one knock sensor may not yield the best result. One for each bank will be much better. For me, I would prefer to have ionic sensor at the spark plug cause this will enable me to control every cylinder ignition timing.