SpeedTalk Store - Opinion Columns

question on knock sensing

General engine tech -- Drag Racing to Circle Track

Moderator: Team

question on knock sensing

Postby Belgian1979 » Tue Jan 15, 2013 5:50 am

This is a question about the use of a knock sensor in a performance engine.

Currently I mounted a factory SBC knock sensor just above the pan rail in the cooling bleeding hole (factory location) in the (maybe idle) hope that it will function correctly. This was done as was suggested by many as the right way.
Scott (Dieselgeek) already said that it might be a problem using a more generic sensor. That it might be useless.

I would like to detect knock as best as I can to avoid any problems. The plan is to use my MS to monitor the sensors and see what I can get away with in terms of ignition, the results later on to be implemented in the ignition tables when going EFI.

As far as I know these sensors can be tuned to a specific frequency. However I would like to know how this is exactly done. Is the engine used in such a way that audible knock is detected and then the sensor is tuned accordingly. Does one rely on the plugs to show detonation ?

Thanks
Belgian1979
Guru
Guru
 
Posts: 3218
Joined: Sat Mar 26, 2011 10:34 am
Location: Belgium - Koersel

Re: question on knock sensing

Postby RednGold86Z » Tue Jan 15, 2013 6:12 am

Is it a flat response sensor, or a resonant tuned sensor? A flat response will transmit all of the frequencies to the ECU's knock chip (typically), while a tuned sensor will resonate at the supposedly correct frequency for which it was designed.

The knock chip is typically a programmable band-pass filter device. A rough range of knock frequency is 5-8kHz, depending primarily on the cylinder bore (rings like a bell).

It's up to the software to determine whether the knock-like sounds are feasible. For example - it's not going to knock well before TDC for a given cylinder. Also, they can measure the background noise of the engine in that position in order to compare it with the noises in the right frequency near and after TDC.

Some old softwares aren't built to decide much - just waiting for a knock input from a knock box.

If you have a flat response sensor, you can connect it to a sound card on a laptop (microphone input), and listen to the output on some noise cancelling earphones and actually hear knock if you're lucky. There are also ways to amplify the sounds before you plug it into the laptop if it's just too quiet for you. You'll need to use software to play the mic right back out the ear phone jack (or just un-mute the microphone via the sound control settings). You can even do some analysis on the sound that comes in - if you find some good software.
RednGold86Z
Expert
Expert
 
Posts: 854
Joined: Sun Nov 04, 2007 8:47 pm
Location: China, Iowa

Re: question on knock sensing

Postby vwchuck » Tue Jan 15, 2013 7:39 am

Number one you need to have the correct software to do knock control properly. You need to be able to look at the firing cylinder for around 50 degrees after TDC.Then you run that signal through the standard knock chip that is available. This is time consuming and a pain to get it all working and get decent software to control the chip properly.
OR
The easiest way to detect knock is to hook your knock sensor directly to an Oscope. Run one channel to a once per 720 degree(cam signal) and the other channel from the knock sensor. You can find where TDC will be for whatever cylinder with the cam signal. Once you can see ATDC for the firing cylinder on the scope run it with no knock and then make it knock. You will see the detonation on the sensor. It iks very easy to spot. The noise after TDC will kind of popp in amplitude.
vwchuck
Expert
Expert
 
Posts: 609
Joined: Thu May 28, 2009 10:06 pm

Re: question on knock sensing

Postby Force Fed Mopar » Tue Jan 15, 2013 8:35 am

Most modern cars have the knock sensor installed either in the valley, or near the top of a cylinder in the block (usually one for each bank if a V engine).
Rob M
Force Fed Mopar
Pro
Pro
 
Posts: 215
Joined: Sun Nov 04, 2012 10:59 am
Location: Upstate SC

Re: question on knock sensing

Postby Belgian1979 » Tue Jan 15, 2013 8:36 am

Ok, so this is performed when standing still. How do you get it to knock ?

Flat rate : honestly I don't know. This is a GM type sensor for this block ( Gen I SBC)
Belgian1979
Guru
Guru
 
Posts: 3218
Joined: Sat Mar 26, 2011 10:34 am
Location: Belgium - Koersel

Re: question on knock sensing

Postby David Redszus » Tue Jan 15, 2013 11:05 am

As R&G stated earlier, there are basically two types of knock sensors available. The broadband type requires the correct software in order to properly identify the knock signal (compared to normal background noise). Broadband sensors are typically found on OE applications that have knock control programmed into the ECU.

Some knock sensors are attenuated to respond only in specific frequency ranges. The output will be an analog voltage that can be read by a volt meter, scope, earphones, or fed into a data logger. Now it becomes necessary to know the knock frequency for a specific engine and operating condition.

Knock can be induced by the addtion of a knock precursor to the fuel such as jet fuel JP-8.

Beware of knock control when using high octane race gas. While knock may be avoided with excessive ignition advance, pre-ignition will not be avoided which can produce substantial problems with rod bearings and piston wrist pins.
David Redszus
Guru
Guru
 
Posts: 5827
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago

Re: question on knock sensing

Postby Runit » Tue Jan 15, 2013 12:15 pm

Here's a link to an article on the Motec site about setting up a their knock sensor. http://www.motec.com/skm/skmdownload/
Runit
Pro
Pro
 
Posts: 419
Joined: Mon Dec 26, 2011 10:10 am

Re: question on knock sensing

Postby In-Tech » Tue Jan 15, 2013 12:36 pm

Belgian1979 wrote:Ok, so this is performed when standing still. How do you get it to knock ?

Flat rate : honestly I don't know. This is a GM type sensor for this block ( Gen I SBC)



Sounds like you have a resonant sensor. I'll dig up a TI chip number later so you can play on the bench. Your question does not have an easy answer. Flat response IMO is much better and faster but software is the key in either. Anything can be a "speaker"...understanding what you hear is the difference.
-Carl
In-Tech
Guru
Guru
 
Posts: 1321
Joined: Wed Apr 21, 2010 3:35 am

Re: question on knock sensing

Postby MadBill » Tue Jan 15, 2013 12:56 pm

The exposure I had to GM sensors in the late eighties was largely related to false retard from such things as exhaust system grounding out. Tapping the block with a big screwdriver near the sensor would also trigger retard. The only sensitivity 'tuning' we had available was how much Teflon tape we wrapped around the sensor's threads and how tightly we torqued it.

If like me, your eyes glaze when the topic of electrons comes up, there are easily adjustable stand alone knock systems; ew.g: http://www.holley.com/534-136.asp Here's a thread re another system: http://www.mgexperience.net/phorum/read ... 580,page=1
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.
User avatar
MadBill
Guru
Guru
 
Posts: 12006
Joined: Tue Nov 15, 2005 10:41 am
Location: The Great White North

Re: question on knock sensing

Postby Belgian1979 » Tue Jan 15, 2013 5:36 pm

After reading comments here and reading up on the subject apparently the human ear is the best to detect knock.

It was suggested to hook up the sensor to the mic input of the laptop. Can this be done directly or do I have to pass via the ESC ?
Belgian1979
Guru
Guru
 
Posts: 3218
Joined: Sat Mar 26, 2011 10:34 am
Location: Belgium - Koersel

Re: question on knock sensing

Postby user-z68766209 » Tue Jan 15, 2013 6:19 pm

Belgian1979 wrote:Ok, so this is performed when standing still. How do you get it to knock ?

Flat rate : honestly I don't know. This is a GM type sensor for this block ( Gen I SBC)


If you have it hooked up to a labtop with the motor running idle..stand still.
If you take a small hammer or even a wrench and tap the block above the sensor you will
"see" or "here" or will do somthing on the labtop..
user-z68766209
 

Re: question on knock sensing

Postby RednGold86Z » Tue Jan 15, 2013 6:36 pm

I've only done the laptop thing with a flat response sensor - if you're using an old GM sensor that goes through an ESC module, it's almost surely a resonant sensor - in which case I don't know the answer. Maybe google it for some more info.
RednGold86Z
Expert
Expert
 
Posts: 854
Joined: Sun Nov 04, 2007 8:47 pm
Location: China, Iowa

Re: question on knock sensing

Postby David Redszus » Wed Jan 16, 2013 1:00 am

If the knock sensor being used is a Delco (GM) product, knowing the part number can reveal the knock frequency for that specific sensor.

Do you have a part number?
David Redszus
Guru
Guru
 
Posts: 5827
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago

Re: question on knock sensing

Postby Belgian1979 » Wed Jan 16, 2013 5:38 am

Standard motor product KS2, which should be equivalent to a 1986 knock sensor on iron gen I block.

I've found a formula to calculate the frequency based on the bore size. It pointed out 2.730 kHz.
Belgian1979
Guru
Guru
 
Posts: 3218
Joined: Sat Mar 26, 2011 10:34 am
Location: Belgium - Koersel

Re: question on knock sensing

Postby Force Fed Mopar » Wed Jan 16, 2013 8:20 am

Belgian1979 wrote:After reading comments here and reading up on the subject apparently the human ear is the best to detect knock.

It was suggested to hook up the sensor to the mic input of the laptop. Can this be done directly or do I have to pass via the ESC ?


Yes it can, it can also be plugged into your car radio if you have an auxiliary input. This thread has a lot of good info in it:

http://www.turbo-mopar.com/forums/showt ... post946458
Rob M
Force Fed Mopar
Pro
Pro
 
Posts: 215
Joined: Sun Nov 04, 2012 10:59 am
Location: Upstate SC

Next

Return to Engine Tech

Who is online

Users browsing this forum: af2, ap72, Bing [Bot], bobalattie, dfarr67, englertracing, F-BIRD'88, Google [Bot], gvx, hpetew, John Wallace, kosky racing, PbFlinger, Steve.k and 35 guests