Knock Sensor Applications

[Walters Chris]

>I read a lot of e-mail about "knock sensors" and get the distinct
>impression that they are greatly misunderstood. This is especially so in
>the area of adding sensors to vehicles not normally fitted with one. I
>have an article on engine knock detection in the SAE magazine of December
>'93 which really threw me when I read it. I'd love to quote large passages
>from it but I don't want the SAE suing my butt off for breach of copyright
>so I'll summerize in my own words.
>
>There are 3 ways to detect knock in order of cost and complexity: auditory
>(ear), accelerometer knock sensor and pressure transducer. The authors of
>the article say none is without disadvantages regarding detection
>certainty.
>
>When refering to placement, they talk of "nodes" where knock can not be
>detected and "anti-nodes" where background noise swamps the signal
>entirely. You wouldn't want to place a sensor at either of these points.
>To make matters worse, on some engines, these ??nodes are not stationary
>but move around in some RPM related pattern. Peter reported once that GM
>spent millions on the problem and no one seemed to pick up on the
>significance of the fact. I doubt that corporations like GM would spend
>that sort of money if they didn't have to.
>
>But wait, there's more. There are 3 bands of knock noise to be considered
>at 6-8kHz, 11-13kHz and 17-20kHz. The significant band can vary with RPM
>and any number of other characteristics. Broad-band sensors are not the
>answer since they have problems with knock detection at mid range (3000
>rpm) and require non trivial DSP (eg, 256 tap floating point FFT) to
>determine the presence of knock.
>
>OEMs use an advance determination method called MBT (Minimum spark advance
>for Best Torque), unfortunately knock can sometimes set in before best
>torque is realized.
>
>So far, things do not look very good. All of the above leads me to a
>number of conclusions (all mine):
>
>    Knock sensors are not magic. They are basically a microphone in the
>    noisiest (electrically & mechanically) environment possible. A knock
>    sensor should be considered only in conjunction with an amplifier
>    which is designed to match its characteristics and only on the engine
>    for which it was designed.
>
>    If a motor was never fitted with a knock sensor, don't bother. Just
>    set as conservatively or agressively as you dare. It's your engine.
>    You have to rebuild it if you break ring lands or burn a hole in a
>    piston.
>
>    If the motor is fitted with knock detection then duplicate the circuit
>    exactly. Most pizo knock sensors have some bias voltage or current
>    applied which influences the sensor responce characteristics.
>
>    Learn to read spark plugs for signs of knock. (I can't, yet)
>
>    I believe the OEMs are probably using the output of the knock sensor
>    at points where it is known to be reliable and otherwise being very
>    conservative. It has been reported that GM units over retard under
>    some conditions. This may be the reason.
>
>Now that I have let the fox into the hen house, let's have some feed-back.
>I've seen the results of undetected knock and it ain't pretty and I
>wouldn't wish that on my worst enemy. I look forward to reading responces
>to this.

Greetings all,
I'm Chris Walters aka Snake. I've recently joined this list, having drifted 
over from the fordnatics list. BSEE with 10 yrs programming  and 1 year 
schematic-capture / PCB layout design experience. Also a gearhead, seeking 
to combine the two fields in my fleet of 70s-vintage Fords.

I've checked out the list's web site and have only a layman's knowledge  (so 
far) but one question comes immediately to mind: since detonation is caused 
by a too-lean condition couldn't you simply test the exhaust from each 
cylinder to determine if its running lean?
If oxygen sensors don't provide enough accuracy perhaps some other type or 
combination of types, coupled with calculations from a uP, could. But if the 
answer was that simple you'd think the OEMs wouldn't've put so much effort 
into knock-detection - unless  sensor technology available at the time 
wasn't up to the job.

My D3VE-A2A   heads have a small hole in each exhaust port for EGR - perfect 
for installing such a sensor.

Snake
no cool Fords yet, one cool Dodge