Knock sensors, was Re: Ignitition Controllers

[Brian Knowles]

On Fri, 25 Jul 1997, Bruno Marzano wroto:
>
>On Fri, 25 Jul 1997, Rob Skala wrote:
>
>[snippity-snip]
>
>> incorporate a few additional features one of which in knock control. 
>> 
>> Does anyone out there have any advice as to the best method to implement
>> this ? Ideally what I would like to do is advance the timing to the onset
>> of knock and then retard it slightly back into the safe zone. The
>> controller will be used on a fuel injected 12A turbo rotary engine.
>
>If your engine needs to be run with the ddis before you implement knock,
>you could have the controller emulate a distributor initially, and (dons
>the asbestos underwear) gradually tweak the timing so that you might be
>able to get the engine to ping. If not, you may be able to set the dizzy
>to do so. You could record the engine sounds by bolting a microphone to
>the engine somewhere (or maybe even 3 or 4, near the valves :)) to find
>the best point where to "listen" for this condition. I've read that this
>should not continue for very long, if you want the engine to survive. 
>
>The signal could then be filtered out and cleaned up to give a knock
>sensor "switch". Make sure that other egine noises do not make the sensor 
>think it's knocking when it isn't.

It's my understanding (means I don't remember where I heard it) that the 
manufactures of knock sensors make them in only 4 frequencies, and you 
make your choice of one that comes closest to working on your engine, and 
that a lot (not all) aluminum OHC and DOHC engines, 'specially those with 
chain driven cams, make so much general thrashing noise as to prevent use 
of any of the available knock sensors.  4 cylinder Alfas, for instance.

Its my further understanding that because there are only four to choose 
from, many available implementations don't catch ALL detonation, 
especially under high boost, and that an engine with factory knock 
sensing can still detonate and damage itself because of this. Buick turbo 
V6s for instance.

I guess this explains why manufacturers put knock sensors in unlikely 
places.  4 cylinder turbo-ford engines have 'em low on the side of the 
inlet manifold.  I speculate that they got a sensor of the frequency that 
comes closest, and then optimised the location of the sensor to best 
detect that frequency.

Brian