madteg
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| posted on 27/1/08 at 05:29 PM |
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knock sensor
What do they do, how do they work
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scotty g
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| posted on 27/1/08 at 05:37 PM |
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they detect engine knock, better known as pinking? (sp) not a clue how they work but i know the one on my Carina-e is knackered and will cost me over
£90 +VAT!!! 
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MkIndy7
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| posted on 27/1/08 at 05:37 PM |
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Basically there a Microphone that the ECU listens to.. then when the engine knocks it detects the set frequency and retards the timing accordingly.
The frequency that the engine knocks at can be different for different makes and models of engine depending on their bore and stroke etc I should
imagine.
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turbodisplay
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| posted on 27/1/08 at 05:40 PM |
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Most have a heavy weight with a piezo (creates electricity when squeezed)material between senor body, and weight.
Any vibrations are picked up and coverted into electrical signal.
To detect knock special frequency filtering is required, plus windowing (just after ignition event for x degrees).
Quite hard to do as engine noise, valve train noise is quite significant.
Darren
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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madteg
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| posted on 27/1/08 at 05:43 PM |
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What would happens if it fails
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Tim 45
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| posted on 27/1/08 at 05:55 PM |
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Your ECU would not retard the timing in instances of knocking. For a normal car with a standard manufacturers ECU this shouldnt be much of a problem
if an adequate fuel grade is used (95 or higher in the uk) as they dont run that much spark advance.
However if your using a timing system running a modified spark advance (megasquirt or other), that relies on the knock sensor to allow max. advance
then you may find yourself needing a new set of pistons after a very short space of time.
[Edited on 27/1/08 by Tim 45]
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MkIndy7
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| posted on 27/1/08 at 06:01 PM |
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Surely in a case of a failure like if the Sencor went open circuit or it recieved no signal back then it would just stick to its base "safe
map"?
Does it not use a base map, then advance and listen for knock.. then advance a bit more and so on.
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Tim 45
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| posted on 27/1/08 at 06:05 PM |
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Perhaps, i was going off concequences of a megasquirt...which alas - has resulted in the car having to have 2 new sets of pistons as a result of
knocking 
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BenB
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| posted on 27/1/08 at 09:53 PM |
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Two types of knock sensors. The microphone type (piezo) knock sensors. They just listen for pinking. Trouble is you have to try and filter out all the
other noises going on.....
Second type is ion sensing. They look at the resistance across the spark plug during the non-firing phase to look for free radicals... Very clever and
no major filtering needed... Very few cars use them yet...
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