Whats a Knock Sensor do anyway?

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KNOCK SENSORS


Some engines come with knock sensors integrated into the electronics. Knock sensors use a microphone—usually on the engine block or intermediate housing. The mike feeds electronics which are tuned to recognize knock from the engine. Once identified as knock, the computer intervenes by retarding the ignition timing. Why does this work? Gasoline engines are usually set to fire the spark plugs before the combustion chamber reaches its smallest size (maximum compression). On a piston engine maximum compression is when the piston is at the top of the compression cycle. In other words, the mechanical compression cycle is not complete at the time the plug fires. Thus, during combustion, the total pressure in the chamber is a combination of the remaining part of the mechanical compression cycle plus the pressure from expansion caused by the burning fuel/air mixture. If you delay the firing of the spark plug, more of the mechanical compression cycle will have passed at the time the intake charge is lit, so the overall amount of pressure (and heat) in the combustion chamber reduces. This reduction in pressure should be enough to ease the tendency for the end gasses to detonate. This is why the more you retard the spark, the more relief you get from detonation.
Those with aftermarket turbo kits can add a knock sensor. J&S makes one that intercepts the firing signal going to the leading plugs and delays it in proportion to any knock that is sensed. The unit is very sophisticated, and can identify which cylinder is associated with the detonation. It then retards the spark to that cylinder only. If more than one cylinder is involved each one is treated independently. Owners of highly modified factory turbo cars can also benefit from such a device since the range and capabilities of stock knock sensors may not be enough to fully protect an engine that is exceeding factory output.