Knock Sensors

The whole idea of the APC system is to allow for maximum boost without detonation or knock. Knocking is an explosion of the air fuel mixture as opposed to a normal controlled burn. When running at high boost pressures, there is a tendency for violent knocking. In order to push the limit of maximum boost without damaging the engine, you must protect the engine from this danger. Knocking combustion causes greater thermal and mechanical stresses on the engine that could damage it due to pressure fluctuations and increased heat loads. The knocking noise heard is caused by gas vibrations. (Here is some more info on Knock and Detonation)

The knock sensor is an acceleration sensor that converts vibrations into electrical signals by way of a piezo-ceramic element. The piezo-ceramic element generates surface charges when mechanical forces act upon it. You will find the knock sensor mounted to the side of the block between cylinders 2 and three.

Knocking causes vibrations in the engine block that exhibit a characteristic frequency between 5 and 10 kilohertz (see below). In order to filter out erroneous noises caused by other mechanical components in the engine the APC system's brain has the ability to read the signal from the knock sensor, decode it, and make a knock/no knock decision. If the APC hears knocking it then retards boost in increments of 1.5psi until knock goes away.

Below are two graphs of a knock sensor reading and cylinder pressure. Notice how the cylinder pressure rises with knock.

Normal engine running. Engine Knocking

Color Key: Filtered Pressure Signal | Signal From Knock Sensor | Cylinder Pressure

Here is a picture of the knock sensors for both the APC and the EZK ignition system
stacked one on top of the other. The APC sensor in on top.

Proper tightening torque of the knock sensor is:

14 +/- 5 Nm
(10.5 +/- 3.7 lb ft.)


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