Theory Of Operation

Knock is the spontaneous auto-ignition of the remaining fuel/air mixture in the engine combustion chamber that occurs after normal combustion has started. It can occur under extreme vehicle operating conditions such as;

• excessive spark advance for the given engine operating conditions.
• high engine temperature.
• high Manifold Absolute Pressure (MAP) Sensor value.
• low humidity and heavy loads to the engine.

Severe, continuous knock may be caused by any of the following, but not limited to;

• carbon deposits.
• bad gasoline and/or low octane fuel.

Engine power is maximized when the spark advance is calibrated to the maximum torque point or the knock limited point. In order to calibrate spark advance as close as possible to its optimum point, a knock feedback system is used to protect the engine from damage.

The analog output voltage of the Knock Sensor represents the strength of the engine knock. If the engine is running, Knock Sensor voltage will not be at zero, even when knock is not present. This is due to the background noise of the engine. Background noise varies from engine to engine depending upon;

• age.
• temperature.
• speed.
• load.
• and other engine operating conditions.

An adaptive background noise curve is established by learning the knock voltage versus engine speed under non-knock conditions. When the engine is operated under high load conditions where knock is possible, the knock voltage is tested to decide if it exceeds the knock voltage threshold. Knock has occurred when the knock voltage is at or above this calibrated voltage threshold.

When knock is detected, short term spark advance is reduced by a calculated amount. The amount of spark advance reduction is based off a calibrated severity of the knock event. This reduction in spark advance is used in the next ignition event to prevent further knock events. If the knock continues, an additional reduction in short term spark advance occurs. When the knock stops, the short term reduction in spark advance is ceased. Once ceased, the long term knock compensation is reduced by a calibrated amount to recover some of the reduction in spark advance. This decreases the amount of short term spark advance reduction to optimize engine performance.