Final ignition timing
Depending on whether ignition works in start, idling or normal mode, after all compensation the value will be converted to a signal that regulates the power amplifiers of the ignition coils for the six cylinders.
Scheme 778
Scheme 779
Combustion in an Otto engine can be beneficial (normal combustion) or destructive (knocking).
During normal combustion, the air/fuel mixture is ignited by the spark form a spark plug. The flame expands in the combustion chamber originating from the centrally located spark plug.
Normal combustion takes place in a controlled fashion at a speed in the range 20-40 m/s, depending on the prevailing operating conditions.
The pressure in the cylinder now rises in a controlled manner as the combustion proceeds and maximum pressure will be attained at approx. 16-17 degrees ATDC. Occasionally, the pressure and temperature in the cylinder are so high that the unburned air/fuel mixture can self-ignite, either before or after the spark has ignited. This self-ignition usually takes place at very high speed and therefore releases large amounts of energy into the cylinder in a very short time.
This generates pressure waves that can move at supersonic speed. Uncontrolled knocking could damage the engine through, for example, heat damage to pistons and cylinder heads. Pressure waves can cause fatigue damage to pistons and cylinder heads. It is necessary to distinguish between controlled knocking that is handled by the engine management system and uncontrolled knocking that can occur if the engine is run on poor quality fuel at high air temperature and high altitude.
A high compression ratio is advantageous to engine efficiency and consequently fuel consumption.
The engine's propensity for knocking increases as the compression ratio increases. The engine compression ratio is thus selected with both engine knock propensity and fuel knock propensity in mind.
Knock control in modern engines is not a safety function but a standard function. Consequently, it is normal for knock control to be active during normal driving. At times in certain operating conditions, the engine can be heard knocking. This is controlled knocking that can be considered normal.
Knock control is individual to each cylinder and is adaptive.
