Knock Control
The new knock control system is incorporated for the first time on a U.S. BMW model which also contributes to the improved performance of the E36 M42 engine.
If an engine runs for any length of time with knock occurring, it may suffer severe damage.
Knock is encouraged by:
- a high compression ratio
- a high level of cylinder filling
- poor-quality fuel (low octane number)
- high intake-air and engine temperatures.
An artificial increase in the compression ratio may also be caused by carbon deposits in the combustion chambers, or as a result of an unfavorable combination of manufacturing tolerances.
If the engine has no form of knock control, the ignition timing has to include a safety margin to allow for such unfavorable influences, so that efficiency is automatically lowered in the full-load operating range.
Knock control enables the engine to run right up to the knock limit, since it retards the ignition (on the affected cylinder only) when the actual risk of knock is detected. The normal ignition timing can therefore be chosen for optimum fuel consumption and operating efficiency, without allowing any safety margin for influences which could cause the knock limit to be exceeded.
The knock control system adjusts the ignition timing sufficiently to avoid engine knock and will allow the engine to run satisfactorily (but with reduced power) on fuel of a lower octane rating than the recommended unleaded premium. Thus, if the owner accidentally gets a tank of inferior gasoline, the engine will not be damaged immedately.
Advantages of knock control:
- avoid damage caused by knock even in the most unfavorable circumstances
- maximum economy, since the energy in the available grade of fuel is fully utilized and the engine's operating condition is taken into account
- fuel consumption is reduced and torque maximized throughout the upper engine-load range, particularly when the DISA system increases the mixture charge entering the cylinders.
To ensure that knock is reliably identified, BMW installed two knock sensors on its four-cylinder engines (many otherwise comparable systems for four-cylinder engines use only one sensor).
A multiplexer circuit in the engine control module analyzes the signals. This ensures that only the signal from the cylinder in which combusion is actually taking place is transmitted to the adjacent knock sensor. The multiplexer is switched correctly by evaluating the signal from the cylinder identification sensor (on the camshaft).
The knock sensor is a piezo-electric conductor-sound microphone with a broadband characteristic. A piezo-ceramic ring is clamped by a spring washer between a seismic mass and the sensor body. If the seismic mass is accelerated, it exerts a force on the piezo-ceramic element. Opposed electrical charges build up on the upper and lower ceramic surfaces, and generate a voltage at the contacts. In this way, acoustic vibrations can be converted into electrical signals. These in turn are transmitted by shielded wires to the engine control module for processing.
The knock sensors are bolted to cast bases on the intake side of the engine block, between the 1st and 2nd and between the 3rd and 4th cylinders.
These locations ensure that even when knock is only slight that the acoustic vibrations emitted from the combustion chambers are transmitted reliably to the knock sensors.
If the actual value exceeds a predetermined value, the combustion process in that cylinder is identified as "knock". The ignition timing is then retarded immediately in that cylinder so that knock is eliminated.
After this, the ignition is advanced again step by step until the optimum value as stored in the mapped igniton program is reached, or until knock is once again detected.
The ignition is retarded even if knock is only slight and does not yet represent any acute hazard for the engine.
Knock control is out of action at engine temperatures below 35~C (95~F) and at low loads (below approx. 1/3 volume air flow sensor load signal).
If noise from the engine is unusually loud, reliable knock identification cannot be guaranteed (for instance above-average valve closing noise caused by a defective hydraulic valve lifter (HVA) or one which is malfunctoning due to lack of oil).
The knock control system's self-diagnosis function consists of the following tests:
- sensor signal malfunction, open circuit (broken wire), defective plug etc.
- self-test of complete analysis circuit
- checking the basic engine noise level as registered by the knock sensors.
If a fault is detected in one of these tests, the knock control is shut down. An emergency program takes over ignition timing control, and the fault is stored in the fault memory.
The emergency operating program ensures that the engine can run without risk on fuel with a minimum octane number (Research Method) of 91. It takes load, engine speed and engine temperature into account.
The diagnosis routine is unable to detect whether the plugs for the two sensors or the sensors themselves have been accidentally interchanged.