Principle of Operation

Breakerless ignition system with knock sensor (EZK)

General




Due to the variations in the fuel quality and the environmental demands, the octane rating may vary widely from one country to the next.

A fuel system incorporating a knock sensor enables the special properties of a given fuel to be utilised to achieve optimum performance and minimised fuel consumption. Knocking will occur when the engine load is high and if the fuel is of poor quality (e.g. with a low octane rating).

Knocking is the result of pre-ignition of the fuel/air mixture and may cause serious damage to the engine if it persists. In addition to the audible knocking occurring when the engine is temporarily overloaded, high-speed knocking which is inaudible to the human ear may also occur. It is this type of knocking which is most detrimental to the engine.

In order to avoid engine damage under such conditions, manufacturers have been obliged to allow generous margins to take into account the variations in fuel quality. As a result, the engine is unable to convert all of the energy in the fuel into propulsion power, and much of the energy is wasted as heat losses.

Note: Even if a control system incorporating a knock sensor is used, the knocking occurring during normal running of the engine cannot possibly be eliminated. But this knocking is harmless to the engine.

The ignition system adjusts its timing to suit the load, the engine speed and any knocking tendencies detected.

Since the spark is always fired at the correct instant, the engine performance will always be a maximum, regardless of the fuel grade used at any particular time. This reduces the fuel consumption and minimises the pollutants in the exhaust gases.


Principle of operation

A knock sensor detects any knocking tendencies in the engine and applies a signal to the electronic control unit, which will then automatically adjust the ignition timing in accordance with the programmed parameters, and the engine load and speed input signals received.

When the engine is started, the ignition is always set to 50 before top dead centre (BTDC), and will remain at this value up to an engine speed of about 700 r/min. At engine speeds above 700 r/min, the system will automatically adjust the ignition timing in accordance with the data programmed into the memory of the electronic control unit. The ignition timing is set to suit the engine load, the engine speed and any knocking tendencies, and is varied between 5° and 22° BTDC.

The knocking-related ignition timing is controlled individually on each of the cylinders. The ignition timing may thus vary from one cylinder to the next.

On the other hand, the load-dependent timing is the same for all cylinders.


Knocking-dependent control




When knocking is detected in one cylinder, the ignition for that particular cylinder will be retarded in steps of about 3° until knocking has ceased or until the ignition has been retarded by a total of 13°. The ignition timing will revert to normal in steps of 0.350 per speed-dependent period.


Load-dependent control




If an engine load change should exceed the predetermined value stored in the memory of the electronic control unit, the ignition will be retarded by about 6° on all cylinders simultaneously. When the load has ceased to increase, the ignition will be advanced back to the normal timing in steps of around 1°.


Knock sensor




The knock sensor records any knocking occurring in the engine, convents the resulting vibrations into electrical signals and transmits them to the electronic control unit. The knock sensor is located on the left-hand side of the engine block, below the intake manifold.


Electronic control unit




The electronic control unit receives signals from the knock sensor, the control unit of the LH system and the Hall sensor in the distributor. The signals are analysed in the electronic control unit and are compared with the information stored in its memory. The control unit then resets the ignition timing by transmitting signals to the ignition system, which opens the primary circuit of the coil and triggers the ignition spark.


Distributor




The distributor is equipped with a Hall sensor and has no centrifugal or vacuum advance mechanism.