Combustion knock is a problem that is associated with engine operation. Early motor vehicles were equipped with a hand control that enabled the driver to retard the ignition when the characteristic ‘pinking’ sound was heard. After the pinking had ceased the driver could move the control lever back to the advanced position. Electronic controls permit this process to be done automatically and a knock sensor is often included in the make-up of an electronic ignition system.
The knock sensor on the engine.
The piezoelectric effect is often made use of in knock sensors and the tuning of the piezoelectric element coupled with the design of the sensor’s electronic circuit permits combustion knock to be selected out from other mechanical noise. The combustion knock is represented by a voltage signal which is transmitted to the ECM and the processor responds by retarding the ignition to prevent knocking. The ECM retards the ignition in steps, approximately 2Ž at a time, until knocking ceases. When knocking ceases the ECM will again advance the ignition, in small steps, until the correct setting is reached.
Adaptive Ignition.
The computing power of modern ECMs permits ignition systems to be designed so that the ECM can alter settings to take account of changes in the condition of components, such as petrol injectors, as the engine wears. The general principle is that the best engine torque is achieved when combustion produces maximum cylinder pressure just after TDC. The ECM monitors engine acceleration by means of the crank sensor, to see if changes to the ignition setting produce a better result, as indicated by increased engine speed as a particular cylinder fires. If a better result is achieved then the ignition memory map can be reset so that the revised setting becomes the one that the ECM uses. This ‘adaptive learning strategy’ is now used quite extensively on computer controlled systems and it requires technicians to run vehicles under normal driving conditions for several minutes after replacement parts and adjustments have been made to a vehicle.
This review of ignition systems gives a broad indication of the technology involved and, more importantly, it highlights certain features that can reasonably be said to be common to all ignition systems. These are: crank position and speed sensors, an ignition coil, a knock sensor, and a manifold pressure sensor for indicating engine load. In the next section, computer controlled fuelling systems are examined and it will be seen that quite a lot of the technology is similar to that used in electronic ignition systems.
