136 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

1. Harness connector 2. Knock sensor

Fig. 1.11 The knock sensor on the engine 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.

1.4 Computer controlled petrol fuelling systems

Computer controlled petrol injection is now the normal method of supplying fuel - in a combustible mixture form - to the engine's combustion chambers. Although it is possible to inject petrol directly into the engine cylinder in a similar way to that in diesel engines, the practical problems are quite difficult to solve and it is still common practice to inject (spray) petrol into the induction manifold. There are, broadly speaking, two ways in which injection into the induction manifold is performed. One way is to use a single injector that sprays fuel into the region of the throttle butterfly and the other way is to use an injector for each cylinder, each injector being placed near to the inlet valve. The two systems are known as single-point injection (throttle body injection), and multi-point injection. The principle is illustrated in Fig. 1.12.

Fig. 1.12 (a) Single-point injection. (b) Multi-point injection
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