Ignition and injection control

Ignition and injection control

These values have been obtained experimentally, by means of a long series of practical tests carried out on prototypes at the engine test bench, to identify the advances which give the best compromise between the conflicting requirements of maximum power and a reduction in fuel consumption and harmful exhaust emissions.

These optimum advances have then been memorized in the system control unit. During the operation of the engine, the control unit is constantly informed of the (engine) speed and load conditions and on this basis it "selects" the optimum advance from its memory to strike the spark at the spark plug for the cylinder in the explosion stroke with the optimum advance.

In addition, the control unit corrects this value, depending on other factors such as the engine coolant temperature, intake air temperature, detonation and butterfly valve position, so that the ignition point is always optimum.

The information required by the control unit to operate the single coils is transmitted by means of electrical signals sent by the following sensors:

1. An rpm sensor which produces a single phase alternating signal whose frequency indicates the engine speed.
2. An air flow meter with a temperature sensor which, on the basis of the quantity/temperature of the air drawn in by the engine, transforms these figures into electrical signals, sending them to the electronic control unit.
3. Two detonation sensors which, located in the upper part of the cylinder block/crankcase, one between cylinders 1 and 2 and the other between cylinders 4 and 5, respectively, allow the control unit to recognize the cylinder where detonation (or the onset of detonation) is taking place and only correct the ignition advance for the spark plug of the cylinder concerned.
4. An accelerator pedal potentiometer which transforms the angular value of the actual pedal into an electrical signal allowing the control unit to recognize minimum, partial and full load conditions.

Functions of the injection/ignition system

In addition to electronically controlling the moment of ignitionand the air flow rate during idling, in order to allow the engine to run smoothly when the ambient parameters and loads applied vary, the control unit should control and manage the injection in such a way that the stoichiometric (air/fuel) ratio is always within the optimum value.

The electronic control unit establishes the operating «time» for the injectors using a relatively simple rule which can be summarized as follows.

Taking the physical properties of the fuel (viscosity and density) and the difference in pressure between the pressure of the fuel and the pressure in the inlet manifolds as constant, the quantity of fuel to be injected only depends on the injector «opening time».

The injection/ignition system basically carries out the following functions: