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 at which it is possible to achieve the best compromise between the conflicting requirements of maximum power and low consumption and harmful exhaust emissions.

The optimum advance values 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 advance value from its memory to strike the spark at the spark plug for the cylinder during the explosion stroke with the optimum advance.

In addition the control unit corrects this value depending on further factors such as engine coolant temperature, intake air temperature, detonation and position of the butterfly valve, 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 emitted by the following sensors:

1. An rpm sensor which generates a single phase alternating signal, whose frequency indicates the engine rpm.
2. An air flow meter which, on the basis of the quantity of air drawn in by the engine, transforms this value into an electrical signal, sending it to the electronic control unit.
3. An air temperature sensor which transforms the value of the intake air temperature into an electrical signal, sending it to the electronic control unit.
4. Two detonation sensors which are positioned in the upper part of the crankcase, one between cylinders 1 and 2 and the other between cylinders 4 and 5, allowing the control unit to recognize the cylinder which is detonating (or is in the early stages of detonation) and to correct the ignition advance for the spark plug for the cylinder involved only.
5. A butterfly valve position sensor which transforms the angular value of the actual butterfly into an electrical signal allowing the control unit to recognize the conditions of minimum, partial or full load.

Functions of the system

In addition to electronically controlling the moment of ignition and the air flow rate during idling in order to allow the engine to run smoothly when the environment parameters and the load conditions vary, the control unit must also control and manage the injection in such a way that the stoichiometric ratio (air/fuel) is always within the optimum values.

The electronic control unit establishes the «time» for the injectors following a relatively simple rule which can be summed up as follows.

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

The functions carried out by the injection/ignition system are basically as follows: