The conventional ignition distributors of the transistor based systems with mechanical and pneumatic correction of the advance can make only very simple correction curves, that satisfy very approximately the needs of the engine.
With the electronic ignition the mechanical corrector of the advance of the ignition distributor has been eliminated.
In its place as information of the number of rpm the signal of the transducer for the trigger of the ignition is used.
A supplementary pressure sensor detects the load of the engine.
The microcomputer calculates the necessary correction of the ignition advance and correspondingly modifies the output signal, that is transmitted to the final power module.
The correction of the ignition can be adapted in the best possible way to the numerous individual needs imposed to the engine (figure 24).

With the electronic ignition it is possible to include new control parameters (for example the engine temperature); there is a better behaviour at starting, a better regulation of the minimum load and reduced fuel consumption; there is the detection of a greater number of operating data and the possibility to make the regulation of the head-beat.
The diagram of the ignition angle allows to illustrate in the best way the advantages provided by the electronic ignition.
It contains the most suitable ignition angle chosen as a compromise in engine design phase, for each possible operating point of the engine, that is for each number of rpm and load. The ignition 'angle for a given operating point is chosen keeping into account fuel consumption, torque, discharge gas, distance from the beat limit, engine temperature, driving comfort, etc.
Such factors assume greater or smaller importance according to the optimization criterion.

Therefore, the diagram of an electronic correction of the ignition often has a very rough surface with respect to the diagram of a centrifugal masses or depression type mechanical correction system of the ignition.
If we had to represent also the usually non linear effect of the temperature or of another correction function, we should have used a four dimension diagram that could not be represented.

Operating principle
The signal emitted by the depression sensor is used as a load signal for the ignition. On the basis of this signal and of the number of rpm a three dimension diagram of the ignition is drawn; this allows to program the most favourable ignition angle (on the vertical axis) for the discharge gas and the consumption in correspondence of each point of the number of rpm and of the load (horizontal axes).
The map of the characteristic curves is composed, as a function of the specific needs, of 1000 to 4000 ignition angles that can be recalled one by one.
With the throttle valve closed the special characteristic line of the minimum and of the release phase is used.
For rpm numbers lower than the nominal rpm number of the minimum the ignition angle can be corrected toward advance, to obtain a stabilization of the minimum through the increase of the torque.
For the release phase the ignition angles are programmed with particular respect to the discharge gas emission and to a good running behaviour.
At full load the characteristic line of full load is used, because on it the ignition value is programmed in consideration of the limits of the head-beat.

In some systems, for the starting it is possible to program a behaviour of the ignition angle as a function of the number of rpm and of the temperature of the engine, but independent from the ignition program.
In this way a higher torque at starting is obtained, without the creation of reaction torques. The programming complexity of the diagrams changes according to the needs; there are also programmed diagrams with few correction curves.

There are two possibilities for measuring the number of rpm and the synchronization on the engine shaft: the signal can be taken directly on the engine shaft or on the cam shaft or on an ignition distributor equipped with a Hall barrier.
The advantages offered by an ignition diagram of the shown shape can be exploited with the greatest accuracy by a transducer of the number of rpm on the engine shaft.

Input signals (fig. 25)
Number of rpm, position of the engine shaft and pressure of the aspiration manifold are the main control parameters of the ignition point.

Number of rpm and position of the engine shaft
The number of rpm is detected by a pulse inductive transducer, which feels the teeth of a special cogwheel mounted on the engine shaft (figure 26).

	Fig. 26: toothed disc (on the engine shaft) with inductive transducer With this system we have a variation of the magnetic flux that induces an alternate voltage (figure 27), which is evaluated by the electronic board. Fig. 27:  Behaviour of the inductive voltage

The cogwheel has an opening that is detected by the pulse inductive sensor and elaborated by a special circuit for the exact detection of the angle of the engine shaft.

Load (pressure in the aspiration manifold)
The pressure in the aspiration manifold operates on the pressure sensor through a pipe.
In addition to the pressure of the aspiration manifold, that is only useful for an indirect measurement of the load, mainly the mass and the quantity of air in the time unit are suitable to be used as load signals, because they offer a better index of the cylinder filling range, which represents the true load.
The load signal used for the preparation of the mixture can therefore be used also for the ignition in engines equipped with an electronic ignition system.

Position of the throttle valve
A switch on the throttle valve sends switching signals corresponding to the minimum and full load regime of the engine.

Temperature
The sensor of the temperature of the cooling fluid, mounted in the engine block sends to the electronic board a signal corresponding to the engine temperature.
The temperature of the aspired air can be measured by another sensor in the same way.

Battery voltage
Also the voltage of the battery is a correction parameter that is detected by the electronic board. With low battery voltage, the dwell time is lengthened (and consequently reduced the opening time).

Signal processing (fig. 28).
Pressure of the aspiration manifold, engine temperature, air temperature and battery voltage are analog parameters that are digitized by the analog-digital converter, while number of rpm, positions of the engine shaft and locks of the throttle valve, which are digital parameters, enters directly in the microcomputer.
The signal processing is done in the microcomputer, which is made of a microprocessor with quartz oscillator for the formation of the cadence.
The processor re-calculates for each ignition the temporary values of the ignition angle and of the dwell time, to send, in the shape of an output quantity, the optimum ignition point in each operating phase of the engine.

Fig. 28:  processing of the signals in the ignition electronic control board (block diagram). 1   RPM  e  PMS 2   Switch signals on the throttle valve 3   Pressure of the aspiration manifold 4   Engine temperature 5   Aspired air temperature 6   Battery voltage 7   Microcomputer 8   A/D converter 9   Final ignition stage

Ignition output signal
The primary circuit of the ignition coil is switched from the power final stage of the electronic board.
The dwell time is regulated so that the voltage of the secondary remains almost constant independently from the number of rpm and the battery voltage.
Since the dwell time (dwell angle) is determined for each couple of values, number of rpm and battery voltage, it is necessary another diagram: the diagram of the dwell angle, represented in figure 29.

Fig. 29:  characteristic diagram of the dwell angles. It contains a network of reference points; among them there is an interpolation as for the ignition angle diagram. The energy stored in the ignition coil can be imeasured with the same accuracy obtained in the regulation of the dwell angle by using a similar diagram.

However, there are also electronic ignition systems, where the diagram is still subordinated to the regulation of the cam angle, that optimizes the dwell angle of each cylinder independently from the other cylinders.

Electronic board
As shown in the block diagram of figure 28, the nucleus of a board for an electronic ignition is composed of a microcomputer.
This microcomputer contains all the data, including the diagrams, and the programs for the detection of the input quantities and for the calculation of the output quantities.
Since the sensors are usually electromechanical components adapted to the difficult engine operating conditions, it is necessary to prepare the signals for the processor.
The signals coming from the transducers in the form of pulses (for example the signal of the transducer of the number of rpm) are converted to digital signals defined in the circuits making the pulses.
The output quantities of the sensors (as the temperature and pressure sensors) often consist of an analog electric signal .
This analog signal is transformed by an analog digital converter and sent to the processor in the form of a digital quantity.
The analog - digital converter can also be integrated in the microcomputer.

Final ignition stage
The electronic boards are made in printed circuit or hybrid technique.
Consequently, the ignition final stage can be integrated in the electronic board (as shown in the block diagram of figure 28) or mounted externally, usually in combination with the ignition coil.
If the ignition final stage is external, the electronic board is usually mounted in the inside, while such assembling is more unusual for electronic boards with integrated ignition final stage.
The electronic boards with integrated ignition final stage which are mounted in the engine space must have a particularly good heat dissipation.
Consequently, the hybrid technique is used.
The semiconductor elements, hence also the final stage, are mounted directly on the cooling body which guarantees the thermal contact with the vehicle body.
Then, the electronic boards can be used at ambient temperatures up to values higher than 100 °C.
Furthermore, the hybrid circuit boards have the advantage of being small and light.

Other output quantities
In addition to the ignition final stage, there are other outputs for other quantities that depend on the different type of use, for example the outputs for the signals of the number of rpm and the signals of the variables for other electronic boards such as the injection, the diagnosis signals, the switching signals for the control of injection pumps or relays, etc.
The electronic ignition can be very well combined with other engine control functions. Together with the electronic injection it makes the basic version in a single electronic board.

Another widely used solution consists in joining together the electronic ignition with the beat regulation. Such combination is particularly advantageous because the correction of the ignition angle toward delay represents the most efficient and the quickest intervention to avoid engine beat.