a. Fuel metering

a. Fuel metering

With the aquisition of the information concerning the engine speed (n) and the opening of the butterfly valve (alfa), the control unit determines the basic injection time (ti) from a specific map.

Given the differing usage conditions of the engine the injector opening values are optimum when the times are corrected on the basis of the following variations: the coolant temperature (1), the air temperature (2) and the signal sent by the Lambda sensor (3), always using a synchronous type injection timed with the moment of ignition.

By means of the special maps selected by interpolation, the control unit always tries to keep the metering values close to the stoichiometric value (14.7 parts of air to 1 part of petrol).

In order to keep this ratio constant, the electronic control unit uses two different methods to control the opening of the injector.

The first method is synchronous operation where the injector is opened each time a high tension impulse is sent to the spark plugs (diagram A).

The second method is asynchronous operation where the electronic control unit operates the opening of the injector irrespective of the number of high tension impulses sent to the spark plugs (diagram B).

This takes place because in certain conditions, (with too short basic injection times, ≥ 1.4 milliseconds) the mechanical inertia characteristics (hysteresis) of the injector do not allow suitable openings and closures; as a result of this a special strategy must be adopted which is appropriate for the mechanical properties of the injector.