Foreword

Foreword

The Weber-Marelli systems fitted to 1108 c.c. and 1242 c.c engines of the “Punto” belong to the category of integrated systems: ignition with static electronic advance and distribution and intermittent electronic single-point petrol injection, i.e. with only one injector.

PRINCIPLE OF OPERATION

In this system, fuel is injected by the injector upstream from the throttle valve upon each engine TDC, with a low output pressure (about 1 bar).

In order for a petrol engine to work efficiently, the mixture ratio (air-fuel weight ratio) should be maintained constant throughout all service speeds without being affected by changes in coolant temperature, air intake or absolute pressure - except under certain service conditions.

The amount of fuel to be injected is therefore directly proportional to the amount of air taken in by the engine. In fact the engine uses this parameter to control the injector opening time. In this system, the stoichiometric dose (= air-fuel weight ratio = 14.8) is also maintained constant by means of a Lambda probe. This continually monitors the amount of oxygen present in the exhaust gas to enable the ECU to correct the amount of fuel to be injected continuously in order to achieve a stoichiometric ratio throughout the required service range.

This injection system is termed an “angular engine rotation speed - intake air density - retro-active concentration control” type, more commonly known as Speed-density-lambda.

Because for certain service conditions the amount of fuel required for each engine cycle is too little and thus difficult to calculate, the system is able to govern an asynchronous injection in addition to the usual synchronous injection (upon each ignition at TDC). The asynchronous injection is achieved by maintaining injection time for longer than calculated and modulating injector closure time.

The ignition is fully static inductive discharge type, i.e. without a distributor, with a power module transferred inside the ignition-injection ECU. The ignition system consists of two coils with two high tension outlets connected directly to spark plugs (1-4 and 2-3). The primary winding of each coil is connected to both the power relay (i.e. supplied from the battery) and terminals 1 and 19 (respectively) of the ECU. The ECU activates the coils and computers, on the basis of information from the sensors, the correct instant for establishing the internal earth connection for the time necessary for activating the coils.

The optimum ignition system advance is calculated by the ECU on the basis of engine speed and absolute pressure in the inlet manifold and therefore implemented taking into account the time required to charge the ignition coil.

The high tension used to supply the spark plugs differs in intensity due to the nature of the secondary circuit (spark plugs in line). This is due to the fact that one of the two spark plugs activated is alternately positioned in a high pressure environment (compression stage) whereas the other is in a low pressure environment (exhaust stage). Because the current must overcome a greater dielectric gradient in the spark plug undergoing compression, a more powerful spark is produced, whereas the spark from the other is negligible.