Foreword

Foreword

The Weber-Marelli I.A.W. 18FD system fitted on the Fire 16v engine, belongs to the semi-sequential intermittent type electronic fuel injection systems integrated with static advance, digital,电子 ignition systems, where the injectors are operated in pairs every T.D.C.

GENERAL DESCRIPTION OF THE SYSTEM

The smooth operation of a petrol engine requires the mixture strength (air/fuel weight ratio) to be kept constant for all operating speeds without being affected by the variations in the coolant temperature, intake air and absolute pressure according to the operating conditions.

The quantity of fuel to be injected is therefore directly proportional to the quantity of air drawn in by the engine and, in effect, it is this parameter that the system takes as a reference to implement the injector opening time.

The fuel is injected in FULL-GROUP during the initial stages, in other words for each revolution of the engine the injectors are operated simultaneously, then once the system is synchronized, the fuel is injected semi-sequentially, i.e. for each pair of cylinders at a time established similarly to the firing order.

In this system the stoichiometric metering (= air/petrol weight ratio = 14.7) is also kept constant by means of a Lambda sensor which, by continuously analyzing the quantity of oxygen in the exhaust gases, enables the electronic control unit for the system to constantly correct the quantity of fuel to be injected so that the stoichiometric metering is achieved in the desired operating range.

This injection system is known as the "angular engine rotation speed - intake air density - mixture control" type, more commonly referred to as Speed-Density-Lambda.

The inductive discharge ignition is completely static, in other words there is no distributor, with the power module transferred inside the injection/ignition control unit. The ignition system consists of two coils with high tension twin outlet terminals connected directly to the spark plugs (1-4 and 2-3).

The primary winding for each coil is connected to both the power relay (therefore supplied by the battery voltage) and to terminals 1 and 19 (respectively) of the injection/ignition control unit. The latter controls the supply of the coils calculating the moment to establish internal earth contact for the time required to supply the coils by means of information from the sensors.

The optimum advance for the ignition system is calculated by the control unit on the basis of the engine operating speed and the value of the absolute pressure in the inlet manifold and is then implemented taking into account the time required for charging the ignition coil.

Given the closing characteristics of the secondary circuit (spark plugs in series), the high tension destined to supply the spark plugs will differ in intensity. This is due to the fact that one of the two spark plugs supplied is alternately in a high pressure ambient (compression stroke) whilst the other one is in a low pressure ambient (exhaust stroke).

The current, having to overcome a greater dielectric in the spark plug which is in the compression stroke, creates a more powerful spark, whilst in the other one it is insignificant.