Injection-ignition system diagram (page 2 of 2)

Two essential conditions must be satisfied during air-fuel mixture preparation for efficient operation of controlled ignition engines:

1. metering (air/fuel ratio) must be maintained as close as possible to a stoichiometric level in order to ensure swift combusion and avoid pointless waste;
2. the mixture must be made up of petrol vapour distributed through the air as finely and evenly as possible.

In the ACG MULTEC-XM system, the injector nozzles distribute the petrol by nebulising it into tiny droplets. Because the absolute pressure of intake air may vary, the amount of fuel to be injected must be adjusted in order not to alter the air-fuel weight ratio. This ratio is maintained constant by altering the fuel feed pressure by means of a regulator according to inlet manifold vacuum level. In this way, the difference between the two pressures is maintained constant under all engine service conditions.

Optimal metering is calculated on the basis of the following measurements:

• Absolute pressure in inlet manifold
• Coolant temperature
• Inlet air temperature
• Level of oxygen in exhaust gas
• EGR percentage

Base pulse width is calculated through indirect measurement of engine load, achieved by measuring absolute pressure in the inlet manifold and rpm (Speed Density).

The following main parameters are considered when calculating injection time:

• Base pulse constant
• Absolute pressure
• Air temperature
• Metering (A/F)
• Volumetric efficiency
• Battery voltage
• Self-adaptive factor
• Lean-burn factor during over-run
• Closed-loop correction"
• Enrichment during acceleration.

The part of the equation directly concerned with the Speed Density measurement is given by the product of base pulse constant (BPC) and absolute pressure and volumetric efficiency divided by the product of air temperature and metering (A/F) carried out by the microprocessor every 15.6 ms.

The ECU governs the injectors electrically. These are connected in pairs in parallel, and inject two by two in asynchronous fashion, with injection start instant, mappable up to 180° after the 7th tooth (cylinders 2-3) and the 37th tooth (cylinders 1-4) of the phonic wheel.

Injection delay depends on engine speed and throttle angle.