Injection management

INJECTION MANAGEMENT

The ignition management strategies are designed to provide the engine with the correct amount of fuel at the right time in accordance with engine service conditions.

The Marelli I.A.W.-IAF.13 injection-ignition system uses an indirect measurement system known as «SPEED DENSITY - LAMBDA», i.e. angular speed of rotation, intake air density, retro-active concentration check.

In practice this system uses ENGINE SPEED (revs per minute) and AIR DENSITY (pressure and temperature) to measure the amount of air taken in by the engine.

The amount of air taken in by each cylinder upon each engine stroke depends on intake air density and also on standard displacement and volumetric efficiency.

Air density is taken to be the density of air taken in by the engine and calculated as a function of absolute pressure and temperature - both measured in the inlet manifold.

Volumetric efficiency is a parameter relating to the cylinder filling coefficient. It is calculated on the basis of experimental tests carried out on the engine throughout its service range and then memorized in the ECU.

Once the amount of air taken in has been established, the system must provide sufficient fuel to make up the required fuel mixture concentration.

The injection trigger pulse or time at which fuel delivery begins is mapped in a control unit and alters according to engine speed and intake manifold pressure. In practice, the the ECU performs calculations in order to control sequential, phased opening of the four injectors, one per cylinder for the length of time strictly necessary to form an air-fuel mixture as close as possible to a stoichiometric ratio.

Fuel is injected directly into the manifold close to the intake valves at a differential pressure of some 3 bar.

Speed (no. of revolutions per minute) and air density (pressure and temperature) are used to measure intake air quantity, which is used in turn to calculate the amount of fuel needed to achieve the required mixture proportions. The other sensors in the system (coolant temperature, throttle valve position, battery voltage, etc.) allow the ECU to correct the baseline strategy for each specific engine service condition.

This specification is fitted with a catalytic converter to abate polluting emissions. In order to keep this working efficiently, the air-fuel ratio must be maintained at near stoichiometric levels.

A stoichiometric ratio is achieved by using a hot lambda probe. This probe continually monitors the amount of oxygen in the exhaust gas and then informs the ECU. This unit in turn uses mapped data to correct the fuel air mixture on-line if proportions are no longer stoichiometric.

Idle speed, exhaust gas CO level and butterfly valve sensor position are not adjusted on this system.

Control of mixture concentration (retro-active control)

NOTE Mixture ratio is defined and indicated by the Greek letterα (alfa) as follows:

α = quantity of air taken in by engine / quantity of fuel injected