Fiat Bravo Service Manual page 152

Bravo-Brava
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 concen-
tration check.
In practice this system uses ENGINE S P E E D (revs per minute) and AIR DENSITY (pressure and temper-
ature) 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 ac-
cording to engine speed and intake manifold pressure. In practice, the the E C U 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, bat-
tery voltage, etc.) allow the ECU to correct the baseline strategy for each specific engine service condi-
tion.
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 E C U . 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 C O 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 lettera (a/fa) as follows:
Copyright Fiat Auto
&|
lev
quantity of air taken in by engine
a =
quantity of fuel injected
Engine
Fuel system
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