Fiat 1995 Brava, Service Manual

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Bravo-Brava
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Engine
Fuel system
10.
The ignition coil consists of two primary windings connected to control unit (A) via terminals 34 and 35
supplied by battery voltage, and two high tension secondary windings whose outputs are connected di-
rectly to the spark plugs of cylinders 1 -4 and 3-2.
The two spark plug pairs are supplied with high voltage whenever current is cut off in the primary w i n d -
ing by the power module. The high tension solid state distribution exploits the different pressure condi-
tions existing simultaneously in cylinder pairs 1 -4 and 3-2.
In practice, if an air/fuel mixture is present, the compression phase of one cylinder corresponds to the
exhaust phase of the other.
If, under these conditions, the voltages required to set off a spark between the electrodes of a spark plug
are examined, it may be noted that the voltage is very high during the compression stage ( 1 0 - 1 5 kV),
but extremely low during the exhaust stage (in the order of 500 V).
Two sparks therefore cross the secondary circuit of each coil. One of these generates about 500 V and is
in essence a wasted spark because it is triggered in the cylinder in exhaust phase, the other generates 10
- 1 5 or more kV in the cylinder in compression-exhaust stage in order to burn the air fuel mixture in the
cylinder.
ENGINE IDLE CONTROL
The main aim of this strategy is to maintain engine speed at around the mapped setting (warm engine:
850 rpm): the position assumed by the actuator is dependent on engine conditions/rpm and vehicle
speed.
Start-up stage
When the key is inserted, the actuator takes up a position dependent on engine temperature and battery
voltage (open-loop position).
Warming-up stage
The rpm is mainly corrected on the basis of coolant temperature.
With the engine warm, idle management is dependent upon the signal from the rpm sensor. The E C U in-
troduces sustained idle when external loads are activated.
When the accelerator is released when the car is not idling, the ECU governs the actuator (step motor)
by measn of special delivery curve (dash-pot curve), i.e. delays the return of the plunger to its housing to
bring about a reduction in engine braking effect.
F U E L VAPOUR R E C I R C U L A T I O N M A N A G E M E N T
Fuel vapours (legally recognised pollutants) are directed to an active carbon filter and then on to the en-
gine where they are burnt. This is achieved by means of a solenoid controlled by the E C U only when e n -
gine load conditions are appropriate for permitting effective combustion without operation being im-
paired. The E C U compensates for the extra fuel by reducing fuel flow to the injectors.
C O N T R O L L I N G KNOCK
This function detects the presence of knock by processing a signal from a knock sensor. The E C U c o n -
tinually compares signals from the sensor with threshold values, which are in turn continually updated
to take into account background noise levels and engine age.
The E C U is therefore able to detect the presence of knock (or incipient knock) and reduce the ignition
advance until the effect disappears. The advance is then gradually restored to baseline levels.
Over-run
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