Fiat Bravo Service Manual page 355

Engine Bravo ign ^
Fuel system
10.
Before the idle speed is reached the dynamics of the engine speed decrease are checked.
If they exceed a certain value, the fuel supply is partly reactivated on the basis of a logic which involves
the "soft accompaniment" of the engine at idle speed.
When this condition is reached the normal idle functions are reactivated and the cut off during decelera­
tion is only reactivated if the fuel cut off level is exceeded to ensure that the engine runs smoothly. The
levels at which the fuel supply is restored and the cut off activated depend on the temperature of the en­
gine.
Another fuel cut off logic is developed in the engine which intervenes during partial deceleration, i.e.
when lower engine loads are required.
The function is only activated if the new condition persists for a pre-set time and after the ignition ad­
vance angle has been adjusted to the new situation.
Checking and managing engine idle speed
The adjustment of the idle speed is controlled in all operating conditions by means of the idle speed ac­
tuator which acts on the butterflyl by-pass.
In addition to controlling the actual idle speed it also carries out the functions of a supplementary air
valve and regulator for switching on the various consumers (e.g. the climate control compressor); with
the butterfly in the end of travel position, in effect the actuator regulates the by-pass opening compen­
sating for the power required by the consumers to ensure an engine idle speed of 750 rpm.
The actuator fitted on this version guarantees great responsiveness to adjustment because the opening
and the closing of the by-pass are both operated by magnetic windings.
The correction of the idle is carried out not only by the actuator but also by the adjustment of the ignition
(advance) angle because it has a more rapid effect.
NOTE The self-adjustment function makes it possible to dispense with all idle speed adjustments with
the butterfly casing sensor recognizing the "butterfly in the end of travel" position and correct­
ing any wear which occurs in time and is detected by the butten'ly closure position.
Limiting maximum number of revs (protection outside of revs)
When the engine speed exceeds 6,800 rpm, the electronic control unit reduces the injector operating
times so that this value is not exceeded.
If the engine speed exceeds 7,013 rpm, the control unit activates the "fuel cut off" strategy restoring the
operation of the injectors when the speed goes below 6,800 rpm.
Checking combustion via the Lambda sensor
The Lambda sensor informs the control unit of the quantity of oxygen present at the exhaust and there­
fore the correct air/fuel metering.
The optimum mixture is obtained with a Lambda coefficient of 1 (optimum stoichiometric mixture). The
electrical signal which the sensor sends to the control unit undergoes a sharp variation when the com­
position of the mixture differs from Lambda= 1. When the mixture is "lean" (^>1) the control unit in­
creases the quantity of fuel, when the mixture is "rich" (X< 1) it decreases it: in this way the engine op­
erates as close as possible to the ideal Lambda value.
The Lambda sensor signal is processed, inside the control unit, by a special integrator which prevents
sharp variations in the injection times to correct the mixture strength.
The sensor is heated by an electrical resistance in order to be able to reach the correct operating temper­
ature of around 300 °C rapidly.
By using this sensor it is possible to regulate the engine carburation with precision. This makes it possi­
ble, amongst other things, to operate at the limits laid down for emissions.
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Print no. 506.670