Fiat 1995 Brava, Service Manual

Page: 353 / 544

Engine Bravo ft**
Fuel system
10.
These values have been obtained experimentally, by means of a long series of practical tests carried out
on prototypes at the engine test bench to identify the advances at which it is possible to achieve the best
compromise between the conflicting requirements of maximum power and low consumption and harm-
ful exhaust emissions.
The optimum advance values have then been memorized in the system control unit. During the opera-
tion of the engine, the control unit is constantly informed of the engine speed and load conditions and,
on this basis, it "selects" the advance value from its memory to strike the spark at the spark plug for the
cylinder during the explosion stroke with the optimum advance.
In addition the control unit corrects this value depending on further factors such as engine coolant tem-
perature, intake air temperature, detonation and position of the butterfly valve, so that the ignition point
is always optimum.
The information required by the control unit to operate the single coils is transmitted by means of electri-
cal signals emitted by the following sensors:
a. An rpm sensor which generates a single phase alternating signal, whose frequency indicates the
engine rpm.
b. An air f l o w meter which, on the basis of the quantity of air drawn in by the engine, transforms this
value into an electrical signal, sending it to the electronic control unit.
c. An air temperature s e n s o r which transforms the value of the intake air temperature into an elec-
trical signal, sending it to the electronic control unit.
d. T w o detonation s e n s o r s which are positioned in the upper part of the crankcase, one between
cylinders 1 and 2 and the other between cylinders 4 and 5, allowing the control unit to recognize the
cylinder which is detonating (or is in the early stages of detonation) and to correct the ignition ad-
vance for the spark plug for the cylinder involved only.
e. A butterfly valve position sensor which transforms the angular value of the actual butterfly into
an electrical signal allowing the control unit to recognize the conditions of minimum, partial or full
load.
Functions of the system
In addition to electronically controlling the moment of ignition and the air flow rate during idling in order
to allow the engine to run smoothly when the environment parameters and the load conditions vary, the
control unit must also control and manage the injection in such a way that the stoichiometric ratio (air/¬
fuel) is always within the optimum values.
The electronic control unit establishes the «time» for the injectors following a relatively simple rule
which can be summed up as follows.
Taking the physical characteristics of the fuel (viscosity and density) and the difference in pressure be-
tween the fuel and the pressure in the inlet manifold as constant, the quantity of fuel injected depends
only on the «opening time» of the injector.
The functions carried out by the injection/ignition
- adjusting the injection times;
- adjusting ignition advances;
- checking cold starting;
- checking the enrichment during
acceleration;
- fuel cut off during deceleration (Cut-off);
- checking and managing engine idle speed;
- limiting the maximum number of revs;
- checking combustion via the Lambda sen-
sor;
system are basically as follows:
- fuel vapour recovery;
- checking detonation
- checking the phase transformer
- connection with the climate control system
(where fitted);
- self-adjustment of the system
- connection with the engine starting immo-
bilizing device (Fiat C O D E ) ;
- autodiagnosis.
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