Adjustment Of Injection Times; Ignition Advance Adjustment; Fuel Cut-Off During Overrun - Fiat 1998 Marea Service Manual

Engine
Fuel system
10.

Adjustment of injection times

Digital technology has made it possible to ensure optimum consumption and performance using pro-
grammed maps, stored inside the control unit memory, according to the engine speed and load conditions.
The control unit operates the injectors with great speed and precision, calculating the opening time on the
basis of the engine load (number of revs and air flow rate), also taking into account the battery voltage
and the temperature of the engine coolant.
The injection is sequential and phased for each cylinder (the moment of injection is not simultaneous for
all cylinders) and takes place at the optimum injection point.

Ignition advance adjustment

Thanks to a map stored in its memory, the control unit is capable of calculating the advance according to
the engine load (minimum, partial, full - on the basis fo the engine speed and air flow rate), the tempera-
ture of the intake air and the temperature of the engine coolant.
The ignition can be selectively delayed at the cylinder as required, recognized by means of the combina-
tion of values recorded by the cam angle and detonation sensors.
Controlling cold starting
Under these circumstances there is a natural weakening of the mixture as a result of the poor turbulence
of the particles of fuel at low temperature and reduced evaporation and condensation on the internal walls
of the inlet manifold, all of which is exacerbated by the increased viscosity of the lubricant oil.
The electronic control unit recognizes this condition and corrects the injection time according to the signal
for the coolant temperature, the intake air temperature, the battery voltage and the engine speed.
The ignition advance is only dependent on the number of revs and the temperature of the engine coolant.
During cold starting, the control unit carries out an initial simultaneous injection for all the injectors (full
group) and, after recognizing the reference on on the flywheel, it switches to normal sequential, phased
operation. Warm starting takes place immediately in a sequential, phased manner.
Controlling the enrichment of the mixture during acceleration
The control unit detects the request for acceleration from the signal from the accelerator pedal potenti-
ometer and consequently increaes the injection times and modifies the opening of the butterfly valve to
reach the number of revs required quickly.
If, following an acceleration request, the variation in the signal from the air flow meter exceeds a pre-set
increase, the control unit, not only adapts the injection to the new requirement, but it also increases it
further for several seconds to improve the responsiveness.

Fuel cut-off during overrun

When the accelerator pedal is released and beyond a pre-set engine rpm level, the injection control unit
cuts off the supply to the injectors.
If there is no supply, the number of revs will start to decrease at a rate dependent on the vehicle driving
conditions.
Before the idle speed is reached, the dynamics of the decreaes in the engine revs is verified.
If this is above a certain value, the fuel supply is partly restored according to a logic which involves the
"soft accompaniment of the engine" during idling.
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SYSTEM MANAGEMENT STRATEGIES
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