Control Of The Mixture Strength (Feed-Back Control); Self-Adjustment - Fiat Bravo/Brava Workshop Manual

Bravo- Brava ®»«* Engine
1999 update Fuel system
la
Control of the mixture strength (feed-back control)
NOTE This is defined as the mixture strength and the following ratio is denoted by the Greek letter a
(alfa):
quantity of air drawn in by the engine
quantity of fuel injected
This is defined as the stoichiometric mixture and the ratio is defined by a st:
theoretical quantity of air required to burn all the fuel injected
quantity of fuel injected
This is defined as the mixture strength and the ratio is defined by the Greek letter X (lambda):
quantity of air drawn in by the engine
theoretical quantity of air required to burn all the fuel injected
It is easy to deduce that a / a st = X.
The stoichiometric ratio depends on the type of fuel: for current unleaded petrol (lead-free) it is around
14.7 - 14.8, which corresonds to a Lambda of 1.
A mixture is known as rich () when the quantity of air is below the stoichiometric level and in this case
the Lambda is < 1:
a mixture is known as poor (or weak) when the quantity of air is higher than the stoichiometric level and
in this case the Lambda is > 1.
The strategy is designed to correct the "basic" injection times so that the mixture strength constantly fluc­
tuates at high frequency between 0.98 and 1.02.
The fluctuation frequency varies according to the engine load and speed: it is in the order of tens of Hertz.
NOTE 1 Hz = 1 fluctuation per second
In the following conditions:
- cut-off,
- butterfly opening of more than 70° and high engine load,
- engine temperature below 25°C,
- the strategy is disabled.
Self-adjustment
The control unit is equipped with a self-adjustment function which has the task of memorizing any differ­
ences between the basic map and the corrections imposed by the Lambda sensor which may persistently
occur during operation. These differences (due to the ageing of the system and engine components) are
permanently memorized, allowing the adaptation of the system operation to the gradual alterations in the
engine and the components compared with when they were new.
The strategy is disabled whilst the charcoal filter solenoid valve is open.
If the control unit is replaced, the vehicle must be road tested allowing the engine to reach operating tem­
perature and the control unit to intervene in the self-adjustment mode (especially during idling stops).
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