Controlling The Mixture Strength; Self-Adjustment - Fiat Bravo Service Manual

Bravo-Brava i § i
98 range

Controlling the mixture strength

N O T E The mixture strength is defined and the ratio is indicatd by the greek fetter a (a/fa).
The stoichiometric ratio is defined and the ratio is indicated by ctst.
theoretical quantity of air required to burn all the fuel injected
The mixture strength is defined and the ratio is indicated by the greek letter X (lambda):
theoretical quantity of air required to burn all the fuel injected
It can easily be deduced that ast= X.
The stoichiometric ratio depends on the type of fuel: for current unleaded petrol it is about 14.7 - 14.8
and corresponds to a Lambda of 1:
A mixture is called rich when the quantity of air is lower than the stoichiometric figure and in that case
the Lambda is X , < 1 :
A mixture is called poor (or lean) w h e n the quantity of air is higher than the stoichiometric figure and in
that case the Lambda is X > 1.
The strategy is designed to correct the "basic" injection times so that the mixture strength constantly os-
cillates at a high Hertz frequency:
NOTE 1 Hz - 1 oscillation per second
In
- cut-off conditions,
or
- in full load engine conditions,
or
- with the engine cold (coolant and air temperature below around 20 °C)
the strategy is disabled.

Self-adjustment

The control unit is equipped with a self-adjustment function w h i c h has the task of memorizing any dif-
ferences between the basic map and the corrections set by the Lambda sensor w h i c h occur repeatedly
during operation. These differences (due to the ageing of the system and engine components) are per-
manently stored in the memory, allowing the adjustment of the system operation to the gradual alter-
ationsin the engine and components compared to when they were new.
The strategy is disabled
If the control unit is replaced, a road test must be carried out w h i c h allows the engine to reach operating
temperature and the control unit to intervene (especially during stops whilst idling) imposing the self¬
-adjustment function.
Copyright by Fiat Auto
quantity of air drawn in by the engine
quantity of fuel injected
quantity of fuel injected
quantity of air drawn in by the engine
during the opening of the charcoal filter
E n g i n e
Fuel system
10.
solenoid valve.
1