Fiat Bravo Service Manual page 217

Bravo-Brava
Checking mixture strength
NOTE The following ratio is defined as the mixture ratio and is denoted by the Greek lettercx (a/fa):
The following ratio is defined as the stoichiometric mixture and is denoted by cxst :
The following ratio is defined as the mixture strength andis denoted by the Greek letter X (lambda):
It can easily be deduced that a / ast = X .
The stoichiometric ratio depends on the type of fuel: for current unleaded petrols it is around 14.7 - 1 4 . 8
which corresponds to a Lambda strength of 1.
We talk of a mixture being rich when the quantity of air is lower than the stoichiometric ratio in which
case the Lambda < 1:
we talk of a mixture being poor (or lean) when the quantity of air is higher than the stoichiometric ratio
in which case the Lambda >1,
The strategy has the function of correcting the "basic" injection times so that the mixture strength c o n -
stantly oscillates at a high frequency between 0.98 and 1.02.
The oscillation frequency varies according to the engine load and speed: it is in the order of tens of
Hertz.
NOTE iHz-1 oscillation per second
In conditions of:
- cut-off,
- butterfly opening above 70°,
- engine temperature below 25°C,
the strategy is disabled.
f
Self-adjustment
The control unit is equipped with a self-adjustment function which has the task of memorizing any dif-
ferences between the basic map and the corrections set by the Lambda sensor which may occur during
operation. These differences (due to the ageing of the system and engine components) are memorized
permanently, allowing the adaption of the operation of the system to gradual alterations of the engine
and the components in relation to the characteristics when new.
The strategy is disabled when the charcoal filter solenoid valve is open.
If the control unit is replaced it is necessary to carry out a road test which allows the engine to reach op-
erating temperature and the control unit (above all during idling stops) to operate in the self-adjustment
mode.
Copyright by Fiat Auto
quantity of air drawn in by the engine
quantity of fuel injected
theoretical quantity of air to burn all the fuel injected
quantity of fuel injected
quantity of air drawn in by the engine
theoretical quantity of air to burn all the fuel injected
Engine
Fuel system
10.
7