Fiat 1996 Brava Service Manual page 416

Engine Bravo-Brava >£p i
6v
Fuel system '98 range
10.
The stoichiometric ratio is obtained using a heated Lambda probe. This probe, by means of constant
analysis of oxygen present in the exhaust gases, informs the control unit which, on the basis of data
memorized in it, can correct in real time the mixture strength (air-petrol) if it is not stoichiometric.
This system has no method of recording the engine idle speed and CO level in the exhaust gases, or of
the throttle valve position.
Mixture strength control (feedback control)
NOTE The following ratio is defined as mixture ratio, and it is indicated by the letter a (alpha):
quantity of air drawn in by engine
a =
quantity of injected fuel
The following ratio is defined as stoichiometric mixture and is indicated by ast:
quantity of theoretical air for burning all the injected fuel
ast=
quantity of injected fuel
The following ratio is defined as mixture strength and is indicated by the Greek letter X (lambda):
quantity of air drawn in by the engine
quantity of theoretical air for burning all the injected fuel
It is easy to observe that oc/ocst=X.
The stoichiometric ratio depends on the type of fuel: for the current non-ethylated (unleaded) petrols, it
is about 14.7 -14.8, which corresponds to a Lambda strength = 1 (ratio 14.8:1 means that 14.8 parts air
are required to burn 1 part petrol).
The mixture is rich when the quantity of air is less than the stoichiometric value, and in this case lambda
< 1;
The mixture is weak when the quantity of air is greater than the stoichiometric value, and in this case
lambda > 1 ;
The strategy has the function of correcting the "basic" fuel injection times so that the mixture strength
fluctuates continuously between 0.98 and 1.02.
The oscillation frequency varies depending on the engine load and speed; it is in the region of a few
Hertz (about 0.5 - 4 Hertz).
NOTE
1 Hz = 1 oscillation per second
In the following conditions:
- cut-off,
- throttle 60% open (at low engine speeds this value is lower)
- engine temperature below 25 °C
the strategy is disabled.
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Publication no. 506.670/14