Fiat 1996 Brava Service Manual page 434

Engine Bravo-Brava l(pi «*
Fuel system '98 range
10.
DIAGRAM OF EXHAUST ASSEMBLY
P4A22IJ01
On the I.A.W. system, the closed-loop control of the mixture strength is activated by the Lamba sensor
which measures the oxygen content present in the exhaust gases upstream of the catalytic converter.
The Lamba probe's measurements enable the electronic control unit to make continuous corrections of
the mixture strength, keeping the air/fuel ratio constant.
In this way, there is control of the harmful emissions in the exhaust, which is completed by the trivalent
catalytic converter.
The efficient operation of the catalytic converter and consequently reduction in toxicity of the exhaust
gases depends on the air/fuel ratio with which the engine is supplied.
The trivalent catalytic converter simultaneously reduces the three polluting gases present in the exhaust
gases: unburnt hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx).
Two types of chemical reactions take place inside the converter:
- oxidation of the CO and HC, converted into carbon dioxide (C0
2
) and water (H
2
0)
- reduction of NOx, converted into nitrogen (N
2
).
The causes which quickly and irreparably put the catalytic converter out of use are:
- presence of lead in petrol, which reduces the level of conversion such that its presence in the system is
futile;
- presence of unburnt petrol in the converter; a flow of petrol lasting 30 s in an environment of 800°C
(silencer's internal temperature) is sufficient to cause the converter to melt and break. It is absolutely
necessary for the ignition system to be working perfectly, so under no circumstances should the
spark plugs be removed when the engine is running; during tests, the silencer must be replaced
with an equivalent piece of pipe.
22
Publication no. 506.670/14