Fiat 1996 Brava Service Manual page 286

Bravo-Brava £ p i
6v
Engine
98 range Fuel system
10.
EMISSION CONTROL DEVICES
The devices used for this purpose have two aims:
- to keep down the levels of pollutant substances in the exhaust, through the catalytic silencer;
- to eliminate the dispersion towards the outside of unburnt hydrocarbons, through the (fuel) anti-e­
vaporation system and the (lubricant) oil vapour recirculation system.
CATALYTIC SILENCER
The catalytic silencer is a device which makes it possible to simultaneously keep down the levels of the
three main pollutants in the exhaust: unburnt hydrocarbons (HC), carbon monoxide (CO) and nitrogen
oxide (NOx).
Two types of chemical reaction take place inside the catalyzer:
- oxidation of the CO and the HC, converted into carbon dioxide (CO2) and water (H2O);
- reduction of the NOx, converted into Nitrogen (N2).
These reactions can take place extremely quickly thanks to the presence, inside the catalyzer structure
(ceramic support), of a layer of active substances (platinum and rhodium) which greatly accelerate the
conversion speed of the harmful substances.
The effectiveness of this conversion process is, however, affected by the fact that the mixture strength
with which the engine is operating is constantly oscillating around the stoichiometric value, which is
achieved thanks to the feed-back control carried out by the control unit on the basis of the Lambda sen­
sor.
Lastly, the conversion processes are activated at temperatures in excess of 300-350 °C: it is therefore vi­
tal for the catalyzer to reach this temperature as quickly as possible in order to work properly.
P4A35JJ01
1. Ceramic monolith
2. Metal support
3. Steel outer casing
When work has to done near the catalytic silencer, the vehicle should be left to rest for some time
because the operating temperature (inside) the catalyzer is between 500 and 850 °C.
There are basically two things which destroy the inside of the catalyzer:
- the presence of lead in the fuel, which lowers the degree of conversion to practically nil ("lead
poisoning") and also irreparably damages the Lambda sensor;
- the presence of completely unburnt fuel in the exhaust gases, due to failed ignition, which
causes an increase in temperature which melts the ceramic support. Consequently, the hight
tension leads should not be disconnected with the engine running for any reason: in the case
of tests, the silencer should be replaced with an equivalent section of pipe.
m
0
Copyright by Fiat Auto
35