Marea- Marea Weekend ©
99 u p d a t e
16v
Engine
Fuel system
10.
EMISSION CONTROL DEVICES
These devices have two aims:
- to keep down the levels of pollutant substances present in the exhaust, by means of the catalytic si-
lencer;
- to eliminate the dispersion into the atmosphere of the unburnt hydrocarbons, through the (fuel) anti-
evaporation system and the (lubricant) oil vapour recirculation system.
CATALYTIC SILENCER
The catalytic silencer is a device which makes it possible to keep down the levels of the three main pollut-
ant compounds present in the exhaust simultaneously: unburnt hydrocarbons (HC), carbon monoxide
(CO) and nitrogen oxides (NOx).
Two types of chemical reactions take place inside the catalyzer:
- oxidation of the CO and 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 structure (ceramic
support) of the catalyzer, 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 conditioned by the fact that the mixture strength on which
the engine is running is continuously fluctuating 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 sensor signals.
Lastly, the conversion processes are activated at temperatures in excess of 300 - 350°C: it is therefore vital
for the catalyzer to reach this temperature as quickly as possible in order to be able to work properly.
1. Ceramic structure
2. Metal support
3. Outer steel casing
when work hs to be carried out in the vicinity of the catalytic silencer, the vehicle must be left for
a certain length of time because the operating temperature inside the catalyzer is between 500
and 850'C.
There are basically two causes which can destroy the inside of the catalyzer:
- the presence of lead in the fuel which lowers the degree of conversion to practially zero (lead
poisoning) and irreparably damages the Lambda sensor as well;
- the presence of completely unburnt fuel in the exhaust gases, due to failed ignition, which
causes an increase in temperature which leads to the ceramic support melting. As a result, the
connector for the coils must never, under any circumstances, be disconnected with the engine
running: in the case of tests, the silencer must be replaced with an equivalent length of pipe.
Copyright Fiat Auto
41
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