Marea-Marea Weekend®
99 range
20v
Engine
Fuel system
ENGINE EXHAUST ASSEMBLY DIAGRAM
10.
1. Exhaust manifold
2. Lambda sensor
3. Catalytic silencer
4. Silencers
5. CO intake
The closed-loop control of the mixture strength is activated by the Lambda senor which measures the
oxygen content of the exhaust gases upstream of the catalytic silencer.
The Lambda sensor measurements allow the electronic control unit to continuously correct the quantity of
petrol injected keeping the air/fuel ratio constant.
This means that the harmful exhaust emissions are controlled, a task completed by the three-way catalytic
converter (silencer).
The efficient operation of the catalytic silencer and the consequent restriction of the toxic exhaust gases
depends on the air/fuel ratio being received by the engine.
The three-way catalytic converter makes it possible to simultaneously keep down the levels of the three
pollutant gases present in the exhaust gases: unburnt hydrocarbons (HC), carbon monoxide (CO) and ni-
trogen oxides (NOx).
Two types of chemical reaction take place inside the converter:
- oxidation of the CO and HC, converted into carbon dioxide
(CO2)
and water
(H2O);
- reduction of the NOx, converted into nitrogen
(N2)
The following can put the catalytic converter out of action and damage it irreparably:
- the presence of lead in the petrol which lowers the degree of conversion to such an extent that the pres-
ence of the catalytic converter in the system is rendered superfluous;
- the presence of unburnt petrol in the converter: in effect a flow of petrol lasting 30s in an ambient of
800 °C (temperature inside the silencer) is sufficient to cause the catalyzer to melt and break.
It is vital for the injection/ignition system to be working perfectly, therefore
the spark plug leads
should not be disconnected, for any reason, with the engine running and the catalytic con-
verter should be replaced with an equivalent length of pipe when carrying out tests.
Copyright by Fiat Auto
13
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