Emission control devices

EMISSION CONTROL DEVICES

The devices used for this purpose have two aims:

• - to keep down the levels of pollutant substances in the exhaust via the catalytic silencer;
• - to eliminate the dispersion of unburnt hydrocarbons into the atmosphere, by means of 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 simutaneously reduce the levels of the three main pollutant compounds present at the exhaust: unburnt hydrocarbons (HC), carbon monoxide (CO) an nitrogen oxides (NOx).

Two types of chemical reaction 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 (N₂O).

These reactions 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 efficiency of this conversion process is affected by the fact that the mixture strength on which the engine runs is continuously fluctuating around the stoichiometric value which is achieved through the feedback control carried out by the control unit usnig 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 operations have to be carried out in the vicinity of the catalytic silencer, the vehicle must be left to rest for a while since the operating temperature inside the catalyzer is between 500 and 850°C.

There are basically two things which can destroy the inside of the catalyst, namely:

• - the presence of lead in the fuel which lowers the degree of conversion to practically 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 should not be disconnected, for any reason, with the engine running: in the case of tests, the silencer must be replaced, as a precaution, with a suitable length of pipe.