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, through the catalytic silencer;
• to eliminate the dispersion towards the outside of 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 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 catalyst:

• 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 catalyst 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 sensor.

Lastly, the conversion processes are activated at temperatures in excess of 300-350 °C: it is therefore vital for the catalyst to reach this temperature as quickly as possible in order to work properly.

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 catalyst is between 500 and 850 °C.

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

• 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.