Treatments of exhaust gases

Conversion of air pollutants to unharmful substances before they are released to atmosphere from outlet of the exhaust system. This can be achieved by fitting special devices to engine systems.

Catalytic Converter

Aim of catalytic converter is to chemically treat the pollutants and convert them into harmless gases like Co to com, HC to H20 and splitting of NO to and .

A catalyst is a material which promotes/enhances chemical change without entering into chemical reaction. The materials, which are used as catalysts are Platinum, Palladium, Rhodium, Iridium etc.

Chemical reaction of conversion of Co to Con and HC to, which normally takes place at temperatures of 5000 to 600 C, occurs at 300° to 350°C in the presence of catalyst.

Fig .1. Representation of catalytic converter

This suits as sending air() to exhaust may not convert CO and HC as the air will be at lower temperature and the temperature of exhaust also comes down quickly while passing through the exhaust tube from exhaust manifold.

Platinum and Palladium are good catalysts for conversion of CO and HC and Rhodium is used for reduction of.

Catalytic converter as shown in Fig.1 is a tubular device integrated into exhaust system. Catalytic converter is filled with thin walled mesh/matrix coated with catalysts.

As the exhaust gases pass through this mesh, HC and CO gets converted to and C. When the mesh is coated with Rhodium, is reduced to and.

Since all the three major pollutants i.e. CO, HC and are to be converted into Rhodium, are used in one converted only.

As three pollutants are converted by one catalytic converter, this type is called 3-way catalytic converter as shown in Fig.2

Fig.2. Three way catalytic converter

Catalysts do not take part in chemical reaction, their life is long but if gasoline contains lead, then then the mesh gets coated with this poisonous lead and converter becomes useless. Therefore, automobiles fitted with catalytic converter should always use unleaded fuel.