cataltic converter

8/3/2019 Cataltic Converter

1/4

The Operation of a Three-way Catalytic Converter used in a Spark-ignition

(Gasoline) Engine

The three-way catalyst converter is so named because of the number ofexhaust pollutants it converts, these being;

1. Nitrogen Oxides (NOx)

2. Carbon Monoxide (CO)

3. Unburnt Hydrocarbons (HC)

The catalysts used in a 3-way catalytic converter are palladium, rhodium, and

platinum, and their purposes are noted below:

1st Stage Reduction Catalyst. This stage uses a combination of platinum

and rhodium to reduce the volume of NOx emissions. As the NOx gases pass

over the catalysts the nitrogen atoms are attracted to and held by the

catalyst, where they bond with other captured nitrogen atoms forming N2.

The oxygen molecules carry on through the converter in the form of O2 whichwill be used in the oxidization of the unburnt hydrocarbons in the next stage.

-


2/4

2nd Stage Oxidizing Catalysts. The second stage consists of oxidation, here the

exhaust gases containing carbon monoxide and unburnt hydrocarbons passes throughthe matrix where the platinum and palladium are embedded. These react with the

gases converting carbon monoxide to carbon dioxide, and unburnt hydrocarbons to

carbon dioxide and water as shown below;

Carbon Monoxide (CO) + Oxidation = Carbon Dioxide (CO2)

Unburnt Hydrocarbons (HC) + Oxidation = Carbon Dioxide (CO2) and water (H2O).

3rd Stage - Control System. This consists of a sensor that is fitted to the exhaust pipe

before the catalytic converter which monitors the exhaust gases and sends a signal

back to the on-board computer. The computer then adjusts the air/fuel ratio to

maintain stoichiometric conditions. The computer also ensures that there is enough

oxygen in the exhaust gases to allow oxidization of the unburnt hydrocarbons andconversion of CO to CO2 by the platinum and palladium catalysts.


3/4

The Operation of a Two-way Catalytic Converter used in a Compression-ignition

Engine

The two-way catalytic converter is used in compression ignition engines such asdiesels using platinum and palladium as catalysts. The 2-way catalytic converter

operates similar to the 3-way counterpart in that it converts the exhaust gases through

oxidation:

1. Carbon Monoxide (CO) + Oxidation = Carbon Dioxide (CO2)

2. Unburnt Hydrocarbons (HC) + Oxidation = Carbon Dioxide (CO2) and water (H2O).

As can be seen from above, the two-way catalytic converter treats the carbon

monoxide and unburnt hydrocarbons, but does not reduce the NOx emissions from a

diesel engine exhaust gases. When diesel oil is used in a compression ignition engine,

the resultant exhaust gases contain a large quantity of sooty particulates including

elemental carbon; these pollutants must be removed from the exhaust gases.This is accomplished by fitting an NOx trap or absorber to the exhaust system. The 2-

way catalytic converter also has a sensor fitted which transmits the condition of the

exhaust gases to the on-board computer as in the 3-way converter.


4/4

Advantages and Disadvantages of Catalytic Converters

Advantages of Catalytic Converters

Conversion of noxious gases, especially those which form acid rain and

ground ozone (smog) in city centers.Catalytic converters can be recycled, the catalysts being fully reclaimed and

reused.

Disadvantages of Catalytic Converters

Catalytic converters do not fully operate until they warm up so short runs

should be avoided.Catalytic converters increase fuel consumption Possibility of poisoning the

catalysts due to use of leaded fuel or contamination from lube-oil or coolant

into exhaust system.

Possibility of catalysts becoming overheated and partially melting due to theunburnt hydrocarbons build-up. However, modern catalytic converters have

an overheating warning system alarm on the dashboard.

Catalysts used are becoming scarce and more expensive, so much so that gold

has been used to replace platinum.

cataltic converter

Documents