United Nations Environment Programme

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Tool 12: Emissions controls for petrol vehicles

The 3-way catalyst: Today, in nearly all parts of the world, petrol vehicles are equipped with a 3-way catalytic converter, which reduces emissions by over 90%. Effective operation of a 3-way catalyst requires unleaded petrol, because lead contaminates the catalyst. Lowering sulphur levels in petrol (petrol also contains sulphur but at levels substantially lower than diesel) reduces emissions of SOx and increases the efficiency of the 3-way catalyst.

Emissions reduction:The 3-way catalytic converter reduces CO, HC and NOx emissions (hence the name “3-way”). These catalysts require that engines run with just enough air to burn the fuel (called the stoichiometric air to fuel ratio), and are therefore combined with an advanced electronic fuel and air injection system that constantly measures and controls this ratio. Sulphur levels in unleaded petrol are generally between 150 and 1500 ppm; in some countries, notably Japan, petrol with near-zero sulphur levels is sold. Lower fuel sulphur levels result in better emissions control, because the sulphur competes with the other reactants. However, sulphur does not poison the 3-way catalyst in the same way as lead does, and most 3-way catalysts are effective even at relatively high sulphur levels

Installed when you buy the vehicle: 3-way catalysts are installed as standard equipment by the vehicle manufacturer. Globally, 85% of new petrol vehicles are equipped with 3-way catalysts, as are a large number of 2- and 3- wheel vehicles.

3-way catalysts can be retrofitted into older vehicles; however, this requires the installation of advanced air-to-fuel measurement equipment and an electronic fuel injection system. This makes retrofits of petrol vehicles (mostly passenger cars) prohibitively expensive, and is therefore unusual. However, a poisoned 3-way catalyst can be exchanged relatively easy for a new well-functioning one. For cars that once had a catalyst installed (such as cars imported into developing countries that previously did not have unleaded petrol), it is easily to re-install the catalyst.

For more info on emission reduction technologies >> www.epa.gov