Three-way catalyst (TWC)

Operation of a three-way catalyst

A three-way catalyst oxidizes exhaust gas pollutants - both hydrocarbons (HC) and carbon monoxide (CO) - and reduces nitrogen oxides (NO_x) into the harmless components water (H₂O), nitrogen (N₂), and carbon dioxide (CO₂).

Depending on the operating conditions of the engine and the exhaust gas composition, conversion rates upwards of 98% can be achieved at close to stoichiometric (lambda one) conditions. The necessary reaction conditions can be reached after less than a minute by the introduction of special cold-start measures, especially a fast heat-up of the exhaust gas after engine cranking. This is especially important for city driving, characterized by frequent start-stop events.

Lambda controlled three-way catalysts

In lambda controlled three-way catalysts the following three basic reactions are catalyzed in parallel. 

• 2 NO + 2 CO → N₂ + 2 CO₂
• 2 CO + O₂ → 2 CO₂
• 2 C₂H₆ + 7 O₂ → 4 CO₂ + 6 H₂O 

High conversion rates for all three reactions can be achieved in a stoichiometric exhaust gas, with a so-called lambda 1 composition. Lambda describes the mass ratio of air to fuel during the combustion and is slightly dependent of the fuel composition / octane number (ON). For an ON of 95, lambda 1 is 14.7; for an ON of 91 this number moves to 14.8.

Especially lean excursions (i.e. lambda > 1, excess oxygen) lead quickly to a significantly reduced NO_x conversion. So a balance between catalyst and lambda control is absolutely necessary in order to guarantee high conversion levels over the lifetime of the vehicle.

Conversion characteristics of TWCs:
(e.g. Pd/Rh TWC)

Narrow range of air to fuel ratio with high conversion of all regulated gaseous components.

NO_x adsorber for gasoline fueled vehicles

The working principle of NO_x adsorbers for gasoline vehicles is comparable to the mechanism already explained in the NO_x adsorber diesel section. Differences can be realized in the operating conditions, especially a higher exhaust gas temperature and a different exhaust gas composition – lower oxygen content - for gasoline applications.

This can lead both to adjusted washcoat formulations and to adjusted engine management strategies.