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 (NOx) 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 H2

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 NOx 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.

NOx adsorber for gasoline fueled vehicles

The working principle of NOx adsorbers for gasoline vehicles is comparable to the mechanism already explained in the NOx 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.