Chemical Reaction

The task of the SCR system is to substantially reduce the nitrogen oxides (NO_x ) in the exhaust gas. Nitrogen oxides occur in two different forms:

• Nitrogen monoxide (NO)
• Nitrogen dioxide (NO₂ ).
  Fig 1: Nitrogen Monoxide And Nitrogen Dioxide - Chemical Reaction

  Courtesy of BMW OF NORTH AMERICA, INC.

Ammonia (NH₃ ) is used for the purpose of reducing the nitrogen oxides in a special catalytic converter. The ammonia is supplied in the form of a urea-water solution.

Fig 2: Ammonia - Chemical Reaction

Courtesy of BMW OF NORTH AMERICA, INC.

Urea-water solution 

Fig 3: Urea-Water Solution - Chemical Reaction

Courtesy of BMW OF NORTH AMERICA, INC.

The urea-water solution is injected by the metering system into the exhaust system downstream of the diesel particulate filter. The required quantity must be metered exactly as otherwise nitrogen oxides or ammonia would emerge at the end. The following description of the chemical processes explains why this is the case.