To control and reduce the pollution from combustion of fossil fuel, governments in EU, USA and Asian countries have legislated on the reduction of emissions. All legislation requires reduction of Carbon Monooxide (CO), Carbon Dioxide (CO2), Hydrocarbons (HC), Particulates (PM) and Nitrogen Oxides (NOx).

How to reduce emission of Nitrogen Oxides?
One group of the legislated pollutants, Nitrogen Oxides called NOx (for NO and NO2) are mainly produced by the combustion in diesel engines.
Automotive standards drastically reduce the emission of NOx in the exhaust gases. For instance, in Europe, Euro IV lowers the NOx emission by 30% vs. Euro III and again Euro V reduces by 60% vs. Euro III for heavy-duty diesel engines like the one used in trucks and buses.



In order to fulfill the required reduction, after treatment systems were developed as engine settings cannot provide such low emissions.

Among the technologies to reduce NOx, SCR (Selective Catalytic Reduction) has demonstrated high reduction performance and improved fuel economy.

SCR technology converts NOx exhaust fumes into harmless water and nitrogen by chemical reaction of a reactant, very often urea, in a catalyst in the exhaust gas stream.

The chemical reaction in the exhaust converts urea into Nitrogen and Ammonia. The Ammonia itself is stored on the catalyst and reacts to the Nitrogen Oxides of the exhaust gases. The final product is nitrogen and water.

As NOx emission depends on engine operating conditions, the amount of reactant to be injected has to be fully controlled.

Too much dosed reactant will lead to the slip of ammonia that pollutes at least as the NOx themselves, too little dosed reactant will lead to reduced system performance (low NOx conversion ratio).