MKW Motors explains Selective Catalytic Reduction (Part 1 of 3)

The reduction of pollutant emissions is an important challenge in automotive technology. In the first of three articles, MKW Motors explains Selective Catalytic Reduction.

Selective Catalytic Reduction
The reduction of pollutant emissions is an important challenge in automotive techno|ogy.
The SCR system is a new exhaust gas aftertreatment system. It is used to reduce the nitrogen oxides contained in exhaust gas. The abbreviation SCR stands for Selective Catalytic Reduction.
With this technology, the chemical reaction of reduction is selective. This means that, out of all exhaust gas, only the nitrogen oxides are specifically reduced.

In the reduction catalytic converter, the nitrogen oxides (NOX) contained in the exhaust gas are converted into nitrogen (N2) and water (H20). To achieve this, a reducing agent is continuously injected into the exhaust gas flow upstream of the reduction catalytic converter. The reducing agent is contained in a separate, additional tank.
In the automotive industry, SCR technology has already been in use for some time in commercial vehicles and buses.

Emissions standards
The table below shows the extreme reduction required for NOx and PM (particle matter) from Euro 4 to Euro 6. Any car sold from January 2015 must meet Euro 6 levels.

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Screen shot 2015-04-08 at 09.55.33Nitrogen oxides
Nitrogen oxides are the collective term forchemical compounds consisting of nitrogen and oxygen (elg. N0, N02 …).

They occur during combustion in the engine as a result of high pressure, high temperatures and surplus oxygen.

The nitrogen oxides are partly responsible for forest damage caused by“acid rain”and forsmog formation.


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Nitrogen oxide reduction measures

Before the SCR catalyticconverter system was designed to help reduce nitrogen oxide, a range of different technical solutions were in place to reduce nitrogen oxide emissions.

Emission reduction can be achieved via measures within the engine. Effective optimisation of combustion ensures that pollutants do not occur in the first place.

The measures within the engine include:

  • Designing intake and exhaust ports for optimal flow conditions
  • High injection pressures for good mixture formation
  • The design ofthe combustion chamber, e.g. the design ofthe piston recess and the reduction of the compression ratio






Exhaust Gas recirculation valve

Exhaust Gas recirculation valve

Exhaust gas recirculation
During exhaust gas recirculation, some of the exhaust gas is returned to the combustion process.

The reduction in the fuel-air mixture’s oxygen concentration which is achieved in this process slows combustion down. This leads to a reduction in the peak combustion temperature and therefore reduces nitrogen oxide emissions.






Cooler for exhaust gas recirculation

Cooler for exhaust gas recirculation


Exhaust gas recirculation cooling

In order to reduce the nitrogen oxides even more effectively during exhaust gas recirculation, the recirculated exhaust gases are passed through a cooler when the engine has reached its operating temperature.

This additionally reduces the combustion temperature and enables an increased volume of exhaust gases to be recirculated.



Operating principle of the SCR system

The reduction catalytic converter has reached its operating temperature at approximately 200 ‘C. Information on the reduction catalytic converter’s exhaust gas temperature is received by the engine control unit from exhaust gas temperature sender.

The AdBlue° reducing agent is sucked out of the reducing agent tank by the reducing agent pump and send through the heated supply line to the injector for reducing agent at a pressure of approx. 5 bar.

The injector for reducing agent is actuated by the engine control unit and injects the metered reducing agent into the exhaust system. The injected reducing agent is carried along by the exhaust gas flow and is evenly distributed in the exhaust gas by the mixer. On the route to the reduction catalytic converter, which is called the hydrolysis section, the reducing agent is broken down into ammonia (NH3) and carbon dioxide (CO2).

In the reduction catalytic converters, the ammonia (NH3) reacts with the nitrogen oxides (NOX) to form nitrogen (N2) and water (H20). The SCR system’s efficiency is registered by NOX sender.

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Operating principle
The following prerequisites must be met for the engine control unit to inject the reducing agent

  • The reduction catalytic converter has reached its operating temperature of approx. 200 ‘C.
  • There must be guaranteed that sufficient liquid reducing agent is available for injection at low exterior temperatures.

Under the following conditions, injection of the reducing agent is interrupted by the engine control unit:

  • lf the mass exhaust gas flow is too low, e.g. during idling.
  • If the exhaust gas temperature falls too far and drops below the reduction catalytic converter’s operating temperature.

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The design of the reduction catalytic converters corresponds tothat ofan oxidation catalytic converter with a honeycomb-shaped ceramic body.

The reduction catalytic converter’s coating consists of copper zeolith. This accelerates the speed of the nitrogen oxide reduction process.

Hydrolysis section
The hydrolysis section is located between the injector for reducing agent and the reduction catalytic converter.

The ammonia(NH3) required to reducethe nitrogenoxides is formed therefromthe reducing agen (urea solution). This is carried out viaathermolysis and hydrolysis reaction on the part of the injected reducing agent.

When the reducing agent is injected into the hot exhaust gas flow, the water initially evaporates.

During thermolysis, the reducing agent (urea solution) is broken down into ammonia and isocyanic acid.

CO(NH2)2 b NH3 +HNCO
Urea bammonia+ isocyanic acid

This is followed by hydrolysis, in which the isocyanic acid reacts with the water contained in the exhaust gas. This leads to the creation of an additional ammonia and carbon dioxide molecule.

lsocyanic acid +water bammonia+ carbon dioxide

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Thermolysis = thermolysis is a chemical reaction in which an initial substance is broken down into several products by means of heating.

Hydrolysis = hydrolysis is the cleavage of a chemical compound via its reaction with water.

Good mixing and the even distribution of red ucing agent and exhaust gas are very important! Before entering
the SCR catalytic converter, the reducing agent must have evaporated completely.The more even the
distribution,the higher the efficiency of the reduction catalytic converter.

Reduction catalytic converter

Functional principle of nitrogen oxide reduction

Nitrogen oxide reduction takes place in the reduction catalytic converters. This means that the nitrogen oxides (NOX) give u ptheiroxygen molecules du ring the reduction process or, to express it differently,the oxygen molecules are withdrawn from the nitrogen oxides. In the reduction catalytic converter, the nitrogen oxides (NO + N02) react with the ammonia (NH 3) to form nitrogen (N2)and water(H2O).

The correct NO and N02 ratio in the exhaust gas for the reduction process is formed in the oxidation catalytic converter. The oxidation catalytic converter’s coating is designed for the SCR system.
NO+ N02 + 2NH3 b 2N2 + 3H2O
Nitrogen monoxide + nitrogen dioxide + ammonia b nitrogen +water

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(Part 2 of 3) next month

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Published on: April 8, 2015

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