Future-proof

EU standardisation of waste gas

The 88/77/EEC directive already specified limiting values for exhaust gases from commercial vehicles, and these values have been made even more stringent in subsequent directives. The next stages, EURO 4 and EURO 5, come into force in October 2005 and October 2008, respectively (Directive 99/96/EC). The limiting values for nitrogen oxides, in particular, were lowered in each stage.

Exhaust gas aftertreatment

While the standards up to EURO 3 could all be fulfilled technically by engine improvements alone, the limiting values from EURO 4 onwards cannot be met without exhaust gas aftertreatment systems. At present, of the several competing technologies, only the SCR technology is able to comply with both EURO 4 and EURO 5. It is likely that a EURO 6 will also be based on this technology. The complexity of finding a suitable technical process is due to the necessity for a simultaneous reduction of the limiting values for the particle emissions and for the nitrogen oxide emissions, although only one of these parameters can be changed by the combustion process in the engine at a time.

Technologies for future emission standards

Operating principle of SCR technology

The engine is optimised until only a low quantity of soot particles is generated by combustion while copious amounts of nitrogen oxides are produced intentionally. The reducing agent, AdBlue®, is then injected into the stream of exhaust gas. Because of the high exhaust gas temperature, AdBlue® is hydrolysed to ammonia and CO2. The ammonia reacts with the nitrogen oxides on the surface of the catalyst to yield nitrogen and water, which are natural components of our atmosphere.

Operating principle of the SCR catalyst

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How does SCR exhaust gas aftertreatment work?