IAA 2005: Tackling nitrogen oxides: Concept for cleaner diesel cars
September 2005
Pollutants almost totally eliminated by SCR technology
Diesel passenger cars are facing tougher exhaust emission limits in Europe with the pending Euro5 standard. In North America, extremely demanding limits, in particular for nitrogen oxide emissions, will already be entering into force from 2007 - 2009. As an exhaust aftertreatment strategy, Eberspächer has developed an exhaust system with SCR (Selective Catalytic Reduction) to supplement the upstream particulate filter. This system will be ready for full-scale production in model year 2009. Particulate matter and nitrogen oxides can thereby be reduced by more than 90%.
The main challenges for cleaning diesel vehicle exhaust emissions are soot particles and nitrogen oxides. Today, most diesel exhaust systems have oxidising converters and particulate filters for reducing HC, CO and particulate emissions to well below the legal limits. In the current state of the art, nitrogen oxides (NOx) are controlled by systems acting inside the engine.
Worldwide toughening of exhaust emission limits may in the medium term also require nitrogen oxides to be further reduced by means of the exhaust cleaning system. In North America, exhaust after treatment to reduce NOx emissions from heavy diesel vehicles (e.g. SUVs) will be required in only a very few years' time under the TIER-2 limits of the Environmental Protection Agency. Eberspächer has developed a concept for this purpose, drawing on its experience of the commercial vehicle industry. This system is scheduled to be ready for full-scale production for the model year 2009
The system uses the Selective Catalytic Reduction (SCR) process. As reducing agent, an additive (aqueous urea solution), named AdBlue, is injected into the exhaust gas stream from a separate container. The injector is located downstream of the particulate filter. This urea solution is converted to ammonia, which reacts with the nitrogen oxide on the coated surface of the catalytic converter to form nitrogen and water.
Geometry of the exhaust gas system significant
The key components of this system are the metering device for the reducing agent and the catalytic converter for chemical conversion. In addition, for the overall operation of the system, the geometry of the exhaust gas system is essential: here, the pipe routing of the mixing section up to the SCR converter and the gas flow are configured to ensure that the reducing agent is optimally mixed and distributed over the catalytic converter surface. In addition, mixing elements can assist in the preparation of urea. The pipes to the SCR catalytic converter are double-walled; this air gap minimises heat loss and thereby ensures rapid light-off of the catalytic converters and preparation of the reducing agent. A system of this kind also offers excellent medium and long-term prospects for establishing the diesel passenger car as a viable environment-friendly alternative.
(Recommended Photo : SCR-Filter System.jpg)