Abstract
To cope with tighter emission regulations and with the urge to increase fuel efficiency many heavy duty vehicle constructors and some car manufacturers see SCR-systems on diesel engines as the way to go. This choice leads to new challenges for the automotive industry. SCR-systems require a good urea injection strategy as well as the appropriate sizing of the catalyst. Both are important to obtain a high reduction of NOx. Moreover, excess urea will result in ammonia spill unless an additional cleanup catalyst is used.
To assist the automotive industry with working out highly performing SCR-systems VITO has recently expanded its VOEMLow dynamic emission measurement system with an accurate ammonia measurement system. VOEMLow combines the best available exhaust gas analysers with an in-house developed sampling and data acquisition system to provide accurate measurements of regulated emissions at 1Hz. The benefit of accurate, dynamic emission measurements for the development of efficient SCR-systems is illustrated with elements of a retrofit case study.
The case study deals with the retrofitting of a EURO-II bus with a combined SCR+DPF system. This paper will highlight how VOEMLow measurement were used to develop the SCR-system from scratch. In one step a well performing SCR-system was developed because with the retrofit the bus was already made EURO-V compliant. Furthermore, a detailed analysis of the emission measurements reveal a potential for further possible improvements.
At first sight, these further improvements may seem to be overkill. But very efficient SCR-systems may offer the possibility for engines to run in very efficient, high engine out NOx modes. Next to better fuel economy, these modes are characterised with low CO, HC and last but not least low PM emissions.