SCR technology has proven its ability to significantly reduce NOx emissions. Hence it has become an outstanding solution to be used to meet future emission limits in Europe, Japan and North America. To add even more confidence towards the introduction of SCR technology, the implementation of NOx sensors will become mandatory for on-board emission control systems.
To demonstrate the usefulness of NOx sensors, a typical North American On-Highway Truck was chosen as a demonstration vehicle. It was equipped with vertically mounted, extruded SCR catalysts, a urea dosing system, and NOx-sensors.
In order to achieve EURO 5 or even lower emission limits in NOx and PM, attempts were made to equip the system with oxidation catalysts upstream of the SCR catalyst as well as upstream of the turbine. Some changes had to be made to the model year 2000 engine calibration to reduce PM emissions to EURO 4 and 5 levels.
The system was calibrated and then tested according to different HD test procedures on a transient test bench. NOx emissions were determined by CLD and NDIR analyzers and by state of the art NOx sensors.
After completion of the test bench work, the catalyst system was transferred to the test truck and the final system calibration was copied from the test bench ECU to the vehicle ECU. On-road NOx measurements were gathered by using NDIR-analyzers and NOx sensors.
Based on our previous experience with HD trucks, it can be stated that the introduction of NOx sensors is an appropriate method to monitor NOx conversion performance of SCR systems and also to adapt the system to compensate for various disturbances.
Ongoing evaluation of the test truck in the future will include NOx sensor performance, implementation of the NOx signal into the dosing control algorithms, as well as on-road durability testing.