Investigating Euro 6 diesel vehicle emissions after-treatment using chassis dynamometer and CAN-bus data

Abstract

Euro 6 Diesel vehicles employ technologies such as DPFs and Urea-SCR in order to mitigate NOx and particulate matter emissions. Aiming to analyse the effectiveness of each, the accurate measurement of NOx and O2 concentrations in the vehicle’s exhaust gases was essential. It had been noted that the test vehicle at hand did not relay accurate information from the engine control unit regarding these two parameters specifically. As such, CAN messages from both sensors located upstream and downstream of the SCR catalyser containing information on these types of emissions were sniffed and calibrated, allowing emissions tests to be performed. Calibration was performed using a UniNOx sensor produced by Continental which had been donated as a laboratory type sensor and was measuring accurate readings for both NOx and O2 concentrations. Both the upstream OEM NOx sensor and the UniNOx sensor were exposed to the same stream of exhaust gases using an exhaust gas sampling box to ensure an accurate calibration could be performed. Through testing on the chassis dynamometer, it was found that tailpipe NOx concentrations remained constant at 24ppm throughout testing and were not dependant on engine loading, thus showing the emissions reduction benefit of the Urea-SCR system utilising AdBlue. The AdBlue injection quantity was also modelled to compare to that recorded during testing. This led to the conclusion that the vehicle also takes into account the storage of ammonia within the SCR catalyser due to the large discrepancy between modelled and actual values, albeit managing to maintain high NOx conversion efficiency. It was noted however that the SCR system fails to operate effectively during active DPF regenerations events, causing tailpipe NOx concentrations to reach values similar to engine-out concentrations for a brief period of time. This was caused by the significant increase in exhaust gas temperatures, leading to highly weakened sorption of ammonia in the SCR catalyst. During said DPF regeneration event, a difference between engine-out and tailpipe O2 concentrations had been noted, this allowed a calculation of soot oxidation quantity to be performed and compared to the reduction of soot measured during the event, which resulted in high congruity with the DPF soot quantity reported by the scan tool.