SCR and traps can revitalise old buses - Ricardo

Latest research from Ricardo shows that “some of the oldest vehicles” can also be the cleanest, if retrofitted with SCR and regenerating particulate traps.

Ricardo, working with Horiba Automotive Test Systems and Horiba UK of Moulton Park, Northampton, UK, have carried out measurements on an older Euro III bus that has been recently retrofitted with a selective catalytic reduction (SCR) and a continuously regenerating particulate trap system.  

The tests, examining real-world emissions of buses operating through a known pollution hot spot in Brighton city centre, form a follow-up to Ricardo’s published results earlier in the year demonstrating the important role that improving traffic flow can have upon reducing NOx emissions.

The study focused on a range of buses of the Go-Ahead Group including Euro IV, Euro V conventional and Euro V hybrid powered vehicles based in Brighton and Hove on the south coast of the UK.

Testing of the older retrofitted vehicle was carried out on the same route used in the previous study. This route traverses Brighton and Hove through the ‘pollution hot spots’ and covers a total of 18km (9km in each direction) with significant gradients throughout.

The bus, instrumented with Horiba’s advanced portable emissions monitoring system (PEMS), was artificially loaded with ballast representing a 70 per cent passenger load.

Multiple trips were conducted in normal traffic during business/shopping hours, stopping at regular bus stops in a similar manner to normal passenger service.

Tests showed that when averaged across the route, total emissions results of the Euro III retrofitted bus were substantially below those of all of the other buses tested, including the Euro V hybrid vehicle.

Data indicated that of the nitrogen oxides remaining in the exhaust, the NO2 fraction was substantially reduced to below 10 per cent of total NOx.

However, the retrofitted after-treatment system required a considerable period of warm-up before its full emissions control functionality could be achieved (typically 5 - 10 minutes from cold start).

This could have important operational implications for bus operators having depots in, or close to, low emissions zones. Scope was also seen both for further optimization of the system calibration by improving the dosing of the SCR system in uphill stop-start traffic, and improving thermal management of the exhaust.

“Results of this additional study challenge the conventional received wisdom that newer vehicles are always better in terms of their emissions,” noted Ricardo manager of after-treatment and chemical analyses, Jon Andersson. “While there is some scope for improvement of the installed system, the Euro III retrofit bus equipped with SCR (selective catalytic reduction) and continuously regenerating particulate trap, produced significantly lower NOx emissions than all other vehicles tested, including a Euro V hybrid.”

“As bus operators attempt to balance their fleet replacement cycles with the imperative to reduce pollution and hence improve urban air quality, the optimal use of retrofit clean technologies of this nature may be an attractive and highly effective alternative to the early replacement of older vehicles,” he added. “Local authorities are examining the potential of such retrofit solutions in the rules governing future low emissions zones, as these may provide a highly practical path to reducing emissions at source.”