Research has shown that some of the oldest vehicles can also be the cleanest if retrofitted with the latest low emissions exhaust aftertreatment technology.
Ricardo published results earlier this year demonstrating the important role that improving traffic flow can have upon reducing NOx emissions. The study focused on a range of buses including Euro 4, Euro 5 conventional and Euro 5 hybrid powered vehicles.
As a follow-up to this work, it has since carried out measurements on an older Euro 3 bus that had recently been retrofitted with a selective catalytic reduction and continuously regenerating particulate trap system. Testing was carried out on exactly the same route as used as the basis of the previous study. Service 7 traverses Brighton and Hove through the air quality hot spot of North Street in the city centre, and covers a total of 18km (9km in each direction) with significant gradients throughout.
The retrofitted bus was instrumented with HORIBA’s Portable Emissions Monitoring System (PEMS) equipment and artificially loaded with ballast representing a 70 percent passenger load. Multiple trips were conducted in normal traffic during business/shopping hours, stopping at regular bus stops in a similar manner to the normal passenger service.
In addition to highlighting the significant influence of route topology and traffic congestion in driving NOx emissions, the previous study had observed the expected trend of the latest vehicles generating the lowest emissions when the whole of route 7 was considered. In contrast, however, when averaged across the route – and also for the section through the North Street pollution hot spot – the total emissions results of the Euro 3 retrofitted bus were substantially below those of all of the others tested, including the Euro 5 hybrid. Data also indicated that of the nitrogen oxides remaining in the exhaust, the NO2 fraction was substantially lowered to below 10% of total NOx.
Significantly from an operational standpoint, the retrofitted aftertreatment system required a considerable period of warm-up before its full emissions control functionality could be achieved (typically five to 10 minutes from cold start). This may have implications for bus operators with 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.
