The government recently published its report on selective catalytic reduction (SCR) bus retrofit performance.
At first glance, it appears that these retrofit systems are struggling to perform as expected. An 11% NOx reduction seems quite alarming, but read a bit deeper, and the tests conducted indicate that when systems are maintained, a NOx reduction of 80-90% is attainable.
Automotive standard Portable Emission Measurement System tests conducted by Zemo Partnership and the Energy Saving Trust (EST) as part of the Clean Vehicle Retrofit Accreditation Scheme (CVRAS) in-service monitoring programme have shown that performance can vary.
But when systems are maintained, they can, and do, perform on-road in real-word passenger operations as they would on a test track over the same drive cycle as used for the CVRAS test.
So where is the problem? It is clear that these systems need looking after as they are not a ‘fit and forget’ solution. CVRAS requirements specify either a robust Euro VI-level on-board diagnostics system or added SCR system telemetry fitted to monitor and report performance in real time.
The former is very difficult to replicate on a Euro IV or Euro V bus, so suppliers (and operators by default) settled for telemetry for CVRAS approval. It should be noted that this was the preferred solution required by Transport for London before CVRAS was launched. CVRAS later adopted the concept.
Active telemetry on each CVRAS-retrofitted bus is now required for Clean Air Zone compliance. It is not a new stipulation, but one that is now being actively monitored and enforced.
This might seem a bit scary for operators that may have neglected their Euro VI SCR retrofitted buses, but if NOx or temperature sensors have failed, urea filters or injectors have blocked, or the urea pump has stopped working, then NOx reduction will be very low. This is certainly what the government report has concluded.
The overriding aim here is to ‘fix not fine’, and EST is supporting operators in giving them the oversight needed and providing reports on buses that need attention. EST receives daily data via suppliers’ telemetry portals – information that is also available to the operator should they have the time, resource or inclination to look at it.
EST is taking some of the legwork away by providing oversight reports directly to operators, something that has been done for over a year now for those operators with the most retrofitted buses.
Other activity to support operators has included conducting depot workshops with the company’s retrofit supplier and Joint Air Quality Unit officials in attendance. Those have highlighted the importance and benefit of SCR telematics data in ensuring the best performance for NOx reduction.
In addition, a CVRAS SCR retrofit maintenance guide for operators has been written to highlight the components of the system and potential issues to look out for to get the SCR performing at optimal levels. It will be published soon.
The key message is that retrofit SCR systems have demonstrated they can work. It is the same technology used in all Euro VI diesel vehicles, and like those, needs to be properly maintained. The installed telemetry is a great tool to identify developing problems early before they become more costly to resolve, and both EST and system suppliers are available to help.
If an operator prides itself on having a clean and efficient fleet, then it can demonstrate that by how its SCR retrofitted buses are performing.
Government SCR bus retrofit report in full available here.
