If you’re a driver or fleet manager, you can use the in-cab displays and telematics to monitor system performance and identify any issues or possible faults.
Continuing to operate a bus with a faulty or disabled system can lead to higher repair costs, so it’s important to fix these early on. A failed system also emits pollutants that are harmful to health.
Here’s our advice for maintaining retrofit emission reduction systems.
Maintain the vehicle and base engine in line with the manufacturer’s guidelines to make sure the retrofit system works effectively. High fuel or oil consumption, or contaminated compressed air, can damage the system.
Check exhaust insulation for correct fitment and oil or fuel contamination.
The telematics system must be working. If it’s not, the vehicle is non-compliant. Without the telematics data, it’s much harder to target maintenance where and when needed.
Monitor Adblue levels and consumption. This can be an early sign of system issues such as blocked filter pumps, injectors or failing sensors.
NOx sensors are highly sensitive and critical components that need replacing at set intervals. The telematics data can help you identify NOx sensor issues early.
Maintain and regularly clean diesel particulate filters (DPF).
Follow the guidance on regular engine-out smoke testing to prevent DPF failures.
Check your vehicle is operating on the right route. Retrofit exhaust systems work best within a specific temperature range. Running a vehicle over very short or low speeds routes can result in lower-than-expected retrofit performance. Using telematics data can help identify if a vehicle is unsuited to the route.
Replacing components with non-approved parts or disconnecting components could affect the system’s warranty and performance.
Work with the system supplier – they’re the experts and can help support and maintain your system.
Make sure vehicles are switched off correctly at the end of a shift so the systems can purge as designed.
If a malfunction indicator light comes on, either on the vehicle dashboard or the retrofit display, report it to the depot and get it checked.
How do I become Clean Air Zone compliant with CVRAS approved retrofit systems?
Clean Air Zones improve air quality in traffic hotspots by encouraging the replacement of old, polluting vehicles with lower emission vehicles. Cleaner vehicles can enter these zones for free, while older vehicles must pay a charge (or are issued a penalty for non-payment).
The emissions standard for diesel buses to comply with Clean Air Zones is Euro VI. Retrofit technology has been designed to treat the exhaust emissions of older vehicles, reducing harmful emissions and making the vehicle compliant with the Euro VI emissions standard. Only retrofit systems accredited under the CVRAS are allowed to enter the zones.
To help the bus industry meet the emission requirements, funding was provided to fit CVRAS retrofit systems to over 9,000 buses across the UK. To make sure they continue to operate effectively and retain their low emission status, the systems and buses must be monitored and maintained.
Suppliers have developed retrofit solutions that can lower the NOx and particulate emissions of older vehicles to levels similar to Euro VI. Retrofit systems approved through CVRAS are exempt from emission zone restrictions in the UK.
There are four main suppliers of CVRAS approved systems to UK bus market:
Systems are monitored through a telematics monitoring unit and web portal and the core retrofit vehicle technology:
CVRAS approved systems are required to have continuous telematics-based monitoring systems for emissions. These systems store data on the vehicle’s operation and retrofit system to diagnose faults or failures. The telematics and its web-based data portal should be working at all times.
We work with system suppliers to monitor bus retrofits daily and give regular updates to bus operators who aren’t reporting or performing. The data reports go to the operator, supplier and the Joint Air Quality Unit.
What to watch
Some systems can automatically report retrofit faults via the telematics, so switch this on.
Review our regular reports to identify any non-reporting and underperforming vehicles.
Get the telematics systems repaired or replaced if not reporting correctly.
Review the low NOx performance bus list we provide to identify which vehicles to schedule for diagnosis and repair.
Work with system suppliers to monitor vehicles for developing problems.
Review the telematics regularly to see if NOx conversion is happening. Also check the bus’s operating conditions allow for good emissions performance.
The diesel particulate filter (DPF), or continuously regenerating trap from HJS and Eminox, is usually made up of two key elements. These are positioned in the exhaust flow:
A diesel oxidation catalyst which oxidises the exhaust gas pollutants (NO to NO2, CO to CO2, and HC to H2O and CO2) creating a NO2 rich gas. While NO2 is not desirable in exhaust emissions, in this case it is needed in the next stage of the system.
The DPF traps all the particulates, such as soot and ash. It uses the NO2 to oxidise the soot into CO2 and reduce the NO2 back to NO.
The DPF gradually builds up with ash so needs regular cleaning or exchanging. This is a standard service activity. The control system can monitor this. It measures the back pressure across the DPF and indicates if the pressure is too high due to the filter getting blocked.
The DPF can be expensive to replace. To reduce the build-up of particulates and prevent DPF damage, make sure the bus engine is in good condition and use the right oil and fuel.
What to watch
DPF back pressure is a key indicator of filter condition. Regularly monitor the telematics portals as these can show back pressure levels.
Regular engine-out smoke checks can identify high particulate engines, which could lead to DPF issues.
Inspect for leaks (signs of smoke from the connections can also indicate DPF blockages).
Broken DPFs can quickly lead to contamination of the selective catalytic reduction (SCR) elements. These may be indicated by smoke at the tailpipe or signs of the DPF overheating.
Any black smoke from the tailpipe or black build-up around the tailpipe is a sign of significant faults.
Adblue or diesel exhaust fluid is needed to convert the NOx gases (NO and NO2) in the exhaust gas into harmless nitrogen (N2) and water (H2O) within the catalyst.
Adblue is stored in a separate tank and the level and quality is monitored. When the exhaust conditions are correct the fluid is injected into the exhaust stream after the DPF to convert the exhaust NOx gas. The active component is ammonia. This is produced when the Adblue evaporates in the hot exhaust, so injection only happens when the temperature is high enough.
Adblue systems typically have in-line filters to protect the injector and an air supply to pressurise the Adblue pump. Some Adblue systems purge to prevent blockages from happening when the vehicle is parked. It’s important to follow the correct switch off procedure specified.
What to watch
Adblue consumption varies with route, load, temperature and other conditions. Keep a regular check on the Adblue level to make sure it doesn’t run out. The system display will flag when the Adblue level is low.
The system filters need regular cleaning or replacement in line with service intervals to protect the injector from blockage. Injectors are expensive to replace.
The monitoring system checks the level and quality of Adblue, so don’t be tempted to use non-approved Adblue.
Check the filters for the air supply to the pump as contamination can be expensive to fix. Adblue systems often need a service ‘flush’ to prevent blockages.
Follow the correct vehicle switch-off routine to prevent Adblue being left in the pipe or injector overnight. Adblue crystallises easily and can form a blockage.
The SCR catalyst, and ammonia slip catalyst where fitted, are the last step to clean the exhaust gas NOx pollutants before the vehicle’s tailpipe.
Effective conversion of NOx requires a high enough temperature in the exhaust and an exact amount of ammonia. The control system calculates the injection amount multiple times per second based on an array of sensors and inputs from the engine control unit. A sensor downstream of the SCR checks NOx emissions in real time.
If the conditions are right and the system is working correctly, NOx emissions can be reduced significantly. Any unused ammonia is kept in the ammonia slip catalyst and will be used in the next reaction. This stops any ammonia emissions.
What to watch
The SCR shouldn’t need replacing or cleaning if the system is working correctly.
If the SCR catalyst is contaminated with oil or fuel from a poorly maintained engine, it can become inactive and will be expensive to replace.
The telematics system is constantly monitoring SCR performance and can give an early indication of SCR catalyst issues.
Where fitted, it’s important to insulate the SCR to maintain an effective operating temperature.
The retrofit systems rely on a wide variety of sensors to monitor and control the Adblue injection accurately. Some of the sensors may be originally fitted to the vehicle, while many will be new as part of the retrofit.
Pressure sensors monitor the DPF and identify any blockages.
Temperature sensors in front of each catalyst monitor the exhaust gas temperature to determine when the catalysts will work best.
NOx sensors measure the concentration of NOx in the exhaust to establish how much Adblue to inject. A second NOx sensor monitors and optimises system performance in real time.
All the sensors are monitored by the control unit and any failures are flagged via the telematics. The detailed data in the telematics portal can indicate where sensors are deteriorating before they fail.
What to watch
The drivers display will usually indicate if a sensor has failed, so keep it visible. If the malfunction indicator light on either the retrofit display or the vehicle dashboard illuminates, report it to the depot for diagnosis.
NOx sensors are often a service item and must be replaced at set intervals with the correct replacement part. Using non-standard parts can negatively affect the system operation.
During servicing, visually check the retrofit system wiring for damage or friction indications.
Where fitted, the in-cab display indicates system faults or failures. Systems use either the vehicle dashboard display or a bespoke retrofit display. All systems have indicator lights to flag to the driver. When parked, the driver can often interrogate the displays to get detailed information on the fault or use. For example, low Adblue level or sensor failure.
You should encourage drivers to check these displays at appropriate points during their driving shifts, even if the display is out of immediate sight. This is a key monitor for the retrofit system and can prevent costly repairs.
What to watch
Drivers’ displays can help to indicate vehicle maintenance needs or faults but should not be a distraction to the driver.
The malfunction indicator lights are an essential part of the emission control system and should be acted on if illuminated.
Don’t disconnect or disable the displays as this can lead to system malfunctions.
Advice to drivers
Drivers should be aware that most emissions can’t be seen, so a poorly performing retrofit is unlikely to be visible from the appearance of the exhaust.
Checking the Adblue level and the driver’s display at the start and end of a shift is a key requirement.
Some systems need a purge sequence at the end of the day. Make sure drivers follow the operating instructions to switch off the vehicle correctly at the end of their shift.
Advice to depot managers
Service buses according to set schedules. This includes servicing NOx sensors and Adblue system components. Make sure you have the right service contracts in place.
Review the fleet report we issue and check the telematics of buses flagged in the report.
Contact the system supplier for expert advice.
Don’t disconnect the system or components. This can cause major issues and lead to expensive repair costs.
Case studies
Coach use case was operating school runs.
On vehicle handover the telematics had been switched off and on a morning school run NOx emissions measured at more than 20 times the CVRAS limit (10.6 g/km). An average NOx reduction of 5% was seen.
The system supplier attended the operator depot over lunch for one hour of basic maintenance. This included replacing fuses and NOx sensors and flushing the Adblue injector.
On the afternoon school run emissions measured much lower than the morning run at 1.25 g/km with a NOx reduction seen of nearly 87%.
Double decker bus use case was operating between city centre CAZ and airport.
Vehicle telematics not reporting any emission data.
Emission testing of the vehicle as handed over showed NOx emissions at over 6g/km, 12x the CVRAS limit. Particle number levels were nearly 1000 times the limit. A NOx reduction performance of 6% was seen.
The retrofit system was fully serviced in four hours, with replacement of NOx sensors, Adblue system components and the DPF.
Emissions were measured post-repair and a NOx reduction of 92% was seen with tailpipe NOx at 0.2g/km. Also, the bus was seen to be reporting NOx data via telematics and an improvement in particulate emissions to below the Euro VI limit.
Last updated: 28 October 2024
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