Dirty work: Keeping diesel exhausts clean

Diesel vehicle exhaust systems can run clean if they are given the proper care, and vital components like the DPF are properly serviced

Published:  11 October, 2017

The exhaust is a lot more than just an exit route for waste gases for some time now. Tim Howes, deputy general manager – supply chain and technical service, NGK Spark Plugs (UK) Ltd, provides some context: “In 2009, The Euro V emissions standard for passenger cars demanded a significant reduction in NOx, HC and particulate matter and in 2014 the Euro VI standard brought a further tightening of these emissions, primarily for diesel engines.”

For diesel powered vehicles this has meant a significant increase in the complexity of exhaust gas recirculation (EGR) and after treatment resulting in the fitment of various combinations of diesel oxidising catalyst (DOC), selective catalytic reduction (SCR), lean NOx trap (LNT),  diesel particulate filter (DPF) and other associated devices and control systems.
All these additional components have led to an increased need for sensors in the system.

“NTK’s range of oxygen sensors has evolved significantly since their introduction several decades ago,” according to Tim. “This continual development and performance enhancements are key in allowing vehicle manufacturers to meet the standards. Oxygen sensors used on diesel applications will usually be of the wide band type due to the lean air/fuel ratios of compression ignition engines. Amongst other particular features of these sensors, special soot protection technology is employed to protect the ceramic sensor element from contamination as they are installed upstream of the DOC/DPF.

“Temperature sensors located within the exhaust gas stream are increasingly being employed as part of the exhaust gas treatment architecture and are used to monitor the performance and control the function of several components such as the turbo charger, DPF and DOC.  These exhaust gas temperature sensors (EGTs) ensure the highest operational efficiency of the engine in terms of power output and reduced emissions and they are also used to protect certain components from overheating thus maintaining their service life expectation.

 “NTK’s EGTs are NTC thermistor devices whose resistance value changes relative to the temperature of the exhaust gas and are a popular choice for vehicle manufacturers as they provide extreme resilience against heat and vibrations, have high measuring accuracy, fast light-off times and an exceptionally wide measuring range typically -40 to +900 degrees Celsius.”  

Checking and replacing
You will need the right equipment to check if they are functioning correctly: “Typical resistance values can be from 0.1 ohm up to megaohm range,” says Tim, “and their function can be checked by the use of a suitable ohmmeter. One vital aspect of checking and replacing these sensors is ensuring that the correct sensor has been identified on the vehicle as multiple locations are often used by visually similar components. The exhaust system is now a highly complex and expensive piece of equipment that is expected to work faultlessly and for an extended period of time in an extremely harsh environment. There will be an increased need to closely manage the gases emitted from the combustion chamber and significantly more complex systems will become the norm.”

The DPF on its own is something garages need to be up to speed on: “Over recent years, we have seen the DPF become a separate check on an MOT test,” says BM Catalysts’ commercial director Mark Blinston,  “and that will be changed further in the future to not just be a visual check following recent government announcements surrounding the Roadworthiness Directive.”

Captures, contains and converts
It’s worth reminding ourselves what the DPF actually does. Mark takes over: “The DPF is the part of the exhaust system which removes particulate matter, or soot, from the exhaust gases; it captures, contains and converts these soot particles into carbon dioxide via regeneration. Exhaust gasses containing soot enter the DPF channels, which are closed off at alternating ends. The channel walls are porous; allowing exhaust gasses to pass through. Particulate matter is trapped inside the DPF, while clean exhaust gasses exit. Trapped soot particles are eliminated during passive or active regeneration cycles triggered by the ECU to prevent blockages.

“To help future-proof our products, we make a number of recommendations regarding the prevention, recovery and repair of DPFs to avoid premature replacement, including; correct driving styles, use of quality fuels and fuel additives and regular servicing.”

Main reasons
When a DPF becomes blocked, the cause is very unlikely to be the DPF itself. “Soot problems begin long before they reach the DPF,” Mark points out.“Unsuitable driving styles and mechanical defects in other components within the engine are the main reasons for a DPF blockage. Replacing a blocked DPF without correctly diagnosing the genuine fault will only cause the new DPF to also become blocked. There are a number of components used in the control of soot combustion and all could contribute to a DPF failure; all should be checked before assuming a replacement is required e.g. air mass meter, lambda sensors, EGR value etc.”
Mark adds: “In 2016, we released an extensive range of DPF pressure pipes in a bid to further help reduce the unnecessary replacement of DPFs, caused by impaired pressure pipes producing false readings. Due to their slim form, and being exposed to the elements, DPF pressure pipes are susceptible to damage. Faulty pressure pipes could be mistaken for a faulty DPF, causing wasted time and money for the motorist.“

Richard Collyer, product and equipment specialist at Launch UK comments:  A lot of vehicles are suffering from a high amount of contamination in all post combustion areas, exhaust, EGR, diesel turbo and especially the DPF. This eventually becomes a running problem and can throw a light up on the dashboard as failed regeneration can occur. This means that the DPF must be cleaned using a forced regeneration through a diagnostic tool. If the unit fails to regenerate this way, then the technician can remove the DPF and send away for cleaning or replace it.

“Removing a DPF can be time consuming and can end up as a costly job for the customer. Some DPFs are £1,500 or more, so coupled with labour it can be a £2,000 job.”

Launch UK has added a new DPF gun to its range of diagnostic equipment. Richard adds: “Launch has also introduced a point of sale poster informing motorists of critical information surrounding DPFs and potential treatment to resolve any issues, helping to keep them in line with the law, as well as making them aware of the tool’s core benefits.”

Making sure that DPFs don’t become clogged in the first place will also help the situation.

“A critical point for cars with diesel particulate filters is the motor oil,” says Ibrahim Memis who is in charge of UK operations at LIQUI MOLY. “It must be a low ash oil. When conventional oils burn in the combustion chamber, ash forms. This ash lands in the diesel particulate filter, clogging it after a short time. This is why cars with diesel particulate filters need so-called low SAPS motor oils.

“At www.liqui-moly.com LIQUI MOLY offers a free oil guide to help you be sure which oil is the right one for
any vehicle.”

Ibrahim continues: “The DPF’s regeneration program only works when the filter has reached a minimum temperature. Unfortunately this temperature is not reached on short trips. The result; more and more soot collects, until the filter is completely clogged up and the engine no longer runs.“

Diesel particulate filters clogged with soot can be freed up quickly and easily with the LIQUI MOLY Diesel Particulate Filter Cleaner. “By offering this service workshops can prove they are state-of-the-art and simultaneously help customers save money,” adds Ibrahim.


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