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.


Related Articles

  • Highs and Lows 

    When faced with diagnosing a fault, in order for us to be able to test the system it is crucial we understand the system’s layout, components and function. We recently faced a fault in a system we had little experience on, so it was an ideal opportunity for a bit of studying.

    Technical information is readily available from many sources, be it manufacturer or generic information, and does not take too long to find. While Google isn’t really a substitute for diagnostics, in situations like this it can be very useful for generic information. The fault on this vehicle turned out to be something so trivial I won’t bore you with it. What I would like to share is the valuable information I picked up along the way.

    Main purpose
    Exhaust gas recirculation (EGR) is nothing new, it’s been used on petrol and diesel engines for many years and while layout and control has varied in design the principle has remained the same. It is important to understand that manufacturers use different methods and configureuration, and for this article I’ve studied several and have tried to demonstrate a generic system.

    The main purpose of EGR is to reduce the level of harmful Nitrogen Oxide (NOx) gases emitted from the vehicle’s exhaust. NOx is present in exhaust emissions due to high combustion temperatures and pressures. Under light load/cruising conditions the EGR system directs a proportion of the exhaust gas back into the engine’s air intake. This reduces the oxygen levels which in turn reduces combustion temperature resulting in a lower NOx emission. When power is required from the engine the EGR system closes to insure a more efficient combustion (see figure 1).

    EGR on/off
    This is the conventional system in its closed (off) position.  During operation exhaust gases are taken from the exhaust manifold (pre-turbo), passed through a cooler (10) up to the EGR Valve (6). The cooler is a heat exchanger that not only uses the engine coolant to cool the gases to increase the mass but utilises the heat to warm up the coolant faster which helps the interior heater warm-up faster. The EGR Valve (6) can be either electrical of vacuum operated. The  powertrain control module (PCM) commands the EGR valve to open by a specified amount dependent on engine conditions (see figure 2).

    Some EGR valves have a position sensor that provides feedback to the PCM to ensure the correct position has been achieved. In a system where the EGR valve is not equipped with a position sensor, the PCM monitors the Mass Airflow signal in order to regulate EGR flow. This is achievable due to the fact that as the EGR valve is commanded open and gases start to flow, the air flowing in to the Mass Airflow Sensor will decrease. The calculation is made using tables of data (mapping) within the PCM’s software. Understanding this is crucial when diagnosing running faults as a fault in the Mass Airflow can easily affect the EGR system and vice versa.

    Understanding and diagnosing airflow and EGR faults I find can be easier if you look at it pressure differential. If air is flowing through a tube with a restriction in it, the air pressure after the restriction will always be lower than the pressure before the restriction. The difference in pressure will vary depending on the mass or pressure of the air and the size of the restriction.

    Air intake/throttle flap
    The air intake/throttle flap (see figure 3) generally defaults to the fully open position while the EGR valve defaults to the closed position. The purpose of the flap is to reduce the pressure on the engine side. As the intake flap starts to restrict the airflow, the pressure decreases to a pressure lower than that of the EGR pressure and the EGR gases start to flow into the engine’s air intake. If the exhaust gas pressure was slightly lower than the air pressure entering the engine then the gases would flow in the wrong direction.
        When in good working order this system serves its purpose. However, due to the fact that there is particulate matter in the exhaust gases, the system and components will slowly become blocked, causing reduced flow and valves starting to jam or not seal correctly. The air intake system often contains oil residue from the engines breathing system and slight oil loss from the turbo itself. When this oil is mixed with the particulates in the EGR gases it makes a very sticky gunk that starts to block the inlet manifold and intake ports.

    When the engine is under load and turbo boost pressure is required, the EGR valve needs to close and seal. If an EGR valve isn’t sealing correctly when closed then boost pressure will be lost into the exhaust system. The lower boost pressure and reduced oxygen level affects the combustion which in turn causes more particulate matter which only adds to the issue. If the EGR valve is stuck wide open then in most cases the engine will barely run.

    High pressure system    
    Euro 6 was introduced in September 2014 which demanded much tighter emissions than previous which required an advance in emission control technology. While the precise control of the fuel side of the engine management system has gained precision with higher fuel pressure and multiple injections within the cycle, the air intake, exhaust and emission control systems have too. Most manufactures use a high and a low pressure EGR system.  Prior to this most EGR systems were relatively simple and fell under the ‘High Pressure EGR’ title (see figure 4 and figure 5).

    The high pressure system is similar in layout to previous systems but serves a slightly different purpose. The system is only used during the warm-up phase of the engine from cold start. There is a pre-turbo passage from the manifold directly to the high pressure EGR valve (6). As the system is only used in the warm-up phase there is no need for a cooler. In this particular system there is a distribution channel that directs the gases equally into each inlet port. The purpose of this system is to raise the intake air temperature in order to improve combustion and reduce the warm-up time for the catalytic convertor/NOx storage catalyst (7) allowing them to function sooner. Once at operating temperature the system is pretty much redundant.

    Low pressure system
    The low pressure system (is active under most engine operating conditions and its purpose replaces that of the older systems- to reduce NOx gases (see figure 6). A proportion of the exhaust gas is collected after the Diesel Particulate Filter (8) and passes through a Wire Mesh Filter (9), through the EGR Cooler (10), up to the Low Pressure EGR Valve (11). The EGR valve then controls the flow through a channel up to the intake side of the turbocharger. The wire mesh filter ensures there is no particulate matter entering the system and also in the event of the particulate filter substrate breaking up, it also protects the rest of the system including the turbocharger, air intake and engine internals from damage. The cooler reduces the gas temperature which in turn increases the mass allowing a higher volume of exhaust gas to be recirculated. Due to the exhaust pressure after the particulate filter being quite low and also the air intake pressure before the turbo charger also being low there is and Exhaust Flap (12) fitted. By closing this slightly the exhaust pressure increases which causes the gases to flow back towards the turbocharger.

    Key benefits
    These systems usually have between three and four  exhaust gas temperature sensors each placed at key points of the exhaust system and two pressure differential sensors. The first is measuring pressure before and after the particulate filter (to calculate soot loading) and second between the DPF outlet and the point after the EGR valve, before the turbo. Coupling these six signals with the Mass Airflow sensor, the positions of both EGR valves and the intake flap, the turbo variable-vane position and the intake pressure (MAP), using the mapping within the PCM’s software means it can also make all calculations necessary. This provides an extremely high intake pressure and exhaust after treatment control.

    The key benefits of this system are that the exhaust gases are free of any particulate matter which keeps the entire system much cleaner and therefore reliable. The gases are also cooler meaning a greater mass can be used in a more effective way. Finally the gases re-enter the system before the turbocharger, allowing for the increase in boost pressures at lower engine load and RPM.

    Does this make diagnosis harder than before? Not if you take the time to study the purpose of each component and how it works. I’ll openly admit it wasn’t that long ago that I would have taken one look at this system and sent it on its way! Nobody likes being beaten by a job but neither should we have to waste too many hours trying to guess what’s wrong with it, worse still start throwing parts at it. It took me half an hour to locate this info, an hour studying it and a further hour planning what tests I was going to conduct and what results I was expecting to see. What was wrong with it in the end? A faulty sensor confirmed with no more than a voltmeter! After replacing the sensor I wanted to confirm the repair and monitor the function of the components using serial data. Something I highly recommend doing is picking five lines of serial data on every car you work on that requires an extended road test and monitoring them to see how they behave and what effect driving style (engine load) has on them. I guarantee after 10 cars you’ll know what to expect and be far more confident in diagnosing related faults. It works for me!

  • Diesel EGR and Turbo Cleaner 

    Lucas Oil is extending its additive solutions range with the introduction of a solvent-based fast action Diesel EGR and Turbo Cleaner spray. When sprayed into the air intake system, Diesel EGR and Turbo Cleaner removes carbon, lacquer, tar and varnish deposits and restores engine performance. To use, remove the flexible hose between the turbo charger/intercooler and the air intake manifold while a warm engine is idling. Then direct short bursts of spray into the air intake manifold. In extreme cases, the EGR valve-housing can be removed and the spray directed straight on to the unit itself. First stocks are expected to be available at UK motor factors from 1 November 2017.

  • ECP says prepare for Euro 6 now and profit later 

    Euro Car Parts is advising repairers that carry out MOT testing to make sure their emissions analysers comply when new emissions standards come in later this year.

  • Euro Car Parts announces free lifetime clutch warranties  

    Repairers buying a clutch product from Euro Car Parts before 30 March 2018 will benefit from a free lifetime warranty. The offer covers every clutch product supplied by the company, including such brands as LUK, SACHS, Transmech, Valeo and more.

  • DENSO diesel engine efficiency with i-ART  

    DENSO is using its i-ART pressure sensors to deliver optimal fuel injection control. The design of the miniature pressure sensors, which monitor injection activity from within the structure of the injector itself, allows a high level of fuel injection accuracy, monitoring the amount and timing of the process with 1/100,000 second precision. The technology ensures modern diesel engines offer a cleaner, quieter and more fuel-efficient drive.i-ART has already become diesel technology of choice to some of the largest manufacturers in the world, including becoming a key selling-point for Volvo’s eco-friendly Drive-E engine.


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