LIVING WITH Diesel particulate filters

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From February 2014, DPFs are mandatory MoT test items. ROB MARSHALL and KIM HENSON explain how to stay on the right side of the law and reduce the chance of DPF breakdowns, while uncovering what cost-effective alternatives to removal are available.

he demands placed on diesel engines, by both customers and legislators, to be more powerful, refined, economical and less

polluting, have seen complexity levels soar, while reliability has nosedived. Located within the exhaust system, the diesel particulate filter (DPF) started to appear on cars from the early 2000s but became especially popular by mid-decade, prior to becoming mandatory as a result of Euro V emissions legislation in 2009. If DPFs were not fitted to new cars from this time, carmakers could not sell them within Europe.

DPFs & AIR QUALITYDPFs fulfil environmental directives by capturing soot, which includes particles of unburned hydrocarbons and oil. The trapped matter is then super-heated at pre-determined times, when it combusts into less harmful emissions, during a self-cleaning process called regeneration.

While many people feel they are unaffected by vehicle emissions, excessive

quantities carry a social cost in built-up areas. Certain sized particulates, especially PM2.5, are so small (approximately 1/30th of a human hair’s width) that not only can they be inhaled but their toxins can also be absorbed straight into the bloodstream.

Should a DPF be removed, tailpipe pollutants would increase, as witnessed in the City of London, which has struggled recently to meet EC-established clean air standards. In March 2013, it was reported that the European Commission was investigating the use of pollution suppressants that had been found around air monitoring stations to ‘disguise’ high levels of particulates, which might lead a reasonable person to conclude that the city is still struggling to meet its targets. Obviously, the situation is not helped by an increasing number of passenger cars with their DPFs removed.

WHAT GOES WRONG?DPF blockages can be caused by poor maintenance, which leads to high engine wear, or by the use of a non low-SAPS lubricating oil. However, stop-start journeys, short, low-speed trips and erratic use of the accelerator at cruising speeds tend to be the main culprits. The resultant excessive back-pressure can also damage the turbocharger in extreme cases.

On some models where the DPF is fitted close to the engine, passive regeneration can occur on a fast motorway trip, where the temperatures of the fast-flowing exhaust gas are sufficient to vaporise the trapped soot. Otherwise, the car has to regenerate its DPF manually, a process that, on most models, involves diesel being injected into the engines’ cylinders on the exhaust stroke. Yet, regeneration causes some fuel to contaminate the lubricating

LIVING WITH

Diesel particulate filters The DPF’s ceramic filter medium is housed within

a metal canister, situated either beneath the vehicle or close to the turbocharger.

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NOx storage catalyst

Diesel particulate filter with oxidation

catalyst and H2S slip catalyst coating

The Euro V ‘dry’ system from Volkswagen, with the DPF mounted close to the engine.

From next year, the NOx trap will be mandatory to comply with Euro VI

requirements, but it is already found on many current models.

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oil, thereby shortening its life. A number of cars, especially those that use a ‘dry’ DPF system, are prone to having their oil levels rise above a safe limit, which can, in extreme cases, cause the engine to consume its lubricating oil, making it rev

uncontrollably, often to destruction.The excess fuel evaporates inside the exhaust manifold and burns within the DPF, generating temperatures of approximately 600°C, which incinerates the

soot into, mainly, carbon dioxide and water. This active regeneration

cycle takes place every 300 miles typically. It lasts between 10 and 30 minutes, and is triggered when the DPF exceeds approximately 40% of its soot-carrying capacity. For an active regeneration to be completed successfully, engine coolant, exhaust gases and oil must be at their nominal operating temperatures, the vehicle road speed needs to be in excess of 30mph, the fuel tank should be at least

a quarter-full and the engine speed must be above 1850rpm, with the accelerator pedal movement not erratic, as releasing it suddenly could cancel the regeneration cycle. On some models, should more than a certain number of regenerations be aborted prematurely, the car might require a forced regeneration to be carried out at a workshop. A dashboard warning lamp, the type of which varies between models, tends to illuminate once the DPF soot capacity exceeds 55%, and ‘limp-home’ mode might be activated should the loading pass 70%.

MAINTENANCE REQUIREMENTSContrary to car-makers’ claims, DPFs require maintenance. ‘Wet’ DPF systems are equipped with an extra additive reservoir, situated close to the fuel tank, containing a fluid that reduces the

temperature at which the soot burns. By sensing how much fuel is added to the main tank, a corresponding quantity of additive is dosed into the diesel. Extra sensors are used, including a magnet built into the fuel cap, the accidental dropping of which will destroy the magnet and might result in the cap being undetected. Pioneered by Peugeot in the early 2000s and first fitted to the 607, wet additive systems tend to be specified for models that do not have their DPFs situated close to the exhaust manifold.

The fluid requires topping up at certain service intervals, adding to the costs, but ignoring any dashboard warning will cause the filter to fill quickly and result in unsuccessful regenerations. In addition, both the soot and fluid leave behind ash deposits that are not vaporised during regenerations, which is why many companies recommend that the DPF is either cleaned chemically or renewed every 80,000-150,000 miles.

Differential pressure sensor

Oxygen sensor

NOx trap

Temperature sensor

Diesel particulate filter

Temperature sensor

Oxygen sensor

Catalyst inlet temperature sensor

ECU

Several pressure and temperature sensors constantly inform the ECU of the DPF’s soot loading and temperature. Other parameters, such as the engine coolant temperature, throttle position and speed sensors are also considered.

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THE LEGALITIES OF REMOVALA nationwide DPF removal industry has sprung up, offering an inexpensive service that removes the filter totally and then reprogrammes the engine’s ECU so that no more active regeneration cycles are triggered. As the filter acts as a restriction within the exhaust system, this results in a slight increase in torque and, because active regenerations are negated, fuel consumption is likely to decrease. Obviously, with no filter to block, it cannot fail again.

The work can be carried out in several ways, depending on the car model. The simplest method is to remove the DPF, including sensors, and fit a bypass pipe. Alternatively, on models with additive systems that have a separate catalytic converter, the DPF is removed and its filter smashed out, prior to the empty metal canister being refitted. On wet DPF systems, the additive dosing unit tends not to be removed, although some specialists fill the reservoir with diesel. The other method, used commonly on DPFs that combine the filter and catalyser (including most VAG models), is to cut the canister open and remove the

filter, although most removal companies leave the catalyst in place; the filter housing is then welded back together and refitted. However, because the catalytic converter’s purpose is to promote Nitrogen Dioxide (NO2) production, which not only raises the temperature within the DPF but also reacts with the soot to form less toxic emissions, tailpipe output of not only particulates but also harmful NO2 will increase.

While the most recent European Directive (2010/48/EU) states that any diesel car’s emission control equipment should neither be missing nor defective, the UK is obliged, as a member state, to enforce it. When Car Mechanics investigated this issue in October 2013, a spokesperson for the Department for Transport (DfT) advised: “Drivers should not disable a diesel particulate filter in a vehicle fitted with this device.”

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The more complicated ‘wet’ system (right and below), which utilises an additive to reduce regeneration temperatures. Pioneered by Peugeot 15 years ago, the system remains in use by a variety of manufacturers.

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Body computer (BSI)

Additive tank

Additive valve

Additive pump

Fuel tank

Engine ECU

Fuel gauge sender/diesel

pump unit



However, the Vehicle and Operator Services Agency (VOSA) admitted that while it should enforce the DfT’s interpretation of the law, it was not implementing the European Directive on diesel emission systems at the time. Furthermore, according to Regulation 61A of the Road Vehicles (Construction and Use) Regulations 1986, it is an offence to use a car on the

road that has been modified so that it cannot meet the emissions requirement that applied to it when new and this refers not to the MoT smoke opacity inspection but the European Emissions Standards.

The thorny issue of DPF removal was raised in Parliament several times last year and, in early December 2013, VOSA stated that the presence of a standard-fit DPF must

be checked for its existence as part of the MoT from February 2014, matching the same requirement for a petrol car’s catalytic converter. This does not mean that only Euro V models will be inspected. In the early 2000s, some models were sold with the option of a particulate filter – if one of those cars came with a DPF from new, it must still be fitted in order to pass its MoT.

KIM HENSON takes a look at how a DPF can be cleaned to extend its effective operating life and save cash.

The gradual clogging of a DPF filter is inevitable over time, with the effects being far worse in vehicles used mainly

for short journeys. Preventative measures – in the form of cleansing additives poured into the fuel tank on a regular basis – can be adopted to help keep the DPF clean for longer, thereby postponing the time when deeper cleaning or the costly renewal of the filter is required.

One such cleansing product is Cataclean, which is available from motor factors. Developed in 1999 and the flagship product of System Products, it is a fuel system cleaner for petrol, diesel and hybrid engines. The makers recommend adding a 475ml bottle of Cataclean to the fuel tank when it’s about one-quarter full, then running the engine for about 20 minutes (either driving the vehicle or with it stationary).

It is claimed that Cataclean can help prevent problems in DPF systems by reducing soot emissions by up to 60%, assisting the DPF to regenerate and therefore reducing the risk of it becoming clogged. However, it is recommended

that the product is used about three or four times a year as a preventative treatment, rather than trying to use it to deal with problems of clogging after they have already arisen.

It’s said by the firm that further benefits of using Cataclean include improvements in fuel efficiency and performance.

DEEP CLEANINGIn cases where the car’s DPF system will not regenerate and/or the filter has reached its limit in terms of the accumulation of soot and ash deposits, there are several options to consider.

One is to have the filter renewed, which, at main dealer prices, will typically cost in excess of four figures (although aftermarket filters are less expensive). A cheaper but still effective alternative to replacing the original filter is to have it removed from the vehicle and cleaned.

Again there are several options with regard to the method adopted for cleaning the filter. Whichever route you choose, check that the firm offering the service is a properly established business taking its customers seriously and operating with environmental aspects in mind.

Many approaches involve liquid-based cleaning processes, but some tend to be comparatively inefficient (including jet washing), resulting in a short-term solution to the problem simply because the method doesn’t always remove enough soot and ash. In the case of certain dodgy operations, the deposits released from the filter are then simply released into the atmosphere, which negates the point of the DPF in the first place. Other water-based cleaning methods are much more efficient in removing soot and ash, but it is still wise to check that the sludge which results from the process is disposed of in an environmentally-friendly manner.

Thermal cleaning processes are also available. These have been used for many years on off-road and heavy goods vehicles, and the techniques employed are now being applied to passenger cars and smaller commercial vehicles. With thermal cleaning, initially loose materials are carefully removed

DPF CLEANING

Emptying a bottle of Cataclean into a diesel engine’s fuel tank can reduce the DPF’s soot content by as much as 60%.

The catalytic converter is separate to this Euro IV DPF, meaning that the filter can be renewed independently and, therefore, the cost of the replacement part is likely to be cheaper than a filter and catalyser in a single housing. On this model, the cat is situated to the right of the clamp.

Active regeneration takes place once a set number of parameters have been met, one of which is the exhaust gas temperature that is detected by this sensor.


However, as removal companies argue correctly, removing a DPF will still permit a vehicle to overcome the MoT’s smoke opacity emissions test. In addition, unless a car is fitted with an obvious decat pipe, an MoT tester will be unable to inspect whether or not a DPF canister is empty. While a diagnostic check, or removal of a pressure sensor and inserting a camera into the housing, might be possible, those operations introduce a level of dismantling to the inspection, which could be a very slippery slope. Worryingly, to circumnavigate the legislation, many DPF removal companies have added a codicil to their services, stating that DPF removal is for off-road use only.

Tricking the testerPosing as potential customers, we contacted several DPF removal companies at random, whose staff told us that not only would they continue to remove these filters but they would also intentionally leave the empty canister in place, so that an MoT tester would be tricked into thinking the filter medium was present inside.

While the new test requirement might not deter some motorists from going ahead with DPF removal, insurance companies are clear on the subject. Even prior to the filter being included in the MoT’s scope, insurers were advising that they would need to be informed if a DPF were removed, although 80% of the removal companies that we approached admitted that they never advised their customers to do so. Interestingly, the Direct Line Group, owner of both Churchill and Privilege brands, stated that it would decline cover in all cases where a standard-fit filter was removed. However, since the item was inspected for its presence (and, by implication, that it must work) at MoT time, a spokesperson from the Association of British Insurers told CM: “If motorists remove their diesel particulate filters, then have an accident, they will need to consider the consequences of not holding valid insurance.” Obviously, this would include driving a car with an empty DPF canister.

It is possible that you could buy a vehicle that has had its DPF removed by a former owner and remain uninformed. When we quizzed them on this, some

insurance companies stated that a degree of leniency might be applied. However, this should not be seen as a reason to assume automatically that a converted car would be insured. We would advise any buyer to protect themselves and get the vendor to state in writing that the standard-fit emissions control equipment remains intact.

from the filter and captured by pulsing with air. The filter is then placed in a purpose-designed oven, to be baked for up to eight hours at temperatures in excess of 700°C. On completion of this part of the treatment, the filter is then again pulsed with air to remove the resulting ash.

A major player in this type of thermal cleaning business is DPF Genie (Tel: 020 7998 7702; www.dpfgenie.co.uk). The firm charges from £234 (inclusive of VAT), which includes collection and delivery of the filter. No jet-washers, soaking, chemicals or liquids are used in the DPF Genie cleaning process.

CLEANING IN SITUAnother option for cleaning the DPF involves chemicals being introduced to the exhaust system/DPF, usually via the Lambda (oxygen) sensor aperture, having first carefully removed the sensor. The idea is to reduce internal contamination so that normal regeneration can then take place. With this approach

the filter is still fitted to the vehicle. This avoids – or, at least, reduces – the need for time-consuming and possibly damaging dismantling/reassembly.

The precise procedural details vary from system to system, but typically, with the exhaust system at a low temperature (below about 40°C), the Lambda sensor is unscrewed and removed, then a chemical cleaning agent is injected under pressure into the DPF using purpose-designed equipment, and allowed to soak into the soot/debris contamination within the DPF.

A second, purging chemical liquid is then injected into the DPF and the Lambda sensor is refitted. This is followed by running the engine at tickover for around 15 minutes, until normal operating temperature has been reached.

It is said that the particles of soot freed up by the cleaning and purging process are distributed within the DPF and can then be burned off during a regeneration, which, in the final stage of this cleaning process, needs to be triggered in accordance with the car manufacturer’s recommendations.

DPF Genie offers a DPF cleaning service. You’ll need to remove the filter from the vehicle, but it collects as part of the deal.

Up to eight hours in an oven gives your DPF a new lease of life. DPF Genie will return the unit with a certificate of cleanliness.

BEFORE CLEANING AFTER CLEANING

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To prevent high levels of sulphated ash and phosphorus residue from blocking the DPF prematurely, a low SAPS engine oil must be used at every lubrication service, including top-ups (ACEA C grade). Check that any oil you buy conforms to your model’s requirements.


WHAT ARE THE ALTERNATIVES?As DPF removal contravenes Construction and Use, the European Directive and MoT regulations, as well as voiding your insurance policy, removal is no longer a practical option.

The obvious alternative is to replace a blocked DPF with a new one. Several years ago, this was uneconomical, but aftermarket companies have developed more affordable options. Bear in mind that a DPF, which includes the catalytic converter, will be more expensive. For example, a main dealer price for a combined DPF and catalyser unit for a 2007 Audi A4 2.0-litre is approximately £1150, but the same aftermarket part could be bought from a supplier such as Euro Car Parts for around £400. It is hard to ascertain whether or not an aftermarket part is inferior to the OE main dealership item, because both parts have to be Type Approved, to comply with the same performance and longevity standards. The ability of the more expensive DPF to exceed these requirements over the alternative part is harder to qualify.

Depending on the severity of the blockage, DPFs can be treated, without removal, by either dosing the fuel tank manually with a cleansing additive, or by unscrewing one of the pressure pipes and injecting cleansing fluids into the filter, to help soften and disperse the soot.

In more extreme cases of blockage, and to purge the filter of any additive ash, the filter will have to be removed from the car and cleaned chemically. Fortunately, cleaning companies can perform this operation by mail order, many of them returning your DPF within 48 hours.

RIGHT: Competent diagnostic software should be able to identify precisely how the car has been driven and the estimated remaining life of its DPF. This car has spent 40% of its 60,000-miles being driven on either fast roads or motorways, which usually means fewer active regenerations have been carried out and that there is longer DPF life expectancy.

Many Euro V engines employ two oxidising catalysts to help produce nitrogen dioxide (NO2),

which promotes rapid heating within the DPF.

ABOVE: Real-time diagnostics can reveal if any sensors are faulty – on ‘wet’ systems they also indicate how much additive is remaining. It is running low on this example.

Catalyser

Pressure sensor

Temperature sensor

FAP Silencer

DPFs can be troublesome but

removing them is now illegal. Yet,

providing that they are maintained and the car is not used for short

journeys continually, the part can be both

long-lived and reliable.

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LIVING WITH Diesel particulate filters