Although sights like this diesel vehicle belching clouds of smoke and soot have become rare since DPF (Diesel Particulate Filter) devices have become standard (and legally mandated) equipment on all diesel vehicles in 2009, there is still a lot of confusion among non-diesel technicians about DPF's in terms of what they do, and how they work. In many cases, non-diesel techs confuse DPF devices with catalytic converters, and in one recent case, a non-diesel technician of this writer's acquaintance had needlessly replaced an expensive SCR catalytic converter on a 2017 Ford pick-up truck with severe driveability issues because as the technician said at the time, he did not think that the truck was equipped with a DPF. Thus, if you do not specialise in diesel repairs, or have not had much exposure to diesel exhaust after-treatment systems and/or technology, this article should answer all the questions you have ever wanted to ask. Let us start with this question-
While both petrol and diesel engines produce carbon dioxide, water, and nitrogen as by-products of the fuel combustion process, some waste products are unique to diesel exhaust. We need not delve into the chemistry of diesel exhaust here, but two aspects are worth mentioning. The first is that diesel engines produce less carbon monoxide than petrol engines because diesel fuel is always burned in excess air, and the second is that the high combustion temperature of diesel fuel produces about 20 times as much NOx* (oxides of nitrogen) as petrol engines.
* While the nitrogen in atmospheric air remains largely unchanged during the combustion process in petrol engines, the complex chemical reactions that occur during diesel combustion cause the nitrogen and oxygen molecules in atmospheric air to combine to form nitrogen oxide, which along with sulphur in diesel exhaust, causes urban smog when it combines with water and sunlight.
However, modern advances in diesel exhaust after-treatment technology, mainly in the form of SCR (Selective Catalytic Reduction) technology, in combination with EGR (Exhaust Gas Recirculation)* have now made it possible to convert as much as 90% of the harmful substances in diesel exhaust to water, carbon dioxide, and other innocuous substances.
* The introduction of metered amounts of exhaust gas into the cylinders of diesel engines has a quenching effect on the combustion flame. In practice, the amount of exhaust gas that is fed into the combustion chambers is calculated to reduce the combustion temperature of diesel fuel to below 1 800 degrees Celsius to prevent the combination of nitrogen and oxygen.
One further aspect of diesel fuel combustion needs mentioning, though. This aspect relates to stoichiometric air/fuel ratios, which for petrol engines, is 14.7 parts of air, to one part of fuel. In this ratio, all the available air is used to combust all of the fuel, but this principle does not apply to diesel engines.
In theory, the stoichiometric ratio or diesel fuel is 14.5 parts of fuel to one part of air, but since diesel engines are not throttled, a diesel engine’s power output is controlled by varying the amount of fuel that is injected into the cylinders, as opposed to limiting or controlling the amount of air that enters the engine. This is true for both unit and common rail fuel injection systems/engines, but what does this mean in practice?
It simply means that under some operating conditions, a diesel engine can run on an air/fuel mixture that is as lean as 45 (or more) parts of air to one part of fuel, which greatly reduces the load on the exhaust after-treatment system. However, not all of the fuel is combusted fully even at a mixture of 45 parts of air to one part fuel and the unburned, or partially combusted fuel exits the exhaust system in the form of a haze of exceedingly fine solid particles.
The solid particles are about the same size as the particles in cigarette smoke and although individual particles may not be visible to the naked eye in low concentrations, in high concentrations, they become visible as black smoke on diesel vehicles whose exhaust after-treatment systems are inadequate, defective, or have been tampered with.
So, unlike petrol engines that for the most part*, produce only gaseous exhaust products, diesel engines create two distinctly different forms of exhaust emissions; one is purely gaseous, and the other consists of fine, solid particles. Therefore, a targeted method was required to remove the solid particles in diesel exhaust, hence the introduction of diesel particulate filters that can remove as much as 90% to 95% of soot from diesel exhaust, which brings us to-
* The introduction of direct petrol injection technology has seen a steep rise in the concentration of solid particulate matter in petrol exhaust. As a result, all major automotive markets now require the use of petrol particulate filters in the exhaust after-treatment systems of all new petrol vehicles to meet Euro 6 emissions standards. In practice though, it will take several more years before legally required petrol particulate filters are in universal use around the globe.
Before we get to the specifics of DPF operation, we need to clear up a major conception that many members of the motoring public and non-diesel techs struggle with. This misconception involves the differences between SCR (Selective Catalytic Reduction) and DPF operation in general, and the respective roles these two technologies play in diesel exhaust after-treatment systems, in particular. Let us first look at-
Selective Catalytic Reduction
Selective catalytic reduction takes place in a catalytic converter in a process that requires a liquid reductant, which is not to be confused with a catalyst. Catalysts are substances that are not consumed during chemical reactions such as the conversion of one substance into another and in the case of catalytic converters, the catalysts are the various precious metals that coat a porous substrate.
Nonetheless, selective catalytic converters are so-called because the conversion of oxides of nitrogen into water, carbon dioxide, and other harmless substances is selective in the sense that the process targets the destruction of nitrogen oxide, specifically. To do this, the process requires the ammonia content of the reductant, commonly known as ADBLUE, or DEF (Diesel Exhaust Fluid), to initiate and maintain a chemical reaction (that takes place on the surface of the catalytic metals) to convert the oxide of nitrogen to harmless substances.
To do this effectively and consistently, a dedicated injection system is employed to introduce precisely metered amounts of ADBLUE into the catalytic converter. However, if the injection system is defective and too little ADBLUE is injected, the chemical conversion process either does not initiate or complete. On the other hand, if too much ADBLUE is injected the catalysts can be “deactivated”, in the sense that the excess ammonia can prevent contact between the exhaust gas and the catalysts, which when it happens, typically requires replacement of the catalytic converter. This might be mildly interesting to some readers, but the point of the above is that the sole function of selective catalytic reduction is to destroy oxides of nitrogen, and as such, the process does nothing to remove solid particulate matter from diesel exhaust, which brings us to-
DPF operation
In its simplest form, a diesel particulate filter is a simple mechanical filter that uses a porous ceramic medium to trap soot particles as small as one micron in diameter. In practice, the only chemical reaction that takes place in a DPF occurs when the temperature of the substrate is raised to about 600 degrees Celsius by one of several means to oxidise or burn off the accumulated soot particles by using the excess oxygen in the exhaust stream to “fuel” the process. Once oxidised, the oxidised soot particles resemble fine powdery ash, which is then expelled from the DPF by the normal exhaust flow.
The method employed to heat the DPF substrate to the required temperature depends on the vehicle, but the methods in current use include-
NOTE: For reasons that are not entirely clear, some DPF substrates may be coated with an ultra-thin layer of one or more catalytic metals.
One other important distinction between SCR and DPF systems is that while a diesel vehicle will still start and run, albeit poorly when a DPF device is overloaded with soot. On the other hand, the starting circuits on a diesel vehicle with SCR that uses ADBLUE will be disabled when a), the ADBLUE storage tank runs dry, or b), when some types of failures, defects, or malfunctions are present in the ADBLUE injection system.
In practice, SCR and DPF technologies serve completely different purposes, and while either system can operate independently of each other, modern diesel exhaust after-treatment systems depend equally on both technologies to be compliant with Euro 6 emissions regulations, which brings us to-
As stated elsewhere, a DPF device is a simple mechanical filter, and like any filter, it eventually becomes overloaded or clogged with the substance it is intended to trap or remove. Therefore, to monitor the level of soot accumulation in the device, dedicated pressure sensors are mounted on either side (directly upstream and immediately downstream) of the device to provide the ECU with input data.
Since the ECU cannot monitor the condition of the substrate directly, it uses the difference in pressure readings before and after the DPF device to infer the level of soot accumulation in the substrate. Note though that due to the construction of DPF devices, there will always be a slight pressure difference before and after the device, but this is factored into the lookup tables the ECU uses to determine soot loads in a DPF.
In practice, DPF devices are at their most efficient when the soot load is above about 30%, but under 75%, at which point an ECU will typically illuminate dedicated warning light, and initiate a regeneration process to burn off the accumulated soot, but this is where things can become complicated, so let us look at the various-
The word “regeneration” refers to the process of clearing accumulated soot from the DPF device to restore its efficiency. However, while all manufacturers state that the regeneration process on their products will typically occur after “X” amount of driving at “Y” kilometres per hour, or after “Z” number of hours driving in urban conditions, these statements mean very little in practice and should be taken as guidelines, at best.
Under real-world driving conditions, DPF regeneration intervals are greatly influenced by fuel quality, driving style, how the vehicle is used, the overall mechanical condition of the engine, and perhaps most importantly, by the aggregate amount of driving at both highway and urban speeds. Moreover, the degree to which any given regeneration event manages to restore a DPF devices’ efficiency is greatly dependent on a) whether or not the previous generation event completed successfully, and b) how many previous regeneration events did not complete successfully, which happens more often than you might think. Nonetheless, let us look at the different regeneration processes, starting with-
Passive regeneration
This process happens automatically every time the ECU calculates a soot load that approaches about 75% or so, with the vehicle travelling at a predetermined speed, which is different for different vehicles. During the process, which happens without the need for driver inputs, the exhaust gas temperature is raised to about 600 degrees Celsius to initiate the process, and this temperature will be maintained until the pressure difference across the DPF device returns to an acceptable level. In practice, a passive regeneration process can take anything between about 5 minutes, and about 20 minutes to complete.
Active regeneration
As with passive regeneration, active regeneration occurs automatically, and without the need for inputs from the driver. Depending on the system, the ECU will either deliver a pulse width modulated current to the DPF's heater element or perform several injections of small amounts of raw fuel (and alter the exhaust valve timing) into the DPF to increase the substrate's temperature to the point where the accumulated soot is oxidised. This process will also take about 20 minutes or so to complete.
Hybrid active/passive regeneration
While both active and passive regeneration methods are effective under suitable conditions, vehicles used in urban environments have the potential problem that a) the regeneration process can be interrupted during short trips, and b), that the exhaust gas never gets hot enough to initiate a passive regeneration, in the first place.
In cases where a regeneration process is interrupted, the process will not be re-initiated automatically when the engine starts again, and in cases where regenerations happen seldom because conditions are rarely suitable, the DPF will relatively quickly clog up to the point where the exhaust system becomes effectively plugged.
To prevent this, newer vehicles use a hybrid system in which active regeneration is initiated when a certain number of passive regenerations either did not initiate or did not complete. However, while this strategy has greatly increased the useful life of DPF devices on vehicles that are primarily used in urban environments, predominantly urban driving still clogs up DPF devices on all applications quicker than vehicles that are occasionally used on highways.
Forced regeneration
In practice, a DPF is deemed clogged when its soot load approaches about 90%, at which point a forced regeneration can be performed with a scan tool under controlled conditions. However, the bad news is that there is never a guarantee that a DPF’s efficiency will be restored with a forced regeneration, and in these cases, replacement of the DPF with an OEM part or OEM-equivalent part is the only reliable long-term remedy.
Moreover, a forced regeneration is not only very much a hit-and-miss affair; it is also a potentially dangerous one if it is not done strictly according to prescribed procedures since the heat that is generated during a forced regeneration is localised. Therefore, it is very easy to set a vehicle on fire during the process, so use extreme caution whenever you have to perform this procedure, which brings us to this question-
Despite claims made by various service providers, the simple fact is that unlike DPF devices on heavy trucks and construction equipment, diesel particulate filters that are designed and intended for use on light vehicles cannot be repaired and/or cleaned.
There are several reasons for this, not the least of which is the fact that heavy-duty DPF’s are specifically designed so that they can be purged with specialised equipment to reduce equipment maintenance costs. For instance, the pores in heavy-duty DPF substrates are significantly bigger than the pores in light-duty devices, which makes it possible to force various liquid chemical cleaners through a heavy-duty device under extreme pressure after an off-vehicle regeneration using a kind of baking oven. While this process is expensive, cleaning a heavy-duty DPF comes to only a fraction of the cost of a new unit.
Nonetheless, most manufacturers of passenger and light-duty vehicles guarantee their DPF devices for around 160 000km, or between 8 and 10 years, which means that DPF replacements are not common occurrences on most diesel vehicles. However, be sure to check with the dealer before you replace a DPF on a customer’s vehicle because the DPF might still be under warranty, which makes it important to-
Since all DPF faults, failures, malfunctions, and/or defects can potentially affect exhaust emissions, all DPF faults, failures, malfunctions, and/or defects will set fault codes and/or illuminate warning lights.
However, some fault codes occur more often than others do but bear in mind that many DPF-related fault codes are set as secondary indicators that a failure, malfunction, or defect is present in a related system. This, will, however, always be reflected in the order in which trouble codes are set and stored, such as when an EGR-related is stored first, with (for example) one or more DPF catalyst efficiency codes stored after it. In such a case, the original problem involves the EGR system, and it is almost certain that a long-term defect in the EGR system had overwhelmed the DPF’s ability to regenerate successfully.
Similarly, excessive oil consumption due to excessive engine wear might overwhelm both the catalytic converter and the DPF, but in this particular case, there will be additional signs such as loss of power, rough running, a hard or no-start condition, or either white or black smoke from the exhaust to point you in the right diagnostic direction. Limited space precludes listing other possible examples of DPF related problems arising as secondary (or consequential) failures here, so we strongly recommend that you consult this resource to get a more complete overview of specific DPF-related problems and diagnostic issues. In fact, we suggest that this resource should be required reading even for seasoned diesel techs since we could all benefit from an occasional short refresher course.
For the most part, though, most DPF-related issues involve nothing more complex than simple wiring issues such as abnormally high or low resistances/currents/voltages in DPF related wiring. Other common issues involve range or performance issues in DPF pressure sensors, or catalyst efficiency issues on old(er) DPF filters- all of which are easy to diagnose and repair without specialised equipment or access to specific OEM service information.
However, since light-duty DPF filters cannot be repaired or cleaned internally, these units are often collected from recyclers and just cleaned off and polished somewhat on the outside to make them look “new”. Therefore, this writer strongly recommends that you a) avoid so-called “rebuilt” or “refurbished” DPF filters when you have to replace one.
Moreover, since you never get a meaningful warranty with a so-called rebuilt DPF device, you can never know what you are getting for your customer’s money. Thus, we suggest that you stick with OEM parts, or at least, with new aftermarket units supplied by reputable suppliers that are known to be equivalent to OEM parts in terms of fit, form, function, and performance, which leaves us with this-
We hope that this article has both given you new insights into modern diesel exhaust after-treatment systems, and cleared up much of the confusion and misinformation about DPF filters that abound online. We also hope that this article has inspired you to become more involved in diagnosing and repairing diesel exhaust after-treatment systems, because a) they are far less complicated than you might have believed, and b), because they all work in the same way regardless of the make, model, or price tag of the vehicle.
