Whether you own a diesel-powered vehicle or aspire to become a diesel technician, it’s important to understand diesel particulate filters (DPFs) and how they work.
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What is a DPF system? DPFs are crucial in reducing emissions from diesel vehicles by trapping soot and ash. They are made from a ceramic material and require periodic regeneration to burn off the accumulated soot. DPF regeneration is essential for maintaining the filter's functionality. There are two types of regeneration: passive, which occurs automatically during normal vehicle operation, and active, which involves additional measures to increase exhaust temperatures to burn off soot.
If passive and active regeneration fail, a parked regeneration may be necessary. This process requires the vehicle to be stationary and can take up to an hour, depending on various factors. It's important to ensure the exhaust is directed away from flammable materials during this process.
Symptoms of a blocked DPF include reduced vehicle performance and an orange warning light on the dashboard. Blockages can be caused by short, low-speed journeys; poor servicing; incorrect oil; low-quality fuel or running the vehicle on a low fuel level.
Replacing a DPF can be expensive, ranging from $3,000 to $10,000. Regular maintenance and understanding how DPFs work can prevent costly replacements. For those interested in diesel engines, training to become a diesel technician can lead to a fulfilling career path, with programs available at various Universal Technical Institute (UTI) campuses.1
Follow along as we share what diesel particulate filters are, types of regeneration and more.
A DPF is an exhaust aftertreatment device that traps particulate matter such as soot and ash.
To reduce emissions from diesel vehicles, diesel particulate filters capture and store exhaust soot, which must be periodically burned off to regenerate the filter. The regeneration process burns off excess soot deposited in the filter, which prevents harmful exhaust emissions and the black smoke you commonly see emitted from diesel vehicles when accelerating.
Engine manufacturers use DPFs to trap particulate matter to meet the EPA emission standards.
The DPF system captures and removes soot and particulate matter from diesel engine exhaust. As exhaust gases pass through the filter, fine particles are trapped in the porous walls, preventing them from being released into the air. Over time, the filter undergoes a process called regeneration, where high temperatures burn off the accumulated soot, converting it into ash and keeping the DPF functioning efficiently. Regular maintenance and proper operation are essential to prevent clogging and ensure optimal engine performance.
Often, blocked diesel particulate filters are caused by short journeys at low speeds. Vehicles operating at low speeds on short journeys are unable to meet the requirements for the filter to clean itself.
DPFs can also fail due to poor servicing. The lifespan of a diesel particulate filter varies based on the application. For example, the Cummins ISX15 engine’s filter has an interval for cleaning up to 400,000 to 600,000 miles—although it will need to regenerate before hitting the 400,000-mile mark.
DPFs may fail sooner if they are not well maintained. Additionally, filter blockage can be caused by using the wrong type of oil, performance modifications, using low-quality fuel, or even running the vehicle frequently on a low fuel level.
Using the wrong oil or fuel can contribute to DPF blockages and failures. Low-quality fuel or high-ash engine oil can produce excessive soot and residue, leading to faster clogging of the filter. To prevent issues, always use manufacturer-recommended low-ash engine oil and ultra-low sulfur diesel (ULSD) to ensure cleaner combustion and reduce particulate buildup in the DPF.
So how can you tell if your filter is blocked? Typically, when the filter becomes clogged or an error occurs in the system, an orange warning light will appear on your dashboard. This light varies based on the manufacturer but commonly appears like the image below. When this lights up, you know your filter is most likely blocked and regeneration may be required.
Just like two main particulates are being filtered, there are two types of cleanings that are required. Regeneration cleans out the soot by converting carbon to carbon dioxide, and the ash is removed by removing the filter and cleaning it in a machine with compressed air.
Read more: Diesel Truck Maintenance Guide
The key to maintaining a DPF is to ensure regeneration is possible when it fills with soot (triggering the warning light). The two types of regeneration include passive and active.
Inside the aftertreatment device (ATD), the exhaust first passes over the diesel oxidation catalyst (DOC), then passes through the particulate filter, which traps soot particles. Passive regeneration happens when heat in the engine builds to the point where soot, or carbon, is combined with oxygen to create carbon dioxide. Since carbon dioxide is a gas, it can pass through the filter.
Ash, on the other hand, is already a byproduct of combustion, so no amount of heat from the engine can convert it. Over time, the ash will build up to the point where the filter has to be removed and cleaned. This filter can then be reinstalled and reused.
Passive regeneration occurs as the vehicle is driven normally under load; the driver is not aware that it is happening. It may not always keep the DPF clean so the filter may have to undergo active regeneration.
Passive regeneration is part of normal engine operation; however, active regeneration requires the engine to take action. For example, a truck fully loaded with 80,000 pounds moving down the highway will create enough heat in the engine for a chemical reaction to occur—which is passive regeneration.
Active regeneration takes place when the engine isn’t creating the heat it needs. For example, this may occur in a truck that’s not fully loaded. Once the soot level reaches a certain point, fuel is injected into the exhaust stream, which goes over the oxidation catalyst and oxidizes the fuel to create heat. The heat created from the fuel oxidizing is then used to convert soot to carbon dioxide.
Both active and passive regeneration happen automatically and without driver input. Active regeneration can occur automatically any time the vehicle is moving. The exhaust gas temperature could reach 1,500 F (800 C). Active regeneration is unknown to the driver except for some additional dashboard lamps being lit. The biggest sign to look for to determine if it is taking place is the “high-exhaust temp” light, which will illuminate once the aftertreatment doser starts to inject, increasing the temperature in the aftertreatment device.
When operating conditions do not allow for DPF cleaning by active or passive regeneration, the vehicle may require an operator-activated parked regeneration.
For this to take place, the vehicle must be standing still. The driver or technician brings the engine to operating temperature and initiates the parked regeneration by activating the dash controls. This may take anywhere from 20 minutes to an hour, depending on ambient conditions and the engine type or DPF system.
Before initiating a parked regeneration, it’s critical for the driver or technician to ensure the exhaust outlets are directed away from structures, vegetation, trees, flammable materials and anything else that may be damaged or injured by exposure to high heat. Not all DPF systems have a parked regeneration feature.
Replacing a diesel particulate filter can be pricey. A new filter from a car manufacturer can cost anywhere from $3,000 to $10,000.
It’s no secret that as cars and trucks age, their value decreases. Often, the price associated with replacing a DPF in an older, higher-mileage car or truck is more than the value of the vehicle itself. It’s much easier (and more affordable!) to clean a diesel particulate filter than it is to replace it, which is why understanding how these filters work and performing regular diesel maintenance is so important.
If a parts supplier charges less for a DPF, be wary—the filter must be the correct type for your vehicle. Otherwise, you will most likely end up spending more on repairs.
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If you’re fascinated by the complexity of diesel-powered systems and love the roar of a diesel engine, consider turning your passion into a career by training to become a diesel technician. At Universal Technical Institute (UTI), you can prepare for an exciting career in the field in less than a year.1,7
Once students complete their core training program, they can decide to further their education with one of our specialized diesel programs. One of these is the 12-week Cummins Engines program, where students have access to the manufacturer's full line of diesel equipment for hands-on training.9 Students can earn Cummins qualifications that prepare them to do warranty work at an authorized Cummins dealer or distributor!
SCR is an active emissions control system. Hot exhaust gases flow out of the engine and into the SCR system where aqueous urea (known as Diesel Exhaust Fluid, or DEF) is sprayed onto a special catalyst. The DEF sets off a chemical reaction in the exhaust on a special catalyst that converts nitrogen oxides into nitrogen, water, and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe. The exhaust also passes through a particulate filter somewhere in the system and then is then expelled through the vehicle tailpipe.
The design of SCR technology is such that it permits nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called "selective" because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The chemical reaction is known as "reduction" where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water, and tiny amounts of CO2. The DEF is rapidly broken down to produce the oxidizing ammonia in the exhaust stream.
SCR technology is one of the most cost-effective and fuel-efficient technologies available to help virtually eliminate emissions from diesel engines. Since , all heavy-duty diesel truck engines utilize SCR technology to comply with the latest EPA emissions standards.
These standards require reducing particulate matter (PM) and nitrogen oxides (NOx) to near zero levels.
SCR can reduce NOx emissions up to 90% while simultaneously reducing HC and CO emissions by 50-90%, and PM emissions by 30-50%. SCR systems can also be combined with a diesel particulate filter to achieve even greater emission reductions for PM.
The high efficiency of SCR systems in reducing NOx emissions allows manufacturers to balance engine performance and maximize fuel economy, while still achieving near zero emissions. Some SCR-equipped heavy-duty commercial truck operators report fuel economy gains of more than 4%.
Diesel Exhaust Fluid (DEF) is a non-toxic fluid composed of 32% automotive grade aqueous urea and purified water. DEF is available with a variety of storage and dispensing methods. Storage options consist of various size containers such as bulk, totes and bottles or jugs. The American Petroleum Institute rigorously tests DEF to ensure that it meets industry-wide quality standards. DEF is sometimes referred to as AdBlue in Europe and in the US.
A nationwide DEF distribution infrastructure is in place making DEF is readily available in bulk, at retail stores, online and nearly all fueling stations that carry diesel fuel. On-board tanks to store DEF typically range in size from 5 to 22 gallons and are easily identified by a blue cap and/or labeling “DEF ONLY.” For pick-up trucks and SUVs, the DEF filler port is typically located alongside the fuel filler nozzle area. (SEE IMAGE) For heavy duty tractor trailers, the DEF tank is typically alongside the diesel fuel tank on the side of the vehicle near the driver’s door. Due to the diverse nature of off-road engines and equipment, the location of the DEF tank and filler port is variable.
The DEF tank fill opening is designed to accommodate a DEF fill nozzle only to ensure only DEF is put into the tank. Diesel fuel should never be put in DEF tank and vice-versa. To protect against this misfuelling, a diesel fuel nozzle will not fit into the DEF tank opening. In addition, the DEF tank has a blue lid to differentiate it from the diesel tank which may have a yellow or green fuel cap.
For light-duty vehicles, DEF refill intervals typically occur around the time of a recommended oil change, while DEF replenishment for heavy-duty vehicles and off-road machines and equipment will vary depending on the operating conditions, hours used, miles traveled, load factors and other considerations. Typically, DEF consumption is around 3% of fuel consumption: (example:100-gal fuel consumption would consume about 3 gallons of DEF).
DEF is an integral part of the emissions control system and must be present in the tank at all times to assure continued operation of the vehicle or equipment. Low-DEF supply triggers a series of escalating visual and audible warning indicators to the driver or operator. If the DEF is not replenished, the series of operator inducements progresses and eventually can lead to derating the engine and limiting vehicle speed, and ultimately locking out the starting system.
Proper storage of DEF is required to prevent the liquid from freezing at temperatures below 12 degrees Fahrenheit. Most vehicle DEF tanks and dispensing systems have warming devices.
Whether you're looking to improve efficiency, reduce costs, or stay ahead in the market, diesel emission parts can offer significant value, and you can make better choices tailored to your specific needs.It’s been 56 years since the Clean Air Act was passed, but the last half of those years have been the busiest for diesel engines. Beginning in , more stringent federal emission standards were introduced to get engine manufacturers to cut down on particulate matter (PM). By , a further 60-percent reduction in PM was made mandatory. Then in , stricter limits on nitrogen oxides (NOx) were imposed, with maximum limits set to gradually tighten up. The next crackdown on NOx emissions came in , prompting Ford to debut its Navistar-built 6.0L Power Stroke with exhaust gas recirculation (EGR) and GM to add EGR to the 6.6L Duramax.
As of January 1, , PM emission limits were lowered once more, this time to 0.01 g/bhp-hr—a 96-percent reduction from the standard. At the same time, manufacturers had to meet a NOx level that was 90-percent lower than the standard that had just taken effect in , although it didn’t take effect immediately. Manufacturers were allowed to use the period from to as a phase-in period for the new, extremely stringent NOx standard. As a result, Ford and GM both waited until to meet the standard, which required the use of selective catalytic reduction (SCR), and that Ford and GM debuted on the new 6.7L Power Stroke and LML Duramax (launched as models).
To meet the ever-increasing regulations mentioned above, a plethora of pollution-combatting technology has been employed. In the early days (s), changing piston designs, altering injector spray angle or switching to electronically controlled injection systems helped curb a lot of emissions, but not all of them. As a result, higher injection pressures and especially high-pressure common-rail systems were turned to in order to meet PM requirements. Then came the aforementioned EGR systems, which helped drop NOx levels. Further PM and NOx-fighting technologies such as the diesel oxidation catalyst (DOC), diesel particulate filter (DPF), active and passive regeneration, selective catalytic reduction (SCR) and diesel exhaust fluid (DEF) were soon to follow.
Confused by all the fancy acronyms? Scroll down to find exactly what they mean and how they function.
The first pollutant on the EPA’s radar was particulate matter (PM). In a diesel engine, PM is the result of unburned fuel, as in fuel that isn’t completely used during the combustion process and is allowed leave the cylinder through the exhaust valves, flow through the exhaust system and out the tail pipe in the form of black smoke. Mechanical (i.e. older) fuel systems with less precise injection events occurring in-cylinder were notorious for producing PM. PM itself is a complex makeup of sulphates, carcinogenic compounds, elemental carbon and heavy metals.
Nitrogen oxides (NOx) are a direct result of the high operating temperatures diesels are capable of and are a primary ingredient in smog. They form when nitrogen is released during combustion and conjoined with oxygen. Various oxides of nitrogen are produced when extreme in-cylinder heat is present, namely nitrogen dioxide and nitric oxide, which means a fine balance between NOx (achieved with lower in-cylinder temps) and PM (achieved with complete combustion/high in-cylinder temps) is paramount if a modern diesel engine is to keep both pollutants at reasonable levels.
In the early s, PM standards were met through the use of electronically controlled injection systems and higher in-cylinder injection pressures. However, the higher combustion temperatures resulting from controlling PM are counteractive to controlling NOx. To cool in-cylinder combustion temps and reduce NOx, high-pressure loop exhaust gas recirculation quickly became the norm. Being that EGR gases are almost completely depleted of all oxygen content, the engine is denied the oxygen atoms required to facilitate the development of NOx.
The component responsible for controlling the flow rate of exhaust gases destined to reenter the intake stream is the EGR valve. There are two types of EGR valves, cold-side and hot-side, and the type of activation varies from pneumatically to hydraulically to electrically (the latter being most common today). Cold-side EGR valves direct exhaust gases into the intake after they’ve left the EGR cooler(s). A hot-side EGR valve performs its flow duties before the EGR cooler(s). Use of a hot-side EGR valve is believed to help cut down on the kind of soot and grime buildup that leads to valve failure (i.e. sticking).
With exhaust gas temperatures exceeding 1,200 degrees F in most engine applications, the portion of exhaust gases being routed back to the intake tract have to be cooled. The job of dropping these temperatures is handled by the EGR cooler. An air-to-liquid heat exchanger, the EGR cooler uses circulated engine coolant to lower the temperature of the exhaust gases passing through it. On engines such as the 6.4L Power Stroke and 6.7L Power Stroke, there are two EGR coolers at work.
When the federal government tightened the noose around PM emissions in , not even the maximized efforts of high-pressure common-rail injection were efficient enough to meet the new standard. This led to the modern exhaust aftertreatment systems we know today. At the heart of the aftertreatment system is the wall-flow style catalyst known as the diesel particulate filter (DPF). Its primary function is to keep PM from exiting the tailpipe by storing it. The soot collected by the DPF periodically triggers a regeneration cycle (more on that below), which involves the combination of diesel fuel and exhaust gases within the diesel oxidation catalyst (DOC) transforming the soot into a fine ash. While DPF’s have proven very effective, they do require occasional cleaning and eventual replacement.
In modern diesel exhaust aftertreatment systems, the DOC’s job is to provide the extreme heat that’s necessary to convert soot accumulation into the fine ash that remains trapped in the DPF. By oxidizing diesel fuel supplied to it from the engine, the DOC provides the means of increasing DPF temperature, which effectively incinerates the soot buildup within the DPF, breaking the soot down into the finer, noncombustible particles (i.e. ash) that remain.
At a predetermined level of soot accumulation (usually triggered by pressure differential across the DPF), the DPF regeneration process is initiated to convert soot into ash. During this active regeneration cycle, fuel is introduced into the DOC. The diesel fuel comes by way of excess fuel being injected on the engine’s exhaust stroke (example: ’08-’10 6.4L Power Stroke) or through a downstream ninth fuel injector (example shown above: ’11-’16 LML Duramax). During regeneration, the combination of fuel being introduced to the DOC and retarded injection timing taking place in-cylinder causes exhaust gas temperature to climb higher than 1,000 degrees F.
Going beyond what EGR systems are capable of, SCR has become the most effective technology in getting NOx levels low enough to meet the current, stringent standard. The NOx problem that can’t be solved in-cylinder is dealt with in the exhaust aftertreatment system by injecting urea-based diesel exhaust fluid (DEF) upstream of an SCR catalyst. During this process, harmful NOx is chemically converted in harmless nitrogen.
Although it must be carefully blended, the chemical makeup of DEF is pretty simple: 32.5-percent high purity urea and 67.5-percent deionized water. Urea itself is a nitrogenous compound that turns to ammonia when exposed to heat. Ammonia, along with the SCR catalyst, is paramount in converting NOx into nitrogen. The upside to SCR is that a more efficient combustion process can be employed in-cylinder without the NOx produced as a result of it being a problem or overtaxing the EGR system. SCR’s downside is further complexity within the diesel exhaust aftertreatment system and the fact that DEF freezes before diesel fuel does. The latter is the reason behind all SCR systems featuring DEF heaters, as well as DEF’s chemical makeup of exactly 32.5-percent urea (a ratio that provides the lowest possible freeze point of 12 degrees F).
Believe it or not, VGT’s are a necessary part of curbing both PM and NOx emissions. By acting like a smaller, more restrictive turbo at low rpm, its transient response is unmatched by any other type of turbo. This responsiveness means that the engine is kept in the meat of its power band, where fuel is more efficiently burned and instances of being “under the charger” (where a puff of smoke leaves the tail pipe) are eliminated. As it pertains to the EGR system, the VGT assures that positive pressure exists between the exhaust and intake manifold(s) so that sufficient EGR flow is available when it’s required.
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