A diesel particulate filter, also known as a DPF, is an emissions reduction device designed to remove diesel particulate matter or soot from the exhaust gases of a diesel engine. This particulate matter is the byproduct of incomplete combustion mostly made up of unburned hydrocarbons (Hc) and the non-combustible residue of lubricating oils. These particles also provide a vehicle for the many trace chemicals that are also produced by diesel fuel and the combustion process which are known to cause environmental problems and believed to cause public health issues. The DPF is designed to physically trap, store and then oxidize or burn off particulate matter effectively reducing particulate matter emissions. This process of burning off the collected particles is called regeneration. What remains after the regeneration process is ash which is the noncombustible residue of engine lubricating oil. The use of ultra low sulfur fuel (ULSD) and low phosphate engine oils is required on DPF equipped engines. Sulfur and phosphates will foul the DPF quickly causing performance loss and dramatically reducing the life of the DPF.

It is important to note that the chemistry shown is simplified to show the basic function. The chemistry that takes place in an after treatment system is complex, varies between manufacturers and is still being explored. You need to understand that the process of burning off the particulate matter and determining what chemicals and gasses that exit the tail pipe involves several chemical reactions, not just heat. In fact, depending on the chemical changes that take place, the temperatures require to burn off particulate matter can be reduced. Most after treatment systems combine a diesel oxidation catalyst or a diesel oxidation converter with the DPF and both may be"catalyst-coated." The application of a base, or precious metal coating, to the surface of the catalyst and the filter will alter the chemistry of the exhaust that can reduce the ignition temperature necessary for oxidation of the particulate matter. This passively burns off some of the soot during normal operation of the vehicle and helps in reducing the volume of soot that reaches the DPF. Used in-line with a DPF, a diesel oxidation catalyst will also help boost exhaust temperatures required for active regeneration. A system like that of the 6.4L PSD, extra fuel is added to the exhaust gasses by injecting fuel into the cylinders during the exhaust stroke. The added fuel is burned off in the catalyst effectively raising the exhaust temperature to heat the DPF during active regeneration.
Worth noting- Urea injection, also called Selective Catalytic Reduction (SCR), works by an ammonia-like acid being injected into a special catalyst to reduce NOx in diesel emissions. This will further complicate after treatment systems and increase maintenance for the operator in that urea it will add another fluid to be regularly maintained.
There are different types of DPF's but the most common type is a double walled flow through design made with a cordierite core. This core is similar to a full flow catalytic converter honeycomb design with half of the channels blocked at the inlet and the other half blocked at the outlet forcing the exhaust gasses to flow though the walls between the channels. As the exhaust gasses flow though the walls, the particulate matter is trapped where it remains until it is burned off during regeneration. After regeneration, the resulting minute amount of ash remains where over time it too will build up and require removal. Ash removal can only be done manually which requires removal of the DPF to be cleaned in a reverse flow machine designed to remove ash and collect it for proper disposal. The substrate cores of both catalytic converters and particulate filters are similar in composition and construction.
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6.4L Catalyst
(Oxi-Cat) |
Take A Closer Look
The core on the left reveals the open passages of a catalytic converter which exhaust gasses flow directly through. On the right, the alternating pattern of blocked passages in the core of a particulate filter is quite apparent. The opposite ends of the open cores is blocked off and the opposite ends of the blocked passages are open. This means that exhaust gasses enter the open passages and must pass through the substrate to the alternating passages and exit out the other end. |
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6.4L DPF
(Particulate Filter) |
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Wall Flow Technology
The image to the right shows how the exhaust enters the DPF, flows through the substrate which filters the particulate matter and allows the filtered exhaust to exit the other end. |
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Regeneration is the process of burning the collected soot trapped by the DPF. This process restores or maintains the DPF's ability to allow exhaust gasses to flow through it while preserving engine performance and efficiency. Regeneration is achieved by elevating the exhaust temperature in the DPF to around 600ºC (1112ºF). The type and method of regeneration an engine is equipped with is largely determined by the way it is used and the conditions it is intended to be used. Most on highway and off road diesel vehicles will require some type of active regeneration capability. If back pressure caused by the collected soot is allowed to get to high, damage to the engine and the DPF itself will result. The use of low sulfur fuel and low ash oil is required for use in a DPF equipped vehicle otherwise the DPF will become clogged quickly causing frequent regenerations and decreasing the lifespan of the filter.
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Inside the DPF
![]() The image above shows the heated exhaust entering the DPF. The hot exhaust gasses heat the DPF substrate igniting the soot that has collected and built up in the cells. When regeneration is complete, all that remains is a tiny amount of ash. This process effectively restores the flow through the cell walls, or regenerating its ability to do so. |
The ash that remains in the DPF after regeneration is a very small amount of matter but it will
build up eventually reducing the DPF's capacity and performance. The EPA regulation mandates DPF's must
allow a nominal 150,000 mile interval for ash cleaning. When high mileage is reached and regenerations become
more frequent, the engine control software may also detect that the DPF requires cleaning or replacement. The ash
can only be removed by physical means such as washing, pulsed or swirled compressed air. Depending on the type of
DPF and the manufacturer, a high temperature baking process may also be utilized. The machines that are used to
remove ash are expensive and may not be widely available. Some manufacturers like Ford for example will offer an
exchange program where a dirty DPF is removed from a vehicle and a DPF that was removed from another vehicle and
has been serviced and certified is installed in its place. Large fleets or busy service centers will find having
one to be cost effective.

Failures of the after treatment can
range from a check engine light to a plugged system that will prevent an engine from running. These faults can be
the result of a fuel injection concern, or a base engine failure that has loaded or contaminated the system. It
is also possible to detect a system that has been tampered with or modified by monitoring the temperature and
pressure sensors however a simple visual inspection is all you need to perform to know that the Diesel
Particulate Filter or after treatment system is not functioning as designed.![]() |
LET IT BE KNOWN |
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