Diesel Particulate Filters
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.
The Regulated Diesel Emissions
- Particulate Matter (PM)
- carbon from incomplete combustion
- soluble organic fractions from fuel and lubricating oils
- sulfates formed from the sulfur in the fuel
- Oxides of Nitrogen (NOx)
- composed of nitric oxide (NO) and nitrogen dioxide (NO2)
- Hydrocarbons (HC)
- regulated either as total hydrocarbon emissions (THC) or as non-methane hydrocarbons (NMHC)
How It Works
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.
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.
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.
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.
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.
Passive - regeneration takes place while driving when engine
load elevates exhaust temperatures enough to burn small amounts of soot. These temperatures can range from
200ºC (392ºF) to nearly 600ºC (1112ºF) and requires no action from the driver or engine
Active - regeneration can occur while driving or when the
vehicle is not moving and the engine is idling to burn large amounts of soot. Active regeneration is
initiated by the engine control software when regeneration is determined to be required and only when certain
conditions are met. Typical exhaust temperatures will range from around 400ºC (752ºF) to more than
600ºC (1112ºF) and requires no action from the driver.
Passive/Active - regeneration is a combination of both types of
regeneration. Due to varying engine cycles and inconsistent exhaust temperatures, passive regeneration alone
is not capable of burning all of the soot produced and it slowly builds up. Periodic active regeneration is
necessary to clean the DPF when the engine control software determines it is needed.
Manual - regeneration is essentially the same as active
regeneration however it is typically initiated using a diagnostic tool by a technician for service or
diagnostic purposes. Some manufactures of medium and heavy duty trucks will allow a driver to manually
disable regeneration if conditions are not favorable. These vehicles are equipped with a disable switch and
some will also have a force regen switch to manually initiate regeneration when certain conditions are
Dosing - or HC combustion, is a method of raising exhaust
temperature for regeneration by introducing fuel to the exhaust where it reacts with an oxidation catalyst
before it enters the DPF. This is done by enriching the combustion process, injecting fuel during the exhaust
stroke or by injecting fuel directly into the exhaust. This is a common method and it is a fairly simple way
to achieve elevated exhaust temperatures but it does have some drawbacks. Dosing can allow unburned fuel to
pass through the system known as hydrocarbon slip. Secondly, temperatures cannot be as closely controlled as
other methods. Use of ultra low sulfur diesel fuel (15 ppm sulfur maximum) is required in diesel vehicles
equipped with an oxidation catalytic converter (OC) and diesel particulate filter. Using low sulfur (16-500
ppm) or high sulfur (500 ppm or greater) diesel fuel can effectively poison the catalyst destroying it,
rendering it useless.
ARD - or After treatment Regeneration Device which is an
exhaust component that creates heat for regeneration. There are many different types of ARD's but they
all do the same thing. A self contained component that meters fuel with an injector, has its own supply of
air for combustion and a method for igniting the injected fuel. These types of systems are controlled by the
engine control system and can be very complex which raises the cost of the engine and requires additional
maintenance. The advantages of this type of system are complete control of exhaust temperatures and it can
regenerate under a wide range of conditions. An ARD is more commonly used on larger, medium and heavy duty
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.
Using The Proper Engine Oil And Fuel
All 2007 and newer diesel engines equipped with DPF systems require the use of Ultra Low Sulfur Diesel fuel
(ULSD) and engine lubricating oil that meets CJ-4 specifications. Ash, phosphorus, and sulfur, which are commonly
found in the exhaust of internal combustion engines, comes directly from the oil and fuel and can cause damage to
after treatment devices. The required fuels and oils have been refined and formulated to remove most of these
contaminants making them safe to use with after treatment systems. Exposure to such contaminants is known as
"poisoning." It is important to use ULSD and CJ-4 oil together. Switching to CJ-4 oil while still using
LSD 500 ppm fuel will allow the buildup of sulfuric acid which could cause serious oil deterioration and possible
engine damage. The use of CI-4 in a new engine could increase regeneration intervals and shorten the life of a
DPF requiring frequent ash removal.
ASH from the lubricant can potentially block the pores of the
diesel particulate filter, leading to an increase in back pressure. This can have a negative impact on fuel
economy and power. Excessive amounts of ash in the filter can cause reactions and high temperatures that can
lead to permanent damage to the DPF substrate.
Phosphorus can reduce the efficiency of catalysts in exhaust
after treatment systems. Catalysts used in these exhaust after treatment devices can include metals such as
platinum, rhodium, or palladium. Phosphorus can render catalysts and DPF's useless by blocking these
metal coatings causing irreversible damage that accumulates over time.
Sulfur itself does not permanently damage the DPF but it will
increase hydrocarbon, oxide of nitrogen and particulate matter emissions enough to clog the DPF in a
relatively short time. In the United States, ULSD fuel is defined as having less than 15 ppm sulfur content.
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.
Since it's primary function is to eliminate soot from the exhaust, finding soot in the tail pipe or observing
heavy smoke is a positive sign that the system is not working. Some minor staining may be considered normal but
if you see a tail pipe that looks like the one pictured then something is wrong. That may be the only indication
if the fault is a crack in the substrate. The picture shows the result of a broken DPF that has either cracked or
has broken loose and is no longer anchored and sealed to the metal case. This article may be revised as more
examples and conditions are documented.