DIESEL PARTICULATE FILTER (DPF)
Two processes are used to regenerate the DPF; passive and active.
Passive Regeneration
Passive regeneration requires no special engine management intervention and occurs during normal engine operation. The
passive regeneration involves a slow conversion of the particulate matter deposited in the DPF into carbon dioxide. This
process occurs when the DPF temperature exceeds 250°C (482°F) and is a continuous process when the vehicle is being
driven at higher engine loads and speeds.
During passive regeneration, only a portion of the particulate matter is converted into carbon dioxide. This is because the
chemical reaction, which utilises nitrogen dioxide, is slower than the rate of engine production of particulate matter and is
effective from 250°C (482°F).
Above 580°C the conversion efficiency of the particulates into carbon dioxide rapidly increases. These temperatures are
generally only be achieved using the active regeneration process.
Active Regeneration
Active regeneration starts when the particulate loading of the DPF reaches a threshold as monitored or determined by the DPF
control software. The threshold calculation is based on driving style, distance travelled and back pressure signals from the
differential pressure sensor.
Active regeneration generally occurs every 250 miles (400 km) although this is dependant on how the vehicle is driven. For
example, if the vehicle is driven at low loads in urban traffic regularly, active regeneration will occur more often. This is due to
the rapid build-up of particulates in the DPF than if the vehicle is driven at high speeds when passive regeneration will have
occurred.
The DPF software incorporates a mileage trigger which is used as back-up for active regeneration. If active regeneration has
not been initiated by a back pressure signal from the differential pressure sensor, regeneration is requested based on distance
travelled.
Active regeneration of the DPF is commenced when the temperature of the DPF is increased to the combustion temperature of
the particles. The DPF temperature is raised by increasing the exhaust gas temperature. This is achieved by introducing
post-injection of fuel after the pilot and main fuel injections have occurred.
It is determined by the DPF software monitoring the signals from the two DPF temperature sensors to establish the
temperature of the DPF. Depending on the DPF temperature, the DPF software requests the Engine Control Module (ECM) to
perform either one or two post-injections of fuel:
The first post-injection of fuel is associated with retarded combustion to increase the temperature of the exhaust gas
and therefore allow the oxidation catalyst to reach it's operational temperature.
The second post-injection of fuel is injected late in the power stroke cycle. The fuel is not intended to combust in the
cylinder, and hence unburnt fuel passes into the exhaust where it creates an exothermic event within the catalytic
converter, further increasing the temperature of the DPF.
The active regeneration process takes up to 20 minutes to complete. The first phase increases the exhaust gas temperature to
ensure the catalytic converter is active. The second phase further increases the DPF temperature to the optimum temperature
for particle combustion. This temperature is then controlled for 15-20 minutes to ensure complete oxidation of the particles
within the DPF. The oxidation process converts the carbon particles to carbon dioxide.
The active regeneration temperature of the DPF is closely monitored by the DPF software to maintain a target temperature at
the DPF inlet. The temperature control ensures that the temperatures do not exceed the operational limits of the turbocharger
and the catalytic converter. The turbocharger inlet temperature must not exceed 830°C (1526°F), the catalytic converter brick
temperature must not exceed 800°C (1472°F) and the exit temperature must remain below 875°C (1382°F).
Diesel Particulate Filter Side Effects
The following section details some side effects caused by the active regeneration process.
Engine Oil Dilution
Engine oil dilution can occur due to small amounts of fuel entering the engine crankcase during the post-injection phases. This
has made it necessary to introduce a calculation based on driving style to reduce oil service intervals if necessary. The driver
is alerted to the oil service by a message in the instrument cluster.
The DPF software monitors the driving style and the frequency of the active regeneration and duration. Using this information
a calculation can be made on the engine oil dilution. When the DPF software calculates the engine oil dilution has reached a
predetermined threshold (fuel being 7% of engine oil volume) a service message is displayed in the IC.
Depending on driving style, some vehicles may require an oil service before the designated interval. If a service message is
displayed, the vehicle will be required have a full service and the service interval counter will be reset.
Fuel consumption
During the active regeneration process of the DPF, there will be an increase in fuel consumption.
However, because active regeneration occurs infrequently, the overall effect on fuel consumption is approximately 2%. The
additional fuel used during the active regeneration process is accounted for in the instantaneous and average fuel
consumption displays in the instrument cluster.