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I found this information on the DPF systems fitted to the Jaguar 2.7 V6 diesels, it's incredibly detailed and covers issues including active & passive regenerations, oil dilution and more. The general principles apply to any coated DPF system (i.e. ones that don't use a liquid catalyst additive.
Diesel Particulate Filter
Diesel emissions are 90% lower than they were in the 1980s; however exhaust regulations based on statistical
studies dealing with the health impact of exhaust emissions continue to demand even lower gaseous and particulate
diesel emissions. Particulate emissions are responsible for the characteristic black exhaust fumes emitted from the diesel
engine. They are a complex mixture of solid and liquid components with the majority of particulates being carbon
microspheres on which hydrocarbons from the engines fuel and lubricant condense.
In order to comply with the strict European Stage IV emission standard, which now stipulates a further 50% reduction in
particulate emissions, an exhaust emission control system is used on the XJ 2.7 liter and S-TYPE 2.7 liter diesel vehicles.
The primary component of the system is the diesel particulate filter (DPF), which has been proven to be effective in reducing particulate emissions to negligible levels. The main ability of the particulate filter is its capacity for regeneration; that is burning the particulates trapped in the filter at calculated intervals in such a way that the process is unnoticed by the driver of the vehicle.
Operation of the Diesel Particulate Filter
To enable the exhaust emission control system to store and when conditions determine, burn the particulates, the diesel particulate filter uses new filter technology based on a filter with a catalytic coating. Made of silicon carbide the filter is packaged into a steel container installed in the exhaust system of the vehicle. The filter has good thermal shock resistance and thermal conductivity properties, plus a closely controlled porosity. The filter is tailored to the engines requirements to maintain the most favorable exhaust backpressure.
The porous substrate in the filters interior consists of thousands of small parallel channels running in the exhausts longitudinal direction, adjacent channels in the filter are alternately plugged at each end. This arrangement forces the exhaust gases to flow through the porous walls, which acts as the filters medium. Particulates that are too big to pass through the porous walls are left behind and stored in the channels. To prevent the particulates creating an obstruction to the exhaust gas flow, the filter system provides a regeneration mechanism, which involves raising the temperature of the filter to such an extent that the particulates are incinerated and as a result removed from the filter.
The most important parameters influencing filter regeneration is the temperature of the exhaust gases and filter. With this in mind the composition of the filter also includes a wash coating to the surface of the filter comprising platinum and
other active components; materials used in the manufacture of oxidation catalytic converters. At certain exhaust gas and filter temperatures the catalytic coating promotes combustion and therefore burning of the particulates, while also oxidizing carbon monoxide and hydrocarbon emissions.
Exhaust gas and filter temperatures are controlled by the diesel particulate filter (DPF) module, which is incorporated in the powertrain control module (PCM). The DPF module monitors the load status of the particulate filter based on driving style, distance driven, and signals from the differential pressure sensor. When the particulate loading in the filter reaches a threshold, the filter is actively regenerated by adjusting, in accordance with requirements various engine-control functions; such as:
.fuel injection,
.intake-air throttle,
.glow-plug activation,
.exhaust-gas recirculation, and
.boost-pressure control.
This control function is made possible by the flexibility of the common-rail fuel injection engine in providing the precise
control of:
.fuel-flow
.fuel pressure, and
.injection timing,
all essential requirements for an efficient regeneration process.
Two processes are used to regenerate the particulate filter,'passive regeneration' and 'active regeneration' both of which are discussed below:
Passive Regeneration
Passive regeneration involves the slow environment-protecting conversion of the particulates deposited in the filter into carbon dioxide. This regeneration process comes into effect when the filters temperature reaches 250*C and occurs continuously when the vehicle is being driven at higher engine loads and speeds. No special engine management intervention is initiated during passive regeneration, allowing the engine to operate as normal.
Only a portion of the particulates are converted to carbon dioxide during passive regeneration and due to chemical
reaction this process is only effective within the temperature range of 250*C to 500*C. Above this temperature range the
conversion efficiency of the particulates into carbon dioxide subsides as the temperature of the filter increases.
Active regeneration
Active regeneration commences when the particulate loading in the filter reaches a threshold as monitored and determined by the DPF module. This calculation is based on driving style, distance driven and exhaust backpressure signals supplied by the differential pressure sensor. Active regeneration generally occurs approximately every 400 kilometers (250 miles) although this will depend on how the vehicle is driven.
For example, if the vehicle has operated for a length of time at low-loads for instance in urban traffic, active regeneration will be initiated more often. This is due to a more rapid build up of particulates in the filter than if the vehicle has been driven periodically at greater speeds, where passive regeneration would have occurred.
A mileage trigger incorporated within the DPF module is used as a backup for initiating active regeneration. If after a threshold distance has been driven and regeneration has not been activated by backpressure signals; regeneration will then be requested on the basis of distance driven.
Active regeneration of the particulate filter is started by raising the temperature in the particulate filter up to the
combustion temperature of the particulates. A principal method of increasing the exhaust gas temperature is by
introducing post-injection of the fuel, that is after the pilot and main fuel injections have taken place. This is achieved
by the DPF module processing signals from the temperature sensor to determine the temperature of the particulate filter and depending on the filters temperature, the DPF module commands either one or two post-injections:
.First post-injection retards combustion inside the cylinder to increase the heat of the exhaust gas.
.Second post-injection injects fuel late in the power stroke cycle; fuel partly combusts in the cylinder but also sweeps down the exhaust where unburned fuel triggers an exothermal event in the catalyst, raising the filters temperature further.
Active regeneration takes approximately 20 minutes to complete. The first phase is to raise the temperature of the filter to particulate combustion temperature of 500*C.
In the second phase the temperature is raised to 600*C, the optimum particulate combustion temperature. This temperature is maintained for 15 to 20 minutes to ensure complete incineration of the particulates captured in the filter. The incinerated particulates produce carbon dioxide and water.
Active regeneration is controlled to achieve a target temperature of 600*C at the inlet of the particulate filter without exceeding the temperature limits of the turbochargers and close-coupled catalysts; refer to 'Active Regeneration
Protection Limits' below.
During the active regeneration period:
.The turbochargers are maintained in the fully open position to minimize heat transmission from the exhaust gas to the turbochargers and to reduce the rate of gas flow through the particulate filter. This enables optimum heating of the particulate filter. If the driver demands a higher torque the turbochargers will respond by closing the vanes as required.
.The throttle is closed as this assists in increasing the exhaust gas temperature and reducing the rate of exhaust gas flow,
both of which increase the speed at which particulate filter is heated.
.The exhaust gas recirculation (EGR) valve is closed as the use of EGR lowers exhaust gas temperatures and therefore makes it difficult to achieve the regeneration temperature in the particulate filter.
.The glow plugs are sometimes activated to provide additional heat in raising the temperature of the particulate filter. To maintain glow plug serviceability the activation period of the glow plugs is restricted to 40 seconds.
The regeneration process also compensates for ambient temperature changes.
WARNING: Due to the high temperatures which can occur in the particulate filter, care should be taken when working within the vicinity of the filter.
WARNING: Due to the high temperatures which can occur in the particulate filter, it is advisable not to park the vehicle:
.Where the filter can come into contact with flammable materials underneath the vehicle.
.Where exhaust gasses emitted from the exhaust tail pipe can come into contact with flammable materials.
Engine Oil Dilution
A disadvantage of active regeneration is engine oil dilution caused by small amounts of fuel entering the engine crankcase during the post-injection phases. This has made it necessary, in some circumstances to reduce the oil service intervals; the driver of the vehicle is alerted to this by the instrument-cluster message centre.
An algorithm programmed in the DPF module monitors driving style, active regeneration frequency and duration.
Using this information the module predicts the level of oil dilution. When the oil dilution level reaches a threshold value (the fuel being 7% of engine oil volume), a red warning lamp and 'Service Required' message is displayed.
Depending on driving style, a small percentage of vehicles will require an oil change before the standard 15,000 miles service interval. If an engine oil dilution event does occur the vehicle will undergo its full service and the service mileage counter will be reset to zero by the service technician. Refer to GTR for further information on resetting the service mileage counter.
Fuel Consumption
There will be a small increase in fuel consumption due to active regeneration of the particulate filter.
During regeneration the fuel consumption approximately doubles. However, because regeneration happens relatively
infrequently the overall increase in fuel consumption is small.
This is accounted for in both the instantaneous and average fuel economy displayed in the instrument cluster.
(XJ) Powertrain
Driver and Dealer Intervention
For drivers who make frequent short journeys at low speeds, it may not be possible to effectively regenerate the particulate filter. In this case, the DPF module will detect a particulate overload condition and a warning message will displayed to the driver via the message centre. This message will read ‘DPF Full - See Handbook’ accompanied by an amber warning light. The driver will be given the opportunity to regenerate the particulate filter by driving the vehicle until the engines normal operating temperature is attained, and then for an additional 20minutes at a speed of 48 km/h (30 mile/h) or above. Successful regeneration of the filter is indicated to the driver by both the message and amber warning light being extinguished.
If the message is ignored and no action is taken there is the possibility that the DPF will become blocked. If this occurs the vehicle must be taken to an authorized dealer for the filter to be force regenerated. Refer to GTR for further information.
NOTE: There is no requirement to manually remove ash or other stubborn compounds during the life of the filter under normal operation.
Diesel Particulate Filter Module
The diesel particulate filter (DPF) module is incorporated in the powertrain control module (PCM).
The DPF module monitors and supervises the operation of the DPF system while also monitoring diagnostic data. The
DPF module is divided into three sub-modules controlled by a coordinator module. The DPF coordinator module manages the operation of different features when a forced regeneration is requested or cancelled.
.The DPF supervisor module is a subsystem of the coordinator module.
.The DPF fuel-management module calculates the timing and quantity of four fuel injections as well as the injection pressure during regeneration.
.The DPF air-management module contains the control for EGR, boost pressure, air temperature and pressure in the intake manifold.
In the following, the functionality of each sub-module is explained:
DPF Coordinator Module
The DPF coordinator responds to a regeneration request from the supervisor module by initiating and coordinating the
following DPF regeneration specific requests:
.EGRcut off
.Boost pressure control
.Engine loadincrease
.Control of gas pressure and temperature in the intake manifold
.Fuel injectioncontrol.
Once a regeneration request is set by the supervisor module the coordinator requests EGR cut off, and regeneration specific boost pressure control. It awaits a feedback signal from the EGR system indicating that the valve is shut. Once this occurs, the coordinator initiates requests to increase engine load by activating electric consumers and controlling the intake air temperature and pressure. Once it receives a confirmation that intake conditions are adequately controlled or expiration of a calibratable time, it switches to a state waiting for an accelerator pedal release manoeuvre from the driver. If this occurs or a calibratable time elapses, the coordinator initiates a request to control fuel injections to increase exhaust gas temperature.
DPF Fuel Management Module
The fuel management module controls:
.Timing and quantity of four split injections per stroke (pilot, main, and two post injections).
.Injection pressure and transition between three different levels of injection.
All of which are dependent on the state of the close-coupled catalysts and the state of the particulate filter.
The control injection determines the required injection level as well as an indication of the activity of the close-coupled
catalyst and particulate filter. The injection management calculates the quantity and timing for the four split injections,
each for the three calibration levels for injection pressure, and manages the transition between levels.
The two-post injections are required to de-couple the functionality of elevating in-cylinder gas temperature and
production of hydrocarbons (to be burnt in the particulate filter). The first post injection is used to generate higher
in-cylinder gas temperature and at the same time retain the same torque produced under normal operation mode (non
regeneration mode). The second post is used to generate hydrocarbons which are burnt partly in-cylinder and partly
over the close-coupled catalyst, but without producing increased engine torque.
(XJ) Powertrain
DPF Air Management Module
The DPF air management module consists of:
.EGRcontrol
.Boost pressure control
.Intake air temperature and pressure control.
During regeneration, the EGR feature is shut off, and the closed-loop activation of the boost controller is calculated.
The module controls the state of the air in the intake manifold to a predetermined level of pressure and temperature. This is required to achieve correct in-cylinder conditions for a stable and robust combustion of the post-injected fuel.
The module controls the intake air pressure during regeneration by actuating the EGR throttle and adjustment of
boost pressure control.
Active Regeneration Protection Limits
For engine and other component protection and durability the DPF module implements some limits during the active
regeneration phase, in particular:-
.Temperature before the turbocharger inlet must remain below 830 C for turbocharger protection.
.Close-coupled catalyst in-brick temperatures must not exceed 800 C and exit temperature must remain below 750*C.
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