DPF

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A diesel particulate filter (or DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. Wall-flow diesel particulate filters usually remove 85% or more of the soot, and under certain conditions can attain soot removal efficiencies approaching 100%. Some filters are single-use, intended for disposal and replacement once full of accumulated ash. Others are designed to burn off the accumulated particulate either passively through the use of a catalyst or by active means such as a fuel burner which heats the filter to soot combustion temperatures. This is accomplished by engine programming to run (when the filter is full) in a manner that elevates exhaust temperature or produces high amounts of NOx to oxidize the accumulated ash, or through other methods. This is known as "filter regeneration". Cleaning is also required as part of periodic maintenance, and it must be done carefully to avoid damaging the filter. Failure of fuel injectors or turbochargers resulting in contamination of the filter with raw diesel or engine oil can also necessitate cleaning.[1] The regeneration process occurs at road speeds higher than can generally be attained on city streets; vehicles driven exclusively at low speeds in urban traffic can require periodic trips at higher speeds to clean out the DPF. If the driver ignores the warning light and waits too long to operate the vehicle above 40 miles per hour (64 km/h), the DPF may not regenerate properly, and continued operation past that point may spoil the DPF completely so it must be replaced. Some newer diesel engines, namely those installed in combination vehicles, can also perform what is called a Parked Regeneration, where the engine increases RPM to around 1400 while parked, to increase the temperature of the exhaust.

Variants of DPFs

Unlike a catalytic converter which is a flow-through device, a DPF retains bigger exhaust gas particules by forcing the gas to flow through the filter;[citation needed] however, the DPF does not retain small particles and maintenance-free DPFs break larger particles into smaller ones. There are a variety of diesel particulate filter technologies on the market. Each is designed around similar requirements:

1. Fine filtration
2. Minimum pressure drop
3. Low cost
4. Mass production suitability
5. Product durability
break down of the dpf filter

Cordierite wall flow filters

The most common filter is made of cordierite (a ceramic material that is also used as catalytic converter supports (cores)). Cordierite filters provide excellent filtration efficiency, are (relatively) inexpensive, and have thermal properties that make packaging them for installation in the vehicle simple. The major drawback is that cordierite has a relatively low melting point (about 1200 °C) and cordierite substrates have been known to melt during filter regeneration. This is mostly an issue if the filter has become loaded more heavily than usual, and is more of an issue with passive systems than with active systems, unless there is a system break down.

Cordierite filter cores look like catalytic converter cores that have had alternate channels plugged - the plugs force the exhaust gas flow through the wall and the particulate collects on the inlet face.

Silicon carbide wall flow filters

The second most popular filter material is silicon carbide, or SiC. It has a higher (2700 °C) melting point than cordierite, however it is not as stable thermally, making packaging an issue. Small SiC cores are made of single pieces, while larger cores are made in segments, which are separated by a special cement so that heat expansion of the core will be taken up by the cement, and not the package. SiC cores are usually more expensive than cordierite cores, however they are manufactured in similar sizes, and one can often be used to replace the other. Silicon carbide filter cores also look like catalytic converter cores that have had alternate channels plugged - again the plugs force the exhaust gas flow through the wall and the particulate collects on the inlet face.

The characteristics of the wall flow diesel Particulate filter substrate are as follows: Broad band filtration (the diameters of the filtered particles are 0.2-150 μm); High filtration efficiency (can be up to 95%); High refractory; High mechanical properties. High boiling point.

Ceramic Fiber Filters

Fibrous ceramic filters are made from several different types of ceramic fibers that are mixed together to form a porous media. This media can be formed into almost any shape and can be customized to suit various applications. The porosity can be controlled in order to produce high flow, lower efficiency or high efficiency lower volume filtration. Fibrous filters have an advantage over wall flow design of producing lower back pressure. Ceramic wall-flow filters remove carbon particulates almost completely, including fine particulates less than 100 nanometers (nm) diameter with an efficiency of >95% in mass and >99% in number of particles over a wide range of engine operating conditions. Since the continuous flow of soot into the filter would eventually block it, it is necessary to 'regenerate' the filtration properties of the filter by burning-off the collected particulate on a regular basis. Soot particulates burn-off forms water and CO2 in small quantity amounting to less than 0.05% of the CO2 emitted by the engine.

Metal fiber flow through filters

Some cores are made from metal fibers - generally the fibers are "woven" into a monolith. Such cores have the advantage that an electrical current can be passed through the monolith to heat the core for regeneration purposes, allowing the filter to regenerate at low exhaust temperatures and/or low exhaust flow rates. Metal fiber cores tend to be more expensive than cordierite or silicon carbide cores, and generally not interchangeable with them because of the electrical requirement.

See Also -

1. Dpf Removal And Procedure Guide
2. Dpf Additive Systems Explained
3. EGR System Explained


Safety

In 2011, Ford recalled 37,400 F-Series trucks with diesel engines after fuel and oil leaks caused fires in the diesel particulate filters of the trucks. No injuries occurred before the recall, though one grass fire was started.A similar recall was issued for 2005-2007 Jaguar S-Type and XJ diesels, where large amounts of soot became trapped in the DPF. In affected vehicles, smoke and fire emanated from the vehicle underside, accompanied by flames from the rear of the exhaust. The heat from the fire could cause heating through the transmission tunnel to the interior, melting interior components and potentially causing interior fires


DPF Regeneration

Regeneration is the process of removing the accumulated soot from the filter. This is done either passively (from the engine's exhaust heat in normal operation or by adding a catalyst to the filter) or actively introducing very high heat into the exhaust system. On-board active filter management can use a variety of strategies:

Engine management to increase exhaust temperature through late fuel injection or injection during the exhaust stroke Use of a fuel borne catalyst to reduce soot burn-out temperature A fuel burner after the turbo to increase the exhaust temperature A catalytic oxidizer to increase the exhaust temperature, with after injection (HC-Doser) Resistive heating coils to increase the exhaust temperature Microwave energy to increase the particulate temperature All on-board active systems use extra fuel, whether through burning to heat the DPF, or providing extra power to the DPF's electrical system, although the use of a fuel borne catalyst reduces the energy required very significantly. Typically a computer monitors one or more sensors that measure back pressure and/or temperature, and based on pre-programmed set points the computer makes decisions on when to activate the regeneration cycle. The additional fuel can be supplied by a metering pump. Running the cycle too often while keeping the back pressure in the exhaust system low will result in high fuel consumption. Not running the regeneration cycle soon enough increases the risk of engine damage and/or uncontrolled regeneration (thermal runaway) and possible DPF failure.

Diesel particulate matter burns when temperatures above 600 degrees Celsius are attained. This temperature can be reduced to somewhere in the range of 350 to 450 degrees Celsius by use of a fuel borne catalyst. The actual temperature of soot burn-out will depend on the chemistry employed. The start of combustion causes a further increase in temperature. In some cases, in the absence of a fuel borne catalyst, the combustion of the particulate matter can raise temperatures above the structural integrity threshold of the filter material, which can cause catastrophic failure of the substrate. Various strategies have been developed to limit this possibility. Note that unlike a spark-ignited engine, which typically has less than 0.5% oxygen in the exhaust gas stream before the emission control device(s), diesel engines have a very high ratio of oxygen available. While the amount of available oxygen makes fast regeneration of a filter possible, it also contributes to runaway regeneration problems.

Some applications use off-board regeneration. Off-board regeneration requires operator intervention (i.e. the machine is either plugged into a wall/floor mounted regeneration station, or the filter is removed from the machine and placed in the regeneration station). Off-board regeneration is not suitable for on-road vehicles, except in situations where the vehicles are parked in a central depot when not in use. Off-board regeneration is mainly used in industrial and mining applications. Coal mines (with the attendant explosion risk from coal damp) use off-board regeneration if non-disposable filters are installed, with the regeneration stations sited in an area where non-permissible machinery is allowed.

Many forklifts may also use off-board regeneration - typically mining machinery and other machinery that spend their operational lives in one location, which makes having a stationary regeneration station practical. In situations where the filter is physically removed from the machine for regeneration there is also the advantage of being able to inspect the filter core on a daily basis (DPF cores for non-road applications are typically sized to be usable for one shift - so regeneration is a daily occurrence).


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: .EGR cut off .Boost pressure control .Engine load increase .Control of gas pressure and temperature in the intake manifold .Fuel injection control. 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 calibrator time, it switches to a state waiting for an accelerator pedal release man-oeuvre from the driver. If this occurs or a calibrator 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.


DPF Air 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.


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.