EMISSION CONTROL BY DPF DIESEL PARTICULATE FILTER EXPLAINED WITH IMAGES AND VIDEOS

• A diesel particulate filter (DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine.

Mode of action

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, in conjunction with an extra fuel injector in the exhaust stream that injects fuel to react with a catalyst element to burn off accumulated soot in the DPF filter, 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. Te 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.

Diesel engines produce a variety of particles during combustion of the fuel/air mix due to incomplete combustion. The composition of the particles varies widely dependent upon engine type, age, and the emissions specification that the engine was designed to meet. Two-stroke diesel engines produce more particulate per unit of power than do four-stroke diesel engines, as they burn the fuel-air mix less completely.

Diesel particulate matter resulting from the incomplete combustion of diesel fuel produces soot (black carbon) particles. These particles include tiny nanoparticles—smaller than a thousandth of a millimeter (one micron). Soot and other particles from diesel engines worsen the particulate matter pollution in the air and are harmful to health.

New particulate filters can capture from 30% to greater than 95% of the harmful soot. With an optimal diesel particulate filter (DPF), soot emissions may be decreased to 0.001 g/km or less.

The quality of the fuel also influences the formation of these particles. For example, a high sulfur content diesel produces more particles. Lower sulfur fuel produces fewer particles, and allows use of particulate filters. The injection pressure of diesel also influences the formation of fine particles.

Variants of DPFs

Cordierite Diesel Particulate Filter

Unlike a catalytic converter which is a flow-through device, a DPF retains bigger exhaust gas particles by forcing the gas to flow through the filter 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

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 greater than 95% in mass and greater than 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.

Paper

Disposable paper cores are used in certain specialty applications, without a regeneration strategy. Coal mines are common users – the exhaust gas is usually first passed through a water trap to cool it, and then through the filter. Paper filters are also used when a diesel machine must be used indoors for short periods of time, such as on a forklift being used to install equipment inside a store.

Partial filters

There are a variety of devices that produce over 50% particulate matter filtration, but less than 85%. Partial filters come in a variety of materials. The only commonality between them is that they produce more back pressure than a catalytic converter, and less than a diesel particulate filter. Partial filter technology is popular for retrofit.

Maintenance

Filters require more maintenance than catalytic converters. Ash, a byproduct of oil consumption from normal engine operation, builds up in the filter as it cannot be converted into a gas and pass through the walls of the filter. This increases the pressure before the filter. Warnings are given to the driver before filter restriction causes an issue with drive-ability or damage to the engine or filter develop. Regular filter maintenance is a necessity.

DPF filters go through a regeneration process which removes this soot and lowers the filter pressure. There are three types of regeneration: passive, active, and forced. Passive regeneration takes place normally while driving, when engine load and vehicle drive-cycle create temperatures that are high enough to regenerate the soot buildup on the DPF walls. Active regeneration happens while the vehicle is in use, when low engine load and lower exhaust gas temperatures inhibit the naturally occurring passive regeneration. Sensors upstream and downstream of the DPF (or a differential pressure sensor) provide readings that initiate a metered addition of fuel into the exhaust stream. There are two methods to inject fuel, either downstream injection directly into the exhaust stream, downstream of the turbo, or fuel injection into the engine cylinders on the exhaust stroke. This fuel and exhaust gas mixture passes thru the Diesel Oxidation Catalyst (DOC) creating temperatures high enough to burn off the accumulated soot. Once the pressure drop across the DPF lowers to a calculated value, the process ends, until the soot accumulation builds up again. This works well for vehicles that drive longer distances with few stops compared to those that perform short trips with many starts and stops. If the filter develops too much pressure then the last type of regeneration must be used - a forced regeneration. This can be accomplished in two ways. The Vehicle operator can initiate the regeneration via a dashboard mounted switch. Various signal interlocks, such as park brake applied, transmission in neutral, engine coolant temperature, and an absence of engine related fault codes are required (vary by OEM and application) for this process to initiate. When the soot accumulation reaches a level that is potentially damaging to the engine or the exhaust system, the solution involves a garage using a computer program to run a regeneration of the DPF manually.

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. Asimilar 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.

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:

1. Engine management to increase exhaust temperature through late fuel injection or injection during the exhaust stroke
2. Use of a fuel borne catalyst to reduce soot burn-out temperature
3. A fuel burner after the turbo to increase the exhaust temperature
4. A catalytic oxidizer to increase the exhaust temperature, with after injection (HC-Doser)
5. Resistive heating coils to increase the exhaust temperature
6. 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). Dieselparticulate filters remove particulate matter found in diesel exhaust by filtering exhaust from the engine. In order to meet the stringent particulate emissions that are required for diesel light duty vehicles starting with the 2007 model year, the highest efficiency particulate filter is required. The filters are commonly made from ceramic materials such as

cordierite,

aluminum titanate,

mullite or

silicon carbide.

The basis for the design of wall flow filters is a honeycomb structure with alternate channels

plugged at opposite ends. As the gasses passes into the open end of a channel, the plug at the

opposite end forces the gasses through the porous wall of the honeycomb channel and out

through the neighboring channel. The ultrafine porous structure of the channel walls results in greater than 90% percent collection efficiencies of these filters. Wall flow filters capture particulate matter by interception and impaction of the solid particles across the porous wall. The exhaust gas is allowed to pass through in order to maintain low pressure drop.

Particulate Filter Regeneration

Since a filter can fill up over time by developing a layer of retained particles on the inside surface of the porous wall, engineers that design engines and filter systems must provide a means of burning off or removing accumulated particulate matter and thus regenerating the filter. A convenient means of disposing of accumulated particulate matter is to burn or oxidize it on the filter when exhaust temperatures are adequate. By burning off trapped material, the filter is cleaned or "regenerated" to its original state, this is called “regeneration”. The frequency of regeneration is determined by the amount of soot build-up resulting in an increase in back pressure. Two simple pressure sensors will provide feed back to ECU as a part of regeneration control. To facilitate decomposition of the soot, a catalyst is used either in the form of a coating on the filter or a catalyst added to the fuel. Filters that regenerate in this so-called "passive" fashion cannot be used in all situations. The experience with catalyzed filters indicates that there is a virtually complete reduction in odor and in the soluble organic fraction of the particulate. Despite the high efficiency of the catalyst, a layer of ash may build up on the filter requiring replacement or servicing. The ash is made up of inorganic oxides from the fuel or lubricants used in the engine and will not decompose during the regular soot regeneration process.In some applications or operating cycles, the exhaust never achieves a high enough temperature to completely oxidize the soot even in the presence of a catalyst. In these instances, an "active" regeneration system must be employed. Active regeneration utilizes a fuel burner or a resistively heated electric element to heat the filter and oxidize the soot. Active regeneration can be employed either in-place on the vehicle or externally.

? Active regeneration - In place vehicle regeneration, the DPF will periodically undergo active regeneration. In this process, a small mist of diesel fuel is injected into the exhaust stream at the turbocharger outlet; the mist travels through the exhaust pipe to wet the DPF's precatalyst. This causes a chemical reaction which raises DPF temperatures to the level required to convert the soot into CO2. Active regeneration normally takes about 15 minutes and the operation is not noticeable to the driver. This event is triggered when sensors – in most cases pressure sensors located at the inlet and outlet end of the DPF that alert the vehicle computer (ECU) that the restriction across the particulate trap is increasing (a back pressure change) and the particulate trap is becoming full.

Prevention of DPF replacement

Depending on how blocked the DPF is, it may be the case that these regeneration procedures cannot clean the DPF completely.

The only way to carry out complete removal of particles is to remove the filter from the car and have it professionally cleaned, or replace the filter altogether.

This is an extremely expensive exercise, as replacement DPF units can cost good amount of money.

What can prevent normal regeneration taking place?

• Frequent short journeys, such as stop-start city driving, that do not allow the engine to reach correct operating temperature
• Using the wrong oil type – DPF equipped vehicles require oil of a ‘low Ash, low Sulphur’ grade, to prevent excess build-up occurring
• An issue with another emissions control device, such as the exhaust gas recirculation (EGR) system, or a problem with the inlet or fuel systems
• Low fuel level – most vehicles will not carry out a regeneration cycle if the fuel level is under ? of a tank
• Overdue service interval – low oil quality or level will prevent regeneration from occurring
• Engine warning light on – a warning light or a diagnostic trouble code stored in the ECU may prevent regeneration

 

DPF additives

Some manufacturers have special additive systems to supplement the DPF system.

These usually include injecting a small amount of liquid – usually Cerium Oxide – to the fuel mixture, to allow regeneration to occur at lower temperatures.

This additive is usually located in a separate tank next to the fuel tank and is usually topped up during logbook service operations. 

If your vehicle is not fitted with this system, there are many aftermarket fuel treatments you can purchase and add to your fuel tank to carry out the desired effect.

Always check with your vehicle manufacturer before using these products, as some may not be compatible with your vehicle.

Filter Failures

Some diesel exhaust filter failures are a result of not allowing the regeneration to take place. This will inadvertently clog the DPF to the point that replacement is the only option. Although it can be cleaned to some degree, a portion of functionality is still lost due to the severity of the restriction. Another problem is when it is in regeneration and the excess heat combined with the clog causes the metal casing of the DPF to expand and rupture. Which, of course, means the only solution is to replace the DPF. The DPF requires professional cleaning every 150,000–250,000 miles or 5000 hours.