Diesel vehicles throughout the world have become more popular as diesel engine technology has advanced due to their excellent fuel economy over petrol vehicles. However, diesel vehicles have a drawback as they have high NOx and particulate emissions when compared to petrol vehicles. To counteract this, the existing Euro Emissions Standards were tightened. In order for vehicles to meet these guidelines many green initiatives were introduced by vehicle manufacturers, such as Start/Stop technology. One of the most important was the introduction of diesel particulate filters, or DPFs (FAPs).
What is it made from?
DPFs are normally made from two different materials.
The first, Cordierite is most commonly used and is found in our standard range of DPFs. It is a ceramic material with excellent filtration efficiency and thermal properties. The only drawback of cordierite is its relatively low melting point of 1200°C, and they have occasionally been known to melt during regeneration if the filter is extremely blocked – although these instances are very rare. Cordierite DPFs are commonly used in the aftermarket and in additive systems in conjunction with diesel oxidisation catalysts.
The second material is Silicon Carbide (SiC). This is found in our Premium range of DPFs and is a compound of silicon and carbon. It has a melting point of 2700°C so is extremely unlikely to melt during regeneration. It has to be made from small segments fixed together with special cement to enable it to cope with heat expansion. They also boast 99% filtration efficiency. They are commonly used in O.E and catalysing DPFs.
How do they work?
A DPF is NOT a “flow-through” device, the gasses are forced through. Unlike a catalytic converter, the channels of the filter are blocked at alternate ends, forcing the gasses to flow through the cell walls in order to exit the filter. As the cell walls are porous, the clean gasses can pass through, but the holes are not large enough to let particulate matter pass through. Instead this is deposited on the cell walls and trapped in the filter.
The Engine Management System (ECU) constantly monitors the filter and will carry out a regeneration to stop it blocking.
What is Regeneration?
As a DPF is a “soot trap” it has to be able to clean itself to prevent it becoming blocked and affecting the running of the vehicle. This process is known as regeneration. There are three different types of regeneration:
Passive Regeneration occurs during normal driving conditions when the DPF becomes hot enough to burn off some of the trapped particulates naturally at 550°C.
Active Regeneration is an ECU led process. When the level of soot in the filter reaches around 45%, the ECU will make small adjustments to the fuel injection timing and increases the exhaust gas temperature. The optimum temperature required for particle combustion is 600°C. Active regeneration normally occurs around every 450 miles, but it does depend on how the vehicle is driven. Vehicles that are primarily driven on short urban journeys will regenerate more often than those primarily driven on motorways. This is due to a greater build up of particles at lower speeds. The regeneration cycle will generally be triggered by the back pressure. Failing this, mileage is used as a back up.
During active regeneration it is common to see smoke being emitted from the exhaust as the particulates are burned off. The fuel economy of the vehicle will also decrease during an active regeneration phase.
Forced Regeneration is carried out by garages with Diagnostic equipment.
Whilst all DPFs work in the same way car manufacturers use slightly different variations of the technology.
Fuel Additive Systems – used in conjunction with a diesel catalyst, this type of DPF uses a fuel additive in order to aid the regeneration process. The additive enables regeneration to take place at a lower temperature of 400°C in conditions where regeneration would not normally be possible. It is advised that this type of DPF needs to be replaced as a service item at around 75,000 miles. Fuel additive systems are favoured by French manufacturers, ie Peugeot and Citroen, along with Volvo.
Coated DPFs – or a “non-additive” DPF, uses a precious metal coating to aid the regeneration process. The coated DPF catalyses as well as filters, so the vehicle doesn’t necessarily need a separate catalytic converter. The coating of precious metals also lowers the point of “Regeneration” to 400°C. Using this type of system, the DPF can expect to last around 150,000 miles under normal operating conditions. Coated DPFs are favoured by German manufacturers such as BMW, VAG and Mercedes along with Jaguar and Land Rover.
Fault & Maintenance Checks
The DPF is unlikely to be the cause of the vehicle fault. Make sure the fault is rectified before fitting a replacement part.
“The DPF light comes back on within a few miles” – was the ECU set up properly? Are the pressure pipes blocked? Have the pressure sensors been checked?
“The DPF keeps blocking up” – is the vehicle getting long enough journeys to regenerate? Has the fuel additive tank been filled?
Carrying out the correct diagnostic checks is very important when replacing a DPF to ensure the replacement works as it should. The following should be checked as routine when replacing a DPF:
- DPF Pressure Pipes and Sensors
- Oil Level
- Oil Specification
- Fuel Additive level (where applicable)
- Sensors – check all are operating correctly
- EGR System
- Air flow Meter
- Engine and Turbo Wear
- Glow Plugs
- Air Filter
- ECU Malfunctions