How DPFs Work And How To Select The Correct DPF?

Both The Particulator^® and the Johnson Matthey range of DPFs use ceramic wall flow filters. These are commonly used by truck and machine manufacturers on their new engines and are the most effective technology in reducing Diesel particulate from exhaust. The exhaust gas has to pass through the porous walls of many channels in the filter in order to exit the exhaust outlet. A very high filtration rate is achieved to remove nearly all the particulate by mass and size.

Johnson Matthey DPFs come in a wide range of filter sizes. The filter is sized to meet the back pressure requirements of the engine manufacturer at the maximum exhaust flow rate for that application. This assessment must also take into account the likely soot loading of the filter. Johnson Matthey have many years experience to draw on when selecting the correct filter size for a particular engine and application.

Although wall flow filter technology is very effective at taking Diesel particulate from exhaust, without regular regeneration the particulate would accumulate in the filter leading to higher exhaust back pressure and possible damage to the engine. That is why all DPF systems have some means of regenerating or cleaning the filter of its trapped particulate.

There are two types of regeneration possible, active, that requires the operator to initiate a regeneration cycle periodically, or passive, that uses catalysts to continuously burn the soot from the filter during normal engine operation. The active systems involve heating the filter to above the ignition temperature of the particulate so that it burns in air. Heating systems can be electrically powered or use Diesel fuel in a burner. Passive systems may use an exhaust catalyst in front of, or on the filter to generate NO2 that removes the particulate at lower temperatures, or a catalyst that is added to the fuel that promotes the burning of particulate with oxygen at lower temperatures.

The CRT^®, a Johnson Matthey invention, is the most widely used passive device and uses a catalyst in front of the filter. Like all passive devices, the CRT^® relies on certain exhaust temperatures being exceeded on a regular basis for the continuous regeneration mechanism to be successful. In the CCRT^® the filter is also coated with catalyst to achieve regeneration at lower exhaust temperatures than needed for the CRT^®. Fuel borne catalysts require quite hot exhaust temperatures to work successfully and can only be used in limited applications.

When selecting the best regeneration type for a particular application the preferred option is a passive system using the CRT^® technology as this minimises input from the machine operator. Whether or not a passive system can give reliable regeneration and low build up of soot on the filter depends on a number of factors. The most important of these is exceeding a minimum exhaust temperature for a certain percentage of the operating time. This depends on how hard the engine is normally working and how close to the engine the filter can be fitted. For example, an excavator usually works hard and is often a good application for a passive system, whereas a crane will often only work hard intermittently and the engine is idling for prolonged periods and should generally not be fitted with a passive DPF. The NOx to PM ratio also needs to be considered before using a passive system as it relies on NOx to remove soot from the filter. The EU emissions level usually gives a good indication of this ratio. If the application is not considered suitable for a passive system, then the correct active system must be selected. The two main types use heating from electrical elements or from a Diesel burner. The electrical system requires an external power supply whereas the burner system uses fuel from the machine’s Diesel tank.