"Incoming ash and soot pass into the dead end holes on the inlet side of the DPF and lodge against the cell walls," he says. "The soot burns inside the hot cell and passes as a gas through the micro-pores in the cell wall. The unburnable ash stays in the cell and builds up as an accretion similar to a stalagmite in a cave.
"Ash build-up occurs over a long period of time and takes either a lot of mileage or higher consumption of engine oil. Eventually, the ash forms into a long square ash plug that is called an ash noodle. Ash plugs are difficult to remove and bad news if left to long in the DPF.
Constant thermal regenerations can cause some of the elements in ash such as potassium to react with the ceramic wall to form a low level grade of glass, adds Taylor. This is commonly called sintering or glazing of the cell walls. Glazing the cell walls is permanent damage.
Build up of ash in the cells is eventually detected by emission system sensors and the truck tries to correct the problem by going into active regeneration mode. This mode burns fuel, but doesn't cure the problem. Eventually, the truck will stop.
DPF can also plug with soot, notes Waldo. Soot plugging typically occurs quickly and is a result of an event or unfavorable operating conditions. A bad injector or too low of operating temperatures can cause the problem.
"While active regenerations can typically cure the symptom," he says, "the problem must be cured to stop the constant regenerations. Under extreme cases, the DPF can become so plugged with soot it will require removal of the DPF for cleaning.
"If an unfavorable operating condition exists and is not corrected, you should expect the DPF to quickly plug up again."
Following the recommended DPF cleaning schedule will assure best operation in the majority of applications and duty cycles, says Navistar Engine's Shick. In applications requiring more frequent cleaning, the driver will know a cleaning is required if the vehicle regeneration process does not adequately clean the DPF.
As the DPF needs cleaning, he says, the DPF status indicator lamp on the instrument panel will change from off to solid on to flashing, and an audible alarm will beep. Normally, the vehicle regeneration process would address this but if regeneration is not adequate, the monitor lamp and audible alarm will remain active.
Another indicator that it is time to clean the DPF is the driver will notice ever-decreasing intervals between a system commands for an active regeneration. Some vehicles will have filter service monitors. Service technicians will have diagnostic tools that they can use to determine a DPF's "health."
For a typical Class 8 truck application, Cummins has recommended a DPF cleaning interval of 200,000 to 400,000 miles. The exact mileage at which a fleet will need to actually clean the DPF is impacted by several factors, including duty cycle.
There are various levels of cleaning for a diesel particulate filter. The first level of cleaning is a pulse air cleaning. The next level uses a thermal process.
Pneumatic cleaning is the first cleaning process a DPF needs to go through, says FSX's Taylor. The method is used to clean each cell by bi-directionally scanning high-pressure air into the filter from both ends.
The next cleaning stage is thermally processing the DPF to burn off any remaining soot and to loosen remaining ash from the cell walls, Waldo of FSX says. Once the soot is burned and the ash is loosened, it is important to do a second pneumatic cleaning to blow out the remaining ash.
"The key to successful thermal cleaning is using the correct temperature ramp rate, not exceeding the maximum temperature and avoiding pumping air through the DPF during processing," he says. "Even a little air pumped through the DPF is like fanning the flames. At higher temperatures the oxygen can cause the soot to flash leading to uncontrolled regeneration and damaging temperature spikes."
"Always thoroughly clean a DPF with a pneumatic cleaner before processing thermally," emphasizes Taylor. "Pneumatic cleaning removes as much soot (fuel) as possible so it cannot ignite."
Some DPFs do not need to go on to thermal cleaning, observes Waldo. "If testing proves the OEM specifications have been met with just the pneumatic cleaning, the DPF is ready to go."