Active regeneration becomes necessary when the temperature is too low to completely burn off the soot. The process is initiated by injecting fuel into the exhaust automatically, with no driver interaction needed.
However, if soot continues to accumulate in the DPF, a parked regeneration is required. The driver is alerted to this through the DPF indicator lamps on the instrument panel. He must park the vehicle and initiate the regeneration process by pushing a button on the dash or holding the accelerator at a high idle rpm for up to 30 minutes.
Although the DPF is designed for long life, to ensure that an optimum level of performance is maintained, the filter must be cleaned at regular intervals. This is necessary as small amounts of inert debris, mainly accumulated mineral and metal particulate from lubricating oil and engine wear, build up in the filter substrate. This inert, unburnable material is known collectively as ash.
Ash builds up and occupies space in the cells intended for soot. However, unlike soot, ash does not burn off. It must be physically removed from the DPF.
Typically, most engine OEMs recommend this be done between approximately 200,000 to 250,000 miles for less severe service applications, such as Class 8 linehaul operations.
Some fleets think that by ignoring ash buildup in their DPF they can avoid another operating expense. Besides, if the DPF fails under warranty it is covered.
Other fleets have discovered that by allowing the DPF to fill up with ash it will eventually crack and rot out to the point that the ceramic material breaks up, crumbles and blows out the exhaust stack. Provided the truck has not been forced to the side of the road with backpressure warnings, the ceramic core can partially or totally disappear.
This sudden loss of backpressure will result in a slight improvement in engine performance and mileage.
One problem comes when the truck is resold and a new replacement DPF must be installed costing several thousand dollars.
Since the primary function of a DPF is to eliminate soot from diesel exhaust prior to escaping to the atmosphere, the DPF is vital to helping prevent serious air pollution, says Drew Taylor, national sales manager for FSX Equipment, a company that provides filter cleaning systems and services for cleaning diesel particulate filters.
He says a good rule of thumb is: If you observe soot (black) in the exhaust stack/pipe or observe heavy black smoke coming from a truck’s stacks, this is a sure sign the DPF is not working and harmful pollutants are being emitted into the atmosphere.
Since 2007, to meet EPA heavy duty diesel engine emission requirements, vehicles now come standard with aftertreatment system warning lights and indicator symbols. These alert the operator to actions being taken or to actions that need to be taken.
“One of the most common DPF myths circulating is that the DPF status light indicates when the DPF needs to be cleaned of ash,” says Taylor. “That is false. This light keeps the driver informed about the active regeneration system as it periodically clears out soot buildup to relieve backpressure created by soot.
“By the time the DPF status light shows signs of ash buildup in a DPF, the condition is so far advanced that substrate cracking and other damage are well under way,” he points out. “The only way the driver can see this is by noticing the regenerations are occurring one after another as the sensors detect DPF backpressure that does not respond to thermal regeneration.”
This is why ash removal must be done proactively as a scheduled maintenance item, stresses Taylor. The best DPF rule of thumb: Take it off and clean it sooner than later.
“If you leave ash in the DPF too long it becomes like a cancer that grows in the filter over time,” he says. “If left in place, the ash becomes hardened, making removal even more difficult. The solution is to get the ash out before it has the time to solidify.
“Because the ash is cooler than the surrounding material, it causes cracking of the brittle ceramic. Once the cracking starts, it is irreversible damage that will continue to worsen and eventually destroy the filter.”