DPF maintenance
With DPFs in service for up to six years now, long-term maintenance of these expensive, and delicate, pieces of equipment is critical for proper function and long life

By Paul Abelson, senior technical editor

When the Environmental Protection Agency regulated engine exhaust emissions years ago, it altered our industry forever and forced the development of new technologies. The second of the phased-in emission standards brought the industry diesel particulate filters, or DPFs.

The devices, introduced in 2007, reduce particulate matter in the exhaust by 90 percent. Now with these units in service for up to six years, the long-term maintenance and concerns come into play.

DPFs replace traditional mufflers. They are made from high-temperature-resistant porous ceramics. The ceramic resists damage from extremely high operating temperatures and allows exhaust gasses to flow with minimal back pressure, while capturing as many solids as possible.

But like all the other filters on trucks, DPFs require periodic attention.

How does a DPF get clogged up?
Most of the solids are soot, unburned or partially burned fuel that has become a solid, mostly carbon. Additional solids in exhaust form around incombustible metals, found in oil and some fuel and oil additives.

Oil is generally well controlled, but some must get into cylinders and combustion chambers. Oil is on cylinder walls to lubricate piston movement. It flows past valve guides. If it didn’t, engines would seize.

The soot and ash are created when oil is burned with the fuel. They enter the exhaust stream and are captured by the DPF. Inside the porous structure, they accumulate, restricting and eventually clogging the filter if not cleaned out.

The DPF structure is a series of adjacent tunnels. The tunnels alternate between being blocked at the inlet end or the outlet end. Exhaust flows through the open inlets into the tunnels until it is blocked, diverting it through the porous ceramic material separating the tunnels. The pores in the ceramic material allow the gas to flow through to tunnels with open outlets, where the gas exits the tunnel system. Solids, the soot and ash, are too large to flow through the pores in the ceramic material and are left behind in the tunnels. Eventually the tunnels need to be cleaned out when the solids restrict exhaust flow.

Since most of the solids are soot made of carbon, they can be converted to gas, carbon dioxide, by a high-temperature combustion process we know as regeneration. Fuel is added to the exhaust and passed over a catalyst that raises the exhaust temperature above 1,100 degrees. At that temperature, all the carbon becomes carbon dioxide and flows out the exhaust, while the incombustible ash is left behind.

Keeping it clean
Regeneration takes care of the soot, freeing the filter from much of its backpressure. But to get the ash out requires more effort. Because of the costly, specialized equipment, the work should be done by an authorized dealer. They have the cleaning machines and the equipment to safely handle DPFs. Cleaning will remove about 85 to 90 percent of the ash plugging the DPF.

How many times can you clean these things before you need a new one? There are no limits, but if cleaned to an assumed level of 70 percent, life before cleaning efficiency theoretically drops to 70 percent of new after the first cleaning, 49 percent after the second, 35 percent after the third, etc. That is theoretical. In practice, I believe subsequent cleanings will be more effective and life-to-cleanout may be at 50 percent after four or five cleanings. In theory, you can keep cleaning until the ceramic breaks, which will depend on handling. The longer between cleanings, the tighter the ash will be packed in and the less efficient each cleaning will be.

A cleaning may cost from $350 to $550. But if you bump, drop, dent or damage the brittle ceramic filter core, you’ve bought a $6,000 (or more) component. Let the dealer do it, so the dealer can assume warranty responsibility.

Paccar and Navistar have networks of dealers with cleaning equipment. Drivers can schedule cleaning just as they would any other service. It usually takes a half-day or less. The cleaning process for the two OEMs includes reversed flow with controlled high-pressure flow at the outlet end and filtered suction at the inlet end. For DPFs with stubborn clogging, a baking process may be needed to loosen the solids. When needed, baking may add several hundred dollars to cost and about eight hours to the total time.

Remanufactured DPFs
You may want to consider a remanufactured unit. Remanufactured units also last longer than units that were just machine cleaned and many have a warranty. A good cleaning will restore 65 percent to 75 percent life. A reman unit will have 85 percent to 90 percent life. A remanufactured exchange unit could run $800 to $1,000 with a core credit.   

Detroit’s Reliabilt DPF Cleaning Process is up to 95 percent effective and is claimed to last up to 550,000 miles on Series 60 engines and 300,000 miles on DD engines. The one-year warranty covers any progressive engine damage.

Cummins’ ReCon DPFs are made for ISX engines. They use back pressure, vacuuming and baking to eliminate maximum residue. All ReCon DPFs are validated to be restored close to original capacity and functionality.

Volvo and Mack have had a remanufacturing program based at their Middletown, Pa., remanufacturing center since 2008. The DPF ceramic filter elements are restored to 90 percent of original capacity. They are then exchanged at dealer locations. LL