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Exhaust Topics: Good maintenance protects aftertreatment system

By on November 2, 2012

There was a time when trucks rolling down the highway billowed heavy black smoke from two exhaust stacks. Sure, it looked cool, but that smoke contained tiny bits of junk known as diesel particulate matter and oxides of nitrogen (NOx). The collateral damage it caused — smog, UV radiation and multiple health risks — was too much to bear. Federal regulations on truck emissions were enacted, with a 90 percent reduction required by 2007. Manufacturers developed exhaust aftertreatment systems that have been very effective in reducing particulate matter and lowering NOx emissions.

For the past few years, diesel particulate filters (DPF), diesel oxidation catalyst devices (DOC), regenerating technologies, Selective Catalytic Reduction (SCR) systems and many other emission-reducing technologies have been part of the standard equipment on the nation’s trucks. Now, about five years into regular use of this new equipment, many drivers are facing the first service trip for a significant cleaning and maintenance check on a key part of the aftertreatment system, the DPF.

Ash buildup

Cleaning up a truck’s exhaust causes some dirt of its own. The DOC, located at the inlet of the muffler, oxidizes or burns off hydrocarbons created by fuel combustion before they reach the atmosphere. The DPF, a ceramic filter located directly behind the DOC, traps any remaining soot and ash. To remove buildup in the DPF, the temperature in the muffler is raised, turning the soot into a harmless carbon dioxide gas and ash through a process called regeneration.

A “passive” regeneration typically occurs while the vehicle is in operation. An “active” regeneration requires the operator to stop the vehicle in a safe location and press the “Regen” button in the dash. The driver can also choose to assist the passive regeneration cycle by heading to the highway and driving at a speed that will generate high exhaust temperatures. Ignoring the Regen indicator light will lead to an engine derate or the vehicle shutting down and more costly repairs.

Eventually enough ash builds up in the DPF that it must be brought in for cleaning. The driver may see a “Check Engine” lamp, indicating that the vehicle must be brought in to a qualified technician. The “Regen” light might also come on, and stay on even after the regeneration process is done. If one or both of these lights comes on, it’s important to bring the truck in.

Cleaned & inspected

Technicians must remove the DPF to clean it, so maintenance involves downtime — possibly overnight. The filter must go through a thorough inspection to ensure that it has not been damaged from the heat of regeneration, impact breaks, or contaminated by oil or coolant. If it is in good shape, it will be placed in a machine that will remove the majority of contaminants.

After cleaning, a technician will do a final inspection. If it meets EPA certification requirements, the DPF goes right back in service.

If the DPF has been damaged or contaminated, it will have to be replaced. You do not want to wait for or pay for a replacement, and following key maintenance habits could save the life of your DPF. It is imperative that internal coolant leaks are discovered and corrected in a timely manner. Get your vehicle serviced on time and pay close attention to fluid levels. If you notice that you are losing either oil or coolant, get your vehicle in for service as soon as possible.

There are many points on the modern engine where coolant could enter the exhaust system, causing catastrophic failure to your aftertreatment system. Low coolant conditions must be investigated to include pressure testing to determine root cause and ensure a timely repair. Coolant that enters the aftertreatment system will cause detrimental damage. Equally important is the prevention of oil entering the exhaust system. The aftertreatment system is not capable of absorbing large quantities of oil.

These systems do require a shift in how a vehicle is operated and maintained. Take a little time to familiarize yourself with the additional indicator lights on the dash and always know what happened to any oil or coolant that comes up missing. With a few changes in how you operate and maintain the vehicle, your aftertreatment system should give you years of good service.

Homer Hogg, Senior Technical Trainer for TA and Petro, has worked as a truck technician for 30 years. He is ASE-certified, a Daimler Certified Trainer and a member of the Nashville Auto Diesel College Hall
of Fame

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