Modern medium- and heavy-duty trucks have sophisticated exhaust aftertreatment systems that can include a diesel oxidation catalyst (DOC), diesel particulate filter (DPF) and selective catalytic reduction (SCR) system consisting of the SCR catalyst, ammonia oxidation catalyst (AOC), diesel exhaust fluid (DEF) tank, DEF injector and pressure and nitrogen oxide (NOx) sensors. (See the poster following page 16 for specific information on these components.) While these systems require little maintenance, they cannot be ignored without costly consequences.
Starting with 2007 model trucks, a DPF was added to remove particulate matter. “The DPF has two functions: to collect particulate matter and to store the residual ash after the soot has been oxidized,” says David McKenna, director of powertrain sales, Mack Trucks.
The DPF traps and removes up to 95% of particulate matter from the diesel engine exhaust. It contains a ceramic filter element that utilizes a honeycomb structure with long channels that are plugged off on alternate ends. Exhaust gas is forced through the porous channel walls, allowing the gas to escape through the neighboring channel wall while the particulates are trapped.
Over time, soot builds up on the DPF channel walls. The soot is cleaned and reduced to ash by heating either through passive or active regeneration. During this process, the heat inside the filter can reach 1,125° F.
Passive regeneration occurs automatically when the exhaust gases are hot enough during vehicle operation to cause a significant oxidation (a.k.a. burn-off) of the accumulated soot. Active regeneration injects fuel into the exhaust system to elevate exhaust temperatures and burn off soot collected on the DPF.
Volvo Trucks uses active regeneration on its pre-2010 technology trucks. “There is a seventh injector that sprays a mist of fuel downstream from the engine exhaust to the DPF during active regeneration on pre-2010 units,” notes John Moore, powertrain products manager, Volvo Trucks. While this system requires little in the way of maintenance, proper injector operation is critical. “If the seventh injector clogs and cannot burn the soot off the DPF, the filter will have to be pulled off the vehicle and cleaned using a machine, along with repairing the injector.”
Cleaning the DPF
The byproduct after regeneration is ash. “As the ash accumulates within the DPF housing, the effective filtering surface area is reduced, which can cause excessive active (fuel-consuming) DPF regeneration events,” McKenna points out. “Should the filter reach 100% of the design of the ash loading capacity, there is little to no filter area left to collect soot, and significant exhaust backpressure will arise.”
After many regeneration cycles, the ash begins to build up in the DPF and the ceramic filter must be removed and professionally cleaned. “Clean at the appropriate intervals,” advises Brad Williamson, manager of engine and component marketing, Daimler Trucks North America. “Don’t push the regeneration limits. Ensure the cleaned DPF meets the OEM specifications. If a DPF is not properly cleaned, it will have less efficiency and capacity.”
Volvo trucks recommends that the DPF on its trucks be cleaned at 250,000 miles, or 4,500 hours, whichever comes first. Performance will decline if the DPF is not serviced at the proper intervals. “Backpressure will continue to increase in the exhaust system, thus reducing engine efficiency and decreasing fuel economy,” says Moore.
Frequency of DPF cleaning intervals can vary. “This is largely dependent on the vehicle duty cycle,” says McKenna. “Trucks operating at highway (steady state) speeds for extended amounts of time can go as long as 350,000 miles. Whereas, a truck that operates in a steady stop-and-go (transient) duty cycle will not see that type of mileage accumulation before an ash cleaning is required.”