January 2008, Work Truck - Feature
What Fleet Managers Should Know About Diesel Particulate Filters
By Sean Lyden
By now, fleet managers should be up to speed on the 2007 federal diesel emissions standards. Diesel engines built Jan. 1, 2007 and thereafter are required to reduce nitrogen oxide (NOx) emissions by 50 percent and particulate matter (PM) emissions, also known as soot, by 90 percent, relative to the 2004 standard.
A critical technology engine manufacturers developed to achieve these requirements is the diesel particulate filter (DPF), which traps and removes particulate matter from diesel exhaust.
What should be expected when operating trucks with a DPF? How does it affect maintenance schedules? In what ways, if any, does the DPF require driver involvement? What should fleet managers know to reduce risk of premature DPF clogging that leads to costly repairs and downtime? Here are the basics.
DPF Features Honeycomb Composition to Trap Soot
What exactly is a diesel particulate filter and how does it work? According to the Manufacturers of Emissions Controls Association (www.meca.org), DPFs are commonly made from ceramic materials such as cordierite, aluminum titanate, mullite, or silicon carbide.
Inside the DPF is a honeycomb structure with alternate channels plugged at opposite ends. (See Figure 1.) The diesel exhaust gases pass through the open end of a channel. At the opposite end is a plug that forces the gases through the porous channel wall, allowing the exhaust to escape through the neighboring channel, while trapping as much as 90 percent of the solid particle matter (soot).
Figure 1
Regeneration Removes Particulate Buildup
Over time, soot builds up on the DPF channel walls. How is that soot removed to keep from clogging the DPF?
The process is called regeneration. The filter is cleaned — "regenerated" to its original state — by burning off the soot at a high temperature (could be as high as 1,000 degrees F inside the filter). What’s left over is harmless ash residue.
Most 2007-compliant engines feature two types of regeneration.
- Passive regeneration. This oc-curs automatically when exhaust gases are hot enough during vehicle operation, usually with highway driving or higher load applications, to burn off accumulated soot. An ongoing process, it occurs when conditions permit and is not noticeable to the driver.
- Active regeneration. This occurs when enough soot accumulates in the filter to raise exhaust backpressure to a certain limit. The process is initiated by injecting fuel into the exhaust automatically, requiring no driver interaction. With International medium-duty trucks, for example, active regenerations occur at speeds over 17 mph and last 15-20 minutes. The driver is not alerted and doesn’t notice a change in performance.
How often does active regeneration occur? City driving with light load applications can require active regeneration as much as every 300-400 miles.