DPF & Emissions
What is a DPF?
Diesel particulate filters (DPF) are devices that physically capture diesel particulates to prevent their release to the atmosphere. Diesel particulate filter materials have been developed that show impressive filtration efficiencies, in excess of 90%, as well as good mechanical and thermal durability. Diesel particulate filters have become the most effective technology for the control of diesel particulate emissions—including particle mass and numbers—with high efficiencies.
Due to the particle deposition mechanisms in these devices, filters are most effective in controlling the solid fraction of diesel particulates, including elemental carbon (soot) and the related black smoke emission. Filters may have limited effectiveness, or be totally ineffective, in controlling non-solid fractions of PM emissions—SOF and sulfate particulates. To control total PM emissions, DPF systems are likely to incorporate additional functional components targeting the SOF—typically oxidation catalysts—while ultra low sulfur fuels may be required to control sulfate particulates.
The term “diesel particulate trap” is sometimes used as a synonym for “diesel particulate filter”, especially in older literature. The term “trap” covers a wider class of particle separation devices. Several particle deposition mechanisms other than filtration are commonly employed in industrial dust separation equipment. Examples include gravity settling, centrifugal separation, or electrostatic trapping. None of these techniques could be adopted to control diesel PM emissions, due to the small particle size and low density of diesel soot.
It may be noted that particle oxidation catalysts (POC)—sometimes called partial filters—can also capture diesel particulates, but provide a much lower overall efficiency than diesel particulate filters. In their common designs, POCs capture particulates only from a fraction of the flow, whereas the total flow is filtered in diesel particulate filters. In the case of some filter media, however, the distinction may not be very clear and the devices can be classified as either a POC or a (depth) particulate filter.
Why Do We Need a DPF?
Diesel engines (well, all engines really) emit pollutants into the air we breathe. And by “pollutants” we mean chemicals and compounds that wouldn’t otherwise end up in our air in the quantity and concentration we create with our vehicles. To reduce the amount of these pollutants in our air, ’07-and-newer diesel engines have been fitted with a filter in the exhaust pipe to capture soot particles. This matrix of materials (a composite of cordierite, silicon carbide, or metal fibers), called a diesel particulate filter (DPF), traps the particulates flowing out the exhaust pipe.
What Happens To The Trapped Particles?
All DPFs capture soot until they fill up and create too much backpressure. At that point, DPFs use one of two approaches to regenerate (clean) themselves: active or passive. Passively regenerating DPF systems are commonly found in the retrofit marketplace for construction equipment. In these applications, the vehicle’s duty cycles and temperature profiles can be observed prior to filter installation. Passive systems rely on precious metal-coated substrates to heat the normal exhaust flow to temperatures that trigger regeneration. Although more sensitive to duty cycle and temperature, once installed, these DPF systems are considered to be more user friendly, as they do not require fuel additives, have no moving components, and only need to be serviced for annual cleaning.
Maintaining and Servicing Diesel Particulate Filters
Diesel particulate filters trap soot from the exhaust and ash from motor oil. Most soot is burned off in the course of a truck's operations, but ash stays in the filter's honeycomb substrate and is removed through periodic servicing.
Exhaust in highway trucks is usually hot enough to burn off most soot in a process called "passive regeneration." However, those involved in stop-and-go operations or that idle a lot don't get their exhaust hot enough. The latter must "actively" burn out soot from the substrate by injecting extra fuel just upstream of an oxygen catalyst or by plugging in an electric heater when parked.
Active removal of soot through on-board regeneration can occur one or more times a day, depending on its type of operation, manufacturers say. Often it goes unnoticed by the driver, who may or may not see the indicator light in the instrument panel. But sometimes a warning gets more insistent through the light indicators and the driver must stop and initiate an active regeneration.
Ash from motor oil stays in the filter's substrate and must be periodically blown or washed out. This is done by removing the DPF from the truck and placing it on a special machine. It sends compressed air through the substrate in a series of measured puffs. Detroit Diesel's filters are pressure-washed with de-ionized water, then dried.
Most filters seen by Cleaire Advanced Emissions Controls, which air-cleans DPFs it has previously made and sold, collect about 200 grams (7 ounces) of ash. Some come in with as much as 600 grams (21 ounces).
Blowing or washing out of ash must be done every 50,000 miles or once a year. This, too, is indicated by a warning light. Large fleets have purchased machines to do the cleaning themselves, but most owners take filters or entire trucks to dealers and independent shops for cleaning.
Some owner-operators running out of the Port of Oakland, Calif., have used state money to retrofit their older trucks with DPFs, only to discover they need frequent removal and cleaning. So it's important to ask upfront what the cleaning requirements will be, how long they will take and how much they will cost.
Sometimes an engine produces extra soot or spits out unburned fuel and burned motor oil. These, too, are captured by the DPF, and usually it can't burn them out through normal processes. So occasionally the substrate must be removed from the DPF, then heated and baked in a special machine to burn out the crud. About 10% of filters brought in for servicing need this, manufacturers say.
Once your mechanic at Circle M gets at a truck, it usually takes about 60 to 90 minutes to remove the DPF, set it on a machine, clean it for about a half hour, then reinstall the filter on the truck. If baking is required, it may take up to eight hours in the machine. The advantage is that the original DPF stays with the truck, which might add to resale value because its owner can show complete servicing records.