What Are Diesel Particulate Filters?
Diesel engines emit significant amounts of particulate matter (PM) and oxides of nitrogen (NOx) into the atmosphere. They also emit toxic air pollutants that adversely affect human health and contribute to acid rain, ground-level ozone, and reduced visibility. Studies show that exposure to diesel exhaust causes lung damage and respiratory problems. There is also increasing evidence that diesel emissions may cause cancer.
Diesel retrofit involves the addition of an emission control device that removes emissions from engine exhaust. Retrofits can be effective at reducing emissions and can eliminate up to 90 percent of pollutants.
Diesel Particulate Filter (DPF) Overview
Diesel particulate filters remove particulate matter by filtering diesel exhaust from the engine. DPFs can be installed on vehicles or stationary diesel engines. Since a filter can fill up over time, engineers who design filter systems must provide a means of burning off or removing accumulated particulate matter. A convenient means of disposing of accumulated particulate matter is to burn or oxidize it on the filter when exhaust temperatures are adequate. By burning off trapped material, the filter is cleaned or “regenerated”. Filters that regenerate in this fashion cannot be used in all situations.
In some nonroad applications, disposable filter systems are often used. A disposable filter is sized to collect particulate during a working shift or a predetermined period of time. After a prescribed amount of time or when backpressure limits are approached, the filter is removed and cleaned or discarded.
To ensure proper operation, filter systems are designed for the particular vehicle and vehicle application.
Diesel Particulate Filter Material
A number of filter materials have been used in diesel particulate filters including ceramic and silicon carbide materials, fiber wound cartridges, knitted silica fiber coils, ceramic foam, wire mesh, sintered metal structures, and temperature resistant paper in the case of disposable filters. Collection efficiencies of these filters range from 50 to over 90 percent. Filter materials capture particulate matter by interception, impaction, and diffusion.
Filter efficiency has rarely been a problem with the filter materials listed above, but work has continued to optimize filter efficiency and minimize backpressure, improve the radial flow of oxidation in the filter during regeneration, and improve the mechanical strength of filter designs. Below is a diagram of a typical high-efficiency, wall-flow filter system. High-efficiency, wall-flow filters have demonstrated the ability to reduce diesel particulate emissions by more than 90 percent in retrofit applications.
In the diagram, particulate-laden exhaust enters the filter from the left. Because the cells of the filter are capped at the downstream end, exhaust cannot exit the cell directly. Instead, the exhaust gas passes through the porous walls of the filter cells. In the process, particulate matter is deposited on the upstream side of the cell wall. Cleaned exhaust gas exits the filter to the right.
Diesel Particulate Filter Regeneration
Many techniques can be used to regenerate a diesel particulate filter. Some of these techniques are used together in the same filter system to achieve efficient regeneration. Both on- and off-board regeneration systems exist. The major regeneration techniques are listed below.
- Catalyst-based regeneration uses a catalyst applied to the surfaces of the filter. A base metal or precious metal coating applied to the surface of the filter reduces the ignition temperature necessary to oxidize accumulated particulate matter.
- Catalyst-based regeneration using an upstream oxidation catalyst. In this technique, an oxidation catalyst is placed upstream of the filter to facilitate the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). The nitrogen dioxide reacts with the collected particulate, substantially reducing the temperature required to regenerate the filter.
- Fuel-borne catalysts. Fuel-borne catalysts reduce the temperature required for the ignition of trapped particulate matter. These can be used in conjunction with both passive and active filter systems.
- Air-intake throttling. Throttling the air intake to one or more of the engine cylinders can increase the exhaust temperature and facilitate filter regeneration.
- Post top-dead-center (TDC) fuel injection. Injecting small amounts of fuel in the cylinders of a diesel engine after pistons have reached TDC introduces a small amount of unburned fuel in the engine’s exhaust gasses. Fuel can also be injected into the exhaust pipe. This unburned fuel can then be oxidized in the particulate filter to combust accumulated particulate matter.
- On-board fuel burners or electrical heaters. Fuel burners or electrical heaters upstream of the filter can provide sufficient exhaust temperatures to ignite the accumulated particulate matter and regenerate the filter.
- Off-board electrical heaters. Off-board regeneration stations combust trapped particulate matter by blowing hot air through the filter system.
The experience with catalyzed filters indicates that there is a virtually complete reduction in odor and in the soluble organic fraction of the particulate, but some catalysts may increase sulfate emissions. Companies utilizing these catalysts to provide regeneration for their filters have modified catalyst formulations to reduce sulfates emissions to acceptable levels. Ultra-low sulfur diesel fuel (15 ppm sulfur maximum) is now available in the U.S. and has greatly facilitated these efforts.
In some situations, the installation of a filter system on a vehicle may cause a slight fuel economy penalty. This fuel penalty is due to the backpressure of the filter system. As noted above, some filter regeneration methods involve the use of fuel burners and to the extent, those methods are used, there will be an additional fuel economy penalty. Many filter systems, however, have been optimized to minimize, or nearly eliminate any noticeable fuel economy penalty. Experience in the New York City Transit program and in the San Diego school bus program has shown that fuel penalties for filters are zero or less than one percent. During the required retrofit technology verification protocols established by either the U.S. EPA or the California ARB, fuel penalties have been documented at about 1 percent for high-efficiency filter systems.