The U.S. Environmental Protection Agency (EPA) has put in place increasingly stringent standards for particulate and NOx emissions. For example, the standards that take place in Octuber, 2002 include 0.1 g/hp-hr for particulates and 2.0 g/hp-hr for NOx. In 2007 these will be further reduced to 0.01 g/hp-hr for particulates and 0.2 g/hp-hr or NOx. Industry has been conducting an intensive program toward achievement of these requirements.
PCT Publication WO 03086580 identifies a method of filtering or trapping the particulate from the exhaust and periodically disposing of the collected soot and ash. In common with most other approaches, the system disclosed in the WO 03086580 publication uses a monolithic ceramic trap having passages with porous walls through which exhaust is passed to filter out the smoke particles at very high (90-97%) trapping efficiency. These systems use either wall flow or cross flow traps in multi-trap or single trap configurations. Each of these systems is capable of achieving the EPA particulate standards for 2002 and 2007.
FIGS. 1A and 1B show the wall-flow single trap particulate trap system from the WO 03086580 publication. The principal parts are a trap structure 166, a wall flow particulate trap 160, a rotating duct/valve 162, a rotating duct/valve drive 163, a remote actuated relief valve 173, a seal 166a, a purge duct 169 and a mode valve 168.
The wall-flow particulate trap systems disclosed in the WO 03086580 publication use cordierite traps of the well known wall-flow type to filter the exhaust gas by passing it through the porous walls of trap channels. This action removes 90-98% of the particulate and this collects on the inside surfaces of the passages as a layer or cake which after a few hours of operation increases the engine backpressure and must be removed to prevent adverse affect on engine performance.
Most competitive trap systems remove this layer of soot by burning it in the trap. To avoid excessive temperatures during this operation, expensive noble metal catalytic coatings are required and ultra low sulfur fuel must be used which will not be broadly available for a number of years. Also, the engines must be operated at a relatively high average load factor or auxiliary heating methods must be used to assure that burn-out occurs before too much soot is collected. Finally, the incombustible ash remains in the traps resulting in increased backpressure and the traps must be periodically cleaned in an expensive and disruptive maintenance operation.
The particulate trap systems disclosed in the WO 03086580 publication preclude the above problems by using a reverse flow of pre-filtered exhaust gas to create a constant reverse pressure drop across the trap, or portions thereof, to dislodge and erode the accumulated soot and ash cake and to transport the dislodged particles to an external chamber where in the soot and/or ash are separated from the purge flow for combustion of the soot and storage of the incombustible ash. This approach permits the use of traps of low cost cordierite and the regeneration process has little or no adverse affect on engine performance. This system will provide the 0.01 g/hp-hr particulate emissions standards required by the EPA regulations in the future.
There remains a need in the art for a wall-flow single trap particulate trap system that is more compact, less expensive and adaptable to a broad range of vehicles, such as highway trucks, transit buses, school buses, off-highway and many other vehicles.