Exhaust from diesel engines on buses, trucks, etc. contain particulate matter, NOx (nitrogen oxides), etc. The particulate matter in turn contains insoluble organic fractions such as soot (carbon or C) and sulfates that are generated as the result of oxidation of sulfur in gas oil, as well as soluble organic fractions (SOF) such as HC either unburned or contained in lubricants. If released into atmospheric air, these fractions cause air pollution or adversely affect the human body, which are by no means desirable. To deal with this problem, a need has recently come to be realized to require by laws and regulations that diesel-powered vehicles such as buses and trucks should be equipped with devices that can control or eliminate PM and other harmful materials in diesel emissions.
In order to trap diesel particulate matter (hereunder sometimes abbreviated as PM) within the exhaust system, honeycomb filters shaped from ceramic materials were developed and have been known as diesel particulate filters (DPF). These honeycomb filters are available in two types, the straight flow type and the whirl flow type. In the former type, a large number of cells are formed within a matrix as partitioned by thin porous walls, with a catalyst being carried on the wall surfaces such that PM, CO, HC, etc. in the exhaust stream passing through the cells are reduced in concentration or rejected as they come into contact with the wall surfaces (prior art technology 1).
In the latter, whirl flow type, the matrix itself is a large number of cells that are made of a porous material and which are closed at their inlet and outlet alternately so that the exhaust stream entering one cell at the inlet passes through the thin porous partition to come into another cell from which it emerges through the outlet.
The soot component of PM is trapped by the partition on its surface or within pores in it. Honeycomb filters of the whirl flow type are classified in two sub-types, one having the catalyst carried both on the surfaces of cell partitions and within pores in the partitions and the other having no catalyst supported (prior art technology 2). In the former case, PM trapped on the surfaces of cell partitions and in their interior are catalytically removed by oxidation and in the latter case, the trapped PM is removed by combustion with a burner or a heater.
Also known is an exhaust cleanup apparatus using two types of honeycomb filter in combination, one being of the straight flow type and the other being of the whirl flow type, that are arranged in the same direction as the emission flow (Japanese Patent No. 3012249). The straight flow type honeycomb filter which is loaded with a regenerating oxidation catalyst system is provided in the upstream area of the tailpipe on a diesel engine and the whirl flow type honeycomb filter which is adapted to trap PM is provided in the downstream area. The regenerating oxidation catalyst system in the straight flow type honeycomb filter oxidizes NO (nitrogen monoxide) in the exhaust to generate more oxidative NO2 (nitrogen dioxide) whereas the downstream, whirl flow type honeycomb filter oxidizes the trapped PM with NO2 to generate CO2, thereby reducing the level of PM.
According to this technique, the concentration of PM on the filters is continuously reduced, thereby ensuring that PM will not be so much deposited on the filters as to make further trapping of PM impossible. This offers the advantage of allowing for continuous regeneration of the filters (prior art technology 3).
However, the prior art technologies described above have their own problems. In prior art technology 1, the soot (carbon or C) in PM is not oxidized but simply released into the atmosphere. Further, if the exhaust temperature is low as on engine start-up, PM is directly deposited at the inlets of cells or the inner surfaces of their walls to plug the cell pores, thereby increasing the pressure loss.
In prior art technology 2, if no catalyst is supported on the surfaces of cell partitions or in their interior, PM deposited on the surfaces of cell partitions is removed by combustion with a burner or a heater. This presents various problems including the need to provide a heating and combustion means such as a burner or a heater, overall complexity of the apparatus, high failure rate and high cost. In addition, the use of a heater can cause abnormal combustion of PM deposited on the filter, often leading to fusion and cracking of the filter matrix.
If a catalyst is supported on the cell partitions, PM deposited on the filter is removed by oxidation at comparatively low temperature, so there will be no fusion or cracking of the matrix. On the other hand, when the exhaust temperature is low as on engine start-up or while the vehicle is driving at low speed or under small load, PM is oxidized insufficiently and prone to be deposited on the surfaces of filter cell partitions or in the cell interior. The exhaust passing through the pores in the cell partitions can cause various other problems such as increased chance of clogging, higher exhaust temperature due to increased back pressure of the exhaust, abnormal combustion of the deposited PM and fusion of the filter.
In prior art technology 3, the exhaust passes through the cell partitions in the filters for such a very short time that the remainder of NO2 that has been spent to oxidize PM is not reduced to NO but simply discharged to the outside. If the exhaust temperature is low, say at 250° C. or less, the filters allow for only insufficient PM oxidation with NO2 and the PM is deposited on the surfaces of cell partitions in the filters to cause various problems such as clogging, greater burden on the engine due to increased back pressure of the exhaust, abnormal combustion of PM due to increased exhaust temperature, fusion of the filters and their failure.
The present invention has been accomplished under these circumstances and has as an object providing an exhaust cleanup filter which, even at low exhaust temperature as is encountered during vehicular driving in a city, can achieve efficient reduction in the concentration of PM in the exhaust from diesel engines without being plugged by PM deposits.
Another object of the invention is to provide a cleanup filter that can achieve efficient reduction of the concentration of PM in the exhaust from diesel engines without using any burners or heaters to remove PM.
A further object of the invention is to provide a cleanup filter that can achieve efficient reduction of the concentration of PM in the exhaust from diesel engines without suffering increased exhaust temperature due to clogging and in which abnormal combustion due to PM deposits and filter fusion are less likely to occur.
A still further object of the invention is to provide an exhaust cleanup filter which, even if the engine is running at high rpm (under high load) during high-speed vehicular driving, is less likely to experience a blow-off of the PM trapped in it but can be regenerated efficiently.