With any internal combustion engine it is desirable that exhaust gases be treated so that they can be safely discharged into the atmosphere. In some engines, particularly of the diesel type, among the most prevalent operating problems is the presence of particulates of varying size, which are carried in the exhaust gas stream.
The particulates are normally bits of carbon. They result from the incomplete combustion of hydrocarbon fuels under certain engine operating conditions. However, the operating efficiency of the engine is also a contributing factor to the amount of carbon produced.
The presence of relatively large amounts of carbon particles in any exhaust gas stream is evidenced by a dark, smoky, undesirable effluent. Such smoke is not only offensive aesthetically; in large quantities it can be unhealthy. The problem is most evident in the operation of diesel engines.
Means have been provided and are known to the prior art, for the elimination or minimization of the particulate content in exhaust discharge streams. It is known, for example, particulates can be eliminated by a suitable smoke filter of proper construction. Eventually, however, the latter can become saturated and/or inoperable, a result that stems from excessive particulate accumulations which block flow passages.
It is further known that the overall engine exhaust gas treating process can be expedited to a degree. This is achieved primarily by passing the hot gas stream through a filter medium, a process which is expedited by providing the filter with a catalyst which is capable of promoting combustion of retained particles.
It should be appreciated that the generation of carbon particles is prevalent under virtually all diesel engine operating conditions. It is further appreciated that the quality of an exhaust gas stream in any internal combustion engine will vary in accordance with the immediate operating characteristics of the engine.
For example, the temperature range experienced by a diesel exhaust gas stream can vary between slightly above ambient air temperature, to temperatures in excess of 1200.degree. F. When the exhaust gas is hot enough, the carbon particles trapped in a filter will, upon contact with the hot gas, be combusted and the filter rejuvenated. However, in diesel powered passager cars, engine operating conditions under which this automatic rejuvenation can occur is seldom reached.
Where it is determined that an engine continuously operates under circumstances that the particulates are continuously produced and accumulated in the filter, the particulate retaining bed must be occasionally rejuvenated.
Under some circumstances, and as noted above, rejuvenation will consist of merely introducing the hot exhaust gas stream, containing sufficient oxygen, into the filter bed to contact and ignite the retained carbon particles. The combustion of any large and confined carbon accumulation tends to produce temperatures greatly in excess of that of the exhaust gas. The result is that at such excessively high tempeatures, parts of the filter bed or even the filter casing are susceptible to thermal shock, damage or distortion.
Stated otherwise, the filter bed, when formed of metallic fibers or of a similar filter media, is constructed in a manner to be characterized by a sufficient fiber density to remove solid particulate matter. The filter mass, however, should not be so dense as to establish too great a back pressure against the source of the gas.
To withstand the high temperatures which can be expected in internal combustion engine operation, the filter media here contemplated is preferably formed of metallic fibers such as stainless steel or the like. Such fibers are capable of being readily bent or deformed. Thus, a mass of randomly disposed fibers can be compressed to a desired density, received within a reaction chamber, and perform the desired particulate removing function. The degree to which it is compressed will determine the size of the gas flow passages therethrough.
When a filter bed of metallic fibers is utilized in an exhaust gas treatment system, the hot gas stream will tend to follow the path of least resistance from the casing inlet to the outlet. As a general rule, for a uniform density filter bed, this path will be through the center of the bed, or along a path determined by the relative positioning of the inlet and outlet.
As a consequence, the bed's outer or peripheral edges will receive a minimal amount of retained particulate matter. As a further consequence, the particulate accumulations will be concentrated in those areas or those parts of the bed where gas flow is most concentrated.
During the rejuvenation period when the combustible accumulations are burned off, the thermal intensity of incineration will be concentrated at certain parts of the bed. Thus, while the rejuvenation process is found to be successful, it might have been achieved at the expense of the parts of the bed which have been permanently deformed or otherwise thermally damaged due to prolonged exposure to the excessive heat.
This thermal damage is most noticeable at the downstream end of the gas's flow path. Generally, the filter bed will be found to have developed a conically shaped defective area adjacent to the filter casing outlet. It results from the combined effect of the incineration combustion, together with the hot exhaust gas as the latter becomes progressively heated while transversing the filter bed.
Toward overcoming the stated problems endemic to exhaust gas filters, there is presently provided a filter having a bed formed of randomly disposed and compressed metallic fibers. The latter are sufficiently compacted to define a plurality of passages through which the hot exhaust gas will flow.
The physical composition of the overall bed is relatively uniform. However, the packed density of the mass is varied in different segments of the bed. Thus, the exhaust gas flow path through the respective bed passages, in following a path of minimal resistance will urge much of the stream toward the filter's peripheral edges.
It is therefore an object of the invention to provide an effective exhaust gas filter for removing particulate combustible matter from an exhaust gas stream.
A further object is to provide an exhaust gas filter which can be safely rejuvenated without jeopardizing the integrity of the filter medium.
Another objective is to provide a filter which utilizes a frangible though compressible filter media which has the capability of being compressed to a desired density whereby to regulate the gas flow path through the filter bed.