Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
2000-11-13
2004-12-07
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S177000, C060S299000
Reexamination Certificate
active
06827909
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a catalytic converter for cleaning exhaust gas from an internal combustion engine, in particular a diesel engine or a lean-burn engine, including at least one honeycomb body which is coated with catalytically active material and has passages through which the exhaust gas can flow and walls separating the passages from each other. The converter has a first zone and a second zone disposed in succession in a flow direction and the first zone has a lower thermal capacity per unit of volume of the honeycomb body than the second zone.
In view of an ever-increasing strictness of environmental legislation in many countries, catalytic converters for cleaning exhaust gas have to be adapted with ever-increasing precision to internal combustion engines of motor vehicles and the mode of operation thereof.
In regard to Otto-cycle engine exhaust gas cleaning, it is the known state of the art, for the purposes of reducing the level of pollutant emissions in a cold-start phase, to use a catalytic converter which has a first zone with the lowest possible level of thermal capacity per unit of volume. That is done so that that zone heats up quickly, triggers off the catalytic reaction and, due to heat energy which is produced in that situation, heats up parts of the catalytic converter which are disposed further downstream. An example of such a catalytic converter is described in International Publication No. WO 92/02716, corresponding to U.S. Pat. No. 5,328,774.
Honeycomb bodies with a thermal capacity which is reduced in a front region are also described in U.S. Pat. No. 5,549,873 and European Patent Application 0 705 962 A1, corresponding to U.S. Pat. No. 5,599,509. What is common to all three of those publications is that the first zone involves a smaller surface area than the second zone, thereby substantially achieving the reduction in thermal capacity.
Besides monolithic honeycomb bodies with a first zone having a reduced thermal capacity, many exhaust systems involve structures having two honeycomb bodies. In that case, the first zone has a smaller number of passages per cross-sectional area and thus a lower level of thermal capacity.
A general trend in development in recent years is towards thinner walls in all honeycomb bodies and towards a thermal capacity which overall is as small as possible. The reason for that, besides saving weight, is in particular achieving a desired, minimum possible pressure drop in the exhaust system, in conjunction with the largest possible surface areas per unit of volume.
Those realizations which were essentially obtained in relation to and applied to Otto-cycle engines, relate to conventional engine constructions in which the mixture of exhaust gas and fuel is regulated as accurately as possible to be stoichiometric.
However, those structural concepts cannot be readily transferred to diesel engines or so-called lean-burn engines, that is to say engines which are operated with a considerable excess of air, in particular with direct gasoline injection. In that case, because of the large excess of air in the exhaust gas, substantially lower temperatures result in operation than when stoichiometric operation is involved. It will be appreciated that there is admittedly still a wish for the operating temperature necessary for the catalytic conversion effect to be achieved as quickly as possible in the catalytic converter after the cold-start of the internal combustion engine. However, in subsequent operation and in particular in idle phases, such low exhaust gas temperatures occur, that under some circumstances a catalytic converter is cooled down to below the operating temperature necessary for the catalytic conversion effect, whereupon in a subsequent load phase pollutants are initially discharged into the environment.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a catalytic converter for cleaning exhaust gas, a diesel engine and a lean-burn engine having a catalytic converter, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and in which the catalytic converter rapidly reaches its operating temperature after a cold-start of the engine, but does not rapidly lose that temperature again in subsequent phases involving a lower exhaust gas temperature.
With the foregoing and other objects in view there is provided, in accordance with the invention, a catalytic converter for cleaning exhaust gas from an internal combustion engine, in particular a diesel engine or a lean-burn engine, comprising at least one honeycomb body coated with catalytically active material, the at least one honeycomb body having passages for conducting an exhaust gas flow, walls mutually separating the passages, and first and second zones disposed in succession in a flow direction, the first zone having a lower thermal capacity per unit of volume of the honeycomb body than the second zone, and the second zone having a particularly high thermal capacity of at least 800 joules per liter and Kelvin [J/lK], preferably at least 900 joules per liter and Kelvin [J/lK].
A decisive feature of the present invention is that of providing a second zone in the catalytic converter with a particularly high level of thermal capacity per unit of volume of the honeycomb body, downstream of the usual zone of low thermal capacity, which is also desired in the present case. Contrary to the development trend, in this case a higher weight and a slightly higher pressure drop is tolerated, so that considerably improved exhaust gas cleaning values are achieved.
The effect of the high thermal capacity in the second zone lies in the storage of heat which can maintain the catalytic reaction for a certain time, when a low exhaust gas temperature is involved.
After the cold-start of the engine, the first zone is rapidly heated up, as is known from the state of the art, and begins with the catalytic conversion of harmful exhaust gas constituents, in particular hydrocarbons and carbon monoxide. That reaction is exothermic and supports the rapid heating effect with respect to the second zone which then begins to store heat. The surface area of the first zone of the catalytic converter is very substantially sufficient for the conversion effect in that cold-start phase, so that the slow increase in temperature of the second zone does not involve any disadvantages in terms of pollutant emissions. In return, in a subsequent phase during operation involving a low exhaust gas temperature, the second zone can continue to maintain the catalytic reaction by virtue of the stored heat while the first zone, because of its low thermal capacity, is rapidly cooled down to below the necessary conversion temperature. In a phase involving a higher exhaust gas temperature, which then follows in turn, the exothermic reaction is very rapidly displaced into the first zone again, whereby the second zone is heated up again and can store fresh heat. That procedure overall results in a uniformly effective exhaust gas cleaning action, even when the engine involves a fluctuating mode of operation, as can occur in particular when driving within towns and urban areas and in corresponding test cycles.
In principle, there are a number of ways of producing a catalytic converter with a second zone having a particularly high thermal capacity. The thermal capacity can be achieved by increasing the wall thickness of the carrier structure and/or by increasing the thickness of the coating. It will be appreciated that in addition it is possible to increase the number of cells per cross-sectional area in relation to the first zone.
In accordance with another feature of the invention, the catalytic converter includes two individual honeycomb bodies which can be disposed in closely adjacent relationship or spatially somewhat separated. In this case, the second honeycomb body has thicker walls than the first honeycomb body.
In accordance with a further feature of the invent
Brück Rolf
Konieczny Jörg-Roman
Emitec Gesellschaft fuer Emissions-technologie mbH
Greenberg Laurence A.
Locher Ralph E.
Stemer Werner H.
Tran Hien
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