US20050076637A1

US20050076637A1 - Process for heating a catalytic converter and/or particle filter mounted in an exhaust system of a diesel internal combustion engine of a vehicle, a motor vehicle in particular, to a desulfation and/or decarbonization temperature and a catalytic converter, a nitric oxide storage catalytic converter in particular, for exhaust systems of internal combustion engines

Info

Publication number: US20050076637A1
Application number: US10/937,393
Authority: US
Inventors: Bodo Odendall; Armin Burkardt; Stefan Kamm; Volker Boestfleisch
Original assignee: Audi AG
Current assignee: Audi AG
Priority date: 2003-09-11
Filing date: 2004-09-10
Publication date: 2005-04-14
Also published as: EP1515028A3; DE10341930A1; EP1515028A2

Abstract

The invention relates to a process for heating a catalytic converter, a nitric oxide storage catalytic converter and/or particle filter in particular mounted in an exhaust system of a diesel internal combustion engine of a vehicle, to a desulfation and/or decarbonization temperature, a process in which the catalytic converter is heated to a desulfation and/or decarbonization temperature at the beginning of the desulfation and/or decarbonization phase as regeneration phase. A rich exhaust gas flow is delivered periodically to the catalytic converter for the purpose of direct heating of this catalytic converter to a regeneration temperature, in such a way that the unburnt exhaust gas components, hydrocarbons and carbon monoxides in particular, react to the oxygen stored in the catalytic converter and the thermal energy released in the process heats the catalytic converter to a regeneration temperature. A catalytic converter is also proposed in which the oxygen storage component is unevenly distributed over the catalytic converter in the direction of flow of the exhaust gas, in such a way that, as viewed in the direction of flow, the highest oxygen storage capacity is present at the exhaust gas inlet of the catalytic converter as viewed in the direction of flow.

Description

The invention relates to a process for heating a catalytic converter and/or particle filter mounted in an exhaust system of a diesel internal combustion engine of a vehicle, a motor vehicle in particular, to a desulfation and/or decarbonization temperature, of a nitric oxide storage catalytic converter in particular, as specified in the preamble of claim 1, and to a catalytic converter, a nitric oxide storage catalytic converter in particular, for exhaust systems of internal combustion engines in vehicles, motor vehicles in particular, as specified in the preamble of claim 10.
It is generally known that nitric oxide storage catalytic converters are employed in exhaust systems of diesel internal combustion engines of a motor vehicle. As soon as the nitric oxide storage capacity in the nitric oxide storage catalytic converter has been exhausted, regeneration of the nitric oxide storage catalytic converters is effected such that rich-fuel operation of the internal combustion engine is conducted for a certain period of time. In this rich-fuel operation the nitric oxides are released again and then reduced to nitrogen on the basis of the reduction agents (CO and HC reduction agents in particular) present in the rich-fuel exhaust gas at that time.
However, the problem of sulfur contamination occurs in conjunction with such nitric oxide storage catalytic converters in that such nitric oxide storage catalytic converters also tend toward accumulation of sulfur as a result of formation of a corresponding sulfate. This contamination of the nitric oxide storage catalytic converter is undesirable, so that a nitric oxide storage catalytic converter thus contaminated must be regenerated. So-called desulfation is effected for this purpose in which the nitric oxide storage catalytic converter must be heated to a minimum desulfation temperature of >550° C.
Removal of particles from the exhaust gas is of importance in the case of diesel internal combustion engines in addition to removal of nitric oxides. For this purpose particle filters, also called carbon filters, are combined in the exhaust system in particular with a nitric oxide storage catalytic converter. These particle or carbon filters as well must be regenerated from time to time in order to remove particle deposits from them. Temperatures >550° C. are also regularly required for such combustion cleaning of the carbon filter.
A process for desulfation and/or decarbonization of such a catalytic converter in which the catalytic converter is heated to a desulfation and/or decarbonization temperature as regeneration temperature before commencement of the desulfation and/or decarbonization phase as regeneration phase is carried out for heating of such a catalytic converter of an exhaust system of a diesel internal combustion engine of a motor vehicle has been disclosed ill generic DE 101 26 455 A1.
Specifically, heating of the nitric oxide storage catalytic converter is effected by means of lean-fuel internal combustion engine operation and by secondary injection of the fuel into the combustion chamber of the internal combustion engine following primary injection of the fuel. A residual amount of unburnt fuel is introduced into the lean exhaust gas flow which is introduced into the catalytic converter in the form of a nitric oxide storage catalytic converter, so that exothermal oxidation of the unburnt fuel occurs under what on the whole are even leaner exhaust gas conditions as a result of the oxygen residue present in this lean exhaust gas flow along with the post-injected unburnt fuel, and as a result the exhaust gas is heated. The exhaust gas as thus heated then flows through the nitric oxide storage catalytic converter and heats the latter to the regeneration temperature. Hence that which occurs in this instance is indirect heating of the catalytic converter by way of the previously heated lean exhaust gas flow.
The object of the invention is to provide an alternative process for heating of a catalytic converter, in particular a nitric oxide storage catalytic converter or particle filter, mounted in an exhaust system of a diesel internal combustion engine of a vehicle, a motor vehicle in particular, to the desulfation and/or decarbonization temperature, a process by which heating to regeneration temperature may be effected by simple and reliably operating means. An additional object of the invention is creation of a catalytic converter, a nitric oxide storage catalytic converter in particular, for exhaust systems of internal combustion engines of vehicles, of motor vehicles in particular, a catalytic converter which may be heated as uniformly as possible.
With respect to the process this object is attained with the characteristics specified in claim 1.
Claim 1 specifies that a rich-fuel exhaust gas flow is periodically introduced into the catalytic converter for the purpose of direct heating of this catalytic converter to regeneration temperature in such a way that the unburnt exhaust gas components, the hydrocarbons and carbon monoxides in particular, react to the oxygen stored in the oxygen storage component of the catalytic converter and the thermal energy thereby released heats the catalytic converter to regeneration temperature.
Direct heating of the catalytic converter is effected by simple and reliable means with the process as thus carried out, that is, in contrast to the generic state of the art there is no direct heating of the storage catalytic converter by the heated exhaust gas flow. In addition, the thermal losses sustained in such direct heating of the catalytic converter are consequently substantially lower than in indirect heating in accordance with the generic state of the art.
In one especially preferred embodiment of the process, as specified in claim 2, a lean and a rich exhaust gas flow are alternately introduced into the catalytic converter for the purpose of heating to regeneration temperature as a function of the extent of oxygen charging of the oxygen storage component. Switching from lean to rich exhaust gas flow is always effected at a switching moment at which the oxygen storage component is more or less full, while switching from a rich to a lean exhaust gas flow is always effected at a second switching moment at which the oxygen storage component is more or less empty. A criterion for switching between individual rich and lean phases, a criterion which is simple and can be reliably monitored, is obtained by this coordination of the calorific value to be introduced into the exhaust system with conversion of the entire oxygen storage component of the catalytic converter during the rich-fuel operating phase.
The heat of reaction per time unit which is released is the greater the more rapidly the oxygen storage component of the catalytic converter can be charged and emptied again. Claim 3 specifies that a large lambda stroke in both rich-fuel and lean-fuel operation is necessary in order to meet this requirement. Claim 4 additionally specifies that the consecutive lambda strokes may be equal at least ill extent. That is to say, the lambda strokes may always be the same in lean switching from a rich-fuel to a lean-fuel exhaust gas flow and, similarly, the lambda strokes may always be the same in switching from a lean-fuel to a rich-fuel exhaust gas flow, but the stroke values of rich-fuel and lean-fuel switching may differ from each other.
Claim 5 specifies that the lambda value of the rich-fuel exhaust gas flow is at the maximum approximately λ=0.6, preferably approximately λ=0.8 to 0.95, while the lambda value of the lean-fuel exhaust gas flow is at least λ=1.2, preferably approximately λ=1.5 to 3.5. The process may be conducted satisfactorily under these conditions.
In another preferred conduct of the process as specified in claim 6 provision is made such that, in order to achieve uniform heating of the catalytic converter in the direction of flow, preferably axial, of the exhaust gas, the oxygen stored is not uniformly distributed over the catalytic converter, specifically in such a way that, as viewed in the direction of flow, the greatest oxygen storage capacity is present at the inlet of exhaust gas into the catalytic converter. Such distribution yields the effect desired, that conversion of the heat of reaction occur primarily at the beginning of the catalytic converter and that the heat subsequently be conducted by way of transfer of matter through the catalytic converter itself. As is to be seen, this results in more uniform temperature distribution in axial direction in the catalytic converter.
It is especially preferable for the process claimed for the invention to be conducted with an exhaust system in which the catalytic converter is a main catalytic converter, preferably an underbody catalytic converter, upstream from which a preliminary catalytic converter, preferably a three-way catalytic converter, is mounted. With an upstream preliminary catalytic converter such as this, in a preferred conduct of the process as specified in claim 8 it is provided that the engine lambda is set at approximately λ=1 in order to minimize the exothermic reaction in the preliminary catalytic converter. The residual oxygen after combustion is to be kept as low as possible after combustion in order to minimize the exothermic reaction in the preliminary catalytic converter, that is, the lambda value of the engine should be in a range of values smaller than or equal to 1.
Claim 9 provides that, for the purpose of additional enrichment of the rich exhaust gas flow coming from the internal combustion engine, fuel is subsequently injected into this flow. The process as thus conducted makes allowance for the diesel engine principle, in which rich-fuel operation is restricted by engine operation. One possibility of minimizing this problem may consequently consist to advantage of additionally enriching the exhaust gas lambda value through specific subsequent injection after combustion proper and thus of additionally delivering unburnt fuel to the exhaust gas.
With respect to the catalytic converter the object is attained by the characteristics specified in claim 10.
Claim 10 specifies that the oxygen storage component is unevenly distributed over the catalytic converter in the direction of flow of the exhaust gas, which direction preferably is axial, in such a way that the highest oxygen storage capacity is present at the exhaust gas inlet of the catalytic converter as viewed in the direction of flow.
Uniform heating of the catalytic converter can be effected in this way in the simplest manner especially in conjunction with the conduct of the process as specified in one of the process claims as presented in the foregoing, since such distribution leads to the desired effect, that is, that conversion of the heat of reaction occur from the viewpoint of priority at the beginning of the catalytic converter and the heat is then conducted through the catalytic converter component by transfer of matter. This results in more uniform distribution of temperature, for example, in the axial direction of the component.
As a rule a catalytic converter such as this may also be used with Otto internal combustion engines. For use in diesel internal combustion engines it is then advantageous as specified in claim 11 additionally to integrate a particle filter, a carbon filter, for example, into the catalytic converter. The process may be applied analogously for carbon filters with oxygen storage components.
The invention is described in greater detail in what follows with reference to a drawing, in which
FIG. 1 presents a diagram illustrating the structure of an exhaust system of a diesel internal combustion engine,
FIG. 2 a diagram illustrating the cyclic alternating rich-lean-fuel operation claimed for the invention for heating the catalytic converter to a regeneration temperature as a function of the oxygen storage component of the catalytic converter mounted in the exhaust system, and
FIG. 3 a diagram illustrating the variation of temperature and HC reduction agent over the length of the exhaust system.
FIG. 1 presents a diagram of the structure of an exhaust system 1 for a diesel internal combustion engine 2, a preliminary catalytic converter 3 near the engine, such as, a three-way catalytic converter which is connected by flow by way of an exhaust gas transfer pipe 4 to a main catalytic converter 5, being mounted downstream from the diesel internal combustion engine 2. This main catalytic converter 5 is in this instance accordingly configured as an underbody catalytic converter and has both a nitric oxide storage catalytic converter not shown here in detail and a particle filter or carbon filter. This main catalytic converter 5 has in addition an oxygen storage component, also not shown here in detail, which as viewed axially in the direction of flow of the exhaust gas is unevenly distributed over the catalytic converter 5, in such a was that, as viewed in the direction of flow, the highest oxygen storage capacity is provided at the inlet of the exhaust gas into the main catalytic converter 5. The preliminary catalytic converter 3 also has an oxygen storage component.
The conduct of the process claimed for the invention is depicted in FIG. 2, FIG. 2 a illustrating the cyclic alternation of rich and lean exhaust gas flow for heating the main catalytic converter 5 to a desulfation or decarbonization temperature as regeneration temperature as a function of the oxygen content shown in FIG. 2 b of the oxygen storage components of the preliminary catalytic converter 3 and the main catalytic converter 5.
The curve 6 represents the variation of the content of the oxygen storage component in the preliminary catalytic converter 3, while the curve 7 illustrates the variation of the content of the oxygen storage component in the main catalytic converter 5.
As is now to be seen in detail in FIG. 2 a and FIG. 2 b, at moment t1 a lean exhaust gas flow coming from the internal combustion engine is delivered to the two catalytic converters 3, 5, this having the result that the oxygen storage component of the preliminary catalytic converter 3 is charged by moment t2. As soon as the preliminary catalytic converter 3 is fully charged with oxygen, the main catalytic converter 5 is subsequently charged with oxygen in this lean-fuel operation phase. The main catalytic converter 5 has been charged with oxygen by moment t3, so that now change is made from lean exhaust gas flow to rich exhaust gas flow. First the oxygen storage component of the preliminary catalytic converter 3 mounted near the engine is emptied at the beginning of this rich-fuel phase, by moment t4. The content of the oxygen storage component of the main catalytic converter 5 remains constant during this interval between t3 and t4. After the oxygen storage component of the preliminary catalytic converter 3 has been emptied, the oxygen storage component of the main catalytic converter 5 is emptied by moment t5, this at the same time representing the criterion for switching from the rich-fuel phase to the lean-fuel phase, that is, for establishing a lean-fuel exhaust gas flow for charging the oxygen storage component. This cycle is repeated during the heating phase as a whole.
The lambda strokes in transition from rich-fuel to lean-fuel exhaust gas flow and also from lean-fuel to rich-fuel exhaust gas flow correspond in approximation to a lambda value of at least 1.
As a result of emptying of the oxygen storage component in the main catalytic converter 5, the exhaust gas components present in the rich-fuel exhaust gas flow, the hydrocarbons and the carbon monoxides in particular, may react with each other, and the thermal energy released in the process may then be used for direct heating of the main catalytic converter 5 to a regeneration temperature. This variation in temperature as a function of reduction of the HC exhaust gas component is illustrated by the diagram in FIG. 3 a and FIG. 3 b over the length of the exhaust system 1. It is to be seen from FIG. 3 a and FIG. 3 b that the temperature of the exhaust gas flow drops until the preliminary catalytic converter 3 is reached, the content of the BC pollution constituent present in the rich-fuel exhaust gas flow logically remaining constant. Reduction of the hydrocarbons (HC) accompanied by consumption of the oxygen stored then occurs in the preliminary catalytic converter 3, this simultaneously having the result that the exhaust gas flow is heated by the exothermic reaction, as is to be seen in FIG. 3 a. The temperature subsequently drops in the course of flow by way of the exhaust gas transfer pipe 4 until the flow enters the main catalytic converter again, the content of HC reduction agent in the rich-fuel exhaust gas flow remaining constant. There then occurs in the main catalytic converter 5, simultaneously with emptying of the oxygen storage component of the main catalytic converter 5, an exothermic reaction in which the reduction agent is burned by the oxygen, the thermal energy released heating the main catalytic converter 5 to a regeneration temperature.
It is claimed for the invention that, as a result of targeted change in the exhaust gas lambda value between lean-fuel and rich-fuel exhaust gas, it is possible to convey exhaust gas with unburnt exhaust components through the preliminary catalytic converter to the underbody area for the purpose of heating the main catalytic converter 5 directly in this area.

Claims

1. A process for heating a catalytic converter, a nitric oxide storage catalytic converter or particle filter in particular, mounted in an exhaust system of a diesel internal combustion engine of a vehicle, a motor vehicle in particular, to a desulfation and/or decarbonization temperature, wherein the catalytic converter is heated to a desulfation and/or decarbonization temperature as a regeneration temperature at the commencement of the desulfation and/or decarbonization phase as regeneration phase, characterized in that a rich-fuel exhaust gas flow is introduced into the catalytic converter periodically for the purpose of direct heating of such catalytic converter to a regeneration temperature, in such a way that the unburned exhaust gas components, hydrocarbons and carbon monoxides in particular, react to the oxygen stored in an oxygen accumulator of the catalytic converter and the thermal energy released heats the catalytic converter to a regeneration temperature.

2. The process as claimed in claim 1, wherein a lean and a rich exhaust gas flow are alternately delivered to the catalytic converter for the purpose of heating to a regeneration temperature as a function of the extent of charging the oxygen storage component of the catalytic converter, in such a way that switching from a lean to a rich exhaust gas flow is always effected at a first switching moment at which the oxygen storage component has been fully charged, and in such a way that switching from a rich to a lean exhaust gas flow is always effected at a second switching moment at which the oxygen storage component is more or less empty.

3. The process as claimed in claim 2, wherein a large increase in the lambda value of the order of at least λ=1, preferably of at least λ=1.5, is effected as a lambda stroke on transition from rich to lean exhaust gas flow and also on transition from lean to rich exhaust gas flow.

4. The process as claimed in claim 3, wherein consecutive lambda strokes are always equal at least as regards extent.

5. The process as claimed in claim 3, wherein

the lambda value of the rich exhaust gas flow is at the maximum λ=0.6, preferably more or less λ=0.8 to 0.95, and

the lambda value of the lean exhaust gas flow is at the minimum λ=1.2, preferably more or less λ=1.5 to 3.5.

6. The process as claimed in claim 1, wherein, in order to achieve uniform heating of the catalytic converter in the direction of flow of the exhaust gas, which direction is preferably axial, the oxygen storage component is not equally distributed over the catalytic converter, in such a way that, as viewed in the direction of flow, the greatest oxygen storage capacity is present at the exhaust gas inlet of the catalytic converter.

7. The process as claimed in one of claim 1, wherein the catalytic converter is a main catalytic converter, preferably an underbody catalytic converter upstream from which at least one preliminary catalytic converter, preferably a three-way catalytic converter, is mounted.

8. The process as claimed in claim 7, wherein the lambda value of the engine is set at approximately λ=1 in order to minimize the exothermic reaction in the preliminary catalytic converter.

9. The process as claimed in one of claim 1, wherein a fuel is subsequently injected into the rich exhaust gas flow coming from the internal combustion engine for the purpose of additional enrichment of such exhaust gas flow.

10. A catalytic converter, a nitric oxide storage catalytic converter for exhaust systems of internal combustion engines of vehicles, motor vehicles in particular, especially for use in the processes claimed in claim 1, with an oxygen storage component, characterized in that the oxygen storage component of the exhaust gas is distributed unevenly over the catalytic converter, preferably in the axial direction, in such a way that the greatest oxygen storage capacity is present at the exhaust gas inlet of the catalytic converter as viewed in the direction of flow.

11. The catalytic converter as claimed in claim 10, wherein a particle filter is integrated into the catalytic converter.