Desulphurisation of a storage catalytst by heating

Abstract

A method of heating a main catalytic converter positioned downstream from at least one primary catalytic converter by supplying unburned fuel generated by the engine in the exhaust gas upstream from the main catalytic converter while at the same time surplus air is present upstream from the main catalytic converter, where the generation of unburned fuel by the engine takes place at a separate time and/or place from the generation of surplus air in the exhaust gas by the engine.

Description

BACKGROUND INFORMATION

[0001] In internal combustion engines in motor vehicles, when the engine is operated lean (&lgr;>1) to fulfill legally prescribed exhaust emission limits, an accumulating catalytic converter is needed for the nitrogen oxides NOx which are generated during combustion. Sulfur oxides are also generated during combustion. Due to the great affinity of the active centers, i.e. the accumulation spaces of the NOx accumulating catalytic converter for the sulfur oxides (SOx) which are generated during combustion of the fuel, the active centers are occupied primarily by the SOx. These sulfates which thus develop are so stable thermally that they are not released again in normal driving. In consequence, the storing capability of the catalytic converter for the nitrogen oxides drops as the resorption of sulfur increases. At an elevated temperature in the catalytic converter (T>600° C.) under reducing conditions (&lgr;<1) at the same time, the sulfates are no longer thermodynamically stable and are released as hydrogen sulfide (H2S) and sulfur dioxide (SO2). To maintain or restore the storing capability, at certain intervals the accumulating catalytic converter must briefly be operated rich at elevated temperatures. This is known for example from European Patent 580 389.

[0002] For heating to the required temperature for desulfurization of the accumulating catalytic converter, a later ignition angle can be set, causing an elevated exhaust gas temperature by degrading the efficiency of the internal combustion engine, which causes the accumulating catalytic converter to heat up. The heating effect can be amplified by generating combustible mixture upstream from the catalytic converter. The combustible mixture is preferably generated by increasing the unburned raw hydrocarbon emissions of the engine upstream from the catalytic converters in combination with a surplus of oxygen in the exhaust. The combustible mixture which is thus generated upstream from the catalytic converter reacts exothermally in the catalytic converter and heats it up.

[0003] Modern exhaust emission control systems have additional catalytic converters besides the accumulating catalytic converter, in particular a primary catalytic converter positioned close to the engine.

[0004] The primary catalytic converter can be subjected to severe thermal loading by the conventionally used heating of the accumulating catalytic converter. This can cause early deactivation of the catalytic converter.

[0005] The object of the present invention is to make possible a method of heating the accumulating catalytic converter which avoids inadmissible heating of the primary catalytic converter.

[0006] This object is achieved with the features of claim 1.

[0007] At the heart of the present invention is the provision of unburned mixture using engine-based measures and at the same time ensuring that it is not able to react exothermally in the primary catalytic converter or is only able to do so to a small extent.

[0008] This is achieved by temporally or spatially separating the emission of excess air in the exhaust gas and of excess hydrocarbons in the exhaust gas.

[0009] This makes it possible to adjust the necessary temperature in the main catalytic converter relatively independently of the temperatures in the primary catalytic converter and the current load point of the engine. The high thermal load appearing in the primary catalytic converter with the conventionally used methods, which can cause deactivation, is avoided.

[0010] Exemplary embodiments of the present invention are illustrated in the drawing and described further below.

[0011] FIG. 1 represents the related art.

[0012] FIG. 2 illustrates a first embodiment of the present invention and FIG. 2 illustrates a second embodiment.

[0013] The symbol 1 in FIG. 1 represents an internal combustion engine which is supplied with air from an intake manifold 2 and with fuel from a fuel injector system 3. The fuel injector system is triggered by a control device 4 having injection pulse widths. The injection pulse widths here are calculated on the basis of detected operating parameters of the internal combustion engine. Examples of such operating parameters are the quantity of aspirated air, which is detected by a sensor 5, the speed of the engine, which is detected by a sensor 6, and the composition of the exhaust gas or its oxygen content, which is detected by a sensor 7. Along with the fuel injector system, the control device controls ignition system 8, in order to ignite the fuel/air mixture in the individual engine cylinders 9-12 at the right moment in each case.

[0014] The number 13 designates a primary catalytic converter and the number 14 an accumulating catalytic converter.

[0015] In the illustrated system the problem of heating the accumulating catalytic converter using engine-based means becomes clear: increasing the exhaust temperature by retarding the ignition and generating combustible mixture with a mixture composition of lambda 1a directly downstream from the internal combustion engine has an effect not only in the accumulating catalytic converter, but also an unwanted effect even in the primary catalytic converter.

[0016] FIG. 2 illustrates a remedy: FIG. 2 shows, as the essential difference from FIG. 1, two separate primary catalytic converters, each primary catalytic converter being assigned to a particular group of cylinders. The assignment is achieved by separating the exhaust pipes.

[0017] For reasons of clarity, here as in the subsequent FIG. 3 the peripheral equipment of the sensor technology and of the fuel and air supply systems from FIG. 1 are not shown. However this peripheral equipment is present both in the object of FIG. 2 and in the object of FIG. 3, so that in this respect both figures are to be viewed in combination with FIG. 1.

[0018] The separation of the exhaust gas routing of different groups of cylinders makes it possible according to the present invention to bring together a lean and a rich exhaust gas stream upstream from the accumulating catalytic converter.

[0019] This is done for example by supplying the group of cylinders whose exhaust gas flows through preliminary catalytic converter 1 (P.Cat.1) with rich mixture having no surplus air (lambda 1b). As a result, the exhaust gas of this group of cylinders contains unburned fuel and at the same time a deficiency of oxygen. Because of the deficiency of oxygen, the surplus fuel cannot react exothermally in primary catalytic converter 1. Primary catalytic converter 1 is therefore not heated up.

[0020] Furthermore, when primary catalytic converter 1 has exhaust gas with surplus fuel flowing through it, the other group of cylinders, whose exhaust gas flows through primary catalytic converter 2 (P.Cat.2), is operated with a deficiency of fuel and thus with a surplus of oxygen (lambda 2b). That results in an oxygen surplus in primary catalytic converter 2 for which no fuel is available in primary catalytic converter 2 as a reaction partner. Hence no exothermal reaction takes place in primary catalytic converter 2 either, so that primary catalytic converter 2 is also not heated up.

[0021] Merging the air surplus of the second group of cylinders with the fuel surplus of the first group of cylinders into an exhaust gas corresponding to a lambda value of lambda 3b takes place only downstream from the two primary catalytic converters. The exhaust gas having lambda 3b thus contains both unburned fuel and the necessary reaction partner oxygen. Both components react exothermally only in the accumulating catalytic converter and thus heat up the latter as desired.

[0022] In other words: at least two primary catalytic converters are needed to realize the invention shown in this embodiment. In the heat-up phase the two primary catalytic converters are subjected to different lambdas (&lgr;1b and &lgr;2b). Realization is possible without problem due to the differing quantities of fuel injected according to bank (bank 1/bank 2). One lambda value here must be greater than 1 (lean), the other less than 1 (rich). The mix lambda (&lgr;3b), resulting from the individual lambdas (&lgr;1b and &lgr;2b) and the exhaust gas mass flows, should adjust itself to a value of about &lgr;3b=1. Conversion of the heating value present in the exhaust gas, which comes primarily from the rich exhaust gas downstream from the primary catalytic converter, with the oxygen present, which comes primarily from the lean exhaust gas downstream from the primary catalytic converter, then takes place in the accumulating catalytic converter. The temperature increase in the main catalytic converter results from the conversion of the incompletely oxidized components with the oxygen.

[0023] Also conceivable are variants in which more than two primary catalytic converters are used. The combined lambda upstream from the accumulating catalytic converter is then readjusted to approximately 1 by the individual lambdas of the primary catalytic converters. Use is also conceivable with different numbers of cylinders, where according to the present invention at least one two-cylinder arrangement must always be present.

[0024] In the second embodiment, which is illustrated in FIG. 3, mixing of rich and lean exhaust gas packets also does not take place until downstream from the primary catalytic converter.

[0025] In this embodiment, a mixing element (static mixer) is connected upstream from the main catalytic converter which is to be heated. This mixing element includes for example a cavity having flow baffles which are arranged at angles to each other. The flow baffles direct the individual flow volumes into each other and slow down the flow. That achieves a thorough mixing of individual portions of exhaust gas.

[0026] At the heart of the second embodiment is the mixing of the lean and rich portions of exhaust gas upstream from the accumulating catalytic converter, the lean and rich portions of exhaust gas being generated by the engine at separate times.

[0027] This is done by operating the engine in the heat-up phase always alternately rich (&lgr;<1) and lean (&lgr;>1). Alternatively, individual cylinders may also be operated rich and lean by using different injection quantities. The resulting rich and lean exhaust gas packets are only partially mixed back together in the primary catalytic converter, so that lean and rich exhaust gas packets are also present even downstream from the primary catalytic converter.

[0028] In the static mixer downstream from the primary catalytic converter, the exhaust gas is then homogenized by the re-mixing which takes place there (the behavior of the static mixer resembles an ideal agitated vessel with small volume, due to the blurring of the residence time).

[0029] Conversion of the heating value present in the exhaust gas, which comes primarily from the rich exhaust gas packets, with the oxygen present, which comes primarily from the lean exhaust gas packets, then takes place in the accumulating catalytic converter. The temperature increase in the main catalytic converter results from the conversion of the incompletely oxidized components with the oxygen.

[0030] The period length of the rich and lean cycles is based on the proportions of the exhaust gas flow volume to be expected and the volume of the static mixer or the required quality of re-mixing. The goal should be the smallest possible oxygen storing capability of the primary catalytic converter, so that an unnecessary dead time with regard to the rich and lean exhaust gas packets is not introduced into the system.

[0031] Alternatively to heating up an accumulating catalytic converter, the present invention may also be used to heat up a three-way catalytic converter which is positioned downstream from at least one primary catalytic converter. The term main catalytic converter in claim 1 is meant to cover both of these alternatives.

Claims

1. A method of heating a main catalytic converter which is positioned downstream from at least one primary catalytic converter by supplying unburned fuel generated by the engine in the exhaust gas upstream from the main catalytic converter while at the same time surplus air is present upstream from the main catalytic converter,

wherein the generation of unburned fuel by the engine takes place at a separate time and/or place from the generation of surplus air in the exhaust gas by the engine.