EXHAUST AFTER TREATMENT SYSTEM WITH A PASSIVE NOx ADSORBER AND A HEATABLE SCR CATALYTIC CONVERTER

Abstract

The invention relates to an exhaust after treatment system with a passive NO.sub.x adsorber and a heatable SCR catalytic converter. The exhaust gas after treatment system according to the invention for an internal combustion engine comprises an exhaust pipe, a passive NO.sub.x adsorber arranged in the exhaust pipe, an injector arranged in the exhaust pipe, a mixer arranged in the exhaust pipe, an SCR catalytic converter arranged in the exhaust pipe and a heating device arranged in the exhaust pipe, wherein the heating device is set up in such a way that at exhaust gas temperatures below a start-up temperature of the SCR catalytic converter, the temperature in the SCR catalytic converter can be brought to a temperature above the start-up temperature within a period of time by means of the heating device.

Claims

1. Exhaust after treatment system for an internal combustion engine, comprising an exhaust pipe, a passive NO.sub.x adsorber arranged in the exhaust pipe, an injector arranged in the exhaust pipe, a mixer arranged in the exhaust pipe, an SCR catalytic converter arranged in the exhaust pipe and a heating device arranged in the exhaust pipe, wherein the heating device is set up that so at exhaust gas temperatures below a start-up temperature of the SCR catalytic converter, the temperature in the SCR catalytic converter can be brought to a temperature above the start-up temperature within a period of time by the heating device.

2. Apparatus according to claim 1, wherein the heating device is attached directly to the SCR catalytic converter.

3. Apparatus according to claim 1, wherein the heating device is arranged upstream of the SCR catalytic converter between the passive NO.sub.x adsorber and the mixer.

4. Apparatus according to claim 1, wherein the heating device is arranged upstream of the SCR catalytic converter between the mixer and the SCR catalytic converter.

5. Apparatus according to claim 1, wherein the heating device as an electric heat source.

6. Apparatus according to claim 1, wherein the NO.sub.x adsorber is a low-temperature adsorber.

7. Apparatus according to claim 1, including a control unit having a structure performing the following steps: switching on the heating device, determining the temperature in the SCR catalytic converter, switching off the heating device if the temperature in the SCR catalytic converter has exceeded a temperature threshold.

8. Apparatus according to claim 7, wherein the control unit switches on the heating device immediately when the engine is started.

9. Apparatus according to claim 7, wherein the control unit switches on the heating device if it is recognized that desorption of the NO.sub.x adsorber is required at a point in time and that the temperature in the SCR catalytic converter will not exceed a temperature threshold by that point in time without heating.

10. Apparatus according to any one of claim 7, wherein the control unit regulates the power of the heating device depending on the load state of the NO.sub.x adsorber and the temperature in the SCR catalytic converter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Preferred embodiments are explained in more detail using the following figures. In the figures

[0013] FIG. 1 shows an exhaust after treatment system according to the disclosure, wherein the heating device is attached directly to the SCR catalytic converter,

[0014] FIG. 2 shows an exhaust after treatment system according to the disclosure, wherein the heating device is attached directly to the passive NO.sub.x adsorber,

[0015] FIG. 3 shows a method for heating an SCR catalytic converter of an exhaust after treatment system that is carried out by a control unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] FIG. 1 shows an exhaust aftertreatment system according to the disclosure for an internal combustion engine. The exhaust aftertreatment system comprises an exhaust pipe 1, a passive NO.sub.x adsorber 2 arranged in the exhaust pipe 1, an injector 3 arranged in the exhaust pipe, a mixer arranged in the exhaust pipe 4, an SCR catalytic converter 5 arranged in the exhaust pipe and a heating device 6 arranged in the exhaust pipe. The exhaust pipe 1 is fluidically connected to the outlet of the combustion engine and the exhaust gas of the combustion engine flows through the exhaust pipe 1. The exhaust gas reaches the passive NO.sub.x adsorber 2 via the exhaust pipe 1. The passive NO.sub.x adsorber 2 is designed to adsorb NO.sub.x emissions contained in the exhaust gas. When the storage capacity of the passive NO.sub.x adsorber 2 is exhausted and/or a sufficiently high temperature is reached, the NO.sub.x emissions are desorbed. An injector 3 and a mixer 4 are arranged in the exhaust pipe 1 between the passive NO.sub.x adsorber 2 and the SCR catalytic converter 5. A reducing agent, for example a urea solution, is added to the exhaust gas via the injector 3. The mixer 4 allows homogenization of the exhaust gas and the introduced urea solution. The NO.sub.x emissions desorbed in the passive NO.sub.x adsorber 2 are reduced in the SCR catalytic converter 5. A heating device 6 is attached directly to the SCR catalytic converter 5. This heating device 6 is set up in such a way that at exhaust gas temperatures below a start-up temperature of the SCR catalytic converter 5, the temperature in the SCR catalytic converter 5 can be brought to a temperature above the start-up temperature within a period of time by means of the heating device 6. This ensures that the SCR catalytic converter 5 is ready for operation when the storage capacity of the passive NO.sub.x adsorber 2 is exhausted, or that the time interval between desorption of the passive NO.sub.x adsorber 2 and reaching the start-up temperature of the SCR catalytic converter 5 is at least minimized.

[0017] FIG. 2 shows another exemplary embodiment of the exhaust aftertreatment system, wherein the heating device 6 is attached directly to the passive NO.sub.x adsorber 2. This allows the use of heatable passive NO.sub.x adsorbers that are available on the market, so that costs can be reduced. Alternatively, the heating device 6 can be arranged upstream of the SCR catalytic converter 5 between the passive NO.sub.x adsorber 2 and the mixer 4 or upstream of the SCR catalytic converter 5 between the mixer 4 and the SCR catalytic converter 5. This flexibility in the possible arrangement of the heating device 6 allows advantageous adaptation to different exhaust aftertreatment systems.

[0018] In the exemplary embodiments shown of the exhaust aftertreatment system, the heating device 6 is implemented as an electrical heat source. This has the advantage that for some exhaust aftertreatment components such as passive NO.sub.x adsorbers 2, combined heating systems and NO.sub.x adsorbers 2 are available, which can reduce costs. In addition, electrical heat sources allow precise control as they have short delay times. Furthermore, electrical heat sources can be supplied by an external power supply, such as a battery, whereby they can be operated largely independently of the operating state of the internal combustion engine.

[0019] The NO.sub.x adsorber 2 is implemented here as a low-temperature adsorber. Low-temperature adsorbers are understood to be passive NO.sub.x adsorbers that desorb NO.sub.x emissions at relatively low temperatures, for example between 150 and 200 C. The use of a low-temperature adsorber is advantageous, since an empty passive NO.sub.x adsorber 2 at the end of the engine operation is enabled by a purely passive NO.sub.x desorption.

[0020] In the exemplary embodiments shown, the exhaust aftertreatment system includes a control unit. This control unit is set up to perform the steps shown in FIG. 3. In the first step S10 the heating device 6 is switched on, in the second step S20 the temperature in the SCR catalytic converter 5 is determined and in the third step S30 switching off the heating device 6 is initiated if the temperature in the SCR catalytic converter 5 has exceeded a temperature threshold.

[0021] The control unit is set up to switch on the heating device 6 if it is recognized that a desorption of the passive NO.sub.x adsorber 2 is required at a point in time and the temperature in the SCR catalytic converter 5 will not exceed a temperature threshold by that time without heating. This enables adaptive operation of the heating device 6 by switching on the heating device 6 depending on the load state of the passive NO.sub.x adsorber 2 and depending on the thermal state of the SCR catalytic converter 5. This has the advantage that the SCR catalytic converter 5 has highly advantageous conversion rates when desorption of the passive NO.sub.x adsorber 2 is performed.

[0022] Alternatively, the control unit can be set up to switch on the heating device 6 immediately when the engine is started. Immediately when the engine is started means within 10 seconds after the engine is started, preferably within 5 seconds after the engine is started, particularly preferably within 2 seconds after the engine is started. This form of control is easier to implement but is also less accurate.

[0023] In this exemplary embodiment, the control unit is set up to regulate the power of the heating device 6 depending on the load state of the passive NO.sub.x adsorber 2 and the temperature in the SCR catalytic converter 5. This achieves the technical advantage that not only is switching on and off of the heating device 6 used, but a power level of the heating device 6 that is adapted to the operating condition of the combustion engine and/or the thermal condition of the exhaust aftertreatment system is used. Thus, the power of the heating device 6 can be reduced, for example if the difference between the temperature determined in the SCR catalytic converter 5 and a temperature threshold allows the temperature threshold value to at least be reached within an advantageous period of time even with lower power. For this regulation of the heating device 6, models and/or characteristics are particularly suitable that make it possible to describe at least parts of the exhaust aftertreatment system predictively and a heating device 6 that allows an adjustment of the power output that goes beyond mere switching on and off.

[0024] Here the control unit is set up to model the load state of the passive NO.sub.x adsorber 2 and the temperature in the SCR catalytic converter 5. In alternative embodiments that are not shown, a state of the passive NO.sub.x adsorber and/or the SCR catalytic converter 5 is modeled and/or measured based on a temperature, a level of a conversion behavior and/or a storage behavior. The heating device 6 is then controlled on the basis of this modeled and/or measured state.