Patent application title: EXHAUST AFTER TREATMENT SYSTEM WITH A PASSIVE NOx ADSORBER AND A HEATABLE SCR CATALYTIC CONVERTER
Inventors:
IPC8 Class: AF01N320FI
USPC Class:
1 1
Class name:
Publication date: 2020-05-28
Patent application number: 20200165948
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:
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of German Patent Application No. DE 102018129683.5 filed Nov. 26, 2019.
FIELD OF THE INVENTION
[0002] The disclosure relates to an exhaust after treatment system with a passive NO.sub.x adsorber and a heatable SCR catalytic converter.
BACKGROUND OF THE INVENTION
[0003] An exhaust after treatment system for an internal combustion engine is known from U.S. Pat. No. 8,661,790B2 that includes an exhaust pipe, an upstream SCR catalytic converter, an electrically heated catalytic converter, a downstream SCR catalytic converter and a control unit. The oxidation catalytic converter can be heated by the electrically heated catalytic converter. A NO.sub.x adsorber for adsorbing NO.sub.x emissions below a temperature threshold and for desorbing NO.sub.x emissions above a temperature threshold is arranged in the electrically heated catalytic converter and/or the oxidation catalytic converter. The electrically heated catalytic converter is switched on when the operating temperature of the downstream SCR catalytic converter is above the start-up temperature. As a result, the temperature in the NO.sub.x adsorber increases and the desorption of NO.sub.x emissions is initiated.
[0004] The exhaust after treatment system according to the disclosure 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. 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.
[0005] The combination of a passive NO.sub.x adsorber (PNA) and an SCR (selective catalytic reduction) catalytic converter is suitable for reducing emissions of nitrogen oxides (NO.sub.x). The idea is that the PNA adsorbs NO.sub.x emissions during the cold start and warm-up phases of the combustion engine. As soon as the SCR catalytic converter reaches the start-up temperature, the PNA can be desorbed. For the combined use of a PNA and an SCR catalytic converter, the temperature properties of the PNA must be taken into account, as this affects the possibility of having an empty PNA at the end of engine operation. Thus, low adsorption stability of the NO.sub.x can cause very early NO.sub.x desorption, if for example strong acceleration of the internal combustion engine takes place in cold start conditions or in the warm-up phase. This results in high temperatures in the PNA with continued low temperatures in the SCR catalytic converter. The NO.sub.x, that is desorbed in the low-temperature PNA due to the high temperatures cannot therefore be converted in the SCR catalytic converter.
[0006] The exhaust after treatment system according to the disclosure combines the idea of an electrically heated SCR catalytic converter with a PNA. This allows the conversion of NO.sub.x emissions in the SCR catalytic converter by heating the SCR catalytic converter within a period of time, even if very early NO.sub.x desorption in the passive NO.sub.x adsorber takes place at exhaust gas temperatures below the start-up temperature of the SCR catalytic converter. A time span of less than 90 seconds is advantageous, especially advantageous is less than 60 seconds.
SUMMARY OF THE INVENTION
[0007] An advantageous embodiment of the disclosure provides that the heating device is designed as an electrical heat source. An electrical heat source is very easy to control, as switching operations involve hardly any delays. Due to the separate power supply, for example by a battery, it can also be used largely independently of the operating point of the internal combustion engine.
[0008] A further advantageous embodiment of the disclosure provides that the NO.sub.x adsorber is implemented as a low-temperature adsorber. This enables an empty PNA at the end of engine operation as a result of purely passive NO.sub.x desorption, so that a large part of the storage capacity of the PNA is available at the next engine start.
[0009] Preferably, the exhaust after treatment system according to the disclosure comprises a control unit, which is set up to perform 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. The temperature in the SCR catalytic converter can be determined by means of a sensor, a model or a characteristic field. The sensor can either directly measure the temperature in the SCR catalytic converter or is positioned in the exhaust pipe to measure the exhaust gas temperature. Several sensors can also be used, so that, for example, a temperature upstream of the SCR catalytic converter and a temperature downstream of the SCR catalytic converter are determined. The determination of the temperature in the SCR catalytic converter can be supplemented by a model in order to be able to determine the temperature in the SCR catalytic converter more precisely. This is particularly useful if the temperature in the SCR catalytic converter is measured indirectly by a measurement of an exhaust gas temperature. The temperature threshold is preferably in a range of .+-.20.degree. C., particularly preferably in a range of .+-.10.degree. C. of the start-up temperature of the SCR catalytic converter. The starting temperature in a catalytic converter is understood to be the temperature at which the catalytic reactions start to take place. Experience has shown that this is in the range of 200-300.degree. C. for SCR catalytic converters.
[0010] Preferably, the control unit is set up to first determine the temperature in the SCR catalytic converter and to switch on the heating device if the temperature in the SCR catalytic converter falls below a further temperature threshold. This has the advantage that the heating device is only switched on when this is necessary. Particularly preferably, the temperature in the SCR catalytic converter is determined continuously, so that the heating device can be switched on or off depending on the state of the exhaust gas after treatment system. The further temperature threshold is lower than the temperature threshold.
[0011] The dependent claims describe further advantageous embodiments of the disclosure.
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.degree. 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.
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