Patent Application
20200165948
This application claims priority of German Patent Application No. DE 102018129683.5 filed Nov. 26, 2019.
The disclosure relates to an exhaust after treatment system with a passive NOx adsorber and a heatable SCR catalytic converter.
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 NOx adsorber for adsorbing NOx emissions below a temperature threshold and for desorbing NOx 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 NOx adsorber increases and the desorption of NOx emissions is initiated.
The exhaust after treatment system according to the disclosure for an internal combustion engine comprises an exhaust pipe, a passive NOx 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.
The combination of a passive NOx adsorber (PNA) and an SCR (selective catalytic reduction) catalytic converter is suitable for reducing emissions of nitrogen oxides (NOx). The idea is that the PNA adsorbs NOx 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 NOx can cause very early NOx 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 NOx, that is desorbed in the low-temperature PNA due to the high temperatures cannot therefore be converted in the SCR catalytic converter.
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 NOx emissions in the SCR catalytic converter by heating the SCR catalytic converter within a period of time, even if very early NOx desorption in the passive NOx 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.
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.
A further advantageous embodiment of the disclosure provides that the NOx 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 NOx desorption, so that a large part of the storage capacity of the PNA is available at the next engine start.
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° C., particularly preferably in a range of ±10° 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° C. for SCR catalytic converters.
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.
The dependent claims describe further advantageous embodiments of the disclosure.
Preferred embodiments are explained in more detail using the following figures. In the figures
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 NOx adsorbers 2, combined heating systems and NOx 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.
The NOx adsorber 2 is implemented here as a low-temperature adsorber. Low-temperature adsorbers are understood to be passive NOx adsorbers that desorb NOx 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 NOx adsorber 2 at the end of the engine operation is enabled by a purely passive NOx desorption.
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
The control unit is set up to switch on the heating device 6 if it is recognized that a desorption of the passive NOx 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 NOx 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 NOx adsorber 2 is performed.
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.
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 NOx 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.
Here the control unit is set up to model the load state of the passive NOx adsorber 2 and the temperature in the SCR catalytic converter 5. In alternative embodiments that are not shown, a state of the passive NOx 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102018129683.5 | Nov 2018 | DE | national |
