DE102017214444A1 - A method of operating a three-way catalyst having exhaust aftertreatment system of an internal combustion engine and exhaust aftertreatment system - Google Patents

Abstract

A method for operating an exhaust aftertreatment system having a three-way catalytic converter (5) of an internal combustion engine (1) of a motor vehicle and an associated exhaust gas aftertreatment system are described. In the method, a NOx sensor (6) is provided on the three-way catalyst (5) or downstream of the three-way catalyst (5) in an exhaust pipe of the internal combustion engine (1). This measures the NOx emissions during operation of the internal combustion engine (1). If these emissions exceed a threshold, NOx reduction measures will be implemented until the threshold falls below the threshold. As measures to reduce the NOx emissions, for example, an ignition delay at the internal combustion engine (1) and / or an exhaust gas recirculation are performed or the rate of exhaust gas recirculation is increased. As a result, a further reduction of the NOx content in the exhaust gas is achieved without great calibration effort.

Description

The present invention relates to a method for operating a three-way catalyst having exhaust aftertreatment system of an internal combustion engine of a motor vehicle.

It is known to use in gasoline engines three-way catalysts for exhaust aftertreatment to convert carbon monoxide (CO), nitrogen oxides (NOx) and unburned hydrocarbons (HC) to carbon dioxide (CO ₂ ), nitrogen (N ₂ ) and water (H ₂ O). Particularly problematic in this case is the reduction of the amounts of nitrogen oxide in the raw exhaust gas, since due to the relatively high proportion of oxygen in the exhaust gas, a reduction of the nitrogen oxides (NOx) takes place only to a small extent. In particular, in lean-burn gasoline engines therefore a special NOx storage is used according to a common method, which accumulates the nitrogen oxides contained in the exhaust gas of the internal combustion engine. When the capacity of the NOx storage is exhausted, it is usually regenerated by operation of the rich air-fuel mixture engine. This method for reducing the nitrogen oxides in the exhaust gas is therefore quite expensive.

Three-way catalysts are characterized in that they only at a certain temperature, i. the so-called light-off temperature (light-off temperature), start operation. After starting the internal combustion engine, therefore, a certain warm-up phase of the catalyst is required so that it reaches the optimum operating temperature. To shorten this warm-up phase, it is known to take special measures for heating the three-way catalyst. In order to achieve a reduction of the NOx content in the exhaust gas, it is therefore important to carry out this reduction before and after the light-off temperature of the catalytic converter has been reached.

So that the three-way catalyst reaches the light-off temperature as quickly as possible, it is known to carry out a corresponding calibration by means of heating measures, including, for example, an ignition delay counts. The calibration is carried out so that during the corresponding emission cycle, the three-way catalyst is always above the light-off temperature. However, this is not always achieved in normal driving. To guarantee this, a much higher calibration effort must be made so that the catalyst temperature is always above the light-off temperature of the catalyst and the NOx emissions are below the allowable limits.

The present invention has for its object to provide a method for operating a three-way catalyst having exhaust aftertreatment facility available with which the NOx emissions can be reduced with a very low calibration effort.

• - Vorsehen eines NOx-Sensors am Dreiwegekatalysator oder stromab des Dreiwegekatalysators in einer Abgasleitung des Verbrennungsmotors;
• - Messen der NOx-Emissionen im Betrieb des Verbrennungsmotors mit dem NOx-Sensor; und
• - wenn die vom NOx-Sensor gemessenen NOx-Emissionen einen Schwellenwert übersteigen, Durchführen von Maßnahmen zur Reduzierung der NOx-Emissionen bis der Schwellenwert wieder unterschritten wird.

This object is achieved by a method for operating a three-way catalyst having exhaust aftertreatment system of an internal combustion engine of a motor vehicle, comprising the following steps:

• Providing a NOx sensor on the three-way catalyst or downstream of the three-way catalyst in an exhaust pipe of the internal combustion engine;
• - measuring the NOx emissions during operation of the internal combustion engine with the NOx sensor; and
• - if the NOx emissions measured by the NOx sensor exceed a threshold, implement measures to reduce NOx emissions until the threshold falls below the threshold again.

In the method according to the invention, therefore, in addition to the three-way catalyst, a NOx sensor is used, which measures the proportion of NOx in the exhaust gas of the internal combustion engine leaving the three-way catalytic converter. If the NOx content exceeds a specified threshold, appropriate measures are taken to reduce the proportion of NOx until the threshold falls below it again. In this case, for example, as a measure to reduce the NOx emissions, an ignition delay can be performed on the internal combustion engine. As a further measure, an exhaust gas recirculation can be carried out or the rate of such exhaust gas recirculation can be increased. Specifically, in this case, the provided NOx sensor outputs a signal representing the NOx component in the exhaust gas to a control unit which compares the signal with a threshold value and initiates appropriate special measures for NOx reduction or not.

The process according to the invention is carried out in particular during the heating phase of the three-way catalyst. Here, after the engine starts and during the heating phase, the NOx emissions are measured using the NOx sensor. If the measured NOx emissions are above the defined limit, the additional NOx reduction measures already mentioned above are initiated. The usual measures to heat the catalyst (ignition delay and / or electrical heating) are then stopped because the NOx emissions are below the threshold (eg below 50 ppm), which means that the catalyst is in operation.

It is understood that the set threshold for NOx emissions is dependent upon the particular operating points of the engine, such as engine speed and load.

The method according to the invention is preferably designed as a method for regulating the NOx emissions in the exhaust gas.

In order to ensure a high degree of conversion of the pollutants, the lambda combustion air ratio must be kept within a narrow range around lambda = 1 (lambda window) by means of a Lambda control system, since not all of them deviate from the stoichiometric air-fuel ratio to lean lambda> 1 Nitrogen oxides are degraded, because excess oxygen is present. Preferably, therefore, in the method according to the invention, use is made of such a lambda control loop. The method according to the invention is therefore preferably carried out when the internal combustion engine is in the lambda = 1 control phase.

In particular, a NOx sensor is used which, in addition to performing a NOx measurement, also provides lambda information, and these are then used for lambda control. Upon detection that the lambda information signal detected by the NOx sensor is above a voltage threshold, measures are taken to reduce NOx emissions.

During vehicle operation, particularly frequent start / stop of the engine or frequent fuel cut during downhill running, the three way catalyst may be cooled below the light off temperature. According to the invention, the NOx emissions are continuously measured by the NOx sensor, while the engine is in the lambda = 1 control phase. If, for example, the NOx emissions are above a threshold, for example above 70 ppm, and the NOx sensor lambda information signal is still above a voltage threshold, for example above 600 mV, which means that the three-way catalyst loses its efficiency due to the lower catalyst temperature, According to the invention additional catalyst measures are carried out, such as an ignition delay and / or electrical heating of the catalyst until the NOx emissions are again below the threshold, for example below 40 ppm.

In a further variant of the method according to the invention, the NOx sensor is additionally used for the diagnosis of the three-way catalytic converter. In particular, the three-way catalytic converter is diagnosed as defective if the NOx emissions measured by the NOx sensor are above a threshold value and the lambda information signal determined by the NOx sensor is in the form of a jump signal above a voltage threshold value.

The diagnostic method performed according to the invention can be carried out as a supplement to the diagnosis with regard to the oxygen storage capacity. It is especially carried out at constant engine speed and load when the three-way catalyst is safely above the light-off temperature. As a threshold value for the NOx emissions, for example, a value of 100 ppm (depending on the operating point) can be set, and the voltage threshold with respect to the lambda information signal can be, for example, 600 mV.

In order to increase the accuracy of the corresponding diagnosis, in the method according to the invention, an additional NOx sensor upstream of the three-way catalyst can be used. This additional NOx sensor can detect lambda information, for example, and these can be used for lambda control.

Specifically, this can be done so that the efficiency of the three-way catalyst is determined by using the NOx emission values measured by both NOx sensors, wherein the three-way catalyst is diagnosed as defective when the efficiency below a threshold value and the lambda information signal after the three-way catalyst in Form of a jump signal above a voltage threshold.

In general, in the method according to the invention, the measures for reducing the NOx emissions can be carried out before additional heating measures of the three-way catalytic converter if the NOx emissions measured by the NOx sensor exceed a threshold value. As appropriate measures for reducing, for example, the exhaust gas recirculation rate (EGR rate) can be increased and / or an ignition delay can be carried out. In this way, a rapid reduction of NOx emissions is achieved. The corresponding optimization of NOx emissions can be further improved by using the additional NOx sensor upstream of the three-way catalytic converter.

The present invention further relates to an exhaust aftertreatment system of an internal combustion engine of a motor vehicle having a three-way catalytic converter, wherein a NOx sensor is provided on the three-way catalyst or downstream of the three-way catalyst in an exhaust pipe of the internal combustion engine. Furthermore, the exhaust aftertreatment system can be an additional NOx sensor upstream of the three-way catalyst. The exhaust aftertreatment system according to the invention is designed for carrying out the method described above.

• 1 eine erste Ausführungsform einer Abgasnachbehandlungsanlage in schematischer Darstellung;
• 2 eine zweite Ausführungsform einer Abgasnachbehandlungsanlage; und
• 3 eine dritte Ausführungsform einer Abgasnachbehandlungsanlage.

The invention will be explained in more detail below with reference to exemplary embodiments in conjunction with the drawing. Show it:

• 1 a first embodiment of an exhaust aftertreatment system in a schematic representation;
• 2 a second embodiment of an exhaust aftertreatment system; and
• 3 a third embodiment of an exhaust aftertreatment system.

In the schematic representation of 1 is an internal combustion engine 1 shown having a combustion air supply line 8th and a Abgasabführstrang in the form of an exhaust pipe 2 having. For the invention, only the parts arranged in the Abgasabführstrang important, so that will be discussed in the following description only on these parts. All other parts may be formed in a conventional manner.

The through the Abgasabführstrang the internal combustion engine 1 guided exhaust gases drive a turbine wheel 3 an exhaust gas turbocharger and are from there via the exhaust pipe 2 delivered to the atmosphere. They pass a lambda probe on their way 4 , a three-way catalyst 5 , a NOx sensor 6 and another three-way catalyst 7 which, however, is irrelevant to the invention.

With the provided three-way catalyst 5 is an exhaust aftertreatment of trained as gasoline engine internal combustion engine 1 carried out. This converts carbon monoxide, nitrogen oxides and unburned hydrocarbons into carbon dioxide, nitrogen and water. To ensure a high degree of conversion of the pollutants, is using a control loop with the proposed lambda probe 4 the combustion air ratio (air-fuel ratio lambda) is kept within a narrow range around lambda = 1.

With the intended NOx sensor 6 the NOx content in the exhaust gas is measured. Hereby it is determined whether or not the NOx content still contained in the exhaust gas after passing through the three-way catalyst exceeds a specified threshold value. If the threshold is exceeded, additional measures to reduce the NOx content are initiated and carried out, for example, an ignition delay and / or an increase in the rate of exhaust gas recirculation (not shown here).

In particular, after the engine starts and during the heating phase of the three-way catalyst, the NOx sensor 6 the NOx emissions in the exhaust gas and the lambda value measured. The corresponding lambda information is used for the lambda control circuit of the internal combustion engine. The measured NOx emissions are used for a NOx emission control loop. For example, when the NOx emissions are above a defined emission threshold that is dependent on the engine operating point, such as engine speed and load, NOx emissions are reduced in the manner described above. The usual heating actions of the three-way catalytic converter are stopped when the NOx emissions at lambda = 1 are below the threshold value.

The NOx emission measurement performed with the NOx sensor is performed in a permanent manner during engine operation, particularly when the engine is in the lambda = 1 control phase. Then, when the NOx emissions exceed a threshold and the lambda signal of the NOx sensor 6 is above a voltage threshold, additional catalyst actions are performed, as noted above, until the NOx emissions are again below the threshold.

2 shows an embodiment of an exhaust aftertreatment system, which differs from that of 1 only differs in that the NOx sensor 6 not downstream of the three-way catalyst 5 but at the three-way catalyst 5 arranged itself.

3 shows an embodiment of an exhaust aftertreatment system that has no lambda probe, but instead a second NOx sensor 9 own, the upstream of the three-way catalyst 5 at the exhaust pipe 2 is arranged. The corresponding lambda control circuit of this system can corresponding lambda information of this second NOx sensor 9 to use. In this case, the efficiency of the three-way catalyst 5 calculated by the measured NOx emissions before and after the three-way catalyst 5 to be used. If the efficiency is below a threshold and the lambda information signal of the NOx sensor is above a voltage threshold, the catalyst may be diagnosed as faulty.

It can thus work with a NOx sensor 6 at the three-way catalyst 5 or downstream of it or with such a sensor and an additional NOx sensor 9 upstream of the three-way catalyst 5 in addition to a control or regulation of the NOx content in the exhaust gas, a corresponding diagnosis of the three-way catalyst 5 be performed.

LIST OF REFERENCE NUMBERS

1

internal combustion engine

2

exhaust pipe

3

Turbine wheel of the exhaust gas turbocharger

4

lambda probe

5

Three-way catalytic converter

6

NOx sensor

7

Three-way catalytic converter

8th

Verbrennungsluftzuführstrang

Claims (17)

Method for operating an exhaust aftertreatment system having a three-way catalytic converter (5) of an internal combustion engine (1) of a motor vehicle, comprising the following steps: - Providing a NOx sensor (6) on the three-way catalyst (5) or downstream of the three-way catalyst (5) in an exhaust pipe (2) of the internal combustion engine (1); - Measuring the NOx emissions during operation of the internal combustion engine (1) with the NOx sensor (6); and - if the NOx emissions measured by the NOx sensor (6) exceed a threshold, implementing measures to reduce NOx emissions until the threshold falls below the threshold again. Method according to Claim 1 , characterized in that as a measure to reduce the NOx emissions, an ignition delay on the internal combustion engine (1) is performed. Method according to Claim 1 or 2 , characterized in that as a measure to reduce the NOx emissions exhaust gas recirculation performed or their rate is increased. Method according to one of the preceding claims, characterized in that as a measure for reducing the NOx emissions, an electrical heating of the three-way catalyst (5) is performed. Method according to one of the preceding claims, characterized in that it is carried out during the heating phase of the three-way catalyst (5). Method according to one of Claims 1 to 4 , characterized in that it is performed when the internal combustion engine (1) is in the lambda-1 control phase. Method according to one of the preceding claims, characterized in that the NOx sensor (6) determines lambda information and these are used for lambda control. Method according to Claim 7 characterized in that upon determining that the detected lambda information signal is above a voltage threshold, measures are taken to reduce NOx emissions. Method according to one of the preceding claims, characterized in that the NOx sensor (6) is additionally used for the diagnosis of the three-way catalytic converter (5). Method according to Claim 9 , Characterized in that the three-way catalyst (5) is diagnosed as defective if the by the NOx sensor (6) measured NOx emissions detected above a threshold value and from the NOx sensor (6) Lambda-information signal are above a voltage threshold. Method according to one of Claims 9 or 10 characterized in that an additional NOx sensor (9) upstream of the three way catalyst (5) is used for the diagnosis of the three way catalyst (5). Method according to Claim 11 , characterized in that the additional NOx sensor (9) determines lambda information and these are used for lambda control. Method according to Claim 12 characterized in that the efficiency of the three-way catalyst (5) is determined by using the NOx emission values measured by both NOx sensors (6, 9), the three-way catalyst (5) being diagnosed as defective when the efficiency is below a threshold and the lambda information signal is above a voltage threshold. Method according to one of the preceding claims, characterized in that the measures for reducing the NOx emissions are performed before additional heating measures of the three-way catalytic converter (5) if the NOx emissions measured by the NOx sensor (6) exceed a threshold value. Exhaust after-treatment system of an internal combustion engine of a motor vehicle, which has a three-way catalyst, characterized in that the three-way catalyst (5) or downstream of the three-way catalyst (5) in an exhaust pipe (2) of the internal combustion engine (1) is provided a NOx sensor (6). Exhaust after-treatment plant according to Claim 15 , characterized in that it has a additional NOx sensor (9) upstream of the three-way catalyst (5). Exhaust after-treatment plant according to Claim 15 or 16 , characterized in that it is suitable for carrying out the method according to one of Claims 1 to 13 is trained.

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