DE102016213056B4 - Improved operation of lean nitrogen oxide traps in urban driving conditions - Google Patents

Abstract

A method for operating a lean nitrogen oxide trap (11) for an internal combustion engine (3) and comprising the steps: during a storage period of the lean nitrogen oxide trap (11), electrically heating the lean nitrogen oxide trap (11) up to an adsorption temperature of at least 200 degrees Celsius by means of a direct contact electrical heater (12) associated with the lean nitrogen oxide trap (11), wherein the substrate is heated directly rather than indirectly by the exhaust gas while the internal combustion engine (3) is operated lean;at the end of the storage period, electrically heating the lean nitrogen oxide trap (11) to one Flushing temperature of at least 300 degrees Celsius by means of the electric heater (12) and performing a flushing period during which the internal combustion engine (3) is operated rich.

Description

The invention relates to a method for operating a lean nitrogen oxide trap for an internal combustion engine and a motor vehicle with a lean nitrogen oxide trap.

The emissions from motor vehicles are subject to legal regulations that aim to reduce the environmental pollution caused by motor vehicle traffic. Exhaust aftertreatment devices are therefore provided in order to be able to comply with the prescribed limit values. This also applies to the emission of environmental toxins such as nitrogen oxides. In this context, it is known to provide a nitrogen oxide storage catalytic converter which adsorbs nitrogen oxides contained in an exhaust gas stream of an internal combustion engine of the motor vehicle on its surface and thus stores them. Such nitrogen oxide storage catalysts are also called lean NOx traps (LNT) or lean nitrogen oxide traps and are, for example, from DE 196 26 836 A1 known. Since the absorption capacity of the nitrogen oxide storage catalytic converter is limited, the nitrogen oxide storage catalytic converter can be regenerated or flushed. For this purpose, a rich fuel mixture can be generated for the internal combustion engine. It is also possible to add unburned fuel to the exhaust gas flow using late post-injection. In both cases, the hydrocarbons contained in the rich exhaust gas flow react with the nitrogen oxides stored in the nitrogen oxide storage catalytic converter and decompose them in this way.

As a result of the ever more stringent emission limit values, greater effort must be made to comply with the applicable regulations for types of vehicle use such as predominantly inner-city use, in which predominantly short distances are covered with low engine loads. This is due to the fact that the function of exhaust gas aftertreatment devices is temperature-dependent and, with typical inner-city use, the exhaust gas and the exhaust gas aftertreatment devices heated by the exhaust gas mainly have substrate temperatures below the respective temperatures required for optimal operation.

Therefore, for example, disclosed the DE 196 26 836 A1 a method in which an oxidation catalytic converter connected upstream of a nitrogen oxide storage catalytic converter oxidizes fuel while generating heat in order to heat up the nitrogen oxide storage catalytic converter. The nitrogen oxide storage catalytic converter and/or the oxidation catalytic converter can optionally be electrically heatable.
From the publication OTTO, E.; HELD, W.; THURSDAY, A.; KUPLER, PF; PFALZGRAF, B.; WIRTH, A.: The system development of the electrically heatable catalytic converter E-Kat for the LEV/ULEV and EU III legislation. In: MTZ 56 (1995) issue 9, pp. 488-498 is also known to heat a catalyst electrically. Batteries, generators and capacitors are described as possible energy sources.

the DE 10 2013 113 830 A1 discloses the use of a heat source located immediately upstream of an LNT catalyst, e.g. B. an electrically heated catalyst, for adding thermal energy to the exhaust stream to cause regeneration of the LNT catalyst.

The object of the invention is to improve the effectiveness of lean nitrogen oxide traps.

• während einer Speicherperiode der Magerstickoxidfalle, elektrisches Aufheizen der Magerstickoxidfalle bis auf eine Adsorptionstemperatur von wenigstens 200 Grad Celsius mittels einer in direktem Kontakt mit der Magerstickoxidfalle angeordneten elektrischen Heizvorrichtung, wobei das Substrat direkt erwärmt wird anstatt mittelbar durch das Abgas, während die Brennkraftmaschine mager betrieben wird;
• am Ende der Speicherperiode, elektrisches Aufheizen der Magerstickoxidfalle bis auf eine Spültemperatur von wenigstens 300 Grad Celsius mittels der elektrischen Heizvorrichtung und Durchführen einer Spülperiode, während der die Brennkraftmaschine fett betrieben wird.

The invention therefore introduces an improved method of operating a lean nitrogen oxide trap for an internal combustion engine. The method according to the invention has at least the following steps:

• during a storage period of the lean nitrous oxide trap, electrically heating the lean nitrous oxide trap to an adsorption temperature of at least 200 degrees Celsius by means of an electrical heater arranged in direct contact with the lean nitrous oxide trap, wherein the substrate is heated directly instead of indirectly through the exhaust gas while the internal combustion engine is operated lean;
• at the end of the storage period, electrically heating the lean nitrous oxide trap up to a purge temperature of at least 300 degrees Celsius using the electrical heater and performing a purge period during which the engine is operated rich.

The invention has the advantage of increasing the storage capacity of the lean nitrogen oxide trap by increasing its substrate temperature, so that there is good storage capacity almost immediately after the internal combustion engine is started cold. The lean nitrogen oxide trap is heated to at least 200 degrees, which is a threshold temperature for good adsorption properties for common lean nitrogen oxide traps. Due to the electrical heating, nitrogen oxide can also be adsorbed by the lean nitrogen oxide trap during the short driving distances that are usual in city traffic at low speeds, so that even stricter legal limit values for nitrogen oxide emissions can be complied with. In a purge period following the storage period, the invention utilizes the improved conversion properties of the lean nitrous oxide trap at temperatures of at least 300 degrees Celsius, which in turn is achieved by electrically heating the lean nitrous oxide trap be reached. The invention thus enables a reduced emission of nitrogen oxides and a reduction in the substrate size of the lean nitrogen oxide trap or the mass of the expensive catalyst materials used therein.

Preferably, electrical heating is terminated at the end of the purge period until the temperature of the lean nitrous oxide trap has fallen to the adsorption temperature. Continued operation at the even further increased substrate temperatures desired for rinsing can advantageously be dispensed with, as a result of which the need for electrical heating power can be reduced and ultimately the fuel efficiency of the internal combustion engine can be improved.

The adsorption temperature is preferably at most 250 degrees Celsius. At this temperature, very good adsorption properties of the lean nitrogen oxide trap are achieved without unnecessary electrical heating power being required for only a marginal improvement.

The rinsing temperature is preferably at most 350 degrees Celsius. Above 350 degrees Celsius, the conversion properties of the lean nitrogen oxide trap during flushing are not increased to an extent that would justify an even higher expenditure on electrical heating power.

The method may include an additional step of determining a level of the lean nitrogen oxide trap, ending the storage period when the determined level is greater than a predetermined threshold level. The filling level of the lean nitrogen oxide trap can be modeled mathematically, for example, taking into account the operating parameters of the internal combustion engine and the lean nitrogen oxide trap, determined by measurements such as the nitrogen oxide slip or by a combination of the methods mentioned.

A heating output of the electrical heating device is particularly preferably provided by an alternator operated by the internal combustion engine. This has the additional advantage that a load on the internal combustion engine is increased, which results in advantages with regard to the synchronization of the internal combustion engine (NVH - Noise Vibration Harshness) and the stability of the scavenging process.

The temperature of the lean nitrogen oxide trap is preferably determined using operating parameters of the internal combustion engine and/or a measurement of a nitrogen oxide concentration downstream of the lean nitrogen oxide trap. As a result, a separate temperature sensor can be omitted to save costs.

A second aspect of the invention relates to a motor vehicle with a lean nitrogen oxide trap, an electrical heating device which is in thermally conductive connection with the lean nitrogen oxide trap and is arranged in direct contact with the lean nitrogen oxide trap, and a control unit connected to the heating device and an internal combustion engine of the motor vehicle. According to the invention, the control unit is designed to carry out the method of the first aspect of the invention.

The lean nitrogen oxide trap can be arranged in an underbody area of the motor vehicle. A lean nitrogen oxide trap arranged there often has an undesirably low substrate temperature compared to a lean nitrogen oxide trap arranged close to the engine due to the lower exhaust gas temperatures, so that it particularly benefits from the electrical heating. In this way, the invention makes it possible to keep the basically limited space in the immediate vicinity of the internal combustion engine free for other components.

The motor vehicle may be equipped with an additional lean nitrogen oxide trap close-coupled, which is located remote from the electric heater (specifically, upstream of the electric heater). As a result, a particularly advantageous compromise can be achieved between the required heating output and the space requirements in the immediate vicinity of the internal combustion engine.

• 1 ein Ausführungsbeispiel eines erfindungsgemäßen Kraftfahrzeugs zur Durchführung des erfindungsgemäßen Verfahrens;
• 2 ein Diagramm, das beispielhaft urbane Betriebsbedingungen einer Magerstickoxidfalle veranschaulicht; und
• 3 ein Diagramm, in dem die Speicherkapazität einer Magerstickoxidfalle in Abhängigkeit der Substrattemperatur aufgetragen ist.

The invention is explained in more detail below with reference to illustrations of exemplary embodiments. Show it:

• 1 an embodiment of a motor vehicle according to the invention for carrying out the method according to the invention;
• 2 Figure 12 is a diagram illustrating example urban operating conditions of a lean nitrous oxide trap; and
• 3 a diagram in which the storage capacity of a lean nitrogen oxide trap is plotted as a function of the substrate temperature.

1 shows an exemplary embodiment of a motor vehicle 1 according to the invention for carrying out the method according to the invention. The motor vehicle has wheels 2 and is driven by an internal combustion engine 3, for example a diesel engine. The internal combustion engine 3 receives the intake air required for the combustion of the fuel via an intake air manifold 4 , which intake air is supplied through an air filter 8 and compressed by a compressor 6 in the example shown. The resulting during the combustion of the fuel exhaust gas is an exhaust manifold 5 from the internal combustion engine line 3 derived. In the example shown, the exhaust gas flows through an exhaust gas turbine 7, which is rotated by the exhaust gas. The exhaust gas turbine 7 is connected to the compressor 6 via a shaft 10 and drives it. Of course, alternative arrangements without a compressor and exhaust gas turbine or with a compressor driven electrically or directly by the internal combustion engine are also conceivable.

The exhaust gas emerging from the exhaust gas turbine 7 flows through a lean nitrogen oxide trap 11, which adsorbs nitrogen oxides contained in the exhaust gas during a storage period and catalytically reduces them during a scavenging period. The lean nitrogen oxide trap 11 is thermally conductively connected to an electrical heating device 12 which, in the example shown, is arranged in direct contact upstream of the lean nitrogen oxide trap 11 at its inlet. The electric heater 12 is arranged on the substrate or a wall of the substrate so as to heat the substrate directly instead of indirectly through the exhaust gas.

An alternator 9 connected to the internal combustion engine 3 and driven by it can be provided to supply the electrical heating device 12 and other electrical consumers in the motor vehicle 1 with power.

Further exhaust gas after-treatment devices 14, silencers and the like can be provided downstream of the lean nitrogen oxide trap 11. Exhaust gas recirculation can also be provided (not shown). The after-treated exhaust gas is finally released into the environment via an exhaust pipe 15 .

The motor vehicle 1 according to the invention has a control unit 13 which is connected to the internal combustion engine 3 and the lean nitrogen oxide trap 11 and is designed to carry out the method according to the invention. In particular, the control unit 13 is designed to activate the electrical heating device 12 during a storage period of the lean nitrogen oxide trap 11 until the lean nitrogen oxide trap is heated to an adsorption temperature of at least 200 degrees Celsius. At the same time, the control unit 13 initiates lean operation of the internal combustion engine 3. The control unit 13 is also designed to activate the electric heating device 12 for electric heating of the lean nitrogen oxide trap up to a flushing temperature of at least 300 degrees Celsius at the end of the storage period. When the scavenging temperature is reached, the control unit 13 can control the internal combustion engine 3 to perform a scavenging period during which the internal combustion engine 3 is operated rich. The rich exhaust gas, ie an exhaust gas with an excess of unburned CO and unburned hydrocarbons, can then react with the nitrogen oxides stored in the lean nitrogen oxide trap 11 to flush the lean nitrogen oxide trap 11 and convert them into harmless substances.

2 FIG. 12 is a diagram illustrating example urban operating conditions of a lean nitrogen oxide trap. In the diagram, the number of kilometers driven or operating hours plotted at a specific exhaust gas or substrate temperature is plotted against the exhaust gas or substrate temperature for typical inner-city motor vehicle use. It turns out that only a small proportion of motor vehicle use occurs at exhaust gas or substrate temperatures above 200 degrees Celsius and mostly at temperatures just over 100 degrees Celsius. These temperatures are too low for the lean nitrogen oxide trap to function properly, so that it must be designed very close to the engine and/or undesirably large in order to be able to comply with strict nitrogen oxide emission limits.

3 shows a diagram in which the storage capacity of a lean nitrogen oxide trap is plotted as a function of the substrate temperature in degrees Celsius. The storage capacity is normalized to the maximum storage capacity without a unit. It turns out that the storage capacity of the lean nitrogen oxide trap is greatly reduced at temperatures below 200 degrees Celsius. Above 200 degrees Celsius, however, it increases to its maximum at around 300 degrees Celsius. Beyond the maximum, it falls comparatively slowly compared to the increase at low temperatures to low values at over 500 degrees Celsius.

A comparison of 2 and 3 shows that in inner-city use, a large part of the operation of the internal combustion engine takes place at temperatures far below the maximum storage capacity of the lean nitrogen oxide trap. The invention improves the efficiency of the lean nitrous oxide trap by electrically heating the exhaust gas or the lean nitrous oxide trap to at least 200 degrees Celsius during the storage period. For a particularly good catalytic effect of the lean nitrogen oxide trap during the scavenging period, the exhaust gas and substrate are heated to at least 300 degrees Celsius.

Although the invention has been illustrated and described in detail by examples of preferred embodiments, the invention is not limited by the disclosed examples. Variations of the invention can be derived by a person skilled in the art from the exemplary embodiments shown without departing from the protective scope of the invention as defined in the claims.

Reference List

1

motor vehicle

2

wheel

3

internal combustion engine

4

inlet manifold

5

exhaust manifold

6

compressor

7

exhaust turbine

8th

air filter

9

alternator

10

wave

11

lean nitric oxide trap

12

electric heater

13

control unit

14

further exhaust aftertreatment device etc.

15

Exhaust

Claims (10)

A method for operating a lean nitrogen oxide trap (11) for an internal combustion engine (3) and having the steps: during a storage period of the lean nitrous oxide trap (11), electrically heating the lean nitrous oxide trap (11) up to an adsorption temperature of at least 200 degrees Celsius by means of an electric heater (12) arranged in direct contact with the lean nitrous oxide trap (11), the substrate being heated directly instead indirectly through the exhaust gas while the internal combustion engine (3) is operated lean; at the end of the storage period, electrically heating the lean nitrogen oxide trap (11) up to a scavenging temperature of at least 300 degrees Celsius by means of the electrical heater (12) and performing a scavenging period during which the internal combustion engine (3) is operated rich. The method of the preceding claim wherein the electrical heating is terminated at the end of the purge period until the temperature of the lean nitrous oxide trap (11) has fallen to the adsorption temperature. The process of any one of the preceding claims, wherein the adsorption temperature is at most 250 degrees Celsius. The method of any one of the preceding claims, wherein the rinsing temperature is at most 350 degrees Celsius. The method of one of the preceding claims, with an additional step of determining a filling level of the lean nitrogen oxide trap (11), the storage period being terminated if the determined filling level is more than a predetermined threshold filling level. The method of one of the preceding claims, in which a heating output of the electrical heating device (12) is provided by an alternator (9) operated by the internal combustion engine (3). The method of one of the preceding claims, in which the temperature of the lean nitrogen oxide trap (11) is determined using operating parameters of the internal combustion engine (3) and/or a measurement of a nitrogen oxide concentration downstream of the lean nitrogen oxide trap (11). A motor vehicle (1) with a lean nitrogen oxide trap (11), an electrical heating device (12) which is in thermally conductive connection with the lean nitrogen oxide trap (11) and is arranged in direct contact with the lean nitrogen oxide trap (11), and an electric heating device (12) with the heating device (12) and an internal combustion engine (3) of the motor vehicle (1) connected control unit (13), characterized in that the control unit (13) is designed to carry out the method of one of the preceding claims. The motor vehicle (1) of the preceding claim, wherein the lean nitrogen oxide trap (11) is located in an underbody area of the motor vehicle (1). The motor vehicle (1) of the preceding claim, having an additional close-coupled lean nitrogen oxide trap (11) which is remote from the electric heater (12).

Images