DE10127555A1 - Method and device for increasing a catalyst temperature - Google Patents

Abstract

The invention relates to a method and a device for increasing a catalytic converter temperature of a catalytic converter (16, 18) arranged in an exhaust system (12) of an internal combustion engine (10), wherein an energy input (E) into the exhaust system by at least one motor and / or non-motor measure (12) is increased. DOLLAR A According to the invention, the increase in the energy input (E) into the exhaust system (12) is carried out with different intensities, for example in at least two stages. The method, which can be used particularly advantageously for the desulfurization of NO¶x¶ storage catalytic converters (18), enables a shortening of the heating time and associated fuel savings to be achieved. The thermal load on catalysts (16, 18) is reduced.

Description

The invention relates to a method and a device for increasing an Catalyst temperature in an exhaust system of an internal combustion engine arranged catalyst with the features of the preambles of the independent Claims 1 and 17.

In order to carry out exhaust gas aftertreatment of exhaust gases from internal combustion engines, it is known to arrange catalytic converters or catalytic converter systems from various types of catalytic converters in exhaust gas systems of internal combustion engines. The catalysts convert certain pollutants contained in the exhaust gas into harmless or less environmentally relevant products. In addition, storage catalytic converters, for example NO _x storage catalytic converters, are also known which store certain exhaust gas components in normal operating phases and release and convert them again in so-called regeneration phases.

There are several situations in which an increase in catalyst temperature may be required. This is the case, for example, in starting phases of internal combustion engines, in which the catalytic converter temperature is still below a minimum temperature which is necessary for a sufficient conversion rate of the catalytic converter. In order to carry out a regeneration of a storage catalytic converter, a minimum temperature at the catalytic converter is required for the desorption of the stored component, which is often not easily achieved by certain engine types or in low-load operating situations. However, the catalyst heating is of the greatest importance in connection with so-called desulfurization of NO _x storage catalysts, by means of which the storage catalyst is to be freed from undesirably stored sulfur. Desorption of the sulfur in the form of sulfate requires a minimum desulfurization temperature, which is around 600 ° C depending on the catalyst and is therefore very high. Without active heating measures, this is only achieved in exceptional cases.

In order to raise the catalytic converter temperature, engine measures are usually taken with which an energy input into the exhaust system or into the catalytic converter is increased. Suitable engine measures are, for example, spark ignition or an increase in the proportion of fuel in an air-fuel mixture to be fed into the internal combustion engine. Non-motor measures for direct exhaust gas or catalyst heating are also known. The energy input is dimensioned such that predeterminable maximum values for the catalyst temperature are not exceeded. In the case of desulfurization of NO _x storage catalytic converters, these maximum values are based on temperature thresholds, beyond which thermal, usually irreversible, damage to the catalytic converter must be expected. If a further catalytic converter is connected upstream of the storage catalytic converter, in particular a pre-catalytic converter arranged close to the engine, the amount of energy input is generally limited by its temperature resistance. In order to keep the risk of catalyst damage low, a comparatively large safety margin is maintained between the predetermined maximum value and the critical catalyst temperature. However, the comparatively low maximum temperature leads to a long heating process and thus to a high additional consumption. Furthermore, a certain thermal damage (aging) of the catalyst is observed despite compliance with the temperature limits mentioned with each thermal load.

The invention is therefore based on the object of a method for increasing a To provide catalyst temperature, which is short heating times and as short as possible allows thermal loads on the catalyst. It is also intended to be a device for Implementation of the procedure will be provided.

This object is achieved by a method and a device with the features of independent claims 1 and 17 solved. According to the invention it is provided that the Increasing the energy input into the exhaust system with different intensities is carried out. In other words, one is for the purpose of catalyst heating additionally, the amount of energy entered in the exhaust system varies per unit of time. there is within the scope of the present description under the term energy input Sum understood from a thermal energy of the exhaust gas, that is, directly from the Combustion resulting exhaust gas temperature, as well as a chemical, mainly in the form unburned hydrocarbons HC, bound energy at a Afterburning, for example on a pre-catalyst, released as thermal energy becomes. If a non-motor measure is provided, such as an electrical one Resistance heating, this also contributes to the energy input. The variable increase in Energy input enables a desired target to be reached more quickly.  

Minimum temperature of the catalyst compared to conventional one-stage Heating process. In addition, the risk of operating point dependent Overriding a predetermined maximum temperature of the catalyst to be heated or a catalyst upstream of this reduced. Furthermore, the once reached Minimum temperature even with operational temperature fluctuations with a higher one Probability adhered to, so that the method according to the invention also a higher one Has process reliability. This allows the heating process to be limited as a whole a comparatively short duration. This means that not only one can be used for heating additional fuel consumption can be reduced, but also the thermal Load on the catalyst.

It is particularly preferably provided that one by increasing the Energy input additionally depending on the amount of energy introduced into the exhaust system from an operating point-related energy input and / or a current one Catalyst temperature is measured. For example, is due to a current High-load situation of the internal combustion engine is a relatively high one anyway Energy input before, the additionally entered amount of energy can be relatively small be measured.

The increase in energy input can in principle be done gradually or continuously, for example linear, take place, preferably starting from an initially relative low additional energy input this in one way or another is raised. After a preferred embodiment of the method, the lifting takes place of the energy input in at least two stages, preferably in exactly two stages.

According to an advantageous embodiment of the method, the energy input during a first stage dimensioned such that a maximum with the first energy input achievable temperature of the catalyst to be heated approximately the desired Minimum temperature corresponds to or is slightly higher than this. It is also provided that a second stage begins before the maximum in the previous one Stage achievable catalyst temperature is reached. A last step energy input Finally, it should be such that one with the maximum energy input achievable temperature of the catalyst to be heated and / or one of these upstream catalyst does not exceed a predetermined maximum temperature.

It is also preferably provided that the second and / or subsequent stage of the Increasing the energy input is dispensed with if the  desired minimum temperature on the catalyst to be heated has already been reached. If this is the case depends essentially on the current operating situation of the vehicle from.

Although the method can be used in all situations in which a catalyst temperature is to be raised, it is particularly advantageous in connection with desulfurization of NO _x storage catalysts. In this case, the desired minimum temperature corresponds to a minimum desulfurization temperature of NO _x storage catalytic converters. This is usually around 600 ° C. In connection with the desulfurization of NO _x storage catalysts, a two-stage increase in the energy input is likewise preferred, the duration of the first stage being 100 to 800 s, in particular 400 to 600 s, and its energy input at a catalyst temperature of a pre-catalyst upstream of the NO _x storage catalyst from 800 to 1100 ° C, in particular 900 to 1030 ° C, corresponds. The duration of the second stage is advantageously 100 to 800 s, in particular 400 to 600 s, while its energy input corresponds to a pre-catalyst temperature of 900 to 1150 ° C., in particular 960 to 1090 ° C. The periods of the two stages do not necessarily have to be chosen the same.

The device according to the invention provides means by which the increase in Energy input into the catalyst can be carried out with different intensities. The Means preferably comprise a control unit, in which a procedure for control the process steps to carry out the increase in catalyst temperature in digital Form is deposited. The control unit can also be particularly advantageous in one Engine control unit to be integrated.

Further advantageous embodiments are the subject of the remaining subclaims.

The invention is described below in exemplary embodiments on the basis of the associated Drawings explained in more detail. Show it

Fig. 1 shows schematically a structure of an exhaust system of an internal combustion engine with a NO _x storage catalyst and

Fig. 2 shows a time course of an energy input into an exhaust system and the course of a temperature of a catalyst arranged in this.

In Fig. 1, an internal combustion engine 10 is shown with an overall designated 12 exhaust system. The exhaust system 12 includes an exhaust duct 14 , which houses a small-volume pre-catalytic converter 16 , for example a 3-way catalytic converter, at a position close to the engine. Downstream of the pre-catalytic converter 16 is a NO _x storage catalytic converter 18 , the function of which is to store nitrogen oxides of the exhaust gas during lean operating phases of the internal combustion engine 10 , at a position remote from the engine, in particular at an underbody position of the vehicle. Typically, the oxygen content of the exhaust gas is continuously detected by a lambda probe 20 downstream of the internal combustion engine 10 to regulate be supplied to a combustion engine 10, air-fuel mixture (lambda regulation). The state of the NO _x storage catalytic converter 18 can be monitored, for example, by a NO _x sensor 22 connected downstream of the NO _x storage catalytic converter 18 . With the help of the NO _x sensor 22 , a need for regeneration or desulfurization of the storage catalytic converter 18 can then be recognized on the basis of a NO _x breakthrough. Other means or methods for monitoring are known and can also be used within the scope of the invention. Downstream of the pre-catalytic converter 16 and the NO _x storage catalytic converter 18 , temperature sensors 24 , 26 are arranged, which measure an exhaust gas temperature. Taking into account a certain heat loss, the catalyst temperature of the respective catalytic converter 16 , 18 can thus be inferred. Other arrangements or a different number of temperature sensors are also conceivable. It is also possible to calculate the exhaust gas temperature with sufficient accuracy based on known operating parameters (engine speed, engine load, etc.) of the internal combustion engine 10 . The signals of all sensors 20 , 22 , 24 , 26 installed in the exhaust duct 14 and various operating parameters of the internal combustion engine 10 are input into an engine control unit 28 . Depending on these signals, the engine control unit 28 regulates the operation of the internal combustion engine 10 .

NO _X -Regenerationen and desulfurization of the NO _x storing catalyst 18 require exposure of the catalyst 18 with a rich exhaust gas atmosphere and a minimum catalyst temperature. The minimum temperature required for the release of sulfur, at around 600 ° C, is significantly higher than a NO _X desorption temperature. Sufficient exhaust gas temperatures are often not available, so that it is common to take motor and / or non-motor measures to heat the storage catalytic converter 18 . A control unit 30 is therefore integrated in the engine control unit 28 , in which an algorithm is stored in digital form, by means of which the method according to the invention for variably increasing a catalyst temperature can be carried out. In order to raise a temperature of the NO _x storage catalytic converter 18 , the energy input into the exhaust system 12 is increased. For this purpose, the control unit 30 controls in a known manner, for example, an ignition system (not shown here) in order to increase an exhaust gas temperature, for example by means of a late ignition. Other engine measures are aimed at increasing unburned hydrocarbon HC in the exhaust gas, which burns on the pre-catalytic converter 16 and thereby contribute to an increase in the exhaust gas temperature. It is also known to heat NO _x storage catalysts 18 or the exhaust gas flowing through them with the aid of suitable heating devices. Such heating devices can also be controlled by the control device 30 .

The method according to the invention according to a preferred embodiment, in which the catalyst heating is carried out in two stages, will be explained in more detail with the aid of FIG. 2 using the example of desulfurization of the NO _x storage catalyst 18 shown in FIG. 1. The lower part of the diagram shows the time course of the energy input E into the exhaust system 12 , while the upper part shows the development of the catalytic converter temperature T _{Kat of} the NO _x storage catalytic converter 18 determined with the temperature sensor 26 . At a point in time t ₁ at which a desulfurization requirement of the NO _x storage catalytic converter 18 is determined, the energy input E is increased to the level E ₁ in a first stage by the motor and / or non-motor measures described. As a result of the increased temperature of the exhaust gas flowing through the storage catalytic converter 18 , the catalytic converter temperature T _Kat initially rises steeply and gradually increases in the further course. The energy input E _{1 of} the first stage is dimensioned such that the maximum catalyst temperature T _Kat that can be achieved with this slightly exceeds the desired minimum temperature T _min , in this case the minimum desulfurization temperature of approximately 600 ° C. This corresponds to a temperature of the pre-catalyst 16 of 800 to 1100 ° C, preferably from 900 to 1030 ° C. This is recorded with the aid of the temperature sensor 24 arranged downstream of the pre-catalytic converter 26 , taking into account a heat loss of approximately 30 to 70 K. Due to the saturation character of the temperature T _{Kat of} the storage catalytic converter 18 , the minimum desulfurization temperature T _{min would} only be reached relatively late with the energy input E ₁ . This time is indicated by the right arrow in the upper part of the diagram. Furthermore, slight variations in temperature of the exhaust gas could be as a result of changes in operation of the internal combustion engine 10, a temporarily falls below T _min the NO _X - storage catalyst 18 cause. In such phases, the desulfurization process would be interrupted or would only take place with a lack of reliability. According to the invention, therefore, before the final temperature at the storage catalytic converter 18 which can be achieved with the energy input E _{1 is} reached, the energy input E is raised to the level E ₂ in a second stage. This advantageously takes place at a time t ₂ , 100 to 800 s after the start of the first stage, preferably after 400 to 600 s. The energy input E ₂ is dimensioned such that the catalyst temperature of the pre-catalyst 16 is 900 to 1150 ° C., in particular 960 to 1090 ° C. This upper temperature threshold is based on a critical temperature at which irreversible thermal damage to the pre-catalytic converter 16 is to be expected. Of course, the upper temperature threshold is selected with a sufficient safety margin from this critical temperature. As can be seen in the upper part of the figure, the minimum desulfurization temperature T _min at the NO _x storage catalytic converter 18 is reached shortly after the start of the second stage (left arrow). The maximum achievable catalyst temperature T _{Kat of} the second stage naturally exceeds that of the first stage. The probability that the temperature falls below the minimum temperature T _min during this phase is relatively low, so that the desulfurization takes place very reliably and the process has a high level of process reliability. The overall faster heating and the higher final temperature of the catalytic converter 18 can significantly shorten the total desulfurization time. Despite the higher end temperature of the catalytic converter 18 compared to conventional heating methods, fuel savings can therefore be achieved. Furthermore, aging of the pre-catalytic converter 16 and of the NO _x storage catalytic converter 18 is slowed down by the overall lower thermal load.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust system

14

exhaust duct

16

precatalyzer

18

Catalyst / NO _X

storage catalyst

20

lambda probe

22

NO _X

-Sensor

24

temperature sensor

26

temperature sensor

28

control unit

30

control unit
E energy input
E ₁

First stage energy input
E ₂

Second stage energy input
t time
T _cat

Temperature of the catalyst
T _min

Minimum temperature of the catalyst

Claims (20)

1. A method for increasing a catalytic converter temperature of a catalytic converter ( 16 , 18 ) arranged in an exhaust system ( 12 ) of an internal combustion engine ( 10 ), an energy input (E) into the exhaust system ( 12 ) being increased by at least one motor and / or non-motor measure , characterized in that the increase in energy input (E) into the exhaust system ( 12 ) is carried out with different intensities. 2. The method according to claim 1, characterized in that an amount of energy introduced by increasing the energy input (E) into the exhaust system ( 12 ) is dimensioned as a function of an operating point-related energy input (E) and / or a catalyst temperature. 3. The method according to claim 1 or 2, characterized in that the increase in Energy input (E) is carried out in at least two stages. 4. The method according to claim 3, characterized in that the increase in Energy input (E) is carried out in exactly two stages. 5. A method according to 3 or 4, characterized in that a first energy input (E ₁₎ is sized to a first stage such that a with the first energy input (E ₁₎ maximum attainable catalyst temperature (T _Kat) of the to be heated catalyst (16 18 ) corresponds approximately to a desired minimum temperature (T _min ) or is slightly higher than this. 6. The method according to any one of claims 3 to 5, characterized in that a second stage begins before the maximum attainable catalyst temperature (T _Kat ) is reached in the previous stage. 7. The method according to any one of claims 3 to 6, characterized in that an input of energy of a last stage is dimensioned such that a maximum attainable with the energy input temperature of the heated catalytic converter (16, 18) and / or of an upstream catalyst (16 ) does not exceed a predeterminable maximum temperature (T _max ). 8. The method according to any one of the preceding claims, characterized in that the catalyst to be heated ( 16 , 18 ) is a NO _x storage catalyst ( 18 ). 9. The method according to claim 8, characterized in that the desired minimum temperature (T _min ) is a minimum desulfurization temperature of the NO _x storage catalyst ( 18 ). 10. The method according to claim 8 or 9, characterized in that a duration of first stage is 100 to 800 s, in particular 400 to 600 s. 11. The method according to any one of claims 8 to 10, characterized in that the energy input (E ₁ ) of the first stage is dimensioned such that a catalyst temperature of a pre-catalyst ( 16 ) upstream of the NO _x storage catalyst ( 18 ) is from 800 to 1100 ° C, especially from 900 to 1030 ° C, is not exceeded. 12. The method according to any one of claims 8 to 12, characterized in that a Duration of the second and last stage 100 to 800 s, in particular 400 to 600 s, is. 13. The method according to any one of claims 8 to 12, characterized in that the energy input (E ₂ ) of the second and last stage is dimensioned such that a catalyst temperature of the pre-catalyst ( 16 ) upstream of the NO _x storage catalyst ( 18 ) is from 900 to 1150 ° C, especially 960 to 1090 ° C, is not exceeded. 14. The method according to any one of the preceding claims, characterized in that the engine measures for controlling the energy input (E) in the exhaust system ( 12 ) comprise an ignition angle control and / or a regulation of an air-fuel mixture. 15. The method according to any one of the preceding claims, characterized in that the non-motor measures include heating the catalyst ( 16 , 18 ) and / or the exhaust gas by means of a heating device. 16. The method according to any one of the preceding claims, characterized in that a second and / or subsequent step of increasing the energy input (E) is omitted if in the previous step the desired minimum temperature (T _min ) on the catalyst ( 16 , 18 ) was achieved. 17. Device for increasing a catalytic converter temperature of a catalytic converter ( 16 , 18 ) arranged in an exhaust system ( 12 ) of an internal combustion engine ( 10 ), an energy input (E) into the exhaust system ( 12 ) being increased by at least one motor and / or non-motor measure , characterized in that means are provided with which the increase in energy input (E) into the exhaust system ( 12 ) can be carried out with different intensities. 18. The apparatus according to claim 17, characterized in that the means comprise a control unit ( 28 ), in which a procedure for controlling the method for increasing the catalyst temperature (T _Kat ) is stored in digital form. 19. The apparatus according to claim 18, characterized in that the control unit ( 28 ) is integrated in an engine control unit ( 26 ). 20. Device according to one of claims 17 to 19, characterized in that an arrangement of at least one temperature sensor ( 24 , 26 ) is provided in the exhaust system ( 12 ) with which the catalyst temperature (T _Kat ) of the catalyst to be heated ( 16 , 18th ) and / or a catalyst ( 16 ) connected upstream of this.