DE10113382A1 - Method for heating a downstream catalyst in an exhaust gas system of an internal combustion engine - Google Patents

Abstract

The invention relates to a method for heating a downstream catalyst in an exhaust gas system of an internal combustion engine with a plurality of catalysts arranged one behind the other in the exhaust flow direction, the heating of a downstream catalyst (6) taking place in that the exothermic reaction from a downstream catalyst (3) in the downstream catalyst (6).

Description

The invention relates to a method for heating a flow straightener device downstream catalyst in an exhaust system Internal combustion engine with several in series in the exhaust gas flow direction of the arranged catalysts.

In the case of an exhaust gas system with a plurality of catalysts arranged one behind the other, there may be a requirement to heat the downstream catalyst without exposing the upstream catalyst to an excessive temperature. This requirement exists, for example, for the desulfation of a NO _x catalyst. Temperatures of approx. 650 ° C are required for an effective desulfation of the NO _x catalytic converter. If this temperature is provided by an increase in the exhaust gas temperature of the engine, it can be due to the temperature gradient between a close-coupled three-way catalytic converter and the NO _x catalytic converter installed in the underbody area, especially with exhaust gas cooling in front of the NO _x catalytic converter The maximum permissible temperature in the near-engine three-way catalytic converter of approx. 950 ° C may be exceeded

From DE 198 27 195 A1 and DE 199 22 962 A1 it is known, in aufgeheiztem NO _x in an internal combustion engine with three-way catalyst and nachgeschal tetem NO _x adsorber adsorber desulfurization with low secondary emission of H ₂ S and SO _{2 to} perform. The documents do not show how the NO _x adsorber is brought to the operating temperature required for desulfurization.

From DE 199 60 828 A1 it is known to desulfurize the desulfurization with a NO _x adsorber which has already been heated up by means of a λ control, with λ being set cyclically to a constant, slightly rich value. So that the temperature in the NO _x adsorber is kept in the preset heated temperature range, the heat is generated by late ignition and charge movement in the engine and from there pushed into the NO _x adsorber. This requires high energy for the corresponding heating in the engine compartment, taking into account the high heat losses on the way to the NO _x adsorber. In addition, the temperature limit of the three-way catalytic converter must also be taken into account here. How the NO _x adsorber is brought to the high temperature level required for desulfurization cannot be found in this document.

The invention has for its object one with little effort to heat the downstream catalyst without losing the upstream overheating catalyst.

According to the invention, the object is achieved by the method according to Merk paint the claim 1 solved, after which the heating of the downstream arranged catalyst takes place in that the exothermic reaction from an upstream catalytic converter to a downstream one th catalyst is shifted. In this way, heating takes place in a targeted manner of the downstream catalyst. Energy losses due to an introduction of thermal energy through heat transfer and heat from outside and the associated risks of overheating their components - especially upstream catalysts - are avoided the. Since the heating only by shifting the exothermic Re action from the upstream catalyst to the downstream downstream catalyst takes place, only reactions are ge uses, which already take place in the usual exhaust gas cleaning. Consequently can be done in a very simple manner without additional reactions and without additional Liche means to limit the temperature of other downstream components downstream catalyst are heated. The process enables very low energy consumption and therefore low-consumption engines.

The method according to the features of claim 2 is preferred for which in a simple way the shift by a λ control with alternie grease lean operating cycle of the internal combustion engine and thus achieved the heating of the downstream catalyst controlled can be. The exhaust emissions can be controlled and at low be kept according to level.  

The method according to the features of claim 3 is particularly advantageous liable, as this makes it very easy to clean the exhaust gas during normal operation Exhaust gas cleaning carried out in the upstream catalytic converter Reduction of the pollutants HC and CO in the downstream Catalyst is shifted, causing the downstream catalyst sator is heated. The pollutants can be broken down unchanged the.

For heating, λ is preferably regulated so that for λ in the cyclic fat Operation during heating applies: 0.7 ≧ λ ≧ 0.8, preferably 0.74 ≧ λ ≧ 0.76. As a result, due to the very rich operation, the relocation takes place in a short time the exothermic reaction in the downstream catalyst, so that the required temperature is reached in a short time.

The method according to the features of claim 5 enables a very sensitive control of the fat-lean operating cycle and thus both Heating as well as the exhaust gas composition. The Ver drive according to the features of claims 6 and 7, respectively, through which the exhaust gas limit values are reliably observed and yet a simple and reliable temperature increase can be achieved.

The method according to the features of claim 8 makes it possible to heat the NO _x adsorber to initiate the desulfurization to desulfation temperature without overheating the three-way catalyst.

The invention is explained in more detail below with reference to FIGS. 1 to 6 using the example of a direct-injection gasoline engine. Show here:

Figure 1 shows the schematic structure of an exhaust system of a direct injection gasoline engine.

Fig. 2a, b two diagrams to show the heating behavior of the exhaust system of Fig. 1 without the inventive Ver storage of the exothermic reaction in the downstream arranged catalyst, wherein

Fig. 2a, the temperature distribution at a speed of 200 km / h and

FIG. 2b km / illustrating the temperature distribution at a speed of 120 h;

Fig. 3 representation to explain the principle of operation of the λ variation for heating with representation of the λ variation and the resulting changes in the O ₂ storage content of the upstream and downstream catalyst;

Fig. 4 is a diagram showing the temperature distribution with inventive λ-variation for heating at a speed of 120 km / h;

Fig. 5 qualitative representation of the temperature and the measurable HC amounts in the exhaust system over the length of the exhaust system in the heated state; and

Fig. 6 representation of the change in the O ₂ storage content of the upstream and downstream catalyst over time.

In Fig. 1, an exhaust system is shown using the example of a direct-injection Otto engine. From the internal combustion engine 1 , the exhaust gases are derived in a known manner via exhaust pipes 2 , a three-way catalytic converter 3 , an exhaust pipe 4 , a NO _x adsorber 6 and an exhaust pipe 7 . Upstream of the three-way catalytic converter 3 is a wide-wall lambda probe 8 and a lambda probe 9 downstream of the known type, by means of which deviations in the λ value of the exhaust gases upstream and downstream of the three-way catalytic converter 3 from the stoichiometric value λ = 1 are recorded become. Likewise, the NO _x adsorber 6 is arranged in a known manner downstream of a lambda probe 11 which detects deviations of the λ value from the stoichiometric value λ = 1 behind the NO _x adsorber 6 . In a known manner, the exhaust pipe 4 is guided between the three-way catalytic converter 3 and the NO _x adsorber 6 through an exhaust gas cooler 5 , and the type for detecting the inlet temperature of the exhaust gas into the NO _x adsorber 6 is the NO _x adsorber 6 A temperature sensor 10 is arranged upstream. The three-way catalyst 3 is designed in a known manner with an upper temperature limit of 950 ° C., the NO _x adsorber 6 with an upper temperature limit of 750 ° C. The working range of the NO _x adsorber 6 is in a known manner between 250 ° C and 450 ° C. From the gas cooler 5 is designed in a known manner so that it lowers the temperature of the exhaust gas to 750 ° C. even at the maximum speed of the vehicle.

To explain the temperature changes in Fig. 1, the three-way catalyst 3 in its length I _{1 was divided} into three sections of equal length. The position at the beginning of the catalyst is designated A, the position after a third of the length I ₁ with B, the position after two thirds I ₁ with C and the position at the end of I ₁ with D. Likewise, the exhaust gas cooler 5 is its length I _{2 divided} into three sections of equal length, E the input of the exhaust gas cooler 5 , F the position after a third I ₂ , G the position after two thirds I ₂ and H the position at the end of the exhaust gas cooler 5 indicates. In the same way, the NO _x adsorber 6 was divided into three sections of equal length according to its length I ₃ , with J the position at the beginning of the NO _X adsorber 6 , K the position after a third I ₃ , L the position after two thirds I ₃ and M indicates the position at the exit of the NO _x adsorber 6 .

The temporal temperature profile shown in FIGS. 2a, 2b, 4 and 6 can be determined, for example, with the aid of the calorific value entry. The λ signal before the three-way catalytic converter 3 and after the NO _x adsorber 6 is used to determine the calorific value entry in the NO _x adsorber 6 more precisely. The calorific value entry in the NO _x adsorber 6 results from the broadband signal of the lambda probe 8 in front of the three-way catalytic converter 3 and the time between the fat breakthrough of the lambda probe 9 after the three-way catalytic converter 3 and the fat Breakthrough of the Lamb probe 11 after the NO _x adsorber 6 . To avoid a fat breakthrough, a maximum time until shortly before the breakthrough for the fat phase is stored in a map above the exhaust gas mass. The temperature in the NO _x adsorber 6 is calculated with the calorific value entry in the NO _x adsorber 6 and with the temperatures in front of the NO _x adsorber 6 measured with the temperature sensor 10 .

The time until the fat breakthrough can be diagnosed using the in the map stored times can be compared.

FIGS. 2a and 2b show the temperature history TA, TB, TC, TD, TF, TC, TL, TM in the positions A, B, C, D, F, K, L, M, as well as exemplary of the exhaust gas pollutants CO, CH and NO _x the time course of the measured CO values at the input of the three-way catalyst 3 , measured by the broadband lambda probe 8 , and the time course of the measured CO values following the NO _x adsorber 6 , measured by the lambda probe 11 , when trying to achieve a temperature increase without further measures in order to initiate desulfation.

In FIG. 2a, at full load at a speed of 200 km / h, it can be seen that the combustion energy introduced by the internal combustion engine into the three-way catalytic converter 3 near the engine generates temperatures which, based on the inlet temperature TA in the three-way catalytic converter 3 in level A with a constant 900 ° C in the positions downstream in the exhaust gas conveying direction at the beginning of this pure engine heating are still below this temperature, with the temperatures TB, TC and TD due to the exothermic reactions in the three-way catalytic converter 3 after a short time Three-way catalytic converter 3 rise to values between 900 ° C and 950 ° C. The temperatures TK in the K position, TL in the L position and TM in the M position of the NO _x adsorber 6 are also raised and reached from the optimal working range of the NO _x adsorber 6 from 250 ° C. to 450 ° C. Values up to 750 ° C, so that desulfurization in the direct upper temperature limit range is possible in this case of full load.

FIG. 2b shows the same exhaust system at the same engine, but h at partial load at a speed of 120 km /.

It can be seen from the diagrams that the temperatures TA, TB, TC, TD in levels A, B, C, D of the three-way catalytic converter 3 only assume values up to 750 ° C. due to the significantly lower inlet temperature TA the temperatures TK, TL, TM in the positions K, L, M of the NO _x adsorber 6 adjust to temperature values below 550 ° C. Desulphurization in the partial load range does not take place.

In FIGS. 3 to 6, the heating of the present invention is to Desulfatisie annealing temperature of an exhaust system shown in Fig. 1 schematically represents Darge. To heat up the NO _x adsorber 6 , a short cyclic λ variation takes place, as is shown by way of example in FIG. 3. For this purpose, the engine is operated cyclically rich or lean to heat up after specified driving cycles, for example 5,000 or 10,000 km, after which desulfurization is desired. The time period for heating should be minimized as far as possible. For example, it is between 20 seconds and 2 minutes, depending on the load and starting temperature of the NO _x adsorber 6 . During the period of the rich phase Δt _f , the motor is operated with λ, for which the following applies: 0.8 ≧ λ ≧ 0.7, preferably 0.76 ≧ λ ≧ 0.74, for example 0.75. In the short period of time Δt _m of lean operation with 3 ≧ λ ≧ 1.1, as much O ₂ as possible is introduced into the three-way catalytic converter 3 and into the NO _x adsorber 6 . The times in which the engine is to be operated rich or lean are stored in a map over the gas inlet temperature, the engine air mass and the λ values for the rich or lean operation.

As shown in Fig. 3, a cyclic loading and unloading of the oxygen storage takes place in the three-way catalyst 3 and in the NO _x adsorber 6 according to the λ variation, the loading of the three-way catalyst 3 at the beginning the lean phase (λ <1) begins and the discharge of the three-way catalyst 3 begins at the beginning of the rich phase. The loading and unloading of the NO _x adsorber 6 is out of phase with the cycle of the three-way catalyst 3 .

Fig. 4 shows the temperature curve in the three-way catalyst 3 and in the NO _x - adsorber 6 and the CO emissions before (CO A) and after (CO OFF) h of the exhaust system during partial load operation with a speed of 120 km /. The fat-lean cycle is selected such that the fat phase in this example is Δt _f = 1.5 seconds and the lean phase Δt _m = 0.5 seconds. As can be clearly seen for the positions B, C, D of the three-way catalyst 3 and for the positions K, L, M of the NO _x adsorber 6 from FIG. 1 in FIG. 4, the temperatures associated with these positions increase TB, TC, TD, TK, TL, TM in the fat phases. The exothermic combustion in the NO _x adsorber 6 thus leads, after a short time, to a rise in temperature, including the temperatures TK, TL, TM, in a range above 650 ° C. due to the exothermic combustion using the stored oxygen in the three-way catalytic converter 3 and in the NO _x adsorber 6 . In the example shown, about 50 percent of the exothermic combustion components of CO and HC are shifted from the three-way catalytic converter 3 into the NO _x adsorber 6 during heating. In the lean phases, the oxygen stores of the three-way catalyst 3 and the NO _x adsorber 6 are filled with O ₂ . Thus, Fig. 3 is effected by the λ-variation according to that part of the residual calorific value in the rich exhaust-way catalyst Three 3, but in the NO _x adsorber is not converted into heat in 6.

The change over time of the stored oxygen during the rich-lean cycle in the three-way catalytic converter 3 and in the NO _x adsorber 6 is for a section between the times t = 45 seconds and t = 50 seconds from FIG. 4 in Fig. 6 shown enlarged. In the lean phases, for example between the times t = 45.3 to t = 46.0 seconds, the oxygen stores of the three-way catalyst 3 and the NO _x adsorber 6 are charged, starting from the Position B via position C and position D of the three-way catalytic converter 3 via positions K, L, M of the NO _x adsorber 6 from FIG. 1, the charging is delayed. In the fat phases, for example between times t = 45.3 and t = 46.0 seconds, the oxygen stores are emptied in the same order. Cases are conceivable in which a complete emptying of the oxygen storage device of the NO _x adsorber 6 leads to high tailpipe emissions and at the same time the rear part of the NO _x adsorber 6 could be overheated. The oxygen store of the NO _x adsorber 6 is therefore not completely emptied - insofar as this danger exists - but, for example, only 30 percent.

As can be seen in FIG. 4, the three-way catalytic converter 3 is heated to temperatures between approximately 750 ° C. and 890 ° C. in this load case and the NO _x adsorber 6 to temperatures between 650 ° C. and 700 ° C. , so that desulfation can be carried out safely. The desulfation he follows in a known, not shown manner.

As can be seen in FIG. 4, the CO content increases slightly at the end of a fat phase behind the NO _x adsorber 6 . This sign of the fat breakthrough by the NO _x adsorber 6 is determined by the lambda sensor 11 behind the NO _x adsorber 6 and the lean phase is initiated directly when the predetermined threshold value is reached. A break through of the lean phase by the NO _x adsorber 6 is also detected by the lambda probe 9 . When a predetermined threshold value is reached, the fat phase is initiated directly.

Fig. 5 shows the qualitative course of temperature and exhaust gas emission profile at the example of CH over the length of the exhaust system upon reaching the desulfation temperature in the NO _x adsorber. 6

Even if the examples shown refer to the exhaust system of an Otto relate to motors, the method according to the invention is the same for others Engines with similar exhaust gas problems, where the requirement to open heating to initiate detoxification, can be used. example the method is also wise in an exhaust system of diesel engines settable.  

LIST OF REFERENCE NUMBERS

1

internal combustion engine

2

exhaust pipe

3

Three-way catalytic converter

4

exhaust pipe

5

exhaust gas cooler

6

NO _x

adsorber

7

exhaust pipe

8th

Broadband lambda probe

9

lambda probe

10

temperature sensor

11

Combined NO _x

- and O ₂

-Sensor

Claims (8)

1. A method for heating a downstream catalyst in an exhaust system of an internal combustion engine with a plurality of catalysts arranged one behind the other in the exhaust gas flow direction, characterized in that the heating of a downstream catalyst ( 6 ) is carried out in that the exothermic reaction from an in Flow direction upstream catalyst ( 3 ) in the nachgeordne th catalyst ( 6 ) is shifted. 2. Method according to the features of claim 1, the shift by λ control with alternating fat Lean operating cycle of the internal combustion engine takes place. 3. The method according to the features of claim 1 or 2, wherein λ for heating is controlled so that the rich operation with λ <1 is maintained longer than the O ₂ stored in the oxygen storage of the upstream catalyst ( 3 ) the pollutants HC and can convert CO so that the conversion takes place at least in part through the O ₂ stored in the oxygen store of the downstream catalyst ( 6 ), and the two oxygen stores are refilled with λ <1 in lean operation. 4. The method according to the features of claim 2 or 3, where λ is regulated for heating so that for λ in the cyclic fat Operation during heating applies: 0.8 ≧ λ ≧ 0.7, preferably 0.76 ≧ λ ≧ 0.74. 5. The method according to the features of one of the preceding claims, wherein the regulation of the rich-lean operating cycle by means of O ₂ sensors - in particular by means of λ probes - takes place, which is arranged downstream of the catalytic converter ( 6 ) arranged downstream Check position of exhaust gas. 6. The method according to the features of claim 5, wherein in the rich operation λ behind the downstream arranged catalyzer ( 6 ) is measured and when falling below a predetermined upper threshold value for λ is switched from rich operation to lean operation. 7. The method according to the features of claim 5 or 6, wherein in lean operation λ is measured downstream of the downstream catalytic converter ( 6 ) and is switched from lean operation to rich operation when a predetermined lower threshold value for λ is exceeded. 8. The method according to the features of one or more of the preceding claims, in which the upstream catalyst ( 3 ) is a three-way catalyst and the downstream catalyst ( 6 ) is a NO _x adsorber, the NO _x adsorber ( 6 ) is heated to the desulfation temperature by shifting the exothermic reaction from the three-way catalyst ( 3 ) into the NO _x adsorber ( 6 ).