DE19939988A1 - Method for operating a diesel engine - Google Patents

Abstract

The invention relates to a control system for a diesel engine, in the exhaust line of which a storage catalytic converter is arranged, the control system allowing the diesel engine to be operated in a storage mode in which NO x salts and SO x salts are adsorbed in the storage catalytic converter from the exhaust gas of the diesel engine. DOLLAR A In order to regenerate the storage catalytic converter with regard to the NO x salts, the control enables the diesel engine to be operated in a regeneration mode in which a reducing exhaust gas is generated, as a result of which at least the NO x salts are desorbed from the storage catalytic converter. DOLLAR A In order to regenerate the storage catalytic converter also with regard to the SO x salts, the control system according to the invention also enables the diesel engine to be operated in a desalination mode in which a higher temperature is generated in the storage catalytic converter than in the storage mode and regeneration mode and in which an alternating exhaust gas acts and an oxidizing exhaust gas is generated, whereby at least the SO x salts are desorbed from the storage catalyst.

Description

The invention relates to a method for operating a Diesel engine with a storage catalytic converter in its exhaust line is arranged with the features of the preamble of Claim 1.

DE 197 50 226 C1 discloses a method of the type mentioned at the outset, in which a diesel engine, in whose exhaust tract a NO _x storage catalytic converter is arranged, in an overstoichiometric operation (lean operation) and in a substoichiometric operation (rich operation ) can be operated. In the lean operation of the diesel engine, the NO _x salts contained in its exhaust gases are stored in the NO _x storage catalytic converter by adsorption. In this adsorption mode or storage mode, a large part of the nitrogen oxides emitted by the diesel engine can be removed from the exhaust gas. To maintain the storage capacity of the NO _x storage catalytic converter, it is necessary to remove the stored nitrogen oxides from the NO _x storage catalytic converter again. For this purpose, the diesel engine is switched to rich operation for a certain time, in which the unburned diesel fuel acts as a reducing agent for the NO _x salts adsorbed in the NO _x storage catalytic converter, which reduces the nitrogen oxides for desorption from the NO _x storage catalytic converter . During this desorption operation or regeneration operation, the stored nitrogen oxides are removed from the NO _x storage catalytic converter.

In the case of sulfur-containing diesel fuels, during the lean operation of the diesel engine, in addition to the desired adsorption of nitrogen oxides, there is also an undesired adsorption of sulfur oxides (SO _x ) contained in the exhaust gas. These sulfur oxides result from the combustion of sulfur-containing hydrocarbons present in the diesel fuel and are stored in the storage catalytic converter as SO _x salts, in particular as sulfate. However, these SO _x salts (sulfates) are thermodynamically more stable and therefore have a higher chemical binding energy than the NO _x salts (nitrates), with the result that a previously described regeneration process is sufficient to desorb the adsorbed nitrogen oxides, but not is sufficient to desorb the adsorbed sulfur oxides. In this way, the sulfur oxides accumulate in the NO _x storage catalytic converter over time, as a result of which the storage capacity of the NO _x storage catalytic converter for nitrogen oxides gradually decreases. The increasing accumulation of sulfur oxides in the NO _x storage catalytic converter can lead to irreversible damage and is generally referred to as "sulfur poisoning" of the NO _x storage catalytic converter.

The present invention addresses the problem an operating method of the type mentioned in that regard to design that salts, z. B. sulfates, the are thermodynamically more stable than nitrates from which Storage catalyst can be desorbed.

This problem is solved according to the invention by a method with the features of claim 1. Due to the elevated temperature, in conjunction with a reducing exhaust gas, it is possible to use salts with a relatively high chemical binding energy, such as. B. SO _x to desorb from the storage catalyst. The desorbed salt can then be oxidized by the oxidizing exhaust gas, so that on the one hand a renewed adsorption in the storage catalytic converter is prevented and on the other hand the formation of harmful secondary products such as e.g. B. H ₂ S can be avoided. It has been shown that efficient desulfurization of an _NOx storage catalyst can be carried out using the present invention carried out desalination mode so that sulfur poisoning of the _NOx - storage catalyst can be prevented.

This makes a particularly expedient embodiment formed that the diesel engine alternately in the desalination mode superstoichiometric, d. H. with an air-fuel Ratio λ <1, and substoichiometric, i.e. H. with an air Fuel ratio λ <1 is operated. By the The stoichiometric operation becomes oxidizing Exhaust gas generates, while the substoichiometric operation reducing exhaust gas causes.

As an alternative to the aforementioned development, the diesel engine in desalination mode permanently sub-stoichiometric, i.e. with λ <1, operated, then one between the diesel engine and the storage catalytic converter connected to the exhaust system Secondary air supply is switched on and off alternately. When the secondary air supply is switched off, Storage catalyst then that of the diesel engine in the generated substoichiometric operation, reducing effect Exhaust gas available while on Secondary air supply of the supplied oxygen to the exhaust gas Storage catalyst gives the desired oxidizing effect. In particular, this then points into the storage catalytic converter introduced exhaust gas-secondary air mixture superstoichiometric air-fuel ratio.

Has been particularly advantageous in a diesel engine that operated in the storage mode overstoichiometrically with λ values <1 an embodiment is highlighted in which the λ values are always smaller in desalination mode than in storage mode. A lean diesel engine usually works with λ- Values from 1.3 to 10. In contrast, the control according to the invention in desalination mode with λ values <1.3. It preferably works according to the invention  designed diesel engine in desalination mode with λ values < 1.1, in particular ≦ 1.05, e.g. B. λ = 1.03.

For a diesel engine that is in regeneration mode substoichiometric, d. H. operated with λ <1, operated the control system according to the diesel engine Desalination mode so that the λ values are always larger than in Regeneration mode of the diesel engine. For denitration conventional diesel engines work in regeneration mode with λ values from 0.75 to 0.85. In contrast to this actuated the control according to the invention the diesel engine so that this in the desalination mode has λ values that are always greater than 0.85. The one operated according to the invention preferably works Diesel engine in desalination mode with λ values ≧ 0.88, especially with λ ≧ 0.9, e.g. B. λ = 0.97.

If the control according to the invention in the diesel engine Desalination mode is therefore preferably a permanent change of operating phases with about λ = 1.05 and about λ = 0.95. It has been shown that such an operation with λ jumps around the stoichiometric operating point (λ = 1) is particularly advantageous for carrying out a Desalination, especially desulfation.

The temperature in the storage catalytic converter is during the Desalination mode preferably at least 500 to 600 ° C to the To support desalination. The change is preferably made between reducing and oxidizing atmosphere in the Storage catalytic converter with a frequency of approximately 1 to 10 Hz instead of. A temperature increase can be caused, for example, by a late fuel post-injection can be achieved.

It is particularly advantageous if the controller operates the diesel engine only after operation in the regeneration mode in the desalination mode, ie that desorption of salts with a relatively high chemical binding energy, such as. B. SO _x , is only carried out if previously the salts with relatively low chemical binding energy, such as. B. NO _x , have been desorbed from the storage catalyst. This procedure enables a particularly efficient desorption of the salts with a relatively high chemical binding energy.

Other important features and advantages of the invention Procedures result from the subclaims, from the Drawing and from the associated figure description based on the drawing.

It is understood that the above and the features to be explained below not only in the combination given in each case, but also in others Combinations or alone can be used without the To leave the scope of the present invention.

Preferred embodiments of the invention are in the Drawing and are shown in the following Description explained in more detail.

Fig. 1 shows a schematic diagram of an internal combustion engine that can be operated according to the inventive method.

According to Fig. 1 draws an exhaust gas turbocharger to 1 on its compressor inlet side of fresh air indicated by the arrow a. Instead of an exhaust gas turbocharger 1 , another charging device, for. B. a mechanical loader and / or a so-called "booster" can be used. The sucked-in fresh air flows through a heat exchanger 2 , which serves as charge air cooler, at a correspondingly increased pressure, and reaches a throttle point 3 in an intake line 4 of a diesel engine 10 . A throttle valve 5 is arranged in the throttle point 3 and can be actuated by an actuator 7 actuated by an auxiliary force via an actuator 6 . After the throttling point 3 , the fresh air first passes through a suction pipe 16 and then reaches an air collection chamber 8 , from where it is fed to the combustion areas of the diesel engine 10 via separate inlet channels 9 . In the inlet channels 9 , individual throttle valves 11 are arranged, which according to the exemplary embodiment can be actuated by a power actuator 13 via a common actuator 12 .

Downstream of the diesel engine 10 , the exhaust gases formed during the combustion are in an exhaust gas collection chamber 14 with an exhaust gas recirculation line 15 which opens into the intake manifold 16 , ie here after the throttle point 3 and before the air collection chamber 8, in the air intake line 4 . In another embodiment, the exhaust gas recirculation line 15 can also be connected upstream of the throttle point 3 to the intake tract of the diesel engine 10 .

In the mouth area of the exhaust gas recirculation line 15 , an exhaust gas recirculation valve 17 is arranged in the intake manifold 16 , which can be actuated by an auxiliary actuator 19 via an actuator 18 . In the illustrated embodiment, the exhaust gas recirculation line 15 is in heat exchange with a heat exchanger 20 , so that cooling of the recirculated exhaust gas can optionally be achieved.

The turbine inlet cross-section and / or the exhaust gas volume flow flowing through the turbine can be changed with the aid of an actuator 21 , which can be actuated by an auxiliary actuator 22 . After flowing through the turbine of the exhaust gas turbocharger 1 , the exhaust gas is fed in accordance with arrow b to an exhaust gas purification device 28 which is identified in FIG. 1 by a frame shown with broken lines and is described in more detail below.

The diesel engine 10 is controlled or regulated by an engine control or engine regulation 23 , for which purpose this is connected to the corresponding units of the diesel engine 10 via lines. For example, a line 24 is shown in FIG. 1, which connects the engine control system to an injection system 25 of the diesel engine 10 . Additional lines 34 , 35 , 36 and 37 connect the control 23 to the actuators 22 , 13 , 19 and 7 .

The exhaust gas purification device 28 has an adsorber or storage catalytic converter 29 , which is preferably designed as a NO _x storage catalytic converter. Furthermore, the exhaust gas cleaning device 28 comprises an oxidation catalytic converter 30 arranged upstream or downstream of the NO _x storage catalytic converter 29 . The two catalytic converters 29 and 30 are connected to one another by at least one tube 31 which may be heat-insulated and which is, for example, air-gap or mat-insulated. Downstream of the storage catalytic converter 29 , a first λ probe 32 is arranged in the exhaust line of the diesel engine 10 , which is connected to the controller 23 via a corresponding signal line 33 . Furthermore, a first temperature sensor 38 is arranged downstream of the storage catalytic converter 29 and is connected to the controller 23 via a signal line 39 . In addition, a second λ probe 40 and a second temperature sensor 41 are arranged upstream of the storage catalytic converter 29 , which likewise communicate with the controller 23 in a corresponding manner. In addition, further λ probes, not shown here, and temperature sensors can be accommodated in the exhaust line of the diesel engine 10 . In addition, at least one NO _x sensor 42 is provided, which communicates with the exhaust line downstream of the storage catalytic converter 29 and is also connected to the controller 23 .

Furthermore, a secondary air supply 43 can be provided, which introduces fresh air via a supply line 44 connected to the exhaust line downstream of the diesel engine 10 , here downstream of the turbocharger 1 , and upstream of the storage catalytic converter 29 into the exhaust line. The amount of secondary air supplied is adjustable via a controllable supply valve 45 , which is connected to the engine control 23 via a corresponding control line 46 . The secondary air can for example be branched off from the pressure side of the exhaust gas turbocharger 1 . The secondary air can also be made available in another suitable manner.

The storage catalytic converter 29 can be equipped with a heating device 27 , which is symbolized in FIG. 1 by a heating spiral integrated in the storage catalytic converter 29 .

The control according to the invention works as follows:

For normal operation of the diesel engine 10 , the controller 23 actuates the diesel engine 10 so that it is operated in a storage mode in which the diesel engine 10 operates in a stoichiometric manner. In such a lean operation, there is an excess of atmospheric oxygen for the combustion of the diesel fuel, so that λ <1 applies. The diesel engine 10 is operated in its storage mode with λ values from 1.3 to 10, it being possible to change the λ value by varying the amount of fuel injected. In the exhaust gases of the diesel engine 10 are mainly salts with a relatively low chemical binding energy, usually NO _x , and significantly fewer salts with a relatively high chemical binding energy, such as. B. SO _x When flowing through the NO _x storage catalyst 29 , both the NO _x salts and the SO _x salts are adsorbed by the storage catalyst 29 . The storage capacity of the storage catalytic converter 29 decreases over time, so that the storage catalytic converter 29 must be regenerated. The point in time at which such a regeneration has to be carried out can be determined using computing models or, for example, using the NO _x sensor 42 .

To carry out a regeneration, the controller 23 switches the actuation of the diesel engine 10 to a regeneration mode in which the diesel engine 10 works with a substoichiometric ratio of atmospheric oxygen and fuel. In this “rich mode”, the diesel engine 10 can not completely burn the injected fuel, so that the exhaust gas still contains unburned fuel. The diesel engine 10 is operated in its regeneration mode, for example with a λ value of 0.85. The unburned fuel in the exhaust gas serves as a reducing agent, so that the exhaust gas supplied to the storage catalytic converter 29 has a reducing effect. Because of this reducing atmosphere, the nitrates stored in the storage catalytic converter 29 can be desorbed and transported away. The regeneration mode is operated until the nitrates adsorbed in the storage catalytic converter 29 are almost completely desorbed. Since the sulfates have a higher chemical binding energy relative to the nitrates, they are thermodynamically more stable, so that during the denitrification or denitration in the regeneration mode there is virtually no desorption of the SO _x salts. However, the surface of the storage catalytic converter 29 occupied by the sulfates is no longer available for storing the nitrates. Over time, the salts with a relatively high chemical binding energy, that is to say generally the sulfates, accumulate more and more in the storage catalytic converter 29 , as a result of which its NO _x storage capacity decreases more and more. From a certain threshold value, the controller 23 decides that desulfurization or desulfation of the NO _x storage catalytic converter 29 must be carried out.

Before such a desulfation, the controller 23 first causes denitration by switching the operation of the diesel engine 10 to the regeneration mode.

After the denitration has ended, the controller 23 switches the diesel engine operation either directly to a desalination mode or first again to the storage mode and then to the desalination mode. In this desalination mode, the storage catalytic converter 29 is supplied with alternately reducing exhaust gas and oxidizing exhaust gas.

In a first alternative, this alternating change between reduction and oxidation takes place by continuously switching between a lean operation and a rich operation of the diesel engine 10 . It should be noted here that the λ values in fat mode in the desalination mode are always greater than the λ values in fat mode in the regeneration mode. For example, the diesel engine 10 is operated in a fat operating phase of the desalination mode with λ = 0.88 or 0.90 or 0.97. Furthermore, the λ values in the lean mode of the desalination mode are always smaller than the λ values in the lean mode of the storage mode. For example, the diesel engine 10 operates in the lean operating phases of the desalination mode with λ = 1.1 or 1.05 or 1.03. In order to increase the temperature in the storage catalytic converter 29 , the heating coil 27 can be activated, for example. It is also possible to generate a temperature increase in the exhaust system, in particular in the storage catalytic converter 29 , by means of a targeted late post-injection of fuel.

According to a second alternative, the alternating change between oxidizing exhaust gas and reducing exhaust gas can be achieved in that the diesel engine 10 is permanently operated in rich mode in the desalination mode, the secondary air supply 43 being switched on and off alternately. With the secondary air supply 43 switched on , so much atmospheric oxygen is then introduced into the rich exhaust gas that a lean exhaust gas composition results upstream of the storage catalytic converter 29 .

This alternating reducing and oxidizing atmosphere at an overall elevated temperature in the storage catalytic converter 29 makes it possible to reduce the salts in the reduction phases and to oxidize them in the oxidation phases. Harmful secondary emissions, such as B. H ₂ S can be avoided. It is clear that the storage catalytic converter 29 also has an oxidation and reduction function or reducing and oxidizing properties to some extent in order to implement the above-described reduction and oxidation processes.

Claims (10)

1. Control for a diesel engine ( 10 ), in the exhaust line of which a storage catalytic converter ( 29 ) is arranged,
wherein the controller enables operation of the diesel engine ( 10 ) in a storage mode in which from the exhaust gas of the diesel engine ( 10 ) first salts with relatively low chemical binding energy, e.g. B. NO _x , and second salts with relatively high chemical binding energy, for. B. SO _x , adsorbed in the storage catalyst ( 29 ),
wherein the controller ( 23 ) enables operation of the diesel engine ( 10 ) in a regeneration mode in which a reducing exhaust gas is generated, whereby at least the first salts are desorbed from the storage catalytic converter ( 29 ),
characterized by
that the controller ( 23 ) enables operation of the diesel engine ( 10 ) in a desalination mode in which
Storage catalytic converter ( 29 ) generates a higher temperature than in the storage mode and in the regeneration mode and in which a reducing exhaust gas and an oxidizing exhaust gas are alternately generated, as a result of which at least the second salts are desorbed from the storage catalyst ( 29 ). 2. Engine control according to claim 1, characterized in that the diesel engine ( 10 ) is operated in the desalination mode alternately overstoichiometric (λ <1) and substoichiometric (λ <1). 3. Engine control according to claim 1, characterized in that the diesel engine ( 10 ) is operated substoichiometrically (λ <1) in the desalination mode, a secondary air supply ( 43 ) connected between the diesel engine ( 10 ) and storage catalytic converter ( 29 ) being switched on alternately and is turned off. 4. Motor control according to one of claims 1 to 3, characterized in that the diesel engine ( 10 ) in the storage mode is operated stoichiometrically (λ <1) and that the λ values in the desalination mode are always smaller than in the storage mode. 5. Motor control according to one of claims 1 to 4, characterized, that the λ values in desalination mode are always less than 1.3. 6. Engine control according to one of claims 1 to 5, characterized in that the diesel engine ( 10 ) is operated substoichiometrically in the regeneration mode (λ <1) and that the λ values in the desalination mode are always greater than in the regeneration mode. 7. Motor control according to one of claims 1 to 6, characterized, that the λ values in the desalination mode are always greater than 0.85. 8. Motor control according to one of claims 1 to 7, characterized, that in desalination mode with a frequency of about 1 to 10 Hz between the reducing exhaust gases and the oxidizing exhaust gases is switched. 9. Engine control according to one of claims 1 to 8, characterized in that the temperature in the storage catalytic converter ( 29 ) is greater than 500 ° C during the desalination mode. 10. Engine control according to one of claims 1 to 9, characterized in that the diesel engine ( 10 ) is operated only after operation in the regeneration mode in the desalination mode.