GB2344772A

GB2344772A - Method to desulphurise a NOx trap

Info

Publication number: GB2344772A
Application number: GB9925996A
Authority: GB
Inventors: Willibald Schuerz; Hong Zhang; Corinna Pfleger
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1998-11-10
Filing date: 1999-11-04
Publication date: 2000-06-21
Anticipated expiration: 2019-11-04
Also published as: DE19851843B4; DE19851843A1; FR2785555A1; FR2785555B1; GB9925996D0; GB2344772B

Abstract

A method for removing sulphur species from a NO<SB>x</SB> storage catalytic converter ('NO<SB>x</SB> trap') for an internal combustion engine which has been poisoned by SO<SB>x</SB>. Regulator 221 switches the engine to a rich burn mode to effect desulphurisation of the NO<SB>x</SB> trap 18 in response to signals from a binary oxygen sensor 21 downstream of the NO<SB>x</SB> trap. In ordinary use the NO<SB>x</SB> trap is subject to known alternating lean burn (absorption) and rich burn (reduction/desorption) phases. Sulphur species absorbed by the trap are more thermally stable and require a higher temperature regime to be reduced/desorbed. By using an O Þ sensor to regulate the rich burn phase, the production of H Þ S (caused by excess H Þ ) is minimised.

Description

2344772

Description

Method for sulphate regeneration of a NOx storage reduction catalytic converter for a lean internal combustion engine.

The invention relates to a method for sulphate regeneration of a NOx storage reduction catalytic converter for a lean internal combustion engine in accordance with the characteristics of Claim 1.

In order to further reduce the fuel consumption of motor vehicles powered by an internal combustion engine, increasingly frequent use is made of internal combustion engines which are operated with a lean mixture in selected operating states.

In order to meet exhaust gas limit value requirements, special exhaust gas secondary treatment is needed for lean internal combustion engines of this type. NOx storage reduction catalytic converters, referred to simply as NOx storage catalytic converters in the following, are used for this purpose.

NOx storage catalytic converter technology uses the ability of various basic oxides, such as for instance the oxides of the alkali, earth alkali or rare earth metals, to store N02 in super-stoichiometric exhaust gas through the formation of nitrates and to then emit these again under reduced exhaust gas conditions (= rich exhaust gas). As a result of the catalytic activity the released N02 reacts with the reducing agent CO and HC contained in the rich exhaust gas to form the harmless exhaust gas components C02, N2 and H20.

During lean operation of the internal combustion engine an accumulation of sulphate also occurs in addition to the storage of nitrate due to the sulphur contained in the fuel. The basic oxides used as NOx storage components exhibit a strong tendency to form thermally highly stable sulphates. Depending on the sulphur content of the fuel this results in a reduction in NOx storage 2 capacity and storage efficiency, since the sulphates fornled do not decompose due to the regeneration phase for nitrate. In order to ensure that the NOx conversion rate of the NOx storage catalytic converter remains sufficiently high, it is necessary, when a particular quantity of stored sulphate is exceeded, to implement a special sulphate regeneration operation. The sulphate regeneration takes plans analogously to the nitrate r generation, but at a significantly higher temperature level.

DE 197 05 335 Cl describes a method for triggering aisulphate regeneration phase for a storage catalytic converter, in which at prescribed points in time a sulphate regeneration phase is performed. Besides the quantity of the stored c sulphate, the thermal ageing of the storage catalytic converter is also taken into account when triggering the sulphate regeneration. To pe rm the regeneration the storage catalytic converter is heated to a temperature over 60WC and the internal combustion engine is operated with an air ratio k slightly smaller than The object of the invention is to specify a method for sulphate regeneration of a NOx storage catalytic converter for an internal combusti:)n engine, with which the formation of hydrogen sulphide during the regene tion phase is largely avoided and the consumption of regeneration agent is ke t at a low level.

Ilie problem of hydrogen sulphide formation results in the main from an excess of regeneration agent, which besides HC and CO also contains hydrogen. An ideal sulphate regeneration strategy must therefore measure out the quantity of regeneration agent provided in an Apropriate form. This problem is resolved by using a strategy similar to he two-point lambda regulator, based on the signal of a step probe downstream of the NOx storage catalytic converter.

3 The invention therefore comprises the combination of a sulphate regeneration strategy through measures to achieve a NOx storage catalytic converter temperature necessary for desuphatisation using the strategy of a two-point lambda regulator, based on a binary oxygen concentration signal. The parameters (proportional and integral) of the two-point lambda regulator, and the threshold values for switching from lean to rich and vice versa are selected in such a way that the quantity of regeneration agent is supplied in the desired dosage.

The result of such regulation is that a modulation of the air ratio value is obtained upstream of the NOx storage catalytic converter and the frequency and amplitude of this air ratio fluctuation exhibits a fixed connection with the parameters referred to. The parameters thus make it possible to display the air ratio modulation in an appropriate form, i.e. with the correct frequency, amplitude and lambda regulator average position, with the effect that the formation of hydrogen sulphide is largely avoided.

This method thereby guarantees optimum consumption and emissions in sulphate regeneration of the NOx storage catalytic converter.

The invention is explained in greater detail on the basis of the figures. These show the following:

Figure 1: shows a schematic representation of a lean storage catalytic converter, Figure 2: The time progression of the output signal of the binary lambda probe located downstream of the NOx storage catalytic converter, and Figure 3: The output variable of the two-point lambda regulator.

4 In the form of a block diagram, Fig. 1 illustrates a lean internal combustion engine with a NOx exhaust gas secondary treatment system in which the method according to the invention is applied. The diagram shows only the components which are required to understand the invention.

An air/fuel mixture is fed to the internal combustion engine 10 by way of an induction port 11. Viewed in the direction of flow of the intake air, the induction port 11 contains, in sequence, an air mass me r 12. a throttle-valve block 13 with a throttle valve 14 and a throttle-valve sensor (not shown) for determining the angle of opening of the throttle valve 14 and, according to the number of cylinders, a set of injection valves 15, of whiffi only one is shown. The method according to the invention can, however also be used in an internal combustion engine in which the fuel is injected directly into the cylinders in question (direct injection).

On the outlet side, the internal combustion engine 1) is connected to an exhaust gas port 16. This exhaust gas port 16 contains an exhaust gas secondary treatment system for lean exhaust gas. This system consists of a three-way catalytic converter 17 (pre-catalytic converte,,-) located close to the internal combustion engine 10, and a NOx storage reducl ion catalytic converter 18 which is located after the pre-catalytic converter 17 in the exhaust gas flow,direction.

The sensor system for the exhaust gas secondary treatMent system consists of an oxygen sensor 19 upstream of the pre-catalytic converter 17, a temperature sensor 20 in the connecting pipe between pre-catalytic onverter 17 and NOx storage catalytic converter 18 close to the inlet area of the latter, and a further oxygen sensor 21 downstream of the NOx storage catalytic converter 18. Instead of the temperature sensor 20, which measires the exhaust gas temperature and whose signal can be used to calculate the temperature of the NOx storage catalytic converter on the basis of a temperature model, it is also possible to measure the NOx storage catalytic converter temperature directly. In Fig. 1, a temperature sensor 201 of this type which directly measures the monolithic temperature of the NOx storage catalytic converter 18 has been drawn with a broken line.

The temperature of the NOx storage catalytic converter 18 can further be determined by using an exhaust gas temperature model, with the aid of which the temperature of the NOx storage catalytic converter 18 is modelled on the basis of input variables that directly or indirectly affect the temperature of the exhaust gas (such as rotational speed, load, ignition angle, air ratio, exhaust gas feedback, intake air temperature, coolant temperature of the internal combustion engine). In this way it is possible to dispense with the use of the temperature sensor 20.

By preference, a broadband lambda probe is used for the oxygen sensor 19 which provides a steady, e.g. linear output signal depending on the oxygen content in the exhaust gas. The signal provided by this broadband lambda probe 19 serves to control the air ratio during lean operation and during the regeneration phase with a rich mixture in accordance with the predetermined desired values. This function is handled by an already familiar lambda regulation device 22, a lambda regulator 221 exhibiting PI behaviour. The lambda regulation device 22 is by preference integrated into a control unit 23 controlling the operation of the internal combustion engine 10.

Such electronic control facilities which normally contain a microprocessor and which in addition to fuel injection and ignition also handle a large number of other control and regulation functions including control of the exhaust gas secondary treatment system are already known; therefore, their structure and mode of operation are described in the following only to the extent that is 6 relevant to the invention. In particular, the control unit 123 is connected to a memory unit 24 which serves to store information including various characteristic curves and fields, and threshold values.

The output signal from the air mass meter 12 and the signals from the throttlevalve sensor, the oxygen sensors 19, 21, and the temperature sensor 20 are fed to the control unit 23 by way of corTesponding connectin leads.

For controlling and regulating the internal combustion engine 10, apart from being connected to an ignition unit 25 for the air/fuel mi ure, the control unit 23 is also connected by way of a data and control line 26 (shown only in schematic form) to other sensors which are not shown eplicitly, for example for rotational speed and coolant temperature of the internal corn ustion engme, and to other actuators.

For regulating the fuel/air mixture of the internal combustion engine in the optimum lambda window during stoichiometric operation, the signal from the oxygen sensor 21 located as a reference probe downstream of the NOx storage catalytic converter 18 is required. A zirconium oxid Zr02 based binary lambda probe (2-point lambda probe) is preferably used r the oxygen sensor 21; this probe exhibits a step characteristic at a lamb a value of k=l with regard to its output signal. This probe signal from the ambda probe located downstream of the NOx storage catalytic co 18 is also used for controlling the NOx storage regeneration and sulphat6 regeneration and for adapting model variables such as the oxygen storage caacity or NOx storage capacity of the NOx storage catalytic converter 18.

However, other sensors are also suitable for use as an oxygen sensor 21 downstream of the NOx storage catalytic converter IS, or example a NOx 7 sensor that emits a binary signal ftom which a rich or lean composition of exhaust gas can be inferred.

The trigger point for sulphate regeneration is, for example, determined using a known model calculation. If it is found that the NOx storage catalytic converter can store less NOx after NOx regeneration has taken place than the model calculation shows, this is primarily due to the storage of sulphates. The thermally highly stable sulphates can be decomposed at higher temperatures than those in the case of nitrate regeneration with the addition of the same regeneration agent. Thanks to desulphatisation approximately the initial storage capacity for NOx is again obtained.

If desulphatisation is required and if the necessary temperature level has not yet been reached, active heating is used to heat the NOx storage catalytic converter 18 to a temperature of typically over 60WC. This value is primarily dependent on the coating of the monolith of the NOx catalytic converter 18.

Such an additional temperature increase can be achieved using known measures, such as retarding the ignition angle, rich mixture in combination with secondary air injection into the exhaust gas channel or lean mixture in combination with a late in ection start angle in the case of a direct injection j system. If a temperature level necessary for desulphatisation is achieved, which for instance can be detected by evaluating the signal of the temperature sensor 20 and comparing this value with a preset threshold value TS housed in the store 24, the internal combustion engine is operated using a rich air-fuel mixture. Preferably an air ratio will be set which is in the range between),=O, 96-0, 99. A rich mixture is required in order to supply the regeneration agent.

The way in which the air ratio for sulphate regeneration of the NOx storage catalytic converter and thus the quantity of reduction agent can be set so that 8 the formation of hydrogen sulphide is largely avo 1 ded during sulphate regeneration, is explained on the basis of Figures 2 and 3.1 Figure 2 shows the time progression of the output signal, 1 in this case the output voltage UL of the binary lambda probe 21 (Zr02 based step probe) located downstream of the NOx storage catalytic converter 18. The electrical wiring of this binary lambda probe is such that in the event of an excess of oxygen in the exhaust gas (lean operation) the value of the output vol age is lower than the value when there is a deficiency of oxygen (rich operation). However, a reverse arrangement between oxygen concentration and output v% 1 J11tage is also possible. UL_FM designates the threshold value for switching fl.om rich to lean, and UL_MF the threshold value for switching from lean to rich. The maximum output voltage, also designated as rich voltage ULJ, an the minimum output voltage, also designated as UL_M, are also shown.

Figure 3 shows the output variable of the two-point 1 mbda regulator 221. P-POS designates the positive proportion, P_NEG the negative proportion, 1-POS the positive integral and I-NEG the negative inte al.

The parameters that can be influenced when setting the quantity of regeneration agent during the desulphatisation phase are:

Switch thresholds UL_MF, M_FM from lean to ich and from rich to lean respectively Proportion P-POS and P-NEG to predetermine a desired lambda value in the combustion chamber Integral I-POS or I-NEG to predetermine a desired lambda value in the combustion chamber.

9 The desired lambda value can also be adhered to by the output signal of the broadband lambda probe 19 upstream of the precatalytic converter 17 if a difference arises between the desired and actual values of the air ratio.

The setting of the lambda regulator average position across these parameters significantly affects the regeneration agent dosing. A significant influence parameter for setting this average position is the temperature of the NOx storage catalytic converter. The higher the temperature of the storage catalytic converter, the richer the air ratio average value that must be selected to avoid damage to the NOx storage catalytic converter through an excess of oxygen. A further influence parameter for the control parameters is the high-temperature oxygen storage capacity of the NOx storage catalytic converter. The oxygen storage capacity can for example be determined by relating the sensor signals of the two oxygen sensors 19, 21 and performing an operating time examination. This operating time behaviour can be used to make conclusions about the oxygen storage capacity of the NOx storage catalytic converter. The higher this oxygen storage capacity is, the richer the air ratio average value should for instance be set. Depending on the load and the speed these parameters can also be varied. The parameters in characteristic fields KF I -KF6 of the memory unit 24 are stored for this purpose. Because the threshold values UL_MF, UL_FM for the switch from lean to rich and rich to lean can be varied, this provides an additional opportunity for affecting the frequency of the air ratio oscillation.

The opportunity also exists to limit the integral of the PI regulator (such a limit on the negative integral value I-NEG is shown in Figure 3 with a solid line) or to set the integral to zero. This produces a regulator which exhibits only a proportion and thus can no longer adjust a stationary value. In this case the signal of the broadband lambda probe 19 located upstream of the pre-catalytic converter 17 is used as a reference probe, i.e. this probe indicates the lambda regulator average position and the signal UL of the b' 1. lateral probe located downstream of the NOx storage catalytic converter 18 supplies the overriding oscillation.

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Claims

1. Method for sulphate regeneration of a NOx storage catalytic converter for a lean internal combustion engine, in which following the requirement for a sulphate regeneration phase when reaching a temperature level necessary for desulphatisation, the quantity of reduction agent for desulphatisation is regulated by changing parameters (I-NEG, I-POS, P-NEG, P-POS, ULjM, UL_MF) of a two-point lambda regulator (22 1) on the basis of a binary oxygen concentration signal (UL) of a binary oxygen sensor (21) located downstream of the NOx storage catalytic converter (18).

2. Method in accordance with Claim 1, characterised in that in order to regulate the reduction agent quantity at least one of the parameters Proportional (P_POS, P-NEG), Integral Q_POS, I-NEG), Switch Threshold Values (ULjM, UL_MF) is changed from a rich to a lean mixture and vice versa.

3. Method in accordance with Claim 1 or 2, characterised in that by means of the parameters (I-NEG, 1-POS, P-NEG, P_POS, UL_FM, UL_MF) the average value of the air ratio is set, with which the internal combustion engine (10) is operated during the sulphate regeneration phase.

4. Method in accordance with Claim 3, characterised in that the average value of the air ratio is set more in the sub-stoichiometric range the higher the temperature of the NOx storage catalytic converter (18) is.

5. Method in accordance with Claim 3, characterised in that the average value of the air ratio is set more in the substoichiometric range the higher the oxygen storage capacity of the NOx storage catalytic converter (18) is.

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6. Method in accordance with Claim 1 or 2, characterised in that the parameters Q_NEG, I-POS, P-NEG, P-POS, ULjM, UL_NW) are stored in a way which is dependent on the load and the speed of the internal!combustion engine in individual characteristic fields (KFI-KF6) of a memory unit (24) of a control unit.(23) controlling the internal combustion engine (10).

7. Method in accordance with Claim 3, characterised in l[hat in order to adhere to the desired value for the air ratio average value the signal of an oxygen sensor (19) located upstream of the NOx storage cata ic converter (18) is used.

8. Method in accordance with Claim 2, characterised in that the integral (I-POS, I-NEG) is restricted to a predetermined value.

9. Method in accordance with Claim 2, characterised in that the integral (I-POS, I-NEG) is set to zero and the signal of an oxygm sensor (19) located upstream of the NOx storage catalytic converter (18) used as a reference probe and determines the average air ratio value and the signal (UL) of the binary oxygen sensor (21) located downstream of the '.J0x storage catalytic converter supplies the overriding oscillation.