GB2345142A

GB2345142A - Method for operating a NOx sensor in the exhaust system of an internal combustion engine

Info

Publication number: GB2345142A
Application number: GB9929609A
Authority: GB
Inventors: Hong Zhang; Juergen Roessler
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1998-12-22
Filing date: 1999-12-16
Publication date: 2000-06-28
Anticipated expiration: 2019-12-16
Also published as: DE19859580C2; FR2787574B1; FR2787574A1; GB9929609D0; DE19859580A1; GB2345142B

Abstract

The sensor has an electrical heating unit for heating the sensing element to a temperature Tsoll, and has an applied oxygen pump voltage Vp0. To ensure that the applied pump voltage Vp0 does not exceed a maximum value above which premature decomposition of NOx would occur, the operating temperature of the sensor is slowly increased in steps over its operating life. The pump voltage Vp0 is compared at VGL with a maximum value Vp0max, and, if that voltage is exceeded, switching unit SE is operated to incrementally increase the temperature value Tsoll by an amount K(Vp0max-Vp0), (K being a correction factor), so that the electrical resistance R given by the ratio of pump voltage Vp0 to oxygen ion pump current Ip0 is reduced.

Description

2345142

Description

Method for operating an exhaust gas sensor in the exhaust system of an internal combustion engine The invention relates to a method for operating an exhaust gas sensor in the exhaust system of an internal combustion engine in accordance with the precharacterising clause of Claim 1.

For cleaning the exhaust gas of an internal combustion engine, a threeway catalytic converter is conventionally arranged in the exhaust gas tract of the internal combustion engine. Upstream of this catalytic converter, a lambda probe is provided, the emitted signal of which is dependent, as in all lambda probes, upon the residual oxygen contained in the exhaust gas. This residual oxygen component in turn depends on the mixture which was introduced into the internal combustion engine. In the case of an excess of fuel (rich mixture), the oxygen component in untreated exhaust gas is lower; in the case of an excess of air (lean mixture), it is higher.

In the operation of the internal combustion engine, the output signal of the lambda probe which reflects the lambda value of the untreated exhaust gas fluctuates around a predetermined mean value which may be assigned lambda = 1. Since a three-way catalytic converter has optimal catalytic properties for untreated exhaust gas with a certain lambda value ko, the predetermined mean value or the value assigned to ko should also correspond to the actual ko. Depending on the catalytic converter, lambda value ko for optimal catalytic effect can deviate slightly from lambda = 1; for example, it can be around lambda = 0.99.

The dynamic and static properties of the lambda probe upstream of the threeway catalytic converter, however, are changed through ageing and contamination. As a result. the position of the signal level corresponding to 2w is shifted. Therefore in the state of the art, downstream of the three-way catalytic converter is positioned an additional lambda probe which is less susceptible to contamination. It serves as a monitor probe for monitoring the catalytic conversion and facilitates fine control of the mixture in that the signal level of the upstream lambda probe assigned to the value ko is corrected such that the lambda value ko most favourable for the conversion can always be maintained. This method is designated pilot or trim regulation. A lambda probe or an NO, sensor can be used as the monitor probe.

To further reduce pollutant emissions of modem internal combustion engines, an NQ, catalytic converter can be provided in addition to the three-way catalytic converter. This NO, catalytic converter can also be integrated into the three-way catalytic converter. For optimal operation of such a catalytic converter, which by way of example can be a storage catalytic converter which in one operating state of the internal combustion engine stores NOx and in another operating state converts the stored NOx, an NO,,-serisitive sensor is present downstream of the NOx catalytic converter. The output signal of this NO, sensor can be used to measure the charge level of the NQ, catalytic converter, and to determine the beginning and end of the regeneration phase.

The use of an NQ, sensor for trim regulation is particularly advantageous in the case where such a sensor is present anyway for regulating an NQ, catalytic converter.

A thick-film sensor, as described for example in the publication by N. Kato et al., "Performance of Thick Film NQ, Sensor on Diesel and Gasoline Engines", 3 Society of Automotive Engineers, Publication 970858, 1997, can be used as the NO,, sensor.

Figure 3 shows a cross-sectional view of such an NO,, sensor 29 in schematic form. It comprises a solid electrolyte 2, which is surrounded by the exhaust gas to be measured and is heated by an electric heating unit 13. The exhaust gas diffuses through a diffusion barrier 3 into a first measuring cell 4. The oxygen content is measured in measuring cell 4 by means of a first Nernst voltage VO between a first electrode 5 and a reference electrode I I exposed to ambient air. Reference electrode I I is arranged in an air channel 12 into which ambient air is admitted through an opening 14. Both electrodes are conventional platinum electrodes. The measured value of first Nernst voltage VO is used for setting a control voltage VpO. Control voltage VpO, also known as pump voltage, drives a first oxygen ion pump stream IpO through solid electrolyte 2 between first electrode 5 and an exterior electrode 6. The regulating intervention of first Nernst voltage VO on control voltage VpO, depicted by a dashed line, results in first oxygen ion pump stream IpO being regulated such that a predetermined first oxygen concentration is present in first measuring cell 4.

The first measuring cell 4 is connected to a second measuring cell 8 via a further diffusion barrier 7. The gas present in measuring cell 4 diffuses through this diffusion barrier 7. The second oxygen concentration in second measuring cell 8 is in turn measured by means of a second Nernst voltage V1 between a second electrode 9, which also is a platinum electrode, and reference electrode I I and is used for regulating a second oxygen ion pump stream lpl. The second oxygen ion pump stream Ipl from second measuring cell 8 passes from second electrode 9 through solid electrolyte 2 to external electrode 6. With the aid of second Nernst voltage V1, it is regulated such that a predetermined slight second oxygen concentration is present in second 4 measuring cell 8. Upon application of voltage V2 between measuring electrode 10 and reference electrode 11, the NO,, which has been not effected by the processes to this point in measuring cells 4 and 8, is now decomposed at measuring electrode 10, which is configured to be catalytically effective, and the released oxygen is pumped through solid electrolyte 2 in a third oxygen ion pump stream lp2 toward reference electrode 11. With sufficiently low residual oxygen content at measuring electrode 10, this third oxygen ion pump stream Ip2 is carried only by oxygen ions which originated from the decomposition of NO, Stream Ip2 is thus a measure of NO, concentration in measuring cell 8 and thus in the exhaust gas to be measured and represents the output signal of the NO,, sensor 29.

As described above, a pump voltage VpO is thus applied to cause an oxygen ion pump stream IpO to flow. The first pump cell is so designed that excessive pump voltages VpO cannot occur, because otherwise NO,, would be decomposed at the first electrode 5 of the first measuring cell 4 and would then no longer be available at the actual measuring electrode 10 in the second measuring cell 8. Too high a pump voltage VpO would thus result in a measurement error. On the other hand, ageing of the sensing element causes an increase in the electrical resistance R=VpO/IpO. Since under identical external conditions the same pump stream IpO must flow for the reference variable VO to be reached, a higher pump voltage VpO must be applied in the case of an aged sensing element. This has the result, however, that NO,, is decomposed at the first electrode 5, causing the above-mentioned measurement error to occur.

This problem does not only apply to the NO,, sensor described, but also to other exhaust gas sensors, where the oxygen concentration in a pump cell is to be reduced without at the same time causing decomposition of other exhaust gas components, for example HC sensors, which are used to measure HC emissions and to determine whether specified HC limits are being maintained.

The object of the invention is therefore to specify a method which prevents measurement errors due to ageing occurring in an exhaust gas sensor of the type described at the outset.

Claims

This object is achieved by the features described in Claim 1.

According to the invention, the working temperature of the sensing element of the NO, sensor is slowly increased over the life of the sensing element. This temperature adjustment lowers the resistance of the sensor element, because the ion conductivity of Zr02 increases with temperature. By increasing the working temperature of the sensing element it is ensured that the pump voltage does not exceed a maximum value. This, on the one hand, prevents measuring errors from arising in the NO,, sensor due to premature NO,, decomposition, and it ensures, on the other hand, that a long operating life of the sensing element is maintained.

Advantageous embodiments of the invention are detailed in the subclaims.

The invention will be described in the following by way of an embodiment with reference to the drawings. In the drawings, Figure I shows a block diagram of an internal combustion engine together with an emission control system, Figure 2 shows a block diagram of an arrangement for temperature adjustment of the NO, sensor, and Figure 3 shows a cross-sectional view in schematic form of a sensor for measuring the NO,, concentration 6 Figure I shows in schematic form an internal combustion engine 20 having an NO,, exhaust gas aftertreatment system in which the method described by the invention is used. The engine may be of the mixture-intake or the directinjection type, and is in particular a lean-buming internal combustion engine (Otto engine at least partially working with excess air, or Diesel engine). Only the components needed for understanding the invention are shown.

The operation of the internal combustion engine 20 is controlled, or regulated, by a control unit 21. A fuel supply system 22, which may, for example, be a fuel injection system, is controlled from the control unit 21 via lines not identified in detail and handles the fuel metering for the internal combustion engine 20, the latter having an intake tract 23 and an exhaust tract 24. In the exhaust tract 24, a preconverter 25 (three-way catalytic converter) is mounted near the internal combustion engine, and an NO,, storage catalytic converter 26 is connected in series with the preconverter 25 in the downstream direction of the gas flow. The three-way catalytic converter 25 has its optimum cleaning effect at a lambda value of ko. The value ?,o can lie between 0.99 and 1 1, depending on the type of converter.

In place of two separate catalytic converters, a single catalytic converter with an NOx-reducing function in addition to the three-way function can be provided.

The sensor arrangement for the exhaust gas aftertreatment system comprises an oxygen sensor 27 upstream of the preconverter 25, a temperature sensor 28 located in the connecting pipe between the preconverter 25 and the NO,, storage catalytic converter 26 at a point near the input of the latter, and a further exhaust gas sensor in the form of an NOx sensor 29 downstream of the 7 NO,, storage catalytic converter 26. The NOx sensor 29 contains an electrical heating unit 13 which is controlled by signals from the control unit 2 1.

Instead of using the temperature sensor 28, which measures the exhaust gas temperature and from whose output signal the temperature of the NO,, storage converter 26 can be calculated by means of a temperature model, it is also possible to measure the temperature of the NO,, storage converter directly.

The calculation or measurement of the temperature of the NO,, storage converter 26 is necessary for optimum control of the system in terms of fuel consumption and exhaust gas emission. Measures for the heating and protection of the catalytic converters are also initiated on the basis of this temperature signal.

The oxygen sensor 27 is preferably a broadband lambda probe, which produces a continuous, e.g. linear, output signal depending on the oxygen content of the exhaust gas. The signal from this broadband lambda probe is used to regulate the air ratio during lean-mixture operation, and during the rich-mixture regeneration phase, in accordance with specified target values. This function is performed by a known lambda controller 30, which is preferably incorporated in the control unit 21 controlling the operation of the internal combustion engine 20.

Electronic control units of this kind, which normally contain a microprocessor and perform a variety of other control and regulation tasks in addition to fuel injection control, such as the control of the exhaust gas aftertreatment system, are generally known and their design and operation is therefore referred to in the following only in the context of the invention. The control unit 21 contains, in particular, a memory 3 1, in which a variety of characteristic curves and data as well as correction factors are stored.

8 The control unit 21 also receives measured data from other sensors not shown in the figure, in particular values for engine speed, load, coolant temperature, etc. The control unit 21 controls and regulates the operation of the internal combustion engine 20 on the basis of these measured values.

The output signal of the NQ, sensor 29 located downstream of the NQ, storage catalytic converter 26 in the exhaust gas tract 24 is used for controlling the regeneration of the NQ, storage catalytic converter 26 and for the adaptation of model characteristics such as the oxygen or the NQ, storage capacity, as well as for determining the state of ageing of the NO, storage catalytic converter.

In addition,, the output signal is used for trim regulation, for example as described in non-prepublished German Patent Application DE 198 52 244. 4 of older priority from the same applicant. For trim regulation, a trim regulator 32 is provided. This can be a free-standing unit or an integral part of the control unit 21, and it receives, among other inputs, the output signal of the NQ, sensor 29 and the oxygen ion pump stream 1p0. The trim regulator 32 can from these signals identify a shift of the,o signal level of the oxygen sensor 27, caused for example by ageing, and compensates for it, thus making sure that the internal combustion engine 20 is controlled by the control unit 21 in such a way that the lambda value of the raw exhaust gas in the exhaust gas tract 24 upstream of the three-way catalytic converter 25 corresponds on average to the desired value ko.

Figure 2 shows in block diagram form an arrangement illustrating how the measuring error caused by ageing can be corrected by temperature adjustment of the NOx sensor 29. The temperature of the sensing element of the NO, sensor 29 is increased in steps slowly over the life of the sensing element. This 9 temperature adjustment reduces the resistance R=Vp0ApO, because the ion conductivity of Zr02 increases with temperature. The aim of the temperature adjustment is to increase the temperature of the sensing element over its life in such a way that the maximum pump voltage in lean operation of the internal combustion engine remains below a specified threshold.

During operation of the internal combustion engine, a comparator VGL checks whether the value of the first regulating voltage VpO is below or above a threshold value VpOmax, above which NO, decomposition would occur. The threshold value VpOmax is determined experimentally for a given NQ, sensor 29 and is stored in the memory 3 1. The output of the comparator VGI, drives a switching unit SE in dependence on the result of the comparison.

If during operation the value of the regulating voltage VpO remains always below or equal to the threshold value VpOmax, the switching element of the switching unit SE stays in the position marked I and the setpoint value of the sensor temperature Tsoll remains unchanged. No change takes place in the heating power for the heating unit 13 in the NO, sensor 29.

In the event of the value of the first regulating voltage VpO being larger than the threshold value VpOmax, however, the switching element in the switching unit SE changes to the position marked 11 and the temperature of the sensing element is increased by a certain amount. For this purpose, the difference is formed between the threshold value VpOmax and the first regulating voltage VpO and is fed to a multiplier MUL. In this multiplier MUL, the difference value VpOmax - VpO is multiplied by a correction factor K determined experimentally and stored in the memory 3 1. The result is a correction value K (VpOmax - VpO), which is fed to an adder stage ADD where the sum is formed of this correction value and the older, i.e. preceding temperature setpoint Tsoll (n-1). The new temperature setpoint Tsoll (n) is thus given by Tsoll (n) = K(VpOmax - VpO) + Tsoll (n-1), where n is a running variable. The setting of the new temperature setpoint Tsoll (n) is performed by increasing the electrical heating power supplied to the heating unit 13.

It should be noted that the NQ, sensor 29, and thus the sensing element, has to work at a certain minimum operating temperature in order to ensure, for example, that the ion conductivity of the Zr02 is sufficiently high. On the other hand, excessive temperatures reduce the operating life of the sensing element. This is the reason why a step-by step increase in temperature as proposed here is better than a permanent setting at a constant, higher operating temperature.

The method according to the invention has here been explained with reference to an NQ, sensor, but it can equally be applied to other exhaust gas sensors, in particular HC sensors located in the exhaust tract of an internal combustion engine for measuring the HC concentration in the exhaust emission.

It is also possible for the signal evaluation (in particular the pumpvoltage-tothreshold comparison and the heating control) not to be performed in the control unit of the internal combustion engine, but instead to integrate the evaluation unit into an "intelligent interface" of the exhaust gas sensor, which is accommodated in a cable between the exhaust gas sensor and the control unit of the internal combustion engine, in particular in a connector housing, as described e.g. in nonprepublished German Patent Application DE 198 072 15.5 from the same applicant.

Claims 1. Method for operating an exhaust gas sensor in the exhaust system of an internal combustion engine (20), with - the exhaust gas sensor (29) containing an electrical heating unit (13) for heating the sensing element of the exhaust gas sensor (29) to a setpoint temperature (Tsoll), said heating unit being connected to a control unit (21), and - a pump voltage (VpO) being applied to the exhaust gas sensor (29) for the purpose of causing an oxygen ion pump stream to flow, characterised in that - the pump voltage (VpO) is compared to a specified threshold value (VpOmax), and if this threshold value (VpOmax) is exceeded, - the value of the setpoint temperature (Tsoll) of the sensing element in the exhaust gas sensor (29) is increased in steps of a certain value over the operating life of the sensing element by means of the electrical heating unit (13), so that the electrical resistance (R), given by the ratio of pump voltage (VpO) to oxygen ion pump stream (IpO), is reduced as a result.
2. Method in accordance with Claim 1, characterised in that - the difference between the threshold value (VpOmax) and the pump voltage (VpO) is formed, - said difference is multiplied by a correction factor (K), and 12 - the new value for the setpoint temperature (Tsoll (n)) is formed by taking the sum of the correction value (Kc(VpOmax - VpO)) obtained above and the previous value of the setpoint temperature (Tsoll (n-1)).
3. Method in accordance with Claim 2, characterised in that the new temperature setpoint Tsoll (n) is determined in accordance with the following relationship:

Tsoll (n) = Ke(VpOmax - VpO) + Tsoll (n-1),
4. Method in accordance with one of the preceding claims, characterised in that the correction factor (K) is determined experimentally and is stored in a memory (3 1) of the control unit (2 1).
5. Method in accordance with Claims 1-4, characterised in that an NO, sensor is used as an exhaust gas sensor.
6. Method in accordance with Claims 1-4, characterised in that an HC sensor is used as an exhaust gas sensor.
7. Method in accordance with Claim 2, characterised in that the setting of the new temperature setpoint Tsoll (n) is carried out by increasing the electrical heating power for the heating unit (13).
8. Method in accordance with Claims 1 and 4, characterised in that the increase in the heating power is controlled via signals from a control unit assigned to the internal combustion engine (20).
9. Method in accordance with Claims 1 and 4, characterised in that the increase in the heating power is controlled via signals from a control unit assigned to an interface of the exhaust gas sensor (29).