DE102011003493B4 - Method and control device for diagnosing a catalytic converter system of an internal combustion engine - Google Patents

Abstract

Method for checking the functionality of a catalytic converter (10) in the exhaust system of an internal combustion engine, wherein the catalytic converter (10) is prepared by introducing a quantity of reducing agent into the catalytic converter (10) and an oxygen storage capacity of the catalytic converter (10) is then determined, and one of the catalytic converter ( 10) the amount of oxygen (16) supplied is continuously increased, characterized in that a dynamic diagnosis (20) of a primary lambda sensor (12) is carried out in advance and a dynamic with which the supplied oxygen flow (16) is increased, a dynamic of the primary lambda sensor (12) is adapted.

Description

State of the art

The invention relates to a method according to the preamble of claim 1 and a control device according to the preamble of claim 6. Such a method and such a control device are in each case from the publication “Otto engine management, Motronic systems”, publisher: Robert Bosch GmbH, 1 . Edition, April 2003, there p. 58, known.

Due to demands from legislators worldwide, exhaust-relevant components of motor vehicles must be monitored during operation of the motor vehicle; the so-called on-board diagnosis (OBD). The exhaust gas-relevant components include, in particular, catalytic converters, which convert, among other things, hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas with oxygen into water and carbon dioxide. It is assumed that there is a correlation between the HC conversion ability and the oxygen storage capacity of the catalyst. The oxygen storage capacity can be determined during operation of the motor vehicle by evaluating signals from oxygen-sensitive exhaust gas sensors and signals from other sensors from which an exhaust gas mass flow can be determined.

The well-known method is based on the direct measurement of oxygen uptake or oxygen storage during the transition from rich to lean mixture. A continuous broadband lambda sensor is installed in front of the catalytic converter, which measures the oxygen content in the exhaust gas. Behind the catalytic converter there is a second lambda sensor that detects the condition of the oxygen storage. Both types of probes are known to those skilled in the art and are described, for example, in the book “Kraftfahrtechnikes Taschenbuch”, Verlag Friedrich Vieweg & Sohn, Verlagsgesellschaft m. b. H., Braunschweig/Wiesbaden, 1999, ISBN 3-528-03876-4, there pp. 522 - 524 explained together with a lambda control.

In a first step of the known method, the oxygen storage is emptied by a reducing exhaust atmosphere. This occurs because the internal combustion engine is operated with a rich air-fuel mixture (lambda < 1). The reducing exhaust atmosphere introduces reducing agents made up of unburned hydrocarbons (HC) and carbon monoxide (CO) into the catalytic converter. The probe signal of the second lambda probe behind the catalytic converter indicates this by a voltage that corresponds to a rich air-fuel mixture after the catalytic converter, for example greater than 650 mV. In the next step, oxygen is introduced into the catalytic converter and the oxygen mass introduced until the oxygen storage overflows is calculated using the air mass flow and the primary lambda sensor signal. The oxygen introduced comes from an oxidizing exhaust atmosphere that is generated by operating the internal combustion engine with a lean mixture (lambda > 1). The overflow of the oxygen storage is due to the drop in the probe voltage behind the catalytic converter to values smaller than e.g. B. 200 mV, corresponding to a lean air-fuel mixture.

The calculated integral value of the oxygen mass indicates the amount of oxygen absorbed, i.e. the oxygen storage capacity, of the catalytic converter. This value must exceed a reference value, otherwise the on-board diagnostics (OBD) system will generate an error message.

Usually, the internal combustion engine is suddenly switched to a lean mixture and the oxygen flow introduced into the catalytic converter is measured with the first lambda sensor in front of the catalytic converter. Dynamically slow lambda sensors, hereinafter also referred to as sluggish, cannot follow this jump from rich to lean mixture and signal a lower oxygen content than is actually present. The lambda control based on the signals from the first lambda sensor then reacts by making the mixture leaner, which results in the actual oxygen content in the exhaust gas overshooting and the mixture becoming significantly leaner. In extreme cases, depending on the controller setting, among other things, the mixture can become leaner until the value measured by the primary lambda sensor corresponds to the specified target value, which leads to a significantly leaner actual mixture.

Because the lambda sensor signal, which in this case is too low, is used to calculate the amount of oxygen stored, the amount of oxygen actually stored by the catalytic converter is greater than that calculated by the OBD system. This leads to the catalytic converter being systematically rated worse and, under certain circumstances, being diagnosed as faulty.

From the publication DE 10 2005 057 957 A1 a method for monitoring an exhaust gas purification system of an internal combustion engine is known. In the exhaust gas purification system, an exhaust gas probe is arranged behind a catalytic converter and is connected to a control device with jump characteristics. To determine the oxygen storage capacity of the catalytic converter, a lambda pilot control is used to specifically switch between a rich and a lean fuel-air mixture. A pilot control error is detected by a changing time of a jump in the signal curve of the exhaust gas probe. The fuel-air mixture is pilot-controlled in such a way that the lambda pilot control takes a flat course in the range λ = 1.

From the publication DE 10 2005 058 524 A1 a method for monitoring an exhaust gas purification system of an internal combustion engine with an exhaust gas probe with jump characteristics arranged in the exhaust gas purification system behind a catalytic converter and connected to a control device is disclosed. To determine the oxygen storage capacity of the catalytic converter, a modeled lambda pilot control is used to specifically switch between a rich and a lean fuel-air mixture. The modeled lambda pre-control is specified in such a way that a lambda value passes through a region of constant increase after a minimum lambda value or a region of constant decrease after a maximum lambda value over time. In the area of constant increase, the lambda value is pre-controlled in a linearly increasing manner in the form of a ramp over time of the modeled lambda pre-control.

From the publication DE 41 124 78 A1 a method for assessing the aging state of a catalytic converter to which the exhaust gas of a lambda-controlled internal combustion engine is supplied is known. The lambda values in front of and behind the catalytic converter are measured and it is examined whether, in the event of a control oscillation of the lambda value in front of the catalytic converter from rich to lean or vice versa, the lambda value behind the catalytic converter shows a corresponding transition, and then, if this is the case, the catalytic converter gas mass flow flowing through is determined. The time integral of the product of the gas mass flow and the lambda value in front of the catalytic converter and the time integral of the product of the gas mass flow and the lambda value behind the catalytic converter are calculated. Either the difference between the two integrals or the quotient of the two integrals or the quotient of the difference and one of the two integrals are used as a measure of the aging state of the catalyst.

From the publication DE 100 64 665 A1 It is known that in order to avoid vehicle jerks, the transition between different operating modes of an internal combustion engine should not be carried out suddenly, but rather the air ratio should be changed using a ramp function,

Disclosure of the invention

The problem on which the invention is based is solved by a method according to claim 1 and by a control device according to the independent claim 6. Advantageous further developments are specified in subclaims. Features important for the invention can also be found in the following description and in the drawings, whereby the features can be important for the invention both alone and in different combinations, without this being explicitly pointed out again.

An advantage of the invention is that due to the slow, successive increase, instead of a sudden increase, of the oxygen content in the exhaust gas, dynamically slow lambda sensors can also follow the increase, so that the oxygen component flow measured with the aid of the first lambda sensor also largely corresponds to the actual oxygen flow . Firstly, this avoids overshooting of the lambda control. Secondly, this makes it possible to reliably carry out the legally required catalytic converter diagnosis correctly, even with sluggish lambda sensors. It is intended to carry out a dynamic diagnosis of the primary or first lambda sensor in advance. If the dynamics of the primary lambda sensor are determined using a known method before carrying out the method according to the invention, lambda sensors can also be used which generate an error message using known methods by adapting the lambda change from rich to lean air-fuel mixture to the dynamics of the lambda sensor . This makes the overall system more reliable. In addition, the requirements for the dynamics of the first lambda probe can be reduced. This prevents OBD error messages that are not due to a faulty catalytic converter, but rather to a sluggish first lambda sensor.

It is particularly helpful if the supplied oxygen flow is increased linearly, exponentially or sinusoidally if the first lambda probe is relatively sluggish. By selecting different lambda increases, the speed at which the supplied oxygen flow is increased can be individually adapted to the probe dynamics.

Therefore, one embodiment of the method according to the invention also provides that the oxygen flow supplied is increased as quickly as possible. In the limit case, that is, if the previously carried out dynamic diagnosis of the first lambda probe shows no abnormalities, the lambda change from low to high supplied oxygen flow occurs suddenly.

Brief description of the drawings

• 1 das technische Umfeld der Erfindung;
• 2 das Regelverhalten mit einer trägen Lambda-Sonde und OBD nach dem Stand der Technik,
• 3 das Regelverhalten mit einer trägen Lambda-Sonde und einer erfindungsgemäßen OBD und
• 4 ein Flussdiagramm als Ausführungsbeispiel eines erfindungsgemäßen Verfahrens;

Exemplary embodiments of the invention are shown in the drawings and are explained in more detail in the following description. Show it, each in schematic form:

• 1 the technical environment of the invention;
• 2 the control behavior with a slow lambda sensor and OBD according to the state of the art,
• 3 the control behavior with a slow lambda probe and an OBD according to the invention and
• 4 a flowchart as an exemplary embodiment of a method according to the invention;

Embodiments of the invention

1 shows schematically an embodiment of a catalytic converter 10, as it can be arranged, for example, in an exhaust tract of conventional internal combustion engines. The catalyst 10 is upstream - based on an in 1 Flow direction from left to right of the exhaust gas emitted by the internal combustion engine, not shown, is assigned a primary or first lambda sensor 12, which is designed to determine a first lambda value of the exhaust gas supplied to the catalytic converter 10. Furthermore, a secondary or second lambda sensor 14 is assigned downstream to the catalytic converter 10, which is designed to determine a second lambda value.

The second lambda value is used to assess whether rich or lean exhaust gas leaves the catalytic converter 10.

The in 1 Block arrow designated with reference number 16 on the input side of the catalytic converter 10 symbolizes an oxygen stream which is supplied to the catalytic converter 10. Or rather, the arrow 18 denotes an oxygen stream that emerges from it again. At the in 1 In the operating state shown as an example, a larger oxygen flow 16 is supplied to the catalytic converter 10 than is taken from it in the same time; it is therefore an oxygen storage phase in which oxygen is stored in at least one oxygen-storing layer (not shown) of the catalytic converter 10.

The catalytic converter 10 is also assigned a control device 20, which z. B. can evaluate the sensor signals of the lambda sensors 12, 14 and is designed to do so, with reference to below 2 carry out the method described in order to evaluate the oxygen storage capacity of the catalyst 10. The control device 20 can be, for. B. also involves a control device that regulates the operation of the internal combustion engine, the so-called engine control unit.

2 shows in a diagram 22 the relationship between an amount of oxygen measured by the first lambda sensor 12 and the amount of oxygen actually present according to a step function 24 imposed by the control device 20 to check the catalytic converter 28. This step function is currently used in on-board diagnosis (OBD). of the catalyst 10 used.

Both values are plotted as curves 26 and 28 over time t. When the diagnosis of the catalytic converter 10 starts, at a time t ₀ , the control device 20 suddenly switches from a rich mixture (λ < 1) to a lean mixture (λ > 1). This process is shown in diagram 22 by curve 24. A sluggish first lambda sensor 12 cannot follow this jump and therefore signals output values that correspond to curve 26. The control device 20 then reacts with a further leaning of the mixture. The result of this is that the amount of oxygen actually contained in the exhaust gas overshoots, as shown in curve 28.

Because to determine the oxygen storage capacity of the catalytic converter, the area (the integral) under the curve 26 is calculated from a first time t ₁ to a second time t ₂ , but the actual amount of oxygen stored corresponds to the area under the curve 28, resulting in a difference a hatched area 30. The hatched area 30 is therefore a measure of a systematic error in the determination of the oxygen storage capacity of the catalytic converter 10. The size of the hatched area 30 depends on the one hand on the dynamics of the first lambda probe 12 and on the other hand on a controller setting of the control device 20 away. This means that when using a slow first lambda sensor 12, the catalytic converter 10 must have a greater oxygen storage capacity in order to be diagnosed as good.

In order to keep the hatched area 30 as small as possible, it is proposed according to the invention to give the curve 24 a flatter slope, as in 3 shown. The diagram 22 is shown and the curve 24 according to the invention is drawn therein. The curve shape can be linear 24a as a ramp, sinusoidal 24b or exponential 24c. All curves with a slope < ∞ are conceivable. A slow first lambda sensor 12 can also follow a flat slope better, and the values signaled by it to the control device 20 correspond to the predetermined ones. With that must the amount of oxygen cannot be readjusted and the actual amount of oxygen 28 overshoots, as in 2 shown is avoided. The slope of the curve 24 should be adapted to the dynamics of the first lambda sensor 12 and should be as steep as possible in order to keep the time required for the diagnosis as short as possible.

An embodiment of a method according to the invention is described below with reference to 4 explained. This embodiment generally relates to testing the functionality of a catalytic volume in the exhaust system of an internal combustion engine, which is arranged between two lambda sensors 12, 14.

First, in a step 32, the dynamics of the primary lambda sensor 12 are determined using known methods. The diagnosis of the catalytic converter 10 is then started in step 34 by first preconditioning the catalytic converter 10 with a rich mixture, as before. The probe dynamics previously determined in step 32 are evaluated in a query 36. If the probe dynamics are unremarkable, the system branches to the standard, sudden increase in oxygen supply after step 38. If the probe dynamics show abnormalities, in the following step 40 the amount of oxygen is increased in accordance with the determined probe dynamics. In a step 42, the amount of oxygen stored in the catalytic converter 10 is calculated using an already known method.

The principle according to the invention represents a robust method for determining the oxygen storage capacity of a catalyst 10. Using the method according to the invention, the oxygen storage capacity can be precisely quantified, so that e.g. B. aging or damage to the catalyst 10 is detected. Since the principle according to the invention is based directly on lambda values, a robust and precise diagnosis of the oxygen storage capacity is possible.

Claims (7)

Method for checking the functionality of a catalytic converter (10) in the exhaust system of an internal combustion engine, wherein the catalytic converter (10) is prepared by introducing a quantity of reducing agent into the catalytic converter (10) and an oxygen storage capacity of the catalytic converter (10) is then determined, and a method for the catalytic converter (10) is determined. 10) the amount of oxygen (16) supplied is continuously increased, characterized in that a dynamic diagnosis (20) of a primary lambda sensor (12) is carried out in advance and a dynamic with which the supplied oxygen flow (16) is increased, a dynamic of the primary lambda sensor (12) is adapted. Procedure according to Claim 1 , characterized in that the supplied oxygen flow (16) is increased linearly (24a). Method according to one of the preceding claims, characterized in that the supplied oxygen flow (16) is increased exponentially (24c). Method according to one of the preceding claims, characterized in that the supplied oxygen flow (16) is increased sinusoidally (24b). Method according to one of the preceding claims, characterized in that the supplied oxygen flow (16) is increased as quickly as possible. Control device (20) for checking the functionality of a catalytic converter (10) in the exhaust system of an internal combustion engine, the control device (20) being set up to first prepare the catalytic converter (10) by introducing a quantity of reducing agent into the catalytic converter (10) and then to absorb oxygen from the catalytic converter (10). To determine the catalyst (10), and the control device (20) is set up to continuously increase a supplied oxygen flow, characterized in that the control device (20) is set up to carry out a dynamic diagnosis (20) of a primary lambda sensor (12) in advance. takes place and a dynamic with which the supplied oxygen flow (16) is increased to adapt to a dynamic of the primary lambda sensor (12). Control unit (20). Claim 6 , characterized in that it is designed to carry out a process according to one of the Claims 1 until 5 to control.

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