EP1180594B1 - Method for testing an exhaust gas recirculation system - Google Patents

Description

The invention relates to a method for checking an exhaust gas recirculation system of an internal combustion engine.

In order to further reduce the fuel consumption of Otto internal combustion engines, more and more internal combustion engines are used, which can be operated except with stoichiometric mixture with lean combustion. In gasoline engines with lean combustion, the excess air is selected to be as large as the load requirement allows the internal combustion engine; When the load demand is low, the fuel / air mixture with which the internal combustion engine is operated can have lambda values of 3 and more in a stratified charge mode.

In order to meet the required exhaust emission limit values special measures are required in such internal combustion engines, since otherwise the emitted NOx masses would be too large. This also applies to diesel internal combustion engines. In addition to the use of NOx storage catalysts, which are due to their coatings able to absorb NOx compounds from the exhaust gas, which arise during lean combustion, in a storage phase and convert in a regeneration phase with the addition of a reducing agent into harmless compounds are also so-called exhaust gas recirculation systems known. In such exhaust gas recirculation systems, part of the exhaust gas stream is added to the stream of fresh gas flowing into the cylinders. Since exhaust gas for combustion is an inert gas, thereby the NOx raw emissions of the internal combustion engine decreases. The control of the recirculated exhaust gas flow, the so-called exhaust gas recirculation rate, usually takes place via an exhaust gas recirculation valve connected in the return line.

Such an exhaust gas recirculation system is an emission-relevant component. Such components are subject to review due to current or future regulations in the operation of an internal combustion engine, as a failure or incorrect operation of the exhaust gas recirculation system could drastically degrade the emission behavior of an internal combustion engine and could lead to exceeding predetermined limits.

A component of an exhaust gas recirculation system which is particularly threatened by failure or, in the case of a failure, particularly leading to emission increases, is the exhaust gas recirculation valve, via which the exhaust gas recirculation rate is set. So far, one uses for the diagnosis of this exhaust gas recirculation valve arranged in a suction pressure sensor, which detects the intake manifold pressure. At the same time, the intake air mass is determined via an air mass meter. From the intake air mass can be calculated at a certain position of the exhaust gas recirculation valve expected intake manifold downstream of a throttle valve of an internal combustion engine. If there is a difference between the measured and calculated intake manifold pressure, a faulty exhaust gas recirculation valve is diagnosed. This principle is described for example in DE 44 06 281 A1.

The document JP 63 263 258 A describes a diagnostic device for an exhaust gas recirculation device of an internal combustion engine. For diagnosis, an exhaust gas recirculation valve is closed. In an intact exhaust gas recirculation, this leads to an increased proportion of fresh air in the exhaust gases, which is measured by means of an O2 sensor. If such a proportion of fresh air is not detected in the exhaust gases, it is concluded that an error has occurred in the exhaust gas recirculation system. With this device can be determined that the exhaust gas recirculation valve does not close properly, an error with respect to a no longer opening exhaust gas recirculation valve can not be determined.

From DE 42 16 044 A1 it is known the function of an exhaust gas recirculation system based on the smoothness of the internal combustion engine to recognize, since the frequencies of dropouts or the rough running of an internal combustion engine increases with the exhaust gas recirculation rate.

It is also known from DE 36 24 441 A1, to change the exhaust gas recirculation rate during idling of an internal combustion engine and to monitor the speed fluctuation of the internal combustion engine. A similar method is also disclosed in EP 0 635 629 A1.

The invention has for its object to provide a method for checking an exhaust gas recirculation system, in which no pressure measurement in the intake manifold is required.

This object is solved by the invention characterized in claim 1.

The invention makes use of the knowledge that changes in the exhaust gas recirculation rate can greatly influence the NOx emissions of an internal combustion engine. If the exhaust gas recirculation rate is changed by a certain amount, it is possible to detect a faulty exhaust gas recirculation system due to a lack of the anticipated change in the NOx emission of the internal combustion engine. This concept is suitable for all equipped with exhaust gas recirculation systems internal combustion engines.

This check is particularly easy to perform if the NOx emission of the engine otherwise would be constant, which is the case especially in static operating conditions of the internal combustion engine, especially if so the change over time of load and / or speed of the internal combustion engine below a certain appropriate limit remains.

In order to detect a lack of an expected NOx concentration in the exhaust gas of the internal combustion engine, various approaches are conceivable. On the one hand, one can form a concentration difference from the NOx concentration measured in the exhaust gas before and after the change of the exhaust gas recirculation rate. Of course, this concentration difference depends on the change made in the exhaust gas recirculation rate. If an expected concentration difference remains despite a change in the exhaust gas recirculation rate, the exhaust gas recirculation system is defective. The NOx concentration can be measured at any point in the exhaust system, in particular upstream of a catalyst.

To determine the concentration difference, one of these NOx concentration measurements can also be determined by a model determination replace the NOx raw emissions, which known models can be used, which estimate from operating parameters of the internal combustion engine, the NOx concentration emitted for this operating condition. With such a model value for the NOx concentration, the concentration difference can be formed together with the measurement of the NOx concentration after the change, and either the model value for the NOx concentration before the EGR rate change or the model value for the operation state after the change of the NOx concentration Use exhaust gas recirculation rate. It is expedient here, however, that otherwise the operating conditions of the internal combustion engine remain largely constant, since then the error in the model determination of the NOx concentration is as low as possible.

If the internal combustion engine has a NOx storage catalytic converter, it is also possible to use a NOx concentration sensor for diagnosis, as is conventionally provided for controlling a NOx storage catalytic converter of this catalytic converter. The same applies to internal combustion engines with a 3-way catalyst in the exhaust system. A known arrangement is, for example, a downstream of the catalyst sensor. Since such a NOx storage catalyst but usually absorbs the NOx compounds in the exhaust gas, it must be ensured in this arrangement to carry out the verification that this absorption does not take place temporarily. This can be achieved in a preferred embodiment of the invention in that the catalyst is saturated before the test up to its maximum storage capacity. The saturation state can be achieved with the NOx concentration sensor arranged downstream, for example by comparing a modeled NOx concentration with a measured NOx concentration or by suitable interpretation of the gradient of the NOx concentration downstream of the NOx storage catalytic converter during a storage process.

If the NOx catalyst is saturated, changes in the NOx concentration upstream of the catalyst to the catalyst outlet so that then a check is possible.

Saturation can be achieved particularly quickly if a high level of raw NOx emission is provided upstream of the NOx storage catalytic converter, for example by setting the exhaust gas recirculation rate below a certain threshold value or particularly preferably close to zero.

In principle, a particularly good diagnosis is obtained if the exhaust gas recirculation rate is set from a maximum value to a minimum value. In order to achieve this in the variant with the accelerated saturation of a NOx storage catalytic converter, if it is determined that the NOx storage catalytic converter is in saturation, first the exhaust gas recirculation rate must be increased from the value lying below the minimum value, then subsequently reduced again can be used to form the concentration difference.

In principle, how the change in the exhaust gas recirculation rate set for checking is carried out in time is irrelevant. If one sets a ramp-like change in the exhaust gas recirculation rate, the diagnosis proceeds with particularly low effects on the operation of the internal combustion engine, since the change in the smooth running of the internal combustion engine, which inevitably accompanies the change in the exhaust gas recirculation rate, proceeds slowly. If you want to make the diagnosis as safe as possible, it makes sense to increase the exhaust gas recirculation rate by leaps and bounds. This approach has the further advantage that the review takes only a very short period of time, so only a very small increase in the emitted NOx mass is obtained by the review.

Advantageous developments of the invention are the subject of the dependent claims.

Figur 1

eine schematische Darstellung einer Brennkraftmaschine mit einer Abgasrückführanlage,

Figur 2

ein Ablaufdiagramm eines Verfahrens zur Überprüfung einer Abgasrückführanlage und

Figur 3

ein weiteres Ablaufdiagramm für ein abgewandeltes Überprüfungsverfahren.

The invention will be explained in more detail below with reference to the drawings in exemplary embodiments. In the drawing shows:

FIG. 1

a schematic representation of an internal combustion engine with an exhaust gas recirculation system,

FIG. 2

a flowchart of a method for checking an exhaust gas recirculation system and

FIG. 3

another flowchart for a modified verification process.

1 shows in the form of a block diagram an Otto internal combustion engine with direct injection, wherein only those components are shown, which are necessary for the understanding of the invention; In particular, the fuel circuit and an exhaust aftertreatment system are not shown.

The internal combustion engine of Figure 1 has an intake manifold 1 with an air collector 2, which opens via a suction pipe 3 in a cylinder 6 of the internal combustion engine. For better clarity, only one cylinder 6 is shown; However, the number of cylinders is not important.

In the cylinder 6 20 fuel is injected via a controlled by a control unit 10 injection valve. In the air collector 2 is a throttle valve 5, which is actuated by a throttle valve actuator 12, which is also controlled by the control unit 10. Further, an air mass meter 4 is provided upstream of the throttle valve 5 in the intake tract 1. Downstream of the throttle valve 5 opens into the intake 1, an exhaust gas recirculation line 8, on the other hand connected to the exhaust gas tract 7 of the internal combustion engine, in which the combustion gases flow from the cylinder 6. In the exhaust gas recirculation line 8 is an exhaust gas recirculation valve 9, which is actuated by an actuator 14, which is controlled by the control unit 10. In this case, a bearing feedback is provided, via which the control unit 10 detects the set on the exhaust gas recirculation valve 9 opening degree.

Furthermore, a crankshaft sensor 13 is provided, which senses the rotational speed of the crankshaft 15.

In the exhaust tract 7, finally, there is still a NOx sensor 16, which measures the NOx concentration in the exhaust gas flowing through the exhaust tract. Any intended in the exhaust system catalysts, NOx storage or 3-way catalysts are not shown for the sake of clarity.

The control unit 10 has a plurality of program modules 11, 17, 19 and 18, which will be discussed later.

To check the exhaust gas recirculation valve 9, the following process, shown as a flow chart in FIG. 2, will now be carried out. In this case, the reference numerals preceded by the letter "S" denote steps of the method.

First, in a step S1, a first NOx concentration NOx1 is measured. This is done with the aid of the measuring module 11 of the control unit 10, which reads the NOx sensor 16. Subsequently, in a step S2 at the exhaust gas recirculation valve 9, a preset in the memory module 17 change in the exhaust gas recirculation rate is set; This is done by the EGR module 18 of the control unit 10. The change is chosen so that the exhaust gas recirculation rate a predetermined jump from high exhaust gas recirculation rate to small exhaust gas recirculation rate, for example, from a high setpoint to 0%, performs. After this change in the exhaust gas recirculation rate, the NOx concentration in the exhaust gas of the internal combustion engine is again measured via the NOx sensor 16 and stored as a value NOx2 in the memory module 17 of the control unit 10. This is again done via the measuring module 11 (step S3). Then, in step S4, it is checked whether the difference between NOx1 and NOx2 exceeds a threshold value SW1 likewise stored in the memory module 17. If this is not the case (N branching), an error of the exhaust gas recirculation system (in particular of the exhaust gas recirculation valve 9) is determined in a step S5. Otherwise (J branch), a proper feedback facility is diagnosed in step S6.

Instead of subtracting the measured NOx concentration in NOx1 and NOx2 in step S4, as the value of NOx1, an NOx concentration determined in a model may also be used. This model determination takes place in the NOx model module 19 of the control unit 10. The NOx model module 19 calculates in a known manner the raw emission of NOx in the exhaust gas of the internal combustion engine. In order for this model calculation to be as accurate as possible, the checking method is performed only when the crankshaft sensor 13 indicates that the speed of the crankshaft 15 and thereby of the internal combustion engine remains within a certain window, preferably constant. The accuracy is further enhanced when, at the same time, the load, i. the indicated by the air mass meter 4, flowing into the engine air mass is constant within certain limits.

The difference formation in step S4 then takes place between the modeled NOx concentration and the measured NOx concentration NOx2 after the change of the exhaust gas recirculation rate in step S2. The step S1 can be omitted in this modification, since no longer the measured NOx concentration NOx1, but a modeled value in step S4 input.

Of course, the stored values (SW1, ...) can also be selected depending on the operating parameters. The memory module 17 then contains suitable maps.

In an internal combustion engine with a NOx storage catalytic converter upstream of the NOx sensor 16, before carrying out the method the steps shown in Figure 3 as a flow chart performed. First, in a step S7, the exhaust gas recirculation rate is set to a value below a threshold, in this case to zero. Then, in step S8, the NOx concentration is measured by means of the NOx sensor 16 and stored in the memory module 17 as the value NOx3. In step S9, it is then checked whether the value NOx3 exceeds a threshold value.

If this is not the case (N branching), a return is made in step S8. Only when the query in step S9 leads to a positive result (J-branch), the NOx storage catalyst is in saturation and it supplied NOx amounts beat to its output. Then, in step S10, the exhaust gas recirculation rate is set to a high value, for example 100%, and the process steps of FIG. 2 follow.

Claims (9)

1. Method for the testing of an exhaust gas recirculation system of an internal combustion engine returning exhaust gas at an exhaust gas recirculation rate from the exhaust line (7) to the intake line (1), in which a specific adjustment of the exhaust gas recirculation rate is set, the NOx concentration in the exhaust gas is measured and in the absence of a specific differential NOx concentration varying as a function of the adjustment of the exhaust gas recirculation rate a defect of the exhaust gas recirculation system is diagnosed.
2. Method according to Claim 1, characterized in that the differential NOx concentration is determined from the NOx concentration measured in the exhaust gas before and after the adjustment of the exhaust gas recirculation rate.
3. Method according to Claim 1, characterized in that the differential NOx concentration is determined from the NOx concentration measured in the exhaust gas after the adjustment of the exhaust gas recirculation rate and an NOx concentration obtained by means of a model from operating parameters of the internal combustion engine for the operating condition of the internal combustion engine prevailing before or after the adjustment of the exhaust gas recirculation rate.
4. Method according to one of the preceding claims, characterized in that the NOx concentration is measured downstream of an NOx storage catalytic converter and the NOx storage catalytic converter is saturated up to its maximum storage capacity before setting the specific adjustment of the exhaust gas recirculation rate.
5. Method according to Claim 4, characterized in that the saturation is detected from a predetermined NOx concentration downstream of the catalytic converter.
6. Method according to Claim 4 or 5, characterized in that for rapid attainment of saturation the exhaust gas recirculation rate is set below a minimum value, preferably close to zero.
7. Method according to one of the preceding claims, characterized in that the specific adjustment of the exhaust gas recirculation rate is a progressive or graduated reduction.
8. Method according to Claims 6 and 7, characterized in that the exhaust gas recirculation rate set below a specific minimum value for the rapid attainment of saturation is increased again and then reduced in the specific adjustment.
9. Method according to one of the preceding claims, characterized in that the test is performed only when operating parameters of the internal combustion engine, especially load and/or speed lie within a certain range and/or have a limited dynamic.

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