DE10310954A1 - Diagnostic procedure for a NOx sensor - Google Patents

Abstract

A method for diagnosing a NOx sensor (15) is proposed. At least one difference (D1-D5) is formed between signal values (NO1-NO5) of the NOx sensor signal (NO) recorded at different times (T1-T6). An error signal (FS) is output if at least one difference (D1-D5) falls below a threshold value (SW1-SW5).

Description

In the DE 198 43 879 A1 describes an operating method of an internal combustion engine, in the exhaust gas area of which a NOx storage catalytic converter is arranged. In a first operating phase, in which the internal combustion engine is operated lean as part of a stratified cylinder charge, the NOx produced is stored in the NOx storage catalytic converter. In a second operating phase, in which the internal combustion engine is operated stoichiometrically or richly as part of a homogeneous cylinder charge, the NOx storage catalytic converter is regenerated. A NOx sensor located behind the NOx storage catalytic converter detects an increasing NOx concentration in the exhaust gas during the first operating phase. A change to the second operating phase is initiated as soon as the NOx concentration exceeds a predetermined threshold. In other exemplary embodiments, there is a change from the first to the second operating phase when the NOx mass flow or the integral of the NOx mass flow in the first operating phase behind the NOx storage catalytic converter exceeds a predetermined threshold value. The NOx mass flow downstream of the NOx storage catalytic converter can be obtained from the signal of the NOx sensor and the exhaust gas mass flow, which can be determined from the measured intake air mass flow.

In the DE 197 39 848 A1 An operating method of an internal combustion engine is also described, in whose exhaust gas area a NOx storage catalytic converter is arranged. A change from the first to the second operating phase is carried out as a function of the NOx mass stored in the NOx storage catalytic converter. The mass is determined from the integral of the NOx mass flow, which in turn is determined from the measured air mass flow or from the known load of the internal combustion engine. If necessary, the speed of the internal combustion engine and / or the exhaust gas lambda and / or the catalytic converter temperature and / or the saturation behavior of the catalytic converter can also be taken into account.

The invention is based on the object Reason to provide a method for diagnosing a NOx sensor that is highly reliable having.

The task is defined in the independent claim specified features solved.

According to the invention it is provided that at least two signal values of the NOx sensor signal different times are considered that at least a difference between the signal values is determined that a Comparison of the difference with a threshold value is made and that an error signal is issued when the difference exceeds the threshold below.

The method according to the invention enables one Diagnosis of the sensor signal emitted by the NOx sensor by monitoring, whether the sensor signal is around an expected at different times Difference changed Has. The diagnosis corresponds to a plausibility check of the NOx sensor signal.

The method according to the invention secures with the Diagnosis compliance with specified exhaust gas limit values by the fact that in the event of a deviation from expected changes in the NOx sensor signal the NOx sensor signal for deciding on one at different times Operating phase change is no longer taken into account. An output The error signal becomes masking or masking, for example of the NOx sensor signal. Furthermore, an invitation to be generated for a service.

Advantageous configurations and Further developments of the method according to the invention result from dependent Claims.

Further training sees the provision of a Masking signal before, preferably at the end of the first operating phase the internal combustion engine begins and in the following first operating phase ends. With this measure can the NOx sensor signal during the second operating phase of the internal combustion engine for the further Signal processing can be blocked because it cannot be ruled out that the NOx sensor signal due to the increased NOx concentration in the exhaust gas during the second phase of operation in saturation goes and therefore no reliable Reflects signal values. The fade-out signal should not be immediate withdrawn at the end of the second operating phase, so that NOx sensor signal can be allowed a recovery time. The fade-out signal therefore advantageously ends only after the beginning of the first Operating phase.

An embodiment of the process provides for the formation of a difference between signal values that occur during the occur in the first operating phase.

A further development of this configuration provides before that the first signal value in the first operating phase to end the blanking signal and the second signal value at the end of the first Operating phase is recorded.

One embodiment provides that the first signal value at the end of the first operating phase and the second Signal value is recorded at the end of the second operating phase.

One embodiment provides that the first signal value at the end of the second operating phase and the second signal value at the end of the fade-out signal is detected.

One embodiment provides that the first signal value at the end of the first operating phase and the second signal value is detected at the end of the fade-out signal.

One embodiment provides that at least one further difference formation is provided, provided in the case of a first difference, the predetermined threshold value was undercut. With this measure, an edition of Error signal after sporadic errors prevented.

Further advantageous configurations and further developments result from further dependent claims and from the following description.

1 shows a technical environment in which a method according to the invention takes place and 2a to 2c show waveforms as a function of time.

1 shows an internal combustion engine 10 which send a speed signal N to an engine control 11 emits. An air quantity sensor or air mass sensor arranged in a suction area 12 gives an air quantity signal or air mass signal LS to the engine control 11 , which continues to be supplied with an accelerator pedal signal PW. In an exhaust gas area of the internal combustion engine 10 is a NOx storage catalytic converter 13 arranged, which is referred to below as Kat. Behind the cat 13 is a lambda sensor 14 arranged the a lambda signal LAM to the engine control 11 emits. Behind the cat 13 is still a NOx sensor 15 arranged that a NOx sensor signal NO to the engine control 11 and to a first to third memory 16 . 17 . 18 emits. In 1 is indicated that the lambda sensor 14 and the NOx sensor 15 can form a structural unit.

The engine control 11 outputs a fuel metering signal KS to the internal combustion engine 10 and an operating phase signal B to a timer 19 as well as to the second and third storage 17 . 18 from. The timer 19 outputs a fade-out signal A to the first memory 16 from. The stores 16 . 17 . 18 pass the stored NOx sensor signal values to a signal evaluator 20 further, which continues to receive threshold values SW1, SW2, SW3, SW4. The signal evaluator 20 outputs an error signal FS.

2a shows the time course of the lambda signal LAM and the NOx sensor signal NO. At a first time T1 there is a first value NO1, at a third time a second value NO2, at a fourth time T4 a third value NO3, at a fifth time T5 a fourth value NO4 and at a sixth time T6 a fifth value NO5 NOx sensor signal NO. At a second time T2 there is a first value LAM1 and at the fifth time T5 there is a second value LAM2 of the lambda signal LAM. A first difference D1 between the second value NO2 and the third value NO3 is entered, a second difference D2 between the third value NO3 and the fourth value NO4, a third difference D3 between the second value NO2 and the fourth value NO4, a fourth difference D4 between the third value NO3 and the fifth value NO5 and a fifth difference D5 between the fourth value NO4 and the fifth value NO5 of the NOx sensor signal NO.

2 B shows the operating phase signal B, the first regeneration phase R1 lying between the first and second times T1, T2, a first lean phase M1 lying between the second times T2 and the fourth time T4, a second regeneration phase lying between the fourth and fifth times T4, T5 R2 and a second lean phase M2 beginning at the fifth time T5.

2c shows that from the timer 16 provided blanking signal A, which occurs between the first and third times T1, T3 and between the fourth and sixth times T6.

The inventive method is based on the in the 2a to 2c The waveforms shown are explained:
The internal combustion engine 10 can according to the above DE 198 43 879 A1 be operated in two different operating phases. The first operating phase is then the lean phase M1, M2 and the second operating phase is the regeneration phase R1, R2. The operating phases are signaled by the operating phase signal B. At the first time T1, when the first value NO1 of the NOx sensor signal NO is reached, a previous lean phase is ended and the first regeneration phase R1 is started. At the same time it becomes the timer 19 started at the first time T1. At the second point in time T2, when the first value LAM1 of the lambda signal LAM is reached, the first regeneration phase R1 is ended and the first lean phase M1 is started.

As an alternative to ending the previous lean phase by reaching the first value NO1 of the NOx sensor signal NO, according to the said DE 197 39 848 A1 the lean phase by a model in Cat 13 stored NOx mass can be ended.

In both documents mentioned embodiments to switch to both the one and the other operating phase described, which should be included in the disclosure.

The method according to the invention can be used to diagnose NOx sensors which are arranged in an exhaust gas area of gasoline or diesel internal combustion engines. In the lean phase of the internal combustion engine, a lift number Lambda> 1, for example of 1.5, is set. In the regeneration phase, the reducing agent required for regeneration can be provided in the engine by reducing the air ratio lambda. The lowering of the lambda air ratio is also possible with diesel internal combustion engines by using a throttle valve. Alternatively, the Reduk tion means are introduced into the exhaust gas after the internal combustion engine.

In some exemplary embodiments, the NOx sensor signal NO is used to determine the operating phase change. The NOx sensor 15 but is not always ready for operation, for example after a cold start of the internal combustion engine 10 can occur when the NOx sensor 15 has not yet reached its operating temperature. There may also be a defect in the NOx sensor 15 occur. In these cases, the operating phases must be changed without including the NOx sensor signal NO, otherwise it could not be ruled out that the proportion of harmful exhaust gas components would increase to an unauthorized degree. Reliable diagnosis of the NOx sensor is therefore essential 15 ,

The change from the first regeneration phase R1 to the first lean phase M1 can take place with the inclusion of the lambda signal LAM. The occurrence of the first value LAM1 of the lambda signal LAM shows a beginning fat breakthrough behind the cat 13 at a fully regenerated cat 13 occurs. With the beginning of the first lean phase M1 at the second time T2, the lambda signal LAM decreases again.

The NOx sensor signal NO decreases comparatively slowly during the first regeneration phase R1 and thereafter. The decrease is due to the real behavior of the NOx sensor 15 determined, which goes into saturation during the first regeneration phase R1 and requires a certain recovery time. During this time, the NOx sensor signal NO reflects an actually non-existing NOx concentration. For this reason, the blanking time A is provided, which ends at the third time T3. The blanking time given with the blanking signal A is to be set to a value, after which the NOx sensor signal NO is again reliable. At this third point in time T3, the second value NO2 of the NOx sensor signal NO is present, which with the negative edge of the blanking signal A is in the first memory 16 is taken over.

In the first lean phase M1, the NOx sensor signal NO increases until the third value NO3 is reached, which signals the end of the first lean phase M1. The end marks the fourth time T4 at which the second regeneration signal R2 begins. At the same time, the blanking signal A occurs. The positive signal edge of the second regeneration signal R2 stores the third value NO3 of the NOx sensor signal NO in the second memory 17 ,

With the presence of the two stored values NO2 and NO3, it is possible to determine the first difference D1, which is in the signal evaluator 20 is made. On the basis of a comparison of the first difference D1 with the first threshold value SW1, the error signal FS is then output when the first threshold value SW1 is undershot.

During the second regeneration phase R2, the NOx sensor signal NO rises to the fourth value NO4, which with the falling edge of the operating phase signal B at the end of the second regeneration phase R2 into the third memory 18 is saved.

With the presence of the further stored value NO4, it is possible to determine both the second and the third difference D2, D3, which are in the signal evaluator 20 is made. On the basis of a comparison of the second difference D2 with the second threshold value SW2 and / or on the basis of a comparison of the third difference D3 with the third threshold value SW3, the error signal FS is output when the value falls below the second and / or the third threshold value SW2, SW3.

During the blanking signal A, the NOx sensor signal NO drops to the fifth value NO5, which again with the falling edge of the blanking signal A into the first memory 16 is saved.

With the presence of the stored fifth value NO5, it is possible to determine the fourth and fifth difference D4, D5, which is also in the signal evaluator 20 is made. On the basis of a comparison of the fourth difference D4 with the fourth threshold value SW4 and / or the fifth difference D5 with the fifth threshold value SW5, the error signal FS is output when the fourth or fifth threshold value SW4, SW5 is undershot.

In principle, only one difference is required from the described differences D1 to D5 and comparisons with the threshold values SW1 to SW5. The multiple difference formation initially increases the detection reliability of a fault of the NOx sensor 13 , Furthermore, it can be provided that, if a deviation is found, at least a subsequent difference formation and a comparison with the associated threshold value is carried out before the error signal FS is output. With this measure, on the one hand, deviations that occasionally occur are hidden. On the other hand, an excessively large deviation of only a difference D1 to D5 from its threshold value SW1 to SW5 that occurs in the long term can be hidden. Such a case can occur if the characteristics of the cat 13 change in the long run. Alternatively, the threshold values SW1 to SW5 can be adjusted.

The functions described are preferably implemented in software that runs in a computer that preferably controls the engine 10 contains.

Claims (9)

Diagnostic procedure for a NOx sensor ( 15 ) behind a NOx storage catalytic converter ( 13 ) is arranged, in which, in a lean phase (M1, M2) of an internal combustion engine ( 10 ) NOx is stored and that in a regeneration phase (R1, R2) of the internal combustion engine ( 10 ) is regenerated, characterized in that at least two sig nal values (NO1-NO5) of the NOx sensor signal (NO) at different times (T1-T6) are considered that at least one difference (D1-D5) between the signal values (NO1-NO5) is determined, that a comparison of the difference ( D1-D5) with a threshold value (SW1-SW5) and that an error signal (FS) is output if the difference (D1-D5) falls below the threshold value (SW1-SW5). A method according to claim 1, characterized in that a fade-out signal (A) is provided, that with the regeneration phase (R1, R2) begins and ends in the lean phase (M1, M2). A method according to claim 1, characterized in that a difference formation (D1, D4) of signal values (NO1, NO2, NO3, NO5) is provided during the lean phase (M1, M2) occur. A method according to claim 2 and 3, characterized in that a signal value (NO2, NO5) in the lean phase (M1, M2) to the end (T3, T6) of the blanking signal (A) and a signal value (NO1, NO3) to the end (T1, T4) of the lean phase (M1, M2) is detected. A method according to claim 2 and 3, characterized in that a signal value (NO1, NO3) at the end (T1, T4) of the lean phase (M1, M2) and a signal value (NO2, NO5) at the end (T3, T6) of the fade-out signal (A) is detected. A method according to claim 6, characterized in that a signal value (NO1, NO3) at the end (T1, T4) of the lean phase (M1, M2) and a signal value (NO4) at the end (T2, T5) of the regeneration phase (R1, R2) is recorded. A method according to claim 1 and 2, characterized in that a signal value (NO4) at the end (T2, T5) of the regeneration phase (R1, R2) and a signal value (NO2, NO5) at the end (T3, T6) of the fade-out signal (A) is detected. A method according to claim 1 and 2, characterized in that a signal value (NO1, NO3) at the end (T1, T4) of the lean phase (M1, M2) and a signal value (NO2, NO5) at the end (T3, T6) of the fade-out signal (A) is detected. Method according to one of the preceding claims, characterized characterized in that at least two differences (D1-D5) are formed and compared to their threshold values (SW1-SW5), and that the error signal (FS) is only output when all Threshold values (SW1 – SW5) undershot are.