DE10062289A1 - Method for diagnosing a NOx sensor in the exhaust tract of an internal combustion engine - Google Patents

Abstract

The NOx sensor has a cross sensitivity to NH¶3¶ and is arranged downstream of a catalytic converter, the catalytic converter producing more NH¶3¶ when the internal combustion engine works with lambda <1 than when it works in lambda = 1 operation. The internal combustion engine operates in lambda = 1 mode for a first period of time. The internal combustion engine is operated with lambda <1 during a second time segment following the first time segment. A defective function of the NOx sensor is diagnosed if the change in the signal (41, 42) of the NOx sensor at the beginning of the second time period is not sufficiently steep and / or the maximum size of the signal of the NOx sensor reached in the second time period is small.

Description

The invention relates to a method for diagnosing a NOx Sensors in the exhaust tract of an internal combustion engine.

For cleaning the exhaust gas of an internal combustion engine is a Catalyst arranged in the exhaust tract of the internal combustion engine.

In internal combustion engines in which fuel is injected directly into the combustion chamber (HPDI internal combustion engines), the fuel consumption can be reduced by burning a lean air / fuel mixture, ie a mixture with excess air or λ <1. This creates high concentrations of NOx compounds. To clean the exhaust gas, the catalytic converter is designed as a NOx storage catalytic converter. The NOx storage catalytic converter stores the NOx compounds that are formed during combustion. This prevents the NOx compounds from being released into the environment. Since the catalytic converter has only a limited storage capacity, regeneration phases are provided in the operating sequence, during which the internal combustion engine is operated with λ <1. The stored NOx compounds are converted into other compounds with the addition of a reducing agent. At the end of the regeneration phase, NH ₃ is formed in the catalyst. In order to determine the start time of a regeneration phase, a NOx sensor is arranged downstream of the catalytic converter. If the NOx sensor detects a NOx concentration above a predetermined value, this means that the storage capacity of the catalyst has been exhausted. Thereupon, an engine control changes the state of the internal combustion engine from λ <1 operation to λ <1 operation.

Except during the regeneration phase, the HPDI Internal combustion engine operated with λ <1 even if that  a torque to be generated by the internal combustion engine agreed maximum threshold or if a destruction The catalyst can be prevented by overheating got to. In certain circumstances, the HPDI Internal combustion engine operated with λ = 1. This is e.g. B. then the case when that to be generated by the internal combustion engine But torque exceeds a certain intermediate threshold is still below the maximum threshold or if the tempe rature of the catalyst is so high that the storage capacity catalyst is lost, but is not as high, that the catalyst can be destroyed.

Internal combustion engines where the fuel is not directly in the combustion chamber, but is injected into an intake manifold (PI Internal combustion engines), usually work in λ = 1- Business. To clean the exhaust gas, a 3- Way catalyst used. So that the cleaning effect of 3-way catalyst is optimal, the lambda value of the raw exhaust gas varies cyclically by λ = 1. To check the Zu composition of the exhaust gas can be downstream of the 3-way A NOx sensor can be arranged. Under certain ten circumstances, the PI internal combustion engine with λ <1 be exaggerated. This is e.g. B. the case when the Brenn Torque to be generated engine a certain smoldering le exceeds or if the catalyst is overheating must be protected.

DE 198 28 929 A1 describes a method for checking the dynamic behavior of a NOx sensor in the exhaust tract of an internal combustion engine. Among other things, the NH ₃ cross sensitivity of the NOx sensor and the fact that NH _{3 is} generated in the catalytic converter at the end of the regeneration phase in an HPDI internal combustion engine are used. A reliable dynamic behavior of the NOx sensor is diagnosed if the change in the signal of the NOx sensor at the end of the regeneration phase does not have a sufficient slope. Since the NH _{3 is} generated in the catalytic converter and is not introduced into the catalytic converter as part of the raw exhaust gas, the change in the NH ₃ signal is not damped by the catalytic converter, so that the change in the NH ₃ concentration is good for a functional NOx sensor is detectable.

However, this method can only be used with HPDI internal combustion engines, since no regeneration phases are provided in other internal combustion engines, at the end of which NH ₃ is formed in the catalyst.

The invention has for its object a method for Diagnosis of a NOx sensor in the exhaust tract of an internal combustion engine machine indicate that both in HPDI internal combustion engines and is also applicable to PI internal combustion engines.

The invention is based on the knowledge that both in HPDI internal combustion engines and in PI internal combustion engines, NH _{3 is formed} in the catalytic converter in λ <1 operation, while NH ₃ and NOx do not or only arise in small amounts in λ = 1 operation.

The object is achieved by a method for diagnosing a NOx sensor in the exhaust tract of an internal combustion engine with the following features: The NOx sensor has a cross sensitivity to NH ₃ and is arranged downstream of a catalytic converter. The catalytic converter is such that it produces more NH ₃ when the internal combustion engine is operating in λ <1 mode than when the internal combustion engine is operating in λ = 1 mode. The internal combustion engine operates in the λ = 1 mode during a first time period. During a second period, which follows the first period, the internal combustion engine operates in the λ <1 mode. A defective function of the NOx sensor is diagnosed if the change in the signal of the NOx sensor at the beginning of the second time period does not have a sufficient slope and / or the maximum size of the signal of the NOx sensor reached is too small in the second time period ,

A PI engine with a 3-way catalytic converter is usually operated in λ = 1 mode. There is no NH ₃ or a negligible amount of NH ₃ in the 3-way catalyst. In λ <1 operation, a significantly larger amount of NH ₃ arises due to the lack of oxygen in the 3-way catalytic converter. The exhaust gas downstream of the 3-way catalytic converter therefore has a significantly higher NH ₃ concentration in λ <1 operation than in λ = 1 operation. This fact is used for diagnosis. For this purpose, the change in the signal of the NOx sensor at the beginning of the second period is considered. If the NOx sensor does not respond quickly enough due to its NH ₃ cross sensitivity to the change in the NH ₃ concentration downstream of the catalytic converter, which must take place during the transition from λ = 1 operation to λ <1 operation, this is so poor behavior of the NOx sensor. If the maximum size of the signal of the NOx sensor reached in the second period is too small, ie the maximum value does not correspond to the value that the NOx sensor would have to display due to the known amount of NH ₃ in the exhaust gas, then the static behavior of the NOx - Sensor defective. The NOx sensor does not correctly indicate the level of the NH ₃ concentration in the second period.

An advantage of the invention is that the method is applied during operation of the internal combustion engine. There are no interferences in the operational sequence of the internal combustion engine machine necessary to carry out the diagnosis. consequently there is no influence on the Ab through the diagnosis gas emissions or fuel consumption. The transition from λ = 1 operation to λ <1 operation is repeatedly caused by the Driver triggered by, for example, pressing the accelerator pedal during a period of slow driving, whereby the torque to be generated by the internal combustion engine ment exceeds a certain threshold.  

The method can also be used with an HPDI Use the internal combustion engine. An HPDI internal combustion engine ar works with λ <1 if it has the required load torque can generate, and the NOx storage catalyst is sufficient appropriate NOx storage capacity. The NOx The storage capacity of the catalyst depends on the temperature. If the temperature is outside one for the storable favorable temperature range, then the HPDI Internal combustion engine switched to λ = 1 operation and the NOx storage catalytic converter is operated as a 3-way catalytic converter ben. If the temperature of the catalyst exceeds a maximum temperature is then the same as for a PI internal combustion engine Catalytic converter overheating protection activated. As a consequence, that the internal combustion engine is operated with λ <1 until the temperature of the catalyst is sufficient again has struggled. The same switches to λ = 1 operation or λ <1 operation are triggered when the required Load torque cannot be generated in λ <1 operation but an intermediate threshold or a maximum threshold below. This is how an HPDI Internal combustion engine due to the driving style of the driver and the distance to be traveled again and again to ensure that the HPDI internal combustion engine in a first time period in the λ = 1 operation works and in a second, subsequent Period is operated with λ <1. It is not that required in the operation of the internal combustion engine intervene to carry out the diagnosis.

Since the NH _{3 is formed} in the catalytic converter and does not flow into the catalytic converter as part of the raw exhaust gas, the change in the NH ₃ signal is not damped by the catalytic converter, so that the change in the NH ₃ concentration can be easily detected by a functional NOx sensor ,

A motor controller determines at least one depending on the value ner system variables the operating mode of the internal combustion engine. A first system variable is, for example, the degree with  which the accelerator pedal is operated by the driver. If a Ma ximalgrad exceeded, the engine control recognizes that the torque to be generated by the internal combustion engine is half the maximum threshold, and causes the force proportion of substance in the air / fuel mixture is increased, d. that is the internal combustion engine is operated with λ <1. At a HPDI internal combustion engine differentiates the engine control between between the maximum degree and an intermediate degree. Is the degree with which the accelerator pedal is operated between the maximum degree and the intermediate degree, the engine control causes the Internal combustion engine with λ = 1 works. In this case lies the torque to be generated by the internal combustion engine between an intermediate threshold and the maximum threshold.

A second system variable is preferably provided so that the engine control can recognize when the catalyst before Ü overheating must be protected. The second system variable is for example the temperature in the catalyst. Will one specified maximum temperature exceeded, then the Engine control that there is a risk that the catalyst can be destroyed by overheating, and causes the Internal combustion engine with λ <1 works.

If it is an HPDI internal combustion engine, it can Detect engine control using the second system variable whether the catalyst has sufficient NOx storage capacity has or not. Becomes a predetermined upper intermediate temperature that is less than the maximum temperature steps or a predetermined lower intermediate temperature, which is higher than the minimum temperature, so the engine management recognizes that the catalytic converter is not off has sufficient storage capacity. In this case the engine control that the internal combustion engine in λ = 1- Operation works unless there is a requirement for a λ <1- Operation.  

The temperature of the catalyst can be adjusted using a tempera tursensor can be determined in or upstream of the Kataly sator is arranged. Alternatively, the temperature can calculate can be determined from a temperature model.

An exemplary embodiment of the invention is described below explained in more detail using the figures:

Fig. 1 shows schematically an internal combustion engine, an exhaust tract, a catalyst, an engine control, an accelerator pedal, a temperature sensor, an evaluation unit and a NOx sensor.

Fig. 2 shows the time course of a signal from a first NOx sensor, a signal from a second NOx sensor and the NH ₃ concentration in the gas stream from the catalyst.

In the exemplary embodiment, an internal combustion engine 1 is provided with a combustion chamber and an adjoining exhaust tract 2 . A catalytic converter 3 is arranged in the exhaust tract (see FIG. 1). In the case of an HPDI internal combustion engine, the catalytic converter 3 is designed as a NOx storage catalytic converter. In the case of a PI internal combustion engine, the catalytic converter 3 is designed as a 3-way catalytic converter.

A NOx sensor 4 is arranged downstream of the catalytic converter 3 . The NOx sensor 4 has a cross sensitivity to NH ₃ .

An engine control unit 5 is provided, which detects and evaluates the position of an accelerator pedal 6 and the temperature of the catalytic converter 3 with the aid of a temperature sensor 7 . The temperature sensor 7 can also be omitted if the temperature of the catalyst 3 using other methods, for. B. a tempera ture model can be determined with sufficient accuracy. The engine control 5 is connected to the internal combustion engine 1 in order to control the composition of the air / fuel mixture in the combustion chamber of the internal combustion engine 1 , ie to determine the value of Lamb da. The engine control 5 and the NOx sensor 4 are connected to an evaluation unit 8 . In an HPDI internal combustion engine, the NOx sensor 4 is additionally connected to the engine control 5 . The evaluation unit 8 can be integrated in the engine control 5 .

The engine controller 5 of an HPDI internal combustion engine operates the internal combustion engine 1 with λ <1 if the torque to be generated and the temperature of the catalytic converter 3 are not too high. For this purpose, the corresponding signals and system sizes, such as. B. the position of the accelerator pedal 6 or the NOx concentration downstream of the catalyst 3 , evaluated by the engine control 5 . If the signal from the NOx sensor 4 rises above a minimum value during this operation, the engine controller 5 λ <1 and initiates the regeneration phase. In the regeneration phase, the NOx compounds stored in the catalyst 3 are converted into other compounds.

If the production of the internal combustion engine 1 required a Drehmo mentes, which can not be generated in the λ <1 operation, then switches the motor controller 5, the internal combustion engine 1 in the λ = 1 or in the λ <1 operation in order.

The driver requests torque via the accelerator pedal 6 . Due to this torque request, the internal combustion engine 1 is operated by the engine control 5 in a first time section from t = 0 to t = t1 with λ = 1. At time t = t1, the driver increases the value of the requested torque by changing the accelerator pedal position 6 . Thereupon, the engine control 5 switches the internal combustion engine 1 into the λ <1 mode. Due to the lack of oxygen in the raw exhaust gas, NH ₃ is now formed in the catalyst 3 . This leads to an increase in the NH ₃ concentration K in the gas downstream of the catalyst 3 (see FIG. 2).

The signal of a functional first NOx sensor 41 consequently rises steeply from t = t1 and then remains at a maximum value.

In comparison, a signal from a faulty second NOx sensor 42 rises significantly more slowly and has a lower maximum value than the first NOx sensor 41 .

Based on the gradient of the signal of the NOx sensor 4 and / or on the basis of its maximum value within the second time segment, it can consequently be determined whether the NOx sensor 4 is functional or has errors.

Claims (6)

1. Method for diagnosing a NOx sensor in the exhaust tract of an internal combustion engine
the NOx sensor ( 4 ) having a cross sensitivity to NH ₃ ,
the NOx sensor ( 4 ) being arranged downstream of a catalytic converter ( 3 ),
the catalyst ( 3 ) producing more NH ₃ when the internal combustion engine ( 1 ) is operating in λ <1 mode than when it is operating in λ = 1 mode,
in which the internal combustion engine ( 1 ) operates in the λ = 1 mode during a first period,
in which the internal combustion engine ( 1 ) is operated with λ <1 during a second time segment following the first time segment,
in which a defective function of the NOx sensor ( 4 ) is diagnosed if the change in the signal ( 41 , 42 ) of the NOx sensor ( 4 ) at the beginning of the second period does not have a sufficient slope and / or the maximum size of the Signal ( 41 , 42 ) of the NOx sensor ( 4 ) in the second period is too small. 2. The method of claim 1, wherein an engine control ( 5 ) determines the operating mode of the internal combustion engine ( 1 ) depending on the value of at least one system variable. 3. The method according to claim 2, wherein the internal combustion engine ( 1 ) is operated with λ <1 when the engine control ( 5 ) recognizes on the basis of the value of at least one first system variable that the torque to be generated by the internal combustion engine ( 1 ) is a maximum paint threshold exceeds. 4. The method according to claim 2 or 3, wherein the internal combustion engine ( 1 ) is operated with λ <1 when the engine control ( 5 ) recognizes on the basis of the value of at least one second system variable that the temperature of the catalyst ( 3 ) is a maximum temperature over steps. 5. The method according to claim 3 and 4,
in which the catalytic converter ( 3 ) is designed as a NOx storage catalytic converter,
in which the internal combustion engine ( 1 ) is operated with λ <1 outside of regeneration phases, if the engine control ( 5 ) detects from the value of at least the first system variables that the torque to be generated by the internal combustion engine ( 1 ) is below an intermediate threshold and if the engine controller ( 5 ) recognizes on the basis of the value of at least the second system variable that the catalytic converter ( 3 ) has sufficient NOx storage capacity,
in which the internal combustion engine ( 1 ) is operated in λ = 1 mode,

• a) if the engine control ( 5 ) recognizes on the basis of the value of at least the first system variables that the torque to be generated by the internal combustion engine ( 1 ) lies between the intermediate threshold and below the maximum threshold, and recognizes on the basis of the value of at least the second system variables that the Temperature of the catalyst ( 3 ) is not above the maximum temperature or below the minimum temperature, or
• b) if the engine controller ( 5 ) recognizes on the basis of the value of at least the second system variable that the temperature of the catalytic converter ( 3 ) is between an upper intermediate temperature that is lower than the maximum temperature and the maximum temperature or between a lower intermediate temperature that is higher than the minimum temperature and the minimum temperature, and recognizes from the value of at least the first system variables that the torque to be generated by the internal combustion engine ( 1 ) is not above the maximum threshold.

6. The method according to any one of claims 2 to 4,
in which the catalyst ( 3 ) is designed as a 3-way catalyst,
in which the internal combustion engine ( 1 ) is operated in the λ = 1 mode when the engine control ( 5 ) recognizes on the basis of the value at least the first system variables that the torque to be generated by the internal combustion engine ( 1 ) is below the maximum threshold, and recognizes from the value of at least the second system variable that the temperature of the catalytic converter ( 3 ) is not above the maximum temperature or below the minimum temperature.