Patent Publication Number: US-2017370264-A1

Title: Method and device for controlling and/or monitoring the function of a secondary air supply in an emission control system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to DE 10 2016 211 595.2, filed in the Federal Republic of Germany on Jun. 28, 2016, the content of which is hereby incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for controlling and/or monitoring the function of a secondary air supply in an emission control system of an internal combustion engine, which in its central section includes at least two catalytic converters situated in succession in an exhaust duct, it being possible for additional exhaust gas-purifying components to be situated upstream or downstream from this central section, a lambda probe being situated, with respect to the direction of flow of the exhaust gas, downstream of an engine block and upstream of the first catalytic converter, and an additional exhaust gas probe being situated upstream from an additional catalytic converter, air being introduced between the first and the additional catalytic converter by the secondary air supply for the optimized operation of the additional catalytic converter. The present invention furthermore relates to a device, in particular an engine control unit, for implementing the method. 
     BACKGROUND 
     In today&#39;s engine control systems, lambda probes are used for detecting the concentration of oxygen in the exhaust gas and for the lambda control of the engine. Wide-band lambda probes and two-point lambda probes are used for this purpose. 
     A wide-band lambda probe makes it possible to control the exhaust-gas lambda continuously in a broad lambda range. By a linearization of the probe characteristic, a continuous lambda control is also possible using a more cost-effective two-point lambda probe, even if in a limited lambda range. 
     Compared to a wide-band lambda probe, a two-point lambda probe has a significantly higher accuracy in a narrow range around lambda=1 due to its stepwise probe characteristic. Outside of this narrow range, at rich or lean lambda, the accuracy of a two-point lambda probe is normally less than that of a wide-band lambda probe due to tolerance and aging effects. 
     Normally, for this reason, wide-band lambda probes are used in engine control systems where rich or lean lambda values are to measured accurately or where a measurement in the range around lambda=1 with limited accuracy is sufficient. Two-point lambda probes are used where the exhaust-gas lambda in the range around lambda=1 is to be measured with great accuracy. 
     Typical applications for a wide-band lambda probe are the lambda control upstream from the catalytic converter and the balancing of the oxygen input and discharge in the diagnosis of the catalytic converter. Typical applications of a two-point lambda probe are the very accurate lambda=1 control downstream from the catalytic converter and the detection of the breakthrough of rich or lean exhaust gas in the diagnosis of the catalytic converter. 
     A typical exhaust system of a gasoline system for today&#39;s strict emission and diagnostic requirements, e.g., in so-called “super ultra-low emission vehicles” (SULEV), is made up of a wide-band lambda probe, a first three-way catalytic converter, a two-point lambda probe, and a second non-monitored three-way catalytic converter. 
     Even stricter future emission and diagnostic requirements (e.g., probably China 6) require exhaust systems, in which not only the second catalytic converter is also monitored, but in which also the particulate quantity in the exhaust gas is limited. The second three-way catalytic converter must therefore be combined with a particulate filter or must be replaced by a coated particulate filter, which is also called a four-way catalytic converter. 
     For an optimized operation of the particulate filter, in particular with respect to reaching the operating temperature quickly and with respect to the regeneration, an introduction of secondary air upstream from the particulate filter may become necessary. This also applies to other components of the emission control system (e.g., NCS or SCR catalytic converter). 
     Such a secondary air introduction is known, for example, from DE 102009043087 A1. It describes an internal combustion engine, in particular a diesel engine, having a catalytic converter and a particulate filter in its exhaust system, at least one secondary air line for supplying secondary air into the exhaust system being developed and situated in such a way that the secondary air line discharges into the exhaust system downstream from the catalytic converter and upstream from the particulate filter, at least one secondary air line being developed and situated in such a way that it branches off from a combustion-air system. For regenerating the particulate filter, the quantity of the secondary air introduced into the exhaust system is selected in such a way that a lambda value for the air-fuel ratio in the particulate filter λ DPF  has a value of 1&lt;λ DPF &lt;x, where 1.4≧x≧1, in particular 1.4≧x≧1.05. This ensures that the regeneration occurs reliably, without an exhaust gas temperature in the particulate filter falling below 600° C. 
     From the perspective of an optimized operating strategy of the particulate filter, it seems expedient to use a wide-band lambda probe instead of a two-point lambda probe for controlling the introduced secondary air quantity upstream from the secondary air supply and the particulate filter. 
     In this connection, currently discussed (and in principle also known from DE 102013226083 A1) are exhaust systems with the following arrangement in the direction of flow of the exhaust gas in the exhaust duct behind the engine block: a first wide-band lambda probe, a first catalytic converter (implemented as a three-way catalytic converter), a second wide-band lambda probe, a second catalytic converter including a particulate filter (implemented e.g., as a coated particulate filter), and a two-point lambda probe. 
     The first wide-band lambda probe is used for the lambda control as well as for balancing for the diagnosis of the first catalytic converter. The second wide-band lambda probe is used for tracking control and for breakthrough detection in the diagnosis of the first catalytic converter as well as for the balancing for the diagnosis of the second catalytic converter. In addition, there is the task of controlling the introduction of secondary air. The two-point lambda probe downstream from the second catalytic converter that includes the particulate filter is used for breakthrough detection for diagnosing the second catalytic converter. In this system, however, the second wide-band lambda probe has functional disadvantages vis-à-vis a two-point lambda probe in this installation position. These are on the one hand a lower accuracy of the tracking control in the range around lambda=1 and on the other hand a poorer suitability for detecting lambda breakthroughs for the diagnosis of the first catalytic converter. 
     SUMMARY 
     It is therefore an objective of the present invention to provide a method that makes it possible to use a more cost-effective two-point lambda probe in the direction of flow of the exhaust gas behind the first catalytic converter, instead of a second wide-band lambda probe, in an emission control system having at least two catalytic converters and a particulate filter or a first catalytic converter and a coated particulate filter as a second catalytic converter in combination with a particulate filter and including a secondary air supply downstream from the first catalytic converter. This is to make possible a lambda control to a setpoint value of λ=1 and λ&gt;1 in active secondary air operation in order to be able to operate the particulate filter in optimized fashion. Additionally, using this two-point lambda probe instead of a second wide-band lambda probe is also to allow for a diagnosis of the secondary air pump or the secondary air system. At the same time, a more accurate tracking control in the range around λ=1, a more accurate diagnosis of the first catalytic converter and, if necessary, a sufficiently accurate diagnosis of the second catalytic converter are to be achieved (see also DE 10 2016 21 506.5, filed by the applicant in the Federal Republic of Germany on Jun. 27, 2016, and U.S. patent application Ser. No. 15/634,280, filed by the applicant on Jun. 27, 2017, the contents of each of which are hereby incorporated by reference herein in their entireties). 
     It is also an objective of the present invention to provide a corresponding device for executing the method. 
     The objective with respect to the method is achieved in that downstream from the first catalytic converter and the secondary air supply, a two-point lambda probe is used for secondary air diagnosis and/or for secondary air control and in that for this two-point lambda probe, specific measures are used for compensating tolerance and aging effects. 
     One variant of the method also provides that downstream from the first catalytic converter and the secondary air supply at least one particulate filter is situated or that the particulate filter is part of the additional catalytic converter and that the secondary air diagnosis and/or the secondary air control is/are performed for the particulate filter using the secondary air supply and the two-point lambda probe. 
     This makes it possible to provide a comparatively simple and cost-effective emission control system. Normally, an engine control unit for an exhaust-gas bank does not offer the possibility of using it to operate a second wide-band lambda probe. On the other hand, the possibility of operating a wide-band lambda probe and two two-point lambda probes normally already exists such that no change is required in the hardware of the control units. The use of a two-point lambda probe for the lambda control with secondary air at λ=1 and at lean exhaust gas lambda (λ&gt;1) presupposes, however, that there exists a definite correlation between the probe voltage and the exhaust gas lambda. It is important that this correlation be definite over the entire service life of the probe since otherwise an erroneous lambda value is set, which may result in an unnecessarily slow soot burn-off by an unnecessarily slow heating of the particulate filter or in its being damaged and in increased emissions. This precondition is usually not fulfilled. Instead, the actual probe characteristic may be shifted with respect to a reference probe characteristic by tolerance and aging effects. For this reason, the application of specific measures for compensating for tolerance and aging effects is particularly important. With the aid of a lambda signal corrected in this manner, a lambda control at an active secondary air operation and a diagnosis of a secondary air pump or the secondary air system are made possible with sufficiently high accuracy. 
     The use of the first two-point lambda probe in this installation position for balancing the storage capacity of oxygen or rich gas of the second catalytic converter is described in DE 10 2016 21 506.5. The exhaust system according to the present invention is better able to fulfill the requirements regarding the accuracy of the tracking control and the diagnosis of the first catalytic converter than the exhaust system mentioned at the outset. In the exhaust system of the present invention, the accuracy of the diagnosis of the second catalytic converter is sufficient in order to allow for a reliable distinction between a catalytic converter that is still in good condition and one that is defective. The accuracy is likewise regarded as sufficient for the lambda control with secondary air at lambda≠1 as well as for a diagnosis of the secondary air pump or the secondary air system. The exhaust system of the present invention therefore makes possible the fulfillment of stricter emission and diagnostic requirements in a cost-effective manner. 
     This central section of the emission control system can be part of a more complex emission control system, in which additional exhaust gas-purifying components are installed upstream or downstream from this central section. Thus, for example, the following arrangements are conceivable: (a) wide-band lambda probe/1st catalytic converter/wide-band lambda probe/2nd catalytic converter/two-point lambda probe/3rd catalytic converter/two-point lambda probe; (b) wide-band lambda probe/1st catalytic converter/two-point lambda probe/2nd catalytic converter/NO x  catalyst/two-point lambda probe; or (c) wide-band lambda probe/1st catalytic converter/two-point lambda probe/2nd catalytic converter/wide-band lambda probe/3rd catalytic converter/two-point lambda probe/4th catalytic converter/two-point lambda probe. 
     For the purpose of compensating for tolerance and aging effects, a particularly preferred variant of the method provides for an offset of the probe characteristic of the first two-point lambda probe to be adapted by an adjustment at high excess air when the internal combustion engine is running or is at a standstill. This makes it possible to adapt a two-point lambda probe for the above-mentioned measuring tasks in a particularly cost-effective manner. 
     In order to increase the accuracy of the secondary air diagnosis and/or the secondary air control, there can be a provision that for adapting an offset, a compensation is performed by shifting the lambda-one point of the probe characteristic of the first two-point lambda probe via a tracking control of the second two-point lambda probe. 
     An extended compensation of tolerance and aging effects can be achieved if a correction of a temperature-related shift of the probe characteristic is applied with the aid of an active measurement of the sensor element temperature while the internal combustion engine is running or is at a standstill. 
     Additionally, there can be a provision for applying a correction of the calculation of the lambda value for adapting an offset by taking into account a current exhaust-gas composition and a varying cross sensitivity vis-à-vis different exhaust-gas components. 
     Depending on the required accuracy, there can also be a provision to use previously described corrective measures in combination. 
     A particularly preferred application of the method with its previously described method variants provides for its use for vehicles having at least two successive catalytic converters in an emission control system for adhering to particularly strict exhaust gas guidelines. By applying the method of the present invention with the method variants for compensating the tolerance and aging effects, it is, in particular, possible to reduce the particulate emission significantly since it is possible to operate the particulate filter in an optimized range when introducing secondary air. 
     The objective with respect to the device is achieved in that for the secondary air diagnosis and/or for the secondary air control, a first two-point lambda probe is situated downstream from the first catalytic converter and the secondary air supply, and in that an engine control unit is provided for carrying out the method of the present invention, to which the signals of the various lambda probes are supplied and which includes devices, such as storage and/or comparator units, which in addition to a catalytic converter diagnosis allow for a secondary air diagnosis and/or secondary air control as well as measures for compensating tolerance and aging effects of the first two-point lambda probe in accordance with the previously described method variants. The functionality of these functions can be implemented at least in part on the basis of software, e.g., in the form of a control software, it being possible for the control unit to be provided as a separate unit or as part of a higher-order control unit. 
     To reduce the particulate emission, a particulate filter can be situated downstream from the first catalytic converter and the secondary air supply, or the particulate filter can be implemented as part of the additional catalytic converter. These can be implemented as catalytically coated particulate filters. 
     The present invention is explained in more detail below with reference to an exemplary embodiment depicted in the FIGURE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The FIGURE is a schematic representation of an internal combustion engine having an emission control system by which the method of the present invention may be implemented, according to an example embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The FIGURE shows in a schematic representation an internal combustion engine  1  that includes an engine block  10 , an exhaust duct  20 , a lambda probe  30  that is, with respect to the direction of flow of exhaust gas, downstream of the engine block  10  and that is, in an example embodiment, implemented as a wide-band lambda probe, and a first catalytic converter  40  situated downstream of the lambda probe  30 . Lambda probe  30  makes possible lambda control  100  and is used for a first balancing  110  for diagnosing the storage capacity of the first catalytic converter. 
     The internal combustion engine  1  further includes, downstream from first catalytic converter  40 , a first two-point lambda probe  50  used for catalytic converter diagnosis-tracking control and for breakthrough detection for the diagnosis of first catalytic converter  40 . The internal combustion engine  1  further includes a second catalytic converter  60  downstream of the first two-point lambda probe  50 . The first two-point lambda probe  50  is additionally used for a balancing of the storage capacity of the second catalytic converter  60 . The second catalytic converter  60  can be implemented as a combination block made up of the catalytic converter and the particulate filter or as a coated particulate filter. A separate particulate filter is also conceivable. 
     Downstream of the additional catalytic converter  60 , the internal combustion engine  1  further includes a second two-point lambda probe  70  for breakthrough detection for the catalytic converter diagnosis  140  of the additional catalytic converter  60 . Lambda probes  30 ,  50 , and  70  are connected to an engine control unit  80 , in which on the one hand the lambda control and on the other hand the diagnostic methods regarding the monitoring of the operability of the emission control system are implemented in hardware and/or software. 
     Additionally, a secondary air supply  90  into exhaust duct  20  is provided in this system between first catalytic converter  40  and first two-point lambda probe  50 . In an example embodiment, the first two-point lambda probe  50  is used additionally for the secondary air diagnosis as well as for the air supply control (in the combination: tracking control/secondary air diagnosis  120  and second balancing/secondary air control  130 ). 
     For quicker heating of catalytic converter  60  having the particulate filter function to its operating temperature, an exothermic reaction is brought about in the coated particulate filter at a rich combustion chamber mixture by introducing secondary air via the secondary air supply  90  upstream from the particulate filter. In order to avoid unnecessary emissions, it is important for an average exhaust gas lambda of at least 1 to be maintained upstream from the particulate filter. At a sufficiently high temperature of the particulate filter, a burn-off of the soot charge and thus the regeneration of the particulate filter can be achieved by excess oxygen. For this purpose, a defined lean exhaust gas lambda is to be maintained upstream from the particulate filter. 
     A lambda probe is then suitable as a probe upstream from the coated particulate filter for adjusting the lambda in the event of the introduction of secondary air and for a diagnosis of the secondary air pump or the secondary air system if it has a sufficiently accurate lambda signal in a broad lambda range (typically at least between λ=1 und λ−1.2). A two-point lambda probe  50 ,  70  does not fulfill this requirement without additional measures. 
     This limitation applies also if the two-point lambda probe  50 , as described in DE 10 2016 211 506.5, is to be used as a probe upstream from the catalytic converter for adjusting the lambda value in the rich preconditioning and for adjusting the lean lambda value as well as for balancing the entered oxygen quantity when measuring the oxygen storage capacity of the catalytic converter. Here in particular a sufficiently accurate lambda signal is required in a lambda range of typically between λ=0.95 und λ=1.05. 
     The use of a first (as shown in the FIGURE) two-point lambda probe  50  for the lambda control using secondary air or for its diagnosis presupposes the compensation of tolerance and aging effects, which result in a shift of the actual probe characteristic vis-à-vis the reference probe characteristic stored in the control unit or the engine control unit  80 . Only then will the lambda signal of this first two-point lambda probe  50  fulfill the mentioned requirements regarding accuracy. 
     DE102012211687A1, DE102012211683A1, DE102013216595A1, DE102014210442A1, and DE102012221549A1 describe methods that allow for a compensation of tolerance and/or aging effects that result in such a characteristic curve shift. The methods described there, one or more of which, according to example embodiments of the present invention, are applied to the first two-point lambda probe  50  individually or in combination, include: adaptation of a constant offset of the probe characteristic by an adjustment at high excess air while the engine is running or is at a standstill; compensation of the shift of the lambda-one point of the probe characteristic via a tracking control using the second two-point lambda probe  50 ; compensation of a temperature-related shift of the probe characteristic with the aid of an active measurement of the sensor element temperature while the engine is running or is at a standstill; and taking into account the current exhaust-gas composition and different cross sensitivities of the probe vis-à-vis various exhaust-gas components in the conversion of the probe voltage into a lambda value. 
     In a particularly preferred variant of the method of the present invention, only a constant offset of the probe characteristic is adapted. This is possible to achieve in a comparatively simple manner by an adjustment at high excess air, such as occurs for example at an overrun fuel cutoff, and in many cases already results in a sufficient accuracy of the lambda signal so as to be able to use it for balancing the oxygen input and discharge for diagnosing the catalytic converter  60 . At the same time, this adaptation improves the accuracy of the breakthrough detection in the diagnosis of first catalytic converter  40  and the accuracy of the tracking control  120  with the aid of the first two-point lambda probe  50 . 
     By combining the adaptation of a constant offset of the probe characteristic with one or more of the above-mentioned methods, it is possible to improve the accuracy of the lambda signal of the first two-point lambda probe  50  downstream from first catalytic converter  40  further, in the event that even higher requirements are placed on the accuracy of this lambda signal. 
     Following the compensation, the lambda signal of the first two-point lambda probe  50  is used, as described above, for the lambda control using secondary air, in order to operate the particulate filter in optimized fashion. In addition, a corresponding diagnosis of a secondary air pump or of the secondary air system can be performed. 
     It is possible to apply the method according to the present invention analogously also to emission control systems that include more than two monitored catalytic converters or coated particle filters.