Patent Application
20170044962
The invention concerns a method for determining the conversion efficiency of at least one component of a catalytic converter box comprising at least one SCR catalytic converter and an oxides of nitrogen storage catalytic converter for aftertreatment of the exhaust gases of an internal combustion engine, wherein the catalytic converter box is associated with a NOx sensor at the downstream end.
The invention further concerns a computer program that can be stored in a control unit for controlling and/or regulating an internal combustion engine. The invention also concerns a control unit for controlling and/or regulating the operation of an internal combustion engine.
In order to reduce the NOx emissions, exhaust gas aftertreatment components for converting NOx into harmless emissions are used with internal combustion engines and especially with gasoline engines with lean burn modes and with diesel engines. The components that are currently mainly used are the oxides of nitrogen storage catalytic converter (NOx Storage-Catalyst, NSC) and the SCR catalytic converter (Selective Catalytic Reduction).
The oxides of nitrogen storage catalytic converter stores the oxides of nitrogen (NOx) present in the flow of exhaust gas. When the storage catalytic converter is sufficiently full, the stored oxides of nitrogen are converted into harmless molecules in a special operating mode of the internal combustion engine, which is referred to as the regeneration phase or as regeneration and in which the internal combustion engine is operated with a rich fuel mixture, and are removed from the storage catalytic converter. The NOx storage of the storage catalytic converter depends on the current loading, the temperature and other parameters, for example the so-called space velocity. If the relative loading of the storage catalytic converter increases, then eventually the delivered NOx material can no longer be stored to a sufficient extent. The part that can no longer be stored exits as so-called slip of the storage catalytic converter and is consequently emitted in the flow of exhaust gas.
The conversion efficiency of the oxides of nitrogen storage catalytic converter depends on the storage behavior. This is usually initially described by a model, which is then adjusted using measurements with a NOx sensor for the storage catalytic converter concerned, which is referred to as adaptation. During the adaptation, a quality factor is calculated that describes the conversion efficiency of the storage catalytic converter. Using the quality factor, the amount of NOx that can be stored and the time from which NOx slip arises can be calculated. An accurate adaptation between the model and the measured sensor value is thus important in order to be able to use the storage catalytic converter optimally and to keep the oxides of nitrogen emissions low.
During the operation of the internal combustion engine, the storage catalytic converter must be diagnosed in order to recognize damage, for example owing to an excessive temperature.
An SCR catalytic converter also enables the NOx present in the exhaust gas to be selectively reduced inter alia to nitrogen (N2) in the presence of residual oxygen in the flow of exhaust gas by catalytic action, for example using ammonia that is fed in. The introduction of ammonia can be carried out actively by means of the targeted dispensing of a urea water solution into the flow of exhaust material or passively by means of an oxidation catalytic converter disposed upstream of the SCR catalytic converter, for example a three-way catalytic converter or an oxides of nitrogen storage catalytic converter, which produces NH3 during an operating phase in which the internal combustion engine is operated with a rich fuel mixture.
Moreover, the SCR catalytic converter can store excess NH3 in a zeolite to a limited extent. The absorption capacity of the catalytic converter depends here on the temperature of the catalytic converter. If the absorption capacity is exceeded, the excess NH3 exits the catalytic converter as slip. The stored NH3 in the SCR catalytic converter also enables the reduction of the NOx at operating points at which no ammonia is introduced.
With a system of a combination of a SCR catalytic converter and an oxides of nitrogen storage catalytic converter, higher overall NOx efficiencies are achieved. A combination of said catalytic converters is referred to below as a catalytic converter box, wherein the two components are not necessarily disposed in a common housing.
For monitoring the NOx conversion rate or the conversion efficiency, NOx sensors are usually used downstream of each individual component.
It is the object of the invention to determine the quality of the individual components of an overall system consisting of a SCR catalytic converter and an oxides of nitrogen storage catalytic converter, and especially the component of the NOx conversion of the SCR catalytic converter, in order to be able to determine the ageing thereof. A possibility is also to be shown to enable the conversion efficiency to be determined with sufficient accuracy even if only one individual NOx sensor is used.
The object is achieved by a method of the aforementioned type by operating the internal combustion engine at least for a limited period of time such that a first of the two components does not affect the signal detected by means of the NOx sensor, and the conversion efficiency for the second component is determined depending on the signal of the NOx sensor that is not affected by the first component.
According to the invention, consequently a state occurs in which at least one of the components, for example the SCR catalytic converter, is placed into a state in which it is not affected by the signal detected by the NOx sensor. Here it should be noted that depending on the current operating mode of the internal combustion engine both NOx (in a lean operating mode) and also NH3 (in a rich operating mode) can be measured in the flow of exhaust gas by means of a NOx sensor.
According to a first possible embodiment, a state is established in which the SCR catalytic converter does not affect the NOx signal. This is achieved by a reduction of the NH3 stored in the SCR catalytic converter. For this purpose, according to one version the internal combustion engine is operated at least for a limited period of time such that a reduction of the NH3 stored in the SCR catalytic converter takes place. If no or almost no more NH3 is being stored in the SCR catalytic converter, the conversion efficiency of the NOx storage catalytic converter is determined depending on the signal of the NOx sensor. In particular, the internal combustion engine can be operated such that the SCR catalytic converter reaches a temperature at which the stored NH3 is released.
According to another possible embodiment, the internal combustion engine is operated such that a delivery of NH3 to the SCR catalytic converter is at least substantially prevented and a reduction of the NOx by means of the stored NH3 takes place.
By means of the method described above, it is achieved that NH3 is no longer stored the SCR catalytic converter, whereby the SCR no longer influences the NOx conversion of the overall system. The signal of the NOx sensor can thus now be used to determine the conversion efficiency of the oxides of nitrogen storage catalytic converter.
According to another aspect of the invention, the internal combustion engine is operated for at least a limited period of time such that saturation of the NOx storage catalytic converter with NOx occurs and, if no or almost no more NOx can be stored, the conversion efficiency of the SCR catalytic converter is determined depending on the signal of the NOx sensor. Here a state is consequently established in which the oxides of nitrogen storage catalytic converter is saturated and thus an input of NOx passes through almost completely unconverted. In particular, if the oxides of nitrogen storage catalytic converter is installed upstream of the SCR catalytic converter, said state can also be established by selecting an engine operating mode in which NOx raw emissions are produced, so that the storage catalytic converter is fully loaded. If the storage catalytic converter is installed in the flow of exhaust gas downstream of the SCR catalytic converter, it can become filled by operating the internal combustion engine in an operating mode in which the NOx concentration in the flow of exhaust gas upstream of the catalytic converter box is so high that even for an optimally operating or a new SCR catalytic converter the NOx contained in the flow of exhaust gas cannot be fully converted. The NOx slip from the SCR catalytic converter is then stored in the oxides of nitrogen-storage catalytic converter until the same is full.
If the storage catalytic converter is fully loaded, an adaptation of the SCR catalytic converter can be carried out, because the SCR catalytic converter as the only component now influences the NOx concentration in the flow of exhaust gas to the NOx sensor. Thus a diagnosis of the efficiency of the SCR catalytic converter can now be carried out, for example by a comparison of the mass of NOx introduced into the SCR in comparison with the mass of NOx at the position of the NOx sensor.
According to an advantageous development of the method according to the invention, the NOx storage catalytic converter is loaded with NOx at least until a NOx slip is measured at the NOx sensor. An operating mode is then selected in which a fully operational or at least sufficiently operational SCR catalytic converter would convert the entire NOx concentration in the exhaust gas. The conversion efficiency of the SCR catalytic converter is then determined depending on the signal of the NOx sensor.
Consequently, the storage catalytic converter is initially loaded here such that a slip occurs. In this case the process can be as previously described. Regeneration of the storage catalytic converter is usually activated in said load state, but is now suppressed or prevented. Instead, a state is established in the overall system in which a NOx concentration upstream of the catalytic converter box is so high at its maximum that the NOx would be completely converted with an optimal new SCR catalytic converter with sufficient stored NH3. In the case of a SCR catalytic converter that is still converting well enough, no NOx slip would now be measured at the NOx sensor. If the SCR catalytic converter has aged however, and thus no longer has the property of converting the NOx sufficiently well, then a NOx slip is measured at the sensor downstream of the NSC. The ratio between the mass of NOx flow upstream of the SCR catalytic converter and the mass of NOx flow measured by means of the NOx sensor downstream of the SCR catalytic converter can now be used for the calculation of the quality factor or the conversion efficiency of the SCR catalytic converter.
According to a further embodiment of the method, the NH3 storage capacity of the SCR catalytic converter is concluded depending on the signal of the NOx sensor. Said development uses the property of NOx sensors of being able to measure NH3 instead of NOx for a rich fuel mixture. Further, with the NOx sensor it can be determined whether a rich or weak fuel mixture is currently available. In order to be able to conclude the NH3 storage capacity of the SCR catalytic converter for the case in which no NOx sensor is present, the information about the NH3 in the flow of exhaust gas upstream of the catalytic converter box could be calculated by suitable NOx raw mass models and/or oxidation catalytic converter models. It should be noted here that when using a NSC as a NH3 source upstream of the catalytic converter box, a high NH3 peak arises through the low storage of the NSC, which may also have to be taken into account in the model being used.
Preferably moreover, NH3 desorption owing to the NH3 arising from the NOx stored in the oxides of nitrogen storage catalytic converter is taken into account when determining the NH3 storage capacity of the SCR catalytic converter. In particular, NH3 desorption is to be seen a short time after the changeover from a lean mode into a mode with a rich fuel mixture.
Said desorption can for example be determined by integrating the NH3 signal up to a defined time and adding to the NH3 storage capacity, or by fitting the curve of the NH3 signal in the region of the NH3 desorption to the later profile. The NH3 storage capacity of the SCR catalytic converter can thereby be determined sufficiently precisely from the difference of the NH3 flowing into the SCR catalytic converter and the NH3 flowing out from the NOx storage catalytic converter.
Alternatively or additionally, it can be provided to inhibit NOx desorption owing to NH3 arising from the NOx stored in the NOx storage catalytic converter by operating the internal combustion engine before the detection of the signal of the NOx sensor such that the stored NOx in the NOx storage catalytic converter is released. This can for example be achieved by a temperature increase before the measurement.
According to a preferred embodiment, the NOx storage capacity of the oxides of nitrogen storage catalytic converter is determined with a preferably only slightly rich fuel mixture, in particular with a lambda value greater than 0.97, because the downstream oxides of nitrogen-catalytic converter can convert the rich components in the mixture (HC and CO) to harmless components (CO2 and H2O) by means of the present oxygen storage capacity and thereby only slight exhaust gas emissions arise. Furthermore, in said region of the composition of the mixture the NH3 produced by the upstream oxidation catalytic converter is greatest.
If the oxides of nitrogen storage catalytic converter is disposed in the flow of exhaust gas upstream of the SCR catalytic converter, the NH3 from the desorption of the oxides of nitrogen storage catalytic converter could pass into the SCR catalytic converter and cause errors in the results. This can be prevented by emptying the NH3 store of the SCR catalytic converter and the NOx store of the oxides of nitrogen storage catalytic converter, and then by operating the internal combustion engine with a rich fuel mixture NH3 is produced in the flow of exhaust gas and the signal of the NOx sensor is analyzed to determine the NH3 storage capacity of the SCR catalytic converter. If discharging the catalytic converter is achieved by means of a temperature increase, the catalytic converter could thus be cooled to the desired temperature in connection with a homogenous phase, for example with a lambda value=1, without NH3 and NOx emissions and NH3 could be produced by an operating phase with a rich fuel mixture, which would not be affected by the empty oxides of nitrogen storage catalytic converter.
The object of the invention is also achieved by a computer program that is stored in a control unit for controlling and/or regulating an internal combustion engine by conducting the method according to the invention if the computer program is executed in the control unit. The object is further achieved by a control unit for controlling and/or regulating the operation of an internal combustion engine by a computer program that is stored in the control unit and the method according to the invention is conducted if the computer program is executed in the control unit.
Further features, possible applications and advantages of the invention are revealed by the following description of exemplary embodiments, which are described using the figures, wherein the features can be important to the invention both on their own and also in different combinations, without explicit reference again being made thereto. In the figures:
A memory area 13 in which a computer program 14 is stored that is programmed to perform the method according to the invention is formed in the control unit 11.
Besides the arrangement shown in
In
In a step 22 the release of the NH3 is ended. In a step 23 the internal combustion engine is operated with a lean fuel mixture. In a step 24 the NOx slip is determined. For this purpose, the NOx component in the flow of exhaust gas upstream of the catalytic converter box 6 is determined and the NOx concentration downstream of the catalytic converter box 6 is measured by means of the NOx sensor 9. The difference of said values gives the NOx slip.
In a step 25 the previously described adaptation is carried out by for example calculating a quality factor or the conversion efficiency of the oxides of nitrogen storage catalytic converter 8. Alternatively, the conversion efficiency can be used to diagnose the oxides of nitrogen storage catalytic converter 8.
In a step 34 the difference between the mass of NOx upstream of the catalytic converter box 6 in the flow of exhaust gas and downstream of the catalytic converter box 6 is determined. In a step 35 the conversion efficiency of the SCR catalytic converter 7 is concluded from said difference.
In a step 43 the regeneration of the oxides of nitrogen storage catalytic converter 8, which would usually begin now, is suppressed because the oxides of nitrogen storage catalytic converter 8 is fully loaded.
In a step 44 a state is established in which a maximum NOx concentration upstream of the catalytic converter box 6 is such that the NOx would be completely converted in the case of an optimally working or a new SCR catalytic converter with sufficient stored NH3. In the case of a passive SCR catalytic converter 7 that is still converting sufficiently well, no NOx slip is now measured at the NOx sensor in the step 45.
The quality is now determined in a step 46. If the SCR catalytic converter 7 has aged and consequently no longer has the property of converting NOx sufficiently well, a slip measured in the step 45 is correspondingly assessed in the step 46 and the conversion efficiency is determined.
In a step 51 the amount of NOx in the flow of exhaust gas upstream of the catalytic converter box 6 is determined. This can either be measured by means of a further NOx sensor or calculated by means of a suitable NOx untreated mass model and an oxidation catalytic converter model. In a step 52, the mass of NH3 in the flow of exhaust gas downstream of the catalytic converter box 6 is determined by means of the NOx sensor 9.
In a step 53, NH3 desorption may be taken into account. This occurs as a result of NH3 being able to arise from the NOx stored in the oxides of nitrogen storage catalytic converter 8. The desorption can be determined by integrating the NH3 signal downstream of the NOx storage catalytic converter 8 until a defined time and adding the result to the NH3 storage capacity.
Alternatively, a graph of the NH3 signal can be calculated and fitted to the later profile in the region of the NH3 desorption. However, it can also be provided to operate the internal combustion engine 1 in the step 53 such that the NH3 desorption is suppressed. This is achieved by emptying the NOx storage catalytic converter 8 or releasing the NOx stored therein before the measurement of the NH3 signal in the step 52, for which purpose for example, a temperature increase before the measurement is the aim.
In a step 54 the NH3 slip is analyzed and the conversion efficiency of the SCR catalytic converter is determined depending on the result.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 215 501.3 | Aug 2015 | DE | national |
