Abstract:
The present invention provides for a method and apparatus for controlling an SCR catalytic converter ( 2 ). The invention describes a model-based control method including a physical model of the SCR catalyst ( 2 ) with more than one NH3 storage cell and a physical model of a NOx sensor ( 4 ). The observer feedback gain forces the estimated sensor outputs from the model to converge to measured ones, therefore, there is not ambiguity in the determination of the operating point.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and device for controlling an SCR catalytic converter of a vehicle, in particular in the field of the vehicular combustion engines. 
       DESCRIPTION OF THE PRIOR ART 
       [0002]    Many combustion engines which have to comply with the current and future emission legislation make use of a selective catalytic reduction (SCR) system in order to reduce the nitrogen oxide (NOx) emissions. 
         [0003]    In current operational systems, a urea solution is injected in the exhaust gas upstream of the SCR catalyst. The urea is transformed into ammonia (NH3), which in turn reduces the NOx to harmless nitrogen (N2) and water (H2O) in the SCR catalyst. The relevant chemical reactions occur after adsorption of the ammonia on the catalyst surface. 
         [0004]    Generally, the NOx conversion efficiency of the SCR catalyst is dependent on the amount of stored (i.e. adsorbed) ammonia, the temperature, the space velocity, i.e. the gas turnover in the catalyst per time unit, the NO2/NO ratio of the NOx, and other conditions. The temperature and the space velocity are usually dependent on the engine operation and cannot be directly influenced by the SCR controller. The amount of stored ammonia is usually adjusted by a dedicated controller, which controls the estimated level of the ammonia. The NO2/NO ratio is dependent on the performance of a diesel oxidation catalyst (DOC) and on a diesel particle filter (DPF) mounted upstream of the SCR catalyst. In current concepts, the NO2/NO ratio cannot be adjusted directly, since it mainly depends on the DOC/DPF temperatures, the space velocity, and on the soot loading of the DPF. 
         [0005]    Current SCR control systems make use of a model, where the SCR catalyst is modeled as one NH3 storage tank. The amount of stored NH3 is calculated from the injected urea, and from the amount of NH3 consumed by the SCR reactions. The amount of stored ammonia is then adjusted such that the desired NOx conversion efficiency is achieved. An outer control loop using an NOx measurement device is then used to adjust the injected urea quantity such that the estimated, by a model, and the measured NOx conversion efficiency converge. 
         [0006]    Known schemes of a SCR controller are described in e.g. Schär: “Control of a Selective Catalytic Reduction Process” (PhD thesis Nr. 15221, ETH Zürich) or in Chi, Da Costa: “Modeling and Control of a Urea-SCR Aftertreatment System, SAE 2005-01-0966, or in Herman, Wu, Cabush, Shost: “Model Based Control of SCR Dosing and OBD Strategies with Feedback from NH3 Sensors”, SAE 2009-01-0911. 
         [0007]    The known scheme of a SCR controller of the prior art includes a closed loop controller based on an NOx sensor downstream of the SCR catalyst. 
         [0008]    The known control approach lacks of accuracy, especially, when NH3 slip should be accounted for, as well. 
         [0009]    Control systems of the prior art aim at controlling the storage level of the entire SCR or at controlling an overall NOx conversion efficiency. Some concepts even include the calculation and limitation of NH3 slip, i.e. of the dispersing of NH3 in the exhaust gas, without reacting with NOx. However, this usually implies a parallel controller, which, then, is combined with the NOx controller by e.g. minimum selection of the urea quantity to be injected. Generally, activation or deactivation criteria of the controllers upon sensor failure or shut off/on are awkward from an implementation point of view. Currently available NOx sensors exhibit a significant cross sensitivity to NH3. 
         [0010]      FIG. 2  of the prior art shows a typical sensor output characteristic dependent on the injected urea. 
         [0011]    Such a characteristic is a pair function, therefore, it is not a bijective function. 
         [0012]    Under normal conditions, the NOx sensor output decreases with increasing urea injection. However, when the NH3 slip starts to increase, the characteristic of NOx sensor output turns and the sensor output increases with increasing urea injection. 
         [0013]    The ambiguous characteristic of the NOx sensor leads to the problem that potential NH3 slip may be interpreted as NOx and vice versa. This might lead to a destabilisation of the controller. For example, when NH3 is interpreted as NOx, the control algorithm will increase the urea injection in order to reduce the NOx emissions. This will lead to a further increase of the NH3 slip, which in turn leads to a further increase of the urea injection, since the NH3 is interpreted as NOx. 
         [0014]    Hence, the sensor signal is ambiguous, and its ambiguity can destabilize the control system. In order to differentiate between NOx and NH3, the urea injection has to be excited, e.g. by toggling between two levels. Known systems rely on fast response, which is not given with large catalyst volumes in the future, and/or on quasi steady-state operating conditions, which hardly occur during normal operation. 
       SUMMARY OF THE INVENTION 
       [0015]    Therefore it is the main object of the present invention to provide a method and device for controlling an SCR catalytic converter of a vehicle which overcomes the above problems/drawbacks. 
         [0016]    An observer device forces the estimation of NOx or NH3 to converge to measured values, i.e. the observer regulates gains/parameters of said estimation using as feedback the difference/error between said estimation values and said measured values 
         [0017]    Hereby, the main advantage of the invention is that the observer knows at all times the polarity of the sensor model, i.e., the observer inherently distinguishes at all times on which side of the characteristic it actually is, whether the NOx signal increases or decreases with increasing amounts of stored ammonia. Hence, the sensor output ambiguity is overcome. 
         [0018]    Therefore, the controller only controls the level of the estimated/modelled sensor output. If no real sensor is available, the corresponding observer feedback gain is switched to zero, which simplifies the implementation significantly, and make the same ECU (Electronic Control Unit of the vehicle) suitable for several engine setups or applications in several working conditions, for example, at the start up of the engines. 
         [0019]    According to a further aspect of the present invention, the SCR catalyst is considered as divided in a plurality of cascaded cells, and only the storage level of the first cell is controlled, in spite of the fact that the set point is calculated from an overall NOx conversion efficiency demand or from the NH3 slip limitation constraint. 
         [0020]    Since ECU calculation and memory resources are limited, an advantageous implementation of the present method provides the step of considering the SCR catalyst as comprising only few storage cells and controlling the NH3 stored only in the first one of said cells. 
         [0021]    According to a preferred embodiment, the controller calculates one of the following set points: 
         [0022]    NOx Control: Amount of stored NH3 in the first cell in order to reach the required NOx conversion efficiency target of the entire SCR catalyst, i.e. taking into account the contributions, in terms of NOx efficiency conversion, of the other storage cells from the second to the n-th. 
         [0023]    NH3 Control: Amount of stored NH3 in the first cell in order to reach the required NH3 level at the outlet of the SCR catalyst, also taking into account the contributions, in terms of NH3 storage levels, of the other storage cells from the second to the n-th. 
         [0024]    According to another embodiment, the controller calculates both set points, then the actual set point is obtained from a minimum selection. 
         [0025]    Therefore, a multiple cell approach is advantageous, since the accuracy can be significantly improved, if the axial distributions of the gas components and the stored ammonia are taken into account. 
         [0026]    According to another aspect of the invention, the controller implementing the method hereby depicted is able to detect and avoid NH3 slip, if an NH3-sensitive NOx sensor is used alone, without any specific NH3 measurement device. The SCR catalyst method incorporates the NOx sensor model, including NH3 cross-sensitivity. 
         [0027]    If a divergence of the controller is detected, because a non-present NH3 slip is detected or because a present NH3 slip is not detected, the controller inverts the stored NH3 and NOx sensor polarity for the calculation of the present operating point on the sensor characteristic. 
         [0028]    Advantageously, no external NH3 slip detection system is necessary, since the method and the controller implementing the method can be used directly. 
         [0029]    These and further objects are achieved by means of a method and device as described in the attached claims, which form an integral part of the present description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The invention will become fully clear from the following detailed description, given by way of a mere exemplifying and non limiting example, to be read with reference to the attached drawing figures, wherein: 
           [0031]      FIG. 1  shows a known ambiguous NOx sensor output characteristic, 
           [0032]      FIG. 2  shows an SCR control scheme according to the present invention, 
           [0033]      FIG. 3  shows an SCR model-based control scheme according to the sketch of  FIG. 2 , based on a NOx efficiency target, 
           [0034]      FIG. 4  shows an SCR model-based control scheme according to the sketch of  FIG. 2 , based on a NH3 slip level set point, 
           [0035]      FIG. 5  shows an error recovery operated according to the present invention when NOx is detected as NH3, 
           [0036]      FIG. 6  shows an error recovery operated according to the present invention when NH3 is detected as NOx. 
       
    
    
       [0037]    The same reference numerals and letters in the figures designate the same or functionally equivalent parts. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The method and device for controlling the SCR catalyst comprises the fact of considering a model of SCR catalyst as divided in multiple storage cells. In each cell, the amount of stored ammonia and the relevant exhaust gas components (NOx, NH3, etc.) are calculated. Additionally, the method can account for the temperature, where the temperature of each cell is calculated. Therefore, the method estimates, by means of a sensor model, the behaviour of the really mounted sensors, being able to distinguish the operating point on the sensor characteristic and being able to invert the curve polarity. 
         [0039]    As a very important feature, the NH3 sensitivity of the NOx sensor is reflected by the model. 
         [0040]    With reference to  FIG. 2 , the physical exhaust line  1  comprises a real SCR catalyst  2 , including optionally an oxidation catalyst to reduce the NH3 slip, a urea dosing module  3 , a NOx sensor  4 , an NH3 sensor  5 , upstream and downstream temperature sensors  6 ,  7 . 
         [0041]    An NH3 storage model  8  is fed with the physically relevant inlet and outlet quantities, such as at the input  9  exhaust mass flow, catalyst upstream NOx (NO and NO2) concentration and temperature, the amount of injected urea, and exhaust mass flow downstream. 
         [0042]    The estimated sensor outputs  10 , respectively of NOx and NH3, are compared to the measured outputs of sensors  4  and  5 . The errors are then used in an observer loop with a given gain 11, to correct the state variables of the estimating models, which are the amounts of stored ammonia in each cell, such that the calculated sensor outputs converge to the measured ones. 
         [0043]    One of the innovative aspects of the present invention is the concept of controlling only the ammonia storage level of the first cell (in the gasses crossing direction) of the multiple cells in which the SCR catalyst is considered divided. 
         [0044]    The controller is the same for the NOx and the NH3 slip control. Only the set points of the two control goals are calculated separately. The actual set point is obtained from a minimum selection, since the NH3 slip controller is actually only an NH3 slip limitation. 
         [0045]    The proposed control concept has two purposes. On the one hand, a target NOx conversion efficiency of the SCR catalyst has to be reached. On the other hand, an NH3 slip limit must not be exceeded. Thereby the NH3 slip limitation is dominant. 
         [0046]    Since the dynamics of the storage cells from the 2-nd to the n-th are slow as compared to the dynamics of the first cell, only the latter is directly controlled. 
         [0047]    Hence, a control scheme can be of NOX control based type and/or NH3 slip limitation type. 
         [0048]    NOx control:  FIG. 3  shows a scheme of the NOx control concept. From the overall NOx conversion efficiency target  31  and from the achieved efficiency of cells  32  from the 2-th to the n-th, calculated from the current storage levels, an efficiency target for the first cell  37  is calculated in block  33 . This efficiency target is converted into a set point for the NH3 storage level of the first cell in block  34 , where the current state of the system  35  (temperature, space velocity, NO2/NO ratio, etc.) is taken into account. The minimum of the NOx control set point  34  and of the NH3 control set point  36  (see below) is selected and compared to the actual value of the NH3 level in the first cell  37 . The offset is then fed to a controller  38 , which adjusts the urea or NH3 quantity  39 . 
         [0049]    NH3 slip limitation: a scheme of the NH3 limitation concept is depicted in  FIG. 4 . From the NH3 limit at the tail of the SCR catalyst  41 , a storage level of the last cell is calculated in block  42  using the temperature and other operating conditions, like space velocity and NO2/NO ratio in block  43 . Starting from the last element, an NH3 storage level is calculated for each cell  44 , which is necessary to reach the desired NH3 storage level of the last cell under steady state current operating conditions (temperature, space velocity, NO2/NO ratio, etc.). Finally, the desired storage level of the first cell  45  is obtained, which is fed into the minimum selection  46  for the NH3 level set point. The minimum of the NH3 control set point and of the NOx control set point  46  (see above) is selected and compared to the actual value of the NH3 level in the first cell  47 . The offset is then fed to a controller  48 , which adjusts the urea or NH3 quantity  49 . Since ECU calculation and memory resources are limited, an advantageous implementation of the method could model the SCR as a 2÷3 storage cells. 
         [0050]    The method can be applied to NOx and NH3, or other nitrogenated species, such as NO2, NO, N2O. Advantageously, implementing the present method, the following details, variations and modifications can be achieved: 
         [0051]    Variable feedback gains. The feedback gains, i.e. the adjustment of the state variables (amount of stored NH3 or others) imposed by the deviations between the measured and calculated sensor outputs, can be varied upon the operating point and/or under special operating conditions, such as temperature, space velocity, stored ammonia or others. Under normal operating conditions, a positive NOx sensor error leads to a decrease of the stored ammonia, in order to increase the calculated NOx sensor output to the level of the measured one. However, if too much urea is injected and the NOx sensor mainly measures NH3, the modelled level of stored ammonia is increased in order to eliminate the sensor deviation. This behaviour is automatically captured when using a non-linear observer method, such as an extended Kalman filter, and similar ones, please see e.g. Welch, Bishop: “An Introduction to the Kalman Filter”, URL http://www.cs.unc.edu/˜welch/media/pdf/kalman_intro.pdf. Further conditions can make a change of the feedback gain necessary: Firstly, if it is known that a sensor output is inaccurate under well-defined conditions, for example, during transients, the feedback can be temporarily weakened, i.e., the correction is reduced. Secondly, feedback gains can be temporarily increased, if required. If, for example, the NH3 slip is detected by the NH3 sensor, the NOx sensor feedback is weakened and the NH3 sensor feedback increased in order to prioritize the NH3 sensor and to ensure a correct NH3 slip estimation by the controller. This is necessary to allow the controller to take appropriate measures (i.e. reduce the urea injection) 
         [0052]    Extension of the control method, by introducing an identification method, for example, an extended Kalman Filter. Thereby, additional state variables are introduced, which represent constant to slowly drifting parameters, such as catalyst storage capacity or offsets of urea injection or sensors, urea quality. The observer feedback loop also corrects these parameters and thus allows the adaptation of the model to long-term changes such as ageing of the system or concentration drift in the urea solution. 
         [0053]    Availability of sensor information: Gas sensors usually cannot be operated under all conditions. Especially during cold start operation, when water droplets are present in the exhaust gas, some sensors must be shut off. Under these conditions, the feedback loop is simply turned off, i.e., the control scheme runs in open-loop and it is not corrected with the sensor information. 
         [0054]    The control method can be extended with any nitrogenated gas sensor, of which output can be estimated by means of a model. Temperature sensors or gas species not discussed here (e.g. N2O) might be an option. 
         [0055]    The shown control concept inherently uses the storage model and the controller to detect whether the NOx (or NH3) sensor signal is interpreted in a correct way. Hereby, the main advantage of the invention is that the controller “knows” at all times the polarity of the sensor estimating model, i.e., whether the NOx signal increases or decreases with increasing amounts of stored ammonia. Hence, the model inherently “knows” at all times on which side of the characteristic in  FIGS. 5 and 6  it actually is. 
         [0056]    The detection of an erroneous output and its recovery method can be formulated as follows: 
         [0057]    Case 1: “Divergence to NOx side”, i.e. NH3 is detected as NOx: 
         [0058]    The detection of the wrong polarity (wrong operating point) is dependent on the following conditions, which must be fulfilled during a (temperature dependent) time period:
       Set point for stored NH3 is persistently increasing   No NH3 slip is estimated   Controller&#39;s feedback is negative, i.e., the signal of the NOx sensor increases with decreasing NH3 storage level. For recovery the wrong polarity: The amount of stored NH3 is increased by a ramp, until the calculated NOx sensor signal equals the measured one on the right hand side (see the arrow on  FIG. 6 ).       
 
         [0062]    Case 2: “Divergence to NH3 side”, i.e. NOx is detected as NH3: 
         [0063]    The detection of the wrong polarity is dependent on the following conditions, which must be fulfilled during a (temperature dependent) time period:
       Set point for stored NH3 is persistently decreasing   Significant NH3 slip occurs in the model   Controller&#39;s feedback is positive, i.e., NOx signal decreases with increasing NH3 storage level       
 
         [0067]    For recovery, the amount of stored NH3 is decreased by a ramp, until the calculated NOx sensor signal equals the measured one on the left hand side (see the arrow in  FIG. 5 ). 
         [0068]    Once the recovery has become effective, the NOx controller or the NH3 slip limitation described above either brings the system to the desired NOx conversion efficiency or limits the NH3 emissions to the maximum level. 
         [0069]    This invention can be implemented advantageously in a computer program comprising program code means for performing one or more steps of such method, when such program is run on a computer. For this reason the patent shall also cover such computer program and the computer-readable medium that comprises a recorded message, such computer-readable medium comprising the program code means for performing one or more steps of such method, when such program is run on a computer. 
         [0070]    Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by this invention. 
         [0071]    Further implementation details will not be described, as the man skilled in the art is able to carry out the invention starting from the teaching of the above description.