FR2853934A1 - Procedure for determining offset signal value from NOx detector in IC engine exhaust system, uses signal values taken during stoichiometric operating mode - Google Patents

Abstract

The procedure, designed to determine the offset value of a signal from a NOx detector (9, 10) positioned after an IC engine consists of operating the engine with a stoichiometric relationship of air and fuel (lambda = 1), with the signal taken during a stoichiometric operating mode and the offset value taken as a representative value of the values determined and authorized if the authorization conditions are fulfilled. In the event of one or more authorization conditions not being fulfilled the determination of the signal values does not take place or is interrupted. The verification of the authorization conditions is effected before the signal values are determined, and the offset signal value is effected only after a predetermined time has elapsed following the verification of the authorization conditions.

Description

The invention relates to a method for

  determining an offset value, called OFFSET, of a detector signal, as well as a device for implementing the method.

  NOx-sensitive devices for the determination of the content of NOx nitrogen oxides in the exhaust gas of an internal combustion engine are known in various forms, and are for example used for monitoring compliance with limit values. 10 NOx of an internal combustion engine likely to operate in a lean regime. The knowledge of the NOx content of the exhaust gas downstream of a NOx accumulation catalyst is particularly necessary in order to be able to determine the moment at which an accumulation of NOx in the NOx accumulation catalyst is no longer possible, because the NOx storage catalyst is saturated, and regeneration is necessary.

  The accuracy of the currently available NOx detector display is affected by individual inaccuracies. These can lead to errors in the control behavior of the engine control unit, so that the degree of filling of the NOx accumulator is, for example, not accurately detected, and that a necessary regeneration is necessary. triggered too late, leading to increased emission of harmful substances.

  The linear characteristic curve of such detectors is characterized by the slope, which is proportional to the sensitivity of the NOx detector, and by the offset value, which designates a detector signal other than zero in the absence of NOx in the exhaust gases. 'exhaust. Depending on the type of detector, the offset value frequently has considerable series dispersions. It is therefore advantageous to determine the offset value of the NOx signal after the installation of the detector on board a vehicle, and to individually correct the NOx signal of the NOx detector with this value.

  Various methods are known for calibrating a NOx detector to remedy such problems. The concentration of NOx in the exhaust gas can, however, be determined with sufficient precision only in a few operating states of the internal combustion engine.

  The publication EP 1 074 833 A1 proposes to determine the offset value of the NOx detector during operation of the internal combustion engine with a mixture of rich fuel, or in the thrust mode in the absence of fuel supply because in these operating states, NOx emissions in the exhaust gases are not expected. Different types of NOx detectors, however, have a signaling behavior in the thrust mode such that a determination of the offset value becomes impossible. Thus, a NOx detector can cool significantly below its operating temperature when it is in contact with exhaust gases which, in the thrust regime, are colder than in other engines. phases of operation, or there is a sensitivity with respect to the change of the oxygen concentration, which can strongly distort the measurement results.

  DE 100 03 228 A1 reports that a NOx detector signal measured immediately after regeneration of the NOx storage catalyst is recognized as an offset value, since at this time the total amount of NOx contained in the The exhaust gas is accumulated with a sufficiently high probability in the freshly regenerated storage catalyst, and no NOx emissions are expected downstream of the NOx accumulation catalyst. However, in the case of aging of the NOx accumulation catalysts, a substantial concentration of NOx may still be contained in the exhaust gas even after regeneration. NOx detectors are also relatively inert, and therefore, if necessary, too slow to detect sufficiently rapidly after regeneration a value that corresponds to almost zero ppm of NOx.

  The object of the present invention is to more reliably determine an offset value of a detector signal of an NOx sensitive device.

  In accordance with a preferred aspect of the invention, an internal combustion engine operates with a stoichiometric ratio of combustion air and fuel, and signal values of an NOx sensitive device are determined during the stoichiometric regime. A value representative of the determined values is identified as an offset value.

  It is preferably checked whether authorization conditions are fulfilled. If these are satisfied, the NOx sensitive device is calibrated with the offset value. The invention avoids the disadvantages described above known methods.

  A NOx detector does not cool during the determination according to the invention since the measurement is made in an operating mode of the internal combustion engine in which the exhaust gases are hotter than in the thrust mode.

  Due to the preferred conditions of authorization, there is no reason to expect rapid changes in the oxygen concentration which could distort the measurement. Problems due to rapid changes in NOx concentration are also avoided.

  The invention allows a more accurate and reliable determination of the offset value of a NOx detector.

  Other characteristics, features and advantages of the invention will emerge more clearly from a reading of the description below, made with reference to the appended drawings, in which: FIG. 1 represents a preferred arrangement of an internal combustion engine with an exhaust gas line, in which NOx-sensitive devices are arranged; and Figure 2 shows a characteristic curve of a NOx detector.

  FIG. 1 serves to illustrate different installation positions of exhaust gas detectors, and shows in a diagrammatic representation an internal combustion engine 1, for example an Otto engine capable of operating in a lean regime or a combustion engine. diesel internal combustion, comprising an exhaust gas line 2 and an engine control apparatus 3, which is preferably intended for the operation of a motor vehicle. The internal combustion engine 1 comprises a number of cylinders 4 (components corresponding to each other are identified by the same numerical reference), downstream of which is provided a pipe 20 of exhaust gas 5 specific to each of them. An exhaust gas cleaning device, comprising a pre-catalyst 6 and a main catalyst 7, is arranged in the exhaust gas line 2 for the conversion of harmful or undesirable compounds of the exhaust gas in the exhaust gas stream. other compounds. The precatalyst 6 is preferably in the form of a 3-way catalyst, and the main catalyst 7 in the form of a NOx accumulation catalyst. NOx detectors 8 are disposed downstream of the cylinders 4 in the exhaust pipes 30 5, with which the concentration of NOx in the exhaust gas passing through the exhaust line 2 of the combustion engine can be measured. internal combustion 1. Another NOx detector 8 'is further disposed upstream of the precatalyst 6. In a zone of the exhaust gas line 2 located between the pre-catalyst 6 and the main catalyst 7, another detector NOx 9 is also disposed downstream of the pre-catalyst 6 and upstream of the main catalyst 7. Another detector 10 is disposed in the exhaust line 2 downstream of the main catalyst 7. It goes without saying that, in a structured exhaust gas purification device 5 having a plurality of partial components, detectors may be arranged upstream and downstream of the respective parts.

  In addition to the detectors mentioned, lambda probes 11 or 12 are provided upstream and downstream of the pre-catalyst 6 and upstream of the main catalyst 7, as well as temperature sensors 13, 13 'for the detection of the temperature of operation of the catalytic devices.

  It is obvious that, alternatively, or in addition, other temperature sensors may be provided for measuring the operating temperature of the exhaust gas cleaning device or parts thereof. For the recirculation of the exhaust gas, the internal combustion engine 1 comprises an exhaust gas recirculation device 14 with a controllable valve 15.

  Through other detectors, not shown, the control unit 3 of the engine detects in the known manner operating parameters of the internal combustion engine 1, such as, for example, the position of the throttle valves. exhaust gas recirculation rate, ignition timing, injection timing of preliminary / main / rear injectors, injection pressure, valve rocker position, inlet pressure, the camshaft phase adjuster 30, the rotational speed, the position of the accelerator pedal, the load, the speed of operation, and the like, and can if necessary influence them via adjusting members (not shown), a cable system 16 or the like being provided for communication between the motor control unit 3 and the sensors or adjusting members.

  The control apparatus 3 of the engine further comprises a lambda regulating device for regulating the oxygen concentration in the exhaust gas or the lambda value. The engine control unit 3 receives NOx signals from the NOx detectors, with which the gross NOx emissions of the internal combustion engine 1, or the NOx concentration downstream of the catalytic devices 6 and / or can be determined. 7.

  The main catalyst 7 usually operates in an accumulation cycle which comprises at least one absorption mode, during which NOx is stored, and a regeneration mode. The absorption accumulation is in this case carried out at a lambda value X> 1, the desorption being carried out subsequently at a lower lambda value, preferably at X <1.

  The invention is preferably implemented for a lean engine capable of operating with stratified charge. The fuel consumption of such engines is particularly low in the stratified charge mode, however they have the disadvantage that a conventional 3-way catalyst is not able to sufficiently purify the poor exhaust gases. According to the state of the art, the use of a NOx storage catalyst is therefore obligatory for such engines. In addition to a poor regime, including a stratified charge regime, such engines however allow a stoichiometric regime at X = 1. The method according to the invention makes use of this.

  According to the method according to the invention, an NOx-sensitive device 30 placed downstream of the internal combustion engine 1 is used for the determination of the NOx concentration in the exhaust gas of the internal combustion engine 1. The device The NOx sensor 9 preferably has the NOx detectors 9, 10. In a particularly preferred manner, the NOx detector 10, with which the concentration of NOx at the exit of the exhaust line 2 can be detected, is placed downstream of the main catalyst 7. If several NOx detectors 9, 10 are used, an offset value is advantageously determined individually for each NOx detector 9, 10. The method is described below using the detectors NOx 9, 10. With regard to the detectors 9, 10, these may be multi-function detectors which, in addition to a NOx signal, detect a lambda signal and a hydrocarbon signal, a SOx signal. , a ammonia and / or other exhaust gas compounds. In such a case, the offset values of one or more of the other signals can also be determined according to the method according to the invention.

  FIG. 2 illustrates by way of example an output signal Sout of a NOx detector as a function of a NOx concentration. The characteristic curve K has an ordinate at the origin OS, which corresponds to the offset value in the absence of NOx in the exhaust gases, and a slope Gain which reflects the sensitivity of the NOx detector in the concentration range. corresponding NOx.

  According to the invention, the internal combustion engine i operates during a time interval with a stoichiometric ratio (2c = 1) of air and fuel. During the stoichiometric regime, values of the NOx signal are determined in the exhaust gas downstream of the precatalyst 6 and / or the main catalyst 7. The lambda value is conveniently adjusted so that 2k varies with an amplitude of 1. 5%, preferably of the order of 2%, with respect to the average value 2. = 1. In the defect-free state, and preferably in stationary state of the internal combustion engine 1, the catalyst 6 to 3 lanes and / or the main catalyst 7 convert all the NOx contained in the exhaust gases, so that the NOx concentration downstream of the pre-catalyst 6 and / or the main catalyst 7 is no longer hardly measurable. A representative value, preferably an average value of the determined values, is identified as an offset value. The determination of the values of the NOx signal is preferably effected over a selectable time interval, which preferably corresponds at most to the duration during which the internal combustion engine operates with X = 1.

  The X = 1 regime of the internal combustion engine may be forced to effect the determination of the offset value; however, it is also possible to wait for the occurrence of X = 1 phases that usually occur during the normal walking regime.

  In addition to the stoichiometric regime of the internal combustion engine 1 with X = 1, other authorization conditions may be verified, the compliance of which is a prerequisite for authorization of the calibration of the NOx sensitive device 10 in this respect. which concerns the offset value of NOx.

  If one or more conditions of authorization are not fulfilled, the determination of the values of the NOx concentration does not take place or is interrupted. Optionally, a predetermined offset value can then be used for calibration, possibly an offset value from a previous measurement, or calibration is not performed. In a particularly appreciated manner, a new verification of the conditions of authorization is carried out after the observation of the non-respect of one or more conditions of authorization, before the average value of the NOx signal is determined within a time interval susceptible to be chosen.

  In a particularly preferred way, the verification of the conditions of authorization is carried out before the determination of the values of the concentration of NOx. Only when all the predetermined permit conditions are fulfilled are values of the NOx detector 10 entered, and an average value giving the offset value is set. It is particularly appropriate to continue to verify the conditions of authorization in parallel with the determination of the NOx signal values.

  At least one of the conditions of the following group is preferably provided as a condition of authorization existence of a stoichiometric ratio (X = 1) air / fuel; existence of lambda control effected actively; uniform speed of the internal combustion engine, compliance with a permissible range of a lambda sensor signal of the NOx-sensitive device 10 or the NOx detector 10, to ensure that X z 1 also applies downstream the main catalyst 7; at least approximately stationary regime of the internal combustion engine 1, such as for example small rotational speed variations and / or small changes in intake manifold pressure; compliance with a permissible temperature range of the main catalyst 7, preferably between 2000C and 8000C, particularly preferably between 3000C and 7000C; operation without detector faults and / or control elements that can influence the air / fuel ratio; and / or a predetermined waiting time elapses after all other selected authorization conditions have been fulfilled. Alternatively or additionally, it is of course possible to predetermine other appropriate authorization conditions.

  A device for carrying out the method according to the invention described above comprises means for the determination of the offset value with which a detection of values of the NOx signal can be carried out in a defined time interval. storing these values and calculating a representative value, e.g., an average value of these values. These means are preferably made as part of the control 3 of the engine. Alternatively, a separate control apparatus is also within the scope of the invention. Such a device may also include means for defining the authorization conditions and for verifying the conditions of authorization, and for controlling the beginning, the end and, if necessary, an interruption of the authorization. determination of the offset value according to the invention. The invention is not limited to the device shown, but includes any suitable device that is suitable for carrying out the method.

  The process is particularly simple and free of complications. The determination of the offset value is carried out particularly favorably during the homogeneous regime of the internal combustion engine 1 at%, = 1. If other permissible conditions of authorization which are conveniently adapted are taken into account. to the respective vehicle, the concentration of NOx in the exhaust gas downstream of the pre-catalyst 6 and / or the main catalyst 7 is so low that an accurate determination of the offset value is possible on board the vehicle. It is thus also possible to detect and compensate, at least partially, changes in the characteristic curve due to the aging of the NOx detectors.

  Although the invention has been particularly presented and described with reference to a preferred embodiment thereof, it will be readily understood by those skilled in the art that changes in form and detail can be made. without departing from the spirit or scope of the invention.

Claims (12)

RESUME ICAT IONS   A method for determining an OFFSET offset value of a detector signal of an NOx sensitive device (9, 10) placed downstream of an internal combustion engine (1) for determining a concentration of NOx in the exhaust gas of the internal combustion engine (1), characterized in that the internal combustion engine (1) is operated with a stoichiometric ratio (X, = 1) of Air and fuel, in that values of the NOx signal are determined during the stoichiometric regime, and in that a value representative of the determined values is identified as the offset value.   2. Method according to claim 1, characterized in that it is verified whether authorization conditions are fulfilled and, if so, the determination of the offset value is allowed.   3. Method according to claim 1 or 2, characterized in that the determination of the signal values is effected within a selectable time interval.   4. Method according to at least one of the preceding claims, characterized in that, if at least one authorization condition is not satisfied, the determination of the signal values does not take place or is interrupted.   5. Method according to at least one of the preceding claims, characterized in that the verification of the permitting conditions is carried out before the determination of the signal values.   f5. Method according to at least one of the preceding claims, characterized in that the determination of the offset value takes place only after a predetermined waiting time after the verification of the permitting conditions.   Method according to at least one of the preceding claims, characterized in that the checking of the permitting conditions is continued in parallel with the determination of the values of the detector signal.   8. Method according to at least one of the preceding claims, characterized in that at least one of the conditions of the following group is provided as a condition of authorization existence of a stoichiometric ratio air / fuel, existence of a actively controlled lambda control, homogeneous internal combustion engine speed, compliance with a permissible range of a lonk probe signal of the NOx sensitive device (9, 10), at least approximately steady state of the engine of the internal combustion (1), compliance with a permissible temperature range of a catalyst (7), faultless operation of detectors and / or regulating members which can influence the air / fuel ratio, and / or a waiting time after other chosen authorization conditions have been met.   9. Method according to at least one of the preceding claims, characterized in that the determination of the representative value is performed during a homogeneous regime of the internal combustion engine (1).   Method according to at least one of the preceding claims, characterized in that, if more than one NOx sensor (9, 10) is used, the offset value is determined individually for each NOx detector (9, 10).   11. Apparatus for implementing the method 30 according to any one of the preceding claims, characterized in that the NOx sensitive device (9, 10) is placed downstream of an internal combustion engine (1), and in that means are provided for carrying out the method according to any one of claims 1 to 10.   12. Device according to claim 11, characterized in that the NOx-sensitive device (9, 10) is placed downstream of a 3-way catalyst (6) or a NOx accumulation catalyst (7).   13. Device according to at least one of claims 11 and 12, characterized in that the NOx sensitive device (9, 10) comprises at least one NOx detector (9, 10).