Abstract:
A method for operating an electro motor-driven secondary air pump which is provided for injecting secondary air into the exhaust system of an internal combustion engine. The secondary air pump is operated in a clocked manner. The induced voltage at the electro motor of the secondary air pump, measurable during the turn-off time of the clocked operation, is used for diagnosing as well as for controlling the speed of the secondary air pump. The diagnosis is carried out based on the evaluation of a parameter (difference quotient, differential quotient) of the induced voltage. The induced voltage is instantaneously a measure for the secondary air pump&#39;s actual speed value, so that a separate detection of the actual speed value is omitted.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for operating an electro motor-driven secondary air pump which is provided for injecting secondary air into the exhaust gas of an internal combustion engine. 
   BACKGROUND INFORMATION 
   A method is known from German Patent Application No. DE 116 09 922 in which the secondary air pump is checked on the basis of a response to a signal change of a lambda sensor situated in the exhaust gas of the internal combustion engine. Provided are an increase in the exhaust gas mass flow rate and a simultaneous switch-on of the secondary air pump. Due to the increased oxygen portion in the exhaust gas, the switch-on of the secondary air pump results in a signal change of the lambda sensor which in turn causes a response of a lambda controller of the engine control which carries out enrichment of the air-fuel mixture supplied to the engine. The increase in the exhaust gas mass flow rate may be achieved, for example, due to deteriorating efficiency of the engine, for example, by retarding the ignition of an externally ignited engine, by increasing the idle speed, or by connecting additional loads. If a response of the lambda controller is able to be determined, the secondary air pump is considered to be working correctly. 
   A further method for operating a secondary air pump is known from German Patent No. DE 199 52 836 in which an evaluation of the secondary air pump&#39;s operational performance is provided. The evaluation takes place based on the air-fuel mixture supplied to the engine, on the measured oxygen content of the exhaust gas, and on the measured air mass flow flowing to the engine. 
   German Patent Application No. DE 199 63 902 describes a method for operating an internal combustion engine, in which the focus is on a diagnosis of a catalytic converter. An increase in the hydrocarbon portion in the exhaust gas and simultaneously an increase in the secondary air result in an exothermic reaction in the catalytic converter. Based on the temperature control in the catalytic converter, it may be concluded that an increase in the secondary air has actually taken place and that the secondary air pump is working correctly. 
   A method and a device for monitoring the operational performance of a secondary air system of an internal combustion engine is known from German Patent Application No. DE 102 05 906 in which the electric operating current of the secondary air pump is analyzed. The secondary air pump may be operated using a variable power which is predefined within the scope of an electrical clocked operation. The electric operating current must lie within a predefined tolerance range, the tolerance range being a function of the pulse duty factor of the clocked operation. Moreover, the atmospheric pressure or the exhaust gas back-pressure may be taken into account. The operating voltage of the secondary air pump&#39;s electro motor may additionally be taken into account when predefining the tolerance range. 
   An object of the present invention is to provide a method for operating an electro motor-driven secondary air pump which makes easy adjustment of the secondary air flow rate and diagnosis of the secondary air system possible. 
   SUMMARY OF THE INVENTION 
   The present invention makes easy adjustment of the secondary air flow rate and easily executed diagnosis of the operational performance of the secondary air system possible. The electro motor-driven secondary air pump is operated in a clocked manner. During the clocked operation, the secondary pump&#39;s electro motor is connected to and disconnected from an electric power source in rapid time-based succession. The cycle period and/or the turn-on time or the turn-off time may be predefined. During the turn-off time, an induced voltage, which is proportional to the secondary air pump&#39;s speed, occurs in the secondary air pump&#39;s electro motor after decay of an inductive voltage portion. Diagnosis of the secondary air system takes place by comparing a parameter of the induced voltage to at least one predefined threshold value. 
   One embodiment provides that the actual value of the induced voltage is used as the parameter. The threshold value is to be defined as an induced voltage which must be exceeded in any case. An alternative or additional embodiment provides that the temporal change of the induced voltage is used as the parameter for the diagnosis. The differential quotient and/or the difference quotient may be analyzed. The analysis of the temporal change corresponds to an analysis of a change in the secondary air pump&#39;s speed during the turn-off time of the clocked operation. 
   One refinement provides that the induced voltage is only detected after a gating time, which follows the turn-on time, has elapsed. The effect of the inductive voltage portion is largely suppressed due to this measure. 
   One refinement provides that the at least one threshold value is corrected as a function of the supply voltage of the secondary air pump&#39;s electro motor. The effect of the supply voltage on the secondary air pump&#39;s speed is taken into account due to this measure. 
   A particularly advantageous embodiment of the method according to the present invention provides that the secondary air pump&#39;s speed is adjusted to a predefined speed setpoint value. This embodiment makes an accurate adaptation of the secondary air mass flow rate to the exhaust gas mass flow rate of the engine possible, thereby achieving efficient heating of an exhaust gas treatment device situated in the exhaust system of the engine under simultaneously minimal exhaust emission. 
   One refinement provides that the secondary air mass flow rate setpoint value is defined as a function of a predefined lambda setpoint value of the engine. An intended exhaust gas composition without a change in the fuel mass supplied to the engine may be maintained via this measure. Variations within the series due to deviating mechanical tolerances may be compensated within the scope of an adaptation, in particular at the initial startup of the engine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a technical environment in which a method according to the present invention is executed. 
       FIG. 2  shows a voltage characteristic as a function of time. 
       FIG. 3  shows in detail how a speed setpoint value is determined. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an internal combustion engine  100 , an airflow rate sensor  102  being situated in its intake system  101  and a lambda sensor  104  as well as an exhaust gas treatment device  105  being situated in its exhaust system  103 . Secondary air, which is delivered by an electro motor-driven secondary air pump  106 , may be injected into exhaust system  103 . 
   An engine control  110  is supplied with an air flow rate signal ml provided by air flow rate sensor  102 , a speed signal N provided by engine  100 , a lambda signal lam provided by lambda sensor  104  and a torque setpoint signal mifa. 
   Engine control  110  transmits a fuel signal mK to a fuel metering device  111  which is assigned to engine  100 . Engine control  110  contains a speed setpoint value-determination  112  which forwards a measure for a speed setpoint value ns of secondary air pump  106  to a speed controller  113 , which in turn transmits a control signal  134  to a pulse width modulator  121 . 
   Using a switch control signal  130 , pulse width modulator  121  controls a switch  131  which connects a power source  132  to the electro motor of secondary air pump  106 . Power source  132 , which is connected to a threshold value selector  122 , has a supply voltage referred to as Ub. 
   A gate circuit  140  is connected to the electro motor of secondary air pump  106 . Using a gate signal  141 , gate circuit  140  is controlled by pulse width modulator  121 . Gate circuit  140  provides an induced voltage Ui which is available to speed controller  113  as well as to a comparator  142 . Comparator  142 , supplied with a threshold value  143  by threshold value selector  122 , provides a diagnostic signal  144 . 
     FIG. 2  shows a voltage characteristic U as a function of time t. At point in time zero, voltage characteristic U has supply voltage Ub of power source  132 . A turn-off time T 10  starts at a first point in time T 1 ; the turn-off time ends at a second point in time T 2  at which a turn-on time T 11  starts. Turn-on time T 11  ends at a third point in time T 3 . Turn-off time T 10  and turn-on time T 11  together form a cycle period T  12 . Starting with first point in time T 1 , a gating time T 13  lies within turn-off time T 10 . The remaining time during turn-off time T 10  is a measuring time T 14 . 
   An inductive voltage peak  200  occurs during gating time T 13 . After gating time T 13  has elapsed, induced voltage Ui occurs which has a maximum voltage U 1  and a minimum voltage U 2 , minimum voltage U 2  coinciding with second point in time T 2 . After second point in time T 2 , voltage U jumps again to supply voltage Ub of power source  132 . A voltage Umin is indicated as an example of a threshold value  143 . 
     FIG. 3  shows in detail the speed setpoint determination  112  contained in engine control  110 . Provided is a setpoint heat quantity selector  300  which gives out a setpoint heat quantity  301  to an adder  302 . Moreover, an engine heat quantity determination  303  is provided which gives out an engine heat quantity  304  to adder  302 . Air flow rate signal ml, fuel signal mK, torque setpoint signal mifa, and an additional input signal  305  are supplied to engine heat quantity determination  303 . 
   Adder  302  forwards an effective heat quantity  310  to a secondary air mass flow rate determination  311  which is also supplied with a setpoint lambda  312 . A secondary air mass flow rate setpoint value mssl, determined by secondary air mass flow rate determination  311 , is supplied to a speed setpoint value predefinition  313  which provides the measure for speed setpoint value ns. 
   The method works as follows: 
   Injection of secondary air into exhaust system  103  of engine  100  is provided for elevating the temperature of exhaust treatment device  105 . Exhaust treatment device  105  may be, for example, at least one catalytic converter and/or a particle filter and/or another device provided for emission control. Exhaust treatment device  105  may have a minimum operating temperature and, for correct operation, exhaust treatment device  105  may not fall below that minimum temperature. In a different case, an elevated temperature of exhaust treatment device  105  may be necessary to regenerate exhaust treatment device  105 . Heating of exhaust treatment device  105  may become necessary in particular at a cold start of engine  100  or at a restart after an extended shut-off phase of engine  100 . 
   The secondary air injected into the exhaust system, in particular into an exhaust manifold, reacts with combustible exhaust gas components which are suitably inserted into the exhaust gas. The additional insertion of combustible exhaust gas components results in increased fuel consumption of engine  100 . Hence, targeted injection of the secondary air is necessary to ensure operation of engine  100  which is as fuel efficient as possible. Moreover, undesirable exhaust gas components, which exhaust treatment device  105  may no longer be able to remove, may occur when the secondary air quantity is too high or too low. Also because of this reason, it is desirable to meter the secondary air as exactly as possible. 
   Engine control  110  determines fuel signal mK for fuel metering device  111  at least as a function of speed N of engine  100  and/or of air flow rate signal ml and/or of lambda signal lam and/or of torque setpoint signal mifa, torque setpoint signal mifa corresponding to a position of an accelerator pedal (not shown) for example. 
   Speed setpoint value determination  112 , preferably contained in engine control  110 , determines the measure for speed setpoint value ns of the electro motor (not shown) of secondary air pump  106 . In the following, reference is made only to secondary air pump  106 . The starting point is setpoint heat quantity selector  300  which, during a calibration of engine control  110  for example, may be set to a value necessary for adequately heating exhaust treatment device  105  in order to meet predefined exhaust gas limiting values at predefined points in time. Part of the necessary heat quantity is provided by engine  100  itself. The contribution is determined by engine heat quantity determination  303 . The determination may take place, for example, as a function of air flow rate signal ml and/or of fuel signal mK and/or of torque setpoint signal mifa and/or speed N (not shown) of engine  100 , as well as of further input signal  305 . Further input signal  305  reflects, for example, the efficiency of the engine at a given working point. Provided that lambda sensor  104  is operational, lambda signal lam may additionally be taken into account. 
   Adder  302  subtracts engine heat quantity  304 , determined by engine heat quantity determination  303 , from setpoint heat quantity  301  of setpoint heat quantity selector  300  and forwards effective heat quantity  310  to secondary air mass flow rate determination  311 . 
   As a function of effective heat quantity  310  and possibly as a function of a predefined setpoint lambda  312 , secondary air mass flow rate determination  311  defines secondary air mass flow rate setpoint value mssl. Setpoint lambda  312  may be adapted during the initial startup of engine  100 , as well as during subsequent operation after lambda sensor  104  became operational. As a function of secondary air mass flow rate setpoint value mssl, speed setpoint value predefinition  313  defines the measure for speed setpoint value ns which is supplied to speed controller  113 . 
   Pulse width modulator  121  defines switch control signal  130  for switch  131  based on control signal  134  provided by speed controller  113 . Switch control signal  130  contains cycle period T 12 , turn-off time T 10 , and turn-on time T 11  lying within that cycle period. Cycle period T 12  is to be adjusted to the electro motor of secondary air pump  106 . In connection with switch  131 , switch control signal  130  selects a clocked operation of secondary air pump  106  in which, by operating at a predefined efficiency level, secondary air pump  106  is supposed to work at the predefined setpoint speed. The periodic connection of secondary air pump  106  to power source  132  results in a selection of a middle operating voltage level of secondary air pump  106 . Cycle period T 12  of switch control signal  130  is between 10 milliseconds and 100 microseconds, for example. A longer cycle period T 12  increasingly reduces the advantages of the clocked operation. A shorter cycle period T 12  results in increased strain on switch  131 . The middle operating voltage level is set via a variation of turn-off time T 10  and turn-on time T 11 . 
   The clocked operation makes it possible to determine the speed of secondary air pump  106  using induced voltage Ui which occurs within turn-off time T 10 . The part of induced voltage Ui, which reflects the speed, occurs after decay of inductive voltage peak  200 . Gate circuit  140  has the task to gate the inductive voltage peak, occurring during gating time T 13 , as well as supply voltage Ub, present during turn-on time T 11 . Induced voltage Ui, which is supplied to both comparator  142  and speed controller  113 , occurs at the output of gate circuit  140 . Induced voltage Ui is simultaneously a measure for the actual speed value of secondary air pump  106 . 
   According to a first exemplary embodiment, induced voltage Ui is instantaneously compared in comparator  142  to at least one threshold value  143  provided by threshold value selector  122 . Threshold value  143  corresponds, for example, to voltage Umin listed in  FIG. 2 . If induced voltage Ui falls below voltage Umin, it may be assumed that the speed of secondary air pump  106  is too low. Sluggishness of secondary air pump  106  could be present. If induced voltage Ui drops to a zero value, then it must be assumed that secondary air pump  106  is blocked or a connection is interrupted. 
   The difference of induced voltage Ui which occurs during measuring time T 14  may additionally or alternatively be evaluated. The difference between maximum voltage U 1  and minimum voltage U 2  may be determined in the shown exemplary embodiment. Moreover, determination of a difference quotient may additionally or alternatively be provided. A reference to measuring time T 14  is not necessary here, provided that measuring time T 14  is constant during comparable measurements. 
   Moreover, the momentary increase in induced voltage Ui may additionally or alternatively be determined, which corresponds to determining a differential quotient. In all of these cases, it is a matter of determining the change in induced voltage Ui. An evaluation of the determined change takes place by correspondingly selecting threshold values for the difference quotient and/or the differential quotient. The determination of the difference quotient and/or the differential quotient provides a measure for the deceleration of secondary air pump  106  during measuring time T 14 . 
   A faulty opening cross section of a secondary air valve, not shown in  FIG. 1 , could be present in addition to sluggishness of secondary air pump  106 . 
   Provided that an excess or a shortfall of the at least one predefined threshold value  143  is detected in comparator  142 , comparator  142  provides diagnosis signal  144  which reflects an error in the secondary air system of engine  100 . 
   Obtaining induced voltage Ui during measuring time T 14  has the considerable advantage that induced voltage Ui may instantaneously be used as a measure for the actual speed value of secondary air pump  106 . Therefore, induced voltage Ui makes it possible without additional expense to implement the speed regulation of secondary air pump  106  using speed controller  113 .