Abstract:
A method for detecting combustion misfires of an internal combustion engine and a device for carrying out the method, in which an evaluation of a lambda signal made available by at least one broadband lambda sensor is provided. The lambda signal is compared to a lambda signal threshold value. A warning signal is made available in response to the exceeding of a threshold in the direction of lean combustion. The procedure especially makes possible the detection of continual misfires which occur, for example, in response to an error in the air supply to the internal combustion engine and/or because of a lack of fuel. The procedure makes possible the detection of dangerous situations for at least one exhaust gas treatment device that is present, which may be removed by introducing protective measures, error warnings and error memory entries.

Description:
BACKGROUND INFORMATION 
   A procedure is described, for example, in European Patent No. EP 793 803, in which a crankshaft angle range designated as a segment is assigned to each cylinder of the internal combustion engine. The segment time in which the crankshaft sweeps over this angle range depends, among other things, on the energy converted in the combustion cycle. Sporadic misfires lead to an increase in segment times registered synchronously with ignition. A misfire may prolong one or more segment times. A measure for the irregular operation of the internal combustion engine is calculated by a suitable determination of filter coefficients of a programmable digital filter, the filtering being able to extend over an angular range of up to four camshaft revolutions of the internal combustion engine. 
   In PCT Publication No. WO 90/02874 a procedure of this type is described in which a misfire is detected with the aid of a lambda signal made available by a lambda sensor. The lambda sensor has a great sensitivity with respect to lambda changes in range of a stoichiometric combustion. A misfire expresses itself by a change in the lambda signal, because of a short-range increase in the oxygen concentration in the exhaust gas connected with it. The misfire may be allocated to a specific cylinder by consideration of the gas running time from the cylinder to the lambda sensor. 
   Since a misfire leads to the appearance of uncombusted fuel in the exhaust duct of the internal combustion engine, an exhaust gas treatment device that is present there may be damaged by an inadmissibly high temperature increase which may appear within the scope of an exothermic reaction that may possibly be taking place. If misfires are detected, suitable measures may be taken up for protecting the exhaust gas treatment device from inadmissibly high temperatures. For example, the fuel supply to the affected cylinder may be eliminated. The appearance of sporadic misfires in a cylinder or a change in regular combustion processes and misfires leads to a detectable increase in irregular operation, so that the misfires are able to be detected using the known methods. An increased irregular operation does not, however, appear if the combustions misfire in all cylinders within a time that lies within the range of seconds. If, for example, the fuel pressure drops down to ten percent, an air ratio lambda of more than 3 may appear, at which combustion of fuel in an Otto internal combustion engine becomes impossible. Because of the heating energy contained in the uncombusted fuel, the temperature in a catalytic converter connected downstream from the internal combustion engine may increase in a few seconds from the operating temperature of, for example, 500° C. to 1300° C., for instance. At this temperature, a thermal destruction of the catalyst structure can no longer be excluded. 
   The present invention is based on the object of providing a method for detecting misfires of an internal combustion engine and a device for carrying out the method, which are especially conditioned on lack of fuel and/or on errors in the fuel supply of the internal combustion engine. 
   SUMMARY OF THE INVENTION 
   The procedure according to the present invention for detecting misfires of an internal combustion engine starts from an evaluation of a lambda signal made available by at least one broadband lambda sensor. The lambda signal is compared to a lambda signal threshold value which is specified within the range of a lean combustion at an air ratio of lambda greater than 1. If the lambda signal exceeds the lambda signal threshold value in the direction of lean combustion, a warning signal is made available. 
   The procedure according to the present invention makes possible the detection of a sequence of several misfires, which may appear especially because of too high an air proportion and/or too low a fuel proportion of the mixture before ignition. An endangering situation for the exhaust gas treatment device may be detected, which may be created by an input of uncombusted fuel which is able to be oxidized in a catalytic converter or a particulate filter. Because of the considerable heating value of the fuel, the temperature of the exhaust gas treatment device may rise to inadmissibly high values in response to the oxidation reaction which, at least if the reaction lasts for a while, may lead to the destruction of the exhaust gas treatment device. 
   The misfires may be caused by a lack of fuel. For example, there may be an error in the fuel supply. In the simplest case, the fluid level in a fuel tank has dropped to a level at which the fuel pump is no longer able reliably to aspirate the fuel. An error condition having several directly subsequent misfires instead of sporadic misfires is noticeable particularly in response to fuel supplies in which a return flow is being done without. In such a system the fuel pressure may sink to a low value, so that, to be sure, fuel is still being conveyed, but the rate of flow is not sufficient for keeping up an orderly combustion. If the fuel pressure falls, for example, to ten percent of the original value, this drop may lead to an air ratio lambda of greater than 3, at which combustion of the air/fuel mixture in an Otto internal combustion engine is no longer possible. 
   An additional cause for error may be an increased air supply. An undetected, increased air supply may occur if, for example, a crankcase breather hose falls off which connects the crankcase to at least one intake port of the internal combustion engine, the feed into the intake port lying downstream of the air sensor in the flow direction. 
   The procedure according to the present invention especially makes possible the detection of continual misfires, which represent an endangering situation for the exhaust gas treatment device because of uncombusted fuel proportions that are still present. 
   Suitable measures for protecting the exhaust gas treatment device may be undertaken by making available the warning signal. 
   One embodiment provides that the warning signal is made available if the lambda signal threshold value is exceeded for a specifiable time period. This measure may be designated as a first plausibility check of the warning signal. Only misfires present for a short period of time, which do not yet represent an endangering situation for the exhaust gas device, are able to be detected. In this situation, protective measures for the exhaust gas treatment device do not have to be undertaken. A design that may possibly be planned in addition provides checking the increase time of the lambda signal after the exceeding of a threshold. The warning signal is made available if the time of increase exceeds a specified threshold value. 
   An additional design, that may possibly be planned in addition, provides checking the gradient of the lambda signal after the exceeding of a threshold. The warning signal is made available if the gradient exceeds a specified threshold value. An undershooting corresponds to a slower increase than an exceeding. In practice, the gradient is ascertained from at least one difference quotient instead of the differential quotient. 
   One embodiment provides that, in specified operating situations of the internal combustion engine, the warning signal is not made available. Such an operating situation is, for instance, an overrun fuel cutoff, in which, because of the complete discontinuation of the fuel, the lambda signal exceeds the specified lambda signal threshold value within a short time. 
   One embodiment provides that the warning signal is made available if, when the threshold is exceeded, a lambda controller is working on an enrichment limit. A further embodiment, which may possibly be also provided, provides for making available the warning signal if, when the threshold is exceeded, the lambda controller, in a specified time period before the exceeding of the threshold, had already at least once worked on an enrichment limit. 
   One embodiment provides that an additional lambda signal of a lambda sensor that is preferably situated after the exhaust gas treatment device is taken into consideration. Using this measure, the functioning of the broadband lambda sensor, that is preferably situated upstream of the exhaust gas treatment device, may be checked. If the lambda signal made available by the broadband lambda sensor does not appear to be plausible, the warning signal is not made available. 
   One other embodiment provides that the enrichment limit of the lambda controller is increased, and that it is subsequently checked whether the lambda signal of the broadband lambda sensor returns to an admissible range. The warning signal is not made available if the return to the admissible range is possible. A constant lack of fuel and/or a constant interference in the air supply is unlikely under these circumstances. 
   An additional embodiment provides that, in an internal combustion engine having more than one bank of cylinders, the warning signal is made available if there is one exceeding of a threshold in at least two banks of cylinders. 
   Still another embodiment provides that the warning signal is only made available if, in addition, a fuel pressure signal made available by a fuel pressure sensor and/or a fluid level signal made available by a fuel fluid level sensor lies below a specified threshold value, respectively. 
   The warning signal made available is preferably deposited in an error memory. In order to warn a driver of a motor vehicle, corresponding information may be indicated (EPCL lamp, electronic power control lamp). 
   One particularly advantageous further development provides that, in case a warning signal has appeared, at least one measure is introduced to protect the exhaust gas treatment device against overheating. Such a measure is, for example, the suppression of a fuel injection signal and/or, for instance, the closing of a throttle valve situated in the intake port of the internal combustion engine. 
   The device according to the present invention for carrying out the method according to the present invention relates first to a control unit that is prepared for carrying out the method. 
   The control unit includes particularly a signal analysis for the signal made available by at least one lambda sensor. 
   The control unit preferably includes at least one electrical memory in which the method steps are stored as a computer program. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a technical environment in which a method according to the present invention is running. 
       FIGS. 2   a - 2   f  show signal curves as a function of time. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an internal combustion engine  10 , which has a first cylinder bank  11 , a second cylinder bank  12  and a crankcase housing  13 . Crankcase housing  13  is connected via a ventilation hose  14  to an intake port  20 , in which is situated a throttle valve  21 . 
   First cylinder bank  11  is connected to a first exhaust duct  30  in which there are situated a first broadband lambda sensor  31 , a first exhaust gas treatment device  32 , and a first abrupt change lambda sensor  33 . Second cylinder bank  12  is connected to a second exhaust duct  35 , in which there are situated a second broadband lambda sensor  36 , a second exhaust gas treatment device  37 , and a second abrupt change lambda sensor  38 . 
   First broadband lambda sensor  31  passes to control system  40  a first lambda signal lam 1 , second broadband lambda sensor  36  passes to control system  40  a second lambda signal lam 2 , first abrupt change lambda sensor  33  a third lambda signal lam 3 , and second abrupt change lambda sensor  38  a fourth lambda signal lam 4 . 
   The first and third lambda signals lam 1 , lam 3  are supplied to a first lambda controller  41 , which, as a function of the comparison with a lambda setpoint value lamSW, makes available a first controlled variable fr 1 , which is supplied to a signal conditioning  43 . The second and fourth lambda signals lam 2 , lam 4  are supplied to a second lambda controller  45 , which, as a function of the comparison with the lambda setpoint value lamSW, makes available a second controlled variable fr 2 , which is supplied to the signal conditioning  43 . 
   Signal conditioning  43  makes available the first and the second fuel signal mE 1 , mE 2 . First fuel signal mE 1  is made available to a first fuel metering device  50  and second fuel signal mE 2  fuel signal mE 1  is made available to a second fuel metering device  51 . 
   The two fuel metering devices mE 1 , mE 2  are connected to a fuel line  53  which is connected to a fuel pump  54 . Fuel pump  54  is situated in a tank  55 , whose fluid level is recorded by a fluid level sensor  56 , which gives off a fluid level signal FT to control system  40 . The fuel pressure present in fuel line  53  is recorded by a fuel pressure sensor  57 , which makes available a fuel pressure signal pK to control system  40 . 
   Control system  40  includes a signal analysis  60  which makes available a warning signal  61  that is supplied to both an error memory  62  and a display  63 . Signal analysis  60  has a guard signal A supplied to it. 
   Control system  40  also has a torque setpoint signal MFa supplied to it and sends a throttle valve signal DK to throttle valve  21 . 
     FIGS. 2   a - 2   f  show signal curves as a function of time. 
     FIG. 2   a  shows a temperature curve of first or second exhaust gas treatment device  32 ,  37 .  FIG. 2   b  shows the first or second lambda signal lam 1 , lam 2 .  FIG. 2   c  shows the first or second controlled variable fr 1 , fr 2 .  FIG. 2   d  shows a guard signal A.  FIG. 2   e  shows the fuel pressure signal pK, and  FIG. 2   f  shows warning signal  61 . 
   In  FIG. 2   b  a lambda signal threshold value Lim 1  and in  FIG. 2   c  an enrichment limit Lim 2  of the first or second lambda controller  41 ,  45  are drawn in. Controlled variable fr 1 , fr 2  first reaches enrichment limit Lim 2  at a first point in time T 1 . 
   At second point in time T 2  it leaves enrichment limit Lim 2  again. Between a third and a fourth point in time T 3 , T 4  guard signal A appears. Shortly before fifth point in time T 5 , fuel pressure signal pK falls off from a predefined fuel nominal value pKN to a lower value. At fifth point in time T 5 , first or second lambda signal lam 1 , lam 2  rises slowly, and exceeds lambda signal threshold value Lim 1  at sixth point in time T 6 . At seventh point in time T 7 , first or second lambda signal lam 1 , lam 2  furthermore demonstrates a high level that lies above lambda signal threshold value Lim 1 . At an eighth point in time T 8 , enrichment limit Lim 2  is raised to a higher value. Between first and fifth point in time T 1 , T 5  there is a time range tB. Between sixth and seventh point in time T 6 , T 7  there is a time duration tD. 
   The method according to the present invention for detecting misfires of internal combustion engine  10  works in the following manner: 
   Control system  40  determines first and second fuel signal mE 1 , mE 2  and/or throttle valve signal DK as a function of torque setpoint value MFa, which is influenced, for instance, by an accelerator of a motor vehicle that is not shown in detail. First fuel signal mE 1  determines the fuel quantity supplied to the cylinders of first cylinder bank  11  and second fuel signal mE 2  determines the fuel quantity supplied to the cylinders of second cylinder bank  12 . 
   The fuel is taken from tank  55 , whose fluid level is recorded by fluid level sensor  56 , and is made available to control system  40  using fluid level signal FT. Fuel pump  54  establishes specified fuel nominal pressure pKN in fuel line  53 , which is able to be monitored by fuel pressure sensor  57 , which makes available fuel pressure signal pK to control system  40 . 
   In the following, at first reference is made only to components and signals that are assigned to first cylinder bank  11 . The exhaust lambda is monitored with the aid of first lambda signal lam 1  made available by first broadband lambda sensor  31 . First lambda signal lam 1  is a first input variable of first lambda controller  41 , which compares first lambda signal lam 1  to the specified lambda setpoint value lamSW, and ascertains first controlled variable fr 1  as a function of the comparison. 
   Signal conditioning  60  includes, for example, correcting elements for influencing first controlled variable fr 1 . In addition, if appropriate, first fuel signal mE 1  is formed from corrected controlled variable fr 1 , and it is conducted to first fuel metering device  50 . First fuel signal mE 1 , for example, specifies the injection period of fuel injections into the cylinders of first cylinder bank  11 . 
   The exhaust gas of first cylinder bank  11  is purified of undesired components in first exhaust gas treatment device  32 . Exhaust gas treatment device  32  is, for instance, a catalytic converter and/or particulate filter. First abrupt change lambda sensor  33 , which makes available third lambda signal lam 3 , is preferably situated downstream of exhaust gas treatment device  32 . With the aid of third lambda signal lam 3 , one may increase the accuracy of a lambda control circuit which, as the lambda actual value detector, includes first broadband lambda sensor  31 . 
   What is important to the execution of the method according to the present invention is the capability of a broadband lambda sensor to continually be able to measure a broad lambda range, such as 0.6-3.0. By contrast, an abrupt change lambda sensor makes available an abrupt change signal, using which the presence of a stoichiometric combustion is able to be recorded with great accuracy at an air ratio lambda of, for instance, 0.995 to 1.0. 
   According to  FIG. 2   a , first lambda signal lam 1  is at least approximately 1, which corresponds to a stoichiometric combustion. After first point in time t 1 , no enleanment of the mixture occurs which is reflected by a measured increase of the quantity of the first lambda signal lam 1  to greater than 1. At the same time, beginning at first point in time t 1 , the temperature of first exhaust gas treatment device  32  rises from an operating temperature of, for instance, 800° C. to, for instance, 850° C. 
   First lambda controller  41  reacts to the increase of first lambda signal lam 1  using first controlled variable fr 1 , in order to enrich the mixture. First controlled variable fr 1 , which is, for instance, supposed to have the relative value 1, is increased, in the exemplary embodiment shown, up to enrichment limit Lim 2 , which is, for example, about 1.2. At second point in time t 2 , the disturbance has been corrected again. The temperature of first exhaust gas treatment device  32  drops off again to the operating temperature of, for instance, 800° C. 
   At third point t 3  a predefined operating state of internal combustion engine  10  occurs, which is signaled using guard signal A. What is involved here is, for instance, an overrun operation of internal combustion engine  10 , in which no more fuel is metered in. One immediate reaction is the increase of the oxygen proportion in the exhaust gas to large values, which is immediately reflected in lambda signal lam 1 . According to the exemplary embodiment, air ratio lambda rises to a value&gt;3. 
   Signal analysis  60  evaluates first lambda signal lam 1  first of all by a comparison to the predefined lambda signal threshold value Lim 1 , which, in the exemplary embodiment shown, is approximately 1.8. Lambda signal threshold value Lim 1  is preferably fixed at a value at which the transition from a stable to an unstable combustion occurs. Preferably, lambda signal threshold value Lim 1  is fixed to a value that is above the threshold at which, in an Otto internal combustion engine, combustion can no longer take place. At third point in time t 3 , first lambda signal lam 1  exceeds lambda signal threshold value Lim 1 . Upon the exceeding of the threshold, warning signal  61  appears per se. The simultaneous presence of guard signal A, which is supplied to signal analysis  60 , may be drawn upon so that warning signal  61  is not ascertained or is at least suppressed. 
   Shortly before fifth point in time t 5 , a pressure drop-off in the fuel pressure occurs, which is reflected in fuel pressure signal PK. As a result, first lambda signal lam 1  rises slowly, until, at sixth point in time t 6 , it exceeds specified lambda signal threshold value Lim 1 . Since guard signal A is not present, warning signal  61  could already be made available at sixth point in time t 6 . 
   Preferably a plausibilization is undertaken. A first advantageous possibility provides the specification of time period tD, during which the exceeding of the threshold has to be present. Only after the expiration of time period tD at seventh point in time t 7  is warning signal  61  made available. Alternatively, or possibly in addition, a change of lambda signal lam 1  is checked after the exceeding of the threshold at sixth point in time t 6 . The time of the increase may be measured, for example. Alternatively or in addition, the gradient may be ascertained at which, in a practical implementation, at least one difference quotient is ascertained. Warning signal  61  is made available when the time of increase is slower than a threshold value or the gradient is smaller than a threshold value. A rapid increase would indicate a sporadic misfire or another mixture interference that is not supposed to lead to making available warning signal  61 . 
   A plausibilization, which could possibly be provided in addition, provides the inclusion of first controlled variable fr 1  of first lambda controller  41 . It is checked whether first lambda controller  41  has worked at least once already at enrichment limit Lim 2 , within the preferably specified time range tB before the sixth point in time t 6 . In the exemplary embodiment shown, this was the case between the first and second point in time t 1 , t 2 , as well as beginning at fifth point in time t 5 . At fifth point in time t 5 , at the same time as the exceeding of the threshold, enrichment limit Lim 2  is present. 
   An additional plausibilization, which may possibly be provided in addition, provides the inclusion third lambda signal lam 3 , that is made available by first abrupt change lambda sensor  33 . The inclusion especially makes possible a plausibilization of first lambda signal lam 1 , made available by first broadband lambda sensor  31 . The increase in first lambda signal lam 1  after fifth point in time t 5 , which indicates lean combustion, must be reflected in a jump of third lambda signal lam 3 , it being necessary to take into consideration a running time of the exhaust gas through first exhaust gas treatment device  32  as well as an oxygen-storing capability of first exhaust gas treatment device  32 . 
   Another plausibilization, which may possibly be additionally provided, provides the inclusion of more than one cylinder bank  11 ,  12  of internal combustion engine  10 . An exceeding of a threshold must be present in more than one cylinder bank  11 ,  12 . Warning signal  61  is only made available when this condition has been satisfied. 
   Another plausibilization, which may possibly be additionally provided, provides the inclusion of fuel pressure signal pK made available by fuel pressure sensor  57  and/or fluid level signal FT made available by fluid level sensor  56 . Warning signal  61  is only made available if fuel pressure signal pK and/or fluid level signal FT are below specified threshold values, respectively. A further plausibilization, which could possibly be provided additionally, provides an increase in enrichment limit Lim 2 . The relevant increase is undertaken at the eighth point in time t 8 , in the exemplary embodiment. Subsequently to the increase in enrichment limit Lim 2  of first and/or second lambda controller  41 ,  45  it is checked whether first or second lambda signal lam 1 , lam 2  returns again to an admissible range. The admissible range may lie below the specified lambda signal threshold value Lim 1 . However, a higher value may also be specified. What is important here is that lambda signal lam 1 , lam 2  falls off to lower values. In the exemplary embodiment shown, the condition is satisfied, because according to  FIG. 2   e  the fuel pressure is supposed to rise again to fuel pressure nominal value pKN as of seventh point in time t 7 . 
   If protective measures had already been taken, these may be taken back. However, an input into error memory  62  should not be canceled. Display  62  activated by warning signal  61  should continue to indicate the warning statement, since a considerable endangerment potential for exhaust gas treatment device  32 ,  37  has occurred at least once. 
   According to one further improvement it may be provided that warning signal  61  is made available only if the exceeding of the threshold has occurred a predefined number of times. 
   Shortly after fifth point in time t 5 , the temperature of exhaust gas treatment device  32 ,  37  may rise. Making available warning signal  61  at sixth point in time t 6  or, at the latest, at seventh point in time t 7 , makes possible the introduction of measures for protecting exhaust gas treatment device  32 ,  37 . It may, for example, be provided that fuel signal mE 1 , mE 2  be suppressed, so that no more fuel is supplied to the cylinders of internal combustion engine  10 . Alternatively or supplementarily, it may be provided that throttle valve  21  is closed. The measures to be introduced have as their aim either to reduce the fuel proportion or the oxygen proportion appearing in exhaust duct  30 ,  35  in order to prevent an exothermic reaction, or to get a reaction that has already started to die down.