Abstract:
In a method for adjusting the fuel/air ratio in an internal combustion engine ( 1 ) comprising a converter ( 2 ) which is associated therewith, a composition of waste gas in the waste gas wing ( 3, 8 ) of the internal combustion engine ( 1 ) is detected by means of sensors ( 4, 5 ) and output signals from at least one of the sensors ( 4, 5 ) are used for producing a control signal in order to influence the fuel/air ratio. The fuel/air ratio is switched back and forth between a lean operating state with surplus oxygen and a rich operating state with an oxygen deficit by means of a characteristic line of the control signal. The characteristic line of the control signal is adapted to a current converter state. A characteristic curve contour of the characteristic line is adjusted according to the addition and/or desorption of an oxidation agent in the converter ( 2 ). The invention also relates to a device for carrying out said method.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation of copending International Application No. PCT/EP2003/014968 filed Dec. 30, 2003 which designates the United States, and claims priority to German application no. 102 61 751.1 filed Dec. 30, 2002 and German application no. 103 10 672.3 filed Mar. 12, 2003. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention concerns a method and device for adjustment of a fuel/air ratio for an internal combustion engine.  
       BACKGROUND  
       [0003]     A catalyst arranged in the exhaust system of an internal combustion engine is ordinarily used to clean the exhaust of an internal combustion engine. This converts harmful components, like hydrocarbons CH, carbon monoxide CO and oxides of nitrogen NO x  essentially to nontoxic gases. It is critical to the so-called degree of conversion of the catalyst that the oxygen content of the exhaust lie within optimal values. For a so-called three-way catalyst these optimal values lie in a narrow range around the value corresponding to a fuel/air mixture of λ=1. In order to be able to maintain this narrow range, it is customary to regulate the fuel/air ratio for an internal combustion engine by means of oxygen sensors arranged in the exhaust system of an internal combustion engine.  
         [0004]     A method for lambda control for internal combustion engine with a downstream catalyst is known from Unexamined Patent Application DE 40 24 212 A1 in which the oxygen fractions of the exhaust of the internal combustion engine are recorded by oxygen sensors upstream and downstream of the catalyst. In stipulated operating ranges a control signal with controllable amplitude is generated by coupling in an outside signal with controllable amplitude. With increasing catalyst aging the amplitude is reduced. The functional state of the catalyst in the exhaust system of the internal combustion engine can be determined with the method by means of lambda regulation and the time for replacement of an aged catalyst determined.  
         [0005]     A method for adjustment of the fuel/air ratio for an internal combustion engine with a downstream catalyst is known from Unexamined Patent Application DE 43 37 793 A1 in which the oxygen fractions in the exhaust of the internal combustion engine are determined by oxygen sensors upstream and downstream of the catalyst. Both sensors influence regulation of the fuel/air ratio. It is initially determined with an amplitude evaluation whether the catalyst has already reached a certain degree of aging. This actual control quantity is issued by the sensor upstream of the catalyst. A switch is then made to frequency evaluation or frequency regulation in which the catalyst yields the actual control quantity downstream of the catalyst. Such evaluations are sensitive per se, but have a relatively strong influence on the operating behavior of the internal combustion engine. This is avoided by only switching to frequency evaluation when the catalyst has already reached a certain state of aging. With increasing operating time the oxygen storage capability of the catalyst declines. The control frequency therefore increases with increasing catalyst aging so that lambda regulation is adjusted to the state of aging of the catalyst. As soon as the determined control frequency downstream of the catalyst is higher than the frequency threshold, aging of the catalyst can be reliably recognized and the catalyst replaced.  
       SUMMARY  
       [0006]     The object of the invention is to improve the method for adjusting the fuel/air ratio for an internal combustion engine with a downstream catalyst according to the prior art and permit greater dynamics, as well as to devise an apparatus for execution of the method.  
         [0007]     This object can be solved by a method for adjustment of a fuel/air ratio for an internal combustion engine with an associated catalyst, comprising the steps of determining an exhaust composition in an exhaust system of the internal combustion engine by means of sensors, generating a control signal to influence the fuel/air ratio as a function of output signals of at least one of the sensors, and making by means of a characteristic curve of the control signal a switch back and forth between an operating state with oxygen excess and an operating state with oxygen deficiency of the catalyst, wherein a shape of the characteristic curve is adjusted as a function of an oxygen and/or NO x  addition and/or desorption capability of the catalyst.  
         [0008]     A course of a transition from operating state to another and/or a course of the characteristic curve within an operating state can be adjusted as a function of oxygen and/or NO x  addition and/or desorption capability of the catalyst. The characteristic curve of the control signal can be adjusted as a function of a catalyst temperature. The characteristic curve of the control signal can also be adjusted as a function of the degree of aging of the catalyst. Adjustment of the characteristic curve of the control signal may occur as a function of the operating parameters of an internal combustion engine. The characteristic curve of the control signal can be adjusted unsymmetrically to a stipulated lambda value over a time range that includes at least several periods of the control signal. The characteristic curve of the control signal may be a sawtooth. The subsequent operating states can be adjusted with different residence time of the control signal. The subsequent operating states can be adjusted with different amplitude of the control signal. The characteristic curve of the control signal may be nonlinear at least in a region. The characteristic curve of the control signal may become leaner or richer degressively. The characteristic of the control signal may initially become leaner around a stipulated amount or richer and then is degressively guided in the direction λ=1.00. The characteristic curve of the control signal can be a rectangular curve with different amplitudes and/or residence times in the adjusted operating states. During fuel cutoff in the overrun or idle of the internal combustion engine, the internal combustion engine may be operated more in the rich operating state than in the lean operating state. In a catalyst the control signal may be adjusted so that increased incorporation of oxygen and/or NO x  in the catalyst occurs temporarily. In a catalyst after a stipulated operating time the control signal can be adjusted so that a phase with increased lean operation follows a phase with at least two periods with mostly rich operation. Before reaching an operating temperature of the catalyst the characteristic of the control signal may deviate from the characteristic after surpassing the operating temperature. In a catalyst at almost operating temperature, the characteristic curve of the control signal may have a sawtooth trend before reaching a stipulated operating temperature. The catalyst state and/or a state of the sensor upstream and/or downstream of the catalyst can be determined from the control signal.  
         [0009]     The object can also be achieved by a device for adjustment of a fuel/air ratio for an internal combustion engine with an associated catalyst, comprising exhaust composition sensors, a control unit for generating a control signal to influence the fuel/air ratio as a function of output signals of at least one of the sensors, said control signal comprising a characteristic curve for switching back and forth between an operating state with oxygen excess and an operating state with oxygen deficiency of the catalyst, and means for adjusting a form of a characteristic curve as a function of oxygen and/or NO x  addition and/or desorption in the catalyst.  
         [0010]     A sensor can be arranged in the exhaust system of the internal combustion engine upstream and downstream of the catalyst. The sensor upstream of catalyst may be a broadband lambda probe with a constant characteristic. The sensor upstream of the catalyst can also be a two-point lambda probe with a transfer characteristic. The sensor downstream of the catalyst may be a two-point lambda probe with a transfer characteristic. The sensor downstream of the catalyst may also be a broadband lambda probe with a constant characteristic. The catalyst can be a three-way catalyst. The catalyst may have a noble metal content of less than 60 g/ft 3 , especially less than 40 g/ft 3 , preferably less than 30 g/ft 3 , optimally less than 20 g/ft 3 , ideally less than 10 g/ft 3 . The catalyst can be an NO x  storage catalyst. The catalyst may have a noble metal content of less than 80 g/ft 3 , especially less than 60 g/ft 3 . The internal combustion engine can be a directly injected internal combustion engine capable of layered charging.  
         [0011]     One advantage of the method is that the time trend of the reference value of lambda value of the exhaust upstream of a catalyst connected after the engine is automatically adjusted to the operating states of the catalyst in which conversion is otherwise not optimum. This leads to better utilization of the catalyst and to increased reliability in maintaining emission values.  
         [0012]     Another advantage of the invention is that because of the improved dynamics of the exhaust system in a vehicle in which the device according to the invention was implemented the driving dynamics are approved.  
         [0013]     In addition, the catalyst during its lifetime is brought to favorable operating ranges and operated with high efficiency so that in comparison with ordinarily regulated catalysts noble metals of the catalyst can be saved and/or the catalyst itself reduced in size. This saves cost and resources.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention is further explained by means of drawings, in which the figures show:  
         [0015]      FIG. 1  shows a schematic view of a preferred device for execution of the method according to the invention,  
         [0016]      FIG. 2  shows a characteristic curve of a control signal according to the prior art for a three-way catalyst with new catalyst ( 2   a ) and with aged catalyst ( 2   b ),  
         [0017]      FIG. 3  shows a first preferred characteristic curve of a control signal according to the invention with a rectangular trend and different transfer height,  
         [0018]      FIG. 4  shows a second preferred characteristic curve of a control signal according to the invention with a sawtooth trend,  
         [0019]      FIG. 5  shows a third preferred characteristic curve of a control signal according to the invention with a degressive trend,  
         [0020]      FIG. 6  shows a fourth preferred characteristic curve of a control signal according to the invention with also a degressive trend,  
         [0021]      FIG. 7  shows a fifth preferred characteristic curve of a control signal according to the invention with a rectangular trend and progressive runout,  
         [0022]      FIG. 8  shows a sixth preferred characteristic curve of a control signal according to the invention with a rectangular trend and a sawtooth runout.  
     
    
     DETAILED DESCRIPTION  
       [0023]     The invention is particularly suited for catalysts in which the air fraction fed to the internal combustion engine of a fuel/air mixture is adjusted by means of a control signal that is periodically set between a minimal value and a maximal value of the air fraction and switched back and forth between a rich operating state with oxygen deficiency and a lean operating state with oxygen excess.  
         [0024]     The invention is particularly favorable for a three-way catalyst in which oxygen and/or NO x  are periodically introduced as oxidizers for the catalyst and desorbed and in which a control signal of a lambda control deviates essentially periodically around the lambda value λ=1. In the lean operating state the oxygen supply in the exhaust is sufficient to oxidize its HC and CO fractions, whereas in the rich operating state NO x  fractions in the exhaust as oxidizers oxidize the HC and CO fractions present. A common control strategy for a three-way catalyst proposes a lambda regulation in which a λ probe is exposed to a control signal with constant frequency. In the lean operating state when λ&gt;1 oxygen is introduced to the catalyst  2 ; in the rich operating state with λ&lt;1 this oxygen is consumed for oxidation processes.  
         [0025]     However, it is also possible to use the invention NO x  storage catalyst that can be operated at higher lambda values in a three-way catalyst. Aged storage catalysts can also be operated at lower lambda values around λ=1.  
         [0026]      FIG. 1  schematically depicts a preferred device for execution of the method according to the invention. A catalyst  2  is connected in the exhaust  3  after the internal combustion engine  1 . The internal combustion engine  1  is supplied in the usual manner with a fuel/air mixture via means not further shown; air supply preferably occurs via an intake line  7 . In the exhaust line  3  upstream of catalyst  2  a sensor  4  is arranged, which detects the composition of the exhaust. The sensor  4  is preferably an oxygen sensor that detects the oxygen content in the exhaust. The sensor  4  is preferably a broadband lambda probe with a constant control characteristic. Downstream of catalyst  2  another sensor is arranged in the exhaust line  8  that can detect the composition of the exhaust purified in catalyst  2 . Preferably, an oxygen sensor is also used here, with particular preference a two-point lambda probe with a transfer characteristic. The invention also includes devices with more than one downstream catalyst  2 .  
         [0027]     In principle, ordinary lambda probes are suitable with sensors  4 ,  5 , like broadband lambda probes, two-point lambda probes or NO x  sensor with lambda probe function. As an alternative, a two-point lambda probe can also be used upstream of catalyst  2  and/or a broadband lambda probe downstream of catalyst  2  has sensors  4 ,  5 . It is also conceivable to determine the lambda value upstream of catalyst  2  from other types of measured quantities, like injected amount of fuel and drawn in amount of air.  
         [0028]     The oxygen storage capability of catalyst  2  varies over the lifetime of the catalyst  2 . The characteristic of a lambda probe, especially a broadband lambda probe can also vary. This can be compensated by adjusting the frequency of the control signal to the state of aging. Expediently, sensor  4  is exposed to a control signal upstream of catalyst  2 . Sensor  5  downstream of catalyst  2  reports to sensor  4  upstream as soon as breakthrough of rich exhaust or lean exhaust is observed behind catalyst  2 . As long as lean exhaust is available up to sensor  5  downstream of catalyst  2  oxygen breakthrough is recognized via the internal combustion engine  1 . A switch is made to the rich operating state of catalyst  2  until breakthrough of the rich component is observed. A switch is then made back to the lean operating state and the sequence is repeated.  
         [0029]     With increasing age the oxygen storage capability of catalyst  2  diminishes, breakthroughs occur more quickly and the control frequency rises. The lambda control is therefore adapted to the state of aging of catalyst. Such regulation is also referred to as natural frequency regulation.  
         [0030]     The sensors  4 ,  5  are connected to a control device  6  that receives their signals and sends them to evaluation. This control device  6  is expediently a component of an ordinary engine control device used for operation of the internal combustion engine  1 . In this control device  6  or via this device operating parameters of the internal combustion engine  1  or a vehicle driven by the internal combustion engine  1  are available. These operating parameters are preferably entered as maps in a corresponding storage medium. Such operating parameters include exhaust temperature upstream of catalyst  2  and/or in catalyst  2 , exhaust temperature downstream of the catalyst, oxygen storage capability of the catalyst  2 , exhaust flow rate, speed of the internal combustion engine  1 , exhaust recirculation rate, position of a camshaft disk, charge movement flap, ignition time and/or charge pressure and the like. Information concerning the operating parameters can be linked to the sensor signals and control therefore conducted as a function of the operating parameters. This is indicated by arrows on control device  6 . Individual operating parameters or different operating parameters can be used in combination with each other.  
         [0031]     Two characteristic curves of an ordinary control signal according to the prior art for a fresh three-way catalyst ( FIG. 2   a ) and an aged three-way catalyst ( 2   b ) are shown in  FIG. 2 . The frequency of the control signal of fresh catalyst  2  is distinctly smaller than that of the aged catalyst  2 . However, otherwise the characteristic curve of the control signal is unchanged, since the characteristic curve shape and especially the amplitude are retained.  
         [0032]     Means are provided according to the invention in order to adjust a characteristic curve of the control signal to an actual catalyst state so that a characteristic curve shape of the characteristic curve is adjustable as a function of addition and/or desorption of an oxidizer in catalyst  2 . Such a characteristic curve shape can involve a transition from one operating state to another and/or a trend of the characteristic curve within an operating state of catalyst  2 . The oxidizer can be oxygen or NO x .  
         [0033]     In this case the frequency of the control signal is not followed simply as described in the prior art according to the state of aging of catalyst  2 , but the characteristic curve shape of the characteristic curve is varied by varying the amplitude and/or characteristic curve, especially a flank steepness, switching point and/or trend in an operating state. This adjustment occurs within a control cycle and can vary with increasing operating time of catalyst  2 . The aging behavior of catalyst  2  can be different in rich and lean operating states so that consideration of the different behavior in the two operating states permits more efficient utilization of catalyst  2  via a corresponding adjustment of the control signal as a function of the catalyst state.  
         [0034]     Depending on the state of aging of catalyst  2  the amplitude of the characteristic curve for a rich and lean operating state of the catalyst can be adjusted. This is shown in  FIG. 3 . An abrupt control signal with variable amplitude is shown there. In addition, the frequency can also be modulated. By varying the amplitude the system can be accelerated. If the sensor  5  downstream of catalyst  2  establishes a strongly depleted exhaust, it can rapidly be adjusted by stronger enrichment. During over-enrichment it can again be rapidly depleted. The system can therefore reach equilibrium more quickly. The amplitudes for the transition from the rich operating state to the lean operating state and from the lean operating state to the rich operating state can be different.  
         [0035]     Such a control strategy is advantageous for catalysts that have already been used for some time but are still useable over a longer time. Here it is favorable to operate over several control periods more strongly in the rich region, followed by a phase with mostly lean fractions and then again richer fractions and to repeat this. Because of this, increased oxygen storage in the catalyst  2  can be temporarily reached up to the limit of the regeneration capability of catalyst  2 .  
         [0036]     A trend according to  FIG. 3  can also be very advantageous between internal combustion engine  1  operated with thrust operated with fuel cutoff in the overrun. In this state, for example on a gradient, no fuel is temporarily fed to the internal combustion engine  1  and the internal combustion engine  1  operates on idle. The exhaust quickly becomes lean. It is favorable here to adjust the control signal so that the internal combustion engine is initially exposed to a fuel/air mixture that over several periods on a time average causes more rich fractions in the exhaust, which is recognizable by the larger amplitudes in the rich operating state. The internal combustion engine  1  is then operated in the lean range. This permits improved dynamics of the system and better driving dynamics of a vehicle operated in this way.  
         [0037]     Over a time region that includes at least several periods of the control signal it can therefore be advantageous to adjust the amplitudes of the characteristic curve of the control signal unsymmetrically to a stipulated lambda value.  
         [0038]     According to a favorable modification of the invention the characteristic curve of the control signal can be sawtooth. This is shown in  FIG. 4 . The transitions between a lean operating state to a rich operating state therefore do not occur abruptly, as in the previous example, but the transitions occur more smoothly with a finite slope of the flanks. This trend of the control signal is suitable for catalyst  2  that has not reached or has still not reached the optimum temperature range, especially in the phase with average temperature between a cold start and the sought operating temperature. It was found that the catalyst  2  can reach its optimal temperature range for normal operation more quickly by means of the sawtooth trend of the control signal. The flanks of the control signal can then have different slopes in terms of amount as well as different amplitudes.  
         [0039]     It can prescribed that the characteristic curve of the control signal is a rectangular curve or a different characteristic curve with different amplitude and/or residence time in the corresponding operating states so that the percentage of rich operating states and lean operating states can be adjusted as a function of needs to the actual state of catalyst  2 .  
         [0040]     It is also possible that adjust consecutive operating states with different duration depending on how dependent the addition process and/or desorption processes of the oxidizer in catalyst  2  are on the operating parameters of the internal combustion engine and/or the lifetime of catalyst  2 .  
         [0041]     A control characteristic curve is shown in  FIG. 5  that is nonlinear and has a degressive trend. A transition from one operating state to another occurs quickly with a relatively steep flank, whereupon the characteristic curve is slightly rising to a maximum or minimum value. The control signal can also have a progressive trend.  
         [0042]     A control characteristic curve is shown in  FIG. 6  that is nonlinear and has a progressive trend. A transition from one operating state occurs initially quickly, but then with a relatively flat flank.  
         [0043]      FIGS. 7 and 8  show examples of control signals, in which different curve forms are superimposed. The transitions between the operating states are abrupt but in the operating state the lean and/or fat fractions in the exhaust still increase nonlinearly or linearly. There are also additional overlaps and combinations of curve forms of the control signal that occur in succession that can be adjusted as a function of need.  
         [0044]     With increasing age the deep storage of oxygen and/or NO x  in catalyst  2  deteriorates so that the desired catalytic processes can no longer occur efficiently. The behavior of the catalyst  2  in lean operating states can be different than in rich operating states. Variation in modulation of the characteristic curve of the control signal therefore permits adjustment to these boundary conditions with a simultaneous increase in efficiency of the catalytic processes. This is advantageous to maintain emission limits.  
         [0045]     It is particularly expedient to conduct the adjustment of the characteristic curve of the control signal as a function of operating parameters in internal combustion engine  1 . This can occur via maps of operating parameters, as already described in  FIG. 1 . It is therefore considered that the behavior of catalyst  2  is strongly influenced by the operating parameters of the internal combustion engine  1 . The rate of conversion changes sharply with exhaust temperature. The catalyst  2  is exposed to pollutants with a high exhaust flow rate so that in the extreme case the purification efficiency of catalyst  2  can decline. If exhaust recirculation is varied, the richness of the fuel/air mixture changes. If the amount of recycled exhaust rises, the NO x  content in the exhaust drops. The ignition point influences the system in similar fashion to exhaust recirculation. By influencing the combustion trend the pollutant and oxygen concentration change at the same lambda value. With low crude emissions or a shift to more readily convertible pollutants (for example more CO, less CH 4 ) the requirements on accuracy of lambda control diminish. Such influences of the operating parameters can be considered by corresponding adjustment of the characteristic curve of the control signal so that maintenance of the emission standard is ensured over broad operating ranges of the internal combustion engine  1 .  
         [0046]     In addition to ensuring the emission standard, in an advantageous embodiment of the invention the catalyst state and/or state of the sensor  4 ,  5 , especially sensor  5  downstream from catalyst  2  can be determined from a change in the characteristic curve of the control signal.  
         [0047]     In addition to increased reliability in maintain in emission values, the invention also permits a reduction in noble metal content of catalyst  2 . The catalyst  2  is mostly operated in regions in which conversion is improved. Because of this the catalyst volume can be correspondingly reduced and/or the noble metal compound of the catalyst can be reduced in order to achieve the same efficiency as in an ordinary control. It is possible to reduce the noble metal content and/or the catalyst volume during use of the method according to the invention without surpassing the pollutant emissions forming without use of the method by at least 10%, especially by at least 20%. In particular, the catalyst  2  in the case of an NO x  storage catalyst has a noble metal content of less 80 g/ft 3 , especially less than 60 g/ft 3 . In a three-way catalyst a noble metal content of less than 60 g/ft 3 , especially less than 40 g/ft 3 , preferably less than 30 g/ft 3 , optimally less than 20 g/ft 3 , ideally less than 10 g/ft 3  is provided.