Abstract:
An internal combustion engine comprising several cylinders provided with injection valves associated with the cylinders admeasuring fuel. An exhaust gas probe is arranged in an exhaust gas tract and the measuring signal thereof is characteristic for the air/fuel ratio in the respective cylinder. A sensing crankshaft angle is determined in relation to a reference position of the piston of the respective cylinder in order to detect the measuring signal according to a variable characterizing air/fuel ratio in the respective cylinder, or an ambient pressure or a degree of opinion of a bypass valve of a bypass associated with a turbine in which the exhaust gas tract is arranged. The measuring signal is detected at the sensing crankshaft angle and allocated to the respective cylinder.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the US National Stage of International Application No. PCT/EP2005/051427, filed Mar. 30, 2005 and claims the benefit thereof. The International Application claims the benefits of German Patent application No. 10 2004 026 176.8 filed May 28, 2004. All of the applications are incorporated by reference herein in their entirety. 
   FIELD OF INVENTION 
   The invention relates to a method for detecting a cylinder-specific air/fuel ratio in an internal combustion engine with a number of cylinders and injection valves assigned to the cylinders, which meter fuel. An exhaust gas probe is disposed in an exhaust gas tract and its measuring signal is characteristic of the air/fuel ratio in the respective cylinder. 
   BACKGROUND OF THE INVENTION 
   Increasingly stringent statutory provisions relating to permitted pollutant emissions from motor vehicles, in which internal combustion engines are disposed, mean that pollutant emissions have to be kept as low as possible during operation of the internal combustion engine. This can be achieved on the one hand by reducing the pollutant emissions, which result during combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. Exhaust gas post-treatment systems can also be used in internal combustion engines, to convert the pollutant emissions produced during the combustion process of the air/fuel mixture in the respective cylinders to harmless substances. Exhaust gas catalytic converters are used for this purpose, converting carbon monoxide, hydrocarbons and nitrous oxides to harmless substances. Both the specific influencing of the production of pollutant emissions during combustion and the conversion of pollutant components in a highly efficient manner using an exhaust gas catalytic converter require a very precisely set air/fuel ratio in the respective cylinder. 
   A method for the cylinder-selective regulation of an air-fuel mixture to be burned for an internal combustion engine with a number of cylinders is known from DE 199 03 721 C1, wherein the lambda values for different cylinders or cylinder groups are sensed and regulated separately. A probe/evaluation unit is provided for this purpose, in which the exhaust gas probe signal is evaluated with time resolution, thereby determining a cylinder-selective lambda value for each cylinder in the internal combustion engine. Each cylinder is assigned an individual regulator, configured as a PI or PID regulator, the controlled variable of which is a cylinder-specific lambda value and the reference variable of which is a cylinder-specific target value for the lambda. The manipulated variable of the respective regulator then influences fuel injection in the respectively assigned cylinder. 
   The quality of cylinder-specific lambda regulation depends to a large degree on how precisely the measuring signal of the exhaust gas probe detected at the respective sampling time is assigned to the exhaust gas of the respective cylinder. 
   An internal combustion engine with a number of cylinders and injection valves assigned to the cylinders is known from EP 0 643 213 A1. An exhaust gas probe is disposed in the exhaust gas tract and its measuring signal is characteristic of the air/fuel ratio in the respective cylinders. The exhaust gas probe generates a measuring signal, which is converted to digital values by means of an A/D converter and stored in a buffer unit. To assign the respective fuel ratio to the respective cylinder, the respective buffered value to be read out is determined as a function of whether a desired air/fuel ratio is leaner than the stoichiometric air/fuel ratio. According to a further embodiment the buffered value to be read out is determined as a function of the atmospheric pressure. 
   It is known from US 2002/0026930 A1 that a measuring signal from an exhaust gas probe can be sampled and one of the sampled values can be selected as a function of the engine speed, engine load and a selected operating mode, which can for example be a stoichiometric mode, a pre-mix combustion mode and a layer mode. 
   SUMMARY OF INVENTION 
   The object of the invention is to create a method, which allows precise detection of the air/fuel ratio to be assigned to the respective cylinder of an internal combustion engine in a simple manner. 
   The object is achieved by the features of the independent claims. Advantageous embodiments of the invention are characterized in the subclaims. 
   The invention is characterized by a method for detecting a cylinder-specific air/fuel ratio in an internal combustion engine with a number of cylinders and injection values assigned to the cylinders, which meter fuel, with an exhaust gas probe being disposed in an exhaust gas tract and its measuring signal being characteristic of the air/fuel ratio in the respective cylinder. A sampled crankshaft angle in relation to a reference position of the piston of the respective cylinder is determined to detect the measuring signal as a function of an opening angle of a bypass valve of a bypass to a turbine, which is disposed in the exhaust gas tract. When the crankshaft angle reaches the sampled crankshaft angle, the measuring signal is detected and assigned to the respective cylinder. Naturally the crankshaft angle and the sampled crankshaft angle can also be expressed as a corresponding time signal to the same effect. 
   The invention makes simple use of the knowledge that the response time of the exhaust gas probe is a function of the absolute pressure acting on it. The absolute pressure acting on the exhaust gas probe is the exhaust gas back-pressure prevailing in the exhaust gas tract in the region of the exhaust gas probe. It is a fact that, the exhaust gas back-pressure is influenced to a large extent by the opening angle of the bypass valve. The response time of the exhaust gas probe is influenced due to a pressure-dependent diffusion of the oxygen molecules in a corresponding chamber of the exhaust gas probe. 
   According to an advantageous embodiment of the invention the sampled crankshaft angle in relation to a reference position of the piston of the respective cylinder is determined to detect the measuring signal as a function of an ambient pressure. This makes particular use of the knowledge that, particularly when the internal combustion engine is in the same load state, the exhaust gas back-pressure is a function of ambient pressure, in other words the pressure prevailing in the region around the internal combustion engine. 
   According to a further advantageous embodiment of the invention, the sampled crankshaft angle is determined as a function of a variable characterizing the air/fuel ratio in the respective cylinder. This allows the dynamic of the exhaust gas probe to be taken into account in a particularly favorable manner, as it is a function of the air/fuel ratio in the respective cylinder. 
   In this context it is particularly advantageous, if the sampled crankshaft angle is determined as a function of whether or not the air/fuel ratio in the respective cylinder corresponds approximately to the stoichiometric air/fuel ratio. This makes use of the knowledge that the dynamic of the exhaust gas probe differs significantly as a function of whether or not the air/fuel ratio in the respective cylinder corresponds approximately to the stoichiometric air/fuel ratio or is correspondingly super- or sub-stoichiometric. It has proven in particular that this narrow range around the stoichiometric air/fuel ratio is approximately in the range λ=0.97 to 1.03. It has also proven that this changed dynamic in the narrow window around the stoichiometric air/fuel ratio in the respective cylinder is due to recharge processes in a chamber of the linear lambda probe, in particular to a changed distribution of the oxygen in the chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are described in more detail below with reference to the schematic drawings, in which: 
       FIG. 1  shows an internal combustion engine with a control facility, 
       FIG. 2  shows a block circuit diagram of the control facility, 
       FIG. 3  shows a flow diagram of a first embodiment of a program for determining a sampled crankshaft angle, 
       FIG. 4  shows a flow diagram of a second embodiment of a program for determining a sampled crankshaft angle, 
       FIG. 5  shows a flow diagram of a third embodiment of a program for determining a sampled crankshaft angle, 
       FIG. 6  shows a flow diagram of a fourth embodiment of a program for determining a sampled crankshaft angle, 
       FIG. 7  shows a flow diagram of a fifth embodiment of a program for determining a sampled crankshaft angle. 
   

   Elements of the same structure and function are identified with the same reference characters in all the figures. 
   DETAILED DESCRIPTION OF INVENTION 
   An internal combustion engine ( FIG. 1 ) has an intake tract  1 , an engine block  2 , a cylinder head  3  and an exhaust gas tract  4 . The intake tract  1  preferably has a throttle valve  11 , also a manifold  12  and an intake pipe  13 , which leads to a cylinder Z 1  via an inlet duct into the engine block  2 . The engine block  2  also has a crankshaft  21 , which is coupled via a connecting rod  25  to the piston  24  of the cylinder Z 1 . 
   The cylinder head  3  has a valve gear mechanism with a gas inlet valve  30 , a gas outlet valve  31  and valve drives  32 ,  33 . The cylinder head  3  also has an injection valve  34  and a spark plug  35 . Alternatively the injection valve can also be disposed in the intake pipe  13 . 
   The exhaust gas tract  4  has an exhaust gas catalytic converter  40 , preferably configured as a three-way catalytic converter. An exhaust gas turbocharger can also be provided, having a turbine  42 , which is disposed in the exhaust gas tract  4 , a bypass duct  50  to the turbine  42  with a bypass valve  51  and a compressor  18 , which is disposed in the intake tract  1 . 
   A control facility  6  is also provided, to which sensors are assigned, which detect different measured variables and determine the measured value of the measured variable in each instance. The control facility  6  controls the final control elements by means of corresponding actuators as a function of at least one of the measured variables. 
   The sensors are a pedal position sensor  71 , which detects the position of an accelerator pedal  7 , an air mass sensor  14 , which detects an air mass flow upstream of the throttle valve  11 , a temperature sensor  15 , which detects the intake air temperature, a pressure sensor  16 , which detects the intake pipe pressure, a crankshaft angle sensor  22 , which detects a crankshaft angle, to which a speed N is then assigned, a further temperature sensor  23 , which detects a coolant temperature, a throttle valve sensor  19 , which detects the opening angle of the throttle valve  11 , and an exhaust gas probe  41 , which detects a residual oxygen content of the exhaust gas and whose measuring signal is characteristic of the air/fuel ratio in the cylinder Z 1 . The exhaust gas probe  41  is preferably configured as a linear lambda probe, thus generating a measuring signal proportional to the air/fuel ratio over a wide air/fuel ratio range. 
   Any sub-set of said sensors or even additional sensors can be present, depending on the embodiment of the invention. 
   The final control elements are for example the throttle valve  11 , the gas inlet and gas outlet valves  30 ,  31 , the injection valve  34 , the spark plug  35  or the bypass valve  51 . 
   As well as the cylinder Z 1 , further cylinders Z 2 -Z 4  can also be provided, to which corresponding final control elements are also assigned. The internal combustion engine can thus have six cylinders for example, three cylinders respectively being assigned to an exhaust gas unit. An exhaust gas probe  41  is preferably assigned to each exhaust gas unit. 
   A block circuit diagram of parts of the control facility  6 , which can also be referred to as a device for controlling the internal combustion engine, is shown in  FIG. 2 . 
   A block B 1  corresponds to the internal combustion engine. An air/fuel ratio LAM_RAW detected by the exhaust gas probe  41  is fed to a block  2 . Then at respectively determined sampled crankshaft angles CRK_SAMP in relation to a reference position of the respective piston of the respective cylinder Z 1  to Z 4  an assignment takes place in block B 2  of the respective currently detected air/fuel ratio at this time, which is derived from the measuring signal of the exhaust gas probe  41 , to the respective air/fuel ratio of the respective cylinder Z 1  to Z 4 , thus assigning the air/fuel ratio LAM_I[Z 1 -Z 4 ] detected in a cylinder-specific manner. 
   The reference position of the respective piston  24  is preferably its top dead center. The determination of the sampled crankshaft angle CRK_SAMP is described in more detail below based on the programs described below. 
   A mean air/fuel ratio LAM_MW is determined in a block B 2   a  by averaging the air/fuel ratios LAM_I[Z 1 -Z 4 ] detected in a cylinder-specific manner. In block B 2   a  an actual value D_LAM_I[Z 1 ] of a cylinder-specific air/fuel ratio deviation is also determined from the difference between the mean air/fuel ratio LAM_MW and the air/fuel ratio LAM_I [Z 1 ] determined in a cylinder-specific manner. This is then fed to a regulator, which is formed by block B 3   a.    
   In a summing point S 1  the difference between the actual value D_LAM_I[Z 1 ] and an estimated value D_LAM_I_EST [Z 1 ] of the cylinder-specific air/fuel ratio deviation is determined and then assigned to a block B 3 , which is part of an observer and comprises an integration element, which integrates the variable present at its input. The I-element of block B 3  then provides a first estimated value EST 1  [Z 1 ] at its output. 
   The first estimated value EST 1 [Z 1 ] is then fed to a delay element, which is also part of the observer and is configured in block B 4 . The delay element is preferably configured as a PT 1  element. The first estimated values EST 1 [Z 2 -Z 4 ] relating to the further cylinders [Z 2 -Z 4 ] respectively are optionally also fed to the delay element. The first estimated value EST 1 [Z 1 ] forms a status variable of the observer. 
   The first estimated value EST 1 [Z 1 ] is also fed to a block B 5 , in which a further integrator element is configured, which integrates the first estimated value EST 1 [Z 1 ] and then generates a cylinder-specific lambda regulation factor LAM_FAC_I[Z 1 ] as a manipulated variable of the regulator at its output. 
   In a block B 6  a second estimated value EST 2 [Z 1 ] is determined as a function of the cylinder-specific lambda regulation factor LAM_FAC_I[Z 1 ]. This can be done particularly simply by aligning the second estimated value EST 2 [Z 1 ] with the cylinder-specific lambda regulation factor LAM_FAC_I[Z 1 ]. The difference between the first estimated value EST 1 [Z 1 ] filtered by way of the delay element of block B 4  and the second estimated value EST 2 [Z 1 ] is then formed in the summing point S 2  and returned as the estimated value D_LAM_I_EST[Z 1 ] of the cylinder-specific air/fuel ratio deviation to the summing point S 1  and subtracted here from the actual value D_LAM_I[Z 1 ] of the respective cylinder-specific air/fuel ratio deviation and thus fed back and then input back into block B 3 . 
   In a block B 8  a lambda regulator is provided, the reference variable of which is an air/fuel ratio predetermined for all the cylinders of the internal combustion engine and the controlled variable of which is the mean air/fuel ratio LAM_MW. The manipulated variable of the lambda regulator is a lambda regulation factor LAM_FAC_ALL. The lambda regulator therefore has the task of ensuring that the predetermined air/fuel ratio is set across all the cylinders Z 1  to Z 4  of the internal combustion engine. 
   Alternatively this can also be achieved by determining the actual value D_LAM_I of the cylinder-specific air/fuel ratio deviation in block B 2  from the difference between the air/fuel ratio predetermined for all the cylinders Z 1  to Z 4  of the internal combustion engine and the cylinder-specific air/fuel ratio LAM_I[Z 1 -Z 4 ]. There is then no need for the third regulator of block B 8 . 
   In a block B 9  a fuel mass to be metered MFF is determined as a function of an air mass flow MAF into the respective cylinder Z 1  to Z 4  and optionally the speed N and a target value LAM_SP of the air/fuel ratio for all cylinders Z 1 -Z 4 . 
   In the multiplication point M 1  a corrected fuel mass to be metered MFF_COR is determined by multiplying the fuel mass to be metered MFF, the lambda regulation factor LAM_FAC_ALL and the cylinder-specific lambda regulation factor LAM_FAC_I[Z 1 ]. A control signal is then generated as a function of the corrected fuel mass to be metered MFF_COR and this is used to control the respective injection valve  34 . 
   In addition to the regulator structure illustrated in the block circuit diagram in  FIG. 2 , corresponding regulator structures B_Z 2  to B_Z 4  for the respective further cylinders Z 2  to Z 4  are also provided for each further cylinder Z 1  to Z 4 . 
   Alternatively a proportional element can also be configured in block B 5 . 
   A number of exemplary embodiments of programs for determining the sampled crankshaft angle CRK_SAMP are described below. The start of the respective programs preferably takes place close in time to start-up of the internal combustion engine. 
   A first embodiment of the program ( FIG. 3 ) is started in a step S 1 . In a step S 2  the sampled crankshaft angle CRK_SAMP is determined as a function of the air mass flow MAF into the respective cylinder, the speed N and the target value LAM_SP of the air/fuel ratio. The value thus determined of the sampled crankshaft angle CRK_SAMP is then supplied to block B 2  for further processing. The program is then kept on hold for a predeterminable waiting period T_W in step S 4 , before step S 2  is reprocessed. Alternatively the program can continue in step S 4  for a predetermined crankshaft angle period. The sampled crankshaft angle CRK_SAMP is preferably first determined in step S 2  as a function of the air mass flow MAF into the respective cylinder and the speed N and then corrected by means of a correction value, which is determined as a function of the target value LAM_SP of the air/fuel ratio. To this end corresponding performance data is preferably stored in the control facility  6 , having been determined beforehand by tests on an engine test bed or by simulation. 
   In a second embodiment of the program ( FIG. 4 ) a start takes place in a step S 6 . In a step S 8  the sampled crankshaft angle CRK_SAMP is determined as a function of the air mass flow MAF into the respective cylinder, the speed N and the mean air/fuel ratio LAM_MW. The mean air/fuel ratio LAM_MW is preferably also filtered before determination of the sampled crankshaft angle CRK_SAMP using a low-pass, in order to filter out higher-frequency fluctuations, particularly close to the stoichiometric air/fuel ratio. The sampled crankshaft angle CRK_SAMP is determined in step S 8  preferably as in step S 2 . Using the target value LAM_SP of the air/fuel ratio has the advantage that there is regularly no need for low-pass filtering. The sampled crankshaft angle CRK_SAMP can also be determined in a particularly simple manner by only differentiating whether or not the mean air/fuel ratio LAM_MW or the target value LAM_SP of the air/fuel ratio is in a narrow range around the stoichiometric air/fuel ratio. 
   A third embodiment of the program for determining the sampled crankshaft angle CRK_SAMP ( FIG. 5 ) is started in a step S 12 . In a step S 14  an ambient pressure AMP is detected or determined. An ambient pressure sensor can be provided for this purpose for example, detecting the pressure outside the internal combustion engine and thus detecting the current air pressure. If the ambient pressure sensor is not present however, the ambient pressure can be determined in a simple manner as a function of the measuring signal of the intake pipe sensor  16  in predetermined operating states. This can be done in a particularly simple manner, when the optionally present compressor  18  is not active, in other words is not compressing the air, and the throttle valve  11  is open so wide that the pressure drop across the throttle valve  11  is negligible. In this instance a very good approximate value of the ambient pressure AMP can be determined as a function of the measuring signal of the intake pipe pressure sensor  16 . 
   In a step S 16  the sampled crankshaft angle CRK_SAMP is determined as a function of the respective air mass flow MAF into the respective cylinder, the speed N and the ambient pressure AMP. This is preferably done according to the procedure in step S 2 . 
   The program is then kept on hold for the predetermined waiting period T_W in step S 18 . 
   In a fourth embodiment of the program for determining the sampled crankshaft angle CRK_SAMP ( FIG. 6 ) a start takes place in a step S 20 . In a step S 22  the sampled crankshaft angle CRK_SAMP is determined as a function of the air mass flow MAF into the respective cylinder, the speed N and the opening angle OG_WG of the bypass valve  51  to the turbine  42  of the exhaust gas turbocharger. This is also preferably done according to the procedure in step S 2 . 
   The program is then kept on hold in a step S 24  for the predetermined waiting period T_W. Processing then continues again in step S 22 . 
   In a fifth embodiment of the program ( FIG. 7 ) a start takes place in a step S 26 . In a step S 28  the ambient pressure is detected. This is done according to the procedure in step S 14 . 
   In a step S 30  the sampled crankshaft angle CRK_SAMP is determined as a function of the air mass flow MAF into the respective cylinder Z 1  to Z 4 , the speed N, the ambient pressure AMP, the opening angle OG_WG of the bypass valve  51 , and either the target value LAM_SP of the air/fuel ratio or the mean air/fuel ratio LAM_MW. 
   In step S 30  the sampled crankshaft angle CRK_SAMP is preferably first determined as a function of the air mass flow MAF into the respective cylinder and the speed N, preferably by means of performance data and optionally corresponding performance data interpolation. Also at least one correction value is determined as a function of the ambient pressure AMP and/or the opening angle OG_WG of the bypass valve  51  and/or the target value LAM_SP of the air/fuel ratio and/or the mean air/fuel ratio LAM_MW. This is also preferably done by means of one or more sets of performance data and optionally performance data interpolation, the performance data preferably being determined beforehand by corresponding tests, for example on an engine test bed or by simulation. 
   The sampled crankshaft angle CRK_SAMP is then corrected by means of at least one correction value and supplied to block B 2 . The program is then kept on hold for the predetermined waiting period T_W in step S 32 , before processing is resumed in step S 30 .