Patent Publication Number: US-6988494-B2

Title: Method for operating an internal combustion engine

Description:
BACKGROUND INFORMATION 
   It is already known that in at least one operating range of the internal combustion engine, a deviation in the air-fuel mixture ratio from a setpoint value is corrected. Systematic errors in the air-fuel mixture composition are corrected at the same time by the mixture adaptation. Essentially a distinction is made between additive and multiplicative errors. These mixture deviations are adapted in the load speed range in which they have the greatest effect. They are then calculated into the entire load speed range. Additive mixture deviations which occur because of leakage air or fuel injector delay times, for example, are adapted in a lower load speed range. Multiplicative mixture deviations which occur due to a characteristic line drift of the air flow meter used, for example, are adapted in a middle to upper load speed range. A correction value is formed for each adaptation range, i.e., each load speed range in which an adaptation was performed, and this correction value is interpreted as a fuel error. In the case of an air error, e.g., due to a leakage in the intake manifold, this error is also corrected in the fuel path instead of in the air path. 
   SUMMARY OF THE INVENTION 
   The method according to the present invention for operating an internal combustion engine has the advantage over the related art that for correcting the deviation in the air-fuel mixture ratio from the setpoint value in the at least one operating range the particular deviation in the air-fuel mixture ratio is determined for at least two setpoint values and an air error and/or a fuel error is/are determined from these deviations. It is possible in this way to differentiate between an air error and a fuel error. It is therefore possible to correct errors in the air path at the correct location, namely in the air path itself. The same thing is true of the correction of errors in the fuel path which are also corrected at the correct location, namely in the fuel path, and their correction does not include the air errors. Air errors therefore need not be compensated by the driver by corresponding operation of the gas pedal. In addition, the correction of the deviation in the air-fuel mixture ratio from the setpoint value is implemented according to the present invention without any additional sensors. 
   It is particularly advantageous if the air error and/or the fuel error is/are determined by using an equation system having at least two equations for the deviation in the air-fuel mixture ratio from the particular setpoint value. In this way the air error and/or the fuel error may be determined precisely and differentiated from one another with little effort. 
   An additional advantage results if the air error is corrected only in an air path of the internal combustion engine. In this way air errors need not be compensated by the driver through corresponding operation of the gas pedal. In addition this makes it unnecessary to correct the air error in the fuel path. 
   An additional advantage results if the fuel error is corrected only in a fuel path of the internal combustion engine. In this way fuel errors need not be compensated by the driver through corresponding operation of the gas pedal. 
   An additional advantage results if only one error from the quantity formed by the air error and the fuel error is determined and corrected and when any remaining deviation in the air-fuel mixture ratio from the setpoint value is interpreted as being based on that error which was not previously determined. It is possible in this way to avoid the calculation of an error in the quantity formed by the air error and the fuel error and thus to eliminate complexity while nevertheless being able to identify and correct this error. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of an internal combustion engine. 
       FIG. 2  shows a flow chart of an exemplary sequence of the method according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an engine  1  in a vehicle, for example. Engine  1  includes an internal combustion engine  30 , which may be designed as a gasoline engine, for example. Internal combustion engine  30  receives fresh air through an air inlet  15 . An air flow meter  20  situated in air inlet  15  may be designed as a hot-film air-mass meter, for example, which measures fresh air mass flow {dot over (m)} air  supplied to internal combustion engine  30  and sends the result of the measurement to a control unit  45 . The direction of flow of the fresh air in air inlet  15  is indicated by arrows in  FIG. 1 . A throttle valve  5  for adjusting and correcting fresh air mass flow {dot over (m)} air  supplied to internal combustion engine  30  is situated downstream from air flow meter  20  in the direction of flow of the fresh air in air inlet  15 . Therefore throttle valve  5  is triggered by control unit  45 . Fresh air mass flow {dot over (m)} air  is then sent through at least one intake valve (not shown in  FIG. 1 ) to a combustion chamber (also not shown) of internal combustion engine  30 . In addition, fuel is supplied to the combustion chamber through at least one fuel injector  10 , with the quantity of fuel supplied also being adjusted and corrected by control unit  45 . According to  FIG. 1  direct injection of fuel into the combustion chamber of internal combustion engine  30  is indicated. As an alternative, fuel could also be injected into the area of air inlet  15  which is situated between throttle valve  5  and the at least one intake valve and is referred to as an intake manifold. In addition the air-fuel mixture in the combustion chamber of internal combustion engine  30  is ignited by at least one spark plug  25  which to this end is also triggered by control unit  45  for adjusting a suitable ignition point. Through combustion of the air-fuel mixture in the combustion chamber of internal combustion engine  30 , engine  1  is driven in a manner with which those skilled in the art are familiar. 
   The exhaust gas formed during combustion is ejected from the combustion chamber into an exhaust line  40  through at least one outlet valve (not shown in  FIG. 1 ), the direction of flow of the exhaust gas in exhaust line  40  also being indicated by an arrow in  FIG. 1 . A lambda-probe  35  is situated in exhaust line  40 , measuring the oxygen content in the exhaust gas and sending the measured value to control unit  45  in which an actual value for air-fuel mixture ratio λ in the combustion chamber of internal combustion engine  30  is then calculated from the measured oxygen content by a method with which those skilled in the art are familiar. 
   Air-fuel mixture ratio λ in the combustion chamber of internal combustion engine  30  is defined as follows: 
             λ   =         m   air     .             m   .     kr     ·   m     ⁢           ⁢     l   min                 (   1   )             
 
where {dot over (m)} kr  is the fuel mass flow and ml min  is a predetermined fixed value indicating the mass in kilograms of air required to burn one kilogram of fuel. For commercial gasoline fuels, this fixed value currently amounts to approximately 14.7. Fuel mass flow {dot over (w)} kr  is calculated from fresh air mass flow {dot over (m)} air  and air-fuel mixture ratio λ from equation (1) as follows: 
                 m   .     kr     =           m   .     air       m   ⁢           ⁢       l   min     ·   λ         .             (   2   )             
 
   Error λ error  of fuel-air mixture ratio λ is described by: 
                 λ   error     =             ∂   λ       ∂       m   .     Air         ·   Δ     ⁢           ⁢       m   .     Air       +           ∂   λ       ∂       m   .     kr         ·   Δ     ⁢           ⁢       m   .     kr           ,           (   3   )             
 
where Δ{dot over (m)} air  is the error in the air path of engine  1  and Δ{dot over (m)} kr  is the error in the fuel path of engine  1 . The air path refers to the supply of fresh air to internal combustion engine  30  through air inlet  15 , air flow meter  20 , and throttle valve  5 . Error Δ{dot over (m)} air  in the air path is caused for example due to a leak in air inlet  15 , e.g., in the area of the intake manifold or due to a characteristic line offset of air flow meter  20 . The fuel path refers to the supply of fuel to internal combustion engine  30  through at least one fuel injector  10 . Error Δ{dot over (m)} kr  in the fuel path is caused for example by fuel injector delay times.
 
   Depending on the operating range, i.e., the load speed range of engine  1 , a corresponding setpoint value λ setpoint  for the fuel-air mixture ratio may be predetermined. A λ regulation (not shown separately in  FIG. 1 ) in control unit  45  regulates an actual value λ actual  for the air-fuel mixture ratio according to setpoint value λ setpoint . To this end, a regulating factor fr is formed in a manner with which those skilled in the art are familiar and is used to correct the fuel supply through at least one fuel injector  10  for readjusting actual value λ actual  for the air-fuel mixture ratio to setpoint value λ setpoint  for the air-fuel mixture ratio. If regulating factor fr=1, then no correction is necessary and actual value λ actual  for the air-fuel mixture ratio already corresponds to setpoint value λ setpoint  for air-fuel mixture ratio λ. There is no mixture deviation then. In the case of a mixture deviation, fr≠1 and the fuel supply is corrected so that actual value λ actual  for the air-fuel mixture ratio largely corresponds to setpoint value λ setpoint  for the air-fuel mixture ratio. Error λ error  of air-fuel mixture ratio λ then ultimately corresponds to the mixture deviation of actual value λ actual  for air-fuel mixture ratio λ from setpoint value λ setpoint  for the air-fuel mixture ratio that would be established for a regulating factor fr=1. Error λ error  of air-fuel mixture ratio λ is calculated here in control unit  45  from actual regulating factor fr in a manner with which those skilled in the art are familiar. To reduce the complexity, error λ error  of air-fuel mixture ratio λ may be determined approximately by the deviation of the actual value for regulating factor fr from value 1. For compensation of fluctuations in the actual value for regulating factor fr, this regulating factor fr may be averaged by an integrator, for example, with a correspondingly large time constant. 
   The derivations in air-fuel mixture ratio λ according to its variables are: 
                 ∂   λ       ∂       m   .     air         =     1           m   .     kr     ·   m     ⁢           ⁢     l   min                 (   4   )                   ∂   λ       ∂       m   .     kr         =     -           m   .     air             m   .     kr     ·       m   .     kr     ·   m     ⁢           ⁢     l   min         .               (   5   )             
 
   Fuel mass flow {dot over (m)} kr  is replaced according to equation (2): 
                 ∂   λ       ∂       m   .     air         =     λ       m   .     air               (   6   )                   ∂   λ       ∂       m   .     kr         =     -         λ   2         m   .     Air       .               (   7   )             
 
   Error λ error  of air-fuel mixture ratio λ is then obtained as follows from equations (3), (6), and (7): 
               λ   error     =           λ       m   .     air       ·   Δ     ⁢           ⁢       m   .     air       -           λ   2         m   .     air       ·   Δ     ⁢           ⁢         m   .     kr     .                 (   8   )             
 
   In the adaptation of the mixture deviation to date, a general error in the composition of the mixture, i.e., the air-fuel mixture ratio, was measured at a constant λ value of 1.0, for example. Since there is only one λ value per load point, with the particular load point being characterized by a corresponding value for fresh air mass flow {dot over (m)} air , it is impossible to differentiate between fuel errors and air errors. However, if two different λ values are set at one load point, this yields two equations with two unknowns. This equation system is solvable. It is thus possible to differentiate between fuel errors and air errors. Fresh air mass flow {dot over (m)} air  for the particular load point is measured by air flow meter  20  and is therefore available in control unit  45  and is used in equation (8). Alternatively, fresh air mass flow {dot over (m)} air  could be derived from an intake manifold pressure determined by an intake manifold pressure sensor using a model and a method with which those skilled in the art are familiar if such an intake manifold pressure sensor is available in the intake manifold of engine  1 . The λ value used in equation (8) is the setpoint value λ setpoint  for the air-fuel mixture ratio. Error λ error  of air-fuel mixture ratio λ obtained for the air-fuel mixture ratio in the conversion of this setpoint value λ setpoint  is determined as described above from the resulting actual regulating factor fr and is also used in equation (8). In equation (8) error Δ{dot over (m)} air  in the air path and error Δ{dot over (m)} kr  in the fuel path are unknown. Therefore if equation (8) is formulated for at least two different setpoint values λ setpoint  for the air-fuel mixture ratio, this yields the desired equation system which is solvable according to error Δ{dot over (m)} air  in the air path, i.e., the air error, and error Δ{dot over (m)} kr  in the fuel path, i.e., the fuel error. 
   Due to the fact that the air error is differentiated from the fuel error, it is possible to correct the air error in only the air path of engine  1 , i.e., through corresponding correction of the setting of throttle valve  5 . Accordingly it is possible to correct the fuel error in only the fuel path of internal combustion engine  1 , i.e., by correcting the injection quantity at the at least one fuel injector  10 . To reduce computation complexity, it is also possible to calculate either only the air error or only the fuel error from equation system (8) having the at least two equations and to correct it in the corresponding path, for example. The remaining deviation, i.e., the remaining error in air-fuel mixture ratio λ, may be definitely identified as the error not calculated previously and may be corrected accordingly in the particular path, for example. The mixture adaptation described here may be performed for one or more load points, in particular in various operating ranges, i.e., in different load speed ranges of internal combustion engine  1 . 
     FIG. 2  shows a flow chart for an exemplary sequence of the method according to the present invention. After the start of the program, control unit  45  checks at a program point  100  on whether the λ regulation is active. If this is the case, then it branches off to a program point  105 ; otherwise the program is terminated. 
   At program point  105 , control unit  45  checks on whether a mixture adaptation is possible. If this is the case, it branches off to a program point  110 ; otherwise the program is terminated. A mixture adaptation is not possible, for example, when tank ventilation is active. In addition, a mixture adaptation is possible only in a certain engine temperature range above a threshold temperature of approximately 60° C., for example. At program point  110 , a first setpoint value λ setpoint  for the air-fuel mixture ratio, e.g., the value 1, is predetermined for a given load point, characterized by a particular fresh air mass flow {dot over (m)} air . First error λ error  of air-fuel mixture ratio λ thus obtained is determined. Fresh air mass flow {dot over (m)} air  first setpoint value λ setpoint  for the air-fuel mixture ratio, and first error λ error  of air-fuel mixture ratio λ are used in a first equation of the equation system according to equation (8). It then branches off to a program point  115 . At program point  115  a second setpoint value λ setpoint  for the air-fuel mixture ratio, e.g., the value 1.2, is predetermined for the given load point. This corresponds to a lean air-fuel mixture ratio. Second error λ error  of air-fuel mixture ratio λ is then determined. Fresh air mass flow {dot over (m)} air , second setpoint value λ setpoint  for the air-fuel mixture ratio, and second error λ error  of air-fuel mixture ratio λ are used in a second equation of the equation system according to equation (8). The system then branches off to a program point  120 . At program point  120  a third setpoint value λ setpoint  for the air-fuel mixture ratio, e.g., the value 0.8, is predetermined for the given load point. This corresponds to a rich air-fuel mixture ratio. Resulting third error λ error  of air-fuel mixture ratio λ is determined. Fresh air mass flow {dot over (m)} air , third setpoint value λ setpoint  for the air-fuel mixture ratio, and third error λ error  of air-fuel mixture ratio λ are used in a third equation of the equation system according to equation (8). The system then branches off to a program point  125 . 
   At program point  125  the equation system formed from three equations according to the above equation (8) is solved for air error Δ{dot over (m)} air  and/or fuel error Δ{dot over (m)} kr  and a corresponding correction is made in the air path and in the fuel path as adaptation of the mixture and error λ error  of air-fuel mixture ratio λ is compensated. 
   In the flow chart according to  FIG. 2 , three different setpoint values λ setpoint  for the air-fuel mixture ratio at the given load point were used. To solve the equation system according to equation (8) for air error Δ{dot over (m)} air  and fuel error Δ{dot over (m)} kr  it is sufficient, however, to predetermine two different setpoint values λ setpoint  for the air-fuel mixture ratio. Alternatively, more than three setpoint values λ setpoint  for the air-fuel mixture ratio may be predetermined per load point to determine air error Δ{dot over (m)} air  and fuel error Δ{dot over (m)} kr  from the equation system according to equation (8).