Abstract:
A method, a control unit, and a computer program for detecting a defective intake-manifold pressure sensor and/or a defective ambient-pressure sensor in an internal combustion engine having a variable valve timing are provided. The desired detection is carried out exclusively on the basis of a direct evaluation of the pressure upstream from the throttle valve and the pressure in the intake manifold. This method eliminates the need for deriving load signals from these pressures, at least for the determination as to whether at least one of the pressure sensors is defective.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method, a control unit, and a computer program for detecting a defective intake-manifold pressure sensor and/or a defective ambient-pressure sensor in an internal combustion engine. 
     BACKGROUND INFORMATION 
     An intake-manifold pressure sensor is used, for example, to diagnose the exhaust-gas recirculation in an internal combustion engine and therefore supplies important information for a control unit to control the internal combustion engine. A load signal representing the current load of the internal combustion engine, or the setting of a correct torque selected by the driver, may be derived from the pressure in the intake manifold. In addition, an optimum injection time in an internal combustion engine having throttle control may be ascertained from the load signal. 
     Because of the high importance of the intake-manifold pressure sensor during the operation of an internal combustion engine, it is desirable for defects in the intake-manifold pressure sensor to be diagnosed early. 
     Diagnostic methods for detecting the defectiveness of intake-manifold pressure sensors are known in the related art, but only in the case of conventional gasoline engines, in particular spark-ignition engines. The conventional internal combustion engines are distinguished in that their load control is implemented via the throttle valve, where there is a fixed relationship between the pressure in the intake manifold and the load, or between the throttle-valve angle and the load. 
     An example of such a diagnostic method for conventional spark-ignition engines is described, e.g., in published German patent document DE 199 46 874. Here, three different signals L 1 , L 2 , and L 3  are produced from different operating parameters, L 1  representing the mass flow rate of air that flows into an intake manifold of the spark-ignition engine, L 2  representing the pressure in the intake manifold, and L 3  representing a fuel signal ascertained from the mass flow rate of fuel. These signals are compared to each other in pairs and united to form combinations when variations occur. Different combinations of deviations are assigned different causes, i.e., different sources of error, for the deviations. Thus, e.g., in a first part of the method, it can initially be deduced that, when a particular deviation is present, either the intake-manifold pressure sensor and/or the exhaust-gas recirculation valve is defective. In a further part of the method, it may then be determined more accurately if the intake-manifold pressure sensor or the exhaust valve is defective. To this end, the pressure in the intake manifold during both the operation of the engine and its stoppage is measured and evaluated during the post-operation of the corresponding engine control unit. If the intake-manifold pressure is the same in both cases, it may be deduced that there is a defect in the intake-manifold pressure sensor; however, if the pressure in the intake manifold while the engine is stopped is less than that when the engine is being operated, it may then be deduced that an exhaust-gas recirculation valve is defective. 
     In addition, ambient-pressure sensors for use in internal combustion engines are known in the related art. In addition to the intake-manifold pressure sensors, they also supply important information for a control unit to control an internal combustion engine. Ambient-pressure sensors are used for, inter alia, ascertaining the maximum torque of the internal combustion engine. Diagnostic methods for ambient-pressure sensors are also known in the art. In conventional gasoline engines in which the load is controlled via the throttle valve, an ambient-pressure sensor can, however, only be checked for its correct method of functioning, i.e., plausibility-checked, while starting or during full load, since in conventional gasoline engines, a pressure that approaches ambient pressure is only present in the intake manifold under these conditions. 
     However, in internal combustion engines having variable valve timing, i.e., in internal combustion engines having throttleless load control, the load of the internal combustion engine is no longer controlled via the throttle valve and, therefore, via the pressure in the intake manifold, but rather via a change in its valve timing and/or its valve lift. The internal combustion engines having fully variable valve timing are distinguished by a lower fuel consumption than conventional gasoline engines. 
       FIG. 3  schematically illustrates such an internal combustion engine  100  having variable valve timing. Internal combustion engine  100  includes an engine block  110  having a piston  112 , which moves up and down in it. Connected to the engine block is an intake manifold  120  having a built-in throttle valve  122  and an exhaust pipe  130 . However, in contrast to conventional spark-ignition engines, throttle valve  122  is not used for controlling load. The control of the air supply and air exhaust through the intake manifold and the exhaust pipe, and therefore the control of the load of the internal combustion engine, is carried out via valves  140 , which are controlled by a control unit  200 , the control being implemented with the aid of fully variable timing edges. Instead of a single control unit  200 , several control units interconnected by any communication link may also be used for controlling valves  140 . The valves may be moved, for example, by electromagnetic or electrohydraulic actuators. 
     Control unit  200  includes an ambient-pressure sensor  210  for supplying a throttle-valve pressure signal, which represents pressure p_before_DK upstream from throttle valve  122 . In this context, ambient-pressure sensor  210  does not directly supply pressure p_before_DK, but it primarily supplies only the ambient pressure, i.e., the air pressure upstream from air filter  150  of the internal combustion engine. Then, actual pressure p_before_DK upstream from the throttle valve may be subsequently derived in either ambient-pressure sensor  210  itself or control unit  200 , by subtracting a pressure drop occurring in air filter  150  of the internal combustion engine from the measured ambient pressure. 
     In throttleless operation of the internal combustion engine, the pressure upstream from the throttle valve must be equal to the pressure in the intake manifold. An intake-manifold pressure sensor  220  is typically provided in the case of the control units of the related art. Intake-manifold pressure sensor  220  provides an intake-manifold pressure signal, which represents pressure p_intake in intake manifold  120  of internal combustion engine  100 . Sometimes, they are additionally provided with an ambient-pressure sensor  210 . 
     However, as explained above, since the load control in internal combustion engines having fully variable valve timing is no longer implemented via the throttle valve, all of the already known diagnostic methods for pressure sensors, which are based on deriving a load signal representing the load of the internal combustion engine from the angular position of the throttle valve or the pressure in the intake manifold, are no longer applicable to internal combustion engines having fully variable valve timing. 
     Therefore, an object of the present invention is to provide a method, a control unit, and a computer program for detecting a defective intake-manifold pressure sensor and/or a defective ambient-pressure sensor in internal combustion engines having fully variable valve timing. 
     SUMMARY 
     This object is achieved by the method, control unit, and program according to the present invention. In contrast to the related art, the present method dispenses with deriving load signals from the position of the throttle valve or from the intake-manifold pressure for detecting if at least one of two pressure sensors, namely the intake-manifold pressure sensor or the ambient-pressure sensor, is defective. Instead, the method of the present invention allows this detection to take place exclusively on the basis of a direct evaluation of the pressure upstream from the throttle valve and the pressure in the intake manifold. 
     According to an example embodiment, the method includes further steps, in order to be able to determine which of the two pressure sensors is defective. To this end, the internal combustion engine having throttleless load control is artificially shifted into an operating state, which simulates throttled load control. In the scope of this simulation, it is also possible to then derive simulated load signals from both the pressure in the intake manifold and the angular position of the throttle valve. According to the present invention, the simulated load signals are used for identifying the pressure sensor that is actually defective. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows an example embodiment of a method of the present invention for detecting defective pressure sensors in an internal combustion engine and a first logic module provided for implementing the method. 
         FIG. 1   b  shows an example embodiment of a control unit of the present invention. 
         FIG. 2  shows another example embodiment of the method according to the present invention and a second logic module provided for implementing the further refinement. 
         FIG. 3  shows a conventional internal combustion engine having variable valve timing, and a corresponding, conventional control unit for controlling the valves. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a  illustrates a method for determining whether or not at least one of two pressure sensors, namely the intake-manifold pressure sensor and/or the ambient-pressure sensor, is defective in the case of an internal combustion engine having variable valve timing as shown in  FIG. 3 . The method steps illustrates  FIG. 1   a  do not allow one to specifically determine which of the two pressure sensors is defective; such a determination may be made with the aid of further steps of the method described below with reference to  FIG. 2 . 
     The method is illustrated in  FIG. 1   a  by showing the functional layout of a first logic module  240  according to the present invention, which may be a component of a control unit  200  for controlling the valves of internal combustion engine  100 , which control unit  200  is shown in  FIG. 1   b . First logic module  240  allows the claimed method to be implemented. The layout of first logic module  240  is described below in detail; the individual steps of the claimed method may also be deduced from this. 
     According to  FIG. 1   a , first logic module  240  receives both a throttle-valve pressure signal, which represents pressure p_before_DK, and an intake-manifold pressure signal, which represents pressure p_intake in the intake manifold of the internal combustion engine. Generation of these signals is described below in connection with  FIG. 3 . According to the present invention, first logic module  240  also receives a first status signal from a first operating-state detector  230 , the first status signal indicating whether or not the internal combustion engine is being operated in a throttleless operating state. As an alternative, the status signal may also be generated by another logic module inside control unit  200 . 
     As shown in  FIG. 1   a , logic module  240  includes a first subtraction unit  242  for calculating a pressure difference delta_p by subtraction of the intake-manifold pressure signal from the throttle-valve pressure signal. This pressure difference is supplied to a first comparator unit  244 , which determines if pressure difference delta_p is greater than a first threshold value Delta_P 1 . A first AND gate  246  ANDs the logical output signal of first comparator unit  244  and the first status signal. The output signal of this first AND gate  246  makes a first statement as to whether or not one of pressure sensors  210 ,  220  is defective; this is exactly the case, when pressure difference delta_p is actually greater than first threshold value Delta_P 1  and internal combustion engine  100  is simultaneously being operated in a throttleless manner, as indicated by the first status signal. In this context, first threshold value Delta_P 1  may be set to approximately zero. 
     Irrespective of this first statement, first logic module  240  includes a second comparator unit  248  for determining if pressure difference delta_p is less than second threshold value Delta_P 2 , where Delta_P 2  may be zero. When this is the case, then, independently of the first statement, it is possible to make a second statement that one of the two pressure sensors  210 ,  220  is operating incorrectly. This statement is physically based on the fact that the pressure in intake manifold  120  can never be greater than the pressure upstream from throttle valve  122 . 
     First logic module  240  also includes an OR gate  249  for ORing the output signal of first AND gate  246  and the output signal of second comparator unit  248 . This OR operation is used for generating a first error signal E_DS_DSU, which represents an error in one of the two pressure sensors  210 ,  220 , when such an error has already been detected at either the output of first AND gate  246  or at the output of second comparator unit  248  or at both outputs. 
       FIG. 2  illustrates another example embodiment, which is a further refinement of the method represented in  FIG. 1 , this further refinement being used to allow an exact determination as to whether the intake-manifold pressure sensor or ambient-pressure sensor  210  is defective. This further refinement is implemented in the form of a second logic module  250 , which may be assigned to control unit  200  as well, as shown in  FIG. 1   b.    
     According to  FIGS. 1   b  and  2 , second logic module  250  implements the above-mentioned further refinement of the method by logically combining first error signal ES_DS_DSU, a first load signal load_from_intake-manifold-pressure, which represents the load of the internal combustion engine derived from the pressure in the intake manifold, and a second load signal load_from_DK, which represents the load of the internal combustion engine derived from the angular position of throttle valve  122 . 
     To implement the further refinement of the method, an operating state that includes throttled load control is simulated in the internal combustion engine  100  having absolutely variable valve timing. To this end, valves  140  are controlled by control unit  200 , using fixed timing edges. This special operating state is represented by a second status signal B_DK_occurred, which is likewise supplied to second logic module  250  as an input variable. 
     Second logic module  250  includes a second subtraction unit  251  for calculating a load difference by subtraction of the second load signal from the first load signal, as shown in  FIG. 2 . An absolute-value generator  252  calculates the absolute value of the load difference before this is supplied to a third comparator unit  253 . Third comparator unit  253  determines if the absolute value of the load difference is greater than a third threshold value Delta_load. 
     A second AND gate  254  ANDs first error signal E_DS_DSU and second status signal B_DK_occurred. The output signal of second AND gate  254  is transmitted, together with the output signal of third comparator unit  253 , to a third AND gate  255 , which means that the output signal of third AND gate  255  represents the result of ANDing the mentioned, inputted signals. In other words: the output signal of third AND gate  255  represents a second error signal E_DS_intake, i.e., it indicates, if applicable, the defectiveness of intake-manifold pressure sensor  220 . Such defectiveness is present, when the absolute value of the difference of the load of the internal combustion engine derived from the pressure in the intake manifold and the load derived from the angle of the throttle valve is greater than third threshold value delta_load, and when, in addition, the presence of a defect in at least one of the pressure sensors, ambient-pressure sensor  210  or intake-manifold pressure sensor  220 , was simultaneously detected in the preliminary procedure described above with reference to  FIG. 1   a , and when an operation that includes throttled load control was simulated in internal combustion engine  100 . The conclusion that the intake-manifold pressure sensor is defective is valid, since signal load_from_DK is reliably monitored by other diagnoses and is therefore correct. 
     Second logic module  250  further includes a fourth AND gate  256  for ANDing the output signal of comparator unit  253  inverted by an inverter  257  and the output signal of second AND gate  254 , as shown in  FIG. 2 . The output signal of fourth AND gate  256  represents a third error signal E_DS_Umg, which indicates, if applicable, a defect in ambient-pressure sensor  210 . Such a defect is present when the absolute value of the difference of the load of the internal combustion engine derived from the pressure in the intake manifold and the load derived from the angular position of the throttle valve is less than or equal to third threshold value Delta_load, and, at the same time, a defect in at least one of pressure sensors  210 ,  220  was already absolutely detected in the preliminary procedure described above with reference to  FIG. 1 , and an operation that includes throttled load control was simulated in the internal combustion engine. 
     Both the first and second logic modules may each be implemented independently of each other in the form of a hardware circuit. 
     The method of the present invention, and therefore also the first and/or second logic module  240 ,  250 , may be implemented in the form of a computer program. In this context, the computer program is executable on a computing element, e.g., on a microprocessor in control unit  200 , and is suitable for carrying out the method of the present invention. In this case, the present invention is therefore implemented by the computer program. The computer program may be stored in a memory element. In particular, an electrical storage medium, e.g., a random-access memory (RAM), a read-only memory (ROM), or a flash memory, may be used as the memory element.