Abstract:
The present disclosure relates to internal combustion engines and the teachings thereof may be embodied in methods and apparatus for detecting a malfunctioning rail pressure sensor. Some embodiments may include a method for detecting a malfunctioning rail pressure sensor comprising: delivering an analog output signal characterizing the rail pressure to a control unit; generating a differential rail pressure signal as a digital output signal; delivering the differential rail pressure signal of the rail pressure sensor to the control unit; analyzing the analog output signal and the differential rail pressure signal in the control unit; and detecting a malfunctioning rail pressure sensor based on the analysis by the control unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2015/064183 filed Jun. 24, 2015, which designates the United States of America, and claims priority to DE Application No. 10 2014 214 452.3 filed Jul. 23, 2014, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to internal combustion engines and the teachings thereof may be embodied in methods and apparatus for detecting a malfunctioning rail pressure sensor. 
       BACKGROUND 
       [0003]    Common-rail injection systems are equipped with a rail pressure sensor for determining the injection pressure and used in control of the injection pressure. Said rail pressure sensor is, therefore an essential component of a common-rail injection system, which inter alia contributes to complying with maximum permissible values for the emissions that occur and the fuel consumption. A malfunction of said rail pressure sensor would result in a significant emission and drivability degradation and would constitute a safety risk. 
         [0004]    In the event of a sensor malfunction with which too low a pressure is measured, an existing pressure control circuit would increase the injection pressure and thus the rail pressure, perhaps to a critical pressure level. As a result, mechanical components of the common-rail injection system would be overloaded, so that for example a fuel leak can occur. Consequently, it is necessary to detect the presence of a rail pressure sensor defect and place an associated fault signal in a fault memory to indicate the source of the fault to service staff and to enable suitable countermeasures to be taken. 
         [0005]    It is already known to perform a pressure equalization with ambient pressure when the common-rail injection system is unpressurized for detecting a rail pressure sensor defect. But this type of detection can only be performed before the start of the respective engine and gives no indication of whether the pressure measurement value provided by the rail pressure sensor is correct or not in the entire measurement range, for example at a system pressure of 1500 bar. Said type of detection also provides no information about any drifting of the measurement values provided by the sensor and also no information about any characteristic curve gradient errors. 
         [0006]    Furthermore, it is also known to carry out a redundant version of the rail pressure measurement by means of two rail pressure sensors and to continuously compare the measurement values obtained in the entire sensor measurement range with each other for detecting a defect of a rail pressure sensor. If the obtained measurement values deviate from each other, then the presence of a malfunction is detected and necessary countermeasures can be initiated. Such a redundant version of the rail pressure measurement is however difficult to implement for structural reasons and furthermore causes comparatively high costs. 
         [0007]    An analysis of the rail pressure measurement signals provided by a rail pressure sensor is generally carried out in a control unit connected to the rail pressure sensor, to which the rail pressure measurement signals provided by the rail pressure sensor are delivered. One possibility is transmitting the rail pressure measurement signals that are provided by the rail pressure sensor to the control unit in digital form. However, the high data rates that are required by the injection system for pressure control cannot be achieved by means of such a digital transmission. 
         [0008]    An alternative possibility is transmitting the rail pressure measurement signals provided by the rail pressure sensor to the control unit in the form of analog signals. Said analog transmission does not however give the possibility of a manipulation lock for the purpose of making difficult or preventing tuning of the rail pressure sensor to achieve an engine performance increase. 
         [0009]    A method and an apparatus for plausibility checking the output signal of a rail pressure sensor are known from DE 10 2008 043 413 A1. With said known method, an analog signal of a rail pressure sensor characterizing the rail pressure is delivered to a control unit and processed there. Furthermore, an additional digital signal characterizing the rail pressure is output by the rail pressure sensor and is compared with the analog signal in the control unit for plausibility checking of the analog signal characterizing the rail pressure. 
       SUMMARY 
       [0010]    The teachings of the present disclosure enable methods and an apparatus for detecting a malfunctioning rail pressure sensor, with which the detection provides more reliable results. 
         [0011]    Some embodiments may include a method for detecting a malfunctioning rail pressure sensor, with which an analog output signal of the rail pressure sensor characterizing the rail pressure and a digital output signal of the rail pressure sensor are delivered to a control unit and are analyzed in the control unit, characterized in that a differential rail pressure signal is output by the rail pressure sensor ( 13 ) as a digital output signal and is delivered to the control unit ( 17 ) and that the differential rail pressure signal is used in the control unit ( 17 ) for detecting the malfunctioning rail pressure sensor. 
         [0012]    In some embodiments, a comparison of the transmitted differential rail pressure signal with a predetermined threshold value is carried out in the control unit ( 17 ) and that the control unit detects the presence of a malfunctioning rail pressure sensor if the transmitted differential rail pressure signal exceeds the predetermined threshold value. 
         [0013]    In some embodiments, generation of the differential of the output signals of two rail pressure measurement values, each output by a rail pressure measurement device ( 18 ,  18   a,    19 ;  18 ,  18   b,    20 ), is carried out in the rail pressure sensor ( 13 ). 
         [0014]    In some embodiments, generation of the differential of the output signals of two rail pressure measurement values detected at the same time is carried out in the rail pressure sensor ( 13 ). 
         [0015]    Some embodiments may include an apparatus for detecting a malfunctioning rail pressure sensor, comprising a rail pressure sensor ( 13 ) and a control unit ( 17 ) connected to the rail pressure sensor, wherein the rail pressure sensor ( 13 ) comprises a first and a second rail pressure measurement device and is designed to output an analog signal characterizing the rail pressure and a digital signal and the control unit ( 17 ) is designed to receive and to analyze the analog signal output by the rail pressure sensor ( 13 ) and the digital signal output by the rail pressure sensor ( 13 ), characterized in that the rail pressure sensor ( 13 ) furthermore comprises a rail pressure differentiator ( 21 ) that forms a differential rail pressure signal from the output signal of the first rail pressure measurement device ( 18 , 18   a , 19 ) and the output signal of the second rail pressure measurement device ( 18 , 18   b , 20 ), the rail pressure sensor ( 13 ) outputs the differential rail pressure signal as a digital signal to the control unit and the control unit ( 17 ) is designed to analyze the differential rail pressure signal. 
         [0016]    In some embodiments, the first and the second rail pressure measurement devices each comprise a measurement transducer ( 18   a , 18   b ). 
         [0017]    In some embodiments, the first and the second rail pressure measurement devices each comprise an ASIC ( 19 , 20 ). 
         [0018]    In some embodiments, the rail pressure sensor ( 13 ) comprises a digital to analog converter ( 22 ) that is connected to the output of the first rail pressure measurement device ( 18 , 18   a , 19 ) and that provides the analog signal output by the rail pressure sensor. 
         [0019]    In some embodiments, the control unit ( 17 ) comprises a filter ( 23 ), an analog to digital converter ( 24 ) connected to the output of the filter and a computing unit ( 25 ) for analyzing the received analog signal. 
         [0020]    In some embodiments, the control unit ( 17 ) comprises a computing unit ( 26 ) that is designed for analyzing the differential rail pressure signal. 
         [0021]    In some embodiments, the computing unit ( 26 ) is designed to compare the differential rail pressure signal with a predetermined threshold value and to detect the presence of a malfunctioning rail pressure sensor if the differential rail pressure signal exceeds the predetermined threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The teachings of the present disclosure are described in detail below using the drawings by way of example. In the drawings: 
           [0023]      FIG. 1  shows a block diagram of a common-rail injection system; and 
           [0024]      FIG. 2  shows a block diagram describing an exemplary embodiment for the method according to the teachings of the present disclosure as well as an example apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    An example method for detecting a malfunctioning rail pressure sensor may include delivering an analog signal of the rail pressure sensor characterizing the rail pressure and a digital output signal of the rail pressure sensor to a control unit and analyzing the signal in the control unit. The digital signal output by the rail pressure sensor is a differential rail pressure signal that is determined in the rail pressure sensor. This is delivered to the control unit and is used there for detecting a malfunctioning rail pressure sensor. 
         [0026]    An example apparatus for detecting a malfunctioning rail pressure sensor comprises a rail pressure sensor and a control unit connected to the rail pressure sensor. The rail pressure sensor contains first and second rail pressure measurement devices and is designed to output an analog signal characterizing the rail pressure and a digital signal. The control unit is designed to receive and analyze the analog signal output by the rail pressure sensor and the digital signal output by the rail pressure sensor. The rail pressure sensor also comprises a rail pressure differentiator that forms a differential rail pressure signal from the output signal of the first rail pressure measurement device and the output signal of the second rail pressure measurement device. The rail pressure sensor outputs the differential rail pressure signal as a digital signal. The control unit is designed to analyze the differential rail pressure signal. 
         [0027]    Comparing the transmitted differential rail pressure signal with a predetermined threshold value may be performed in the control unit. The control unit detects a malfunctioning rail pressure sensor if the transmitted differential rail pressure signal exceeds the predetermined threshold value. The rail pressure sensor may comprise two rail pressure measurement devices that each output a rail pressure measurement value. The differential rail pressure signal is calculated from said two rail pressure measurement values, which may be determined at the same time by the rail pressure measuring devices. 
         [0028]      FIG. 1  shows a block diagram of a common-rail injection system  1 . Said common-rail injection system  1  includes a fuel tank  2 , from which fuel is pumped during operation of the system and is delivered by means of a fuel filter  3  to a pump unit that is bounded by dashed lines in  FIG. 1 . A temperature sensor  4 , by means of which the temperature of the fuel is measured, may be positioned between the fuel filter  3  and the pump unit. 
         [0029]    In the pump unit, the fuel provided via the fuel filter  3  is delivered to a low-pressure chamber  5 , which furthermore is connected to a fuel return line  16 . The fuel output from the low-pressure chamber  5  is delivered via a pump filter  6  and an inlet valve  7  to a high-pressure pump  8  and is brought to a high pressure by means of said pump. The high-pressure fuel output by the high-pressure pump  8  is delivered to a rail  12  via an outlet valve  9  and an outlet choke  10 . As an alternative to the outlet choke  10 , an inlet choke  11  can be used upstream of the rail  12 . The rail  12  is connected by means of a pressure discharge valve  14  to the fuel return line  16 , via which fuel can be delivered back into the fuel tank  2  and/or the low-pressure chamber  5 . 
         [0030]    Furthermore, a rail pressure sensor  13  for the measurement of the fuel pressure occurring in the rail  12  is coupled to the rail  12 . The output signals of the rail pressure sensor  13  are delivered to a control unit  17 , which analyzes said signals and controls the injection processes of the common-rail injection system while taking into account the analyzed output signals of the rail pressure sensor  13 . 
         [0031]    In the example shown, the rail  12  is connected to a total of four injection valves  15 , the opening and closing processes of which are controlled by the control unit  17  by means of suitable control signals, wherein said control signals are determined in the control unit  17  taking into account the output signals provided by the rail pressure sensor  13  and further signals that are not shown in  FIG. 1 . With the common-rail injection system represented in  FIG. 1 , it is important that a malfunctioning rail pressure sensor can be detected rapidly and reliably, so that suitable countermeasures can be initiated. 
         [0032]      FIG. 2  shows a block diagram of an apparatus for such rapid and reliable detection of a malfunctioning rail pressure sensor. Said apparatus comprises a rail pressure sensor  13  and a control unit  17  connected to the rail pressure sensor. 
         [0033]    The rail pressure sensor  13  contains a first rail pressure measurement device and a second rail pressure measurement device. The first rail pressure measurement device comprises a sensor membrane  18 , a first measurement transducer  18   a , and a first ASIC  19 . The second rail pressure measurement device comprises the sensor membrane  18 , a second measurement transducer  18   b  and a second ASIC  20 . A first rail pressure measurement signal is provided at the output of the ASIC  19 , a second rail pressure measurement signal at the output of the ASIC  20 . The measurement transducers can be Wheatstone measurement bridges with strain gauges (DMS). 
         [0034]    A differential rail pressure signal is calculated from said two rail pressure measurement signals by means of the rail pressure differentiator  21 . Said differential rail pressure signal is available at the output of the rail pressure differentiator  21 , is provided at an output of the rail pressure sensor  13 , and is transmitted from there to the control unit  17  as a digital signal via a transmission path. The differential rail pressure signal, which is not a pressure value, e.g., not a digital signal characterizing the rail pressure, but a fault variable, is received there by a signal input unit  27  and passed on from the output thereof to a computing unit  26 . 
         [0035]    Said computing unit is designed to analyze the differential rail pressure signal. During said analysis of the differential rail pressure signal, the computing unit  26  compares the differential rail pressure signal with a predetermined threshold value and detects the presence of a malfunctioning rail pressure sensor if the differential rail pressure signal exceeds the predetermined threshold value. 
         [0036]    The output signal of the ASIC  19  is furthermore also delivered to a digital to analog converter  22  and is converted in the same into an analog signal. Said analog signal is provided at the output of the digital to analog converter  22  and at a further output of the rail pressure sensor  13 . From there it is transmitted via a further transmission path to the control unit  17 . In the control unit  17 , the transmitted analog signal, which is an analog signal characterizing the rail pressure, is delivered via a noise filter  23  to an analog to digital converter  24  and is converted in the same into a digital signal. The digital signal provided at the output of the analog to digital converter  24  is delivered to a computing unit  25  for further analysis. 
         [0037]    The computing units  24  and  25  can be a computing device comprising a plurality of computing units or computing modules—as indicated by the dashed line enclosing said two computing units. Both analog signals and digital signals are transmitted from the rail pressure sensor  13  to the control unit  17 . Said signals have different time constants, wherein the analog signals have a short time constant of for example 0.124 ms and the digital signals have a long time constant of for example 1 ms. 
         [0038]    The transmission of the analog signals ensures very accurate timing in the control unit  17  of high resolution pressure values that are particularly required in diesel applications for controlling the injection system. By an analysis of the digitally transmitted differential rail pressure signals, which also takes place in the control unit  17 , the presence of a malfunctioning rail pressure sensor can be detected. 
         [0039]    As said differential rail pressure signal is already detected in the rail pressure sensor, the rail pressure measurement values, which are used to form the differential rail pressure signal, can be rail pressure measurement signals that are available in the rail pressure sensor at the same time or rail pressure measurement values that are determined at the same time using the two rail pressure measurement devices. Consequently, there is no time shift between the rail pressure measurement values used for rail pressure differentiation. As a result, the safety or reliability of detection of a malfunctioning rail pressure sensor is increased in comparison to the prior art, in which a signal comparison is only carried out in the control unit and thereby for pressure values that are acquired with a time offset owing to the different signal transition times of analog and digital signals. 
         [0040]    In particular, a fault detected during the analysis of two rail pressure measurement signals compared with each other although there is in fact no fault, owing to different transmission times of analog and digital signals, is prevented. In particular, during a comparison of the signals that are to be compared with each other in the control unit, depending on the dynamics of the signal change asynchronous pressure measurement values would be compared with each other, which can result in undesirable errors in the detection. In addition, if said comparison is carried out in the control unit, the different signal paths of the two signals that are to be compared with each other must then be taken into account, which in principle again results in a measurement value shift. For example, during the analog transmission, electromagnetic noise, errors caused by line resistances, errors caused by the necessary A/D conversion and errors caused by signal filtering occur, which can cause measurement value errors. 
         [0041]    The digital signal transmission may be used to transmit further signals from the rail pressure sensor  13  to the control unit  17  on the digital transmission path. As a result, for example the possibility of a manipulation lock can be offered, by means of which tuning of the rail pressure sensor for the purpose of a non-permitted engine performance increase can be prevented or at least detected.