Abstract:
A method for detecting faults in an air system of an internal combustion engine having an intake manifold and an exhaust manifold includes, but is not limited to the steps of measuring an oxygen concentration of the gas flowing in the exhaust manifold, estimating an intake oxygen control value in the intake manifold and estimating an intake oxygen reference value in the intake manifold based on said oxygen concentration of the gas flowing in the exhaust manifold. The method further includes, but is not limited to the steps of calculating an intake deviation value as a difference between the intake oxygen control value and the intake oxygen reference value and verifying if the intake deviation value is greater than a predetermined first threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to British Patent Application No. 0811772.3, filed Jun. 27, 2008, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to fault detection in an air system of an internal combustion engine. 
     BACKGROUND 
     The combustion process in an internal combustion engine produces NO X  (principally NO and NO 2 ), CO, CO 2 , HC (HydroCarbons), and PM (Particulate Matter). The amount of CO 2  depends on the amount of fuel injected into the cylinders and the amount of CO and HC depends on the combustion efficiency of the internal combustion engine. The amount of NO X  depends on the combustion temperature and on the amount of oxygen introduced into the cylinders, while the amount of PM is strictly dependent on the air to fuel ratio (λ). 
     To optimize the amount of PM and NO X  produced, combustion engines are provided with an EGR (Exhaust Gas Recirculation) circuit. The EGR system recirculates exhaust gas from the exhaust manifold to the intake manifold in order to dilute the fresh air introduced into the engine. This leads to emission optimization during the combustion process, because higher amount of H 2 O and CO 2  are introduced, which have a high heat capacity that reduces the combustion temperature. Another effect of diluting the intake flow is that it is possible to control the amount of O 2  in the intake flow. The counter effect of this system is that the more the fresh air is diluted, the more the air to fuel ratio (λ) is reduced. This leads to higher amount of PM emissions. The quantity of exhaust gas flowing into the cylinders is controlled by an EGR valve. 
     In conventional internal combustion engines there are also an air mass sensor (or air flow meter), an air pressure sensor, an air temperature sensor and an oxygen sensor at the intake manifold. The air mass sensor is adapted to measure the fresh air flow entering the intake manifold through a throttle valve. The air pressure and temperature sensors are adapted to measure the pressure and the temperature of the gas entering into the cylinders, respectively. They are placed in the intake manifold downstream the mixing point between the fresh air flow and the recirculated gas flow. 
     In conventional engines there is an electronic control unit (ECU) arranged to estimate the gas flow entering into the cylinders and to control the exhaust gas recirculation in the intake manifold. In order to detect a failure in the engine operation, the ECU performs a deviation error monitoring by calculating the difference between a requested (or setpoint) value for a given entity and a corresponding measured value taken from a sensor, so as to detect a deviation of the air system behavior due to failures inside it. 
     It has been demonstrated that the emissions can be limited by the introduction of the oxygen concentration monitoring in the control of the exhaust gas recirculation in the intake manifold. However, oxygen sensors adapted to measure an actual oxygen quantity in the intake manifold are expensive and do not provide data quickly, this resulting in a delay in obtaining an indication of the deviation of the actual oxygen quantity from a predetermined oxygen setpoint. 
     In view of the above, it is at least one object of the present invention to provide an improved method for detecting faults in the air system which takes into account the oxygen concentration in the intake manifold without using data directly provided by an oxygen sensor. In addition, it other objects, desirable features and characteristics, will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background. 
     This and other objects are achieved according to the present invention by a method for detecting faults in an air system of an internal combustion engine having an intake manifold and an exhaust manifold. The method comprising the steps of measuring an oxygen concentration ([O2]em_UEGO) of a gas flowing in the exhaust manifold; a) estimating an intake oxygen control value ([O2]im_control) in the intake manifold, estimating an intake oxygen reference value ([O2]im_ECU) in the intake manifold based on the oxygen concentration ([O2]em_UEGO) of the gas flowing in the exhaust manifold, calculating an intake deviation value ([O2]im_dev) as a difference between the intake oxygen control value ([O2]im_control) and the intake oxygen reference value ([O2]im_ECU), and comparing if the intake deviation value ([O2]im_dev) with a predetermined first threshold (TH 1 ). The method further comprising the steps of b) estimating an exhaust oxygen control value ([O2]em_control) in the exhaust manifold, calculating an exhaust deviation value ([O2]em_dev) as a difference between the exhaust oxygen control value ([O2]em_control) and the oxygen concentration ([O2]em_UEGO) of the gas flowing in the exhaust manifold; and comparing said exhaust deviation value ([O2]em_dev) with a predetermined second threshold (TH 2 ). The method also including the steps of c) determining an exhaust oxygen concentration setpoint ([O2]spEM) indicative of the oxygen concentration in the exhaust manifold, calculating a fresh airflow setpoint (Airreference) as a function of the exhaust oxygen concentration setpoint ([O2]spEM), measuring ( 400 ) a fresh air mass flow value (mMAF), calculating ( 1400 ) an airflow deviation (Airflowdev) as difference between said fresh airflow setpoint (Airreference) and the fresh air mass flow value (mMAF), and comparing said airflow deviation (Airflowdev) with a third predetermined threshold (TH 3 ) and a fourth predetermined threshold (TH 4 ), and finally detecting faults in the air system as a function of the combination of results of comparisons at step a), b) and/or c). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and: 
         FIG. 1  is a flow chart of the operations to be performed to detect faults in the air system according to an embodiment of the present invention; and 
         FIG. 2  is a flow chart of the operations to be performed to realize an airflow deviation error monitoring. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit application and uses. Furthermore, there is no intention to be bound by any theory presented in the preceding background or summary or the following detailed description. 
     Briefly, the method according to an embodiment of the invention is based on the use of a double check to monitor the oxygen concentration estimation deviation; a first check is made on the oxygen concentration value in the intake manifold, the other one is based on the oxygen concentration value in the exhaust manifold. 
       FIG. 1  shows a flow chart of the operations to be performed to detect faults in the air system according to the embodiment of the method of the invention. The method comprises a first step  100  of measuring the oxygen concentration in the exhaust gas flow through a Universal Exhaust Gas Oxygen (UEGO) sensor placed in the exhaust line of the engine. The UEGO sensor is arranged to provide an analogue output [O 2 ] em     —     UEGO  which is proportional to the oxygen percentage in the exhaust gas. 
     An electronic control unit ECU of the engine estimates, in a step  200 , an intake oxygen control value [O 2 ] im     —     control  in the intake manifold, for example as disclosed in U.S. Pat. No. 7,117,078, which is hereby incorporated in its entirety by reference. 
     In a step  300  the ECU estimates an intake oxygen reference value [O 2 ] im     —     ECU  in the intake manifold according: 
                       [     O   2     ]     im_UEGO     =     0.233   ⁢     (     1   -           m   TOT     -     m   MAF         m   TOT       ⁢     1   λ         )               (   1   )               
Where m MAF  is a fresh air mass flow value measured, in a step  400 , by an air mass sensor adapted to measure the fresh air flow entering the intake manifold through a throttle valve, λ is the air to fuel ratio calculated by the ECU, based on the oxygen concentration [O 2 ] em     —     UEGO  measured by the said UEGO sensor, and m TOT  is the total air mass flow in the intake manifold, calculated in a step  500  according to the following equation:
 
                     m   TOT     =     η   ⁢           ⁢   V   ⁢     P   RT               (   2   )               
Where V is the volume of the cylinder, P is the pressure in the intake manifold measured by the pressure sensor, T is the temperature in the intake manifold measured by the temperature sensor and η is the estimated volumetric efficiency.
 
     In a step  600  the ECU calculates an intake deviation value [O 2 ] im     —     dev  according to the following equation:
 
[ O   2 ] im     —     dev   =|[O   2 ] im     —     control   −[O   2 ] im     —     ECU |  (3)
 
and verifies if said deviation value [O 2 ] im     —     dev  is greater than a first predetermined threshold TH 1 .
 
     According to the subject matter disclosed in U.S. Pat. No. 7,117,078 and the above disclosed equation 1, the intake manifold pressure sensor, the intake manifold temperature sensor and the air mass sensor are used for both estimating the intake oxygen control value [O 2 ] im     —     control  and the intake oxygen reference value [O 2 ] im     —     ECU  in the intake manifold. 
     The failure modes that can affect the common inputs (e.g., drift of the sensors involved in the estimation, measurement of the sensors fixed to a plausible value, etc. . . . ) cannot be therefore detected by this check. 
     The above calculation of the intake deviation value [O 2 ] im     —     dev  is therefore designed so as to detect all the fault types of the air system that can produce a deviation in the intake oxygen concentration estimation, but it is unable to isolate a single fault since there are many faults that can affect such a deviation. 
     For this reason, the ECU further estimates in a step  700  an exhaust oxygen control value [O 2 ] em     —     control  in the exhaust manifold, for example as disclosed in U.S. Pat. No. 7,117,078. In a step  800  the ECU calculates an exhaust deviation value [O 2 ] em     —     dev  according to the following equation:
 
[ O   2 ] em     —     dev   =|[O   2 ] em     —     control   −[O   2 ] em     —     UEGO |  (4)
 
and verifies if said deviation value [O 2 ] em     —     dev  is greater than a second predetermined threshold TH 2 . Where [O 2 ] em     —     UEGO  is the oxygen concentration in the exhaust manifold measured by the UEGO sensor.
 
     According to the subject matter disclosed in U.S. Pat. No. 7,117,078, the intake manifold pressure sensor, the intake manifold temperature sensor and the air mass sensor are used for both estimating the exhaust oxygen control value [O 2 ] em     —     control  in the exhaust manifold and the intake oxygen control value [O 2 ] im     —     control  in the intake manifold. 
     With this double check, it is possible to separate the fault causes in two different groups. 
     If a fault is detected by the second check but it is not detected by the first one, than it could be due to a failure in one of the sensors that has been used as input for both estimating the intake oxygen control value [O 2 ] im     —     control  and the intake oxygen reference value [O 2 ] im     —     ECU  in the intake manifold. Actually, the same sensors are used for estimating the intake oxygen control value [O 2 ] im     —     control , the exhaust oxygen control value [O 2 ] em     —     control  and the intake oxygen reference value [O 2 ] im     —     ECU , while the oxygen concentration in the exhaust manifold [O 2 ] em     —     UEGO  is measured by the UEGO sensor. 
     If the second check shows a deviation, it means that there is a fault in one of the sensors used for estimating the exhaust oxygen control value [O 2 ] em     —     control  because the other term of comparison, that is the oxygen concentration in the exhaust manifold [O 2 ] em     —     UEGO , is a measured value, and therefore not affected by sensors at the intake manifold. 
     If the first check does not show any deviation, it means that a fault in a sensor commonly used as input both for estimating the intake oxygen control value [O 2 ] im     —     control  and the intake oxygen reference value [O 2 ] im     —     ECU  affects both the two estimations, and this leads to not reveal any deviation. 
     Differently, if a fault is detected by both the two checks, than it could be due to a failure of one of the other sensors, actuators or components that belong to the air system. 
     The ECU defines in a step  900  a predetermined intake oxygen concentration setpoint [O 2 ] spIM  and, in a step  1000 , calculates a final oxygen estimation deviation in the intake manifold as a difference between said intake oxygen concentration setpoint [O 2 ] spIM  and the intake oxygen control value [O 2 ] im     —     control  in the intake manifold estimated in step  200 . 
     In order to isolate the faults that affect said final oxygen estimation deviation, the results of the previous two checks are compared with a third check based on a classical airflow deviation error monitoring, as here below disclosed with reference to  FIG. 2 . 
     A fresh airflow setpoint Air reference  is calculated in a step  1100  through the following equation: 
                     Air   reference     =               [     O   2     ]     spEM         [     O   2     ]     air       ⁢     M   fuel       +       C   sr     ⁢     M   fuel_Burnt           1   +     [         [     O   2     ]     spEM         [     O   2     ]     air       ]                 (   5   )               
Where M fuel  is the quantity of injected fuel, M fuel     —     Burnt  is the portion of the injected fuel quantity that takes part to the combustion process, C sr  is the stoichiometric air to fuel ratio, [O 2 ] spEM  is an exhaust oxygen concentration setpoint in the exhaust manifold, calculated by the control unit ECU as herein below disclosed, [O 2 ] air  is the oxygen concentration in the fresh air (e.g. 20.95% in case of volumetric concentration).
 
     The exhaust oxygen concentration setpoint [O 2 ] spEM  is provided by the ECU according to the following two options: 1) it is determined as a function of the engine operating point (engine speed and load); 2) it is calculated in a step  1300  according to the following equation: 
                       [     O   2     ]     spEM     =             (     η   ⁢         p   boost     ⁢     V   eng     ⁢     N   eng           R   im     ⁢     T   im     ⁢   120         )     ⁡     [     O   2     ]       spEM     -       C   sr     ⁢         M   fuel_Burnt     ⁡     [     O   2     ]       air             (     η   ⁢         p   boost     ⁢     V   eng     ⁢     N   eng           R   im     ⁢     T   im     ⁢   120         )     +     M   fuel                 (   6   )               
Where η is the volumetric efficiency, V eng  is the engine displacement, N eng  is the engine rotational speed, R im  is the ideal gas law constant, T im  is an intake manifold temperature setpoint and p boost  is a predetermined boost pressure setpoint.
 
     Alternatively, the boost pressure setpoint p boost  and the intake manifold temperature setpoint T im  may be replaced with respective actual pressure and actual temperature measured by sensors placed in the intake manifold. Alternatively, other combinations of the above cited parameters may be possible. 
     Briefly summarizing, two options are therefore possible: 1) the exhaust oxygen concentration setpoint [O 2 ] spEM  is determined within the control unit ECU and it is then used to calculate the air reference value Air reference  through equation 5; 2) the intake oxygen concentration setpoint [O 2 ] spIM  is determined within the control unit ECU and it is used to calculate a corresponding exhaust oxygen concentration setpoint through equation 6, so as to have a value that can be used in equation 5 to calculate the air reference value Air reference . 
     The fresh airflow setpoint Air reference  is compared with the measured fresh air mass flow m MAF  in order to calculate, in a step  1400 , an airflow deviation Airflow dev , and to verify if said airflow deviation Airflow dev  is comprised between a third threshold TH 3  and a fourth threshold TH 4 , according to the following equations:
 
Airflow dev =Air reference   −m   MAF   &gt;TH 3  (7)
 
Airflow dev =Air reference   −m   MAF   &lt;TH 4  (8)
 
The airflow deviation Airflow dev  is an error term representative of the difference between the desired fresh airflow and the actual one. In this case, the only sensor common to those used by the two previously described checks is the air mass sensor.
 
     Based on the three checks above disclosed, it is possible to detect faults affecting the final oxygen concentration estimation in the intake manifold and furthermore, by means of a cross check, it is also possible to isolate the faults, as shown for example in Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Check 1 
                 Check 2 
                 Check 3 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Sensor 1 
                 x 
                 x 
                 x 
               
               
                   
                 Sensor 2 
                   
                 x 
                 x 
               
               
                   
                 Sensor 3 
                   
                 x 
                 x 
               
               
                   
                 Sensor 4 
                   
                 x 
                 x 
               
               
                   
                 Sensor 5 
                   
                   
                 x 
               
               
                   
                 Sensor 6 
                   
                   
                 x 
               
               
                   
                   
               
             
          
         
       
     
     The sensor 1 is used by check 1, check 2 and check 3. The sensor 2 is used by check 2 and check 3, but not by check 1. If check 2 and check 3 detect a fault, but check 1 does not detect any fault, it is possible to declare that the fault is not on sensor 1, but could be on sensor 2. Depending on the number of sensors common or not to the three checks, it is possible to reduce the number of sensors that can be subject to a fault and, in some cases, it is possible to recognize the sensor responsible of the fault of the whole system. 
     The present invention is applicable in both diesel and gasoline engines. Clearly, the principle of the invention remaining the same, the embodiments and the details of production can be varied considerably from what has been described and illustrated purely by way of non-limiting example, without departing from the scope of protection of the present invention as defined by the attached claims. Moreover, while at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.