Patent Publication Number: US-6983735-B2

Title: Control apparatus for controlling the amount of intake air into an engine

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a control apparatus for controlling the amount of intake air into the engine in accordance with a leakage in a blow-by gas passage. 
   It is known that a control valve, which is disposed in an intake manifold connected to an internal-combustion engine (hereinafter referred to as an “engine”), is clogged with carbon (it may be referred to as carbon deposit) with years of use due to deposition of lubricating oil and combustion products. 
   Japanese Registered Utility Model Publication No. 2558153 discloses a scheme for correcting the amount of intake air in accordance with the degree of clogging of a bypass valve that is disposed in a passage that bypasses a throttle valve. 
   According to the scheme, a valve for increasing/decreasing the amount of intake air is additionally provided in the bypass passage. At a first desired engine rotational speed, an opening degree D 1  of the bypass valve when the additional valve is in a closed position and an opening degree D 2  of the bypass valve when the additional valve is in an open position are learned. If the additional valve is closed when the opening of the bypass valve is fixed to D 2 , the engine rotational speed decreases. The additional valve is then opened to learn an opening degree D 3  of the bypass valve. A characteristic of the intake air amount with respect to the opening degree of the bypass valve is updated so that changes in the intake air amount when the opening degree of the bypass valve changes from D 1  to D 2  are equal to changes in the intake air amount when the opening degree of the bypass valve changes from D 2  to D 3 . Thus, accuracy of controlling the intake air amount is improved by updating the characteristic of the intake air amount. 
   On the other hand, blow-by gas may leak from a combustion chamber to a crankcase of the engine. There is a conventional scheme for returning the blow-by gas into an intake air system of the engine so as to prevent emission of such blow-by gas to the atmosphere. Japanese Patent Application Unexamined Publication (Kokai) No. 2002-130035 discloses a scheme for detecting a leakage (including disconnection and hole) of a passage that is designed to return the blow-by gas into the intake air system. According to the scheme, if a difference between the amount of air that is actually introduced into the engine and a desired amount of intake air that is calculated by a control unit exceeds a predetermined value, it is determined that a leakage has occurred. 
   According to the conventional scheme, when the intake air amount increases due to a leakage in the blow-by gas passage, it may be erroneously determined that clogging of the control valve disposed in the intake manifold has been eliminated. The characteristic of the intake air amount may be inappropriately updated. After the leakage of the blow-by gas passage is repaired, control of the intake air amount may start based on such inappropriate characteristic of the intake air amount. This may cause instability in the operating condition of the engine. 
   If the characteristic of the intake air amount is updated immediately after a leakage occurs, it is determined that an actual intake air amount into the engine has converged to a desired intake amount. Such updating eliminates a difference between the actual and desired intake air amounts, which may make it difficult to detect the leakage. 
   Thus, there exists a need for a control apparatus that is capable of prohibiting updating the characteristic of the intake air amount if a leakage is detected in the blow-by gas passage. There also exists a need for a control apparatus that is capable of adjusting a speed of updating the characteristic of the intake air amount so as to ensure detection of a leakage in the blow-by gas passage. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a control apparatus for controlling the amount of intake air into an engine is provided. The control apparatus comprises a control valve provided in an intake air passage into the engine and a control unit. The control unit updates a clogging efficient based on a feedback correction amount for feedback controlling a rotational speed of the engine during idling operation. The clogging efficient indicates a degree of clogging of the intake air passage. The control unit determines a desired opening degree of the control valve based on the clogging coefficient and causes an opening degree of the control valve to converge to the desired opening degree. The control unit is further configured to prohibit the update of the clogging coefficient if a leakage in a blow-by gas passage connected between the engine and the intake air passage is detected. 
   According to the invention, since updating the clogging coefficient is prohibited if a leakage is detected in the blow-by gas passage between the engine and the intake air passage, it is prevented to update the intake air characteristic based on an erroneous determination that clogging has been eliminated. After the blow-by gas passage is repaired, the intake air amount control can start based an appropriate intake air characteristic. 
   According to one embodiment of the invention, the clogging coefficient is updated so that a difference between a current value of the clogging coefficient and a previous value of the clogging coefficient is within a predetermined range. Thus, a range within which the clogging coefficient is updated is limited. Such limitation prevents the intake air characteristic from instantly changing. Since a rate at which the intake air characteristic is updated is limited, it is ensured that a leakage in the blow-by gas passage is detected. 
   According to one embodiment of the invention, a controlled variable for controlling the opening degree of the control valve is determined based on the feedback correction amount. The desired opening degree of the control valve is determined based on the controlled variable and the clogging coefficient. 
   According to one embodiment of the invention, the feedback correction amount is smoothed to determine a learning value. The clogging coefficient is determined based on the learning value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing an engine and a control unit in accordance with one embodiment of the present invention. 
       FIG. 2  shows a block diagram of an intake air amount control apparatus in accordance with one embodiment of the present invention. 
       FIG. 3  is a graph showing time-dependent changes in first and second learning values in accordance with clogging in an intake manifold in accordance with one embodiment of the present invention. 
       FIG. 4  shows a map for determining a clogging coefficient in accordance with one embodiment of the present invention. 
       FIG. 5  shows a map for determining a desired throttle opening degree THICMD in accordance with one embodiment of the present invention. 
       FIG. 6  shows a flowchart of a process for calculating a first learning value in accordance with one embodiment of the present invention. 
       FIG. 7  shows a flowchart of a process for determining a learning permission range in accordance with one embodiment of the present invention. 
       FIG. 8  shows a flowchart of a process for calculating a second learning value in accordance with one embodiment of the present invention. 
       FIG. 9  shows a flowchart of a process for calculating a clogging coefficient in accordance with one embodiment of the present invention. 
       FIG. 10  shows a flowchart of a process for calculating a desired throttle opening degree in accordance with one embodiment of the present invention. 
       FIG. 11  shows a flowchart of a process for detecting a leakage in a blow-by gas passage in accordance with one embodiment of the present invention. 
       FIG. 12  shows a graph illustrating an effect of an intake air amount control in accordance with one embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, preferred embodiments of the invention will be described.  FIG. 1  is a block diagram showing an internal combustion engine (hereinafter referred to as an engine) and a control unit for controlling idle rotational speed of the engine in accordance with one embodiment of the invention. An engine  10  is, for example, a four-cylinder automobile engine. 
   A throttle valve  14  is disposed in an intake manifold  12 . The throttle valve  14  is driven by an actuator  18  in accordance with a control signal from an electronic control unit (ECU)  60 . Based on an output from an accelerator pedal opening sensor (not shown), the ECU  60  sends the control signal to the actuator  18  for controlling an opening degree of the throttle valve  14 . This scheme is called a drive-by-wire scheme. Another scheme may be used. For example, a wire  16  is connected to the accelerator pedal so that the accelerator pedal directly controls the throttle valve. The amount of air taken into the engine is adjusted by controlling an opening degree of the throttle valve. 
   A throttle valve opening sensor  20  is disposed near the throttle valve  14  to output a signal corresponding to an opening degree θTH of the throttle valve. 
   A fuel injection valve  24  is disposed, for each cylinder, between the throttle valve  14  and an intake valve of the engine  10 . The fuel injection valve  24  is connected to a fuel pump (not shown) to receive a fuel supply from a fuel tank (not shown) through the fuel pump. The fuel injection valve  24  is driven in accordance with a control signal from the ECU  60 . 
   A blow-by gas passage  25  is disposed between a crankcase (not shown) of the engine  10  and the intake manifold  12 . The blow-by gas passage  25  returns the blow-by gas back to the intake manifold  12 . The blow-by gas is a gas leakage into the crankcase of the engine  1 . A PCV (Positive Crankcase Ventilation) valve  26  is disposed at a portion where the blow-by gas passage  25  is connected to the crankcase. 
   An intake manifold pressure sensor  32  and an intake air temperature sensor  34  are disposed downstream of the throttle valve  14  in the intake manifold  12 . These sensors output electric signals representing the absolute pressure Pb and the temperature TA in the intake manifold  12 , respectively. 
   An engine water temperature (Tw) sensor  36  is attached to the cylinder peripheral wall, which is filled with cooling water, of the cylinder block of the engine  10 . A temperature of the engine cooling water detected by the Tw sensor  36  is sent to the ECU  60 . 
   A cylinder discrimination sensor (CYL)  40  is disposed around a camshaft or a crankshaft of the engine  10 , to output a cylinder discrimination signal CYL, for example, at a predetermined crank angle position of the first cylinder. A TDC sensor  42  and a crank angle sensor (CRK)  44  are disposed. The TDC sensor  42  outputs a TDC signal at a crank angle position that is associated with a top-dead-center (TDC) position of the piston for each cylinder. The CRK sensor  44  outputs a CRK signal at a predetermined crank angle position. The cycle length of the CRK signal (for example, 30 degrees) is shorter than the cycle length of the TDC signal. 
   An exhaust manifold  46  is connected to the engine  10 . Exhaust gas from the combustion is purified by a catalyst converter  50  and then emitted. A full range air/fuel ratio (LAF) sensor  52  is disposed upstream of the catalyst converter  50 . The LAF sensor  52  outputs a signal representing the oxygen concentration in the exhaust gas in a wide air-fuel ratio zone, from a rich zone where the air-fuel ratio is richer than the theoretical air-fuel ratio to an extremely lean zone. 
   A vehicle speed sensor  54  is disposed around a driving shaft that drives the wheels, to output a signal per predetermined number of rotations of the driving shaft. An atmospheric pressure sensor  56  is provided in the vehicle to output a signal corresponding to the atmospheric pressure. 
   The outputs of these sensors are sent to the ECU  60 . The ECU  60  is typically implemented by a microcomputer. The ECU  60  has a processor CPU  60   a  for performing calculations, a ROM  60   b  for storing control programs, various data and tables, and a RAM  60   c  for temporarily storing the calculation results by the CPU  60   a  and other data. The outputs of the various sensors are input to an input interface  60   d  of the ECU  60 . The input interface  60   d  includes a circuit for shaping input signals to modify their voltage levels and an A/D converter for converting the signals from analog to digital. 
   The CPU  60   a  counts CRK signals from the crank angle sensor  44  to detect an engine rotational speed NE and counts signals from the vehicle speed sensor  54  to detect a vehicle speed VP. CPU  60   a  performs operations in accordance with the programs stored in the ROM  60   b  to send driving signals to the fuel injection valve  24 , the throttle valve actuator  18  and other elements through an output interface  60   e.    
   Alternatively, a mechanical throttle valve may be used instead of the above-described throttle valve  14  that is electrically driven to open/close. In this case, an electromagnetic valve that is driven to open/close in accordance with a control signal from the ECU is provided in a passage that bypasses the throttle valve. The amount of air taken into the engine can be adjusted by controlling an opening degree of the electromagnetic valve. It should be noted that the term of “intake air passage” includes such a bypass passage. 
     FIG. 2  shows a block diagram of an intake air amount control apparatus in accordance with one embodiment of the present invention. Respective blocks are typically implemented by the ECU  60 . A feedback controller  71  performs a feedback control for controlling the opening degree of the throttle valve so that the engine rotational speed converges to a desired rotational speed when during engine idling. For example, a PID control is used as a feedback control. The feedback controller  71  calculates a controlled variable ICMDTH for controlling the opening degree of the throttle valve. The controlled variable is calculated, for example, according to the following equation (1):
   ICMDTH =( IFB+I LOAD)× KIPA+IPA   (1) 
   In the equation (1), IFB represents a feedback correction amount (or feedback gain). In the case of using the PID control, the feedback correction amount includes a proportional gain, an integral gain and a derivative gain. LOAD represents a load correction term that is set in accordance with an electric load imposed on the engine, a compressor load of an air conditioner, a power steering load, and whether or not an automatic transmission is in-gear. KIPA and IPA are a correction coefficient and a correction term, which are established in accordance with the atmospheric pressure. 
   A learning value calculator  73  calculates a first learning value IXREFN and a second learning value IXREFDBW based on the above integral gain. 
   An example of time-dependent changes of these learning values will be described referring to  FIG. 3 . The first learning value (IXREFN), which is shown by a dotted line, indicates a value obtained by smoothing the integral gain (IAIN). The second learning value (IXREFDBW), which is shown by a solid line, indicates a value obtained by smoothing the first learning value.  FIG. 3  shows a state where the first learning value and the second learning value are changing due to clogging of the intake manifold (including the throttle valve), which may be caused by years of use. The first and second learning values increase because the intake air amount into the engine decreases as the degree of clogging increases. 
   Thus, by calculating the second learning value through use of the integral gain IAIN that is used for feedback-controlling the engine rotational speed during idle operation, it can be determined how clogging of the intake manifold changes. 
   Referring back to  FIG. 2 , a clogging coefficient calculator  74  calculates a clogging coefficient KTHC based on the second learning value IXREFDBW. The clogging coefficient KTHC indicates to what degree the intake manifold is clogging. As the value of the clogging coefficient is greater, the degree of clogging increases. In one embodiment of the present invention, the clogging coefficient KTHC is calculated so that a difference between a current value of the clogging coefficient KTHC, which is calculated in the current operating cycle, and a previous value of the clogging coefficient KTHC, which is calculated in the previous operating cycle, is kept within a predetermined range. 
   A throttle opening degree calculator  72  calculates a desired opening degree THICMD of the throttle valve based on the controlled variable ICMDTH and the clogging coefficient KTHC. The opening degree of the throttle valve is controlled so that it converges to the desired throttle opening degree THICMD. Thus, the throttle valve is controlled to the opening degree set in accordance with the degree of clogging of the intake manifold. The opening degree of the throttle valve is set to be larger as the degree of clogging increases so that the desired air amount can be taken into the engine. 
   A leakage detector  75  detects a leakage (including a hole and a disconnection) of the blow-by gas passage  25 . The detection may be implemented using any appropriate method. If a leakage of the blow-by gas passage  25  is detected, the leakage detector  75  sets a flag F — PCV. If the flag F — PCV is set, the clogging coefficient calculator  74  prohibits the calculation of the clogging coefficient KTHC. 
   If a leakage in the blow-by gas passage  25  occurs, the intake air amount increases. If the calculation of the clogging coefficient is continued, such increase in the intake air amount causes an erroneous determination that the clogging has been eliminated. In order to avoid such erroneous determination, the calculation of the clogging coefficient KTHC is prohibited when a leakage is detected in the blow-by gas passage  25 . 
   Referring to  FIG. 4 , a specific method for calculating the clogging coefficient KTHC will be described. The figure shows a map indicating the opening degree THICMD of the throttle valve that is to be set in accordance with the amount of air taken into the engine. It should be noted the left and right vertical axes indicate the same scale for the purpose of illustration. 
   A reference number  81  indicates a throttle characteristic when there is no clogging in the intake manifold. The throttle characteristic shifts along a direction of an arrow  82  as clogging of the intake manifold increases. A reference number  83  indicates a throttle characteristic when it is determined that there is a maximum clogging in the intake manifold. The maximum clogging indicates a state beyond which the intake air amount control by the throttle valve may be impossible. 
   A reference value IXREFBASE is predetermined. The reference value IXREFBASE is typically determined based on an air amount beyond which clogging may occur in the intake manifold. In other words, if the air amount taken into the engine exceeds the reference value IXREFBASE, it indicates a possibility that clogging has occurred in the intake manifold. 
   A lower limit value of the throttle opening degree at the reference value IXREFBASE is referred to as a reference lower limit value THX. An upper limit value of the throttle opening degree at the reference value IXREFBASE is referred to as a reference upper limit value THMAX. The clogging coefficient KTHC takes a value within a range defined by the reference lower limit value THX and the reference upper limit value THMAX. In this embodiment, the clogging coefficient KTHC is defined so that a value of the clogging coefficient KTHC corresponding to the reference lower limit value THX is zero and a value of the coefficient KTCH corresponding to the reference upper limit value THMAX is 1. As the value of the KTHC is greater, it indicates that clogging in the intake manifold is greater. 
   The air amount taken into the engine is typically represented by the controlled variable ICMDTH. As described above, the controlled variable ICMDTH is calculated based on the feedback correction amount that includes the integral gain. However, the degree of clogging in the intake manifold is reflected in the second learning value that is calculated based on the integral gain. Therefore, in order to calculate the clogging coefficient, the clogging coefficient calculator  74  refers to the map based on the second learning value IXREFDBW. 
   An upper limit value thdbwmax and a lower limit value thdbwx that are corresponding to the second learning value IXREFDBW are calculated based on the throttle characteristics  81  and  83 . Using a clogging coefficient KTHCLAST calculated in the previous operating cycle, a point  85  at which the throttle opening degree corresponding to the second learning value IXREFDBW is determined between the upper limit value thdbwmax and the lower limit value thdbwx. A throttle opening degree thdbwcmd corresponding to the point  85  is output. 
   In order to calculate the clogging coefficient KTHC for the current operating cycle, it is determined where the throttle opening degree thdbwcmd is located between the reference lower limit value THX and the reference upper limit value THMAX. As described above, the clogging coefficient KTHC is defined so that its value on the throttle characteristic  81  at the reference value IXREFBASE is zero and its value on the throttle characteristic  83  at the reference value IXREFBASE is 1.0. Therefore, the clogging coefficient KTHC corresponding to the throttle opening degree thdbwcmd can be calculated by a simple proportional calculation based on the reference lower limit value THX and the reference upper limit value THMAX. Specific calculation equations will be described later. Thus, KTHC having a magnitude shown by a reference number  86  is determined. 
   Referring to  FIG. 5 , a method for calculating the desired throttle opening degree THICMD will be described. A map shown in  FIG. 5  is the same as in  FIG. 4 . The throttle opening calculator  72  refers to the map based on the controlled variable ICMDTH calculated by the feedback controller  71 . 
   The desired throttle opening degree that is to be used for actually controlling the opening degree of the throttle valve may need to be calculated considering not only clogging but also other factors. Therefore, the map is referred to based on the controlled variable ICMDTH that is calculated considering the engine load and the other factors as described above referring to the equation (1). 
   An upper limit value THICMDC and a lower limit value THICMDX corresponding to the controlled variable ICMDTH are calculated based on the throttle characteristics  81  and  83 . By using the clogging coefficient KTHC calculated by the clogging coefficient calculator  74 , the desired throttle opening THICMD corresponding to the clogging coefficient KTHC can be determined by a simple proportional calculation based on the upper limit value THICMDC and the lower limit value THICMDX. Specific calculation equations will be described later. 
   Referring to  FIGS. 6 through 8 , a process for calculating the second learning value will be described. This process is performed at a predetermined time interval. 
   In step S 101 , a subroutine is performed for determining whether the operating condition of the engine is within a learning permission range, that is, whether the operating condition of the engine is suitable for calculating the learning values. This subroutine will be described referring to  FIG. 7 . 
   In step S 103 , it is determined whether a flag indicating a failure in any device on the vehicle has been set to one. If the flag has not been set to one, the process proceeds to step S 105 . If the flag has been set to one, a default value is set in the first learning value IXREFN (S 117 ). An initial value is set in a counter that defines an interval at which the learning values are calculated (S 119 ), and then the process exits the routine. 
   In step S 105 , it is determined whether a learning permission flag has been set to one. The learning permission flag is a flag that is to be set in the subroutine performed in step S 101 . If the learning permission flag has been set to one, the process proceeds to step S 107 . If the learning permission flag has not been set to one, the initial value is set in the counter (S 119 ), and the process exits the routine. 
   In step S 107 , the counter value is decremented by one. In step S 109 , it is determined whether the counter value has reached zero. If the counter value has not reached zero, the process exits the routine. 
   If the counter value has reached zero in step S 109  when this routine is re-entered, the initial value is set in the counter (S 111 ). The process proceeds to step S 113 , in which the first learning value is calculated. The first learning value IXREFN is calculated in accordance with the following equation (2): 
                   IXREFN   =       ⁢       IAIN   ×     “smoothing  coefficient”       +                     ⁢       IXREFN   ⁡     (     n   -   1     )       ×     (     1   -     “smoothing  coefficient”       )                     (   2   )             
 
   IAIN represents the integral gain of the PID feedback control as described above. IXREFN(n−1) represents the first learning value calculated in the previous cycle. The smoothing coefficient is, for example, 0.7. In this embodiment, the learning value is obtained by using the smoothing coefficient. Alternatively, a moving average of the integral gain IAIN may be used as a learning value. Thus, the calculated learning value is stored in the RAM  60   c  ( FIG. 1 ). 
   In step S 115 , a subroutine ( FIG. 8 ) for calculating the second learning value is performed. 
   A process for determining the learning permission range, which is performed in step S 101  of  FIG. 6 , will be described referring to  FIG. 7 . In step S 121 , based on a status code that indicates an operating mode of the engine, it is determined whether the engine is in a mode for performing a feedback control for the idle rotational speed. If the answer of step S 121  is No, that is, if the current mode is a mode where an open-loop control is to be performed, the learning permission flag is set to zero (rejection) in step S 137  and then the process exits the routine. If the answer of step S 121  is Yes, the process proceeds to step S 123 , in which it is determined whether a flag indicating that a predetermined time has elapsed after the engine start has been set to one. If the flag has not been set to one, the learning permission flag is set to zero (S 137 ) and then the process exits the routine. Thus, the learning operation is prohibited because the engine condition is not stable immediately after the engine start. 
   If it is determined that the predetermined time has elapsed after the engine start, the process proceeds to step S 125 , in which it is determined whether the intake manifold pressure PB is greater than a predetermined value. The intake manifold pressure PB indicates engine load. If the intake manifold pressure PB is larger than the predetermined value, it indicates that the engine load is high. Since the engine condition is not suitable for calculating the learning values, the process proceeds to step S 137  and then exits this routine. If the intake manifold pressure PB is equal to or less than the predetermined value, the process proceeds to step S 127 , in which it is determined whether the gauge pressure PBGA, which is a difference between the atmospheric pressure PA and the intake manifold pressure PB, exceeds a predetermined value. If the gauge pressure PBGA is larger than the predetermined value, it indicates that engine load is high. Since the engine condition is not suitable for calculating the learning values, the learning permission flag is set to zero (S 137 ) and then the process exits the routine. 
   If the gauge pressure PBGA is equal to or less than the predetermined value, the process proceeds to step S 129 , in which it is determined whether a variation in the engine rotational speed NE exceeds a predetermined value. If the variation of the rotational speed NE is larger than the predetermined value, it indicates that the engine condition is not suitable for calculating the learning values. The learning permission flag is set to zero (S 137 ) and then the process exits the routine. If the variation in the rotational speed is equal to or less than the predetermined value, the process proceeds to step S 131 , in which it is determined whether a difference between a desired rotational speed NOBJ calculated in the current cycle and a desired rotational speed NOBJ calculated in the previous cycle exceeds a predetermined value. If the difference is larger than the predetermined value, it indicates that the engine rotation is not stable. Since the engine condition is not suitable for calculating the learning values, the learning permission flag is set to zero (S 137 ) and then the process exits the routine. 
   If the difference between the current value and the previous value for the desired rotational speed NOBJ is equal to or less than the predetermined value, the process proceeds to step S 133 , in which it is determined whether the engine water temperature TW is lower than a predetermined value. If the engine water temperature TW is lower than the predetermined value, it indicates that the engine is not stable. Since the engine condition is not suitable for calculating the learning values, the learning permission flag is set to zero (S 137 ) and the process exits the routine. If the engine water temperature TW is equal to or higher than the predetermined value, the learning permission flag is set to one (S 135 ) and the process exits the routine. 
   Referring to  FIG. 8 , a process for calculating the second learning value, which is performed in step S 115  of  FIG. 6 , will be described. In step S 141 , it is determined whether the intake manifold pressure PB is equal to or less than a predetermined value. Since the intake manifold pressure PB represents engine load as described above, a small intake manifold pressure PB indicates that the engine load is low. If the intake manifold pressure PB is equal to or less than the predetermined value, the process proceeds to step S 143 , in which it is determined whether a difference between a maximum value and a minimum value in the first learning value calculated in step S 113  is equal to or less than a predetermined value. This determination is performed so as to calculate the second learning value under a condition where a difference between the maximum value and the minimum value in the first learning value IXREFN calculated over a predetermined time period, which is established by a timer in step S 159 , is equal to or less than the predetermined value. Thus, the learning value can be determined in a range in which the operating condition of the engine is stable. 
   If the answer of step  143  is No, the process proceeds to step S 157 , in which the first learning value is set in both of the maximum value and the minimum value of IXREFN. In step S 159 , a predetermined initial value is set in the timer, and then the process exits the routine. A function of the timer of step S 159  will be described later. 
   When the routine is re-entered, the answer of step S 143  is Yes because the maximum value and the minimum value has been set to the same value in step S 157 . The process proceeds to step S 145 , in which it is determined whether the first learning value IXREFN calculated in step S 113  of  FIG. 6  exceeds the maximum value established in step S 157 . If the answer of the step is Yes, the maximum value is replaced with the current value of the first learning value IXREFN (S 149 ). If the answer of step S 113  is No, it is determined in step S 147  whether the current value of the first learning value IXREFN is less than the minimum value. If the answer of step S 147  is Yes, the minimum value is replaced with the current value IXREFN (S 151 ). When such updating process for the maximum and minimum values are completed, it is determined in step S 153  whether the timer that has been set to the initial value in step S 159  is zero. That is, it is determined whether a condition where a difference between the maximum value and the minimum value is equal to or less than the predetermined value has continued over a time period established by the timer. If the timer is zero, the process proceeds to step S 155 , in which the second learning value is calculated. If the timer has not reached zero, the process exits the routine. 
   In step S 155 , the second learning value IXREFDBW is calculated in accordance with the following equation (3): 
                   IXREFDBW   =       ⁢       IXREFN   ×     smoothing  coefficient       +                     ⁢       IXREFDBW   ⁡     (     n   -   1     )       ×                     ⁢     (     1   -     smoothing  coefficient       )                   (   3   )             
 
   The smoothing coefficient is, for example, 0.7. Alternatively, it may be different from the smoothing coefficient for the first learning value. 
   A process for calculating the clogging coefficient KTHC will be described referring to  FIG. 9 . This routine is performed at a predetermined time interval. 
   In step S 201 , the value of a flag F — KTHCINI is examined. The flag F — KTHCINI has been initialized to zero when an operating cycle, which is a cycle from engine start to engine stop, is started. Therefore, when this routine is first performed, the process proceeds to step S 203 , in which the current value of the clogging coefficient KTHC is stored as KTHCLAST. That is, the last calculated clogging coefficient in the previous operating cycle is stored as KTHCLAST. 
   In steps S 205  and S 207 , throttle characteristics  83  and  81  shown in  FIG. 4  are referred to based on the reference value IXREFBASE to determine a reference upper limit value THMAX and a reference lower limit value THX of the throttle opening degree. As described above, at the reference value IXREFBASE, the value of the clogging coefficient KTHC is zero when the throttle opening degree is equal to THX and one when the throttle opening degree is equal to THMAX. In step S 209 , the flag F — KTHCINI is set to one, indicating that the initial process for the clogging coefficient is completed. 
   When this routine is re-entered, the value of the flag F — KTHCINI is one. The process proceeds to step S 211 , in which the value of the flag F — PCV is examined. The flag F — PCV is a flag that is to be set to one when a leakage of the blow-by gas passage  25  ( FIG. 1 ) is detected. If the value of the flag F — PCV is one, the process proceeds to step S 213 , in which the clogging coefficient KTHCLAST calculated in the previous operating cycle is set in the clogging coefficient KTHC for the current operating cycle. Thus, when a leakage of the blowby gas passage is detected, updating of the clogging coefficient KTHC is prohibited. 
   If the answer of step S 211  is No, a process for updating the clogging coefficient KTHC shown in steps S 215  through step S 224  is performed. In step S 215 , the throttle characteristic  83  of the map as shown in  FIG. 4  is referred to based on the second learning value IXREFDBW calculated in step S 155  of  FIG. 8  to determine an upper limit value thdbwmax. In step S 217 , the throttle characteristic  81  of the map as shown in  FIG. 4  is referred to based on the second learning value IXREFDBW to determine a lower limit value thdbwx. 
   In step S 219 , using the clogging coefficient KTHCLAST calculated in the previous operating cycle, a throttle opening degree thdbwcmd corresponding to the second learning value IXREFDBW is calculated in accordance with the equation (4). The throttle opening degree thdbwcmd corresponding to the point  85  ( FIG. 4 ) is calculated in accordance with the equation (4). 
                     throttle   ⁢           ⁢   opening   ⁢           ⁢   thdbwcmd     =       ⁢     KTHCLAST   ×                     ⁢     thdbwmax   +       (     1   -   KTHCLAST     )     ×                       ⁢   thdbwx                 (   4   )             
 
   In step S 221 , a temporary clogging coefficient kthctmp is calculated by determining where the throttle opening thdbwcmd is located between the reference upper limit value THMAX and the reference lower limit value THX as shown in the equation (5). 
                     temporary   ⁢           ⁢   clogging               coefficient   ⁢           ⁢   kthctmp           =       ⁢       (     thdbwcmd   -   THX     )       (     THMAX   -   THX     )               (   5   )             
 
   As the second learning value IXREFDBW increases, the temporary throttle opening degree thdbwcmd increases and hence the temporary clogging coefficient kthctmp increases. 
   In step S 223 , an updating allowance range is set for the clogging coefficient KTHCLAST calculated in the previous operating cycle. Specifically, an upper limit value kthcmax of the updating allowance range is calculated by adding a predetermined value to the clogging coefficient KTHCLAST and a lower limit value kthcmin is calculated by subtracting the predetermined value from the clogging coefficient KTHCLAST. 
   In step S 224 , the temporary clogging coefficient kthctmp is limited by the updating allowance range. Specifically, when the temporary clogging coefficient kthctmp exceeds the upper limit value kthcmax, the clogging coefficient KTHC is set to the upper limit value kthcmax. On the other hand, when the temporary clogging coefficient kthctmp is below the lower limit value kthcmin, the clogging coefficient KTHC is set to the lower limit value kthcmin. Thus, a range within the clogging coefficient KTCH is updated is limited. 
   Referring to  FIG. 10 , a process for calculating a desired throttle opening THICMD will be described. This routine is performed at a predetermined time interval. 
   In step S 231 , the controlled variable ICMDTH is calculated in accordance with the above-described equation (1). In steps S 233  and step S 235 , throttle characteristics  83  and  81  of the map as shown in  FIG. 5  are referred to based on the controlled variable ICMDTH to determine an upper limit value THICMDC and a lower limit value THICMDX corresponding to the controlled variable ICMDTH. 
   In step S 237 , as shown in the equation (6), the clogging coefficient KTHCLAST calculated in the previous operating cycle is used to perform a proportional calculation upon the upper limit value THICMDC and the lower limit value THICMDX. Thus, the desired throttle opening THICMD is calculated. 
                           desired   ⁢           ⁢   throttle               opening   ⁢           ⁢   THICMD           =       ⁢       KTHCLAST   ×   THICMDC     +                     ⁢       (     1   -   KTHCLAST     )     ×   THICMDX                   (   6   )             
 
   The reason why the clogging coefficient KTHCLAST calculated in the previous operating cycle is used because the updating process for the clogging coefficient KTCH in the current operating cycle is underway at a predetermined time interval and hence the value of the clogging coefficient for the current operating cycle has not been established yet. Since the clogging of the intake manifold changes little in a short time period such as one operating cycle, an appropriate desired throttle opening degree can be determined even by using the clogging coefficient KTHCLAST calculated in the previous operating cycle. 
   Referring to  FIG. 11 , a process for detecting a leakage of the blow-by gas passage will be described. This routine is performed at a predetermined time interval. 
   In step S 301 , it is determined whether a condition for detecting an abnormality of the blow-by gas passage is met. This condition may include, for example, a stable operating condition of the engine. The operating condition of the engine can be determined based on parameters such as engine water temperature, vehicle speed, air/fuel ratio and so on. 
   In step S 303 , a total intake air amount QTOTAL of the engine  1  is calculated in accordance with the following equation (7):
 
 Q TOTAL= TIM× 2 NE×KC/σA   (7)
 
where
 
 KC=KTQ×σG× 14.7
 
σ A=[ 1.293/(1+0.00367 TA )]×( PA/PA   0 )
 
   In the equation (7), TIM represents a basic fuel injection time, KC represents a coefficient for converting the fuel injection time TIM to an intake air amount, and σA represents the density of the atmosphere. KTQ represents a coefficient for converting the fuel injection time to the amount (volume) of fuel, σG represents the density of fuel, and 14.7 indicates the stoichiometric air/fuel ratio. TA represents an intake air temperature detected by the intake air temperature sensor  34  ( FIG. 1 ), PA represents an atmospheric pressure detected by the atmospheric pressure sensor  56  ( FIG. 1 ), and PA 0  represents the reference atmospheric pressure (=101.3 kPa). 
   In step S 305 , an intake air amount QBP taken into the engine  10  through the throttle valve  14  is calculated in accordance with the following equation (8):
 
 QBP=ICMDTH×KIQ   (8)
 
   KIQ is a coefficient for converting the controlled variable ICMDTH to the amount of air. 
   In step S 307 , the throttle intake air amount QBP is subtracted from the total intake air amount QTOTAL to calculate a leakage air amount QL that is introduced into the engine due to a leakage such as disconnection of the blow-by gas passage  25 . 
   In step S 309 , a predetermined map is referred to based on the gauge pressure PBG to calculate a leakage determination threshold value QTH. The map is established so that the threshold value QTH decreases as the gauge pressure PBG increases (that is engine load increases). 
   In step S 311 , if QL&gt;QTH, it is determined that there is a leakage, and then the value of one is set in the flag F — PCV (S 315 ). If QL≦QTH, it is determined that there is no leakage, and then zero is set in the flag F — PCV (S 313 ). 
   The process for detecting a leakage of the blow-by gas passage shown in  FIG. 11  is an exemplary embodiment. As described above, any other appropriate method may be used for detecting a leakage of the blow-by gas passage. 
   Referring to  FIG. 12 , an effect of the intake air amount control in accordance with one embodiment of the present invention when a leakage occurs in the blow-by gas passage will be described. 
   A reference number  91  shows a change in the throttle opening degree in the case where there is clogging in the intake manifold. A reference number  92  shows a change in the throttle opening degree in the case where there is no clogging in the intake manifold. When there is clogging in the intake manifold, the intake air amount decreases. Therefore, the throttle opening degree is controlled to increase so as to compensate such decrease of the intake air amount caused by the clogging. 
   Over the time period from t 1  to t 2 , the throttle opening degree is controlled as shown by the reference number  91 . A disconnection occurs in the blow-by gas passage  25  at t 2 . 
   A reference number  93  shows a change of the throttle opening degree according to the conventional schemes. The intake air amount abruptly increases because the disconnection occurs in the blow-by gas passage  25 . This abrupt increase of the intake air amount leads to an erroneous determination that the clogging has been eliminated. As a result, the value of the clogging coefficient KTHC is made small, and the throttle opening degree is also made small. 
   After the disconnection of the blow-by gas passage  25  is repaired under such a condition, the throttle opening degree is small as shown by the reference number  93  despite the fact that the clogging has not been yet eliminated. This causes a shortage of the intake air amount and hence makes the operating condition of the engine unstable. 
   According to the present invention, updating the clogging coefficient is prohibited when a disconnection of the blowby gas passage  25  is detected. The throttle opening changes as shown in a reference number  94  because the clogging coefficient is not updated. Therefore, after the disconnection of the blowby gas passage  25  is repaired, the intake air amount control can be performed based on the appropriate throttle opening. 
   As described above referring to one example shown in  FIG. 11 , it is typically determined that a leakage occurs in the blow-by gas passage if there is a difference between an air amount that is actually taken into the engine (QTOTAL in the example of  FIG. 11 ) and a desired intake air amount (QBP in the example of  FIG. 11 ). On the other hand, the control unit uses the clogging coefficient to control the throttle valve to cause the actual intake air amount to converge to the desired intake air amount. 
   If the clogging coefficient is updated immediately after a leakage is detected in the blow-by gas passage, the control unit may make an erroneous determination that the clogging has been eliminated. As a result, the control unit may instantly change the throttle opening degree as shown by the reference number  93  in order to cope with the actually increased intake air amount. Since the control unit determines that the actual intake air amount has been adapted to the desired intake air amount by virtue of the change of the throttle opening degree, the control unit cannot identify that such increase in the intake air amount has been caused by the leakage. Therefore, it may be determined that there is no leakage despite the fact that a disconnection has actually occurred in the blow-by gas passage. 
   According to the present invention, a range within which the clogging coefficient KTCH is updated is limited as shown in step S 224  of  FIG. 9 . By limiting such a range within which the clogging coefficient KTCH is updated, the throttle opening degree can be controlled to change as shown by the reference number  95  even if the clogging coefficient is updated. In other words, although the clogging coefficient is updated to decrease due to the increase of the intake air amount, the amount the throttle opening degree decreases can be limited because the amount of update for the clogging coefficient is limited. Therefore, although the actual intake air amount into the engine increases when a disconnection occurs, the throttle opening degree is not necessarily changed to adapt to the increased amount of the intake air. As a result, since a difference is formed between the actual intake air amount into the engine and the desired intake air amount, it is ensured that occurrence of the leakage is detected. 
   The present invention can be applied to a general-purpose engine (for example, an outboard motor).