Patent Publication Number: US-2011077840-A1

Title: Internal combustion engine system, fuel injection control method of internal combustion engine, and vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Japanese Patent Application No. 2009-220381 filed on Sep. 25, 2009, which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an internal combustion engine system, a fuel injection control method of the internal combustion engine system, and a vehicle. 
     2. Description of the Related Art 
     In one proposed internal combustion engine system, feedback control of a fuel supply amount is performed to obtain a target air-fuel ratio according to an output from an air-fuel ratio sensor at a restart timing of an internal combustion engine (see, for example, Patent Document 1). In this internal combustion engine system, air-fuel ratio feedback control is started at the restart timing of the internal combustion engine on condition that the output from the air-fuel ratio sensor is within a predetermined air-fuel ratio range, and starting performance of the internal combustion engine is improved in a vehicle, for example, a hybrid vehicle, that has an operation mode to intermit operation of the internal combustion engine. Patent Document 1: Japanese Patent Laid-Open No. 2007-239482 
     SUMMARY OF THE INVENTION 
     In internal combustion engine systems, it is generally required to prevent exhaust emission of an internal combustion engine from becoming worse, for example, by reducing nitrogen oxides (NOx) at a start timing of the internal combustion engine. In such a sensor as an air-fuel ratio sensor, there is a case where responsiveness of the sensor is reduced or an output of the sensor indicates an abnormal value. It is so required to make determination of a sensor function to reflect the result of the function determination on control. 
     In the internal combustion engine system, a fuel injection control method of the internal combustion engine system, and a vehicle of the invention, the main object of the invention is to prevent exhaust emission from becoming worse using the result of function determination of an air-fuel ratio detector unit when an internal combustion engine is started up. 
     In order to attain the main object, the internal combustion engine system, the fuel injection control method of the internal combustion engine system, and the vehicle of the invention have the configurations discussed below. 
     According to one aspect, the present invention is directed to an internal combustion engine system. The internal combustion engine system, having an internal combustion engine and a motor capable of cranking the internal combustion engine, the internal combustion engine system has: a fuel injector that performs fuel injection into the internal combustion engine; an air-fuel ratio detector that detects an air-fuel ratio of the internal combustion engine; an air-fuel ratio detecting function determination module that performs function determination of the air-fuel ratio detector, the function determination including detection of a responsiveness reduction abnormality that is an abnormality where the air-fuel ratio detector becomes less responsive to a change in the air-fuel ratio of the internal combustion engine from a rich air-fuel ratio to a lean air-fuel ratio, the rich air-fuel ratio being fuel-richer and the lean air-fuel ratio being fuel-leaner both in comparison with a stoichiometric air-fuel ratio; a target fuel injection amount setting module that, when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is not detected, sets a target fuel injection amount to be injected into the internal combustion engine by applying an increase correction to a basic fuel injection amount until a preset timing that is predetermined so that the internal combustion engine is favorably combusted, the basic fuel injection amount being a fuel injection amount based on an intake air amount of the internal combustion engine for bringing the air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio, and then sets the target fuel injection amount by performing an air-fuel ratio feedback correction from a first start timing that is predetermined as a timing when the detected air-fuel ratio by the air-fuel ratio detector reaches a target air-fuel ratio range including the stoichiometric air-fuel ratio without occurrence of the responsiveness reduction abnormality after finishing the increase correction, the air-fuel ratio feedback correction being a correction of the basic fuel injection amount using feedback control for bringing the detected air-fuel ratio by the air-fuel ratio detector to the stoichiometric air-fuel ratio, and when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is detected, the target fuel injection amount setting module setting the target fuel injection amount by applying the increase correction to the basic fuel injection amount until the preset timing, and then setting the target fuel injection amount by performing the air-fuel ratio feedback correction from a second start timing that is later than the first start timing; and a fuel injection control module that controls the fuel injector so that the fuel injection into the internal combustion engine is performed according the set target fuel injection amount. 
     The internal combustion engine system according to this aspect of the invention, performs function determination of the air-fuel ratio detector, the function determination including detection of a responsiveness reduction abnormality that is an abnormality where the air-fuel ratio detector becomes less responsive to a change in the air-fuel ratio of the internal combustion engine from a rich air-fuel ratio to a lean air-fuel ratio, the rich air-fuel ratio being fuel-richer and the lean air-fuel ratio being fuel-leaner both in comparison with a stoichiometric air-fuel ratio. When the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is not detected, the system sets a target fuel injection amount to be injected into the internal combustion engine by applying an increase correction to a basic fuel injection amount until a preset timing that is predetermined so that the internal combustion engine is favorably combusted, the basic fuel injection amount being a fuel injection amount based on an intake air amount of the internal combustion engine for bringing the air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio, and then sets the target fuel injection amount by performing an air-fuel ratio feedback correction from a first start timing that is predetermined as a timing when the detected air-fuel ratio by the air-fuel ratio detector reaches a target air-fuel ratio range including the stoichiometric air-fuel ratio without occurrence of the responsiveness reduction abnormality after finishing the increase correction, the air-fuel ratio feedback correction being a correction of the basic fuel injection amount using feedback control for bringing the detected air-fuel ratio by the air-fuel ratio detector to the stoichiometric air-fuel ratio. When the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is detected, the system sets the target fuel injection amount by applying the increase correction to the basic fuel injection amount until the preset timing, and then sets the target fuel injection amount by performing the air-fuel ratio feedback correction from a second start timing that is later than the first start timing. And the system controls the fuel injector so that the fuel injection into the internal combustion engine is performed according the set target fuel injection amount. When the internal combustion engine is started up while the responsiveness reduction abnormality is detected, the detected air-fuel ratio by the air-fuel ratio detector reaches the target air-fuel ratio range which includes the stoichiometric air-fuel ratio from the rich air-fuel ratio at a later timing than the first start timing. When the internal combustion engine is started up while the responsiveness reduction abnormality is detected, the air-fuel ratio feedback control is performed from the second start timing later than the first start timing. Accordingly, it is prevented to decrease the fuel injection amount into the internal combustion engine from a fuel injection amount corresponding to the stoichiometric air-fuel ratio at the start timing of the air-fuel ratio feedback correction. As a result, this arrangement effectively prevents exhaust emission from becoming worse using the result of function determination of the air-fuel ratio detector when the internal combustion engine is started up. 
     In one preferable application of the internal combustion engine system of the invention, the air-fuel ratio detecting function determination module may detect a reduced degree of responsiveness of the air-fuel ratio detector as a delay time upon the detection of the responsiveness reduction abnormality, and the target fuel injection amount setting module may set, when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is detected, the target fuel injection amount using a later timing by a corresponding time to the detected delay time than the first start timing as the second start timing. This arrangement more appropriately prevents exhaust emission from becoming worse using the result of function determination of the air-fuel ratio detector when the internal combustion engine is started up. 
     According to another aspect, the present invention is directed to a vehicle having any of the above arrangements of the internal combustion engine system and a second motor capable of outputting power for driving the vehicle, the vehicle being driven with an intermittent operation of the internal combustion engine. Here the internal combustion engine system having an internal combustion engine and a motor capable of cranking the internal combustion engine, fundamentally has: a fuel injector that performs fuel injection into the internal combustion engine; an air-fuel ratio detector that detects an air-fuel ratio of the internal combustion engine; an air-fuel ratio detecting function determination module that performs function determination of the air-fuel detector, the function determination including detection of a responsiveness reduction abnormality that is an abnormality where the air-fuel ratio detector becomes less responsive to a change in the air-fuel ratio of the internal combustion engine from a rich air-fuel ratio to a lean air-fuel ratio, the rich air-fuel ratio being fuel-richer and the lean air-fuel ratio being fuel-leaner both in comparison with a stoichiometric air-fuel ratio; a target fuel injection amount setting module that, when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is not detected, sets a target fuel injection amount to be injected into the internal combustion engine by applying an increase correction to a basic fuel injection amount until a preset timing that is predetermined so that the internal combustion engine is favorably combusted, the basic fuel injection amount being a fuel injection amount based on an intake air amount of the internal combustion engine for bringing the air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio, and then sets the target fuel injection amount by performing an air-fuel ratio feedback correction from a first start timing that is predetermined as a timing when the detected air-fuel ratio by the air-fuel ratio detector reaches a target air-fuel ratio range including the stoichiometric air-fuel ratio without occurrence of the responsiveness reduction abnormality after finishing the increase correction, the air-fuel ratio feedback correction being a correction of the basic fuel injection amount using feedback control for bringing the detected air-fuel ratio by the air-fuel ratio detector to the stoichiometric air-fuel ratio, and when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is detected, the target fuel injection amount setting module setting the target fuel injection amount by applying the increase correction to the basic fuel injection amount until the preset timing, and then setting the target fuel injection amount by performing the air-fuel ratio feedback correction from a second start timing that is later than the first start timing; and a fuel injection control module that controls the fuel injector so that the fuel injection into the internal combustion engine is performed according the set target fuel injection amount. 
     The vehicle according to this aspect of the invention has any of the above arrangements of the internal combustion engine system. The vehicle thus has at least part of effects that the internal combustion engine system of the invention has such as an effect of preventing exhaust emission from becoming worse using the result of function determination of the air-fuel ratio detector when the internal combustion engine is started up. 
     According to still another aspect, the present invention is directed to a fuel injection control method of an internal combustion engine in an internal combustion engine system having the internal combustion engine, a fuel injector that performs fuel injection into the internal combustion engine, an air-fuel ratio detector that detects an air-fuel ratio of the internal combustion engine, and a motor capable of cranking the internal combustion engine. The fuel injection control method includes: performing function determination of the air-fuel ratio detector, the function determination including detection of a responsiveness reduction abnormality that is an abnormality where the air-fuel ratio detector becomes less responsive to a change in the air-fuel ratio of the internal combustion engine from a rich air-fuel ratio to a lean air-fuel ratio, the rich air-fuel ratio being fuel-richer and the lean air-fuel ratio being fuel-leaner both in comparison with a stoichiometric air-fuel ratio; when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is not detected, setting a target fuel injection amount to be injected into the internal combustion engine by applying an increase correction to a basic fuel injection amount until a preset timing that is predetermined so that the internal combustion engine is favorably combusted, the basic fuel injection amount being a fuel injection amount based on an intake air amount of the internal combustion engine for bringing the air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio, and then setting the target fuel injection amount by performing an air-fuel ratio feedback correction from a first start timing that is predetermined as a timing when the detected air-fuel ratio by the air-fuel ratio detector reaches a target air-fuel ratio range including the stoichiometric air-fuel ratio without occurrence of the responsiveness reduction abnormality after finishing the increase correction, the air-fuel ratio feedback correction being a correction of the basic fuel injection amount using feedback control for bringing the detected air-fuel ratio by the air-fuel ratio detector to the stoichiometric air-fuel ratio, and when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is detected, setting the target fuel injection amount by applying the increase correction to the basic fuel injection amount until the preset timing, and then setting the target fuel injection amount by performing the air-fuel ratio feedback correction from a second start timing that is later than the first start timing; and controlling the fuel injector so that the fuel injection into the internal combustion engine is performed according the set target fuel injection amount. 
     The fuel injection control method of the internal combustion engine according to this aspect of the invention, performs function determination of the air-fuel ratio detector, the function determination including detection of a responsiveness reduction abnormality that is an abnormality where the air-fuel ratio detector becomes less responsive to a change in the air-fuel ratio of the internal combustion engine from a rich air-fuel ratio to a lean air-fuel ratio, the rich air-fuel ratio being fuel-richer and the lean air-fuel ratio being fuel-leaner both in comparison with a stoichiometric air-fuel ratio. When the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is not detected, the method sets a target fuel injection amount to be injected into the internal combustion engine by applying an increase correction to a basic fuel injection amount until a preset timing that is predetermined so that the internal combustion engine is favorably combusted, the basic fuel injection amount being a fuel injection amount based on an intake air amount of the internal combustion engine for bringing the air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio, and then sets the target fuel injection amount by performing an air-fuel ratio feedback correction from a first start timing that is predetermined as a timing when the detected air-fuel ratio by the air-fuel ratio detector reaches a target air-fuel ratio range including the stoichiometric air-fuel ratio without occurrence of the responsiveness reduction abnormality after finishing the increase correction, the air-fuel ratio feedback correction being a correction of the basic fuel injection amount using feedback control for bringing the detected air-fuel ratio by the air-fuel ratio detector to the stoichiometric air-fuel ratio. When the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is detected, the method sets the target fuel injection amount by applying the increase correction to the basic fuel injection amount until the preset timing, and then sets the target fuel injection amount by performing the air-fuel ratio feedback correction from a second start timing that is later than the first start timing. And the method controls the fuel injector so that the fuel injection into the internal combustion engine is performed according the set target fuel injection amount. When the internal combustion engine is started up while the responsiveness reduction abnormality is detected, the air-fuel ratio feedback control is performed from the second start timing later than the first start timing. Accordingly, it is prevented to decrease the fuel injection amount into the internal combustion engine from a fuel injection amount corresponding to the stoichiometric air-fuel ratio at the start timing of the air-fuel ratio feedback correction. As a result, this arrangement effectively prevents exhaust emission from becoming worse using the result of function determination of the air-fuel ratio detector when the internal combustion engine is started up. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates the configuration of a hybrid vehicle  20  in one embodiment of the invention; 
         FIG. 2  is a schematic view showing the structure of an engine  22 ; 
         FIG. 3  shows one set of examples of output characteristics of an air-fuel ratio sensor  135   a  and an oxygen sensor  135   b;    
         FIG. 4  is a flowchart showing a startup time fuel injection control routine executed by an engine ECU  24  in the embodiment; 
         FIG. 5  is a flowchart showing a function determination routine executed by the engine ECU  24  in the embodiment; 
         FIG. 6  shows one set of examples of time charts of an oxygen signal Vo and an air-fuel ratio Vaf during execution of function determination of the air-fuel ratio sensor  135   a;    
         FIG. 7  shows one set of examples of time charts of an air-fuel ratio of an engine  22 ; 
         FIG. 8  schematically illustrates the configuration of another hybrid vehicle  120  in one modified example; and 
         FIG. 9  schematically illustrates the configuration of still another hybrid vehicle  220  in another modified example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One mode of carrying out the invention is discussed below as a preferred embodiment.  FIG. 1  schematically illustrates the configuration of a hybrid vehicle  20  in one embodiment according to the invention. As illustrated, the hybrid vehicle  20  of the embodiment includes the engine  22 , a three shaft-type power distribution integration mechanism  30  connected via a damper  28  to a crankshaft  26  or an output shaft of the engine  22 , a motor MG 1  connected to the power distribution integration mechanism  30  and designed to have power generation capability, a reduction gear  35  attached to a ring gear shaft  32   a  or a driveshaft linked with the power distribution integration mechanism  30 , a motor MG 2  connected to the reduction gear  35 , and a hybrid electronic control unit  70  configured to control the operations of the whole hybrid vehicle  20 . 
     The engine  22  is an internal combustion engine that consumes a hydrocarbon fuel, such as gasoline or light oil, to output power. As shown in  FIG. 2 , the air cleaned by an air cleaner  122  and taken into an air intake conduit via a throttle valve  124  is mixed with the atomized fuel injected from a fuel injection valve  126  to the air-fuel mixture. The air-fuel mixture is introduced into a combustion chamber by means of an intake valve  128 . The introduced air-fuel mixture is ignited with spark made by a spark plug  130  to be explosively combusted. The reciprocating motions of a piston  132  pressed down by the combustion energy are converted into rotational motions of the crankshaft  26 . The exhaust from the engine  22  goes through a catalytic converter  134  having a three-way catalyst  134   a  to convert toxic components included in the exhaust, that is, carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx), into harmless components, and is discharged to the outside air. At the three-way catalyst  134   a , oxygen is occluded from the exhaust of the engine  22  when the exhaust makes a fuel-leaner atmosphere than a stoichiometric atmosphere, and the occluded oxygen is released to the exhaust of the engine  22  when the exhaust makes a fuel-richer atmosphere than the stoichiometric atmosphere. An air-fuel ratio sensor  135   a  that output value varies linearly according to the air-fuel ratio is mounted at an upstream side of the catalytic converter  134 , and an oxygen sensor  135   b  that output value abruptly varies according to whether the air-fuel ratio is at a rich or lean side of the stoichiometric atmosphere is mounted at an downstream side of the catalytic converter  134 .  FIG. 3  shows one set of examples of output characteristics of the air-fuel ratio sensor  135   a  and the oxygen sensor  135   b.    
     The engine  22  is under control of an engine electronic control unit (hereafter referred to as engine ECU)  24 . The engine ECU  24  is constructed as a microprocessor including a CPU  24   a , a ROM  24   b  configured to store processing programs, a RAM  24   c  configured to temporarily store data, input and output ports (not shown), and a communication port (not shown). The engine ECU  24  receives, via its input port, signals from various sensors designed to measure and detect the operating conditions of the engine  22 . The signals input into the engine ECU  24  include a crank position from a crank position sensor  140  detected as the rotational position of the crankshaft  26 , a cooling water temperature Tw from a water temperature sensor  142  measured as the temperature of cooling water in the engine  22 , an in-cylinder pressure from a pressure sensor  142  located inside the combustion chamber, cam positions from a cam position sensor  144  detected as the rotational positions of camshafts driven to open and close the intake valve  128  and an exhaust valve for gas intake and exhaust into and from the combustion chamber, a throttle position Ta from a throttle valve position sensor  146  detected as the position of the throttle valve  124 , an intake air amount Qa from an air flow meter  148  located in an air intake conduit, an intake air temperature Ti from a temperature sensor  149  located in the air intake conduit, an air-fuel ratio Vaf from the air-fuel ratio sensor  135   a , and an oxygen signal Vo from the oxygen sensor  135   b . The engine ECU  24  outputs, via its output port, diverse control signals and driving signals to drive and control the engine  22 . The signals output from the engine ECU  24  include driving signals to the fuel injection valve  126 , driving signals to a throttle valve motor  136  driven to regulate the position of the throttle valve  124 , control signals to an ignition coil  138  integrated with an igniter, and control signals to a variable valve timing mechanism  150  to vary the open and close timings of the intake valve  128 . The engine ECU  24  establishes communication with the hybrid electronic control unit  70  to drive and control the engine  22  in response to control signals received from the hybrid electronic control unit  70  and to output data regarding the operating conditions of the engine  22  to the hybrid electronic control unit  70  according to the requirements. The engine ECU  24  also performs several arithmetic operations to compute a rotation speed of the crankshaft  26  or a rotation speed Ne of the engine  22  from the crank position input from the crank position sensor  140 . 
     The power distribution integration mechanism  30  has a sun gear  31  that is an external gear, a ring gear  32  that is an internal gear and is arranged concentrically with the sun gear  31 , multiple pinion gears  33  that engage with the sun gear  31  and with the ring gear  32 , and a carrier  34  that holds the multiple pinion gears  33  in such a manner as to allow free revolution thereof and free rotation thereof on the respective axes. Namely the power distribution integration mechanism  30  is constructed as a planetary gear mechanism that allows for differential motions of the sun gear  31 , the ring gear  32 , and the carrier  34  as rotational elements. The carrier  34 , the sun gear  31 , and the ring gear  32  in the power distribution integration mechanism  30  are respectively coupled with the crankshaft  26  of the engine  22 , the motor MG 1 , and the reduction gear  35  via ring gear shaft  32   a . While the motor MG 1  functions as a generator, the power output from the engine  22  and input through the carrier  34  is distributed into the sun gear  31  and the ring gear  32  according to the gear ratio. While the motor MG 1  functions as a motor, on the other hand, the power output from the engine  22  and input through the carrier  34  is combined with the power output from the motor MG 1  and input through the sun gear  31  and the composite power is output to the ring gear  32 . The power output to the ring gear  32  is thus finally transmitted to the driving wheels  63   a  and  63   b  via the gear mechanism  60 , and the differential gear  62  from ring gear shaft  32   a.    
     Both the motors MG 1  and MG 2  are known synchronous motor generators that are driven as a generator and as a motor. The motors MG 1  and MG 2  transmit electric power to and from a battery  50  via inverters  41  and  42 . Power lines  54  that connect the inverters  41  and  42  with the battery  50  are constructed as a positive electrode bus line and a negative electrode bus line shared by the inverters  41  and  42 . This arrangement enables the electric power generated by one of the motors MG 1  and MG 2  to be consumed by the other motor. The battery  50  is charged with a surplus of the electric power generated by the motor MG 1  or MG 2  and is discharged to supplement an insufficiency of the electric power. When the power balance is attained between the motors MG 1  and MG 2 , the battery  50  is neither charged nor discharged. Operations of both the motors MG 1  and MG 2  are controlled by a motor electronic control unit (hereafter referred to as motor ECU)  40 . The motor ECU  40  receives diverse signals required for controlling the operations of the motors MG 1  and MG 2 , for example, signals from rotational position detection sensors  43  and  44  that detect the rotational positions of rotors in the motors MG 1  and MG 2  and phase currents applied to the motors MG 1  and MG 2  and measured by current sensors (not shown). The motor ECU  40  outputs switching control signals to the inverters  41  and  42 . The motor ECU  40  communicates with the hybrid electronic control unit  70  to control operations of the motors MG 1  and MG 2  in response to control signals transmitted from the hybrid electronic control unit  70  while outputting data relating to the operating conditions of the motors MG 1  and MG 2  to the hybrid electronic control unit  70  according to the requirements. The motor ECU  40  also performs arithmetic operations to compute rotation speeds Nm 1  and Nm 2  of the motors MG 1  and MG 2  from the output signals of the rotational position detection sensors  43  and  44 . 
     The battery  50  is under control of a battery electronic control unit (hereafter referred to as battery ECU)  52 . The battery ECU  52  receives diverse signals required for control of the battery  50 , for example, an inter-terminal voltage measured by a voltage sensor (not shown) disposed between terminals of the battery  50 , a charge-discharge current measured by a current sensor (not shown) attached to the power line  54  connected with the output terminal of the battery  50 , and a battery temperature Tb measured by a temperature sensor  51  attached to the battery  50 . The battery ECU  52  outputs data relating to the state of the battery  50  to the hybrid electronic control unit  70  via communication according to the requirements. The battery ECU  52  also performs various arithmetic operations for management and control of the battery  50 . A remaining charge or state of charge (SOC) of the battery  50  is calculated from an integrated value of the charge-discharge current measured by the current sensor. An input limit Win as an allowable charging electric power to be charged in the battery  50  and an output limit Wout as an allowable discharging electric power to be discharged from the battery  50  are set corresponding to the calculated state of charge (SOC) and the battery temperature Tb. A concrete procedure of setting the input and output limits Win and Wout of the battery  50  sets base values of the input limit Win and the output limit Wout corresponding to the battery temperature Tb, specifies an input limit correction factor and an output limit correction factor corresponding to the state of charge (SOC) of the battery  50 , and multiplies the base values of the input limit Win and the output limit Wout by the specified input limit correction factor and output limit correction factor to determine the input limit Win and the output limit Wout of the battery  50 . 
     The hybrid electronic control unit  70  is constructed as a microprocessor including a CPU  72 , a ROM  74  that stores processing programs, a RAM  76  that temporarily stores data, and a non-illustrated input-output port, and a non-illustrated communication port. The hybrid electronic control unit  70  receives various inputs via the input port: an ignition signal from an ignition switch  80 , a gearshift position SP from a gearshift position sensor  82  that detects the current position of a gearshift lever  81 , an accelerator opening Acc from an accelerator pedal position sensor  84  that measures a step-on amount of an accelerator pedal  83 , a brake pedal position BP from a brake pedal position sensor  86  that measures a step-on amount of a brake pedal  85 , and a vehicle speed V from a vehicle speed sensor  88 . The hybrid electronic control unit  70  communicates with the engine ECU  24 , the motor ECU  40 , and the battery ECU  52  via the communication port to transmit diverse control signals and data to and from the engine ECU  24 , the motor ECU  40 , and the battery ECU  52 , as mentioned previously. 
     The hybrid vehicle  20  of the embodiment thus constructed calculates a torque demand to be output to the ring gear shaft  32   a  functioning as the drive shaft, based on observed values of a vehicle speed V and an accelerator opening Acc, which corresponds to a driver&#39;s step-on amount of the accelerator pedal  83 . The engine  22  and the motors MG 1  and MG 2  are subjected to operation control to output a required level of power corresponding to the calculated torque demand to the ring gear shaft  32   a . The operation control of the engine  22  and the motors MG 1  and MG 2  selectively effectuates one of a torque conversion drive mode, a charge-discharge drive mode, and a motor drive mode. The torque conversion drive mode controls the operations of the engine  22  to output a quantity of power equivalent to the required level of power, while driving and controlling the motors MG 1  and MG 2  to cause all the power output from the engine  22  to be subjected to torque conversion by means of the power distribution integration mechanism  30  and the motors MG 1  and MG 2  and output to the ring gear shaft  32   a . The charge-discharge drive mode controls the operations of the engine  22  to output a quantity of power equivalent to the sum of the required level of power and a quantity of electric power consumed by charging the battery  50  or supplied by discharging the battery  50 , while driving and controlling the motors MG 1  and MG 2  to cause all or part of the power output from the engine  22  equivalent to the required level of power to be subjected to torque conversion by means of the power distribution integration mechanism  30  and the motors MG 1  and MG 2  and output to the ring gear shaft  32   a , simultaneously with charge or discharge of the battery  50 . The motor drive mode stops the operations of the engine  22  and drives and controls the motor MG 2  to output a quantity of power equivalent to the required level of power to the ring gear shaft  32   a . Both of the torque conversion drive mode and the charge-discharge drive mode are modes for controlling the engine  22  and the motors MG 1  and MG 2  to output the required level of power to the ring gear shaft  32   a  with operation of the engine  22  and the control in the both modes practically has no difference. A combination of the both modes is thus referred to as an engine drive mode hereafter. 
     In the engine drive mode, the hybrid electronic control unit  70  sets a torque demand Tr* to be output to the ring gear shaft  32   a  or the driveshaft based on the accelerator opening Acc and the vehicle speed V, and sets a power demand Pe* required for the engine  22  by subtracting a charging power that the battery  50  requires from a driving power that is obtained as the required level of power from the product of the set torque demand Tr* and a rotation speed Nr of the ring gear shaft  32   a . The charging power is positive when the battery  50  is discharged. The rotation speed Nr of the ring gear shaft  32   a  is obtained by dividing the rotation speed Nm 2  of the motor MG 2  by a gear ratio Gr of the reduction gear  35  or by multiplying the vehicle speed V by a conversion factor. The hybrid electronic control unit  70  then sets a target rotation speed Ne* and a target torque Te* based on the set power demand Pe* so that the engine  22  is efficiently operated and sends the settings of the target rotation speed Ne* and the target torque Te* to the engine ECU  24 . The hybrid electronic control unit  70  also sets a torque command Tm 1 * of the motor MG 1  so that the engine  22  is rotated at the target rotation speed Ne*, sets a torque command Tm 2 * of the motor MG 2  within a range of the input limit Win and the output limit Wout of the battery  50  so that the hybrid vehicle  20  is driven with the torque demand Tr*, and sends the settings of the torque command Tm 1 * and Tm 2 * of the motor MG 1  and MG 2  to the motor ECU  40 . In response to reception of the settings of the target rotation speed Ne* and the target torque Te*, the engine ECU  24  performs required controls including intake air flow regulation, ignition control, and fuel injection control of the engine  22  to drive the engine  22  at the specific drive point defined by the combination of the target rotation speed Ne* and the target torque Te*. In response to reception of the settings of the torque commands Tm 1 * and Tm 2 *, the motor ECU  40  performs switching control of the switching elements in the inverter  41  and the switching elements in the inverter  42  to drive the motor MG 1  with the torque command Tm 1 * and the motor MG 2  with the torque command Tm 2 *. In the motor drive mode, the hybrid electronic control unit  70  sets the torque command Tm 2 * of the motor MG 2  within the range of the input limit Win and the output limit Wout of the battery  50  to output the torque demand Tr* based on the accelerator opening Acc and the vehicle speed V to the ring gear shaft  32   a  or the driveshaft, and sends the setting of the torque command Tm 2 * to the motor ECU  40 . In response to reception of the setting of the torque command Tm 2 *, the motor ECU  40  performs switching control of the switching elements in the inverter  42  to drive the motor MG 2  with the torque command Tm 2 *. Switching between the engine drive mode and the motor drive mode is done by comparing the power demand Pe* with a starting threshold value for startup of the engine  22  and with a stopping threshold value for operation stop of the engine  22 . When the power demand Pe* becomes lower than the stopping threshold value to satisfy a stop condition during the engine drive mode, the engine drive mode is switched to the motor drive mode by stopping operation of the engine  22 . When the power demand Pe* becomes higher than the starting threshold value to satisfy a startup condition during the motor drive mode, the motor drive mode is switched to the engine drive mode by starting up the engine  22 . According to the above control, the hybrid vehicle  20  of the embodiment is driven with outputting the torque demand Tr* corresponding to the accelerator opening Acc to the ring gear shaft  32   a  or the driveshaft with charge and discharge of the battery  50  while performing an intermittent operation of the engine  22 . 
     The description regards the operations of the hybrid vehicle  20  of the embodiment having the configuration discussed above, especially a series of operation control for startup of the engine  22  while driving the hybrid vehicle  20  with the intermittent operation of the engine  22 .  FIG. 4  is a flowchart showing a startup time fuel injection control executed by the engine ECU  24  to start up the engine  22 , and  FIG. 5  is a flowchart showing a function determination routine executed by the engine ECU  24  to obtain a result of function determination of the air-fuel ratio sensor  135   a . The result of function determination is used for the startup time fuel injection. Starting up the engine  22  is done by motoring the engine  22  with outputting a motoring torque for motoring (cranking) the engine  22  from the motor MG 1  and with receiving the action of the motoring torque by an output torque of the motor MG 2 , and done by starting fuel injection from the fuel injection valve  126  and ignition at the spark plug  130  when the rotation speed Ne of the engine  22  reaches a preset rotation speed for starting the fuel injection and the ignition. Motoring the engine  22  is done by setting the motoring torque as the torque command Tm 1 * of the motor MG 1  and sending the setting of the torque command Tm 1 * to the motor ECU  40  by the hybrid electronic control unit  70 , and done by performing switching control of the inverter  41  to output a corresponding torque to the torque command Tm 1 * from the motor MG 1  by the motor ECU  40  that received the setting of the torque command Tm 1 *. The function determination of the air-fuel ratio sensor  135   a  is explained first, and the startup time fuel injection control is explained next, for convenience of explanation, as follows. The function determination routine of  FIG. 5  is executed in the case that this routine has never been executed since ignition on (before ignition off) of the hybrid vehicle  20  while the engine  22  is in idle operation after warm up of the engine  22  is completed. 
     In the function determination routine, the CPU  24   a  of the engine ECU  24  sets a target air-fuel ratio to a rich air-fuel ratio (for example, value ‘14.1’) that is fuel-richer than a stoichiometric air-fuel ratio (for example, value ‘14.5’, value ‘14.6’, or value ‘14.7’) and start fuel injection control for the function determination (step S 300 ). The CPU  24   a  starts to measure a time tm 1  from value ‘0’ by a timer (now shown) (step S 310 ). The CPU  24   a  inputs the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  and waits until the air-fuel ratio Vaf reaches the target air-fuel ratio (step S 320 ). In the fuel injection control for the function determination of this embodiment, the CPU  24   a  sets a fuel injection amount corresponding to the intake air amount Qa from the air flow meter  148  so that the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  becomes the target air-fuel ratio, and drives the fuel injection valve  126  to be open for a fuel injection time corresponding to the set fuel injection amount. 
     When the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  reaches the target air-fuel ratio, the CPU  24   a  calculates a delay time Td 1 (C) by subtracting a normal response time Tdnm 1  from the time tm 1  (step S 330 ). The delay time Td 1 (C) represents a reduced degree of responsiveness of the air-fuel ratio sensor  135   a  (ability of the air-fuel ratio Vaf from air-fuel ratio sensor  135   a  to track the target air-fuel ratio) in the case where the air-fuel ratio of the engine  22  is changed from the lean air-fuel ratio to the rich air-fuel ratio. The normal response time Tdnm 1  may be predetermined by experiment or the like, according to the characteristics of the engine  22  and the air-fuel ratio sensor  135   a , as a required time (for example, 300 msec or 500 msec) to bring the air-fuel ratio Vaf from the lean air-fuel ratio to the rich air-fuel ratio when the air-fuel ratio of the engine  22  is changed from the lean air-fuel ratio to the rich air-fuel ratio under a normal condition that the responsiveness of the air-fuel ratio sensor  135   a  is not reduced. The variable C is set to value ‘1’ as an initial value and incremented by value ‘1’ in the processing described later. 
     The CPU  24   a  then inputs the oxygen signal Vo from the oxygen sensor  135   b  and waits until the oxygen signal Vo indicates a rich-side value in comparison with the stoichiometric air-fuel ratio (step S 340 ). When the oxygen signal Vo indicates the rich-side value, the CPU  24   a  sets the target air-fuel ratio of the engine  22  to the lean air-fuel ratio and starts the fuel injection control for the function determination (step S 350 ) and starts to measure a time tm 2  from the value ‘0’ by the timer (now shown) (step S 360 ). The CPU  24   a  inputs the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  and waits until the air-fuel ratio Vaf reaches the target air-fuel ratio (step S 370 ). When the air-fuel ratio Vaf reaches the target air-fuel ratio, calculates a delay time Td 2 (C) by subtracting a normal response time Tdnm 2  from the time tm 2  (step S 380 ) and inputs the oxygen signal Vo from the oxygen sensor  135   b  to wait until the oxygen signal indicates a lean-side value in comparison with the stoichiometric air-fuel ratio (step S 390 ). The delay time Td 2 (C) represents a reduced degree of responsiveness of the air-fuel ratio sensor  135   a  in the case where the air-fuel ratio of the engine  22  is changed from the rich air-fuel ratio to the lean air-fuel ratio. The normal response time Tdnm 2  may be predetermined by experiment or the like, according to the characteristics of the engine  22  and the air-fuel ratio sensor  135   a , as a required time (for example, 300 msec or 500 msec) to bring the air-fuel ratio Vaf from the rich air-fuel ratio to the lean air-fuel ratio when the air-fuel ratio of the engine  22  is changed from the rich air-fuel ratio to the lean air-fuel ratio under the normal condition that the responsiveness of the air-fuel ratio sensor  135   a  is not reduced. 
     When the oxygen signal Vo indicates the lean-side value, the CPU  24   a  increments the variable C (step S 400 ) and determines whether the variable C becomes a preset value Cn (step S 410 ). When it is determined that the variable C is not the preset value Cn, the CPU  24   a  returns to the processing of step S 300 . The preset value Cn is a predetermined value (for example, value ‘4’ or value ‘6’) as the number of repeated execution of a series of the processing from setting the target air-fuel ratio to the rich air-fuel ratio followed by the oxygen signal Vo reaching the rich-side value until the oxygen signal Vo reaches the lean-side value after setting the target air-fuel ratio to the lean air-fuel ratio.  FIG. 6  shows one set of examples of time charts of the oxygen signal Vo and the air-fuel ratio Vaf during execution of the function determination of the air-fuel ratio sensor  135   a . In the figure, with regard to the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a , the sold line indicates values under the normal condition and the broken line indicates under an abnormal condition where the responsiveness of the air-fuel ratio sensor  135   a  is reduced in the case of changing the air-fuel ratio of the engine  22  from the rich air-fuel ratio to the lean air-fuel ratio. The target air-fuel ratio is set to the lean air-fuel ratio at the time t 1  to perform fuel injection, the air-fuel ratio Vaf becomes the target air-fuel ratio at the time t 2  that is later than the time t 1  by the normal response time Tdnm 2  under the normal condition while becoming the target air-fuel ratio at the time t 3  that is further later than the time t 2  by the delay time Td 2 (C) under the abnormal condition. Upon the fuel injection with switching the target air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio at the time t 1 , an excess of oxygen is occluded from the exhaust at the three-way catalyst  134   a  of the catalytic converter  134 , and the oxygen signal Vo switches at the time t 4  to a lean-side value crossing the value Vref corresponding to the stoichiometric air-fuel ratio after some time continuing rich-side values. Upon the fuel injection with switching the target air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio at the time t 4 , the occluded oxygen is released to the exhaust at the three-way catalyst  134   a  of the catalytic converter  134 , and the oxygen signal Vo switches at the time t 5  to a rich-side value crossing the value Vref after some time continuing lean-side values. The fuel injection is performed again with setting the target air-fuel ratio to the rich air-fuel ratio. 
     After the calculation of the delay time Td 1 (C) and the delay time Td 2 (C) (the variable C is from value ‘1’ through the preset value Cn), the CPU  24   a  determines the respective averages of the calculated delay time Td 1 (C) and the delay time Td 2 (C) and sets them as a lean-rich delay time Td 1   a  and a rich-lean delay time Td 2   a  (step S 420 ). The CPU  24   a  compares the set lean-rich delay time Td 1   a  with the sum of the normal response time Tdnm1 and a margin ca (step S 430 ). When the lean-rich delay time Td 1   a  is less than the sum of the normal response time Tdnm 1  and the margin α 1 , the CPU  24   a  sets a lean-rich abnormality flag F 1  to value ‘0’ (step S 440 ). When the lean-rich delay time Td 1   a  is more than or equal to the sum of the normal response time Tdnm 1  and the margin α 1 , the CPU  24   a  sets the lean-rich abnormality flag F 2  to value ‘1’ (step S 450 ). The lean-rich abnormality flag F 1  is a flag that is set to value ‘0’ upon no occurrence of an abnormality where the responsiveness of the air-fuel ratio sensor  135   a  is reduced in the case of changing the air-fuel ratio of the engine  22  from the lean air-fuel ratio to the rich air-fuel ratio (hereafter referred to as lean-rich abnormality) and also as an initial value, while being set to value ‘1’ upon occurrence of the lean-rich abnormality, and is stored in a nonvolatile memory (not shown). The margin α 1  is used to determine the occurrence of the lean-rich abnormality and may be predetermined as a time (for example, 500 msec or 700 msec) by experiment or the like according to the characteristics of the engine  22  and the air-fuel ratio sensor  135   a.    
     The CPU  24   a  further compares the set rich-lean delay time Td 2   a  with the sum of the normal response time Tdnm 2  and a margin α 2  (step S 460 ). When the rich-lean delay time Td 2   a  is less than the sum of the normal response time Tdnm 2  and the margin α 2 , the CPU  24   a  sets the rich-lean abnormality flag F 2  to value ‘0’ (step S 480 ). When the rich-lean delay time Td 2   a  is more than or equal to the sum of the normal response time Tdnm 2  and the margin α 2 , the CPU  24   a  sets the rich-lean abnormality flag F 2  to value ‘1’ (step S 490 ). The CPU  24   a  then terminates the function determination routine. The rich-lean abnormality flag F 2  is a flag that is set to value ‘0’ upon no occurrence of an abnormality where the responsiveness of the air-fuel ratio sensor  135   a  is reduced in the case of changing the air-fuel ratio of the engine  22  from the rich air-fuel ratio to the lean air-fuel ratio (hereafter referred to as rich-lean abnormality) and also as an initial value, while being set to value ‘1’ upon occurrence of the lean-rich abnormality, and is stored in the nonvolatile memory (not shown). The margin α 2  is used to determine the occurrence of the rich-lean abnormality and may be predetermined as a time (for example, 500 msec or 700 msec) by experiment or the like according to the characteristics of the engine  22  and the air-fuel ratio sensor  135   a . The above description makes explanation of the function determination of the air-fuel ratio sensor  135   a.    
     The startup time fuel injection control is explained next. The startup time fuel injection control routine of  FIG. 4  is executed when the rotation speed Ne of the engine  22  reaches the preset rotation speed, which is to start the fuel injection and the ignition, by the motoring of the engine  22  with the motoring torque from the motor MG 1  upon satisfaction of the startup condition of the engine  22 . 
     In the startup time fuel injection control routine, the CPU  24   a  of the engine ECU  24  inputs various data required for control, for example, the rich-lean abnormality flag F 2  and the rich-lean delay time Td 2   a  (step S 100 ) and starts to measure a time tmf from value ‘0’ by the timer (not shown) (step S 110 ). The rich-lean abnormality flag F 2  and the rich-lean delay time Td 2   a  may be input by reading the data that is set as results of execution of the function determination routine of the air-fuel ratio sensor  135   a  of the  FIG. 5  and stored in the no volatile memory (not shown). 
     After the data input, the CPU  24   a  checks the input rich-lean abnormality flag F 2  (step S 120 ). When the rich-lean abnormality flag F 2  is equal to value ‘0’, it is determined that the rich-lean abnormality of the air-fuel ratio sensor  135   a  is not in occurrence and the CPU  24   a  sets a start time Taf, that is a time from start of the startup time fuel injection control to start of an air-fuel ratio feedback correction when starting the engine  22 , to a basic start time Tafb (step S 130 ) and the CPU  24   a  sets a basic fuel injection amount Qfb (step S 150 ). In this embodiment, the basic fuel injection amount Qfb is set based on the intake air amount Qa from the air flow meter  148  and the rotation speed Ne of the engine  22  as a basic value of fuel injection to bring the air-fuel ratio of the engine  22  to the stoichiometric air-fuel ratio. The basic fuel injection amount Qfb may be set using the cooling water temperature Tw from the water temperature sensor  142 , the intake air temperature Ti from the temperature sensor  149 , and the throttle position Ta from the throttle valve position sensor  146 . The air-fuel ratio feedback correction is performed by correcting the basic fuel injection amount Qfb using feedback control so that the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  becomes the stoichiometric air-fuel ratio. The basic start time Tafb is explained later. 
     After the setting of the start time Taf for the air-fuel ratio feedback correction and the basic fuel injection amount Qfb, the CPU  24   a  compares the time tmf with the start time Taf (step S 160 ). When the time tmf is less than the start time Taf, the CPU  24   a  determines that the air-fuel ratio feedback correction is not performed and sets an air-fuel ratio feedback correction factor to value ‘1’ (step S 170 ). 
     The CPU  24   a  then compares the time tmf with the increase correction time Tinc that is a time to continue performing increase correction of fuel injection amount (step S 200 ). When the time tmf is less than the increase correction time Tinc, the CPU  24   a  determines to perform the increase correction and sets an increase correction factor ki to a value larger than value ‘1’ (for example, a gradually decreasing value according to the time tmf or a fixed value) (step S 210 ). In this embodiment, the increase correction time Tinc is duration of the increase correction of the basic fuel injection amount Qfb, and may be predetermined by experiment or the like as a smaller value than the basic start time Tafb of the air-fuel ratio feedback correction. The increase correction is so started together with start of fuel injection as to start up the engine  22  favorably. 
     After the setting of the air-fuel ratio feedback correction factor kaf and the increase correction factor ki, the CPU  24   a  calculates a target fuel injection amount Qf* by multiplying the basic fuel injection amount Qfb by the product of the air-fuel ratio feedback correction factor kaf (currently set to value ‘1’) and the increase correction factor ki (currently set to a value larger than value ‘1’) (step S 230 ). The CPU  24   a  drives the fuel injection valve  126  to be open for a fuel injection time corresponding to the calculated target fuel injection amount Qf* (step S 240 ) and determines whether a termination condition to terminate execution of this routine is satisfied or not (step S 250 ). When the termination condition is not satisfied the CPU  24   a  returns to the processing of step S 150 . The termination condition may be, for example, a condition that complete combustion of the engine  22  is determined or a condition that a preset time for shifting to post-startup fuel injection control elapses after startup of the engine  22  is started. Such control enables to perform fuel injection with the increase correction of the basic fuel injection amount Qfb to start up the engine  22  favorably right after start of the fuel injection when starting the engine  22 . 
     When the time tmf is more than or equal to the increase correction time Tinc at the processing of step S 200 , the CPU  24   a  sets the increase correction factor ki to value ‘1’ (step S 220 ). The CPU  24   a  calculates the target fuel injection amount Qf* by multiplying the basic fuel injection amount Qfb by the product of the air-fuel ratio feedback correction factor kaf (currently set to value ‘1’) and the increase correction factor ki (currently set to value ‘1’) (step S 230 ), and drives the fuel injection valve  126  using the calculated target fuel injection amount Qf* (step S 240 ). The CPU  24   a  then determines whether the termination condition of this routine is satisfied or not (step S 250 ). In this embodiment, the increase correction time Tinc is set to be smaller than the basic start time Tafb, and the air-fuel ratio feedback correction is thus started after finishing the increase correction, as explained next. 
     When the time tmf is more than or equal to the start time Taf at the processing of step S 160 , the CPU  24   a  determines to perform the air-fuel ratio feedback correction and inputs the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  (step S 180 ). The CPU  24   a  sets the air-fuel ratio feedback correction factor kaf, according to Equation (1) given below, using feedback control so that the input air-fuel ratio Vaf becomes the target air-fuel ratio Vaf* set as the stoichiometric air-fuel ratio (step S 190 ) and sets the increase correction factor ki to value ‘1’ (step S 200 ). The CPU  24   a  then drives the fuel injection valve  126  using the target fuel injection amount Qf* calculated from multiplying the basic fuel injection amount Qfb by the product of the air-fuel ratio feedback correction factor kaf and the increase correction factor ki (currently set to value ‘1’) (step S 230  and S 240 ) and determines whether the termination condition of this routine is satisfied or not (step S 250 ). Upon determination of the termination condition, the CPU  24   a  terminates the startup time fuel injection control routine: 
         kaf=kaf+k 1( Vaf*−Vaf )+ k 2∫( Vaf*−Vaf ) dt    (1)
 
     In Equation (1) given above, the first term on the right side denotes the air-fuel ratio feedback correction factor kaf that is set by the present time, and ‘k 1 ’ in the second term and ‘k 2 ’ in the third term on the right side respectively denote a gain of the proportional and a gain of the integral term. Upon termination of the startup time fuel injection control routine, a fuel injection control routine for a post-startup time (not shown) is executed. The basic start time Tafb that the start time Taf is set to is explained here. In this embodiment, the basic start time Tafb is predetermined by experiment or the like as a timing that the air-fuel ratio Vaf detected by the air-fuel ratio sensor  135   a  reaches the target air-fuel ratio Vaf* as the stoichiometric air-fuel ratio after finishing the increase correction of the basic fuel injection amount Qfb under the normal condition for the air-fuel ratio sensor  135   a  (under a condition that there is no occurrence of the lean-rich abnormality and the rich-lean abnormality of the air-fuel ratio sensor  135   a ). Accordingly, when the rich-lean abnormality flag F 2  is equal to value ‘0’ denoting no occurrence of the rich-lean abnormality of the air-fuel ratio sensor  135   a , the air-fuel ratio feedback correction of the basic fuel injection amount Qfb is started at a timing when this basic start time Tafb elapses after starting execution of the startup time fuel injection control routine. It is thus prevented that the air-fuel ratio feedback correction is started in a state that the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  deviates from the target air-fuel ratio Vaf* as the stoichiometric air-fuel ratio, and divergence of the air-fuel ratio of the engine  22  is effectively prevented. 
     When the rich-lean abnormality flag F 2  is equal to value ‘1’ at the processing of step S 120 , it is determined that the rich-lean abnormality of the air-fuel ratio sensor  135   a  is in occurrence, and the CPU  24   a  sets the sum of the basic start time Tafb and the product of the rich-lean delay time Td 2   a  and a conversion factor kd as the start time Taf that is a time from start of the startup time fuel injection control for startup of the engine  22  to start of the air-fuel ratio feedback correction (step S 140 ) and sets the basic fuel injection amount Qfb (step S 150 ). The CPU  24   a  then perform fuel injection using the increase correction factor ki and the air-fuel ratio feedback correction factor kaf respectively set according to the elapsed time tmf from start of the startup time fuel injection control (step S 160  through S 240 ), and terminates the startup time fuel injection control routine upon determination of satisfaction of the termination condition of this routine. The conversion factor kd is a factor to convert the rich-lean delay time Td 2   a  into a response delay time against the normal condition of the air-fuel ratio sensor  135   a , and is predetermined by experiment or the like. The response delay time occurs until the air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  with rich-lean abnormality reaches the target air-fuel ratio Vaf* after finishing the increase correction.  FIG. 7  shows one set of examples of time charts of the air-fuel ratio of the engine  22  without occurrence of the lean-rich abnormality but with occurrence of the rich-lean abnormality of the air-fuel ratio sensor  135   a . In the figure, the solid line indicates the air-fuel ratio Vaf detected by the air-fuel ratio sensor  135   a  and the alternate long and short dashed line indicates the actual air-fuel ratio of the engine  22  (the air-fuel ratio Vaf that is assumed to be detected by the air-fuel ratio sensor  135   a  under its normal condition). In the figure, the lower chart shows the exemplified case of this embodiment where the sum of the basic start time Tafb and the rich-lean delay time Td 2   a  is used as the start time Taf for the air-fuel ratio feedback correction, and the upper chart shows an exemplified case for comparison where the basic start time Tafb is used as the start time Taf for the air-fuel ratio feedback correction. Motoring of the engine  22  with the motor MG 1  is started at the time t 11  and the fuel injection into the engine  22  is started at the time t 12 . The air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  changes from a leaner side value to a richer side value than the stoichiometric air-fuel ratio accompanied by the increase correction of the basic fuel injection amount Qfb of the engine  22 . The air-fuel ratio Vaf from the air-fuel ratio sensor  135   a  gradually approaches the stoichiometric air-fuel ratio after finishing the increase correction. In the case for comparison, the air-fuel ratio feedback correction is started at the time t 13  when the basic start time Tafb elapses from the time t 12 , and the air-fuel ratio feedback correction is started using the air-fuel ratio Vaf (having a richer side value at the time t 13 ) from the air-fuel ratio sensor  135   a  with its rich-lean abnormality. As shown by the alternate long and short dashed line, the fuel injection amount is corrected toward the decrease side although the actual air-fuel ratio is close to the stoichiometric air-fuel ratio, and the actual air-fuel ratio of the engine  22  becomes a lean side value. For this reason, the emission of the exhaust is worsened, for example, by discharging nitrogen oxides (NOx), every time the engine  22  is started up during the intermittent operation of the engine  22 . Therefore, in this embodiment, the air-fuel ratio feedback correction is started at the time t 14  later than the time t 13 , and it thus is prevented that the emission of the exhaust is worsened. Moreover, in this embodiment, the air-fuel ratio feedback correction is started at the timing of the time t 14  when the sum of the basic start time Tafb and the product of the rich-lean delay time Td 2   a  and the conversion factor kd elapses from the time t 12 , and this arrangement enables to start the air-fuel ratio feedback correction at a timing on which a response delay time, due to the rich-lean abnormality in occurrence at the air-fuel ratio sensor  135   a , is reflected. As a result, the exhaust emission is effectively prevented from becoming worse using the result of the function determination of the air-fuel ratio sensor  135   a.    
     In the hybrid vehicle  20  of the embodiment described above, the CPU  24   a  performs the function determination of the air-fuel ratio sensor  135   a  including determination whether there occurs the rich-lean abnormality that is an abnormality where the air-fuel ratio sensor  135   a  becomes less responsive to a change in the air-fuel ratio of the engine  22  from the rich air-fuel ratio to the lean air-fuel ratio. When the engine  22  is started up with motoring by the motor MG 1  while the rich-lean abnormality is not determined, that is, while the rich-lean abnormality flag F 2  is equal to value ‘0’, the air-fuel ratio feedback correction for fuel injection into the engine  22  is started at the timing when the basic start time Tafb elapses from the start of fuel injection after finishing the increase correction at the timing when the increase correction time Tinc elapses from the start of fuel injection. When the engine  22  is started up with motoring by the motor MG 1  while the rich-lean abnormality is determined, that is, while the rich-lean abnormality flag F 2  is equal to value ‘1’, the air-fuel ratio feedback correction for fuel injection into the engine  22  is started at a later timing than the timing when the basic start time Tafb elapses from the start of fuel injection after finishing the increase correction at the timing when the increase correction time Tinc elapses from the start of fuel injection. Accordingly, it is effectively prevented the exhaust emission from becoming worse using the result of the function determination of the air-fuel ratio sensor  135   a.    
     In the hybrid vehicle  20  of the embodiment, the start time Taf of the air-fuel ratio feedback correction is set to the basic start time Tafb when the rich-lean abnormality flag F 2  is equal to value ‘0’, and the start time Taf of the start time Taf of the air-fuel ratio feedback correction is set to the sum of the basic start time Tafb and the product of the rich-lean delay time Td 2   a  and the conversion factor kd when the rich-lean abnormality flag F 2  is equal to value ‘1’ . Instead, the start time Taf of the air-fuel ratio feedback correction may be set to the basic start time Tafb when the rich-lean abnormality flag F 2  is equal to value ‘0’ as well as the lean-rich abnormality flag F 1  is equal to value ‘0’, and the start time Taf of the start time Taf of the air-fuel ratio feedback correction may be set to the sum of the basic start time Tafb and the product of the rich-lean delay time Td 2   a  and the conversion factor kd when the rich-lean abnormality flag F 2  is equal to value ‘1’ as well as the lean-rich abnormality flag F 1  is equal to value ‘0’. 
     In the hybrid vehicle  20  of the embodiment, the start time Taf of the air-fuel ratio feedback correction is set to the sum of the basic start time Tafb and the product of the rich-lean delay time Td 2   a  and the conversion factor kd when the rich-lean abnormality flag F 2  is equal to value ‘1’, and the rich-lean delay time Td 2   a  is set as an average of the delay time Td 2 (C) in the function determination routine of the air-fuel ratio sensor  135   a . Instead, the rich-lean delay time Td 2   a  may be set as a maximum value or a median value of the delay time Td 2 (C) in the function determination routine of the air-fuel ratio sensor  135   a . The start time Taf of the air-fuel ratio feedback correction may be set to the sum of the basic start time Tafb and a preset time that is, for example, a fixed value obtained by experiment or the like as a response delay time against the air-fuel ratio sensor  135   a  with its normal condition when the rich-lean abnormality of the air-fuel ratio sensor  135   a  is in occurrence. 
     In the hybrid vehicle  20  of the embodiment, the basic start time Tafb is predetermined by experiment or the like as a timing that the air-fuel ratio Vaf detected by the air-fuel ratio sensor  135   a  reaches the target air-fuel ratio Vaf* as the stoichiometric air-fuel ratio after finishing the increase correction of the basic fuel injection amount Qfb under the normal condition for the air-fuel ratio sensor  135   a . Instead, the basic start time Tafb may be predetermined by experiment or the like as a timing that the air-fuel ratio Vaf detected by the air-fuel ratio sensor  135   a  reaches a target air-fuel ratio range (for example, a range of the air-fuel ratio more than or equal to value ‘14.5’ and less than or equal to value ‘14.7’) after finishing the increase correction of the basic fuel injection amount Qfb under the normal condition for the air-fuel ratio sensor  135   a.    
     In the hybrid vehicle  20  of the embodiment, the increase correction is finished at the timing when the increase correction time Tinc elapses from the start of fuel injection, and the air-fuel ratio feedback correction is started at the timing when the start time Taf elapses from the start of fuel injection. Instead, the increase correction may be finished at the timing when a preset increase correction finish time elapses from the start of engine  22  startup (for example, the timing when a startup condition is satisfied or the timing when the motoring of the engine  22  is started), and the air-fuel ratio feedback correction may be started at the timing when a preset start time elapses from the start of engine  22  startup. 
     In the hybrid vehicle  20  of the embodiment, the power of the engine  22  is output via the power distribution integration mechanism  30  to the side of the drive wheels  63   a  and  63   b  and the power of the motor MG 2  is output to the side of the drive wheels  63   a  and  63   b . The technique of the invention is also applicable to a hybrid vehicle  120  a modified structure shown in  FIG. 8 . In the hybrid vehicle  120  of  FIG. 8 , a motor MG is connected via an automatic transmission  130  to the driveshaft linked to the drive wheels  63   a  and  63 , and the engine  22  is connected via a clutch  129  to the rotating shaft of the motor MG. In the hybrid vehicle  120 , the power of the engine  22  is output via the rotating shaft of the motor MG and the automatic transmission to the side of drive wheels  63   a  and  63   b , and the power of the motor MG is output via the automatic transmission to the side of drive wheels  63   a  and  63 . In this case, the motor MG corresponds to a motor which the engine  22  is cranked by. The technique of the invention is also applicable to a hybrid vehicle  220  of another modified structure shown in  FIG. 9 . The hybrid vehicle  220  of  FIG. 9  has a generator  230  that generates electric power with the power of the engine  22  and a motor MG connected to the driveshaft linked to the drive wheels  63   a  and  63   b . In the hybrid vehicle  220 , the battery  50  is charged and discharged with power generation by the generator  230  using the power from the engine  22 , and the power of the motor MG using the electric power of the generator  230  and the battery  50  is output to the side of drive wheels  63   a  and  63   b  with the charge and discharge of the battery  50 . In this case, the generator  230  corresponds to a motor which the engine  22  is cranked by. The technique of the invention is also applicable to a motor vehicle that does not have a motor to output a driving power and only the power of the engine  22  is output via an automatic transmission to the drive wheels. 
     The embodiment regards application to the hybrid vehicle. The principle of the invention may be actualized by an internal combustion engine system installed in diversity of other applications, for example, mobile bodies such as vehicles other than automobiles, boats and ships, and aircrafts, and may also be installed in fixed equipments such as construction equipments. The principle of the invention may be actualized by a fuel injection control method of an internal combustion engine included in an internal combustion engine system. 
     The primary elements in the embodiment and its modified examples are mapped to the primary constituents in the claims of the invention as described below. The engine  22  in the embodiment corresponds to the ‘internal combustion engine’ in the claims of the invention. The motor MG 1  in the embodiment corresponds to the ‘motor’ in the claims of the invention. The fuel injection value  126  in the embodiment corresponds to the ‘fuel injector’ in the claims of the invention. The air-fuel ratio sensor  135   a  corresponds to the ‘air-fuel ratio detector’ in the claims of the invention. The engine ECU  24  executing the processing in the function determination routine of  FIG. 5  to perform the function determination of the air-fuel ratio sensor  135   a  including determination whether the rich-lean abnormality of the air-fuel ratio sensor  135   a  is in occurrence or not corresponds to the ‘air-fuel ratio detecting function determination module’ in the claims of the invention. The engine ECU  24  executing the processing of step S 100  through S 230  in the startup time fuel injection control routine of  FIG. 4  to calculate the target fuel injection amount Qf*, when the engine  22  is started up with motoring by the motor MG 1  while the rich-lean abnormality flag F 2  is equal to value ‘0’, with the air-fuel ratio feedback correction started at the timing when the basic start time Tafb elapses from the start of fuel injection after the increase correction finished at the timing when the increase correction time Tinc elapses from the start of fuel injection, and calculate the target fuel injection amount Qf*, when the engine  22  is started up with motoring by the motor MG 1  while the rich-lean abnormality flag F 2  is equal to value ‘1’, with the air-fuel ratio feedback correction started at a later timing than the timing when the basic start time Tafb elapses from the start of fuel injection after the increase correction finished at the timing when the increase correction time Tinc elapses from the start of fuel injection corresponds to the ‘target fuel injection amount setting module’ in the claims of the invention. The engine ECU  24  executing the processing of step S 240  in the startup time fuel injection control routine of  FIG. 4  to drive the fuel injection valve  126  to be open for the fuel injection time which corresponds to the calculated target fuel injection amount Qf* corresponds to the ‘fuel injection control module’ in the claims of the invention. The motor MG 2  in the embodiment corresponds to the ‘second motor’ in the claims of the invention. 
     The ‘internal combustion engine’ is not restricted to the engine  22  designed to consume a hydrocarbon fuel, such as gasoline or light oil, and thereby output power, but may be an internal combustion engine of any other design. The ‘motor’ is not restricted to the motor MG 1  constructed as a synchronous motor generator but may be any type of motor capable of cranking the internal combustion engine, for example, an induction motor. The ‘fuel injector’ is not restricted to the fuel injection valve  126  but may be any other unit that performs fuel injection into the internal combustion engine. The ‘air-fuel ratio detector’ is not restricted to the air-fuel ratio sensor  135   a  but any other unit that detects an air-fuel ratio of the internal combustion engine. The ‘air-fuel ratio detecting function determination module’ is not restricted to the arrangement of performing the function determination of the air-fuel ratio sensor  135   a  including determination whether the rich-lean abnormality of the air-fuel ratio sensor  135   a  is in occurrence or not, but may be any other arrangement of performing function determination of the air-fuel detector, the function determination including detection of a responsiveness reduction abnormality that is an abnormality where the air-fuel ratio detector becomes less responsive to a change in the air-fuel ratio of the internal combustion engine from a rich air-fuel ratio to a lean air-fuel ratio, the rich air-fuel ratio being fuel-richer and the lean air-fuel ratio being fuel-leaner both in comparison with a stoichiometric air-fuel ratio. The ‘target fuel injection amount setting module’ is not restricted to the arrangement of calculating the target fuel injection amount Qf*, when the engine  22  is started up with motoring by the motor MG 1  while the rich-lean abnormality flag F 2  is equal to value ‘0’, with the air-fuel ratio feedback correction started at the timing when the basic start time Tafb elapses from the start of fuel injection after the increase correction finished at the timing when the increase correction time Tinc elapses from the start of fuel injection, and calculating the target fuel injection amount Qf*, when the engine  22  is started up with motoring by the motor MG 1  while the rich-lean abnormality flag F 2  is equal to value ‘1’, with the air-fuel ratio feedback correction started at a later timing than the timing when the basic start time Tafb elapses from the start of fuel injection after the increase correction finished at the timing when the increase correction time Tinc elapses from the start of fuel injection, but any other arrangement of, when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is not detected, setting a target fuel injection amount to be injected into the internal combustion engine by applying an increase correction to a basic fuel injection amount until a preset timing that is predetermined so that the internal combustion engine is favorably combusted, the basic fuel injection amount being a fuel injection amount based on an intake air amount of the internal combustion engine for bringing the air-fuel ratio of the internal combustion engine to the stoichiometric air-fuel ratio, and then setting the target fuel injection amount by performing an air-fuel ratio feedback correction from a first start timing that is predetermined as a timing when the detected air-fuel ratio by the air-fuel ratio detector reaches a target air-fuel ratio range including the stoichiometric air-fuel ratio without occurrence of the responsiveness reduction abnormality after finishing the increase correction, the air-fuel ratio feedback correction being a correction of the basic fuel injection amount using feedback control for bringing the detected air-fuel ratio by the air-fuel ratio detector to the stoichiometric air-fuel ratio, and when the internal combustion engine is cranked by the motor and started up while the responsiveness reduction abnormality is detected, the target fuel injection amount setting module setting the target fuel injection amount by applying the increase correction to the basic fuel injection amount until the preset timing, and then setting the target fuel injection amount by performing the air-fuel ratio feedback correction from a second start timing that is later than the first start timing. The ‘fuel injection control module’ is not restricted to the arrangement of driving the fuel injection valve  126  to be open for the fuel injection time which corresponds to the calculated target fuel injection amount Qf*, but any other arrangement of controlling the fuel injector so that the fuel injection into the internal combustion engine is performed according the set target fuel injection amount. The ‘second motor’ is not restricted to the motor MG 2  constructed as a synchronous motor generator but may be any type of motor capable of outputting power for driving the vehicle, for example, an induction motor. 
     The above mapping of the primary elements in the embodiment and its modified examples to the primary constituents in the claims of the invention is not restrictive in any sense but is only illustrative for concretely describing the modes of carrying out the invention. Namely the embodiment and its modified examples discussed above are to be considered in all aspects as illustrative and not restrictive. 
     There may be many other modifications, changes, and alterations without departing from the scope or spirit of the main characteristics of the present invention. 
     The technique of the invention is preferably applied to the manufacturing industries of the internal combustion engine systems.