Patent Publication Number: US-9890724-B2

Title: Control system of engine

Description:
BACKGROUND 
     The present invention relates to a technical field of a control system of an engine in which a purge gas containing evaporated fuel desorbed from a canister is supplied to an intake passage. 
     Conventionally, arts are known in which when it is determined that evaporated fuel easily overflows from a canister during a deceleration fuel cutoff of an engine, a purge gas containing the evaporated fuel desorbed from the canister is supplied to an intake passage of the engine. For example, JP2007-198210A discloses such an art. By supplying the purge gas to the intake passage during the deceleration fuel cutoff as above, the overflow of the evaporated fuel from the canister can be suppressed. Although the evaporated fuel within the purge gas supplied to the intake passage will be discharged unburned to an exhaust passage through the engine, the unburned evaporated fuel can be purified by an exhaust emission control catalyst provided in the exhaust passage. 
     Further, in JP2007-198210A, a linear O 2  sensor for detecting an oxygen concentration within exhaust gas for the purpose of performing a feedback control of an air-fuel ratio within a combustion chamber is provided upstream of the exhaust emission control catalyst, and an O 2  sensor is provided downstream of the exhaust emission control catalyst. 
     Meanwhile, the O 2  sensor located downstream of the exhaust emission control catalyst is normally for detecting whether a state of the air-fuel ratio of the exhaust gas is stoichiometric, rich, or lean. When the air-fuel ratio is stoichiometric or rich, an output value (output voltage) of the O 2  sensor indicates a first voltage (e.g., approximately 1V), and when the air-fuel ratio is lean, the output value indicates a second voltage (e.g., approximately 0V) which is lower than the first voltage. Therefore, in a situation where the exhaust gas passing through the O 2  sensor is assumed to be rich immediately before the deceleration fuel cutoff, when the deceleration fuel cutoff is performed in this situation, due to the deceleration fuel cutoff, the output value of the O 2  sensor changes from the first voltage to the second voltage in a short period of time in an early stage of the deceleration fuel cutoff. 
     Here, there is a case where an abnormality occurs in the O 2  sensor and a speed of a change of the output value of the O 2  sensor (a speed of the change from the first voltage to the second voltage) caused by the deceleration fuel cutoff becomes lower or the output value does not reduce to the second voltage. Therefore, determining whether the O 2  sensor is abnormal (performing an abnormality determination), based on the change of the output value of the O 2  sensor caused by the deceleration fuel cutoff, may be considered. For example, when the speed of the change of the output value of the O 2  sensor caused by the deceleration fuel cutoff (e.g., the changing speed between the first (high) and second (low) voltages (i.e., a changing period of time between the two voltages) is lower than a predetermined speed (longer than a predetermined period of time), the O 2  sensor is determined to be abnormal. 
     However, by supplying the purge gas to the intake passage of the engine during the deceleration fuel cutoff (performing a purge) as JP2007-198210A does, the purge is performed also during the abnormality determination, and due to the existence of the evaporated fuel within the purge gas, the speed of the change of the output value of the O 2  sensor caused by the deceleration fuel cutoff becomes lower. As a result, even if the O 2  sensor is normal, it may be falsely determined as abnormal. 
     SUMMARY 
     The present invention is made in view of the above situations and aims to suppress degradation in accuracy of an abnormality determination of an O 2  sensor of an engine due to a purge during a deceleration fuel cutoff of the engine when an abnormality of the O 2  sensor is determined based on a change of an output value of the O 2  sensor caused by the deceleration fuel cutoff. 
     According to one aspect of the present invention, a control system of an engine in which a purge gas containing evaporated fuel desorbed from a canister is supplied to an intake passage of the engine, is provided. The control system includes a deceleration fuel cutoff module for performing a deceleration fuel cutoff to stop a fuel supply from an injector to the engine when a predetermined deceleration fuel cutoff condition is satisfied in a decelerating state of the engine, a purge unit for performing a purge by supplying the purge gas to the intake passage during the deceleration fuel cutoff, an O 2  sensor provided in an exhaust passage of the engine, an abnormality determining module for performing an abnormality determination by determining an abnormality of the O 2  sensor based on a change of an output value of the O 2  sensor that is caused by the deceleration fuel cutoff, and a purge restricting module for restricting the purge during the abnormality determination. 
     With this configuration, since the purge restricting module restricts the purge during the abnormality determination (e.g., prohibits the purge, or restricts a supply amount of the purge gas to the intake passage), degradation in accuracy of the abnormality determination due to the purge can be suppressed. 
     During the abnormality determination, the purge restricting module may restrict the purge so that an air-fuel ratio within a combustion chamber of the engine exceeds a predetermined ratio. 
     Thus, when the purge is performed during the abnormality determination, the purge restricting module can restrict the purge so as not to influence a speed of the change of the output value of the O 2  sensor caused by the deceleration fuel cutoff. Further, by purging during the abnormality determination, the supply amount of the purge gas to intake passage can be secured as much as possible. 
     The control system may further include an air-fuel ratio estimating module for estimating an air-fuel ratio within a combustion chamber of the engine during the abnormality determination in a case where the purge is performed during the abnormality determination. The purge restricting module may prohibit the purge during the abnormality determination when the estimated air-fuel ratio is below a preset ratio. 
     Specifically, when the air-fuel ratio within the combustion chamber is below the preset ratio, the purge greatly influences the changing speed of the output value of the O 2  sensor caused by the deceleration fuel cutoff. However, in such a case, the purge is prohibited. Therefore, degradation in accuracy of the abnormality determination of the O 2  sensor due to the purge can securely be suppressed. 
     In the control system, the purge unit preferably includes a purge line through which the canister communicates with the intake passage, a purge valve provided in the purge line, and a purge valve controlling module for controlling a supply amount of the purge gas to the intake passage by performing a duty control of the purge valve when the purge is performed. The control system preferably further includes an evaporated fuel concentration estimating module for estimating a concentration of the evaporated fuel within the purge gas when the purge is performed during the abnormality determination. During the abnormality determination, the purge restricting module preferably restricts the supply amount of the purge gas to the intake passage based on the estimated concentration of the evaporated fuel. 
     Specifically, when the concentration of the evaporated fuel within the purge gas is high, the purge easily lowers the changing speed of the output value of the O 2  sensor caused by the deceleration fuel cutoff. However, in such a case, the purge restricting module restricts the supply amount of the purge gas to the intake passage controlled by the purge valve controlling module, based on the estimated concentration of the evaporated fuel. Therefore, the supply amount is restricted so that the changing speed is not lowered, and degradation in accuracy of the abnormality determination of the O 2  sensor can be suppressed. Further, the air-fuel ratio within the combustion chamber easily changes due to the duty control of the purge valve. However, when the purge is restricted by the air-fuel ratio as described above, by taking into consideration the change of the air-fuel ratio, the purge can more suitably be restricted. 
     When the estimated concentration of the evaporated fuel is above a predetermined concentration, the purge restricting module preferably prohibits the purge during the abnormality determination. 
     Specifically, when the concentration of the evaporated fuel is too high, the purge greatly influences the changing speed of the output value of the O 2  sensor caused by the deceleration fuel cutoff. However, in such a case, the purge restricting module prohibits the purge (i.e., the supply amount of the purge gas to the intake passage is reduced to zero). Therefore, degradation in accuracy of the abnormality determination of the O 2  sensor due to the purge can securely be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a schematic configuration of an engine controlled by a control system according to one embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a configuration of the control system of the engine. 
         FIG. 3  is a chart illustrating relationships between an air-fuel ratio within combustion chambers and an integrated weight of hydrocarbons (HC) which have passed through a downstream exhaust emission control catalyst, for cases where a concentration (learned value) of evaporated fuel indicates a high concentration, a medium concentration, and a low concentration, respectively. 
         FIG. 4  is a chart illustrating a first map indicating a relationship between the learned value of the concentration of the evaporated fuel and a target air-fuel ratio (A/F). 
         FIG. 5  shows time charts illustrating changes of an output value of an O 2  sensor (changes in a normal state and an abnormal state) when a deceleration fuel cutoff is performed in a state where the output value of the O 2  sensor is the first voltage during a normal operation of the engine. 
         FIG. 6  is a chart illustrating relationships between the air-fuel ratio within the combustion chambers of the engine and a period of time to required for the output value of the O 2  sensor to reach from a first voltage threshold to a second voltage threshold during an abnormality determination in a case where the purge is performed during the abnormality determination, for cases where the concentration (learned value) of the evaporated fuel indicates the high concentration, the medium concentration, and the low concentration, respectively. 
         FIG. 7  is a chart illustrating relationships between the air-fuel ratio within the combustion chambers of the engine and a period of time tb required for the output value of the O 2  sensor to reach a third voltage threshold from a start of the deceleration fuel cutoff during the abnormality determination in the case where the purge is performed during the abnormality determination, for cases where the concentration (learned value) of the evaporated fuel indicates the high concentration, the medium concentration, and the low concentration, respectively. 
         FIG. 8  is a flowchart illustrating a processing operation regarding the purge, performed by the control system. 
         FIG. 9  is a flowchart illustrating a processing operation of a deceleration-fuel-cutoff purge valve control by the control system. 
         FIG. 10  is a flowchart illustrating a first example of a processing operation of the abnormality determination of the O 2  sensor by the control system. 
         FIG. 11  is a flowchart illustrating a second example of a processing operation of the abnormality determination of the O 2  sensor by the control system. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT 
     Hereinafter, one embodiment of the present invention is described in detail with reference to the appended drawings. 
       FIG. 1  is a view illustrating a schematic configuration of an engine  1  controlled by a control system  100  (see  FIG. 2 ) according to one embodiment of the present invention. The engine  1  is a gasoline engine mounted on a vehicle and having a turbocharger. The engine  1  includes a cylinder block  3  where a plurality of cylinders  2  (only one cylinder is illustrated in  FIG. 1 ) are arranged in a line, and a cylinder head  4  disposed on the cylinder block  3 . A piston  5  defining a combustion chamber  6  together with the cylinder head  4  therebetween is reciprocatably fitted into each of the cylinders  2  of the engine  1 . The piston  5  is coupled to a crankshaft (not illustrated) through a connecting rod  7 . To the crankshaft, a detecting plate  8  for detecting a rotational angular position of the crankshaft is fixed to integrally rotate therewith, and an engine speed sensor  9  for detecting a rotational angular position of the detecting plate  8  to detect a speed of the engine  1  is provided. 
     In the cylinder head  4 , an intake port  12  and an exhaust port  13  are formed for each cylinder  2 , and an intake valve  14  for opening and closing the intake port  12  on the combustion chamber  6  side and an exhaust valve  15  for opening and closing the exhaust port  13  on the combustion chamber  6  side are provided for each cylinder  2 . Each intake valve  14  is driven by an intake valve drive mechanism  16 , and each exhaust valve  15  is driven by an exhaust valve drive mechanism  17 . The intake valve  14  reciprocates at a predetermined timing by the intake valve drive mechanism  16  to open and close the intake port  12 , the exhaust valve  15  reciprocates at a predetermined timing by the exhaust valve drive mechanism  17  to open and close the exhaust port  13 , and thus, gas inside the cylinder  2  is exchanged. The intake and exhaust valve drive mechanisms  16  and  17  have an intake camshaft  16   a  and an exhaust camshaft  17   a  which are coupled to the crankshaft to be drivable, respectively. The camshafts  16   a  and  17   a  rotate in synchronization with the rotation of the crankshaft. Moreover, the intake valve drive mechanism  16  includes a hydraulically/mechanically-driven phase variable mechanism (Variable Valve Timing: VVT) for varying a phase of the intake camshaft  16   a  within a predetermined angle range. 
     An injector  18  for injecting fuel (in this embodiment, gasoline) is provided in an upper (cylinder head  4  side) end part of the cylinder block  3 , for each cylinder  2 . The injector  18  is disposed such that a fuel injection port thereof is oriented toward an inside of the combustion chamber  6 , and directly injects the fuel into the combustion chamber  6  near a top dead center of compression stroke (CTDC). Note that the injectors  18  may be provided to the cylinder head  4 . 
     The injectors  18  are connected to a fuel tank  22  via a fuel supply tube  21 . Inside the fuel tank  22 , a fuel pump  23  is disposed to be submerged in the fuel. The fuel pump  23  has a suction tube  23   a  for sucking the fuel, and a discharge tube  23   b  for discharging the sucked fuel. The suction tube  23   a  has a strainer  24  at its tip. The discharge tube  23   b  is connected to the injectors  18  via a regulator  25 . The fuel pump  23  sucks the fuel with the suction tube  23   a  and then discharges the fuel with the discharge tube  23   b  for a pressure adjustment at the regulator  25 , so as to send the fuel to the injectors  18 . Specifically, the fuel supply tube  21  is connected to a fuel distribution tube (not illustrated) extending in a cylinder row direction; the fuel distribution tube is connected to the injectors  18  of the respective cylinders  2 , and thus, the fuel from the fuel pump  23  is distributed to the injectors  18  of the respective cylinders  2  by the fuel distribution tube. 
     Inside the cylinder head  4 , an ignition plug  19  is disposed for each cylinder  2 . A tip part (electrode) of the ignition plug  19  is located near a ceiling of the combustion chamber  6 . Further, the ignition plug  19  produces a spark at a predetermined ignition timing, and thus mixture gas of the fuel and air is combusted in response to the spark. 
     On one side surface of the engine  1 , an intake passage  30  is connected to communicate with the intake ports  12  of the cylinders  2 . An air cleaner  31  for filtrating intake air is disposed in an upstream end part of the intake passage  30 , and the intake air filtered by the air cleaner  31  is supplied to the combustion chambers  6  of the respective cylinders  2  via the intake passage  30  and the intake ports  12 . 
     An airflow sensor  32  for detecting a flow rate of the intake air sucked into the intake passage  30  is disposed at a position of the intake passage  30  near the downstream side of the air cleaner  31 . Further, a surge tank  34  is disposed near a downstream end of the intake passage  30 . Part of the intake passage  30  downstream of the surge tank  34  is branched into independent passages extending toward the respective cylinders  2 , and downstream ends of the independent passages are connected to the intake ports  12  of the cylinders  2 , respectively. A pressure sensor  35  for detecting pressure inside the surge tank  34  is disposed in the surge tank  34 . 
     Moreover, in the intake passage  30 , a compressor  50   a  of a turbocharger  50  is disposed between the airflow sensor  32  and the surge tank  34 . The intake air is turbocharged by the compressor  50   a  in operation. 
     Furthermore, in the intake passage  30 , an intercooler  36  for cooling air compressed by the compressor  50   a , and a throttle valve  37  are arranged between the compressor  50   a  of the turbocharger  50  and the surge tank  34  in this order from the upstream side. The throttle valve  37  is driven by a drive motor  37   a  to change a cross-sectional area of the intake passage  30  at the disposed position of the throttle valve  37 , so as to adjust an amount of intake air flowing into the combustion chambers  6  of the respective cylinders  2 . An opening of the throttle valve  37  is detected by a throttle opening sensor  37   b.    
     Additionally in this embodiment, an intake bypass passage  38  for bypassing the compressor  50   a  is provided to the intake passage  30 , and an air bypass valve  39  is provided in the intake bypass passage  38 . The air bypass valve  39  is normally fully closed, but for example when the opening of the throttle valve  37  is sharply reduced, a sharp increase and sharp surging of pressure occur in the part of the intake passage  30  upstream of the throttle valve  37  and the rotation of the compressor  50   a  is disturbed, which results in causing a loud noise; therefore the air bypass valve  39  is opened to prevent such a situation. 
     On the other side surface of the engine  1 , an exhaust passage  40  is connected to discharge exhaust gas from the combustion chambers  6  of the cylinders  2 . An upstream part of the exhaust passage  40  is comprised of an exhaust manifold having independent passages extending to the respective cylinders  2  and connected to respective external ends of the exhaust ports  13  of the cylinders  2 , and a manifold section where the respective independent passages are collected together. A turbine  50   b  of the turbocharger  50  is disposed in part of the exhaust passage  40  downstream of the exhaust manifold. The turbine  50   b  is rotated by the flow of the exhaust gas, and the compressor  50   a  coupled to the turbine  50   b  is operated by the rotation of the turbine  50   b.    
     Part of the exhaust passage  40  which is downstream of the exhaust manifold and upstream of the turbine  50   b  is branched into a first passage  41  and a second passage  42 . A flow rate changing valve  43  for changing a flow rate of the exhaust gas flowing toward the turbine  50   b  is provided in the first passage  41 . The second passage  42  merges with the first passage  41  at a position downstream of the flow rate changing valve  43  and upstream of the turbine  50   b.    
     Further, an exhaust bypass passage  46  for guiding the exhaust gas of the engine  1  to flow while bypassing the turbine  50   b  is provided in the exhaust passage  40 . An end part of the exhaust bypass passage  46  on the flow-in side of the exhaust gas (an upstream end part of the exhaust bypass passage  46 ) is connected to a position of the exhaust passage  40  between the merging section of the first and second passages  41  and  42  in the exhaust passage  40  and the turbine  50   b . An end part of the exhaust bypass passage  46  on the flow-out side of the exhaust gas (a downstream end part of the exhaust bypass passage  46 ) is connected to a position of the exhaust passage  40  downstream of the turbine  50   b  and upstream of an upstream exhaust emission control catalyst  52  (described later). 
     The end part of the exhaust bypass passage  46  on the flow-in side of the exhaust gas is provided with a wastegate valve  47  that is driven by a drive motor  47   a . The wastegate valve  47  is controlled by the control system  100  according to an operating state of the engine  1 . When the wastegate valve  47  is fully closed, the entire amount of exhaust gas flows to the turbine  50   b , and when the wastegate valve  47  is not fully closed, the flow rate of the exhaust gas to the exhaust bypass passage  46  (i.e., the flow rate of the exhaust gas to the turbine  50   b ) changes according to the opening of the wastegate valve  47 . In other words, as the opening of the wastegate valve  47  becomes larger, the flow rate of the exhaust gas to the exhaust bypass passage  46  becomes higher and the flow rate of the exhaust gas to the turbine  50   b  becomes lower. When the wastegate valve  47  is fully opened, the turbocharger  50  substantially does not operate. 
     Part of the exhaust passage  40  downstream of the turbine  50   b  (downstream of the position connected to the downstream end part of the exhaust bypass passage  46 ) is provided with exhaust emission control catalysts  52  and  53  constructed with an oxidation catalyst, etc., and for purifying hazardous components contained within the exhaust gas (and unburned evaporated fuel during a deceleration fuel cutoff described later). In this embodiment, the two exhaust emission control catalysts of the upstream exhaust emission control catalyst  52  and the downstream exhaust emission control catalyst  53  are provided. However, just the upstream exhaust emission control catalyst  52  may be provided instead. 
     In the exhaust passage  40 , a linear O 2  sensor  55  having an output property which is linear with respect to an oxygen concentration within the exhaust gas is disposed near the upstream side of the upstream exhaust emission control catalyst  52 . The linear O 2  sensor  55  is an air-fuel ratio sensor for detecting the oxygen concentration within the exhaust gas for the purpose of performing a feedback control of an air-fuel ratio within the combustion chambers  6 . Further in the exhaust passage  40 , an O 2  sensor  56  for detecting a state of the air-fuel ratio of the exhaust gas which has passed through the upstream exhaust emission control catalyst  52  among stoichiometric, rich or lean is disposed between the upstream and downstream exhaust emission control catalysts  52  and  53 . In this embodiment, when the air-fuel ratio is stoichiometric or rich, an output value (output voltage) of the O 2  sensor  56  indicates a first voltage (e.g., approximately 1V), and when the air-fuel ratio is lean, the output value indicates a second voltage (e.g., approximately 0V) which is lower than the first voltage. 
     The engine  1  includes an EGR passage  60  for recirculating part of the exhaust gas from the exhaust passage  40  to the intake passage  30 . The EGR passage  60  connects the part of the exhaust passage  40  upstream of the branched section of the first and second passages  41  and  42  to the independent passages of the intake passage  30  downstream of the surge tank  34 . An EGR cooler  61  for cooling the exhaust gas passing therethrough and an EGR valve  62  for adjusting an amount of the exhaust gas recirculated by the EGR passage  60  are disposed in the EGR passage  60 . 
     The engine  1  also includes first and second ventilation hoses  65  and  66  for returning back to the intake passage  30  blow-by gas leaked from the combustion chambers  6 . The first ventilation hose  65  connects a lower part (crank case) of the cylinder block  3  to the surge tank  34 , and the second ventilation hose  66  connects an upper part of the cylinder head  4  to part of the intake passage  30  between the air cleaner  31  and the compressor  50   a.    
     The fuel tank  22  is connected to a canister  70  containing an adsorbent (e.g., activated charcoal) therein, via a connecting tube  71 . Fuel evaporated inside the fuel tank  22  flows to the canister  70  via the connecting tube  71  and is trapped by the canister  70  (adsorbent). An inside of the canister  70  communicates with ambient air via an ambient air communicating tube  72 . 
     The canister  70  is connected to the intake passage  30  via a purge tube  73  (purge line). In this embodiment, an end part of the purge tube  73  on the intake passage  30  side is connected to the surge tank  34  provided downstream of the compressor  50   a  in the intake passage  30 . 
     The purge tube  73  is provided with a purge valve  75 . When the purge valve  75  is opened and the pressure inside the surge tank  34  is negative (i.e., when the intake air is not turbocharged by the compressor  50   a  of the turbocharger  50 ), the ambient air (air) is introduced into the ambient air communicating tube  72 , the evaporated fuel trapped in the canister  70  is desorbed therefrom by the flow of the air, and then the desorbed evaporated fuel is supplied along with the air as purge gas, to the surge tank  34  (a purge is performed). A supply amount (or a supply flow rate) of the purge gas to the surge tank  34  (intake passage  30 ) is determined based on an opening of the purge valve  75  and a pressure difference Pd between the pressure inside the surge tank  34  (the pressure detected by the pressure sensor  35 ) and atmospheric pressure (pressure detected by an atmospheric pressure sensor  91  described later). 
     As illustrated in  FIG. 2 , operations of the throttle valve  37  (specifically, the drive motor  37   a ), the injectors  18 , the ignition plugs  19 , the purge valve  75 , the flow rate changing valve  43 , the wastegate valve  47  (specifically, the drive motor  47   a ), the EGR valve  62 , and the air bypass valve  39  are controlled by the control system  100 . The control system  100  is a controller based on a well-known microcomputer, and includes a central processing unit (CPU) for executing program(s), a memory  90  comprised of, for example, a RAM and/or a ROM and for storing the program(s) and data, and an input/output (I/O) bus for inputting and outputting electric signals ( FIG. 2  only illustrates the memory  90  thereamong). 
     The control system  100  receives signals indicating output values of various sensors including the airflow sensor  32 , the throttle opening sensor  37   b , an accelerator opening sensor  92  for detecting a stepping amount of an acceleration pedal (accelerator opening) by a driver of the vehicle on which the engine  1  is mounted, the linear O 2  sensor  55 , the O 2  sensor  56 , the pressure sensor  35 , and the engine speed sensor  9 . In this embodiment, the control system  100  is provided with the atmospheric pressure sensor  91  for detecting the atmospheric pressure. The control system  100  controls the operations of the valves described above, based on the output values of the various sensors. Specifically, the operation control of the injectors  18  (fuel injection control) is performed by a fuel injection controlling module  100   a  of the control system  100 , the operation control of the ignition plugs  19  is performed by an ignition controlling module  100   b  of the control system  100 , and the operation control of the purge valve  75  (opening control, i.e., the control of the supply amount of the purge gas to the surge tank  34 ) is performed by one of a normal-operation purge valve controlling module  100   c  and a deceleration-fuel-cutoff purge valve controlling module  100   d  of the control system  100 . Note that the operation control of the purge valve  75  by one of the normal-operation purge valve controlling module  100   c  and the deceleration-fuel-cutoff purge valve controlling module  100   d  of the control system  100  is performed through a control of a duty ratio of a control signal transmitted to the purge valve  75  (a duty control of the purge valve  75 ). 
     The control system  100  also includes a deceleration-fuel-cutoff controlling module  100   e  (deceleration fuel cutoff module), an evaporated fuel concentration estimating module  100   f , an abnormality determining module  100   g , a purge restricting module  100   h , and an air-fuel ratio estimating module  100   i , which are described later in detail. 
     When a predetermined deceleration fuel cutoff condition is satisfied while the engine  1  is in a decelerating state, the deceleration-fuel-cutoff controlling module  100   e  performs a deceleration fuel cutoff to stop the fuel supply from the injectors  18  to the engine  1 . The predetermined deceleration fuel cutoff condition is, for example, a condition in which the opening of the throttle valve  37  is detected by the throttle opening sensor  37   b  to be fully closed and the speed of the engine  1  is detected by engine speed sensor  9  to be above a predetermined speed (slightly above an idling speed). During the deceleration fuel cutoff, the injectors  18  and the ignition plugs  19  are not operated. 
     During the deceleration fuel cutoff, the deceleration-fuel-cutoff purge valve controlling module  100   d  controls the operation of the purge valve  75  (the supply amount of the purge gas to the surge tank  34 ). Specifically, the purge by supplying the purge gas to the surge tank  34  is performed during a normal operation of the engine  1  (operation in which the fuel is injected by the injectors  18  and the injected fuel is ignited by the ignition plugs  19 ) and also during the deceleration fuel cutoff. The operation control of the purge valve  75  during the deceleration fuel cutoff is described later. In this embodiment, the purge tube  73  (purge line), the purge valve  75 , and the deceleration-fuel-cutoff purge valve controlling module  100   d  (purge valve controlling module) constitute a purge unit for purging by supplying the purge gas to the intake passage  30  of the engine  1  during the deceleration fuel cutoff. 
     On the other hand, during the normal operation of the engine  1  (other than the deceleration fuel cutoff), the normal-operation purge valve controlling module  100   c  controls the operation of the purge valve  75  according to the operating state of the engine  1 . In this embodiment, when the engine  1  is in an operating state where the turbocharger  50  is operated to turbocharge the intake air, since the pressure inside the surge tank  34  is not negative, the normal-operation purge valve controlling module  100   c  fully closes the purge valve  75 , and when the engine  1  is in an operating state where the turbocharger  50  is not operated, the normal-operation purge valve controlling module  100   c  performs the purge. 
     When the purge is performed during the normal operation of the engine  1 , the evaporated fuel concentration estimating module  100   f  learns by estimation a concentration of the evaporated fuel within the purge gas based on a feedback correction amount of the air-fuel ratio obtained based on the output value of the linear O 2  sensor  55 , and the evaporated fuel concentration estimating module  100   f  stores (updates) the learned value of the concentration of the evaporated fuel in the memory  90 . The fuel injection controlling module  100   a  corrects the fuel injection amount based on the feedback correction amount and the learned value. 
     In other words, a shift of the air-fuel ratio within the combustion chambers  6  caused by supplying the purge gas (evaporated fuel) to the surge tank  34  of the intake passage  30  is detected by the linear O 2  sensor  55 . The fuel injection controlling module  100   a  performs the feedback correction of the air-fuel ratio (i.e., fuel injection amount) based on the detected value (output value), and corrects the fuel injection amount according to the learned value of the concentration of the evaporated fuel, so as to compensate for a response lag of the feedback correction. 
     In this embodiment, the evaporated fuel concentration estimating module  100   f  estimates the concentration of the evaporated fuel within the purge gas when the purge is performed during the deceleration fuel cutoff, to be the learned value immediately before the deceleration fuel cutoff (the latest learned value stored in the memory  90 ). Even in this manner, a period of time for which the deceleration fuel cutoff is performed continuously is comparatively short and a possibility of the concentration of the evaporated fuel greatly changing during the time period is low, therefore no problem will occur. 
     The deceleration-fuel-cutoff purge valve controlling module  100   d  first calculates a target air-fuel ratio (target A/F) when the purge is performed during the deceleration fuel cutoff.  FIG. 3  is a chart illustrating relationships between the air-fuel ratio within the combustion chambers  6  and an integrated weight of HC which has passed through the downstream exhaust emission control catalyst  53 , for cases where the concentration (learned value) of the evaporated fuel indicates a high concentration, a medium concentration, and a low concentration, respectively. From  FIG. 3 , it can be understood that at each concentration, the integrated weight of HC is reduced as the air-fuel ratio becomes higher, and when the air-fuel ratio exceeds a certain ratio, the integrated weight of HC becomes 0 (zero). Therefore, the target A/F may be set to be a ratio equal to or larger than a smallest air-fuel ratio at which the integrated weight of HC becomes 0 at each concentration (preferably be a ratio equal or close to the smallest air-fuel ratio, in view of increasing the supply amount of the purge gas to the surge tank  34  as much as possible when the purge is performed). The relationship between the learned value and the target A/F is stored in the memory  90  in advance in a form of a first map as illustrated in  FIG. 4 , and by using the first map, the target A/F is calculated based on the learned value obtained immediately before the deceleration fuel cutoff. Note that in the first map, the target A/F is not set for when the learned value indicates a concentration higher than a preset concentration C (the hatched section in  FIG. 4 ), in other words, when the learned value indicates a concentration high enough that the evaporated fuel cannot suitably be purified by the exhaust emission control catalysts  52  and  53 . In this case, the deceleration-fuel-cutoff purge valve controlling module  100   d  does not perform the purge (i.e., it fully closes the purge valve  75 ) during the deceleration fuel cutoff. 
     Further, a mass ratio ra of the evaporated fuel with respect to the entirety of the purge gas is calculated based on the learned value. A total air mass qa sucked into the combustion chambers  6  and discharged to the exhaust passage  40  when the purge is performed during the deceleration fuel cutoff is calculated based on the output value of the airflow sensor  32 , the mass ratio ra, and the output value of the linear O 2  sensor  55 . 
     When a mass of the evaporated fuel inside the combustion chambers  6  (same as the mass of the evaporated fuel within the purge gas) is “ggas,”
 
target  A/F=qa/g gas.
 
Based on such a relationship,
 
 g gas= qa /(target  A/F ).
 
     The mass ggas of the evaporated fuel inside the combustion chambers  6  is calculated by substituting the calculated values of the target A/F and the total air mass qa into this equation. 
     Further, when a mass of air within the purge gas is “gair,”
 
(1− ra ): ra=g air: g gas.
 
Thus,
 
 g air =g gas×(1− ra ) /ra.  
 
Based on this equation, the mass gair of the air within the purge gas is calculated.
 
     When a total mass of the evaporated fuel and the air within the purge gas is “gprg,”
 
 gprg=g gas+ g air.
 
     A purge gas volume qprg corresponding to the total mass gprg converted into volume is, with a density of the purge gas as cp,
 
 qprg=gprg×cp.  
 
     Note that a value corresponding to the mass ratio ra of the evaporated fuel with respect to the entirety of the purge gas is stored in the memory  90  in advance as the density cp of the purge gas. 
     The deceleration-fuel-cutoff purge valve controlling module  100   d  controls the supply amount of the purge gas to the surge tank  34  (the opening of the purge valve  75 ) when the purge is performed during the deceleration fuel cutoff, based on the purge gas volume qprg and the pressure difference Pd. 
     The abnormality determining module  100   g  determines whether the O 2  sensor  56  is abnormal (performs an abnormality determination) based on a change of the output value of the O 2  sensor  56  (specifically, a response time of the output value of the O 2  sensor  56 ) caused by the deceleration fuel cutoff performed by the deceleration fuel cutoff controlling module  100   e.    
     As illustrated in  FIG. 5 , when the deceleration fuel cutoff is performed while the engine  1  is in the normal operation and the output value of the O 2  sensor  56  is the first voltage, due to the deceleration fuel cutoff, the output value of the O 2  sensor  56  changes from the first voltage to the second voltage in a short period of time in an early stage of the deceleration fuel cutoff, as indicated by the solid line (described as “NORMAL”). When an abnormality occurs in the O 2  sensor  56  and the responsiveness degrades, the speed of the change of the output value of the O 2  sensor  56  caused by the deceleration fuel cutoff becomes lower (the response time of the output value of the O 2  sensor  56  becomes longer) as indicated by the dashed line (described as “ABNORMAL 1”). Based on this, the abnormality determining module  100   g  performs the abnormality determination. 
     In a first example of the abnormality determination, in the changing process of the output value of the O 2  sensor  56  caused by the deceleration fuel cutoff, by having as the response time a period of time ta required for the output value of the O 2  sensor  56  to reach from a first voltage threshold V 1  to a second voltage threshold V 2  (in the example of  FIG. 5 , ta=t 1  in the normal state, and ta=t 2  in the abnormal state), the abnormality determining module  100   g  determines that the O 2  sensor  56  is normal if the time period ta is shorter than a first predetermined period of time ta 0 , whereas the abnormality determining module  100   g  determines that the O 2  sensor  56  is abnormal if the time period ta is the first predetermined time period ta 0  or longer. Here, the first voltage threshold V 1  is set between the first and second voltages (when the first voltage is 1V and the second voltage is 0V, the first voltage threshold V 1  is 0.55V, for example), and the second voltage threshold V 2  is between the first and second voltages and below the first voltage threshold V 1  (when the first voltage is 1V and the second voltage is 0V, the second voltage threshold V 2  is 0.2V, for example). The first predetermined time period ta 0  is set between the time period t 1  and the time period t 2 . 
     A case where the output value of the O 2  sensor  56  does not drop to the second voltage threshold V 2  due to the abnormality of the O 2  sensor  56  as indicated by the one-dotted chain line (described as “ABNORMAL 2”) in  FIG. 5 , and a case where the output value of the O 2  sensor  56  drops to the second voltage threshold V 2  after a significantly long period of time from the start of the deceleration fuel cutoff, can be considered. Therefore, in a second example of the abnormality determination, the response time is a period of time tb from the start of the deceleration fuel cutoff until the output value of the O 2  sensor  56  reaches a third voltage threshold V 3  (a voltage between the first and second voltages and equal or close to the second voltage threshold V 2 ). When the output value of the O 2  sensor  56  reaches the third voltage threshold V 3  within a second predetermined period of time tb 0  from the start of the deceleration fuel cutoff, the abnormality determining module  100   g  determines that the O 2  sensor  56  is normal, whereas when the output value of the O 2  sensor  56  does not reach the third voltage threshold V 3  within the second predetermined time period tb 0  from the start of the deceleration fuel cutoff (i.e., the response time is the second predetermined time period tb 0  or longer), the abnormality determining module  100   g  determines that the O 2  sensor  56  is abnormal. 
     The first and second predetermined time periods, ta 0  and tb 0  in the first and second examples, are set in advance under a condition in which the purge is not performed during the abnormality determination. However, when the purge is performed during the abnormality determination, the changing speed of the output value of the O 2  sensor  56  becomes lower (the response time of the output value of the O 2  sensor  56  becomes longer) due to the existence of the evaporated fuel within the purge gas. As a result, even if the O 2  sensor  56  is normal, it may be falsely determined as abnormal. 
     Therefore, the purge restricting module  100   h  restricts the purge performed by the deceleration-fuel-cutoff purge valve controlling module  100   d  during the abnormality determination performed by the abnormality determining module  100   g , so as to suppress the false determination. 
       FIG. 6  illustrates relationships between the air-fuel ratio within the combustion chambers  6  of the engine  1  and the time period ta during the abnormality determination in the case where the purge is performed during the abnormality determination, for cases where the concentration (learned value) of evaporated fuel indicates the high concentration, the medium concentration, and the low concentration, respectively. Further,  FIG. 7  illustrates relationships between the air-fuel ratio within the combustion chambers  6  of the engine  1  and the time period tb required for the output value of the O 2  sensor  56  to reach the third voltage threshold V 3  from the start of the deceleration fuel cutoff during the abnormality determination in the case where the purge is performed during the abnormality determination, for cases where the concentration (learned value) of evaporated fuel indicates the high concentration, the medium concentration, and the low concentration, respectively. 
     Based on  FIGS. 6 and 7 , it can be understood that at each concentration, the time period ta and the time period tb greatly increase once the air-fuel ratio falls below certain ratios (the air-fuel ratios indicated by the star-shaped symbols), respectively. Therefore, in the case of purging during the abnormality determination, by setting the target A/F at each concentration during the abnormality determination to be equal to or larger than the air-fuel ratios indicated by the star-shaped symbols in  FIGS. 6 and 7 , the purge hardly influences the speed of the change of the output value of the O 2  sensor  56  caused by the deceleration fuel cutoff. Specifically, the time period ta and the time period tb in the case where the purge is not performed during the abnormality determination are values indicated by the “NO PURGE” lines in  FIGS. 6 and 7 , respectively, and by setting the target A/F during the abnormality determination to be equal to or larger than the air-fuel ratios indicated by the star-shaped symbols, the time period ta and the time period tb in the case where the purge is performed during the abnormality determination have no significant difference from those in the case where the purge is not performed during the abnormality determination. In view of increasing the supply amount of the purge gas to the surge tank  34  as much as possible, the target A/F during the abnormality determination is preferably equal or close to the air-fuel ratios indicated by the star-shaped symbols. In this embodiment, since the air-fuel ratio within the combustion chambers  6  changes due to the duty control of the purge valve  75 , the target A/F during the abnormality determination is preferably an air-fuel ratio determined by taking into consideration a change amount of the air-fuel ratio caused by the duty control, based on the air-fuel ratio indicated by the star-shaped symbol (an air-fuel ratio obtained by adding, to the air-fuel ratio indicated by the star-shaped symbol, a difference between an average value and a minimum value of the changed air-fuel ratios caused by the duty control). 
     The relationship between the learned value and the target A/F during the abnormality determination is stored in the memory  90  in advance in the form of a second map (a map in which the target A/F becomes higher as the concentration of the evaporated fuel becomes higher, similar to the first map). In the case of purging during the abnormality determination, the purge restricting module  100   h  calculates the target A/F for during the abnormality determination based on the learned value obtained immediately before the deceleration fuel cutoff by using the second map. With the same learned value, the target A/F during the abnormality determination becomes larger than the target A/F calculated based on the first map in  FIG. 4  (the target A/F for other than during the abnormality determination). Further, the purge restricting module  100   h  calculates the purge gas volume qprg based on the calculated target A/F for during the abnormality determination in a manner similar to the manner in which the deceleration-fuel-cutoff purge valve controlling module  100   d  calculates the purge gas volume qprg, and the purge restricting module  100   h  then controls the supply amount of the purge gas to the surge tank  34  (the opening of the purge valve  75 ) based on the purge gas volume qprg and the pressure difference Pd. Thus, the air-fuel ratio within the combustion chambers  6  of the engine  1  exceeds a predetermined ratio (the air-fuel ratio equal or close to the air-fuel ratios indicated by the star-shaped symbols in  FIGS. 6 and 7 ) so that the time periods to and tb do not significantly increase. Therefore, the purge restricting module  100   h  restricts the purge so that the air-fuel ratio within the combustion chambers  6  of the engine  1  exceeds the predetermined ratio. 
     As described above, the evaporated fuel concentration estimating module  100   f  estimates the concentration of the evaporated fuel within the purge gas while the purge is performed during the deceleration fuel cutoff, to be the learned value immediately before the deceleration fuel cutoff (the latest learned value stored in the memory  90 ). Therefore, the concentration of the evaporated fuel within the purge gas when the purge is performed during the abnormality determination is also estimated to be the learned value immediately before the deceleration fuel cutoff. As described above, the purge gas volume qprg calculated by the purge restricting module  100   h  is based on the estimated value (learned value) of the concentration of the evaporated fuel within the purge gas by the evaporated fuel concentration estimating module  100   f . Therefore, the purge restricting module  100   h  restricts the supply amount of the purge gas to the surge tank  34  controlled by the deceleration-fuel-cutoff purge valve controlling module  100   d , based on the concentration of the evaporated fuel estimated by the evaporated fuel concentration estimating module  100   f.    
     Also in the second map used by the purge restricting module  100   h , similar to the first map ( FIG. 4 ), the target A/F is not set when the learned value indicates a concentration higher than a predetermined concentration, in other words, when the purge greatly influences the change of the output value of the O 2  sensor  56  which is caused by the deceleration fuel cutoff, and in such a case, the purge restricting module  100   h  prohibits the purge. 
     In this embodiment, as described above, the purge restricting module  100   h  restricts the purge based on the concentration of the evaporated fuel estimated by the evaporated fuel concentration estimating module  100   f , so that the air-fuel ratio within the combustion chambers  6  of the engine  1  during the abnormality determination exceeds the predetermined ratio; however, the air-fuel ratio estimating module  100   i  may estimate the air-fuel ratio within the combustion chambers  6  of the engine  1  during the abnormality determination in the case where the purge is performed during the abnormality determination, and when the estimated air-fuel ratio is below a preset ratio, the purge restricting module  100   h  may prohibit the purge during the abnormality determination. 
     In this case, the air-fuel ratio estimating module  100   i  estimates the air-fuel ratio within the combustion chambers  6  of the engine  1  during the abnormality determination in the case where the purge is performed during the abnormality determination, to be the target A/F calculated based on the first map used by the deceleration-fuel-cutoff purge valve controlling module  100   d . Note that also here, by taking into consideration the change amount of the air-fuel ratio caused by the duty control, the air-fuel ratio within the combustion chambers  6  is preferably estimated to be an air-fuel ratio obtained by subtracting, from the target A/F calculated based on the first map, a difference between an average value and a minimum value of the changed air-fuel ratios caused by the duty control. The preset ratio is set so that the time periods to and tb significantly increase if the air-fuel ratio within the combustion chambers  6  falls below the preset ratio. 
     Next, the processing operation regarding the purge performed by the control system  100  is described with reference to the flowchart in  FIG. 8 . 
     First at S 1 , the operating state of the engine  1  is read, and then at S 2 , whether the deceleration fuel cutoff condition is satisfied or not is determined. 
     If the determination result of S 2  is positive, the operation proceeds to S 3  where the deceleration-fuel-cutoff purge valve control (the control of the purge valve  75  by the deceleration-fuel-cutoff purge valve controlling module  100   d ) is performed, then returns to the start of the operation. 
     On the other hand, if the determination result of S 2  is negative, the operation proceeds to S 4  where the normal-operation purge valve control (the control of the purge valve  75  by the normal-operation purge valve controlling module  100   c ) is performed, then returns to the start of the operation. 
     The processing operation of the deceleration-fuel-cutoff purge valve control at S 3  is described in more detail with reference to the flowchart in  FIG. 9 . 
     First at S 11 , the learned value of the concentration of the evaporated fuel is read from the memory  90 , the mass ratio ra of the evaporated fuel with respect to the entire purge gas is calculated based on the learned value, and the total air mass qa sucked into the combustion chambers  6  is calculated based on the output value of the airflow sensor  32 , the mass ratio ra, and the output value of the linear O 2  sensor  55 . Further, the density cp corresponding to the mass ratio ra is read from the memory  90 , and the pressure difference Pd between the detected pressure by the pressure sensor  35  and the detected pressure by the atmospheric pressure sensor  91  is calculated. 
     Next at S 12 , whether a purge stop condition is satisfied or not is determined. The purge stop condition is, for example, a condition in which temperatures of the exhaust emission control catalysts  52  and  53  fall below predetermined temperatures when the purge is performed. The predetermined temperatures are set so that purifying performances of the exhaust emission control catalysts  52  and  53  significantly degrade when falling below the predetermined temperatures, respectively (e.g., they are equal or close to activation temperatures of the exhaust emission control catalysts  52  and  53 ). The temperatures of the exhaust emission control catalysts  52  and  53  may be detected by temperature sensors or estimated when the purge is performed. 
     If the determination result of S 12  is positive, the operation proceeds to S 13  where the purge valve  75  is fully closed, then returns to the start of the operation. 
     On the other hand, if the determination result of S 12  is negative, the operation proceeds to S 14  where whether the abnormality determination of the O 2  sensor  56  is performed or not is determined. 
     If the determination result of S 14  is negative, the operation proceeds to S 15  where the target A/F (the target A/F for other than during the abnormality determination) is calculated based on the learned value by using the first map. Here, if the learned value indicates a concentration above the preset concentration C (the hatched section in  FIG. 4 ), the purge is not performed (the purge valve  75  is fully closed). Then the operation proceeds to S 17 . 
     On the other hand, if the determination result of S 14  is positive, the operation proceeds to S 16  where the target A/F (the target A/F during the abnormality determination) is calculated based on the learned value by using the second map. Here, if the learned value indicates a concentration above the predetermined concentration, the purge is not performed (the purge valve  75  is fully closed). Then the operation proceeds to S 17 . 
     At S 17 , the purge gas volume qprg is calculated based on the target A/F set at one of S 15  and S 16 , the mass ratio ra, the total air mass qa, and the density cp, the opening of the purge valve  75  (the duty ratio described above) is calculated based on the purge gas volume qprg and the pressure difference Pd, and the purge valve  75  is controlled to have the calculated opening. Then, the operation returns to the start of the operation. 
     The processing at S 16  to which the operation proceeds when the determination result at S 14  is positive, and the processing at S 17  which follows S 16 , are performed by the purge restricting module  100   h  to restrict the purge so that the air-fuel ratio within the combustion chambers  6  of the engine  1  exceeds the predetermined ratio during the abnormality determination. 
     Next, the first example of the processing operation of the abnormality determination of the O 2  sensor  56  by the control system  100  (abnormality determining module  100   g ) is described with reference to the flowchart in  FIG. 10 . 
     First at S 31 , whether an abnormality determining condition for performing the abnormality determination is satisfied or not is determined. The abnormality determining condition is a condition in which the deceleration fuel cutoff is not performed and the output value of the O 2  sensor  56  is above the first voltage threshold V 1 . 
     If the determination result of S 31  is negative, the determination at S 31  is repeated, whereas if the determination result of S 31  is positive, the operation proceeds to S 32  where whether the operation of the engine  1  is shifted to the deceleration fuel cutoff is determined. 
     If the determination result of S 32  is negative, the operation returns to S 31 , whereas if the determination result of S 32  is positive, the operation proceeds to S 33  where whether the output value of the O 2  sensor  56  has reached the first voltage threshold V 1  or not is determined. 
     If the determination result of S 33  is negative, the determination at S 33  is repeated, whereas if the determination result of S 33  is positive, the operation proceeds to S 34  where a timer count is started. 
     Next, at S 35 , whether the output value of the O 2  sensor  56  has reached the second voltage threshold V 2  or not is determined. If the determination result of S 35  is negative, the operation returns to S 34 , whereas if the determination result of S 35  is positive, the operation proceeds to S 36 . 
     At S 36 , whether the timer count value is counted up to the first predetermined time period ta 0  or not is determined. If the determination result of S 36  is negative, the operation proceeds to S 37  where the O 2  sensor  56  is determined as normal, and then the processing operation of the abnormality determination is ended. On the other hand, if the determination result of S 36  is positive, the operation proceeds to S 38  where the O 2  sensor  56  is determined as abnormal, and then the processing operation of the abnormality determination is ended. 
     The second example of the processing operation of the abnormality determination of the O 2  sensor  56  by the control system  100  (abnormality determining module  100   g ) is as illustrated in the flowchart in  FIG. 11 . 
     Specifically, processing similar to S 31  and S 32  is performed at S 51  and S 52 , respectively, and if the determination result of S 52  is positive, the operation proceeds to S 53  where a timer count is started. 
     Next, at S 54 , whether the output value of the O 2  sensor  56  has reached the third voltage threshold V 3  or not is determined. If the determination result of S 54  is negative, the operation proceeds to S 55 , whereas if the determination result of S 54  is positive, the operation proceeds to S 56 . 
     At S 55 , whether the timer count value is counted up to the second predetermined time period tb 0  or not is determined. If the determination result of S 55  is negative, the operation returns to S 53 , whereas if the determination result of S 55  is positive, the operation proceeds to S 56 . 
     At S 56 , whether the timer count value is counted up to the second predetermined time period tb 0  or not is determined. If the processing at S 56  is performed after the determination at S 55  resulted in being positive, the determination result of S 56  naturally becomes positive. 
     If the determination result of S 56  is negative, the operation proceeds to S 57  where the O 2  sensor  56  is determined as normal, and then the processing operation of the abnormality determination is ended. On the other hand, if the determination result of S 56  is positive, the operation proceeds to S 58  where the O 2  sensor  56  is determined as abnormal, and then the processing operation of the abnormality determination is ended. 
     Therefore, in this embodiment, the purge performed by the deceleration-fuel-cutoff purge valve controlling module  100   d  is restricted (the purge is prohibited or the supply amount of the purge gas to the surge tank  34  is restricted) during the abnormality determination performed by the abnormality determining module  100   g  to determine whether the O 2  sensor  56  is abnormal based on the change of the output value of the O 2  sensor  56  caused by the deceleration fuel cutoff of the engine  1 . Therefore, the degradation in accuracy of the abnormality determination of the O 2  sensor  56  due to the purge can be suppressed. 
     The present invention is not limited to the above embodiment, and may be substituted without deviating from the scope of the claims. 
     The above-described embodiment is merely an illustration, and therefore, the present invention must not be interpreted in a limited way. The scope of the present invention is defined by the claims, and all of modifications and changes falling under the equivalent range of the claims are within the scope of the present invention. 
     The present invention is useful for performing, with a control system of an engine in which a purge gas containing evaporated fuel desorbed from a canister is supplied to an intake passage, a purge during a deceleration fuel cutoff of the engine and an abnormality determination in which whether an O 2  sensor is abnormal is determined based on a change of an output value of the O 2  sensor caused by the deceleration fuel cutoff. 
     It should be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof, are therefore intended to be embraced by the claims. 
     LIST OF REFERENCE CHARACTERS 
     
         
           1  Engine 
           30  Intake Passage 
           40  Exhaust Passage 
           56  O 2  Sensor 
           70  Canister 
           73  Purge Tube (Purge Line) (Purge Unit) 
           75  Purge Valve (Purge Unit) 
           100   d  Deceleration-fuel-cutoff Purge Valve Controlling Module (Purge Valve Controlling Module) (Purge Unit) 
           100   e  Deceleration-fuel-cutoff Controlling Module (Deceleration Fuel Cutoff Module) 
           100   f  Evaporated Fuel Concentration Estimating Module 
           100   g  Abnormality Determining Module 
           100   h  Purge Restricting Module 
           100   i  Air-fuel Ratio Estimating Module