Patent Publication Number: US-9897044-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 during a deceleration fuel cutoff of the engine, when it is determined that evaporated fuel easily overflows from a canister, the purge gas containing the evaporated fuel desorbed from the canister is supplied to an intake passage of an 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 reduced. Although the evaporated fuel within the purge gas supplied to the intake passage is 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, when a temperature of the exhaust emission control catalyst is detected and the detected result indicates a temperature below a predetermined value, the supply of the purge gas to the intake passage is reduced to suppress degradation of emission performance. 
     However, in JP2007-198210A, even when the purge gas is supplied to the intake passage when the temperature of the exhaust emission control catalyst is the predetermined value or higher, depending on the temperature of the exhaust emission control catalyst, if an excessive amount of unburned evaporated fuel reaches the exhaust emission control catalyst, the emission performance may still degrade, which leaves room for improvement. 
     SUMMARY 
     The present invention is made in view of the above situations and aims to secure as much as possible, when purge gas is supplied to an intake passage (when a purge is performed) during a deceleration fuel cutoff of an engine, a supply amount of the purge gas to the intake passage while suppressing degradation of emission performance. 
     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 an exhaust emission control catalyst provided in an exhaust passage of the engine, 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 purging unit for performing a purge to supply the purge gas to the intake passage of the engine during the deceleration fuel cutoff, an evaporated fuel supply amount estimating module for estimating a supply amount of the evaporated fuel to the intake passage when the purge is performed, and a catalyst temperature estimating module for estimating a temperature of the exhaust emission control catalyst when the purge is performed, based on the estimated supply amount of the evaporated fuel. The purging unit controls a supply flow rate of the purge gas to the intake passage when the purge is performed, based on the estimated temperature of the exhaust emission control catalyst. 
     With the above-described configuration, the supply flow rate of the purge gas to the intake passage when the purge is performed can be adjusted according to purifying performance of the exhaust emission control catalyst which is influenced by its temperature, and a supply amount of the purge gas to the intake passage can be secured as much as possible while suppressing degradation of emission performance. 
     The purging unit preferably reduces the supply flow rate of the purge gas to the intake passage when the purge is performed, as the estimated temperature of the exhaust emission control catalyst becomes lower. 
     As the temperature of the exhaust emission control catalyst becomes lower, the purifying performance of the exhaust emission control catalyst degrades more. Therefore, the supply flow rate of the purge gas to the intake passage when the purge is performed can suitably be set corresponding to the relationship between the temperature of the exhaust emission control catalyst and the purifying performance. 
     The purging unit preferably stops the purge when the estimated temperature of the exhaust emission control catalyst falls below a predetermined temperature while the purge is performed. 
     By setting the predetermined temperature so that the purifying performance of the exhaust emission control catalyst significantly degrades when falling below the predetermined temperature (e.g., equal or close to an activation temperature of the exhaust emission control catalyst), the degradation of the emission performance can surely be suppressed. 
     The control system preferably further includes a catalyst temperature increasing amount estimating module for continuously estimating an increasing amount of the temperature of the exhaust emission control catalyst when unburned evaporated fuel accumulated in the exhaust emission control catalyst by the purge performed is assumed to have entirely combusted at once. While the purge is performed, the purging unit preferably stops the purge once the increasing amount of the estimated temperature of the exhaust emission control catalyst exceeds a preset value. 
     When the deceleration fuel cutoff is ended and shifted to a normal operation of the engine (operation in which the injector supplies fuel to the engine and the fuel is combusted), the unburned evaporated fuel accumulated in the exhaust emission control catalyst by the purge during the deceleration fuel cutoff is entirely combusted at once due to exhaust gas at high temperature which is produced by combustion of the fuel injected by the injector. Thus, the temperature of the exhaust emission control catalyst sharply increases. Here, if the temperature increases excessively, deterioration of the exhaust emission control catalyst will be stimulated. With this configuration, the increasing amount of the temperature of the exhaust emission control catalyst when the unburned evaporated fuel accumulated in the exhaust emission control catalyst due to the purge during the deceleration fuel cutoff is assumed to have entirely combusted at once, is continuously estimated. The purge is stopped when the increasing amount of the temperature exceeds the preset value, and after stopped, the unburned evaporated fuel is not accumulated in the exhaust emission control catalyst. Thus, the increasing amount of the temperature of the exhaust emission control catalyst when the deceleration fuel cutoff is ended and shifted to the normal operation of the engine can be a value (the preset value) set so that the deterioration of the exhaust emission control catalyst due to the sharp temperature increase can be suppressed. 
     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. The purging unit preferably further controls the supply flow rate of the purge gas to the intake passage when the purge is performed, based on the estimated concentration of the evaporated fuel. 
     When the concentration of the evaporated fuel within the purge gas is high, the unburned evaporated fuel may not be purified by the exhaust emission control catalyst and the emission performance may degrade. By controlling the supply flow rate of the purge gas to the intake passage when the purge is performed based on the temperature of the exhaust emission control catalyst as well as the concentration of the evaporated fuel, the degradation of the emission performance can more surely be suppressed. 
     The purging unit preferably does not perform the purge during the deceleration fuel cutoff when the estimated concentration of the evaporated fuel is above a predetermined concentration. 
     By not performing the purge during the deceleration fuel cutoff when the concentration of the evaporated fuel is high enough that the evaporated fuel cannot suitably be purified by the exhaust emission control catalyst, suitable emission performance can be secured. 
     The control system preferably further includes an exhaust gas temperature detecting/estimating module for detecting or estimating a temperature of exhaust gas of the engine when the engine is operated by supplying fuel from the injector to the engine and combusting the fuel. The catalyst temperature estimating module preferably estimates the temperature of the exhaust emission control catalyst when the purge is performed, based on the temperature of the exhaust gas detected or estimated immediately before the deceleration fuel cutoff is started, the estimated supply amount of the evaporated fuel, a heat generation amount, and a heat release amount, the heat generation amount produced by combustion, at the exhaust emission control catalyst, of part of the evaporated fuel which has reached the exhaust emission control catalyst when the purge is performed, the heat release amount produced from the exhaust emission control catalyst to air passing through the exhaust emission control catalyst when the purge is performed. 
     With this configuration, the estimation of the temperature of the exhaust emission control catalyst can suitably be achieved. 
     The control system preferably further includes a turbocharger having a compressor disposed in the intake passage of the engine. The purging unit preferably includes a purge line communicating the canister with part of the intake passage downstream of the compressor, a purge valve provided in the purge line, and a purge valve controlling module for controlling the supply flow rate of the purge gas to the intake passage by controlling an operation of the purge valve when the purge is performed. 
     In the case where the turbocharger is provided to the engine as described above, during the normal operation of the engine, the pressure in the intake passage at the connection position with the purge line rarely becomes negative, and thus the purge is rarely performed. However, according to this aspect of the present invention, the supply flow rate of the purge gas to the intake passage when the purge is performed is controlled based on the estimated temperature of the exhaust emission control catalyst, while the purge is performed during the deceleration fuel cutoff. Therefore, the supply amount of the purge gas to the intake passage can be secured as much as possible while suppressing the degradation of the emission performance. As a result, the operations of the present invention can effectively be achieved and the effects can effectively be exerted. 
    
    
     
       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 a total weight of hydrocarbons (HC) after passing 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 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  is a flowchart illustrating a processing operation regarding a purge performed by the control system. 
         FIG. 6  is a flowchart illustrating a processing operation of a deceleration-fuel-cutoff purge valve control. 
         FIG. 7  is a flowchart illustrating a processing operation of estimating a temperature of an upstream exhaust emission control catalyst by a catalyst temperature estimating module when the purge is performed during a deceleration fuel cutoff. 
         FIG. 8  shows time charts illustrating examples of a change of the temperature of the upstream exhaust emission control catalyst when the purge is performed during the deceleration fuel cutoff. 
     
    
    
     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 the rotational angular position of the detecting plate  8  to detect a speed of the engine  1 . 
     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 and exhaust valves  14  and  15  reciprocate at predetermined timings by the intake and exhaust valve drive mechanisms  16  and  17 , respectively, to open and close the intake and exhaust ports  12  and  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 injector  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 , so as to send the fuel to the injector  18  after a pressure adjustment at the regulator  25 . 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 each combustion chamber  6  of the cylinder  2  via the intake passage  30  and the intake port  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 to flow 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 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, 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 O2 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 whether the air-fuel ratio of the exhaust gas which has passed through the upstream exhaust emission control catalyst  52  is stoichiometric, rich, or lean is disposed between the upstream and downstream exhaust emission control catalysts  52  and  53 . 
     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 flow rate (or a supply amount) 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 built therein 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 flow rate 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 supply amount estimating module  100   f , a catalyst temperature estimating module  100   g , a catalyst temperature increasing amount estimating module  100   h , an evaporated fuel concentration estimating module  100   i , and an exhaust gas temperature estimating module  100   j , which are described later in detail. 
     When a predetermined deceleration fuel cutoff condition is satisfied when 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  as fully closed and the speed of the engine  1  is detected by engine speed sensor  9  as 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 flow rate of the purge gas to the surge tank  34 ). Specifically, the purge to supply 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 purging unit for performing the purge to supply 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   i  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   i  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   i  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 ). Also 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 evaporated fuel supply amount estimating module  100   f  estimates the supply amount of the evaporated fuel to the surge tank  34  when the purge is performed during the deceleration fuel cutoff. 
     Specifically, a target air-fuel ratio (target A/F) when the purge is performed during the deceleration fuel cutoff is first calculated.  FIG. 3  is a chart illustrating relationships between the air-fuel ratio within the combustion chambers  6  and a total weight of HC after passing 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 total weight of HC is reduced as the air-fuel ratio becomes higher, and when the air-fuel ratio exceeds a certain value, the total weight of HC becomes 0 (zero). Therefore, the target A/F may be set to be a value equal to or larger than a smallest value of air-fuel ratio at which the total weight of HC becomes 0 at each concentration (preferably be a value 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 map as illustrated in  FIG. 4 , and by using the map, the target A/F is calculated based on the learned value obtained immediately before the deceleration fuel cutoff. Note that in the map, the target A/F is not set when the learned value indicates a concentration higher than a predetermined 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 entire 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 vale 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,”
 
 g prg= 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,
 
 q prg= g prg× 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. 
     Note that the opening of the purge valve  75  can be determined based on the purge gas volume qprg and the pressure difference Pd. In this embodiment, as described later in detail, the opening is determined by also taking the temperature of one or more of the exhaust emission control catalysts (here, the upstream exhaust emission control catalyst  52 ) estimated by the catalyst temperature estimating module  100   g  as described later. 
     The evaporated fuel supply amount estimating module  100   f  estimates the supply amount of the evaporated fuel to the surge tank  34  when the purge is performed during the deceleration fuel cutoff, based on the opening of the purge valve  75  (determined based on the purge gas volume qprg, the pressure difference Pd, and the temperature of the upstream exhaust emission control catalyst  52 ) and the learned value. 
     The catalyst temperature estimating module  100   g  estimates the temperature of the upstream exhaust emission control catalyst  52  when the purge is performed during the deceleration fuel cutoff based on the supply amount of the evaporated fuel estimated by the evaporated fuel supply amount estimating module  100   f.    
     Specifically, the catalyst temperature estimating module  100   g  estimates the temperature of the upstream exhaust emission control catalyst  52  when the purge is performed, based on the temperature of the exhaust gas immediately before the deceleration fuel cutoff is started, the supply amount of the evaporated fuel estimated by the evaporated fuel supply amount estimating module  100   f , a heat generation amount Q 1 , and a heat release amount Q 3 . The heat generation amount Q 1  is produced by combustion (oxidation), at the upstream exhaust emission control catalyst  52 , of part of the unburned evaporated fuel which has reached the upstream exhaust emission control catalyst  52  when the purge is performed during the deceleration fuel cutoff (the entire evaporated fuel supplied to the surge tank  34  reaches the upstream exhaust emission control catalyst  52 ). The heat release amount Q 3  is produced from the upstream exhaust emission control catalyst  52  to air passing through the upstream exhaust emission control catalyst  52  when the purge is performed, and the heat release amount Q 3  is calculated based on the total air mass qa sucked into the combustion chambers  6 . 
     Here, the exhaust gas temperature estimating module  100   j  continuously estimates the temperature of the exhaust gas based on the speed of the engine  1  obtained by the engine speed sensor  9  and a load of the engine  1  (obtained based on the speed of the engine  1  and the accelerator opening detected by the accelerator opening sensor  92 ), during the normal operation of the engine  1 . The exhaust gas temperature estimating module  100   j  then stores (updates) the estimated value in the memory  90 . 
     The temperature of the exhaust gas immediately before the deceleration fuel cutoff is started is the latest estimated value stored in the memory  90  at the start of the deceleration fuel cutoff. Note that as an alternative to the estimated value, the temperature of the exhaust gas may be detected by using a temperature sensor. 
     The catalyst temperature estimating module  100   g  estimates the temperature of the upstream exhaust emission control catalyst  52 , by adding a temperature corresponding to the heat generation amount Q 1  to the temperature of the exhaust gas (estimated value) and then subtracting therefrom a temperature corresponding to the heat release amount Q 3 . 
     Practically, the catalyst temperature estimating module  100   g  continuously estimates the temperature of the upstream exhaust emission control catalyst  52  and stores (updates) it in the memory  90  during the deceleration fuel cutoff. Specifically, immediately after the deceleration fuel cutoff is started, the catalyst temperature estimating module  100   g  adds a temperature corresponding to the heat generation amount Q 1  produced in a period of time from the start of the deceleration fuel cutoff until the estimation is performed (the temperature is 0 (zero) when the purge is not performed) to the temperature of the exhaust gas (estimated value). The catalyst temperature estimating module  100   g  then subtracts therefrom a temperature corresponding to the heat release amount Q 3  produced in the same time period, so as to estimate the temperature thcat of the upstream exhaust emission control catalyst  52  and store it in the memory  90 . When performing the next estimation (latest estimation), the catalyst temperature estimating module  100   g  adds a temperature corresponding to the heat generation amount Q 1  produced in a period of time between the immediately previous estimation and the latest estimation to the temperature thcat of the upstream exhaust emission control catalyst  52  stored in the memory  90  immediately before the latest estimation. The catalyst temperature estimating module  100   g  then subtracts therefrom a temperature corresponding to the heat release amount Q 3  produced in the same time period, so as to estimate a latest value of the temperature thcat of the upstream exhaust emission control catalyst  52  and store (update) it in the memory  90 . 
     The heat generation amount Q 1  is calculated through multiplying a coefficient k (0 or higher but below 1) by a heat generation amount Q 2  which is produced when the evaporated fuel which has reached the upstream exhaust emission control catalyst  52  is entirely combusted (oxidized). Here, for the sake of convenience, the heat generation amount Q 2  is a heat generation amount produced when butane is combusted. The coefficient k is set larger as the temperature thcat of the upstream exhaust emission control catalyst  52  stored in the memory  90  becomes higher, which means a larger part of the evaporated fuel which has reached the upstream exhaust emission control catalyst  52  is combusted as the temperature thcat of the upstream exhaust emission control catalyst  52  becomes higher. Further, when the temperature thcat of the upstream exhaust emission control catalyst  52  is below a preset temperature (substantially the same as a predetermined temperature described later), the coefficient k becomes 0 and the heat generation amount Q 1  also becomes 0. In other words, when the temperature thcat of the upstream exhaust emission control catalyst  52  is below the preset temperature, the unburned evaporated fuel is not combusted and the temperature of the upstream exhaust emission control catalyst  52  does not increase according to the heat generation amount Q 1 . 
     The deceleration-fuel-cutoff purge valve controlling module  100   d  controls the supply flow rate 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, the pressure difference Pd, and additionally the temperature thcat of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature estimating module  100   g . Note that since the purge gas volume qprg is obtained based on the estimated value of the concentration of the evaporated fuel within the purge gas by the evaporated fuel concentration estimating module  100   i , the deceleration-fuel-cutoff purge valve controlling module  100   d  controls the supply flow rate of the purge gas to the surge tank  34  when the purge is performed during the deceleration fuel cutoff, based on the concentration of the evaporated fuel within the purge gas estimated by the evaporated fuel concentration estimating module  100   i , and the temperature thcat of the upstream exhaust emission control catalyst  52 . 
     Specifically, as the temperature thcat of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature estimating module  100   g  is lower, the deceleration-fuel-cutoff purge valve controlling module  100   d  reduces the supply flow rate of the purge gas to the surge tank  34  when the purge is performed during the deceleration fuel cutoff. Moreover, when the temperature thcat of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature estimating module  100   g  is lower than a predetermined temperature, the deceleration-fuel-cutoff purge valve controlling module  100   d  stops the purge (adjusts the opening of the purge valve  75  to 0). The predetermined temperature is set so that the purifying performance of the exhaust emission control catalyst significantly degrades when falling below the predetermined temperature, for example, it is equal or close to an activation temperature of the upstream exhaust emission control catalyst  52 . 
     The catalyst temperature increasing amount estimating module  100   h  continuously estimates an increasing amount of the temperature of the upstream exhaust emission control catalyst  52  when the unburned evaporated fuel accumulated in the upstream exhaust emission control catalyst  52  due to the purge during the deceleration fuel cutoff is assumed to have entirely combusted at once. 
     Specifically, a total heat generation amount Qt when the unburned evaporated fuel accumulated in the upstream exhaust emission control catalyst  52  is assumed to have entirely combusted at once can be obtained based on
 
 Qt =Σ( Q 2− Q 1).
 
     In other words, the heat generation amount Q 1  within the heat generation amount Q 2  is for the evaporated fuel which is already combusted, and the value of Q 2 -Q 1  is a heat generation amount by the unburned evaporated fuel accumulated in the upstream exhaust emission control catalyst  52  without being combusted, and a summation of Q 2 -Q 1  is the total heat generation amount Qt by the unburned evaporated fuel accumulated in the upstream exhaust emission control catalyst  52  from the start of the purge to a current timing. The catalyst temperature increasing amount estimating module  100   h  estimates the increasing amount of the temperature of the upstream exhaust emission control catalyst  52  based on the total heat generation amount Qt. 
     While the purge is performed, when the increasing amount of the temperature of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature increasing amount estimating module  100   h  exceeds a preset value, the deceleration-fuel-cutoff purge valve controlling module  100   d  stops the purge (adjusts the opening of the purge valve  75  to zero). The preset value is set so that deterioration of the upstream exhaust emission control catalyst  52  due to a sharp temperature increase can be suppressed. 
     When the deceleration fuel cutoff is ended and shifted to the normal operation of the engine  1 , the unburned evaporated fuel accumulated in the upstream exhaust emission control catalyst  52  by the purge during the deceleration fuel cutoff is entirely combusted at once due to the exhaust gas at high temperature which is produced by combustion of the fuel injected by the injectors  18 . Thus, the temperature of the upstream exhaust emission control catalyst  52  sharply increases. Here, if the temperature increases excessively, the deterioration of the upstream exhaust emission control catalyst  52  will be stimulated. In order to suppress such deterioration, the purge is stopped once the increasing amount of the temperature of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature increasing amount estimating module  100   h  exceeds the preset value. 
     Next, the processing operation regarding the purge performed by the control system  100  is described with reference to the flowchart in  FIG. 5 . 
     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 satisfied 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 greater detail with reference to the flowchart in  FIG. 6 . 
     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 and the estimated value thcat of the temperature of the upstream exhaust emission control catalyst  52  are read from the memory  90 , and the pressure difference Pd between the pressure detected by the pressure sensor  35  and the pressure detected by the atmospheric pressure sensor  91  is calculated. 
     Next at S 12 , whether a purge stop condition is satisfied or not satisfied is determined. The purge stop condition includes a condition in which the temperature thcat of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature estimating module  100   g  falls below the predetermined temperature when the purge is performed, and a condition in which the increasing amount of the temperature of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature increasing amount estimating module  100   h  exceeds the preset value 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 the target A/F is calculated based on the learned value by using the map in  FIG. 4 . Here, if the learned value indicates a concentration above the predetermined concentration C (the hatched section in  FIG. 4 ), the purge is not performed (the purge valve  75  is fully closed). 
     Next at S 15 , the purge gas volume qprg is calculated based on the target A/F, 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, the pressure difference Pd, and the estimated temperature value thcat of the upstream exhaust emission control catalyst  52 , and the purge valve  75  is controlled to have the calculated opening. Then, the operation returns to the start of the operation. 
     Next, the processing operation performed by the catalyst temperature estimating module  100   g  to estimate the temperature of the upstream exhaust emission control catalyst  52  when the purge is performed during the deceleration fuel cutoff is described with reference to the flowchart in  FIG. 7 . 
     First, at S 31 , the estimated value thcat of the current temperature of the upstream exhaust emission control catalyst  52  is read from the memory  90  (however, when reading immediately after the deceleration fuel cutoff is started, the temperature of the exhaust gas is read instead). 
     Next, at S 32 , the heat generation amount Q 1  produced in the time period between the immediately previous estimation and the latest estimation is calculated. Specifically, the heat generation amount Q 2  produced when the evaporated fuel which has reached the upstream exhaust emission control catalyst  52  is entirely combusted (oxidized) during the same time period is calculated, the coefficient k corresponding to the estimated value thcat is read from the memory  90 , and the heat generation amount Q 1  is then calculated through multiplying the coefficient k by the heat generation amount Q 2 . 
     Then, at S 33 , the heat release amount Q 3  produced in the same time period between the immediately previous estimation and the latest estimation is calculated. Subsequently at S 34 , the temperature corresponding to the heat generation amount Q 1  is added to the estimated value thcat and the temperature corresponding to the heat release amount Q 3  is subtracted therefrom, so as to estimate a latest temperature thcat of the upstream exhaust emission control catalyst  52  and store it in the memory  90  for an update. 
       FIG. 8  shows time charts illustrating examples (a first example indicated by the dashed line and a second example indicated by the solid line) of the change of the temperature of the upstream exhaust emission control catalyst  52  when the purge is performed during the deceleration fuel cutoff. 
     The first example is an example wherein the temperature of the upstream exhaust emission control catalyst  52  falls below the predetermined temperature when the purge is performed. In the first example, the purge is stopped when the temperature of the upstream exhaust emission control catalyst  52  falls below the predetermined temperature. 
     The second example is an example wherein although the temperature of the upstream exhaust emission control catalyst  52  does not fall below the predetermined temperature, the increasing amount of the temperature of the upstream exhaust emission control catalyst  52  when the unburned evaporated fuel accumulated in the upstream exhaust emission control catalyst  52  by the purge is assumed to have entirely combusted at once exceeds the preset value. The line indicated by the one-dotted chain line is the temperature of the upstream exhaust emission control catalyst  52  after the temperature increase. 
     In the second example, the purge is stopped when the increasing amount exceeds the preset value. After stopping, since the unburned evaporated fuel is not accumulated in the upstream exhaust emission control catalyst  52 , the increasing amount becomes the preset value. When the deceleration fuel cutoff is ended and shifted to the normal operation of the engine  1 , the unburned evaporated fuel is entirely combusted at once and the temperature of the upstream exhaust emission control catalyst  52  sharply increases. However, the increasing amount of the temperature here becomes the preset value, and therefore, the deterioration of the upstream exhaust emission control catalyst  52  due to the sharp temperature increase can be suppressed. 
     As described above, in this embodiment, the deceleration-fuel-cutoff purge valve controlling module  100   d  controls the supply flow rate 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 of the engine  1 , based on the purge gas volume qprg (i.e., the estimated value of the concentration of the evaporated fuel within the purge gas), the pressure difference Pd, and the temperature thcat of the upstream exhaust emission control catalyst  52  estimated by the catalyst temperature estimating module  100   g . Thus, the supply flow rate of the purge gas to the surge tank  34  when the purge is performed can be adjusted according to the purifying performance of the upstream exhaust emission control catalyst  52  which is influenced by its temperature, and the supply amount of the purge gas to the surge tank  34  can be secured as much as possible while suppressing degradation of emission performance. 
     In this embodiment, the supply flow rate of the purge gas to the surge tank  34  when the purge is performed during the deceleration fuel cutoff is reduced as the temperature thcat of the upstream exhaust emission control catalyst  52  becomes lower. Further, when the temperature thcat of the upstream exhaust emission control catalyst  52  falls below the predetermined temperature while the purge is performed, the purge is stopped. Therefore, the degradation of the emission performance can securely be suppressed. 
     The present invention is not limited to the above embodiment, and may be substituted without deviating from the scope of the claims. 
     For example, in the above-described embodiment, the engine  1  has a turbocharger; however, the turbocharger may be omitted. 
     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 control systems of engines in which purge gas containing evaporated fuel desorbed from a canister is supplied to an intake passage, and particularly useful when the engine has a turbocharger. 
     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 
           50  Turbocharger 
           50   a  Compressor 
           50   b  Turbine 
           52  Upstream Exhaust Emission Control Catalyst 
           53  Downstream Exhaust Emission Control Catalyst 
           70  Canister 
           73  Purge Tube (Purge Line) (Purging Unit) 
           75  Purge Valve (Purging Unit) 
           100   d  Deceleration-fuel-cutoff Purge Valve Controlling Module (Purge Valve Controlling Module) (Purging Unit) 
           100   e  Deceleration-fuel-cutoff Controlling Module (Deceleration Fuel Cutoff Module) 
           100   f  Evaporated Fuel Supply Amount Estimating Module 
           100   g  Catalyst Temperature Estimating Module 
           100   h  Catalyst Temperature Increasing Amount Estimating Module 
           100   i  Evaporated Fuel Concentration Estimating Module 
           100   j  Exhaust Gas Temperature Estimating Module