Patent Publication Number: US-9850829-B2

Title: Control device and control method for internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a control device and control method for an internal combustion engine, which execute feedback control such that the rate of decrease in engine rotational speed coincides with a target rate of decrease. 
     2. Description of Related Art 
     There is known control over an internal combustion engine, in which, when accelerator operation is released, feedback control that executes feedback control over the rate of decrease in engine rotational speed so as to coincide with a target rate of decrease to thereby stabilize the behavior of a variation in engine rotational speed at the time when the engine rotational speed decreases with the release of accelerator operation. 
     In order to appropriately execute feedback control over the rate of decrease in engine rotational speed so as to coincide with the target rate of decrease, it is necessary to appropriately control the torque of the internal combustion engine on the basis of a deviation from the target rate of decrease. For this purpose, a feedback gain needs to be set at an appropriate value. 
     In contrast to this, Japanese Patent Application Publication No. 2000-073801 (JP 2000-073801 A) describes a control device for an internal combustion engine. When the engine rotational speed is likely to decrease, the control device increases the feedback gain as compared with when the engine rotational speed is not likely to decrease to thereby suppress a decrease in engine rotational speed. 
     The invention described in JP 2000-073801 A is to execute feedback control so as to coincide the engine rotational speed itself with a target engine rotational speed. Such a configuration that the feedback gain is increased when the engine rotational speed is likely to decrease as compared with when the engine rotational speed is not likely to decrease may be applied to feedback control. 
     Incidentally, the rate of decrease in engine rotational speed varies with an external load that acts on the output shaft of the internal combustion engine, so, even when the above configuration described in JP 2000-073801 A is applied to feedback control, an appropriate feedback gain cannot be set when an external load varies. Thus, it may be difficult to appropriately suppress a deviation of the rate of decrease. 
     Note that such a problem not only exists in feedback control during a period from when accelerator operation is released to when the engine rotational speed reaches an idle rotational speed but also it commonly exists in overall feedback control that executes feedback control over the torque of the internal combustion engine so as to coincide the rate of decrease in engine rotational speed with a target rate of decrease in the engine rotational speed. 
     SUMMARY OF THE INVENTION 
     The invention provides a control device and control method for an internal combustion engine, which are able to appropriately execute feedback control over the rate of decrease in engine rotational speed even when an external load varies. 
     A first aspect of the invention relates to a control device for an internal combustion engine, which includes a control unit that executes feedback control for controlling a torque of the internal combustion engine so as to coincide a rate of decrease in an engine rotational speed with a target rate of decrease in the engine rotational speed. The control unit calculates a required torque that is a torque required to keep the engine rotational speed at a constant rotational speed. The control unit increases a feedback gain in the feedback control as the calculated required torque increases. 
     The required torque is a torque require to compensate for a torque consumed by an internal load due to a friction load inside the internal combustion engine and an external load that acts on the internal combustion engine from an outside of the internal combustion engine to thereby keep the engine rotational speed at a constant rotational speed. Therefore, when the required torque is large, it is estimated that the torque consumed by the internal load and the external load is large, the engine rotational speed tends to decrease, and it is estimated that the rate of decrease in the engine rotational speed in the case of insufficient torque also increases. 
     In contrast to this, with the above described configuration, as the required torque increases, the feedback gain in the feedback control is increased. Therefore, when the torque consumed by the internal load and the external load is large and the rate of decrease in the engine rotational speed in the case of insufficient torque is high, the sensitivity of an increase or decrease in torque corrected through feedback control, is further increased. Thus, even when the external load varies, it is possible to set the appropriate feedback gain by which a variation in the rate of decrease in the engine rotational speed due to the variation of the external load may be suppressed. 
     That is, according to the above aspect, even when the external load varies, the rate of decrease in the engine rotational speed may be appropriately subjected to feedback control. 
     In the above aspect, the control unit may set a target rotational speed, at which the engine rotational speed is kept at constant, as the engine rotational speed that is acquired in order to calculate the required torque. In addition, the control unit may calculate the required torque on the basis of the set target rotational speed, an engine temperature, and an operating state of an auxiliary that is driven using an output of the internal combustion engine. 
     The viscosity of lubricating oil increases as the engine temperature decreases, so the internal load due to the friction load inside the internal combustion engine increases as the engine temperature decreases. Furthermore, as the engine rotational speed increases, the amounts of slide at the sliding portions of the components of the engine increase, so the amount of torque consumed by the internal load per unit time increases as the engine rotational speed increases. 
     In addition, the external load that acts on the internal combustion engine varies on the basis of the operating states of the auxiliaries driven using the output of the internal combustion engine. Specifically, the external load that acts on the internal combustion engine increases as the number of operated auxiliaries increases and as the driving amount of each operated auxiliary increases. 
     Therefore, by acquiring the set rotational speed, the engine temperature and the operating state of the auxiliary that is driven using an output of the internal combustion engine, it is possible to calculate the required torque that is a torque required to keep the engine rotational speed at the constant rotational speed. 
     That is, the engine rotational speed acquired at the time of calculating the required torque may be selectively set. In the above aspect, a torque required to keep the engine rotational speed at an idle rotational speed may be calculated as the required torque. 
     In the above aspect, calculation of the required torque may be executed in parallel with the feedback control. 
     In the above aspect, the feedback control may be integral feedback that integrates an update amount of rate of decrease calculated on the basis of a deviation between the target rate of decrease and an actual rate of decrease and the feedback gain to thereby calculate a torque correction amount. 
     In the above aspect, the control unit may control the torque correction amount such that the torque correction amount does not exceed a predetermined upper limit and a predetermined lower limit. 
     In the above aspect, when the torque correction amount is larger than the predetermined upper limit, the control unit may set the predetermined upper limit as the torque correction amount. When the torque correction amount is smaller than the predetermined lower limit, the control unit may set the predetermined lower limit as the torque correction amount. 
     In the above aspect, the control unit may execute the feedback control from when accelerator operation is released to when the engine rotational speed decreases to an idle rotational speed. 
     A second aspect of the invention relates to a control method for an internal combustion engine, which executes feedback control for controlling a torque of the internal combustion engine so as to coincide a rate of decrease in an engine rotational speed with a target rate of decrease in the engine rotational speed. The control method includes: calculating a required torque that is a torque required to keep the engine rotational speed at a constant rotational speed; and increasing a feedback gain in the feedback control as the calculated required torque increases. 
     With the above configuration, it is possible to stabilize the behavior of a variation in the engine rotational speed resulting from the release of accelerator operation by the time when the engine rotational speed reaches the idle rotational speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements; and wherein: 
         FIG. 1  is a schematic view that shows the relationship between an electronic control unit according to an embodiment of the invention and an internal combustion engine that is an object controlled by the electronic control unit; 
         FIG. 2  is a flow chart that shows the flow of a series of processes in feedback control; and 
         FIG. 3  is a flow chart that shows the flow of a series of processes for setting a feedback gain. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment in which a control device for an internal combustion engine according to the aspect of the invention is applied to an electronic control unit  100  that comprehensively controls an internal combustion engine  10  mounted on a vehicle will be described with reference to  FIG. 1  to  FIG. 3 . 
     As shown in  FIG. 1 , a piston  12  is slidably accommodated in a corresponding one of the cylinders  11  of the internal combustion engine  10 . A crankshaft  14  is coupled to each piston  12  via a connecting rod  13 . The crankshaft  14  is the output shaft of the internal combustion engine  10 . 
     The pistons  12  are respectively accommodated in the corresponding cylinders  11  in this way to thereby define combustion chambers  16  by the inner peripheral surfaces of the cylinders  11 , the top surfaces of the pistons  12  and the bottom surface of the cylinder head  15 . Note that the internal combustion engine  10  is a multi-cylinder internal combustion engine having the plurality of cylinders  11 ; however, only one of the plurality of cylinders  11  is shown in  FIG. 1 . 
     Ignition plugs  20  are installed on the cylinder head  15  at positions facing the pistons  12  accommodated in the respective cylinders  11 . Then, an intake passage  30  and an exhaust passage  40  are connected to each of the combustion chambers  16  respectively defined in the cylinders  11 . In addition, as shown in  FIG. 1 , an injector  19  that injects fuel toward the corresponding combustion chamber  16  is provided in the intake passage  30  one by one for each cylinder  11 . 
     As shown in  FIG. 1 , intake valves  17  and exhaust valves  18  are provided on the cylinder head  15 . Each of the intake valves  17  opens or closes so as to provide or interrupt fluid communication between the intake passage  30  and the corresponding combustion chamber  16 . Each of the exhaust valves  18  opens or closes so as to provide or interrupt fluid communication between the exhaust passage  40  and the corresponding combustion chamber  16 . Note that the intake valves  17  are opened or closed by an intake camshaft coupled to the crankshaft  14  via a timing chain (not shown) and the exhaust valves  18  are opened or closed by an exhaust camshaft coupled to the crankshaft  14  via the timing chain. 
     As is shown at the left side in  FIG. 1 , an air cleaner  31  is provided at the most upstream portion of the intake passage  30 . A filter  32  is provided inside the air cleaner  31 . The filter  32  traps dust and dirt contained in intake air. By so doing, air from which dust and dirt are removed through the air cleaner  31  is introduced into the combustion chambers  16  of the internal combustion engine  10  via the intake passage  30 . 
     A surge tank  33  is provided at a portion downstream of the air cleaner  31  in the intake passage  30 . As shown in  FIG. 1 , the flow passage cross-sectional area of a portion at the surge tank  33  is larger than that of the other portion of the intake passage  30 . By so doing, air introduced through the air cleaner  31  passes through the surge tank  33  to thereby equalize pulsation of air that passes through the intake passage  30 . 
     In addition, as shown in  FIG. 1 , a throttle valve  35  is provided at a portion downstream of the air cleaner  31  and upstream of the surge tank  33  in the intake passage  30 . The throttle valve  35  is driven by a motor  34 , and the opening degree of the throttle valve  35 , which is a throttle opening degree Th, is controlled. 
     Control over the opening degree of the throttle valve  35 , fuel injection amount control for controlling the valve open duration Tf of each injector  19  and torque control executed through, for example, ignition timing control using the ignition plugs  20  are executed by the electronic control unit  100  that comprehensively controls the internal combustion engine  10 . 
     The electronic control unit  100  includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), and the like. The CPU executes various processes in order to execute various controls regarding the torque control. The ROM stores computation programs, computation maps and various data. The RAM temporarily stores the results of computations, and the like. 
     Various sensors are connected to the electronic control unit  100 . Various sensors include an accelerator position sensor  50 , an air flow meter  51 , a crank position sensor  52 , a throttle position sensor  53 , a coolant temperature sensor  54 , a cam position sensor  55 , and the like. The accelerator position sensor  50  detects an accelerator operation amount ACCP that indicates an amount by which an accelerator pedal is depressed by a driver. The air flow meter  51  detects the temperature Ta of air introduced into the intake passage  30  through the air cleaner  31  and an intake air mass GA that is the mass of the introduced air. The crank position sensor  52  detects the rotation angle of the crankshaft  14  per unit time. The throttle position sensor  53  detects a throttle opening degree Th that is the opening degree of the throttle valve  35 . The coolant temperature sensor  54  detects an engine coolant temperature THW that is the temperature of engine coolant. The cam position sensor  55  detects the rotation angle of the intake camshaft. Then, the electronic control unit  100  calculates an engine rotational speed NE that indicates the rotational speed of the crankshaft  14  per unit time on the basis of the detected rotation angle of the crankshaft  14 . 
     The electronic control unit  100  loads detected signals from these various sensors  50  to  55 , and executes various controls associated with torque control. For example, the electronic control unit  100  calculates a required torque on the basis of the accelerator operation amount ACCP to change the throttle opening degree Th and controls the fuel injection amount and the ignition timing in the internal combustion engine  10  on the basis of the intake air mass GA detected by the air flow meter  51  to thereby generate torque appropriate to the required torque. 
     In addition, the electronic control unit  100  executes feedback control for controlling the torque of the internal combustion engine  10  so as to coincide the rate of decrease in the engine rotational speed NE from when depression of the accelerator pedal by the driver is released to when the engine rotational speed NE reaches an idle rotational speed with a target rate of decrease in the engine rotational speed. Note that, in this embodiment, a variation ΔNE that is a variation in the engine rotational speed NE per unit time is controlled so as to coincide with a target variation ΔNEtrg to thereby execute feedback control. 
     In this feedback control, the torque of the internal combustion engine  10  is increased or reduced on the basis of the operating states of auxiliaries, such as an air conditioner unit  200  and an alternator  300 , that are driven using the driving force of the internal combustion engine  10 . By so doing, the torque of the internal combustion engine  10  is controlled so as to suppress fluctuations in the variation ΔNE due to fluctuations in external load resulting from a variation in the operating states of the auxiliaries and to coincide the variation ΔNE with the target variation ΔNEtrg. 
     In order to execute such feedback control, various auxiliaries are connected to the electronic control unit  100  in addition to the various sensors  50  to  55  so as to be able to acquire the operating states of the various auxiliaries. For example, as indicated by the broken line in  FIG. 1 , the air conditioner unit  200  coupled to the crankshaft  14  is connected to the electronic control unit  100 , and a signal that indicates whether the air conditioner unit  200  is operating is input to the electronic control unit  100 . 
     Note that the air conditioner unit  200  is configured to reduce the external load that acts on the internal combustion engine  10  by releasing a clutch arranged between a compressor and the crankshaft  14  when the air conditioner unit  200  is not operating. Then, in the electronic control unit  100  according to the present embodiment, the external load that acts on the internal combustion engine  10  is estimated on the basis of whether the air conditioner unit  200  is operating. That is, when the air conditioner unit  200  is operating, the electronic control unit  100  estimates that at least the external load corresponding to the driving load of the air conditioner unit  200  acts on the internal combustion engine  10 ; whereas, when the air conditioner unit  200  is not operating, the electronic control unit  100  estimates that the external load corresponding to the driving load of the air conditioner unit  200  does not act on the internal combustion engine  10 . 
     In addition, as indicated by the broken line in  FIG. 1 , the alternator  300  coupled to the crankshaft  14  is connected to the electronic control unit  100 . The electronic control unit  100  adjusts the magnitude of field current supplied to the alternator  300  to thereby control the amount of electric power generated by the alternator  300 . Therefore, large field current is supplied to the alternator  300 , and, as the amount of electric power generated increases, the external load that acts on the internal combustion engine  10  increases. Then, the electronic control unit  100  estimates the external load due to the alternator  300  on the basis of the magnitude of field current supplied to the alternator  300 . Specifically, the electronic control unit  100  estimates that the external load corresponding to the driving load of the alternator  300  increases as the field current increases. 
     In feedback control, the electronic control unit  100  corrects a basic torque set in order to achieve the target variation ΔNEtrg using a torque correction amount T that is calculated on the basis of the deviation D between an actual variation ΔNE and a target variation ΔNEtrg. By so doing, the electronic control unit  100  controls the torque of the internal combustion engine  10  so as to coincide the variation ΔNE with the target variation ΔNEtrg and to coincide the rate of decrease in the engine rotational speed NE with the target rate of decrease in the engine rotational speed. Note that the torque is varied by adjusting the throttle opening degree Th, adjusting the fuel injection amount, adjusting the ignition timing, or the like. 
     Hereinafter, the details of feedback control according to the present embodiment will be described with reference to  FIG. 2  and  FIG. 3 . Initially, feedback control will be described with reference to  FIG. 2  that shows the flow of a series of processes according to the feedback control. The routine shown in  FIG. 2  is repeatedly executed at predetermined control intervals by the electronic control unit  100  when depression of the accelerator pedal is released during engine operation. 
     As the electronic control unit  100  starts the routine, the electronic control unit  100  initially calculates the deviation D between the target variation ΔNEtrg and the actual variation ΔNE of the engine rotational speed NE in step S 100  as shown in  FIG. 2 . Note that, here, the target variation ΔNEtrg is subtracted from the variation ΔNE, and the absolute value of that difference is set as the deviation D. Then, the process proceeds to step S 110 , and a feedback gain G set through the processes shown in  FIG. 3  is loaded. Note that a series of processes for setting the feedback gain G will be described later with reference to  FIG. 3 . 
     As the feedback gain G is loaded, the electronic control unit  100  calculates an update amount ΔT for the torque correction amount T on the basis of the deviation D and the feedback gain G in step S 120 . Specifically, the electronic control unit  100  multiplies the deviation D by the feedback gain G, and sets the product as the update amount ΔT. 
     As the update amount ΔT is thus calculated, the electronic control unit  100  updates the torque correction amount T in step S 130 . Specifically, the electronic control unit  100  adds the update amount ΔT to the torque correction amount T used in order to correct the torque in the last control interval to thereby update the torque correction amount T. Then, in step S 140 , the electronic control unit  100  determines whether the updated torque correction amount T is smaller than or equal to an upper limit value. 
     When it is determined in step S 140  that the torque correction amount T is smaller than or equal to the upper limit value (YES in step S 140 ), the process proceeds to step S 150 , and the electronic control unit  100  determines whether the torque correction amount T is larger than or equal to a lower limit value. 
     When it is determined in step S 150  that the torque correction amount T is larger than or equal to the lower limit value (YES in step S 150 ), the process proceeds to step S 160 , and the electronic control unit  100  corrects the torque using the torque correction amount T updated in step S 130 . Specifically, the electronic control unit  100  adds the torque correction amount T to a basic torque to calculate a target torque, and controls the torque so as to obtain the target torque to thereby correct the torque. 
     On the other hand, when it is determined in step S 140  that the torque correction amount T is larger than an upper limit value (NO in step  140 ), the process proceeds to step S 145 , and the electronic control unit  100  sets the upper limit value as a new torque correction amount T. That is, the electronic control unit  100  sets the torque correction amount T at a value equal to the upper limit value. In this way, as the electronic control unit  100  sets the torque correction amount T at a value equal to the upper limit value, the process proceeds to step S 160 , and the electronic control unit  100  corrects the torque using the torque correction amount T. 
     In addition, when it is determined in step S 150  that the torque correction amount T is smaller than a lower limit value (NO in step S 150 ), the process proceeds to step S 155 , and the electronic control unit  100  sets the lower limit value as a new torque correction amount T. That is, the electronic control unit  100  sets the torque correction amount T at a value equal to the lower limit value. As the electronic control unit  100  sets the torque correction amount T at a value equal to the lower limit value, the process proceeds to step S 160 , and the electronic control unit  100  corrects the torque using the torque correction amount T. 
     That is, the feedback control according to the present embodiment provides an upper limit and a lower limit for the torque correction amount T to control the torque correction amount such that the torque correction amount T does not become a value larger than the upper limit value or a value smaller than the lower limit value. 
     Note that the feedback control according to the present embodiment sets the lower limit value at “0”. That is, in the feedback control according to the present embodiment, the torque correction amount T is not set at a negative value. 
     As the torque is corrected using the thus set torque correction amount T, the electronic control unit  100  once ends the routine. 
     Next, the process of setting the feedback gain G used in the above feedback control will be described with reference to  FIG. 3 . The routine shown in  FIG. 3 , as well as the routine associated with the above described feedback control, is repeatedly executed at predetermined control intervals by the electronic control unit  100  when depression of the accelerator pedal is released during engine operation. The routine shown in  FIG. 3  may be executed in parallel with the routine associated with the feedback control described with reference to  FIG. 2 . 
     As the electronic control unit  100  starts the routine, the electronic control unit  100  initially calculates a required idle torque Tid in step S 200  as shown in  FIG. 3 . The required idle torque Tid is a value that indicates a torque required to keep the engine rotational speed NE at the idle rotational speed. Here, the electronic control unit  100  calculates the required idle torque Tid in consideration of the internal load of the internal combustion engine  10  and the external load that acts on the internal combustion engine  10 . 
     Specifically, the electronic control unit  100  initially acquires the engine coolant temperature THW as a value for estimating the engine temperature, and estimates the internal load of the internal combustion engine  10 , that is, a friction load due to friction and the viscosity of lubricating oil. Then, the torque consumed by the internal load is estimated on the basis of the friction load and the engine rotational speed NE. In addition, the electronic control unit  100  calculates the torque required to drive various auxiliaries on the basis of the external load estimated on the basis of the operating states of the various auxiliaries, and adds the torque required to drive the auxiliaries to the torque consumed by the internal load to thereby calculate the required idle torque Tid. 
     As the electronic control unit  100  calculates the required idle torque Tid in this way, the process proceeds to step S 210 , and the electronic control unit  100  determines the feedback gain G on the basis of the calculated required idle torque Tid. Here, the electronic control unit  100  sets the feedback gain G at a larger value as the required idle torque Tid increases. 
     In this way, as the electronic control unit  100  sets the feedback gain G on the basis of the required idle torque Tid, the electronic control unit  100  once ends the routine. 
     The required idle torque Tid is a torque required to compensate for the torque consumed by the internal load due to the friction load inside the internal combustion engine and the external load that acts from the outside of the internal combustion engine  10  on the internal combustion engine  10  to thereby keep the engine rotational speed NE at the idle rotational speed. Therefore, when the required idle torque Tid is large, it is estimated that the torque consumed by the internal load and the external load is large, the engine rotational speed NE tends to decrease, and it is estimated that the rate of decrease in the engine rotational speed NE in the case of insufficient torque also increases. 
     In contrast to this, in the above embodiment, as the required idle torque Tid increases, the feedback gain G in feedback control is increased. Therefore, when the torque consumed by the internal load and the external load is large and the rate of decrease in the engine rotational speed NE in the case of insufficient torque is high, the sensitivity of an increase or decrease in torque corrected through feedback control is further increased. 
     According to the above described embodiment, the following advantageous effects may be obtained. 
     (1) In the above embodiment, as described above, the feedback gain G is increased as the required idle torque Tid increases, so, when the rate of decrease in the engine rotational speed NE in the case of insufficient torque increases, the sensitivity of feedback control is further increased. Thus, even when the external load varies, it is possible to set the appropriate feedback gain G by which a variation in the rate of decrease in the engine rotational speed NE due to the variation of the external load may be suppressed. That is, even when the external load varies, the rate of decrease in the engine rotational speed NE may be appropriately subjected to feedback control. 
     (2) The viscosity of lubricating oil increases as the engine temperature decreases, so the internal load due to the friction load inside the internal combustion engine increases as the engine temperature decreases. Furthermore, as the engine rotational speed NE increases, the amounts of slide at the sliding portions of the components of the engine increase, so the amount of torque consumed by the internal load per unit time increases as the engine rotational speed NE increases. 
     In addition, the external load that acts on the internal combustion engine  10  varies on the basis of the operating states of the auxiliaries driven using the output of the internal combustion engine  10 . Specifically, the external load that acts on the internal combustion engine  10  increases as the number of operated auxiliaries increases and as the driving amount of each operated auxiliary increases. 
     Therefore, as in the case of the above embodiment, by acquiring the idle rotational speed, the engine coolant temperature THW and the operating states of the auxiliaries driven using the output of the internal combustion engine  10 , it is possible to calculate the required idle torque Tid required to keep the engine rotational speed NE at the idle rotational speed. 
     (3) When the above described feedback control is executed, the behavior of a variation in the engine rotational speed NE resulting from the release of accelerator operation by the time when the engine rotational speed NE reaches the idle rotational speed may be stabilized. 
     (4) When the air conditioner unit  200  driven using the driving force of the internal combustion engine  10  is operating, the external load that acts on the internal combustion engine  10  increases by the amount of driving force required to drive the air conditioner unit  200 . In contrast to this, in the above embodiment, the magnitude of the external load that acts on the internal combustion engine  10  is estimated on the basis of whether the air conditioner unit  200  is operating. Therefore, by acquiring whether the air conditioner unit  200  is operating, it is possible to estimate the magnitude of the external load that acts on the internal combustion engine  10 . 
     (5) When a large field current is supplied to the alternator  300 , the amount of electric power generated by the alternator  300  increases in comparison with when a small field current is supplied, so a driving force required to drive the alternator  300  also increases. In contrast to this, in the above embodiment, the magnitude of the external load that acts on the internal combustion engine  10  is estimated on the basis of the magnitude of field current supplied to the alternator  300 . Therefore, by acquiring the magnitude of field current supplied to the alternator  300 , it is possible to estimate the magnitude of the external load that acts on the internal combustion engine  10 . 
     Note that the above described embodiment may be modified where appropriate or may be implemented in the following embodiments. 
     In the above described embodiment, the magnitude of the external load that acts on the internal combustion engine  10  on the basis of whether the air conditioner unit  200  is operating is estimated; instead, a method of estimating the magnitude of the external load that acts on the internal combustion engine  10  may be modified where appropriate. 
     For example, in the case of a control device for an internal combustion engine mounted on a vehicle equipped with an air conditioner unit having a variable displacement compressor, the magnitude of the external load may be estimated on the basis of the displacement of the compressor. 
     In addition, in the above described embodiment, the magnitude of the external load caused by the alternator  300  is estimated on the basis of the magnitude of field current supplied to the alternator  300 . In contrast to this, in a control device for an internal combustion engine mounted on a vehicle equipped with a clutch between the crankshaft  14  and the alternator  300 , the magnitude of the external load may be estimated on the basis of the engagement state of the clutch. 
     That is, in a vehicle that is equipped with a clutch and that engages or disengages the crankshaft  14  to or from the alternator  300 , the magnitude of the external load that acts on the internal combustion engine  10  varies on the basis of the engagement state of the clutch. Therefore, it is applicable that the engagement state of the clutch is monitored, and then the magnitude of the external load is estimated on the basis of the engagement state of the clutch. 
     In addition, as the amount of electric power generated by the alternator  300  increases, the driving load of the alternator  300  increases. Therefore, it is also applicable that the amount of electric power generated by the alternator  300  is monitored and then the external load is estimated on the basis of the amount of electric power generated by the alternator  300 . 
     Furthermore, when the amount of electric power generated by the alternator  300  is controlled on the basis of the magnitude of power consumption, the magnitude of power consumption may be monitored to estimate the external load on the basis of the magnitude of power consumption. 
     Note that the magnitude of power consumption varies on the basis of whether the light of a vehicle is lit up, whether an audio is used, or the like, so the magnitude of the external load may be estimated on the basis of whether the light of the vehicle is lit up, whether the audio is used, or the like. 
     The required idle torque Tid that is the torque required to keep the idle rotational speed is calculated as a required torque used as an index that indicates the rate of decrease in the engine rotational speed NE in the case of insufficient torque, and the feedback gain G is set on the basis of the required idle torque Tid. In contrast to this, when the torque required to keep the engine rotational speed NE at a constant rotational speed is calculated as a required torque, the rate of decrease in the engine rotational speed NE in the case of insufficient torque may be estimated on the basis of the required torque. Therefore, the required torque acquired at the time of calculating the feedback gain G is not limited to the required idle torque Tid. That is, the engine rotational speed NE acquired at the time of calculating the required torque may be selectively set. For example, it is applicable that the torque required to keep the engine rotational speed NE at 3000 rpm (revolutions per minute) higher than the idle rotational speed is calculated and the feedback gain G is set on the basis of the required torque. 
     In the above embodiment, feedback control is executed from when accelerator operation is released to when the engine rotational speed NE reaches the idle rotational speed; however, feedback control according to the aspect of the invention is not limited to such control that is executed when accelerator operation is not performed. That is, the aspect of the invention may be applied to feedback control for executing feedback control over the rate of decrease in the engine rotational speed NE when the accelerator operation amount ACCP is reduced. 
     In the above described embodiment, the aspect of the invention is illustrated as the electronic control unit  100  that controls the internal combustion engine  10  equipped with the throttle valve  35  and that adjusts the intake air mass GA by varying the throttle opening degree Th; however, the configuration for adjusting the intake air mass GA may be modified where appropriate. For example, it is applicable that a bypass passage that bypasses the throttle valve  35  provided in the intake passage  30  is provided and an idle speed control valve that is used to adjust the intake air mass GA is provided in the bypass passage. Then, the aspect of the invention may be applied to an internal combustion engine that varies the opening degree of the idle speed control valve to thereby increase or reduce the intake air mass GA while accelerator operation is released. With the above configuration, even when the throttle valve  35  is closed, the opening degree of the idle speed control valve is varied to thereby make it possible to adjust the intake air mass GA. 
     In addition, other than the above, the aspect of the invention may be, for example, applied to an internal combustion engine in which a mechanism for changing the lift and duration of each intake valve  17  is provided to adjust the intake air mass GA and the lift and duration of each intake valve  17  is changed to thereby adjust the intake air mass GA. 
     The engine temperature is estimated on the basis of the engine coolant temperature THW; instead, a method of detecting the engine temperature may be modified where appropriate. For example, it is applicable that the engine temperature is estimated on the basis of the temperature of lubricating oil instead of the engine coolant temperature THW or a sensor that directly detects the engine temperature is provided. In addition, it is also applicable that the engine temperature is estimated on the basis of the accumulated value of the intake air mass GA, or the like, which correlates with the combustion heat in the internal combustion engine  10 .