Patent Publication Number: US-6990953-B2

Title: Idle rotation control of an internal combustion engine

Description:
FIELD OF THE INVENTION 
   This invention relates to idle rotation speed control of an internal combustion engine. 
   BACKGROUND OF THE INVENTION 
   Tokkai Hei 9-68084 published by the Japan Patent Office in 1997 proposes a vehicle internal combustion engine wherein the intake air flow rate is open-loop corrected for predictable loads such as electrical accessories and the air conditioner, and the intake air flow rate is feedback corrected based on the real rotation speed such that a target idle rotation speed is maintained, for loads which cannot be predicted, such as due to external disturbances. 
   SUMMARY OF THE INVENTION 
   As a general characteristic of proportional/integral control in feedback correction, if the feedback gain is too large, hunting or overshoot occur, and if the feedback gain is too small, convergence to the target value is slow. In an internal combustion engine for vehicles, the idle rotation speed does not vary suddenly, so a smaller gain setting which emphasizes control stability is usually used. As a result, when a large load which cannot be predicted acts and the idle rotation speed falls by a large amount, convergence to the target value of the idle rotation speed tends to be delayed. 
   Examples of loads which are difficult to predict are when release of the lockup clutch of an automatic transmission is too late due to sudden braking, or when a large load acts because load changes cannot be detected due to a fault of the power steering switch or oil pressure switch. 
   It is therefore an object of this invention to rapidly return the idle rotation speed to the target value with good response under stable control when the idle rotation speed falls sharply due to a large load fluctuation. 
   In order to achieve the above object, this invention provides an idle rotation speed control device of an internal combustion engine. The control device comprises a mechanism which regulates an intake air flow rate of the internal combustion engine, a sensor which detects an engine rotation speed of the internal combustion engine, and a programmable controller which controls the intake air flow rate regulating mechanism. 
   The controller is programmed to calculate, when the engine rotation speed is different from an target idle engine rotation speed, a feedback correction amount so that the intake air flow rate is gradually varied in a direction such that the engine rotation speed approaches the target idle engine rotation speed, calculate an increase correction amount of the intake air flow rate based on the engine rotation speed, control, when the engine rotation speed drops below the target idle rotation speed, the mechanism based on the sum of the feedback correction amount and increase correction amount, determine whether or not the engine rotation speed satisfies a preset increase correction termination condition, and set, when the engine rotation speed satisfies the increase correction termination condition, the sum of the feedback correction amount and increase correction amount when the termination condition is satisfied, to a new feedback correction amount, while setting the increase correction amount for subsequent control to be zero. 
   This invention also provides an idle rotation speed control method of the internal combustion engine, 
   The control method comprises detecting an engine rotation speed of the internal combustion engine, calculating, when the engine rotation speed is different from an target idle engine rotation speed, a feedback correction amount so that the intake air flow rate is gradually varied in a direction such that the engine rotation speed approaches the target idle engine rotation speed, calculating an increase correction amount of the intake air flow rate based on the engine rotation speed, controlling, when the engine rotation speed drops below the target idle rotation speed, the mechanism based on the sum of the feedback correction amount and increase correction amount, determining whether or not the engine rotation speed satisfies a preset increase correction termination condition, and setting, when the engine rotation speed satisfies the increase correction termination condition, the sum of the feedback correction amount and increase correction amount when the termination condition is satisfied, to a new feedback correction amount, while setting the increase correction amount for subsequent control to be zero. 
   The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an idle rotation control device according to this invention. 
       FIG. 2  is a flowchart describing an intake air flow rate correction routine performed by a controller according to this invention. 
       FIGS. 3A–3E  are timing charts describing the execution result of the intake air flow rate correction routine. 
       FIGS. 4A–4E  are similar to  FIGS. 3A–3E , but showing the execution result of a routine according to a second embodiment of the invention. 
       FIG. 5  is similar to  FIG. 2 , but showing a third embodiment of the invention. 
       FIGS. 6A–6C  are timing charts comparing the execution result of the intake air flow rate correction routine according to the third embodiment, with the execution result of the intake air flow rate correction routine according to the second embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1  of the drawings, an internal combustion engine  11  comprises an electronic throttle  14  which regulates an intake air flow rate supplied to an intake passage  12 . The electronic throttle  14  is operated by a throttle actuator  13  which responds to an incoming signal from a controller  21 . 
   The controller  21  performs feedback control of the idle rotation speed to a target rotation speed through a signal output to the throttle actuator  13  based on incoming signals from various sensors during idle rotation of the internal combustion engine  11 . 
   The controller  21  comprises a microcomputer comprising a central processing unit (CPU), read-only memory (ROM), random access memory (RAM), and an input/output interface (I/O interface). The controller  21  may also comprise plural microcomputers. 
   The various sensors include a throttle position sensor  15  which detects an opening of the electronic throttle  14 , an air flow meter  16  which detects an intake air flow rate of the intake passage  12 , an engine rotation speed sensor  17  which detects a rotation speed NE of the internal combustion engine  11 , and an accelerator pedal switch  18  which detects whether or not the accelerator pedal of the vehicle is in a release state. 
   The controller  21  determines whether or not the internal combustion engine  11  is in an idle running state based on a signal from the accelerator pedal switch  18 . In the idle running state, the idle rotation speed is feedback-controlled to a predetermined target idle rotation speed according to a signal from the rotation speed sensor  17 , by regulating the intake air flow rate via the throttle actuator  13  and electronic throttle  14 . In this process, feedback control of the intake air flow rate is also performed based on a signal from the air flow meter  16 . 
   The basic feedback control of the idle rotation speed is integral control. Further, according to this invention, if a rotation speed deviation is large, the intake air flow rate is corrected irrespective of the feedback control amount so as to recover the engine rotation speed to the target idle rotation speed. 
   Next, referring to  FIG. 2 , the intake air flow rate correction routine performed by the controller  21  will be described. The controller  21  performs this routine at an interval of ten milliseconds during running of the internal combustion engine  11 . When the internal combustion engine  11  is in an idle running state, as described above, feedback control to the target idle rotation speed of an engine rotation speed is performed by another idle rotation speed feedback control routine. 
   The routine shown in this figure corrects the target intake air flow rate under predetermined conditions. It has priority over control of the opening of the electronic throttle  14  which is performed as part of the idle rotation speed feedback control routine, and controls the opening of the electronic throttle  14  based on a corrected target intake air flow rate. 
   First, in a step S 201 , the controller  21  determines whether or not the internal combustion engine  11  is in an idle running state. Specifically, it is determined that the internal combustion engine  11  is in the idle running state when the accelerator pedal is released based on the signal from accelerator pedal switch  18 . 
   When the determination of the step S 201  is negative, the controller  21  terminates the routine immediately without performing subsequent steps. When the determination of the step S 201  is affirmative, the controller  21  performs the processing of a step S 202  and subsequent steps. In the step S 202 , the controller  21  calculates a rotation speed deviation ΔNE of the internal combustion engine  11  by the following equation (1):
 
Δ NE=tNE−NE   (1)
         where, tNE=target idle rotation speed, and
           NE=real rotation speed of the internal combustion engine  11 .   
               

   The real rotation speed NE is the detection speed of the rotation speed sensor  17 . As shown by the equation, when the real rotation speed of the internal combustion engine  11  is less than the target idle rotation speed, the rotation speed rotation speed deviation ΔNE is a positive value. 
   The controller  21  further calculates a feedback correction amount Q FB  of the intake air flow rate in basic feedback control by the following equation (2):
 
for  ΔNE&gt;Y,Q   FB   =Q   FBZ   +Δl , and
 
for  ΔNE&lt;Y,Q   FB   =Q   FBZ   −Δl   (2)
         where, Y=boundary value which specifies a dead zone,
           Q FBZ =Q FB  calculated on immediately preceding occasion the routine was executed, and   Δl=increment.   
               

   An environment is thus obtained wherein the feedback correction amount Q FB  calculated using equation (2) gradually varies, and hunting of the idle rotation speed does not easily occur. Under the usual control conditions, due to feedback control of the opening of the electronic throttle  14  based on the rotation speed deviation ΔNE of the internal combustion engine  11 , the internal combustion engine  11  absorbs a certain amount of load fluctuation, and the real rotation speed is held near the target idle rotation speed. 
   The method of calculating the feedback correction amount Q FB  in the step S 202  is not limited to equation (2). It is sufficient to use a calculation method wherein the feedback correction amount Q FB  varies gradually according to the deviation ΔNE on each occasion the routine is executed. For example, a calculation method of proportional/integral control wherein a proportional gain is set small, can also be applied to calculation of the feedback correction amount Q FB  in the step S 202 . 
   In a next step S 204 , the controller  21  calculates an intake air flow rate increase amount ΔQN by looking up a map having the characteristics shown in the figure which is stored in the internal memory (ROM) based on the rotation speed deviation ΔNE. 
   Specifically, the intake air flow rate increase amount ΔQN increases as the rotation speed deviation ΔNE increases. When the rotation speed deviation ΔNE is smaller than a predetermined deviation W and the rotation speed deviation ΔNE is a negative value, the intake air flow increase amount ΔQN is zero. When ΔQN is zero, control of the intake air flow rate is performed depending on the feedback control based on the rotation speed deviation ΔNE in the step S 202 . 
   In a next step S 205 , it is determined whether or not the rotation speed deviation ΔNE of the controller  21  is equal to or greater than a predetermined value XNE. Here, the predetermined value XNE is set to zero. The predetermined value XNE is a value for determining whether the rotation speed NE of the internal combustion engine  11  has substantially returned to the target idle rotation speed tNE. It is not necessarily zero, and may be a value close to zero. 
   When the determination of the step S 205  is affirmative, in a step S 206 , the controller  21  sets a final increase amount ΔQN MAX  of the intake air flow rate. Specifically, the larger of the intake air flow increase amount ΔQN found by looking up a map in the step S 204  and an immediately preceding value ΔQN MAXZ  of the final increase amount ΔQN MAX  found on the immediately preceding occasion the routine was executed, is taken as the final increase amount ΔQN MAX . 
   When the rotation speed deviation ΔNE is equal to or greater than the predetermined value XNE in the step S 205 , and the rotation speed deviation ΔNE increases on each occasion the routine is executed, therefore, the intake air flow increase amount ΔQN found from the map in the step S 204  is applied to the final increase amount ΔQN MAX  of the intake air flow rate. 
   On the other hand, in the step S 205 , when the rotation speed deviation ΔNE is equal to or greater than the predetermined value XNE, but the rotation speed deviation ΔNE decreases on each occasion the routine is executed, the immediately preceding value ΔQN MAXZ  is always applied to the final increase amount ΔQN MAX  of the intake air flow. In other words, the final increase amount ΔQN MAX  is held at a fixed value. 
   In a next step S 207 , the controller  21  calculates a total intake air flow rate Q TOTAL  supplied to the internal combustion engine  11  by the following equation (3):
 
 Q   TOTAL   =Q   CAL   +Q   FB   +ΔQN   MAX   (3)
         where, Q CAL =basic intake air flow rate during idle running of the internal combustion engine  11 , and
           Q FB =feedback correction amount of the intake air flow rate calculated in the step S 201 .   
               

   The basic intake air flow rate Q CAL  is set beforehand according to the cooling water temperature of the internal combustion engine  11 , and the running state of accessories such as the air conditioner. 
   When the determination of the step S 205  is negative, i.e., when the rotation speed NE of the internal combustion engine  11  has reached or exceeded the target idle rotation speed tNE, the controller  21 , in a step S 208 , sets the sum of the immediately preceding value Q FBZ  of the feedback correction amount of intake air flow rate and the immediately preceding value ΔQN MAXZ , to the feedback correction amount Q FB  of the intake air flow rate. 
   Here, the immediately preceding values mean Q FB  calculated in the step S 201  and the final increase amount ΔQN MAX  calculated in the step S 206  on the immediately preceding occasion the routine was executed. An immediately preceding value ΔQN MAXZ  of the final increase amount corresponds to an increase correction amount when termination conditions are satisfied in the claims. 
   The controller  21  further sets the final increase amount ΔQN MAX  to zero. By setting the final increase amount ΔQN MAX  to zero, the value of ΔQN MAX  used for the calculation performed in the following step S 207 , is zero. 
   The reason why ΔQN MAX  is reset to zero in the step S 208  is as follows. In the step S 208 , the feedback correction amount Q FB  is calculated by adding the immediately preceding value ΔQN MAXZ  of the final increase amount, to the immediately preceding value Q FBZ  of the feedback correction amount. 
   This feedback correction amount Q FB  which was increased by the final increase amount ΔQN MAXZ  is used as the immediately preceding value Q FBZ  on the next occasion the step S 208  is executed. In other words, the immediately preceding value Q FBZ  used on the next occasion the step S 208  is executed, is a value which has already been increase-corrected. Therefore, on the next and subsequent occasions the step S 208  is executed, ΔQN MAX  is reset to zero so that the increase correction is not duplicated. 
   After the processing of the step S 206 , the controller  21  performs the processing of the aforesaid step S 207 , and determines the total intake air flow rate Q TOTAL . When the processing of the step S 207  is performed following the step S 205 , ΔQN MAX  in equation (3) is zero. 
   After the processing of the step S 207 , the controller  21  terminates the routine. 
   The controller  21  regulates the opening of the electronic throttle  14  based on the total intake air flow Q TOTAL  determined in this way. 
   Next, referring to  FIGS. 3A–3E , the function of the above routine when there is a load change of the internal combustion engine, will be described. The solid line in the figure shows the result of executing the routine of  FIG. 2 . The dashed line in the figure shows the result of controlling the intake air flow rate only by feedback control according to equation (1). 
   Referring to  FIG. 3A , if an unexpected load change occurs at a time P during idle running of the internal combustion engine  11 , the rotation speed NE of the internal combustion engine  11  will drop sharply. If the rotation speed NE of the internal combustion engine  11  drops sharply, and only the general feedback control represented by equation (1) is performed, a long time is required for the rotation speed NE to return to the target idle rotation speed tNE, as shown in  FIG. 3A . This is because, as shown in  FIGS. 3C ,  3 D, in feedback control, the intake air flow rate increases only by Δl each time control is performed. 
   Conversely, if the intake air flow rate correction routine of  FIG. 2  is performed, at and after the time P, until the rotation speed NE of the internal combustion engine  11  completely returns to the target idle rotation speed tNE, the feedback correction amount Q FB  of the intake air flow rate is increased in the step S 207  using the final increase amount ΔQN MAX  of the intake air flow rate calculated in the step S 206 . 
   Therefore, immediately after the time P when a decrease of the rotation speed of the internal combustion engine  11  is detected, the total intake air flow Q TOTAL  increases considerably as shown in  FIG. 3C , and the rotation speed NE rapidly approaches the target value tNE as shown in  FIGS. 3A ,  3 B. 
   As a result of this control, at a time R shown in  FIG. 3B , the rotation speed deviation ΔNE is already effectively zero. However, since the rotation speed deviation ΔNE has not become a negative value, in this step, the determination result of the step S 205  of the routine of  FIG. 2  is still affirmative. Therefore, as shown in FIGS.  3 C, 3 D, both the final increase amount ΔQN MAX  of the intake air flow rate and the total intake air flow rate Q TOTAL  are held at a high level. 
   When a time Q is reached, as shown in  FIG. 3B , the rotation speed deviation ΔNE becomes a negative value, and the determination of the step S 205  changes over to negative. 
   As a result, in the step S 208 , the final increase amount ΔQN MAX  is reset to zero, and on the next and subsequent occasions the routine is executed, only the feedback correction amount Q FB  is applied to the total intake air flow rate Q TOTAL . 
   In other words, the control returns to ordinary feedback control by integral control of the intake air flow rate. However, the immediately preceding value Q FBZ  of the feedback correction amount applied in the step S 208  on the next occasion the routine is executed, is a value to which an increase correction has been added as described above. 
   Summarizing this control, after the feedback correction amount Q FB  of the intake air flow rate is increased by a value corresponding to the final increase amount ΔQN MAXZ  at the time Q, it gradually increases in increments of Δl in equation (2). 
   As described above, due to the execution of the routine of  FIG. 2 , even if the rotation speed NE of the internal combustion engine  11  drops sharply during idle running due to a large load fluctuation, the rotation speed NE can be rapidly returned to the target value tNE. 
   Also, as shown in  FIG. 3B , as a result of the increase correction, the rotation speed NE has already returned to the vicinity of the target idle rotation speed tNE at a time R well before the time Q. However, in the routine of  FIG. 2 , the increase correction by the final increase amount ΔQN MAXZ  is not immediately stopped at the time R, and the increase correction is continued as shown in FIGS.  3 C, 3 D until the deviation ΔNE becomes a negative value at the time Q. 
   Therefore, the rotation speed NE, which has returned to the vicinity of the target idle rotation speed tNE, is definitively prevented from dropping again due to interruption of the increase correction, and stable control of the intake air flow rate is achieved. 
   If it were desired to accelerate the response with which the rotation speed NE of the internal combustion engine  11 , which has dropped during idle running, returns to the target idle rotation speed tNE, it would be sufficient to apply proportional/integral control to the feedback control of intake air flow rate, and set the proportional gain large. 
   However, if this control is applied after the rotation speed NE returns to the vicinity of the target idle rotation speed tNE at the time R in  FIG. 3C , the proportional amount is zero or a value close to zero, so this has no effect in suppressing another drop of the rotation speed NE, and the control of the idle rotation speed is not stable. 
   According to this invention, by combining high stability integral control or a similar control with an increase correction of the intake air flow rate corresponding to a sharp drop of the rotation speed NE of the internal combustion engine  11 , the rotation speed NE of the internal combustion engine  11  which has dropped sharply is rapidly returned to the target idle rotation speed tNE, and the engine rotation speed NE after it has returned, is stabilized. 
   In the above embodiment, in the calculation of the step S 204 , the intake air flow rate increase ΔQN is set to be zero until the rotation speed deviation ΔNE reaches a predetermined deviation W. Also, the predetermined value XNE used in the step S 295  is set to zero. 
   However, various variations are possible regarding the setting of the predetermined deviation Wand the value of the predetermined value XNE. 
   Referring to  FIGS. 4A–4E , a second embodiment of this invention will now be described wherein the predetermined deviation W is set to zero, and the predetermined value XNE is set to a positive value. The steps of the intake air flow rate correction routine performed by the controller  21  are identical to those of the first embodiment. 
   According to this embodiment, when the rotation speed deviation ΔNE is equal to or greater than the predetermined value XNE in the step S 205 , in the step S 206 , an increase correction of the intake air flow rate by the final increase amount ΔQN MAX  of the intake air flow rate, is applied. 
   Further, if the rotation speed deviation ΔNE falls below the predetermined value XNE at the time R in  FIG. 4B , the increase correction of the intake air flow rate by the final increase amount ΔQN MAX  is immediately terminated, and subsequent control of the intake air flow rate is performed by the usual feedback control. 
   However, in the step S 208 , by incorporating the final increase amount ΔQN MAX  in the feedback correction amount Q FB  as shown in  FIG. 4E , the feedback correction amount Q FB  is largely increased. As a result, the feedback correction amount Q FB  is held at a high level until the rotation speed deviation ΔNE fluctuates largely in a negative direction at the time Q, i.e., until the rotation speed NE of the internal combustion engine  11  largely exceeds the target idle rotation speed tNE. 
   According to this embodiment, the increase correction of the intake air flow rate by the final increase amount ΔQN MAX  is terminated at the time R, but the final increase amount ΔQN MAX  of the time of termination is incorporated into the feedback correction amount Q FB , so the increase correction of the intake air flow rate actually continues until a time T. 
   Therefore, as in the first embodiment, even if the rotation speed NE of the internal combustion engine  11  drops sharply during idle running due to a large load fluctuation, the rotation speed NE can be rapidly and surely returned to the target value tNE, and drop of the rotation speed NE after return is also prevented. 
   In this embodiment, the predetermined deviation W is set to zero, so there is no dead zone in the calculation of the intake air flow increase amount ΔQN. However, the predetermined value XNE is set to a positive value, so an identical result to that of the first embodiment is obtained regarding the control characteristics of the intake air flow rate. 
   Next, a third embodiment of this invention will be described referring to  FIG. 5 , and  FIGS. 6A–6C . 
   In this embodiment, the controller  21  executes the intake air flow rate correction routine shown in  FIG. 5  instead of the routine of  FIG. 2  of the first embodiment. 
   In this routine, steps S 303 , S 304  are provided instead of the step S 204  of the routine of  FIG. 2 . The remaining steps are identical to those of the routine of  FIG. 2 . The controller  21  executes this routine at an interval of ten milliseconds during running of the internal combustion engine  11 . 
   In the step S 303 , the controller  21  calculates a decrease ratio ΔNR of the rotation speed NE of the internal combustion engine  11  by the following equation (4):
 
 ΔNR=NE   Z   −NE   (4)
         where, NE Z =immediately preceding value of the rotation speed NE of the internal combustion engine  11 .       

   The routine is executed at an interval of ten milliseconds, so the decrease ratio ΔNR obtained in equation (4) corresponds to the variation of the rotation speed NE every ten milliseconds. 
   The controller  21 , in the next step S 304 , calculates an intake air flow rate correction amount ΔQR by looking up a map stored beforehand in the memory (ROM) from the rotation speed deviation ΔNE and the rotation speed decrease ratio ΔNR. 
   Here, the characteristics of this map will be described. As shown by the diagram on the right of the step S 304 , the intake air flow rate correction amount ΔQR increases the larger the rotation speed deviation ΔNE is, or the larger the rotation speed decrease ratio ΔNR is. 
   This map is set by experimentally determining the increase amount of the intake air flow rate required to compensate the decrease of torque due to a given variation of rotation speed, and by considering the increase amount as the intake air flow rate correction amount ΔQR. 
   Except for the value of the predetermined value XNE, the remaining steps of the routine are identical to those of the routine of  FIG. 2 . In the first embodiment, the predetermined value XNE for determining whether or not the engine rotation speed NE has returned to the target idle rotation speed tNE was set to zero, but in this embodiment, the predetermined value is set to a positive value as in the second embodiment. 
   The difference between this embodiment and the second embodiment is therefore that the calculation of the intake air flow rate correction amount ΔQR depends on the rotation speed decrease ratio ΔNR in addition to the rotation speed deviation ΔNE. In other words, even if the rotation speed deviation ΔNE is identical to the second embodiment, if the rotation speed decrease ratio ΔNR is large, the intake air flow rate correction amount ΔQR calculated in the step S 304  is a larger value than in the second embodiment. 
   As a result, as shown in  FIGS. 6A–6C , compared to the second embodiment, the time required to return the rotation speed NE of the internal combustion engine  11  which has dropped sharply, to the idle target rotation speed tNE, can be largely shortened. At the same time, regarding the rotation speed NE after it has returned to the target idle rotation speed tNE, a desirable stability can be maintained as in the second embodiment. 
   The contents of Tokugan 2004-153012, with a filing date of May 24, 2004 in Japan, are hereby incorporated by reference. 
   Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, within the scope of the claims. 
   For example, in the first and second embodiments, the intake air flow increase amount ΔQN is calculated from the deviation ΔNE of the engine rotation speed NE. In the third embodiment, the intake air flow increase amount ΔQN is calculated using both the deviation ΔNE and decrease ratio ΔNR. However, the intake air flow increase amount ΔQN can also be calculated based only on the decrease ratio ΔNR of the engine rotation speed NE. 
   The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows: