Abstract:
A control unit controls both a motor actuated throttle valve and a magnetically actuated variable valve in accordance with an operation condition of an internal combustion engine. The control unit is configured to provide both a first control mode wherein an intake air control is carried out by controlling the open/close timing of the variable valve while keeping the throttle valve at a full-open or near full-open position and a second control mode wherein the intake air control is carried out by controlling the position of the throttle valve while reducing a controllable range of the open/close timing of the variable valve. The first and second control modes are selectively switched in accordance with the operation condition of the engine. The first and second control modes allow the engine to output the same engine torque under the same operation condition of the engine. The first and second control modes are respectively provided by calculating target intake air amounts for the first and second control modes and feeding the engine with the calculated target intake air amounts respectively.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to intake air control systems of engines, and more particularly to intake air control systems of a type that is applied to an internal combustion engine which has both an electronically controlled throttle valve whose angular position can be freely controlled and electromagnetically actuated intake and exhaust valves (viz., variable valves) whose open/close timing can be freely controlled. 
     2. Description of the Prior Art 
     In internal combustion engines, a throttle valve has been widely used for controlling the amount of intake air directed to cylinders (viz., combustion chambers) of the engine. In these days, however, for controlling the intake air amount, usage of electromagnetically actuated intake and exhaust valves has been proposed and put into practical use in place of or in addition to the throttle valve. That is, by controlling the open/close timing of the valves, particularly, the intake valves, the amount of intake air is controlled. One of systems that embodied such idea is disclosed in Japanese Patent First Provisional Publication 8-200025. 
     In the engines having such system, the throttle valve is not used, or even if used, the throttle valve is auxiliarilly used for the control of intake air amount. That is, for controlling the amount of intake air, operation of the intake valves is mainly used by controlling the open/close timing of the same. Under operation of the engine with this control, the interior of the intake passage is permitted to have a slight negative pressure, which lowers a pumping loss and thus improves a net thermal efficiency of the engine. 
     SUMMARY OF THE INVENTION 
     However, due to inevitable limitation of operation speed of the electromagnetically actuated intake valves, in a high speed cruising of an associated motor vehicle, it is difficult to control the intake air amount to a desired value by using only the operation of the intake valves. In view of this, a measure may be thought out wherein in accordance with the operation condition of the engine, the intake air control is switched to a mode wherein the intake air amount is mainly controlled by the throttle valve while reducing a controllable range of the open/close timing of the intake valves. 
     However, even in the above-mentioned measure, it inevitably occurs that in the certain operation condition of the engine inducing the main control by the throttle valve, throttling of the throttle valve inevitably induces a certain pumping loss of the engine, which brings about a larger lowering in engine torque than that induced by the operation of the intake valves. Thus, upon switching of the intake air control mode, undesired torque gap tends to occur which lowers the driveability of the engine. 
     It is therefore an object of the present invention to provide an intake air control system of an engine, which produces no torque gap or at least minimizes the same upon switching of the intake air control mode. 
     According to a first aspect of the present invention, there is provided an intake air control system of an engine, which comprises a throttle valve installed in an intake air passage; a variable valve incorporated with a cylinder of the engine, the variable valve being controllable to have a desired open/close timing; and a control unit which includes a mode providing section that provides a first control mode wherein an intake air control is carried out by controlling the open/close timing of the variable valve while keeping the throttle valve at a full-open or near full-open position and a second control mode wherein the intake air control is carried out by controlling the position of the throttle valve while reducing a controllable range of the open/close timing of the variable valve; an intake air amount calculation section that calculates respective target intake air amounts for the first and second control modes, the respective target intake air amounts thus calculated allowing the engine to output substantially same engine torque in the respective first and second control modes under the same operation condition of the engine; and a mode switching section that carries out a switching between the first and second control modes in accordance with an operation condition of the engine. 
     According to a second aspect of the present invention, there is provided an intake air control system of an engine, which comprises a throttle valve installed in an intake air passage; a variable valve incorporated with a cylinder of the engine, the variable valve being controllable to have a desired open/close timing; and a control unit that controls the throttle valve and the variable valve in accordance with an operation condition of the engine, the control unit being configured to provide both a first control mode wherein an intake air control is carried out by controlling the open/close timing of the variable valve while keeping the throttle valve at a full-open or near full-open position and a second control mode wherein the intake air control is carried out by controlling the position of the throttle valve while reducing a controllable range of the open/close timing of the variable valve, the first and second control modes being selectively switched in accordance with the operation condition of the engine, and the first and second control modes allowing the engine to output the same engine torque under the same operation condition of the engine, the first and second control modes being respectively provided by calculating respective target intake air amounts for the first and second control modes and feeding the engine with the calculated target intake air amounts respectively. 
     According to a third aspect of the present invention, there is provided a method for controlling an intake air amount for an internal combustion engine. The engine includes a throttle valve installed in an intake air passage and a variable valve incorporated with a cylinder of the engine and controllable to have a desired open/close timing. The method comprises providing a first control mode wherein an intake air control is carried out by controlling the open/close timing of the variable valve while keeping the throttle valve at a full-open or near full-open position and a second control mode wherein the intake air control is carried out by controlling the position of the throttle valve while reducing a controllable range of the open/close timing of the variable valve; calculating respective target intake air amounts for the first and second control modes, the respective target intake air amounts thus calculated allowing the engine to output substantially same engine torque in the respective first and second control modes under the same operation condition of the engine; and carrying out a switching between the first and second control modes in accordance with an operation condition of the engine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an internal combustion engine to which an intake air control system of the present invention is practically applied; 
     FIG. 2 is a sectional view of an electromagnetically actuated valve used in the intake air control system of the present invention; 
     FIG. 3 is a flowchart showing operation steps executed in a control unit employed in a first embodiment of the present invention; 
     FIG. 4 is a map depicting a target intake air amount (Q) with respect to an accelerator open degree (APO) and an engine speed (Ne); 
     FIG. 5 is a map depicting a thermal efficiency loss compensation correction value “x” with respect to a reference fuel injection amount (Tp) and an engine speed (Ne); and 
     FIGS. 6,  7  and  8  are flowcharts similar to FIG. 3, but showing second, third and fourth embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to FIG. 1, there is shown a four-cycle internal combustion engine  10  operated on gasoline, to which an intake air control system of the present invention is practically applied. 
     Each cylinder of the engine  10  is equipped with intake and exhaust valves  12  and  14  which are controlled by an electromagnetic valve actuator  16 . That is, the open/close timing of each valve  12  or  14  is controlled by the actuator  16 . An intake port  18  leading to each cylinder is equipped with a fuel injector  20 , and each cylinder is equipped with an ignition plug  22  which is exposed to a combustion chamber  24 . The ignition plug  22  is equipped with an ignition coil  26 . 
     A crank-angle sensor  28  is connected to the engine  10 , which issues a reference signal upon sensing a reference position taken by a piston of each cylinder and issues a unit angular signal for each unit crank angle. An air flow meter  30  is mounted in an air intake passage upstream of each intake port  18 , which detects the amount of air directed toward the engine  10 . A negative pressure sensor  31  is mounted in the air intake passage downstream of the air flow meter  30 , which detects a negative pressure created in the air intake passage. A water temperature sensor  32  is connected to the engine  10  to detect the temperature of engine cooling water. Designated by numerals  34  and  36  are an accelerator sensor and a vehicle speed sensor  36 . That is, the accelerator sensor  34  detects an open degree of an accelerator (more specifically, depression degree of an accelerator pedal) and the vehicle speed sensor  36  detects the running speed of an associated motor vehicle. 
     Information signals from the sensors  28 ,  30 ,  31 ,  32 ,  34  and  36  are all fed to a control unit  38 . By processing these information signals, the control unit  38  feeds each fuel injector  20  with an injection pulse signal to control a fuel injection amount and a fuel injection timing, feeds each ignition coil  26  with an ignition signal to control an ignition timing and feeds each electromagnetic valve actuator  16  with a valve driving signal to control the open/close timing of the intake and exhaust valves  12  and  14 . 
     In the air intake passage at a position between the air flow meter  30  and the negative pressure sensor  31 , there is installed a throttle valve  40  whose open/close pivoting is actuated by an electric motor  42 . The motor  42  is controlled by a control signal issued from the control unit  38 . 
     In FIG. 2, there is shown the detail of the electromagnetic valve actuator  16 . As shown, the actuator  16  comprises a housing  44  made of a non-magnetic material. An armature  46  is movably received in the housing  44 , to which a valve stem  48  of the intake valve  12  (or the exhaust valve  14 ) is integrally connected, as shown. For ease of description, the description will be made on only the intake valve  12 . A valve closing electromagnet  50  is mounted on an upper position in the housing  44 , which, when electrically energized, attracts or pulls up the armature  46  to bring the intake valve  12  to its close position. A valve opening electromagnet  52  is mounted on a lower position in the housing  44 , which, when electrically energized, attracts or pulls down the armature  46  to bring the intake valve  12  to its open position, as shown. A valve closing spring  54  is compressed between the armature  46  and the lower wall of the housing  44  to bias the armature  46  upward, that is, in a direction to close the intake valve  12 . Furthermore, a valve opening spring  56  is compressed between the armature  46  and the upper wall of the housing  44  to bias the armature  46  downward, that is, in a direction to open the intake valve  12 . The spring forces of the two springs  54  and  56  are adjusted so that when the two electromagnets  50  and  52  are both deenergized, the intake valve  12  assumes a slight open position, that is, an intermediate position between the full-open and full-close positions of the intake valve  12 . When only the valve closing electromagnet  50  is energized, the intake valve  12  is brought up to the full-close position against the force of the valve opening spring  56 , while, when only the valve opening electromagnet  52  is energized, the intake valve  12  is brought down to the full-open position against the force of the valve closing spring  54 . 
     The electromagnetic valve actuator  16  actuates the intake valve  12  in such a manner that with the aid of the control unit  38  the open/close timing of the intake valve  12  takes a target value based on the operation condition of the engine  10 . That is, the close timing (IVC) of each intake valve  12  is controlled in accordance with a target intake air amount (Q: viz., target amount of intake air fed to each cylinder: target torque of the engine) derived based on both the accelerator opening degree (APO) detected by the accelerator sensor  34  and the engine speed (Ne) detected by the crank-angle sensor  28 . More specifically, for this control, the open timing (IVO) of the intake valve  12  is fixed in the vicinity of top dead center (TDC) of the corresponding piston, and the close timing (IVC) of the intake valve  12  is derived by looking up a map that represents a relation between the target intake air amount (Q) and the close timing (IVC). When the needed target intake air amount (Q) is small, the close timing (IVC) of the intake valve  12  is set at the side of top dead center (TDC), while when the needed target intake air amount (Q) is large, the close timing (IVC) is set at the side of bottom dead center (BDC). In the present invention, the closing timing (IVC) of the intake valve  12  is set to appear slightly earlier than the time of bottom dead center (BDC) of the piston in intake stroke. If desired, the closing timing (IVC) may be set to appear slightly later than the time of bottom dead center (BDC). 
     In the control for obtaining the target intake air amount, the close timing of the intake valve  12 , which determines the effective intake stroke of the piston, takes a big part in determining the target intake air amount (Q). That is, the position taken by the piston upon completion of the intake stroke (or upon closing of the intake valve  12 ) determines the effective intake stroke of the piston. Besides, the open/close timing of the intake and exhaust valves  12  and  14 , that is, the open timing (IVO) of the intake valve  12  and the close timing (EVO) of the exhaust valve  14 , which determine a valve overlap rate, and the open timing (EVO) of the exhaust valve  14  which participates in the exhaust efficiency, take a larger part in determining the internal EGR (exhaust gas recirculation) rate and thus in determining the intake air amount (viz., amount of fresh air fed to each cylinder). Thus, in the invention, the control of the open/close timing of a variable valve for controlling the intake air amount includes at least a control of the close timing of the intake valve  12 . However, the control of the invention may include the control of the open/close timing of the intake and exhaust valves  12  and  14 . 
     As is described hereinabove, in the present invention, basically, by carrying out the control of the open/close timing of the variable valve, particularly by carrying out the control of advancing or retarding the close timing of the intake valve  12 , the intake air amount is controlled to a target intake air amount. This control will be referred to as “first control mode” hereinafter. 
     However, as has been mentioned hereinbefore, this first control mode is not suitable for the low load and high speed operation of the engine. That is, by this first control mode, precise control to the target intake air amount is not obtained when the engine is under a low load and high speed condition. In fact, in this low load and high speed condition, more precise control to the target intake air amount may be achieved by controlling the throttle valve  40  in accordance with the temperature of the engine cooling water. 
     Accordingly, in the present invention, in such operation zone of the engine, a second control mode is adopted wherein the controllable range of the open/close timing of the intake/exhaust valve  12  or  14  is kept small and the open degree of the throttle valve  16  is freely controlled in accordance with the target intake air amount. More specifically, in the valve  12  or  14 , the valve timing is fixed. That is, the open timing (IVO) of the intake valve  12  is fixed in the vicinity of top dead center (TDC) of the corresponding piston, and the close timing (IVC) of the intake valve  12  is fixed in the vicinity of bottom dead center (BDC). 
     It is thus to be noted that in the first control mode, the intake air amount is mainly controlled by the open/close timing of the variable valve  12  or  14 , while, in the second control mode, the intake air amount is mainly controlled by the open degree of the throttle valve  40 . 
     Upon switching between the first and second control modes, a correction control is applied to the intake air amount to suppress or at least minimize an engine torque gap which would be produced when the same intake air amount is used in either of the two control modes. In fact, under usage of the same intake air amount, the engine torque provided by the second control mode is smaller than that provided by the first control mode. 
     In the following, the correction control to the intake air amount, which is carried out upon switching between the first and second control modes, will be described with reference to flowcharts of the accompanying drawings. 
     Referring to FIG. 3, there is shown a flowchart of operation steps adopted in a first embodiment of the present invention. 
     At step S 1 , based on the accelerator open degree (APO) and the engine speed (Ne), a target intake air amount (Q) needed for obtaining a target engine torque is calculated. In this step, the target intake air amount (Q) is directed to the amount needed by the first control mode wherein the intake air amount is controlled mainly by controlling the open/close timing of the variable valve. Specifically, the target intake air amount (Q) is derived by looking up a map of FIG. 4 that shows a relation between the accelerator open degree (APO), the engine speed (Ne) and the target intake air amount (Q). 
     Preferably, the target intake air amount (Q) is an amount that is derived by adding an intake air amount needed for idling the engine  10  to the above-described target intake air amount (Q). 
     At step S 2 , one of the first and second control modes is selected in accordance with an operation condition of the engine  10 . That is, in a low load and high speed operation zone of the engine  10  detected based on the accelerator open degree (APO) and the engine speed (Ne), the second control mode is selected. While, in the other operation zone of the engine  10 , the first control mode is selected. In this other operation zone, the above-mentioned control of the throttle valve  40  based on the temperature of the engine cooling water may be also used. 
     If, at step S 2 , the first control mode is selected, the operation step goes to step S 3 . At this step, a target open/close timing of the intake/exhaust valve  12  or  14  (or at least the close timing of the intake valve  12 ) is calculated in such a manner that the target intake air amount (Q) derived at step S 1  is obtained by the first control mode, and then the open/close timing thus calculated is actually outputted to the electromagnetic valve actuator  16 . Then, at step S 4 , a target open degree of the throttle valve  40  is calculated in such a manner the target intake air amount (Q) is obtained by the first control mode. Actually, the target open degree is calculated to correspond to a full-open position of the throttle valve  40  or a position close to the full open position. The calculated target open degree is outputted to the electric motor  42 . Thus, in the first control mode, the intake air amount is controlled to the target amount (Q) by the control of the open/close timing of the intake/exhaust valve  12  or  14  keeping the throttle valve  40  at the full-open or almost full-open position. 
     While, if the second control mode is selected at step S 2 , the operation step goes to step S 5 . At this step, a thermal efficiency loss compensation correction value “x” is calculated. That is, for the reason of pumping loss, in case of the second control mode using mainly operation of the throttle valve  40 , an engine torque loss becomes marked (viz., the thermal efficiency is reduced) as compared with case of the first control mode using mainly operation of the intake/exhaust valve  12  or  14 . In order to compensate this thermal efficiency loss in the second control mode, the correction value “x” for compensating the thermal efficiency loss is calculated in a manner to increase the value of the target amount (Q). The pumping loss that is the largest cause of the loss of the thermal efficiency is decided by the intake pressure or the amount of intake air fed to each cylinder (which will be referred to as “cylinder intake air amount” in the following for ease of description). Thus, when an intake pressure sensor is provided or when a reference fuel injection amount (Tp) corresponding to the cylinder intake air amount is calculated separately, the correction value “x” can be approximately calculated from an intake pressure (Pb) detected by the intake pressure sensor or from the reference fuel injection amount (Tp). The intake air flow varied depending on the engine speed has an influence on the combustibility of the engine and thus on the thermal efficiency of the same. Thus, if the calculation is carried out by looking up a map as shown in FIG. 5 that provides the thermal efficiency loss compensation correction value “x” plotted using the reference fuel injection amount (Tp) (or the intake pressure (Pb)) and the engine speed (Ne) as parameters, much precise correction value “x” can be obtained. 
     Referring back to the flowchart of FIG. 3, at step S 6 , a corrected target intake air amount (Q′) is obtained by multiplying “Q” and “x” together. At step S 7 , a target open/close timing of the intake/exhaust valve  12  or  14  is calculated in such a manner that the corrected target intake air amount (Q′) is obtained by the second control mode, and then, the open/close timing thus calculated is actually outputted to the electromagnetic valve actuator  16 . Then, at step S 8 , a target open degree of the throttle valve  40  is calculated in such a manner that the corrected target intake air amount (Q′) is obtained by the second control mode, and the target open degree thus calculated is outputted to the electric motor  42 . Like in a normal control for the throttle valve, the target open/close timing of the intake/exhaust valve  12  or  14  in this second control mode is set to have a value whose variable range due to change of engine operation condition is small. While, the target open degree of the throttle valve  40  in this second control mode is set to have a value whose variable range is relatively large in order that the intake air amount is mainly controlled by the throttle valve  40 . 
     Accordingly, in the second control mode, the intake air amount is controlled to the corrected target intake air amount (Q′) by controlling the open degree of the throttle valve  40  while generally fixedly controlling the open/close timing of the intake/exhaust valve  12  or  14 . With this, even in this second control mode, there is obtained an engine torque that is identical to that obtained in the first control mode wherein the target intake air amount is set at “Q”. Thus, even when switching between the first and second control modes takes place due to change in engine operation condition, undesired torque gap is suppressed or at least minimized. 
     Referring to FIG. 6, there is shown a flowchart of operation steps adopted in a second embodiment of the present invention. 
     Since steps S 1 , S 2 , S 3  and S 4  are the same as those of the above-mentioned first embodiment, description of them will be omitted from the following. 
     If, at step S 2 , the second control mode is selected, the operation flow goes to step S 11 . At this step, a target open/close timing of the intake/exhaust valve  12  or  14  is calculated in such a manner that the target intake air amount (Q) derived at step S 1  is obtained by the second control mode, and then the open/close timing thus calculated is actually outputted to the electromagnetic valve actuator  16 . Then, at step S 12 , a thermal efficiency loss compensation correction value “x” is calculated like in the manner taken at step S 5  of the above-mentioned first embodiment of FIG.  3 . Then, at step S 13 , a corrected target intake air amount (Q′) is obtained by multiplying “Q” and “x” together. Then, at step S 14 , a target open degree of the throttle valve  40  is calculated in such a manner that the corrected target intake air amount (Q′) is obtained by the second control mode, and the target open degree thus calculated is outputted to the electric motor  42 . 
     It is now to be noted that the calculation of the target open degree of the throttle valve  40  carried out at step S 14  differs from that carried out at step S 8  of the first embodiment. That is, in the second embodiment, the calculation of the target open/close timing of the intake/exhaust valve  12  or  14  is made on the non-corrected target intake air amount (Q), and an increased correction part of the corrected target intake air amount (Q′) is not covered by the control of the open/close timing of the intake/exhaust valve  12  or  14  but by an increased correction part of the open degree of the throttle valve  40 , and thus, the target open degree of the throttle valve  40  is calculated somewhat larger than that derived at step S 8  of the first embodiment. In the second embodiment, substantially same operation as the first embodiment is obtained. However, since, in the second embodiment, correction is applied to only the open degree of the throttle valve  40 , set matching is much easily made as compared with the first embodiment. 
     Referring to FIG. 7, there is shown a flowchart of operation steps adopted in a third embodiment of the present invention. 
     Since steps S 1 , S 2 , S 4 , S 12 , S 13  and S 14  are the same as those of the above-mentioned second embodiment of FIG. 6, explanation of them will be omitted from the following description. 
     In the third embodiment, upon switching from the first to second control mode and vice versa, a target value of the open/close timing of the variable valve derived after the switching is converged to a value with a delay. That is, at step S 3 ′, a delayed target open/close timing of the intake/exhaust valve  12  or  14  is calculated in such a manner that the target intake air amount (Q) is obtained by the first control mode, and the delayed target open/close timing thus calculated is actually outputted to the electromagnetic valve actuator  16 . At step S 11 ′, a delayed target open/close timing of the intake/exhaust valve  12  or  14  is calculated in such a manner that the target intake air amount (Q) is obtained by the second control mode, and the delayed target open/close timing thus calculated is actually outputted to the electromagnetic valve actuator  16 . For providing the open/close timing with the delay, a weighted mean calculation method is employed. The delay degree is determined according to the variation of the target open degree of the throttle valve  40 . That is, with increase of the variation, the delay degree of the target open/close timing of the intake/exhaust valve  12  or  14  increases. 
     When switching is made from the first (or second) control mode to the second (or first) control mode, the throttle valve  40  is controlled to pivot from the almost full-open position (or controlled open position taken before the switching) to the target open position (or almost full-open position). That is, upon such switching, the throttle valve  40  is controlled to decrease (or increase) its open degree reducing (or increasing) the intake pressure. Now, in the third embodiment, in response to an inevitable delay of the reducing (or increasing) of the intake pressure, the variation to the target value of the open/close timing of the variable valve after the switching is provided with a delay, so that displacement from the target value of intake air amount actually fed to each cylinder is suppressed or at least minimized. Thus, undesired torque gap, which would be produced upon the mode switching, is assuredly suppressed or at least minimized. 
     Referring to FIG. 8, there is shown a flowchart of operation steps adopted in a fourth embodiment of the present invention. 
     Since steps S 1 , S 2 , S 4 ,  55 , S 6  and S 8  are the same as those of the above-mentioned first embodiment of FIG. 3, explanation of them will be omitted from the following description. 
     Also in the fourth embodiment, upon switching of the control mode, the target value of the open/close timing of the variable valve is converged to a value with a delay. That is, at step S 3 ′, a delayed target open/close timing of the intake/exhaust valve  12  or  14  is calculated in such a manner that the target intake air amount (Q) is obtained by the first control mode, and the delayed target open/close timing thus calculated is actually outputted to the electromagnetic valve actuator  16 . At step S 7 ′, a delayed target open/close timing of the intake/exhaust valve  12  or  14  is calculated in such a manner that the target intake air amount (Q′) is obtained by the second control mode, and the delayed target open/close timing thus calculated is actually outputted to the electromagnetic valve actuator  16 . Due to substantially same reason as that in the third embodiment, undesired torque gap at the mode switching is assuredly suppressed or at least minimized. 
     The entire contents of Japanese Patent Application P11-344215 (filed Dec. 3, 1999) are incorporated herein by reference. 
     Although the invention has been described above with reference to embodiments of the invention, the invention is not limited to such embodiments. Various modifications and variations of the embodiments will occur to those skilled in the art, in light of the above teachings.