EP1904719A1

EP1904719A1 - Variable valve system for internal combustion engine

Info

Publication number: EP1904719A1
Application number: EP06779851A
Authority: EP
Inventors: Shuichi Ezaki
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2005-07-13
Filing date: 2006-07-11
Publication date: 2008-04-02
Also published as: JP2007023814A; CN101223338A; US20090107431A1; WO2007007168A1

Abstract

The invention relates to a variable valve system for an internal combustion engine including an electric motor that can be rotated in any one of a normal rotation direction and a reverse rotation direction; a first cam (56) that is driven by the electric motor; a second cam (50) that has a same shape as the first cam (56), that is driven by the electric motor, and that differs from the first cam (56) in a rotation phase; a first valve that is opened and dosed by the first cam (56); a second valve that is opened and closed by the second cam (50); and a control unit that controls rotation of the electric motor. Each of the first cam (56) and the second cam (50) is symmetric with respect to a straight line passing a cam center and a cam nose.

Description

VAKTABLE VALVE SYSTEM FOR INTERNAL COMBUSTION ENGINE

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The invention relates generally to a variable valve system for an internal combustion engine, which opens and closes, using a cam, an intake valve and an exhaust valve of an internal combustion engine. More specifically, the invention relates to a variable valve system for an internal combustion engine, which uses an electric motor to obtain power to drive a cam. 2. Description of the Related Art

[0002] Japanese Patent Application Publication No, JP-A-2004-183612 describes an internal combustion engine provided with a variable valve system that can change valve opening/dosing characteristics (a valve lift amount, valve opening time, valve closing time, etc.) of an intake valve and an exhaust valve. The variable valve system drives a cam using power from an electric motor instead of power transmitted from a crankshaft. Accordingly, the variable valve system can flexibly control the rotation speed and the rotation direction of the cam independently of the rotation of the crankshaft.

[0003] With the variable valve system described in Japanese Patent Application Publication No. JP-A-2004-183612, for example, the phase of the cam can be changed by increasing/decreasing the rotation speed of the cam while the intake valve or the exhaust valve is closed (phase variable control). Meanwhile, the valve-open duration can be changed by increasing/decreasing the rotation speed of the cam while the intake valve or the exhaust valve is open (valve-open duration variable control). If the rotation of the cam is reversed before the valve is maximally lifted while the valve is open, the actual valve lift amount is less than the maximum valve lift amount (valve lift amount variable control). With this variable valve system, the valve opening/closing characteristics of the intake valve and the exhaust valve can be changed with greater flexibility.

[0004] An electric motor that is used to drive the cams may be shared by all the cylinders. Alternatively, each cylinder may be provided with an electric motor. Also, the cylinders may be divided into multiple groups, and an electric motor may be provided to each group. When one group consists of two cylinders, an electric motor may drive the cams corresponding to two cylinders that are different in the rotation phase of the cams. In this case, if the cams are oscillated by alternately switching the rotation direction of the electric motor between the normal rotation direction and reverse rotation direction_s the valve lift amount variable control described above can be performed simultaneously on the valves of the two cylinders. More specifically, the rotation direction of the electric motor is alternately switched such that each cam corresponding to one of the cylinders opens and closes the valve using, from among two ramp portions thereof, only the ramp portion used to open the valve during normal rotation of the cam, and each cam corresponding to the other cylinder opens and closes the valve using only the ranoip portion used to close the valve during normal rotation of the cam.

[0005] However, a cam commonly employed in a valve system for an internal combustion engine is formed such that the shape of the ramp portion on the valve opening side with respect to the cam nose is different from that on the valve closing side with respect to the earn nose. The ramp portion on the valve opening side is steeply inclined in order to quickly open the valve to reduce the vajve-open duration, thereby increasing the time area (the value obtained by integrating the valve opening amount) per unit valve-open duration. In contrast, the ramp portion on the valve closing side is moderately inclined in order to gradually close the valve to reduce the speed at which the valve contacts a valve seat, thereby reducing vibration and noise that are caused when the valve contacts the valve seat.

[0006] Therefore, if the valve lift amount variable control is performed simultaneously on the valves of the two cylinders, each valve of one of the cylinders is quickly opened and closed by the steeply inclined ramp portion, and each valve of the other cylinder is gradually opened and closed by the moderately inclined ramp portion. This causes a difference in the valve lift curve between the valves of the two cylinders. As a result, the intake air amount varies between the two cylinders, causing torque fluctuation in the internal combustion engine as a whole.

[0007] The variable valve system described above can change the valve opening/closing characteristics with great flexibility. However, the flexibility in the valve opening/closing characteristics is provided under a certain degree of constraint, because alternately switching the rotation direction of the cams causes the difference in the valve lift curve between the valves of the two cylinders.

SUMMARY OF THE INVENTION

[0008] An object of the invention is to provide a variable valve system for an internal combustion engine, which can change valve opening/closing characteristics with greater flexibility.

[0009] A first aspect of the invention relates to a variable valve system for an internal combustion engine. The variable valve system includes an electric motor that can be rotated in any one of the normal rotation direction and the reverse rotation direction; a first cam that is driven by the electric motor; a second cam that has the same shape as the first cam, that is driven by the electric motor, and that differs from the first cam in the rotation phase; a first valve that is opened and closed by the first cam; a second valve that is opened and closed by the second cam; and a control unit that controls rotation of the electric motor, In the variable valve system, each of the first cam and the second cam is symmetric with respect to the straight line passing the cam center and the cam nose.

[0010] According to the first aspect, because each of the first cam and the second cam is symmetric with respect to the straight line passing the cam center and the cam nose, the same valve lift curve can be obtained regardless of whether the cam is rotated in the normal rotation direction or the reverse rotation direction, In addition, the rotation direction and the rotation speed of each cam can be flexibly controlled by the control unit, because the electric motor is used to obtain power to drive the cams. According to the first aspect, the valve opening/closing characteristics of the first valve and the second valve can be changed with greater flexibility,

[0011] In the first aspect, the control unit may alternately switch the rotation direction of the electric motor between the normal rotation direction and the reverse rotation direction such that the first valve is opened and closed using, from among two ramp portions of the first cam, the ramp portion used to open the first valve when the electric motor is rotated in a given direction (hereinafter, referred to as the "opening ramp portion"), and the second valve is opened and closed using, from among two ramp portions of the second cam, the ramp portion used to close the second valve when the electric motor is rotated in the given direction (hereinafter, referred to as the "closing ramp portion")_*

[0012] As described above, the first valve is opened and closed using the opening ramp portion of the first cam, and the second valve is opened and closed using the closing ramp portion of the second cam. Opening/closing each of the valves using only one of the iamp portions of the corresponding cam enables the valve to be driven by a lift amount that is less than that when the cam is rotated in only one direction and the valve is opened and closed using the entirety of the cam. In addition, the first cam and the second cam have the same shape, and each of the first cam and the second cam is symmetric with respect to the straight line passing the cam center and the cam nose. Accordingly, opening/closing one of the valves using the opening ramp portion and the other valve using the closing ramp portion (using the opening ramp portion of the first cam and the closing ramp portion of second cam, or using the closing ramp portion of the first cam and the opening ramp portion of the second cam) does not cause a difference between the two valve lift curves.

[0013] In the first aspect, the control unit may control the rotation speed of the electric motor such that the rotation speed at the start time of the open period of the first valve is higher than the rotation speed at the end time of the open period of the first valve, and the rotation speed at the start time of an open period of the second valve is higher than the rotation speed at the end time of the open period of the second valve.

[0014] As described above, the rotation speed of the electric motor at the start time of the open period of each valve is made higher than that at the end time of the open period. Thus, the valve-open duration of each valve can be reduced to increase the time area (the value obtained by integrating the valve opening amount) per unit valve-open duration. Accordingly, even if the ramp portions of each cam are moderately inclined to have the valve gradually contact a valve scat, the valve lift curve, according to which the valve is opened quickly and closed gradually, can be obtained. [0015] In the first aspect, the control unit may control the rotation speed of the electric motor such that the rotation speed at the end time of the open period of the first valve is lower than the rotation speed at the start time of the open period of the first valve, and the rotation speed at the end time of the open period of the second valve is lower than the rotation speed at the start time of the open period of the second valve. [0016] As described above, the first valve is opened and closed using the opening ramp portion of the first cam, and the second valve is opened and closed using the closing ramp portion of the second cam. Opening/closing each of the valves using only one of the ramp portions of the corresponding cam enables the valve to be driven by a lift amount that is less than that when the earn is rotated in only one direction and the valve is opened and closed using the entirety of the cam, In addition, the first cam and the second cam have the same shape, and each of the first cam and the second cam is symmetric with respect to the straight line passing the cam center and the cam nose. Accordingly, opening/closing one of the valves using the opening ramp portion and the other valve using the closing ramp portion (using the opening ramp portion of the first cam and the closing ramp portion of second cam, or using the closing ramp portion of the first cam and the opening ramp portion of the second cam) does not cause a difference between the two valve lift curves.

[0017] In the first aspect, the variable valve system may further include a camshaft used to take air into cylinders or discharge exhaust gas from the cylinders. The camshaft is driven by the electric motor controlled by the control unit. The electric motor drives the first cam provided onto the camshaft. Also, the electric motor drives the second cam provided onto the camshaft. Thus, the camshaft is directly driven by the electric motor, and the camshaft drives the first cam and the second cam. Accordingly, the variable valve system has excellent response characteristics. As a result, the variable valve system can appropriately perform the opening/closing control for an intake valve and an exhaust valve, thereby considerably improving the operating performance of the internal combustion engine.

[0018] A second aspect of the invention relates to a variable valve system for an internal combustion engine. The variable valve system includes an electric motor that can be rotated in any one of a normal rotation direction and a reverse rotation direction; multiple cams that arc provided to take air into cylinders or discharge exhaust gas from the cylinders,, and that are driven by the electric motor; multiple valves that are opened and closed by the respective cams; and a control unit that controls rotation of the electric motor. Each of the cams is symmetric with respect to the straight line passing the cam center and the cam nose.

[0019] According to the second aspect, because each valve is opened and closed by the corresponding cam that is symmetric with respect to the straight line passing the cam center and the cam nose, the same valve lift curve can be obtained regardless of whether the cam is rotated in the normal rotation direction or the reverse rotation direction.

[0020] In addition, in the second aspect, the rotation direction and the rotation speed of each cam can be flexibly controlled by the control unit, because the electric motor is used to obtain power to drive the cams. According to the second aspect, the valve opening/closing characteristics of valves can be changed with greater flexibility, [0021] In the second aspect, the control unit may alternately switch the rotation direction of the electric motor between the normal rotation direction and the reverse rotation direction such that the cam opens and closes the valve using only one of two ramp portions of the cam. Because the valve can be opened and closed using only one of the two ramp portions, the valve can be driven by a lift amount that is less than that when the valve is opened and closed by rotating the cam in only one direction and using the entirety of the cam.

[0022] In the second aspect, the control unit may control the rotation speed of the electric motor such that the rotation speed at the start time of the open period of the valve is higher than the rotation speed at the end time of the open period of the valve. [0023] As described above, the rotation speed of the electric motor at the start time of the open period of each valve is made higher than that at the end time of the open period. Thus, the valve-open duration of each valve can be reduced to increase the time area per unit valve-open duration. Accordingly, even if the ramp portions of each cam are moderately inclined to have the valve gradually contact a valve seat, the valve lift curve, according to which the valve is opened quickly and closed gradually can be obtained.

[0024] In the second aspect, the control unit may control the rotation speed of the electric motor such that the rotation speed at the end time of the open period of the valve is lower than the rotation speed at the start time of the open period of the valve.

[0025] As described above, the rotation speed of the electric motor at the end time of the open period of each valve is made lower than that at the start time of the open period. Thus, vibration and noise that are caused when the valve contacts the valve scat can be reduced by reducing the speed at which the valve contacts the valve seat. Accordingly, even if the ramp portions of each cam are steeply inclined to increase the time area per unit valve-open duration, the valve lift curve, according to which the valve is opened quickly and closed gradually, can be obtained.

[0026] In each of the first and second aspect, the control unit may receive data from a sensor that detects an engine operating state. [0027] In each of the first and second aspect, the internal combustion engine may be an in-line four-cylinder engine.

[0028] In each of the first and second aspect, the power stroke may take place in the internal combustion engine in the order of a first cylinder, a third cylinder, a fourth cylinder, and a second cylinder. [0029] In each of the first and second aspect, the first cam and the first valve may constitute a valve unit for the fourth cylinder, and the second cam and the second valve may constitute a valve unit for the first cylinder. Li addition, the first cam and the second cam may be arranged such that the phase of the cam nose of the first cam and the phase of the cam nose of the second cam deviate from each other by- ISO degrees in the circumferential direction of the camshaft.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein the same or corresponding portions will be denoted by the same reference numerals and wherein:

FIG. 1 illustrates the perspective view of a variable valve system according to each of a first embodiment and a second embodiment of the invention; FIG 2 illustrates the cross-sectional view of a camshaft taken along the plane perpendicular to the axis of the camshaft, which is used to describe the relative positional relationship between a cam corresponding to a cylinder #1 and a cam corresponding to a cylinder #4;

FIG 3 illustrates the graph showing the cam lift curve of the cam in FlG 1; FIG. 4 illustrates the table showing cam lift curves that are used to describe examples of changes in the valve opening/closing characteristics obtained by the variable valve system in FIG 1;

FIGS. 5A to 5C illustrate the views showing oscillation of the cam when the valve is opened and closed by a small lift amount; FIG 6 illustrates the graph showing the valve lift curve obtained by performing the motor speed control according to the second embodiment of the invention; and

FIG 7 illustrates the graph showing the valve lift curve obtained by performing the motor speed control according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] [First Embodiment]

Hereafter, a first embodiment of the invention will be described with reference to FIGS. 1 to 5.

[0032] [Structure of Variable Valve System] FId 1 illustrates the perspective view showing the structure of a variable valve system 10 according to the first embodiment. The variable valve system 10 drives an intake valve and an exhaust valve of an internal combustion engine 1. In the first embodiment, an in-line four-cylinder engine is employed as the internal combustion engine 1. In FlG. 1 , reference numerals #1 to #4 denote first to cylinders of the internal combustion engine, respectively. The first to cylinders #1 to #4 are arranged in series. The power stroke order of the internal combustion engine 1 is #1 → #3 -* #4 → #2, as in the case of a commonly used internal combustion engine. In the first embodiment, the variable valve system 10 serves as a system that drives the intake valves of the cylinders. In FIG. 1, the structure on the exhaust valve side is not shown. However, the variable valve system 10 may be configured as a system that drives the exhaust valves of the cylinders instead of or in addition to the intake valves of the cylinders.

[0033] In the internal combustion engine 1 shown in FIG. I₅ each cylinder is provided with two intake valves 12. Valve shafts 14 are fixed to the respective valves 12. Valve lifters 16 are fitted to the respective valve shafts 14 at the upper portions thereof. A force of a valve spring (not shown) is applied to the valve shaft 14. The spring force is applied such that the valve 12 is closed.

[0034] Cams 50, cams 52, cams 54, and cams 56 are arranged so as to contact the upper portions of the corresponding valve lifters 16. In the first embodiment, the cams 50 denote the cams contacting the valve lifters 16 provided to the cylinder #1. The cams 52 denote the cams contacting the valve lifters 16 provided to the cylinder #2.

The cams 54 denote the cams contacting the valve lifters 16 provided to the cylinder #3,

The cams 56 denote the cams contacting the valve lifters 16 provided to the cylinder #4,

[0035] The cams 50 contacting the valve lifters 16 provided to the cylinder #1 and the cams 56 contacting the valve lifters 16 provided to the cylinder #4 are fixed to a camshaft 22. The cams 52 contacting the valve lifters 16 provided to the cylinder #2 and the cams 54 contacting the valve lifters 16 provided to the cylinder #3 are fixed to a camshaft 24 that is rotatable with respect to the camshaft 22 and that is arranged coaxially with the camshaft 22. In the structure shown in FIG, 1, each camshaft is shared by the cylinders where the power stroke takes place at different time. The power stroke takes place in one of these cylinders when the crankshaft has rotated by 360 degrees since the power stroke takes place iπ the other cylinder. These camshafts, namely, the camshaft 22 shared by the cylinder #1 and the cylinder #4 and the camshaft 24 shared by the cylinder #2 and the cylinder #3 are configured so as to be able to rotate and oscillate in the circumferential direction independently of each other The camshaft 22 and the camshaft 24 are rotatably supported by a support member, for example, a cylinder head (not shown).

[0036] A first driven gear 26 is coaxially fixed to the camshaft 22. A first output gear 28 is meshed with the first driven gear 26. The first output gear 28 is coaxially fixed to the output shaft of a first motor 30. Thus, torque of the first motor 30 can be transmitted to the camshaft 22 via the first output gear 28 and the first driven gear 26.

[0037] A second driven gear 32 is coaxially fixed to the camshaft 24. A second output gear 36 is meshed with the second driven gear 32 via an intermediate gear 34. The second output gear 36 is coaxially fixed to the output shaft of a second motor 38. With this structure, torque of the second motor 38 can be transmitted to the camshaft 24 via the second output gear 36, the intermediate gear 34, and the second driven gear 32.

[0038] FIQ 2 illustrates the cross-sectional view of the camshaft 22 taken along the plane perpendicular to the axis of the camshaft 22. FIG 2 shows the details of the camshaft 22 in FIG. 1. As described above, the cams 50 corresponding to the cylinder #1 and the cams 56 corresponding to the cylinder #4 are fixed to the camshaft 22. As shown in FIG. 2, the cams 50, 56 are arranged such that a cam nose 50a of the cam 50 and a cam nose 56a of the cam 56 deviate from each other by 180 degrees in the circumferential direction. [0039] As shown in FIG. 2, two ramp portions 50b, 50c are formed in the cam 50. Each of the ramp portions 50b, 50c extends between the base circle portion and the apex of the cam nose 50a. In the first embodiment, the ramp portion that initially contacts the valve lifter 16 during normal rotation of the camshaft 22, namely, the ramp portion used to open the valve 12 (#1) is referred to as the opening ramp portion 50b. The ramp portion that contacts the valve lifter 16 after the apex of the cam nose 50a contacts the valve lifter 16, namely, the ramp portion used to close the valve 12 is referred to as the closing ramp portion 50c. Similarly, the ramp portion used to open the valve 12 (#4) corresponding to the cam 56 (#4) is referred to as an opening ramp portion 56b, and the ramp portion used to close the valve 12 is referred to as a closing ramp portion 56c,

[0040] As shown in FlQ 2, the cams 50, 56 have the same shape. Each of the cams 50, 56 is symmetric with respect to the straight line passing the cam center and the apex of the cam nose. FIG 3 illustrates the cam lift curve of the cam 50 corresponding to the cylinder #1, In FIG 3, the lateral axis indicates the position of the cam in the circumferential direction, and the longitudinal line indicates the cam height (the height of the cam with respect to the cam base circle). In FIG. 3, the cam lift curve is indicated using the straight line connecting the cam center to the apex of the cam nose 50a as the Y-axis. Tn FIG 3, the cam lift curve of the opening ramp portion 50b is shown on the left side with respect to the Y-axis, and the cam lift curve of the closing ramp portion 50c is shown on the right side with respect to the Y-axis. The cam 50 is symmetric with respect to the straight line passing the cam center and the apex of the cam nose 50a. Accordingly, as shown in FIG 3, the cam lift curve of the opening ramp portion 50b and the cam lift curve of the closing ramp portion 50c are symmetric with respect to the Y-axis. Although not shown in the figure, the cam lift curve of the cam 56 corresponding to the cylinder #4 is similar to the cam lift curve of the cam 50 corresponding to the cylinder #1 shown in FIG 3.

[0041] Although not described m detail here, the cam 52 corresponding to the cylinder #2 and the cam 54 corresponding to the cylinder #3 are fixed to the camshaft 24 such that the phases of the cams 52, 54 are deviated from each other by 180 degrees. Each of the cams 52, 54 has the same shape as that of the cam 50 corresponding to the cylinder #1 and the cam 56 corresponding to the cylinder #4. Each of the cams 52, 54 is symmetric with respect to the straight line passing the cam center and the apex of the cam nose.

[0042] The operation of the variable valve system 10 is controlled by an ECU 40 (Electronic Control Unit) 40. The ECU 40 is connected to various sensors (not shown) that detect the operating state of the internal combustion engine, such as a crank angle sensor, a cam angle sensor, and an accelerator angle sensor. In addition, the ECU 40 is connected to various actuators such as an actuator for the first motor 3O₃ and an actuator for the second motor 38, etc. Also, the ECU 40 stores programs used to control the rotations of the first motor 30 and the second motor 38. The ECU 40 processes the data transmitted from the various sensors, and control the rotations of the motors 30, 38 based on the processing result.

[0043] [Operation of Variable Valve System] Next, the operation of the variable valve system 10 will be described with reference to FIG. 4, Here, an example where the camshaft 22 is driven by the first motor 30 will be described.

[0044] The variable valve system 10 can variously change the valve opening/closing characteristics of the valve 12 as shown in FIG 4, by changing the phase of the camshaft 22 with the crankshaft using the first motor 30> In this specification, the "valve opening/closing characteristics" of the valve 12 is a generic term used to refer to the valve lift amount (maximum valve lift amount), the opening time of the valve 12, and the closing time of the valve 12. In addition, the "opening/closing characteristics" includes the valve-open duration that is defined by determining the opening time and the closing time, and the phase that is defined by determining the opening time (or closing lime) while the valve-open duration is maintained constant.

[0045] FIG 4 illustrates the table showing the valve opening/closing characteristics of the valve 12, which are variously obtained by the variable valve system 10. More specifically, the wave-forms indicated by the solid lines in FIG 4 show the valve lift curves obtained when the camshaft 22 is continuously rotated in the normal rotation direction at the basic speed (the rotation speed of the camshaft is half of that of the crankshaft). The wave-forms indicated by the double-dashed lines in FIG 4 show the valve lift curves obtained after the operation of the camshaft 22 is changed. The lateral axis in each graph in FIG. 4 showing the valve lift curve indicates the crank angle (CA). [0046] First, the operation for changing the "phase", shown in FIG 4, will be described. If the rotation speed of the camshaft 22 is made higher than the basic speed while the valve 12 is closed, the phase of the camshaft 22 with respect of the crankshaft is changed relatively to the advance angle side. Therefore, the phase can be changed as shown in FIG 4. After the phase is changed, the desired phase can be maintained by returning rotation speed of the camshaft 22 to the basic speed. On the other hand, if the rotation speed of the camshaft 22 is made lower than the basic speed while the valve 12 is closed, the phase of the camshaft 22 with respect to the crankshaft can be changed to the delay angle side. [0047] The "valve-open duration" can be changed, as shown in FIG 4, by making the rotation speed of the camshaft 22 higher than the basic speed while the valve 12 is open. However, the ratio between the rotation period of the camshaft 22 and the rotation period of the crankshaft is maintained at 2:1 by making the rotation speed of the camshaft 22 lower than the basic speed, while the valve is dosed, by the amount corresponding to the difference between the rotation speed of the camshaft 22 and the basic speed while the valve 12 is open. On the contrary, the valve-open duration can be made longer than that shown by the basic wave-form indicated by the solid line, by making the rotation speed of the camshaft 22 lower than the basic speed while the valve 12 is open. In this case, while the valve^'12 is closed, the rotation speed of the camshaft 22 is controlled to be higher than the basic speed by the amount corresponding to the difference between the rotation speed of the camshaft 22 and the basic speed while the valve 12 is open.

[0048] Next, the operation for changing the "valve-open duration and valve lift amount", shown in FIG 4, will be described, FIG 4 illustrates the example in which the valve lift amount of the valve 12 is controlled to be less than the maximum valve amount. The change can be obtained by controlling the operation of the camshaft 22 as shown in FIGS 5A to 5C FIGS. 5A to 5C illustrate the operations of the cams 50, 56 for changing "the valve-open duration and the valve lift amount". In this operation, the rotation direction of the camshaft 22 is alternately switched between the normal rotation direction and the reverse rotation direction, whereby the cams 50, 56 are periodically oscillated.

[0049] In the operation for changing "tjic valve-open duration and the valve lift amount", first, the camshaft 22 is rotated in the clockwise direction, that is, the normal rotation direction, as shown in FIG. 5A. Thus, the cam 56 corresponding to the cylinder #4 presses down the valve lifter 16 (#4) using the opening ramp portion 56b, The first motor 30 is stopped before the apex of the cam nose 56a of the cam 56 contacts the valve lifter 36. Then, the rotation direction of the first motor 30 is switched such that the camshaft 22 is rotated in the counterclockwise direction, that is, the reverse rotation direction. Thus, the camshaft 22 is rotated in the reverse rotation direction, and the condition shown in FIG.5B is achieved.

[0050] Thus, the cam 56 oscillates while only the opening ramp portion 56b, from among the two ramp portions 56b, 56c, contacts the valve lifter 16 (#4). Accordingly, the valve 12 of the cylinder #4 is operated according to the valve lift curve defined by the cam lift curve of the opening ramp portion 56b of the cam 56. [0051] Next, the camshaft 22 is further rotated in the reverse rotation direction.

Then, as shown in FIG. 5C, the cam 50 corresponding to the cylinder #1 presses down the valve lifter 16 (#1) using the closing ramp portion 50c. The first motor 30 is stopped before the cam nose 50a contacts the valve lifter 16 (#1). Then, the rotation direction of the first motor 30 is switched again, and the camshaft 22 is rotated in the normal rotation direction until the condition shown in FIG. 5B is achieved. Thus, the cam 50 oscillates while only the closing ramp portion 50c, from among the two ramp portions 50b, 50c, contacts the valve lifter 16 (#1), and the valve 12 of the cylinder #1 is operated according to the valve lift curve defined by the cam lift curve of the closing ramp portion 50c of the cam 50, [0052] The cams 50, 56 are periodically oscillated by continuously performing the scries of the operations described above. Accordingly, the valve lift amounts of the valves 12 of the cylinder #1 and the cylinder #4 can be simultaneously changed by the cams 50, 56 both fixed to the camshaft 22. At this time, the valve lift amounts of the valves 12 can be appropriately changed by appropriately changing the amounts of oscillation of the cams 50, 56. Also when the cams 50, 56 are oscillated, only the "valve lift amount" can be changed, without changing the valve-open duration, as shown in FlG. 4, by appropriately changing the rotation speed of the first motor 30. When the operation for changing "the valve-open duration and the valve lift amount" is performed and when the operation for changing the "valve lift amount" is performed, the oscillation period of the cams 50, 56 is set to be twice as long as the rotation period of the crankshaft.

[0053] In the first embodiment described above, the valve 12 of the cylinder #4 is opened and closed using the opening ramp portion 56b of the cam 56 and the valve 12 of the cylinder #1 is opened and closed using the closing ramp portion 50c of the cam 50 by alternately switching the rotation direction of the first motor 30 to periodically oscillate the cams 50, 56.

[0054] Also, in the first embodiment, the valve 12 of the cylinder #1 and the valve 12 of the cylinder #4 arc opened and closed using only the closing ramp portion 50c of the cam 50 and the opening ramp portion 56b of the cam 56, respectively. Accordingly, each of the valves 12 can be operated by a lift amount less than that when the valves 12 are opened and closed using the entireties of the cams 50, 56 by rotating the camshaft 22 only in the normal rotation direction.

[0055] As shown in FIG 2, the cams 50, 56, which are simultaneously oscillated, have the same shape, and each of the cams 50, 56 is symmetric with respect to the straight line passing the cam center and the apexes of the cam noses 50a, 56a. Accordingly, even if the valves 12 are operated using the closing ramp portion 50c of the cam 50 and the opening ramp portion 56b of the cam 56, the valve lift curves of the valves 12 do not differ from each other. Accordingly, even if the valve lift amount of the valves 12 is changed, the intake air amount does not differ between the cylinder #1 and the cylinder #4.

[0056] With the variable valve system 10 according to the first embodiment described above, the operation of the cams 50, 56 can be variously changed, because the rotation speed of the camshaft 22 is appropriately controlled by controlling the first motor 30 using the ECU 40. Also, each of the cams 50, 56 is symmetric with respect to the straight line passing the cam center and the apex of the cam nose. Accordingly, the same valve lift curve can be obtained regardless of whether the cams 50, 56 are oscillated in the norma) rotation direction or the reverse rotation direction. Therefore, with the variable valve system 10 according to the first embodiment, the rotation direction of the camshaft 22 is not limited to a certain direction, and the operation of the cams 50, 56 can be variously changed by appropriately controlliTig the rotation direction of the camshaft 22. Thus, the valve opening/closing characteristics of the valve 12 can be changed with great flexibility. [0057] In the operation for changing the valve lift amount according to the first embodiment, the opening ramp portion 56b is used in the cam 56 corresponding to the cylinder #4, and the closing ramp portion 50c is used in the cam 50 corresponding to the cylinder #1. However, the closing ramp portion 56c may be used in the cam 56 corresponding to the cylinder #4, and the opening ramp portion 50b may be used in the cam 50 corresponding to the cylinder #1. Which ramp portion, from among the opening ramp portion and the closing ramp portion, should be used may be determined depending on the rotation position of each of the cams 50, 56 when a command for starting the operation for changing the valve lift amount is issued.

[0058] In the case of the cylinder #2 and the cylinder #3, the cams 52, 54 have the same shape, and each of the cams 52, 54 is symmetric with respect to the straight line passing the cam center and the apex of the cam nose. Accordingly, the same valve lift curve can be obtained regardless of whether the camshaft 24 is rotated jn the normal rotation direction or reverse rotation direction. Although not described in detail here, the operation of the cams 52, 54 can be variously changed by appropriately controlling, using the second motor 38, the rotation speed and the rotation direction of the camshaft 24 and the angle by which the caius52, 54 are oscillated. Thus, the valve opening/closing characteristics of the valve 12 can be changed with great flexibility. [0059] [Second Embodiment] Next, a second embodiment of the invention will be described with reference to FIG. 6.

[0060] The structure of a variable valve system according to the second embodiment is shown in FIG. 1, as in the first embodiment. The second embodiment is different from the first embodiment in the control of the rotation speed of the motors 30, 38. FIG 6 illustrates the graph showing the valve lift curve that is obtained by controlling the rotation speed of the motors 30, 38 using the ECU 40.

[0061] As described in the first embodiment, each of the cams 5O₃ 52, 54 and 56 is symmetric with respect to the straight line passing the cam center and the apex of the cam nose. Accordingly, when the motors 30, 38 are rotated at a constant speed, the rotation speed at the start time of the open period of the valve 12 is equal to the rotation speed at the end time of the open period of the valve 12, However, the valve 12 is required to be quickly opened Io quickly reduce the valve-open dwation, thereby increasing the time area (the value obtained by integrating the valve opening amount) per unit valve-open duration. Meanwhile, the valve 12 is required to be gradually closed to reduce the speed at which the valve 12 contacts the valve seat, thereby reducing vibration and noise that are caused when the valve 12 contacts the valve seat. However, these two requirements contradict each other. Accordingly, if the shape of the cam is defined such that one of these requirements is fulfilled, the other requirement cannot be fulfilled.

[0062] The second embodiment is made to fulfill both the requirement concerning the speed at which the valve 12 is opened and the requirement concerning the speed at which the valve 12 is closed. According to the second embodiment, the valve 12 is quickly opened by appropriately selecting the shape of the cam, and the valve 12 is gradually closed by controlling the rotation speed of the motors 30, 38, More specifically, both of the opening ramp portion and the closing ramp portion of each of the cams 50, 52, 54 and 56 are steeply inclined so as to increase the time area per unit valve-open duration. In addition, the rotation speed NCclose of the motors 30, 36 at the end time of the open period of the valve 12 is made lower than the rotation speed NCopen at the start time of the open period of the valve 12.

[0063] Thus, the valve lift curve, indicated by the slid line in FIG 6, according to which the valve 12 opens quickly and closes gradually, can be obtained. As a result, the time area per unit valve-open duration can be increased by reducing the valve-open duration of the valve 12, while vibration and noise that are caused when the valve 12 contacts the valve seat can be reduced by reducing the speed at which the valve 12 contacts the valve seat. When the rotation speed of the motors 30, 3S does not vary between the start time and the end time of the open period of the valve 12, the valve lift curve at the end time of the open period of the valve 12 is as indicated by the dashed line in FIG 6.

[0064] The control described above can be applied not only to the case where each of the cams 50, 52, 54 and 56 is rotated in the normal rotation direction but also to the case where the cams 50, 52, 54 and 56 are oscillated by alternately switching the rotation direction of the motors 30, 38. [0065Ϊ [Third Embodiment] Next, a third embodiment of the invention will be described with, reference to FIGL 7. [0066] The variable valve system according to the third embodiment, as well as the variable valve system according to the second embodiment, is characterized in that both the requirement concerning the speed at which the valve 12 is opened and the requirement concerning the speed at which the valve 12 is closed are fulfilled by appropriately selecting the shape of the cam and controlling the rotation speed of the motors 30, 38. FlG 7 illustrates the graph showing the valve lift curve obtained by controlling the rotation speed of the motors 30, 38 using the ECU 40-

[0067] In the third embodiment, both of the requirements are fulfilled m the approach opposite to that in the second embodiment. According to the third embodiment, the valve 12 is closed gradually by appropriately selecting the shape of the cam, and the valve 12 is opened quickly by controlling the rotation speed of the motors 30, 38. More specifically, the shape of both the opening ramp portion and the closing ramp portion of each of the cams 50, 52₃ 54 and 56 are moderately inclined to have the valve 12 gradually contact the valve seat. In addition, the rotation, speed NCopen of the motors 30, 38 at the start time of the open period of the valve 12 is made higher than the rotation speed NCdose at the end time of the open period of the valve 12.

[0068] Thus, the valve lift curve, indicated by the solid line in FIG 7, according to which the valve 12 quickly opens and gradually closes, can be obtained as in the second embodiment. As a result, the time area per unit valve-open duration can be increased by reducing the valve-open duration of the valve 12, while vibration and noise which are caused when the valve 12 contacts the valve seat can be reduced by reducing the speed at which the valve 12 contacts the valve seat. When the rotation speed of the motors 30,

38 does not vary between the start time and the end time of the open period of the valve

12, the valve lift cuTve at the start time of the open period of the valve 12 is as indicated by the dashed line shown in FIG 7.

[0069] The control described above can be applied not only to the case where each of the cams 50, 52, 54 and 56 is rotated in the normal rotation direction but also to the case where the cams 50, 52, 54 and 56 are oscillated by alternately switching the rotation direction of the motors 30, 38. [0070] [Others]

While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. For example, the invention can be realized according to the following modified examples.

[0071] In the embodiments described above, the cams corresponding to two cylinders are driven by one motor. However, the cams corresponding to one cylinder may be driven by one motor. Ia this case as well, the shape of each cam is symmetric with respect to the straight line passing the cam center and the apex of the cam nose, Thus, the same valve lift curve can be obtained regardless of whether the cam is rotated in the normal rotation direction or the reverse rotation direction. Accordingly, the rotation direction and the rotation speed of the cam can be flexibly set, whereby the valve opening/closing characteristics of the valves can be changed with great flexibility.

[0072] In the embodiments described above, the power stroke order of the internal combustion engine 1 is #1 -> #3 -* #4 -* #2. However, the invention can be applied to an internal combustion engine where the power stroke oχdeτ is #1 → #2 → #4 -> #3.

Claims

1. A variable valve system for an internal combustion engine, characterized by comprising: an electric motor that can be rotated in any one of a normal rotation direction and a reverse rotation direction; a first cam (56) that is driven by the electric motor; a second cam (50) that has a same shape as the first cam (56), that is driven by the electric motor, and that differs from the first cam (56) in a rotation phase; a first valve that is opened and closed by the first cam (56); a second valve that is opened and closed by the second cam (50); and a control unit (40) that controls rotation of the electric motor, wherein each of the first cam (56) and the second cam (50) is symmetric with respect to a straight line passing a cam center and a cam nose.

2. The variable valve system for an internal combustion engine according to claim 1, wherein the control unit (40) alternately switches a rotation direction of the electric motor between the normal rotation direction and the reverse rotation direction such that the first valve is opened and closed using, from among two ramp portions (56b, 56c) of the first cam (56), the ramp portion (56b) used to open the first valve when the electric motor is rotated in a given direction, and the second valve is opened and closed using, from among two ramp portions (50b, 50c) of the second cam (50), the ramp portion (50c) used to close the second valve when the electric motor is rotated in the given direction.

3. The variable valve system for an internal combustion engine according to claim 1 or 2, wherein the control unit (40) controls a rotation speed of the electric motor such that the rotation speed at a start time of an open period of the first valve is higher than the rotation speed at an end time of the open period of the first valve, and the rotation speed at a start time of an open period of the second valve is higher than the rotation speed at an end time of the open period of the second valve.

4. The variable valve system for an internal combustion engine according to claim 1 or 2, wherein the control unit (40) controls a rotation speed of the electric motor such that the rotation speed at an end time of an open period of the first valve is lower than the rotation speed at a start time of the open period of the first valve, and the rotation speed at an end time of an open period of the second valve is lower than the rotation speed at a start time of the open period of the second valve.

5. The variable valve system for an internal combustion engine according to claim 1, further comprising: a camshaft (22) used to take air into cylinders or discharge exhaust gas from the cylinders, wherein the camshaft (22) is driven by the electric motor controlled by the control unit (4O)₅ the electric motor drives the first cam (56) provided onto the camshaft (22); and the electric motor drives the second cam (50) provided onto the camshaft (22).

6. A variable valve system for an internal combustion engine, characterized by comprising: an electric motor that can be rotated in any one of a normal rotation direction and a reverse rotation direction,* multiple cams that are provided to take air into cylinders or discharge exhaust gas from the cylinders, and that are driven by the electric motor; multiple valves that are opened and closed by the respective cams; and a control unit that controls rotation of the electric motor, wherein each of the cams is symmetric with respect to a straight line passing a cam center and a cam nose-

7. The variable valve system for an internal combustion engine according to claim 6, wherein the control unit alternately switches a rotation direction of the electric motor between the normal rotation direction and the reverse rotation direction such that the cam opens and closes the valve using only one of two ramp portions of the cam.

8. The variable valve system for an internal combustion engine according to claim 6 or 7, wherein the control unit controls a rotation speed of the electric motor such that the rotation speed at a start time of an open period of the valve is higher than the rotation speed at an end time of the open period of the valve.

9. The variable valve system for an internal combustion engine according to claim 6 or 7, wherein the control unit controls a rotation speed of the electric motor such that the rotation speed at an end time of an open period of the valve is lower than the rotation speed at a Start time of the open period of the valve.