Engine camshaft driven by a variable valve timing mechanism

Abstract: An engine (3) includes a crankshaft (9), a camshaft (14) driven by the crankshaft (9) for opening and closing intake valves (12), a variable valve timing mechanism (VVT) (1) attached to one end of the camshaft (14), which applies torque to the camshaft (14) so as to change the valve timing. The camshaft (14) has an increased outside diameter at a portion adjacent to the VVT (1) as compared to other portions that are more distant from the VVT (1).

Description: TECHNICAL FIELD The present invention relates to an engine camshaft for driving an intake valve and exhaust valve according to the preamble of any of the claims 1 to 3. More specifically, the present invention relates to an engine having a mechanism located on a camshaft that can change valve timing of intake valves or exhaust valves. RELATED BACKGROUND ART Generally, engines have camshafts for driving intake valves and exhaust valves. There are known engines having variable valve timing mechanisms (hereinafter referred to as "VVTs") on the camshafts thereof. The VVT can change the rotational phase of the camshaft with respect to that of a crankshaft so that valve timing of the intake valves or the exhaust valves may be changed. The VVT achieves optimum control of valve timing depending on the operational state (including load, rotational speed, etc.) of the engine which, changes over a wide range, to improve fuel consumption, output and emissions of the engine. U.S. Patent No. 5,483,930 discloses an example of an engine having a VVT. This engine has a camshaft provided with a VVT at one end thereof. The camshaft has substantially uniform outside diameter over its entire length. Torque from the crankshaft is transmitted via the VVT to the camshaft. The VVT is provided with a timing pulley that is rotatable relative to the camshaft, a cover fixed to the pulley to cover the corresponding end portion of the camshaft and a ring gear located between the cover and the camshaft. The cover and the ring gear are connected to each other by a helical gear, while the ring gear and the camshaft are likewise connected to each other by a helical gear. A pair of pressure chambers are defined on each side of the ring gear with respect to the axial direction of the camshaft, and hydraulic pressure is supplied selectively to these pressure chambers. While the engine is under operation, torque from the crankshaft is transmitted to the pulley to rotate the camshaft via the cover and the ring gear. Intake valves are driven by the rotation of the camshaft with a predetermined timing. The VVT is actuated by the hydraulic pressure supplied selectively to the pressure chambers. The ring gear is moved along the teeth of the helical gear by the hydraulic pressure thus supplied. More specifically, the ring gear rotates while it travels axially. This travel of the ring gear is converted to a torque for rotating the camshaft. When the rotational direction of this torque is the same (positive direction) as that of the camshaft, the rotational phase of the camshaft is advanced with respect to the pulley; whereas when it is of the opposite direction (negative direction) to the rotational direction of the camshaft, the rotational phase of the camshaft is retarded with respect to the pulley. Thus, the rotational phase of the camshaft relative to the pulley is changed by the torque generated based on the travel of the ring gear, and the valve timing of the intake valves is changed from a predetermined timing. However, when the VVT is actuated in the engine, the camshaft is subjected to great torque at a location adjacent to the portion adjacent to the VVT. Thus, distortion is caused in the camshaft by the torque. The distortion is greater at locations nearer to the VVT. Based on such distortion, a change in valve timing of the valves, especially those distant from the VVT, is retarded consequently. Accordingly, in order to change valve timing of the valves with an accurate response, it is essential that the camshaft be rigid to withstand the torque. Though the outside diameter of the camshaft can be increased in order to enhance its rigidity to increase the outside diameter of the camshaft over its entire length leads to a significant increase in the volume and weight of the camshaft, which increases in the weight of the engine. Furthermore, from the WO-A-91/05941, an internal combustion engine camshaft drive mechanism is known in which the camshafts are driven through a variable valve timing mechanism which is driven by a drive means comprising a drive pulley and a tooth belt. The variable valve timing mechanism is located at an end portion of the camshaft and the camshaft has an outside diameter at a portion adjacent to the variable valve timing mechanism which is greater than the outside diameter of other portions of the camshaft that are more remote from the control apparatus. DISCLOSURE OF THE INVENTION It is an objective of the present invention to provide an engine having a variable valve timing mechanism which enables changing of valve timing of valves with an accurate response without greatly increasing the weight of the engine. This object is achieved by the features according to any of the claims 1 to 3. Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principals of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings. Figure 1 is a perspective view showing an engine having a variable valve timing mechanism according to a first embodiment of the invention; Figure 2 is a plan view showing the intake-side camshaft and a variable valve timing mechanism of Fig. 1; Figure 3 is a cross-sectional view showing the structure of the variable valve timing mechanism of Fig. 2; Figure 4 is a plan view showing a camshaft and a VVT according to a second embodiment of the invention; and Figure 5 is a plan view showing a camshaft and a VVT according to the prior art. DESCRIPTION OF SPECIAL EMBODIMENT (First embodiment) A first embodiment of the present invention will be described below referring to Figures 1 to 3. Figure 1 shows an outline of an engine 3 having a variable valve timing mechanism (VVT) 1 and also a valve train 2. The engine 3 contains a cylinder block 4, an oil pan 5 fixed to the bottom of the block 4 and a cylinder head 6 fixed to the top of the block 4. The front of the engine is considered to be the side to which the timing belt 21 of Fig. 1 is connected. The oil pan 5 stores therein a lubricating oil to be supplied to various parts of the engine 3. The cylinder block 4 has a plurality of cylinders 8 each forming a combustion chamber 7. In this embodiment, while the engine 3 has four cylinders 8, only one cylinder is shown to simplify the drawing. The cylinder block 4 rotatably supports a crankshaft 9. A piston 10 is fitted in each cylinder 8 to reciprocate vertically and is connected to the crankshaft 9 via a connecting rod 11. In the cylinder head 6, a plurality of intake valves 12 and exhaust valves 13 provided in each cylinder 8 selectively open and close intake ports and exhaust ports (neither of which are shown). A pair of camshafts 14, 15 are rotatably supported in the cylinder head 6 parallel to each other. The camshafts 14 and 15 have a plurality of cams 14a and 15a, respectively. Two adjacent cams 14a or 15a form a pair. The cams 14a and 15a drive the valves 12 and 13, respectively. Injectors 16, one of which is provided for the each cylinder 8 in the cylinder head 6, inject fuel into the intake ports. Ignition plugs 17, one of which is also provided for each cylinder 8 in the cylinder head 6, ignite a combustible fuel/air mixture introduced to the respective combustion chambers 7. Timing pulleys 18 and 19 attached to front ends of the camshafts 14 and 15, respectively, and a timing pulley 20 attached to the front end of the crankshaft 9 are connected to one another via a timing belt 21. The VVT 1 attached to the front end portion of the intake-side camshaft 14 includes the pulley 18 and operates to change valve timing of the intake valves 12. Figure 2 is a plan view showing the intake-side camshaft 14 and the VVT 1. As shown in Figures 1 and 2, the camshaft 14 has a first journal 22a, a second journal 22b, a third journal 22c, a fourth journal 22d and a fifth journal 22e between the respective pairs of cams 14a. These journals 22a to 22e are respectively supported by bearings provided in the cylinder head 6 respectively. The first journal 22a has the largest outside diameter D1, and the outside diameters D2, D3, D4 and D5 of the second, third, fourth and fifth journals 22b, 22c, 22d and 22e are reduced gradually in this order. On the camshaft 14, portions other than the journals 22a to 22e and the cams 14a constitute first to fourth shaft sections 23a, 23b, 23c and 23d, which are tapered. The first shaft section 23a has the largest diameter, and the diameters of the second, third and fourth shaft sections 23b, 23c and 23d are reduced gradually in this order. Thus, the camshaft 14 is relatively thick at parts adjacent to the VVT 1 compared with other portions and is thinner at parts away from the VVT 1. Incidentally, in this embodiment, the exhaust-side camshaft 15 has the same structure as that of the prior art, and the outside diameter of portions other than the cams 15a are substantially the same irrespective of the distance from the pulley 19. While the engine 3 is operating, the torque of the crankshaft 9, which is rotated by the vertical reciprocation of the pistons 10, is transmitted via the belt 21 to the pulleys 18, 19. When the pulleys 18, 19 are rotated with the rotation of the crankshaft 9, the camshafts 14, 15 are rotated synchronously with the rotation of the crankshaft 9. When the camshafts 14, 15 are rotated, the intake valves 12 and the exhaust valves 13 are opened and closed. In this state, the camshafts 14, 15 are rotated synchronously with the rotation of the crankshaft 9, and the valves 12, 13 are driven with a predetermined valve timing based on the rotation of the camshafts 14, 15. The VVT 1 is controlled by a hydraulic pressure supply unit 24 employing lubricating oil as a hydraulic fuel. As shown in Figure 1, this unit 24 contains a pump 25, a filter 26 and a linear solenoid valve (LSV) 27. The pump 25 draws and delivers the lubricating oil from and to the oil pan 5. The filter 26 filters the oil delivered from the pump 25. The LSV 27 supplies the oil passed through the filter 26 to the VVT 1 and also feeds back the oil discharged from the VVT 1 to the oil pan 5. Figure 3 is a cross-sectional view showing the structure of the VVT 1. As shown in Figure 3, the VVT 1 includes, in addition to the pulley 18, a cover 31 fixed to the pulley 18 and a ring gear 32 located between the cover 31 and the camshaft 14. A cylindrical inner gear 35 is fixed to the front end of the camshaft 14 by a hollow bolt 33 and a pin 34. The inner gear 35 thus forms an extension of the camshaft 14. The inner gear 35 has external teeth 35a on the outer circumference thereof. The pulley 18 and the cover 31 are supported rotatably with respect to the camshaft 14. The cover 31 has internal teeth 31a on the inner circumference thereof. The internal teeth 31a and the external teeth 35a are helical splines. The ring gear 32 couples the inner gear 35 to the cover 31. The ring gear 32 has internal teeth 32a and external teeth 32b. These teeth 32a and 32b are helical splines. The internal teeth 32a are meshed with the external teeth 35a of the inner gear 35, while the external teeth 32b are meshed with the internal teeth 31a of the cover 31. Inside the cover 31, to each side of the ring gear 32, is a pressure chamber, i.e., a first pressure chamber 36 and a second pressure chamber 37. An oil passage 38 is defined in the camshaft 14 to communicate through the bore 33a of the hollow bolt 33 to the first pressure chamber 36. Another oil passage 39 is defined in the camshaft 14 and the inner gear 35 to communicate to the second pressure chamber 37. The hydraulic pressure supplied from the hydraulic pressure supply unit 24 (see Figure 1) is fed selectively through the oil passages 38, 39 to the pressure chambers 36, 37. While the engine 3 is operating, hydraulic pressure is selectively supplied to the pressure chambers 36, 37 so as to actuate the VVT 1 and thus rotate the ring gear 32 as it travels along the axis of the camshaft 14. The travel of the ring gear 32 applies a torque to the camshaft 14. This torque changes the rotational phase of the camshaft 14 with respect to the pulley 18. Consequently, the valve timing of the intake valves 12 is changed. The ring gear 32 is able to travel within a predetermined range in the axial direction of the camshaft 14. The amount of change in the valve timing depends on the amount of travel of the ring gear 32. In this embodiment, the hydraulic pressure values in the pressure chambers 36, 37 are controlled to suitably adjust the balance between the hydraulic pressure in the pressure chamber 36 and that in the pressure chamber 37. This adjustment maintains the ring gear 32 at a desired position within the predetermined traveling range. Thus, valve timing of the intake valves 12 is changed controlled. According to the embodiment described above, while the VVT 1 is actuated, the camshaft 14 receives a torque (torsional load) from the VVT 1. However, since the outside diameter D1 of the first journal 22a located adjacent to the VVT 1 is greater than that of each of the other portions, the camshaft 14 has a high rigidity so as to withstand that torque. Further, since the outside diameters D2 to D5 of the other journals 22b to 22e are reduced stepwise from the first journal 22a, an increase in the overall volume and weight of the camshaft 14 is limited. Accordingly, the rigidity of the camshaft 14 against the load applied by the VVT 1 under actuation is increased effectively, and the valve timing of the intake valves 21 driven by the shaft 14 is changed with a very accurate response. Since the first journal 22a has a relatively large outside diameter D1, machining of the oil passages 38, 39 to be defined in the camshaft 14 is relatively easy. Incidentally, the timing pulleys 20, 18, 19 may be replaced with sprockets, and the timing belt 21 may be replaced with a chain. The present invention may be carried out in other embodiments to be described below, which result in similar advantages and effects to those in the foregoing embodiment. To avoid a redundant description, like or same reference numerals are given to those components which are like or the same as corresponding components of the first embodiment. (Second embodiment) A camshaft 41 of the second embodiment has a different shape from that of the camshaft 14 in the first embodiment. As shown in Figure 4, the VVT 1 is attached to the front end of the camshaft 41. The camshaft 41 has three pairs of cams 41a. This camshaft 41 is employed in an engine having three cylinders. In the camshaft 41, only a first journal 42a located closest to the VVT 1 is given an increased outside diameter D1 and only a shaft section 43a located next to the journal 42a is tapered. The outside diameters of the other journals 42b, 42c, 42d and of the other shaft sections 43b, 43c are uniform. According to this embodiment, machining of the camshaft 41 is relatively easy as compared with the camshaft 14 of the first embodiment. (Third embodiment) A camshaft 51 having a different shape from that of the camshaft 14 or 41 in the foregoing embodiments is employed in a third embodiment which does not form part of the present invention. As shown in Figure 5, the VVT 1 is attached to the proximal end of the camshaft 51. This camshaft 51 has a plural pairs of cams 51a. In the camshaft 51, only a first journal 52a located adjacent to the VVT 1 is given an increased outside diameter D1, and the outside diameters of the other journals 52b, 52c and of shaft sections 53a, 53b, 53c are uniform. According to this embodiment, machining of the camshaft 41 is relatively easy. The present invention may be embodied in engines provided with other types of VVTs. While there are various possible types of VVTs, the present invention can be applied to any desired type of VVT so long as it can substantially change the rotational phase between the camshaft and a rotor. For example, a vane type VVT may be employed. The vane type VVT, which is fixed to the end of the camshaft, has a vaned rotor and a housing surrounding the rotor, which is rotatable with respect to the camshaft and the rotor. Further, this VVT has pressure chambers on each side of the vane with respect to the rotational direction of the rotor. The housing has a chain gear on the circumference thereof and is connected thereby to the crankshaft. The VVT 1 may be designed to supply hydraulic pressure to only one of the pressure chambers 36 and 37. In this case, the VVT 1 has a device for urging the ring gear 32 in an opposite direction. Further, the VVT 1 may be fixed not to the intake-side camshaft 14, 41 or 51 but to the exhaust-side cam shaft 15, and the outside diameter of the camshaft 15 is increased at a location adjacent to the VVT 1. Otherwise, the camshafts 14, 41 or 51 and 15 may be given large diameters, and VVTs 1 may be fixed to both of them so as to change valve timing of the intake valves and exhaust valves. In addition, the number of cams 14a, 41a or 51a provided on the camshaft 14, 41 or 51 may be changed depending on the number of valves in the engine 3.

Claims: An engine camshaft for use in an engine (3) provided with a valve timing control apparatus (1) for changing the valve timing of at least one of an intake valve (12) and an exhaust valve (13), wherein the control apparatus is attached to one end of the camshaft (14; 41; 51), wherein the camshaft (14; 41; 51) has an outside diameter at a portion adjacent to the valve timing control apparatus (1) that is greater than the outside diameter of other portions of the camshaft (14; 41; 51) that are more remote from the control apparatus (1), characterized in that each journal (22a, 22b, 22c, 22d, 22e; 42a, 42b, 42c, 42d; 52a, 52b, 52c) has a diameter that is greater than any other of the journals (22a, 22b, 22c, 22d, 22e; 42a, 42b, 42c, 42d; 52a, 52b, 52c) that is more distant from valve timing control apparatus (1). An engine camshaft for use in an engine (3) provided with a valve timing control apparatus (1) for changing the valve timing of at least one of an intake valve (12) and an exhaust valve (13), wherein the control apparatus is attached to one end of the camshaft (14; 41; 51), wherein the camshaft (14; 41; 51) includes a plurality of cams (14a; 41a; 51a) for driving the valves (12), a plurality of journals (22a, 22b, 22c, 22d, 22e; 42a, 42b, 42c, 42d; 52a, 52b, 52c) for supporting the camshaft (14; 41; 51) and a plurality of shaft sections (23a, 23b, 23c, 23d; 43a, 43b, 43c; 53a, 53b, 53c) joining the cams (14a; 41a; 51a) to the valve timing control apparatus (1) and wherein the camshaft (14; 41; 51) has an outside diameter at a portion adjacent to the valve timing control apparatus (1) that is greater than the outside diameter of other portions of the camshaft (14; 41; 51) that are more remote from the control apparatus (1), characterized in that the outside diameter of each shaft section (23a, 23b, 23c, 23d; 43a, 43b, 43c; 53a, 53b, 53c) is greater than any other of the shaft sections which is more distant from the valve timing control apparatus (1). An engine camshaft for use in an engine (3) provided with a valve timing control apparatus (1) for changing the valve timing of at least one of an intake valve (12) and an exhaust valve (13), wherein the control apparatus is attached to one end of the camshaft (41), wherein the camshaft (41) includes a plurality of cams (41a) for driving the valves (12), a plurality of journals (42a, 42b, 42c, 42d) for supporting the camshaft (41) and a plurality of shaft sections (43a, 43b, 43c) joining the cams (41a) to the valve timing control apparatus (1) and wherein the camshaft (41) has an outside diameter at a portion adjacent to the valve timing control apparatus (1) that is greater than the outside diameter of other portions of the camshaft (41) that are more remote from the control apparatus (1), characterized in that the outside diameter of the journal (42a) located closest to the control apparatus (1) is greater than the other portions, while the shaft section (43a) located closest to the control apparatus (1) is tapered, and the other journals (42b, 42c, 42d) and shaft sections (43b, 43c) have uniform diameters. The engine camshaft according to Claim 1, characterized in that the engine (3) has a plurality of valves (12), and wherein the camshaft (14; 41; 51) includes a plurality of cams (14a; 41a; 51a) for driving the valves (12), a plurality of journals (22a, 22b, 22c, 22d, 22e; 42a, 42b, 42c, 42d; 52a, 52b, 52c) for supporting the camshaft (14; 41; 51) and a plurality of shaft sections (23a, 23b, 23c, 23d; 43a, 43b, 43c; 53a, 53b, 53c) joining the cams (14a; 41a; 51a) to the valve timing control apparatus (1). The engine camshaft according to Claim 4, characterized in that the plurality of cams (14a; 41a; 51a) form cam pairs, and each journal (22a, 22b, 22c, 22d, 22e; 42a, 42b, 42c, 42d; 52a, 52b, 52c) is located between the cams (14a; 41a; 51a) forming a corresponding pair, and one of said shaft sections (23a, 23b, 23c, 23d; 43a, 43b, 43c; 53a, 53b, 53c) is located between adjacent cam pairs. The engine camshaft according to any one of the preceding Claims, characterized in that the apparatus (1) has a first rotating member (35), which is rotated in a fixed phase relationship with the crankshaft (9), a second rotating member (18, 31), which is rotated in a fixed phase relationship with the camshaft (14; 41; 51) and an actuating means for changing rotational phase relationship between the second rotating member (18, 31) and the first rotating member (35).

Inventors: NOGUCHI TOSHIHARU, NOGUCHI, TOSHIHARU
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1997
Main classification: F01L1
IPC codes: F01L1