Abstract:
A variable valve timing system for a vehicle includes an intake camshaft for operating an intake valve, an exhaust camshaft for operating an exhaust valve, an intake cam sprocket mounted on the intake camshaft, an exhaust cam sprocket mounted on the exhaust camshaft, a chain interconnecting the intake cam sprocket and the exhaust cam sprocket, and a device for varying phases of the intake and exhaust camshafts by, according to an engine rpm, pushing and pulling the chain to forcedly rotate the intake and exhaust cam sprockets at a predetermined angle while moving in a perpendicular direction with respect to a longitudinal direction of a power transmission member.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Korea patent application No. 1999-66829, filed on Dec. 30, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a variable valve timing system for a vehicle, and more particularly, to a variable valve timing system that can maximize engine output efficiency by adjusting an open/close timing of an intake valve according to an engine rpm. 
     (b) Description of the Related Art 
     As shown in FIG. 4, a valve operation mechanism for intake and exhaust valves for an engine includes an intake camshaft  101  and an exhaust camshaft  103  which are independently provided to open and close the respective intake and exhaust valves in a timely fashion. 
     Furthermore, the exhaust camshaft  103  is driven by power transmitted from a crankshaft  105 , and the intake camshaft  101  is driven by power transmitted from the exhaust camshaft  103  via a chain  107 . 
     Here, the intake and exhaust valves are opened and closed in a timely fashion by the intake camshaft  101  and the exhaust camshaft  103 . This is called valve timing, in which the intake valve is generally opened before an intake stroke (that is, before a top dead center point), and is closed after the intake stroke is completed (that is, after the bottom dead center point), and the exhaust valve is opened before an exhaust store (that is, before a bottom dead center point) and is closed just after the start point of the intake stroke after the finish point of the exhaust stroke (that is, after the top dead center point). 
     In addition, there is a valve overlap time between the finish point of the exhaust stroke and the start point of the intake stroke, during which both the intake and exhaust valves are open to completely exhaust the burned gas and increase the charging efficiency of the mixture. 
     However, in the conventional valve timing, although an overlap angle is uniform, the overlap time is reduced at a high rpm range of the engine, and is increased at a low rpm range. That is, the overlap time is varied according to the engine rpm. Particularly, in the high rpm range, since the overlap time is too short, the mixture being taken in cannot sufficiently expel the burned gas out of the combustion chamber. That is, the conventional valve operation mechanism cannot properly perform its function. 
     In addition, the chain is designed to be subject to vibration from the crankshaft and the camshaft to generate lateral vibration, causing impact noise with a chain guide and a sprocket. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to solve the above problems. 
     It is an objective of the present invention to provide a variable valve timing system that can maximize engine output efficiency by properly adjusting an open/close timing of an intake valve according to an engine rpm. 
     It is another objective of the present invention to provided a variable valve timing system that can prevent impact noise between a chain and a chain guide by absorbing lateral vibration of the chain. 
     To achieve the above objectives, the present invention provides a variable valve timing system for a vehicle, comprising an intake camshaft for operating an intake valve, an exhaust camshaft for operating an exhaust valve, an intake cam sprocket mounted on the intake camshaft, an exhaust cam sprocket mounted on the exhaust camshaft, a chain interconnecting the intake cam sprocket and the exhaust cam sprocket, and means for varying phases of the intake and exhaust camshafts by, according to an engine rpm, pushing and pulling the chain to forcedly rotate the intake and exhaust cam sprockets at a predetermined angle while moving in a perpendicular direction with respect to a longitudinal direction of a power transmission member. 
     According to an embodiment of the present invention, the means for varying comprises an auxiliary cam sprocket disposed between the intake and exhaust cam sprockets and engaged with the chain, and means for moving the auxiliary cam sprocket in a perpendicular direction with respect to a longitudinal direction of the chain to push and pull the chain in the perpendicular direction, thereby varying the phase angles of the intake and exhaust camshafts. 
     Preferably, the means for moving comprises a slide bar for rotatably supporting the auxiliary sprocket, the slide bar being slidably disposed in a cavity formed in a cylinder head and defined upper and lower oil chambers with its ends against the cavity, each of the upper and lower oil chambers connected to an oil pump and an oil tank through intake and exhaust oil passages formed in the cylinder head, respectively, first and second check valves respectively disposed in the intake oil passages communicating with the upper and lower oil chambers, the check valves allowing the supply of oil from the oil pump to the oil chambers and disallowing the supply of oil from the oil chambers to the oil pump, first and second solenoid valves respectively disposed on the exhaust oil passages communicating with the upper and lower oil chambers, the first and second solenoid valves selectively opening and closing the exhaust oil passages, and an electronic control unit for controlling the first and second solenoid valves according to the engine rpm to selectively open and close the exhaust oil passages, thereby sliding the slide bar along the cavity. 
     The cavity is provided with at least one large diameter portion in which a projection formed on the slidebar is disposed to limit the slide movement of the slide bar to a predetermined length. 
     The slide bar is provided with a sprocket hole, the auxiliary chain sprocket is rotatably supported in the sprocket hole by a shaft, and a bearing is disposed between the shaft and the auxiliary chain sprocket. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
     FIG. 1 is a sectional view of a variable valve timing system according to a preferred embodiment of the present invention; 
     FIG. 2 is a sectional view taken along a line II—II of FIG. 1; 
     FIG. 3 is a sectional view of a circled portion A of FIG. 1; and 
     FIG. 4 is a sectional view of a conventional valve operation mechanism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows a variable timing system according to a preferred embodiment of the present invention. 
     In the inventive variable timing system, an exhaust camshaft  1  for operating an exhaust valve (not shown) and an intake camshaft  3  for operating an intake valve (not shown) are separately formed. Mounted on the exhaust and intake camshafts  1  and  3  are respectively exhaust and intake cam sprockets  5  and  7  connected to each other by a chain  9  to transmit rotational power of the exhaust camshaft  1  to the intake camshaft  3 . 
     Between the exhaust and intake cam sprockets  5  and  7 , an auxiliary chain sprocket  11  is rotatably disposed and engaged with the chain  9 . A slide bar  15  is slidably disposed over upper and lower cavities  28  and  28 ′ of a cylinder head  13 , while defining upper and lower oil chambers  17  and  19  in the cylinder head  13 . 
     The slide bar  15 , as shown in FIG. 2, is provided with a sprocket hole  21  at its central portion such that the auxiliary chain sprocket  11  can be rotatably mounted on the slide bar  15  by a shaft  23 . Preferably, a bearing  25  is disposed between the shaft  23  and the auxiliary chain sprocket  11 . 
     The upper and lower oil chambers  17  and  19  communicate with an oil pump through intake oil passages  27  and  27 ′, respectively. Check valves  29  and  29 ′ are disposed in the intake oil passages  27  and  27 ′ such that the oil can be supplied only in a direction from the oil pump to the upper and lower chambers  17  and  19 . That is, as shown in FIG. 3, each of the check valves  29  and  29 ′ include a check ball  31  disposed in the oil passage (e.g.,  27 ′), a valve housing  33  disposed enclosing the check ball  31 , and a guide member  35  disposed inside the valve housing  33  to guide the check ball  31 . A plurality of oil holes  37  are formed on the valve housing  33 . The check ball  31  is biased by an elastic member  39 , disposed between an inner wall of the valve housing  37  and the check ball  31 , in a direction where the oil passage is blocked when the oil is not supplied from the oil pump to the chamber. 
     Referring again to FIG. 1, the upper and lower oil chambers  17  and  19  further communicate with an oil tank through exhaust oil passages  41  and  41 ′, and solenoid valves  43  and  43 ′ are respectively provided in the exhaust oil passages  41  and  41 ′ so as to exhaust oil from within the oil chambers  17  and  19  to the oil tank. 
     That is, as shown in FIG. 3, an exhaust chamber  45  communicating with the exhaust oil passage  41 ′ is formed in the cylinder head  13 , and a valve spool  49  is provided on an operation rod  51  of a solenoid  53  which is controlled by an electrode control unit (ECU). The valve spool  49  is biased toward an exhaust passage blocking position by an elastic member  47 . 
     Referring again to FIG. 1, the upper and lower cavities  28  and  28 ′ are respectively provided with larger diameter portions  55  and  55 ′, and guide projections  57  and  57 ′ are formed on the slide bar  15  such that they are movable in the larger diameter portions  55  and  55 ′, respectively. The larger diameter portions  55  and  55 ′ function as stoppers on which the projections  57  and  57 ′ of the sliders get caught such that the slide bar  15  does not slide over the exhaust oil passages  41  and  41 ′. 
     The operation of the above-described variable timing system will be described in detail hereinbelow. 
     When a signal that the engine rpm is in a predetermined low/medium rpm range is transmitted from an engine rpm sensor (not shown) to the ECU, the ECU controls the solenoid valves  43  and  43 ′ such that the exhaust oil passage  41  is closed and the exhaust oil passage  41 ′ is opened. Accordingly, the oil supplied from the oil pump to the lower oil chamber  19  through the intake oil passage  27 ′ is exhausted to the oil tank through the exhaust oil passage  41 ′. 
     Therefore, the slide bar  15  is moved downward to block the exhaust oil passage  41 ′, then stopped by the association of the larger diameter portion  55 ′ and the projection  57 ′. At this point, oil pressure formed by oil filled in the lower oil chamber  19  by blocking the exhaust oil passage  41 ′ functions as an absorber against the sliding movement of the slide bar  15 . 
     By the downward movement of the sliding bar  15 , the auxiliary chain sprocket  11  between the intake an exhaust cam sprockets  7  and  5  biases the chain  9  downward to rotate the intake and exhaust cam sprockets  7  and  5  in opposite directions, as a result of which phase angles of the intake and exhaust cam sprockets  7  and  5  are varied such that the open/close operation of the intake valve is retarded with respect to the open/close operation of the exhaust valve, reducing the overlap section where the intake and exhaust valves are simultaneously open. 
     When a signal that the engine rpm is in a predetermined high rpm range is transmitted from an engine rpm sensor (not shown) to the ECU, the ECU controls the solenoid valves  43  and  43 ′ such that the exhaust oil passage  41  is opened and the exhaust oil passage  41 ′ is closed. Accordingly, the oil supplied from the oil pump to the upper oil chamber  17  through the intake oil passage  27  is exhausted to the oil tank through the exhaust oil passage  41 . 
     Therefore, the slide bar  15  is moved upward to block the exhaust oil passage  41 , then stopped by the association of the larger diameter portion  55  and the projection  57 . At this point, oil pressure formed by oil filled in the upper oil chamber  17  by blocking the exhaust oil passage  41  functions as an absorber against the sliding movement of the slide bar  15 . 
     By the upward movement of the sliding bar  15 , the auxiliary chain sprocket  11  between the intake and exhaust cam sprockets  7  and  5  biases the chain  9  upward to rotate the intake and exhaust cam sprockets  7  and  5  in an opposite direction, as a result of which phase angles of the intake and exhaust cam sprockets  7  and  5  are varied such that the open/close operation of the exhaust valve is retarded with respect to the open/close operation of the intake valve, increasing the overlap section to improve output efficiency of the engine by improving the volumetric efficiency of the cylinder. 
     In the above, the auxiliary chain sprocket  11  functions as a chain guide. Therefore, in the present invention, no additional guide member is required. The lateral vibration transmitted from the camshaft and the crankshaft is absorbed by the oil pressure of the upper and lower oil chambers  17  and  19  when the slide bar  15  is moved. 
     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.