Variable valve lift mechanism used in an internal combustion engine
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a variable lift mechanism used in an internal combustion engine, especially to a variable lift mechanism of an internal combustion engine wherein a hydraulic tappet is utilized. 
     BACKGROUND ART OF THE INVENTION 
     It is well known to utilize a hydraulic tappet so that noise generated by valve mechanism can be reduced. Such a hydraulic tappet has an outer case which is in contact with a cam rotated in synchronism with a crankshaft of the engine. The outer case has an inner case, slidably and sealingly engaged therein, which actuates a valve of the internal combustion engine via rocker arm. The outer case also has a pressure chamber formed therewithin. A check valve is disposed inside or outside the pressure chamber and oil under pressure is supplied into the pressure chamber through the check valve. Accordingly, when the oil under pressure is supplied into the pressure chamber, the outer case is urged outwardly and is in contact with the cam, and the inner case is in contact with the rocker arm. When a force is applied to the outer surface from the outside thereof, due to the actuation of the check valve, the supply of the oil under pressure is stopped, and then, the hydraulic tappet serves to transmit the movement of the cam to the rocker arm. 
     If an orifice of a small diameter is opened to the pressure chamber, the oil under pressure flows out through the orifice. Since the amount of the flow of the oil is proportional to the time interval which is required for one cycle of the cam, the amount of the oil flow at a low rotational speed of the engine is large because the time interval for one cycle is long. As a result, the hydraulic tappet is contracted a large amount, and the lift of the valve is reduced a large amount. On the other hand, at a high rotational speed of the engine, the amount of the lift reduction of the valve is small. As mentioned above, the valve lift of the engine can be varied in accordance with changes in rotational speed of the internal combustion engine. 
     However, the above-mentioned mechanism can vary the valve lift of an internal combustion engine in accordance with only changes in the rotational speed of an internal combustion engine and the valve lift of the engine cannot be varied in accordance with other factors. As a result, the desired operating characteristics of the engine cannot be obtained by varying the valve lift in accordance with changes in predetermined factors, except for the rotational speed, of the engine. 
     To overcome the above mentioned problem, U.S. Pat. No. 3,439,661 discloses a controlled displacement hydraulic lifter, wherein the pressure chamber of the hydraulic tappet is communicated with a sliding spool control valve via an additional check valve, so that the lifting action of the lifter is controlled by the sliding spool control valve. However, in this lifter, the spool control valve is not controlled in conjunction with changes in operating conditions of the engine, such as the engine load and the temperature of the engine. 
     SUMMARY OF THE INVENTION 
     The principal object of the present invention is to provide a variable valve lift mechanism used in an internal combustion engine which can control the valve lift in accordance with changes in operating characteristics of the engine, and in which the pressure chamber of the hydraulic tappet is communicated with a valve device which is controlled in accordance with changes in operating conditions of the engine. 
     In an embodiment of the present invention, an electromagnetic valve is utilized as the valve device. Signals which are obtained, based on the engine load and the rotational speed of the engine, are passed through a circuit which includes a fundamental algebraic component, and then, another signal is obtained. The latter obtained signal is modified on the basis of the temperature of the oil under pressure, and the modified signal is utilized to actuate the electromagnetic valve. In addition, the pressure level of the oil under pressure between the pressure chamber and the electromagnetic valve is feed-back controlled at a predetermined level, so that the valve lift is maintained at an optimum level. 
     In a preferred embodiment of the present invention, the electromagnetic valve is ON-OFF controlled. Due to the combination result of the ON-OFF control and the above-mentioned feed back control, the control according to the preferred embodiment is easier and more precise than that of the above-mentioned U.S. Pat. No. 3,439,661, wherein the flow is controlled by means of position control of the spool valve. 
     In an embodiment of the present invention which will be described later with reference to the accompanying drawing, the present invention is applied to an engine of over head cam type (OHC type). The present invention is also applicable to an engine of over head valve type (OHV type), which is not illustrated herein. In the latter case, a push rod is disposed between the cam and the rocker arm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment according to the present invention will be explained hereinafter with reference to the accompanying drawing, wherein the FIGURE is a cross sectional elevational view of the embodiment according to the present invention. 
    
    
     DISCLOSURE OF THE INVENTION 
     Referring to the accompanying FIGURE, a cylinder block (not shown) has a cylinder bore formed therein and a piston (not shown) is reciprocatingly and sealingly engaged within the cylinder bore. A cylinder head 1, having a recess 1b formed thereon, is fixedly mounted on the cylinder block so that a combustion chamber 3 is formed at a space surrounded by the cylinder bore, the recess 1b and the piston. The cylinder head 1 has an intake port 5 and an exhaust port (not shown) formed therein. The intake port 5 and the exhaust port are communicated with the combustion chamber via an intake valve 7 and an exhaust valve (not shown), respectively. 
     The intake valve 7 and the exhaust valve have constructions similar to each other, and therefore, only the actuating mechanism of the intake valve 7 will now be explained hereinafter. The intake valve 7 comprises a valve body 7a and a valve rod 7b fixed to the valve body 7a. The valve body 7a is so constructed that it is capable of being in abutment with a valve seat 1a formed on the cylinder head 1. A retainer 9 is fixed at the upper end of the valve rod 7b. A compression spring 11 is disposed between the retainer 9 and the cylinder head 1, so that the intake valve 7 is urged upwardly and that the valve body 7a is pressed against the valve seat 1a. A knocker 17, adjustably threaded to a rocker arm 13, is so constructed that it can press the upper end of the valve rod 7b. The length of the knocker can be adjusted to a desired length by means of a lock nut 19. The rocker arm 13 is swingable about a rocker shaft 15 and has a rocker pad 13a projected therefrom at one end thereof. The rocker pad 13a is in abutment with the upper end of an inner case 53 of a hydraulic tappet 50. The hydraulic tappet 50 is engaged in a hole 23 formed in a support 21. The support 21 is fixed on the cylinder head 1. 
     The hydraulic tappet 50 comprises: an outer case 51 slidably engaged in the hole 23 and having an inner wall 51a and a pressure chamber 51b; the inner case 53 slidably and sealingly engaged in the inner wall 51a and having an oil chamber 53b formed therein; a compression spring 55 which urges the inner case upwardly with respect to the outer case 51, and; a check valve 57 disposed between the pressure chamber 51b and the oil chamber 53b. The check valve 57 is urged to a valve seat 53a formed at the lower surface of the inner case 53 by means of a compression spring 61. The compression spring 61 is supported by a retainer having a U shaped cross section and secured to the lower end of the inner case 53. An orifice 62 having a small diameter is formed in the proximity of the valve seat 53a, so that the pressure chamber 51b is communicated with the oil chamber 53b by the orifice 62. The orifice 62 may be omitted in some cases. 
     An oil supply (not shown) which is driven by the engine is communicated with a journal 24 formed within the support 21 in a direction perpendicular to the sheet on which the FIGURE is illustrated. The journal 24 is communicated with the oil chamber 53b through a supply passage 25, annular groove 27 formed around the surface of the inner case 53 and a small hole 53c formed in the wall of the inner case 53, so that the oil under pressure is supplied into the oil chamber 53b from the oil supply. 
     An orifice 63 of a small diameter is formed in the wall of the outer case 51, at a position higher than the valve seat 53a, which cooperates with the check valve 57 and is communicated with an annular groove 29 which surrounds the outer case 51. The annular grooves 29 (only one is illustrated) of all the cylinders are communicated by a single pipe 40 via oil relief ports 31 (only one is illustrated) and check valves 33 (only one is illustrated). The pipe 40 is communicated with an electromagnetic valve 43. 
     The check valve 33 comprises a slidable valve body 35, which is urged to a valve seat 39 by a compression spring 37 so that a reverse oil flow toward the hydraulic tappet is prevented. 
     A bottom end of the outer case 51 is in abutment with the cam 41 which is rotated, in a direction designated by an arrow A, in synchronism with a crankshaft (not shown) of the engine. 
     The electromagnetic valve 43 comprises a valve seat 44 and a valve body 45, which cooperates with the valve seat 44. A valve rod 46 fixed to the valve body 45 is urged toward the valve seat 44 by means of a compression spring 47 and is also reversely urged by means of an electromagnetic coil 48 when the electromagnetic coil is energized. The electromagnetic coil 48 is energized by a signal emitted from a control 71. 
     A temperature detecting sensor (not shown), which is of conventional type and which is disposed in a cooling system (not shown) of the engine, detects the temperature of the engine and transmits a signal a. A signal b is transmitted from an engine load sensor (not shown) which is of a conventional type and which is disposed at an intake pipe (not shown) of the engine, so that the sensor can detect the engine load by detecting the vacuum in the intake pipe. A sensor of tachometer type (not shown) transmits a signal c which corresponds to the rotational speed of the engine. The signals a, b and c are input into an electric computer 73, and then, these signals are processed in accordance with predetermined algebraic equations. The result from the computer 73 is transmitted to the controller 71 as a target value of the energizing signal for the electromagnetic coil 48. 
     A signal d is transmitted in accordance with changes in oil pressure in the pipe 40 from a conventional pressure sensor disposed in the pipe 40 and is input into the controller 71. As a result, the electromagnetic coil 48 of the electromagnetic valve 41 is so actuated that the signal d can be maintained at the predetermined target value transmitted from the computer 73. In some cases, the actuating signal introduced into the electromagnetic coil 48 may be an ON-OFF signal, wherein ON and OFF signals are continued for certain time intervals, respectively. In some cases, the actuating signal of the electromagnetic coil 48 may be a pulse width modulation signal, so that the amount of flow passing through the electromagnetic valve 43 is continuously controlled in accordance with the signal level. 
     The oil under pressure, supplied from the journal 24, flows into the oil chamber 53b and the pressure chamber 51b,  so that the bottom end of the outer case 51 is in abutment with the cam and that the upper end of the inner case 53 is in abutment with the rocker arm pad 13a. As a result, since no gaps are generated between the hydraulic tappet 50 and the cam 41 and between the hydraulic tappet 50 and the rocker arm pad 13a, the generation of noise is reduced relative to a conventional fixed type tappet. 
     When the cam 41 rotates, it presses the outer case 51 upwardly, and the oil pressure in the pressure chamber 51b is increased. As a result, the check valve 57 is closed, so that the oil under pressure in the pressure chamber 51b is prevented from flowing therefrom to the oil chamber 53b through the check valve 57. Accordingly, in this case the oil tappet 50 moves as if it were almost a solid body, and the movement of the cam 41 is transmitted to the rocker arm pad 13a almost without effecting any changes thereto. 
     On the other hand, the oil under pressure in the pressure chamber 51b flows into the oil chamber 53b through the orifice 62. Since the amount of the oil flow is proportional to the time interval per one cycle of the engine, a large amount of oil flows out when the rotational speed of engine is low, because the time interval per one cycle is long then, and then the length of the hydraulic tappet is contracted and the valve lift is shortened. As a result, the valve lift is varied in accordance with changes in the rotational speed of the engine. 
     However, if the oil is permitted to flow out only through the orifice 62, the valve lift of the engine at a certain rotational speed is determined only depending on the rotational speed of the engine. Therefore, the valve lift of the engine cannot be controlled in accordance with changes in engine load and temperature of the engine. In the present invention, the electromagnetic valve 43 is controlled in conjunction with the operating conditions of the engine, such as the engine load, the temperature of the engine and the rotational speed of the engine, so that the oil under pressure flows from the pressure chamber 51b in conjunction with the operating conditions of the engine and that the valve lift is controlled. As a result, in the present invention the optimum valve lift in conjunction with the operating conditions of the engine can be obtained. In addition, a feed-back control, wherein the pressure of the oil between the electromagnetic valve 43 and the hydraulic tappet 50 is controlled so that it maintains a predetermined value is applied such that, the valve lift can be optimum for a wide range of operating conditions of the engine. 
     In the illustrated embodiment, since the orifice 63 is positioned at a higher position than the valve seat 53a of the check valve 57, the oil under pressure does not leak from the pressure chamber while the electromagnetic valve 43 is deactuated, and therefore, a shock of the hydraulic tappet, which shock is inherent in a hydraulic tappet having no oil therein, does not occur when the engine is restarted.