VALVE OPERATING MECHANISM

Abstract: A valve operating mechanism for an internal combustion engine having a plurality of exhaust valves (1,2) per cylinder comprises means for driving the exhaust valves at such a valve timing that makes opening of the exhaust valves cause blowdown pressures with a phase difference (0), i.e., at different timings (E1,E2) relative to a crank angle. In one aspect of this invention, the exhaust valves open at the different times and close at the same time.

Description: BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to a valve operating mechanism for an internal combustion engine having a plurality of exhaust valves per cylinder. II. Description of the Prior Art In an internal combustion engine for automotive vehicles, etc., having a plurality of exhaust valves and a plurality of intake valves per cylinder, it has been proposed to operate at the different valve timings the exhaust valves and the intake valves, respectively. A prior art valve operating mechanism of the above described kind, as for example disclosed in Japanese Patent Publication No. 49-17967, is adapted to operate two exhaust valves and two intake valves of a four-stroke cycle four-valves-per-cylinder internal combustion engine at the valve timings as shown in Fig. 9. That is, the valve timings E1 and E2 of the two exhaust valves and the valve timings I1 and I2 of the two intake valves respectively have such predetermined phase differences that at the positions of the piston adjacent the top dead center in the transition from the exhaust stroke to the intake stroke a set of one exhaust valve and one intake valve are closed and a set of the other exhaust valve and the other intake valve are opened with a view to increaing the velocity of flow of the fresh intake mixture within the cylinder and thereby improving the scavenging efficiency. Further, at the position of the piston adjacent the bottom dead center in the transition from the intake stroke to the compression stroke, only one of the intake valves is opened to cause a swirl of the intake mixture for thereby improving the combustibility during high-speed high-load operation of the engine. Such a prior art valve operating mechanism is also known from DE-A 33 32 789, which shows a plurality of exhaust valves per cylinder and comprises means for driving said exhaust valves at such an off timing that makes the said exhaust valves open at different times. EP-A-0 258 061 shows a valve operating apparatus in an internal combustion engine, in which plural intake valves and exhaust valves are associated with each cylinder. The respective valves are operated in one engine-operating condition in response to the action of a first set of cams and in another engine-operating condition in response to the action of a second set of cams coupled to the first cam set. The cams operating the intake valves impart a different valve operating action than those operating the exhaust valves. From EP-A-0 213 759 a valve operating mechanism is known, which is provided for operating a pair of valves of an internal combustion engine, including a camshaft rotatable in synchronism with the rotation of the internal combustion engine and having a first low-speed cam, a second low-speed cam, and a high-speed cam which have different cam profiles. The first and second low-speed cams are disposed one on each side of the high-speed cam, a rocker shaft, the first, second and third rocker arms are rotatably mounted on the rocker shaft and held in sliding contact with the first low-speed cam, the second low-speed cam and the high-speed cam, respectively, for operating the valves according to the cam profiles of the cams. A selective coupling is operatively disposed in and between the first, second and third rocker arms for selectively interconnecting the first, second and third rocker arms to allow angular movement thereof in unison and disconnecting the first, second and third rocker arms to allow separate angular movement thereof. In the valve operating mechanism of the above described type, a phase difference is provided to the timings at which the two exhaust valves fully close. Due to this, the effective area of the opening defined by the exhaust valves at the time of the valve overlap in which the intake and exhaust valves are held open simultaneously is small, thus resulting in that a long valve overlapping period is necessitated in order to fully attain an inertia supercharging effect of the intake mixture during high-speed high-load operation of the engine. However, when the valve overlapping period is set to be long, the exhaust gases are caused to flow reversely into the cylinder being drawn by a large intake vacuum though the area of the opening defined by the exhaust valves is small, thus increasing the quantity of the residual gases during low-speed low-load operation of the engine in which the supply of intake mixture is small, thus largely deteriorating the combustibility. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a valve operating mechanism for an internal combustion engine having a plurality of exhaust valves per cylinder, which can reduce the quantity of the exhaust gases reversibly flowing into the cylinder due to the valve overlap during low-speed, low load operation of the engine and attain the idling stability. This object is achieved by the characterizing features of claim 1. The subclaims contain preferred embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a valve timing diagram according to an embodiment of the present invention; Fig. 2 is a side elevation of an exhaust valve operating camshaft utilized in the embodiment of Fig. 1; Fig. 3 is a diagram illustrating a variation amplitude of a pressure wave in an exhaust passage in the embodiment of Fig. 1 in comparison with that in the comparable prior art device; Fig. 4 is a diagram illustrating variations of an exhaust port pressure in the embodiment of Fig. 1 in comparison with that in the comparable prior art device; Fig. 5 is a graph depicting variations of an enging output torque and a volumetric efficiency relative to an engine speed in the embodiment of Fig. 1 in comparison with that in the prior art device; Fig. 6 is a valve timing diagram according to another embodiment; Fig. 7 is a valve timing diagram according to a further embodiment; Fig. 8 is a schematic view of exhaust cam lobes employed in the embodiment of Fig. 7; Fig. 9 is a valve timing diagram according to the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, the curves E1 and E2 and the curve I depict valve timings for a four-stroke cycle four-valves-per-cylinder internal combustion engine according to an embodiment of the present invention. The curves E1 and E2 represent the valve timings of exhaust valves 1 and 2, respectively. The exhaust valves 1 and 2 start opening in advance of the bottom dead center (BDC) after which the exhaust stroke begins and with a phase difference ϑ. The exhaust valves 1 and 2 then reach the maximum lift points in the middle of the exhaust stroke, respectively and fully close synchronously with each other in retard of the top dead center (TDC) after which the intake stroke begins. Closing movements of the exhaust valves 1 and 2 are synchronous with each other, i.e., performed at the same speed. The curve "I" represents a valve timing common to intake valves 3 and 4. The intake valves 3 and 4 start opening in advance of the top dead center, reach a maximum lift point in the middle of the exhaust stroke and fully close in retard of the bottom dead center after which the compression stroke begins. With those valve timings, the exhaust valves 1 and 2 and intake valves 3 and 4 are given a predetermined valve overlapping angle about the top dead center. Fig. 2 shows a camshaft 5 for the exhaust valves 1 and 2. The camshaft 5 has cams 6 and 7 which are so profiled as to have a phase difference of ϑ/2 at the opening valve points and a phase difference of 0 (zero) at the closing valve points. In Fig. 3, the dotted line curve indicates the pressure wave occurring in the exhaust passages downstream of the exhaust valves 1 and 2. The pressure wave results from the composition of a blowdown pressure (indicated by the one-dot-chain line curve) produced by the opening of the exhaust valve 1 and a blowdown pressure (indicated by the two-dot-chain line curve) produced by the opening of the exhaust valve 2. Therefore, the vibration amplitude of the pressure wave in the embodiment of this invention can be smaller than that (indicated by the solid line curve) in the prior art device of which exhaust valves open at the same time, i.e., with the same phase. Fig. 4 shows by the dotted line curve the pressure variations of the pressure wave measured under the condition that the engine is operated at the speed of 3600 rpm so that the pressure wave becomes positive in the place spaced by 120 mm downstream of the exhaust valves 1 and 2 upon closure of the exhaust valves 1 and 2. In the figure, the solid line curve represents the pressure variations of the pressure wave in the comprable prior art device in which the exhaust valves open with the same phase. From this test result, it will be understood that according to the present invention the variation amplitude of the reflected wave (the portion indicated by "B") can be made smaller by the amount corresponding to the amount by which the blowdown (the portion indicated by "A") is reduced. Fig. 5 shows, with respect to the embodiment of this invention and the prior art device, how the engine output torque and volumetric efficiency vary with variation of the engine rpm. From this graph, it will be understood that while in case of the prior art device there occurs a considerable drop or fall of the output toruqe and volumetric efficiency at or near the engine speed of 3600 rpm, a corresponding drop is smaller in case of this embodiment. The drop of the output torque is caused by the closure of the exhaust valves 1 and 2 at or near the engine speed of 3600 rpm which, as having been described above, causes a positive pressure on the downstream side of the exhaust valves 1 and 2. In case of this embodiment, the downblow pressures are produced with a predetermined phase difference, i.e., at the different timings relative to the crank angle by the opening of the exhaust valves 1 and 2, thus making smaller the vibration amplitude of the pressure wave occurring in the exhaust passage, reducing the pumping loss caused by the consumption of the engine output for discharging the exhaust gases, reducing the residual gases whilst increasing the volumetric efficiency of the intake mixture and making it possible to prevent a drop of an output torque during middle speed operation of the engine. Further, by making the exhaust valves 1 and 2 fully close at the same time, i.e., synchronously with each other, it becomes possible to increase the effective area of opening defined by the exhaust valves 1 and 2 at the time of the valve overlap in which the exhaust valves 1 and 2 are opened together with the intake valves 3 and 4, thus making it unnecessary to set the valve overalpping peiord to be so long. As a result, it becomes possible to retain the volumetric efficiency by utilizing the inertia of the intake mixture during high-speed high-load operation of the engine and at the same time prevent the exhaust gases from reversely flowing into the engine cylinder being drawn by the intake vacuum during low-speed low-load operation of the engine, thus preventing the combustibility from being deteriorated by the residual exhaust gases and thereby improving the idling stability, etc. Fig. 6 shows another embodiment in which the engine is of the type having three exhaust valves per cylinder, and the exhaust valves start opening at different timings so as to provide phase differences ϑ1 and ϑ2 and fully close at the same time, i.e., synchronously with each other though closing movments of the exhaust valves are performed at different speeds. When this is the case, since three exhaust valves start opening with phase differences from each other, the blowdown pressures are produced at the three different timings, thus making it possible to further reduce the variation amplitude of the pressure wave occuring in the exhaust passage. Fig. 7 shows a further embodiment in which the valve timings E1 and E2 cause the exhaust valves 1 and 2 to start opening at the same time, i.e., synchronously with each other in advance of the bottom dead center after which the intake stroke begins but increase in degree of opening at different speeds so as to reach different maximum lift points in the middle of the exhaust stroke. To this end, as shown in Fig. 8, exhaust cam lobes 6' and 7' are constructed so that the maximum lift L2 of one exhaust valve 2 is about one half of the maximum lift L1 of the other exhaust valve 1. Further, the cam lobes 6' and 7' are constructed so that the valve opening speed of the exhaust valve 2 is about half of that of the exhaust valve 1. In the meantime, the maximum lift L1 of the exhaust valve 1 is set so as to be substantially equal to the maximum lift of the intake valves 3 and 4. Except for the above, this embodiment is substantially similar to the previous embodiment described with reference to Figs. 1 to 5 and can produce substantially the same effect.

Claims: A valve operating mechanism for an internal combustion engine having a plurality of exhaust valves (1,2) per cylinder, comprising: means (camshaft 5) for driving said exhaust valves (1,2) at such a valve timing that makes said exhaust valves (1,2) open at different times, characterized in that said exhaust valve driving means (camshaft 5) fully close said exhaust valves (1,2) at the same time. A valve operating mechanism as set forth in claim 1, wherein said engine comprises two exhaust valves (1,2) per cylinder, and closing movement of said exhaust valves are performed at the same speed. A valve operating mechanism as set forth in claim 2, wherein said exhaust valves (1,2) effect a valve overlap together with an intake valve (3;4) of said engine. A valve operating mechanism as set forth in claim 1, wherein said engine has 3 exhaust valves per cylinder, and closing movements of said exhaust valves are performed at different speeds. A valve operating mechanism as set forth in claim 4 wherein said exhaust valves effect a valve overlap together with an intake valve of said engine.

Inventors: MATSUMOTO, YASUO, TAKEYAMA, SATOSHI
Applicants: NISSAN MOTOR CO., LTD.
Application year: 1988
Main classification: F01L1
IPC codes: F02D13, F01L1, F01L13, F02F1, F02B29