The present invention relates to a continuously variable valve actuation apparatus in an internal combustion engine which has charge exchange valves. The apparatus is able to continuously vary stroke, lifting duration, and phase.
Continuously variable valve actuation systems are known in the art, and disclosures of mechanical, hydraulic, and electromagnetic systems are known. The advantages of such systems are numerous. Most importantly, such systems can control the charge filling of a four-cycle spark ignition engine without the conventional throttle valve, thus reducing the pumping loss and improving the efficiency. Another advantage is the ability to generate valve lift curves that suit a wide range of operating conditions. The following discussion of prior art will focus on mechanical systems that presently appear to be relevant for the invention being disclosed here.
A first type of apparatus is disclosed by U.S. Pat. No. 6,029,618 (Nara et al.) This apparatus consists of an eccentric crank and a rocking cam placed coaxially on a driveshaft, and operatively connected by a link mechanism that can vary stroke and lifting duration of the valves. U.S. Pat. No. 6,390,041 (Nakamura et al.) points out that this system is sensitive to dimensional errors of the components when it operates in the region of low lift. This incurs higher requirements for machining accuracy and hence higher production cost. The latter patent discloses a microcomputer-based controller to help compensate for the problem, but it does not solve the fundamental mechanical issue.
A second type of apparatus is disclosed by U.S. Pat. No. 6,425,357 (Shimizu et al.) This apparatus consists of a rocker assembly inserted between a camshaft and a follower connected to a valve of the engine. The rocker assembly includes an input portion driven by the cam, and an output portion contacting the follower. By varying the angle between the input and output portions with a sliding gear, stroke and lifting duration can be varied. U.S. Pat. No. 6,823,826 (Sugiura et al.) points out that the sliding gear is difficult and expensive to manufacture, as it is difficult to machine the inner parts of the helical splines for the sliding gear, and as it must be machined with high accuracy, in order to avoid a situation where only a small fraction of the teeth carry the entire load. The solution devised by this patent is to replace the sliding gear with a single pin, contacting a diagonal hole or a diagonal surface. This will, however, shift the entire load of the sliding gear to a single point of contact. US Pat. Application Pub. No. 2007/0163523 (Miyazato et al.) points out another problem with this type of apparatus. If the engine block is manufactured from an aluminum alloy, and the control shaft for moving the sliding gear is manufactured from steel, then the different temperature expansion coefficients of these materials will cause a difference between the adjustment among the cylinders in the engine block. This hinders accurate control of valve lift. The devised solution is to manufacture the engaging parts of the control shaft (the sliding gear) from steel, and the parts in between from an aluminum alloy. This solution will add to the manufacturing cost and complexity of the system as well. Finally, this type of apparatus is adding to the total spring load supported inertia of the valve train, thus limiting the maximum operation speed and increasing friction due to the requirement for higher spring loads.
A third type of apparatus is disclosed by U.S. Pat. No. 6,907,852 (Schleusener et al.) This apparatus consists of a spring loaded pivoting lever, the top end having a roller contacting a control path and an eccentric control shaft which controls the position where the roller contacts the path. The camshaft presses against another roller in the middle of the lever to rock it sideward. At the other end of the lever, a ramp contacts a follower connected to the valve. The eccentric control shaft is rotated by an electric stepper motor, varying stroke and lifting duration. This type of apparatus is also adding to the total spring load supported inertia of the valve train, thus limiting the maximum operation speed and increasing friction due to the requirement for higher spring loads. Moreover, the requirement for space in the engine head is high.
Common for all three types of apparatuses above is the requirement for a separate additional device to vary the phase. This adds to the total cost and complexity of embodiments, and introduces the extra complication of synchronizing the two systems, especially during transient states. In the worst case, poor synchronization can cause a valve to collide with a piston top, resulting in major damage to the engine. U.S. Pat. No. 6,820,579 (Kawamura et al.) discloses an electronic control system for synchronizing the two systems, exemplified by the first type of apparatus above. This system accounts for the rate constants of change for the respective systems during transient states. U.S. Pat. No. 7,434,553 (Nakano) discloses a combined hydraulic circuit for controlling the two systems in the case of the second type of apparatus above. These extra control systems also add to the cost and complexity of embodiments.
Examples of apparatuses which can vary valve stroke, lifting duration, and phase simultaneously are disclosed in U.S. Pat. No. 6,968,819 (Fujii et al.) and U.S. Pat. No. 7,299,775 (Tateno et al.). Common properties for these inventions are that they add to the spring load supported inertia of the valve train, and that they are relatively complicated mechanisms. Presently, no apparatus which can vary stroke, lifting duration, and phase simultaneously is known to be in mass production.
Therefore, some objects of the present invention are to provide a continuously variable valve actuation apparatus, which is relatively simple and robust, which is not very sensitive to temperature change and dimensional errors of the manufactured parts, which has low friction to benefit fuel economy, and which can vary stroke, lifting duration, and phase simultaneously without requiring a separate valve phasing device. Further objects are that the spring load supported parts of the valve train do not have more inertia than the corresponding parts for a traditional overhead camshaft mechanism, and that the fully supported parts have low inertia as well.
The present invention provides a continuously variable valve actuation apparatus, comprising:
The driving shaft and the first gear wheel drives the second gear wheel and the valve-lifting crankshaft. The connecting rod transmits motion from the valve-lifting crankshaft to the rocker cam assembly. The rocker cam assembly actuates the cam follower and the valve. Stroke, lifting duration, and phase can be controlled by the angular position of the frame.
The above and further objects, features and advantages of the present invention will become understood from the following description with reference to the accompanying drawings, in which like reference numerals and characters are used to represent like or similar elements.
A first exemplary embodiment is shown on
Gear wheels 22 and 24 of the first embodiment are elliptical and rotate about an elliptical focus. This is an example of an optimized gear wheel geometry. Some advantages of gear wheels with optimized geometries will be discussed later.
In order to see how the angular position of frame 23 controls stroke and lifting duration, attention is directed to
In order to see how the angular position of frame 23 also controls phasing, attention is directed to
The amount of phase change relative to the change of stroke and lifting duration is determined mainly by the sizes of the gear wheels relative to the length of crank arm 26.
A fourth embodiment for actuating exhaust valves will be described subsequently. If a four stroke piston engine is running with partial charge filling, then there may be a vacuum when the piston reaches the bottom position after a power stroke. It may therefore be desirable to retard the timing of exhaust valve opening until the point during the exhaust stroke where neutral pressure in the combustion chamber is reached. If the rotational directions of the gear wheels are reversed, as compared to the first embodiment, such that first gear wheel 22 is rotating counterclockwise and second gear wheel 24 is rotating clockwise, then the lift curves of the first embodiment will be “played backward”. Hence, the timing of valve closing will be approximately constant, and the timing of valve opening will be advanced as stroke and lifting duration are increased. Three lift curves generated by such a fourth embodiment are shown on
Gear wheels with optimized geometries, such as e.g. the elliptical geometry chosen for the previous embodiments, can have some advantages over circular gear wheels as will be discussed in the following.
A fifth embodiment, shown on
Therefore, some additional changes are required for the fifth embodiment, if lift curves similar to the desirable lift curves generated by the first embodiment are to be obtained. In the following it will be discussed how these changes can be made, and what disadvantages these changes will incur, as compared to the first embodiment with optimized gear wheel geometry.
The lifting duration of the first embodiment is shorter than the lifting duration of the fifth embodiment, because for the first embodiment, the angular velocity of second gear wheel 24 and valve-lifting crankshaft 25 is higher when the valve is open than when the valve is closed, due to the gear wheel geometry.
So far, the issue with lifting duration has been discussed. Subsequently, the issue with asymmetry of the lift curve will be discussed.
The above comparison between the first and the sixth embodiments illustrates the advantages that can be gained by having gear wheels with optimized geometries, instead of ordinary circular gear wheels. Although the description above has covered the special case of elliptical gear wheel geometries, it is obvious that other gear wheel geometries are possible, and that a predetermined gear wheel geometry which is neither circular nor elliptical may be optimal for a specific embodiment. The optimal geometry for a specific embodiment can be found with numerical methods using a computer.
A ninth embodiment includes two separate mechanisms for actuating two intake valves of the same cylinder. Each mechanism is similar to the seventh embodiment shown on
While in the above exemplary embodiments, driving shaft 21 is driven over a timing belt, other means are also possible, such as e.g. gear wheels or a chain.
While frame 23 is shown as a simple construction in the above exemplary embodiments, other frame designs are possible, such as e.g. a frame spanning all the cylinders of an engine block, or a frame with reinforcements to increase its torsional stiffness. Having multiple points of support from the device controlling the angular position of frame 23 (such as e.g. control linkage 60 shown on some exemplary embodiments) can be advantageous to prevent warping of frame 23.
While one specific type of control linkage has been shown for controlling the angular position of frame 23, other arrangements are also possible, such as e.g. a rack and pinion or a spindle motor. Instead of a control linkage allowing one servo mechanism to control a plurality of valves, individual servo mechanisms each controlling a frame for one valve or cylinder are also possible. The servo mechanism can be powered by e.g. an electric motor or a hydraulic system.
While the above exemplary embodiments include a valve-lifting crankshaft providing a crank arm, other options such as e.g. a valve-lifting crankshaft providing an eccentric instead of a crank arm, are also possible.
While embodiments of rocker cam assembly 29 providing one or two cam lobes have been shown, it is also possible to have rocker cam assemblies providing three or more cam lobes.
While the means for urging the cam follower against the rocker cam assembly is a conventional steel spring in the above exemplary embodiments, other types of spring load devices, such as e.g. a pneumatic device, are also possible.
While the cam follower has been directly connected to a valve stem in the above exemplary embodiments, other types of valve trains, such as e.g. pushrods and rocker arms or hydraulic systems, are also possible.
While a specific arrangement of lever 44 has been shown in some exemplary embodiments, other arrangements for providing valve lash adjustment by means of a movable support for rocker cam assembly 29 are also possible. The arrangement chosen for a specific embodiment will depend on the specific space requirements of parts in the engine head.
While the above description of embodiments has made no mention of bearings, it is understood that pivotal joints or supports of rotating and rocking elements can be implemented with bearings of various types.
While in the description of the above exemplary embodiments, no attention has been paid to the rotational balance of parts and vibration control, it is understood that embodiments can incorporate counter weights and the like to address such issues.
The foregoing description and exemplary embodiments have been set forth merely to illustrate the invention, and are not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations falling within the scope of the appended claims and equivalents thereof.
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Number | Date | Country | |
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20120227694 A1 | Sep 2012 | US |