Patent Application
20090283062
The present invention relates to variable valve lift systems for combustion valvetrains of internal combustion engines; more particularly, to such systems wherein the valve may be lifted to any height in a continuous range of heights, defined herein as “continuously variable valve lift” (CVVL); and most particularly, to such a system having a high-efficiency actuator employing a self-locking helical gear arrangement.
It is well known in the internal combustion engine arts to provide an engine with means for varying the lift of one or more combustion valves to improve engine efficiency and/or decrease emissions under certain engine operating conditions. Some known systems vary valve lift between two steps, for example, fully open (activated) and non-opening (deactivated); such systems can be thought of as switchable systems. Other systems, especially in the compression-ignited engine arts, benefit from mechanisms which are capable of lifting a valve to any desired height within a continuous range of heights; such systems may be thought of as continuously variable.
An important consideration in variable valve lift systems is the mechanical rigidity of the valvetrain during a partial lift event. When the variable lift is actuated directly by an electric solenoid or a hydraulic system that may contain air bubbles, torque fluctuations imposed on a camshaft by the sequential and overlapping opening and closing of valves can cause variation in the desired lift and hence in the intended gas flow profile.
To overcome these shortcomings, it is known in the art of CVVL systems to provide a mechanically self-locking actuation system employing a worm gear actuator. See, for example, U.S. Pat. No. 7,174,887 to Shuichi Ezaki. An inherent drawback of worm gear actuators is very low mechanical efficiency; that is, because of the high sliding velocity due to a small worm lead angle, a high percentage of the actuator torque, typically more than 50%, is consumed in friction with the pinion gear. Further, the mechanical self-locking feature becomes practical only at relatively high gear ratios, on the order of 80:1 and higher. For gear ratios lower than about 80:1, the backdrive efficiency remains low but the system is not completely self-locking. Because of the low mechanical efficiency, the actuator requires a relatively large and expensive motor to drive the worm gear.
What is needed in the art is a self-locking continuously variable drive system for variable valve lift that provides higher forward efficiency, a smaller and less expensive motor, and is fully mechanically self-locking.
It is a principal object of the present invention to improve the self-locking capability of a drive mechanism for a CVVL system.
It is a further object of the present invention to increase the efficiency of the self-locking actuator of a CVVL system.
It is a further object of the invention to reduce the size and cost of a CVVL system.
Briefly described, a CVVL system in accordance with the present invention includes a self-locking helical gear pair with, optionally, an additional gear system to increase output torque of the actuator. The self-locking helical gear pair provides higher forward efficiency, being at least twice that of a conventional worm gear actuator, and a fully mechanically self-locking feature. The actuator therefore requires a smaller motor to perform the same actuation as a prior art worm-gear system, which reduces the cost and volume of the CVVL system. Thus, a CVVL system in accordance with the present invention is significantly less bulky and costly than a comparable prior art system and is fully mechanically self-locking over the entire range of action.
The CVVL system includes a self-locking gear pair comprising two helical gears that have a high radial pressure angle, between about 45° and about 75°, and a high helix angle, between about 60° and about 80°. These gears require high addendum modification (positive and negative profile shift) such that the pitchpoint is beyond the active portion of the contact line, which creates a mechanically self-locking condition against backdrive. Tooth profiles may be symmetric, although an asymmetric profile is presently preferred to reduce contact stress and permit higher torque density. Preferably, the helical gears are discontinuous and comprise a plurality of spur gear slices which can produce smoother power transmission and lower transverse contact ratio. Such discontinuous helical gears are significantly less costly to manufacture.
Various configurations are possible within the scope of the present invention, including but not limited to a single stage gear system; a multiple stage gear system; a planetary gear system; and an internal or external gear system.
The principle of self-locking helical gears is disclosed in a Russian engineering paper from 1969 (“Self-locking in gear mechanism”, by T. G. Ishakov; T R. Kazanskova Aviation Institute of A. H. Typoleva, 1969 Ed. 105, pp 3-15 (in Russian)). However, this principle has not been applied previously to an automotive CVVL system such as is the subject of the present invention. Further, the use of asymmetric gear profiles in self-locking helical gears as disclosed herein is also novel.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.
Self-locking helical gears are known in the prior art from relatively few publications that demonstrate the possibility of designing a helical gear pair to be self-locking and provide general guidelines on how to obtain the self-locking feature. Despite these early publications, however, self-locking helical gears have not been widely reduced to practice, especially in automotive applications, such as the present invention. Further, the CVVL system disclosed herein includes improvements that are novel in the art, including: laminated helical gears to reduce fabrication costs; a preferred range of pressure angle and helical angles appropriate for steel gears.
Referring to
Prior art CVVL system 10 typifies any CVVL system wherein changing the rotational position of a control shaft causes variation in the operating angle and lift amount of an associated engine valve. The shortcomings of such a prior art CVVL system including a worm gear drive are described above. The present invention overcomes these shortcomings.
Still referring to
Referring to
Helical gearing within the scope of the present invention may be either external or internal. “External” refers to gearing wherein the teeth are on the outside of both gears, and the centers of rotation are on opposite sides of the mesh point. “Internal” refers to gearing wherein the teeth are on the outside of one gear and on the inside of the other gear, as for example in a planetary gear system, and the centers of rotation are on the same side of the mesh point.
Referring now to
Helical drive gear 114 having rotational center O1 includes helical teeth 132. Helical driven gear 116 having rotational center O2 including helical teeth 134. The teeth of both gears are modified involute teeth. Gears 114,116 are meshed along a center line 140 between O1 and O2. Drive gear 114 rotates clockwise, exerting torque T1. Thus, the leading flanks 142 of teeth 132 are the driving flanks, and trailing flanks 144 are the coast flanks. Conversely, for driven gear 116, the trailing flanks 146 of teeth 134 are the driven flanks, and leading flanks 148 are the coast flanks. The driving and driven teeth meet at a drive point 150, and the coast flanks meet at a corresponding driven point 152. The pressure exerted by driving gear 114 at drive point 150 is orthogonal to the contact tangents, defining a contact force direction 154 within driving gear 114 that in turn defines a driving torque arm 155, at a radial pressure angle {acute over (α)}, that is the radius of a torque circle 156 for torque T1. In the absence of friction, contact force direction 154 creates a response force direction 158 in driven gear 116 again normal to the contact tangents. However, sliding friction between the helices turns the response force direction through a friction angle γ such that the effective response force direction is vector 160 from the driven torque arm 162, creating a counterclockwise response forward torque T2 in driven gear 116.
Considering now the geometry of back-drive in driven gear 116, when gear 116 is urged in a clockwise direction as by a reversal of torque in an associated automotive cam as would pertain in a CVVL system 24 (
Of special interest is the fact that reverse force line 166 departs extensively from forward force line 154 to the extent that line 166 lies across the rotation center O1 at response torque arm 168, creating a response torque T′1 in the same direction as driving torque T1 and counter to torque T′2. Thus, back torque T′2 in gear 116 is effectively opposed by forward torque T′1 in gear 114, preventing reverse rotation of gear 116. Helical gear system 100 is thus self-locking, in accordance with the present invention.
Important elements of the self-locking feature include the pressure angle α, and the helix angle β. Also of importance are material selection, surface finish selection and appropriate lubrication design to achieve higher gear efficiency. These latter elements lead to values of friction coefficients for the driving and driven wheel. In conventional gear designs, the angle α is typically in the range of about 14.5° to about 25°, and usually about 20°. Similarly, angle β is usually less than 45°. In order to achieve a self-locking condition according to the present invention, however, values of α and β outside these ranges must be considered. With these common upper limits removed, values of α, and β may be found and appropriate material and surface finish may be chosen, that will satisfy this self-locking condition and higher gear efficiency. Specifically, it was found that angles α between 45° and 75°, and angles β between 60° and 80°, are satisfactory. Preferable, the gears are made of steel having a grinded surface finish resulting in a coefficient of friction on the order of between 0.11 and 0.18. Together, angle values within these ranges and material selection to produce the desired coefficient of friction will generally satisfy the necessary self-locking condition according to the present invention.
Referring still to
In another aspect of the invention, the helical gears can be formed from designed as discontinuous and comprise laminated spur gear slices, bolted or pinned together, to form a discontinuous tooth profile. Formed in this manner, the gears do not generate axial forces and allows lower transversal contact ratio on the slice for higher tooth strength. Manufacturing costs for helical gears in this form are substantially reduced as compared to cutting the individual gear teeth of a continuous helical gear. The number of slices used to form the laminated helical gear may be from two to as many as practical. More slices provide smoother transmission, creating more nearly continuous helix. Referring to
While the invention was described specifically for a CVVL mechanism, it should be understood to be applicable to any similar positioning mechanism where self-locking is desirable for internal combustion engine or any other application.
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Further, Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
