Patent Application
20120024247
The present invention relates to an electric variable cam phaser (eVCP) which uses an electric motor and a harmonic drive unit to vary the phase relationship between a crankshaft and a camshaft in an internal combustion engine; more particularly, to an eVCP with phase authority stops which limit the phase authority of the eVCP.
Camshaft phasers (“cam phasers”) for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from an engine's crankshaft. A second element, known generally as a camshaft plate, is mounted to the end of an engine's camshaft. A common type of camshaft phaser used by motor vehicle manufactures is known as a vane-type cam phaser. U.S. Pat. No. 7,421,989 shows a typical vane-type cam phaser which generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is supplied via a multiport oil control valve, in accordance with an engine control module, to either the advance or retard chambers, to change the angular position of the rotor relative to the stator, as required to meet current or anticipated engine operating conditions. In prior art cam phasers, the rotational range of phaser authority is typically about 50 degrees of camshaft rotation; that is, from a piston top-dead-center (TDC) position, the valve timing may be advanced to a maximum of about −40 degrees and retarded to a maximum of about +10 degrees. The phase authority of a vane-type cam phaser is inherently limited by the vanes of the rotor which will contact the lobes of the stator. Limiting the phase authority is important to prevent over-advancing and over-retarding which may, for example, result in undesired engine operation and engine damage due to interference of the engine valves and pistons.
While vane-type cam phasers are effective and relatively inexpensive, they do suffer from drawbacks. First, at low engine speeds, oil pressure tends to be low, and sometimes unacceptable. Therefore, the response of a vane-type cam phaser may be slow at low engine speeds. Second, at low environmental temperatures, and especially at engine start-up, engine oil displays a relatively high viscosity and is more difficult to pump, therefore making it more difficult to quickly supply engine oil to the vane-type cam phaser. Third, using engine oil to drive the vane-type cam phaser is parasitic on the engine oil system and can lead to requirement of a larger oil pump. Fourth, for fast actuation, a larger engine oil pump may be necessary, resulting in additional fuel consumption by the engine. Lastly, the total amount of phase authority provided by vane-type cam phasers is limited by the amount of space between adjacent vanes and lobes. A greater amount of phase authority may be desired than is capable of being provided between adjacent vanes and lobes. For at least these reasons, the automotive industry is developing electrically driven cam phasers.
One type of electrically driven cam phaser being developed is shown in U.S. patent application Ser. No. 12/536,575; U.S. patent application Ser. No. 12/825,806; U.S. Provisional Patent Application Ser. No. 61/253,982; and U.S. Provisional Patent Application Ser. No. 61/333,775; which are commonly owned by Applicant and incorporated herein by reference in their entirety. The electrically driven cam phaser is an electric variable cam phaser (eVCP) which comprises a flat harmonic drive unit having a circular spline and a dynamic spline linked by a common flexspline within the circular and dynamic splines, and a single wave generator disposed within the flexspline. The circular spline is connectable to either of an engine camshaft or an engine crankshaft driven rotationally and fixed to a housing, the dynamic spline being connectable to the other thereof. The wave generator is driven selectively by an electric motor to cause the dynamic spline to rotate past the circular spline, thereby changing the phase relationship between the crankshaft and the camshaft. Unlike vane-type cam phasers in which the phase authority is inherently limited by interaction of the rotor and stator, there is no inherent limitation of the phase authority of the eVCP. The eVCP is also capable of provide a phase authority of 100 degrees or even more if desired for a particular engine application.
U.S. Pat. No. 7,421,990 discloses an eVCP comprising a harmonic drive unit. The eVCP of this example uses a phase range limiter that is bolted to the camshaft. The phase range limiter protrudes through an arcuate slot formed in a sprocket wheel. The two ends of the arcuate slot constrain movement of the phase range limiter and thereby limit phase authority of the eVCP. However, this arrangement for limiting the phase authority of the eVCP requires additional components and assembly time. Additionally, since the phase range limiter is external to the eVCP, it may be susceptible to damage which would affect the phase authority of the eVCP.
What is needed is an eVCP with means for limiting the phase authority of the eVCP. What is also needed is a robust means for limiting the phase authority of the eVCP which does not require the addition of components to the eVCP.
Briefly described, a camshaft phaser is provided for controllably varying the phase relationship between a crankshaft and a camshaft in an internal combustion engine. The camshaft phaser includes a housing having a bore with a longitudinal axis and a harmonic gear drive unit is disposed therein. The harmonic gear drive unit includes a circular spline and a dynamic spline, a flexspline disposed radially within the circular spline and the dynamic spline, a wave generator disposed radially within the flexspline, and a rotational actuator connectable to the wave generator. One of the circular spline and the dynamic spline is fixed to the housing in order to prevent relative rotation therebetween. A hub is rotatably disposed within the housing and attachable to the camshaft and fixed to the other of the circular spline and the dynamic spline in order to prevent relative rotation therebetween. A first arcuate input stop member is provided having a first length and rotatable with one of the circular spline and the dynamic spline. A first arcuate output opening having a second length is defined by at least a first arcuate output stop member having a third length. The first arcuate output opening and the first arcuate output stop member are rotatable with the other of the circular spline and the dynamic spline. The first arcuate input stop member is received within the first arcuate output opening and the first length of the first arcuate input stop member is less than the second length of the first arcuate output opening to establish a predetermined phase authority of the camshaft phaser. An anti-rotation means is provided for temporarily fixing the circular spline to the dynamic spline in order to prevent relative rotation therebetween when the hub is being attached to the camshaft.
This invention will be further described with reference to the accompanying drawings in which:
Referring to
Harmonic gear drive unit 12 comprises an outer first spline 28 which may be either a circular spline or a dynamic spline as described below; an outer second spline 30 which is the opposite (dynamic or circular) of first spline 28 and is coaxially positioned adjacent first spline 28; a flexspline 32 disposed radially inwards of both first and second splines 28, 30 and having outwardly-extending gear teeth disposed for engaging inwardly-extending gear teeth on both first and second splines 28, 30; and a wave generator 36 disposed radially inwards of and engaging flexspline 32.
Flexspline 32 is a non-rigid ring with external teeth on a slightly smaller pitch diameter than the circular spline. It is fitted over and elastically deflected by wave generator 36.
The circular spline is a rigid ring with internal teeth engaging the teeth of flexspline 32 across the major axis of wave generator 36. The circular spline serves as the input member.
The dynamic spline is a rigid ring having internal teeth of the same number as flexspline 32. It rotates together with flexspline 32 and serves as the output member. Either the dynamic spline or the circular spline may be identified by a chamfered corner 34 at its outside diameter to distinguish one spline from the other.
As is disclosed in the prior art, wave generator 36 is an assembly of an elliptical steel disc supporting an elliptical bearing, the combination defining a wave generator plug. A flexible bearing retainer surrounds the elliptical bearing and engages flexspline 32. Rotation of the wave generator plug causes a rotational wave to be generated in flexspline 32 (actually two waves 180° apart, corresponding to opposite ends of the major ellipse axis of the disc).
During assembly of harmonic gear drive unit 12, flexspline teeth engage both circular spline teeth and dynamic spline teeth along and near the major elliptical axis of the wave generator. The dynamic spline has the same number of teeth as the flexspline, so rotation of the wave generator causes no net rotation per revolution therebetween. However, the circular spline has slightly fewer gear teeth than does the dynamic spline, and therefore the circular spline rotates past the dynamic spline during rotation of the wave generator plug, defining a gear ratio therebetween (for example, a gear ratio of 50:1 would mean that 1 rotation of the circular spline past the dynamic spline corresponds to 50 rotations of the wave generator). Harmonic gear drive unit 12 is thus a high-ratio gear transmission; that is, the angular phase relationship between first spline 28 and second spline 30 changes by 2% for every revolution of wave generator 36.
Of course, as will be obvious to those skilled in the art, the circular spline rather may have slightly more teeth than the dynamic spline has, in which case the rotational relationships described below are reversed.
Still referring to
Output hub 20 is fastened to second spline 30 by bolts 52 and may be secured to engine camshaft 22 by central through-bolt 54 extending through output hub axial bore 56 in output hub 20, and capturing stepped thrust washer 58 and filter 60 recessed in output hub 20. In an eVCP, it is necessary to limit radial run-out between the input hub and output hub. In the prior art, this has been done by providing multiple roller bearings to maintain concentricity between the input and output hubs. Referring to
Back plate 66, which is integrally formed with input sprocket 16, captures bias spring 24 against output hub 20. Inner spring tang 67 is engaged by output hub 20, and outer spring tang 68 is attached to back plate 66 by pin 69. In the event of an electric motor malfunction, bias spring 24 is biased to back-drive harmonic gear drive unit 12 without help from electric motor 14 to a rotational position of second spline 30 wherein engine 18 will start or run, which position may be at one of the extreme ends of the range of authority or intermediate of the phaser's extreme ends of its rotational range of authority. For example, the rotational range of travel in which bias spring 24 biases harmonic gear drive unit 12 may be limited to something short of the end stop position of the phaser's range of authority. Such an arrangement would be useful for engines requiring an intermediate park position for idle or restart.
The nominal diameter of output hub 20 is D; the nominal axial length of first journal bearing 70 is L; and the nominal axial length of the oil groove 72 formed in either output hub 20 (shown) and/or in sprocket housing 40 (not shown) for supplying oil to first journal bearing 70 is W. In addition to journal bearing clearance, the length L of the journal bearing in relation to output hub diameter D controls how much output hub 20 can tip within sprocket housing 40. The width of oil groove 72 in relation to journal bearing length L controls how much bearing contact area is available to carry the radial load. Experimentation has shown that a currently preferred range of the ratio L/D may be between about 0.25 and about 0.40, and that a currently preferred range of the ratio W/L may be between about 0.15 and about 0.70.
Oil provided by engine 18 is supplied to oil groove 72 by one or more oil passages 74 that extend radially from output hub axial bore 56 of output hub 20 to oil groove 72. Filter 60 filters contaminants from the incoming oil before entering oil passages 74. Filter 60 also filters contaminants from the incoming oil before being supplied to harmonic gear drive unit 12 and bearing 46. Filter 60 is a band-type filter that may be a screen or mesh and may be made from any number of different materials that are known in the art of oil filtering.
Extension portion 82 of output hub 20 receives bushing 78 in a press fit manner. In this way, output hub 20 is fixed to bushing 78. Input sprocket axial bore 76 interfaces in a sliding fit manner with bushing 78 to form second journal bearing 84. This provides support for the radial drive load placed on input sprocket 16 and prevents the radial drive load from tipping first journal bearing 70 which could cause binding and wear issues for first journal bearing 70. Bushing 78 includes radial flange 80 which serves to axially retain back plate 66/input sprocket 16. Alternatively, but not shown, bushing 78 may be eliminated and input sprocket axial bore 76 could interface in a sliding fit manner with extension portion 82 of output hub 20 to form second journal bearing 84 and thereby provide the support for the radial drive load placed on input sprocket 16. In this alternative, back plate 66/input sprocket 16 may be axially retained by a snap ring (not shown) received in a groove (not shown) of extension portion 82.
In order to transmit torque from input sprocket 16/back plate 66 to sprocket housing 40 and referring to
Now referring to
Now referring to
In order to establish the phase authority of eVCP 10, first and second arcuate input stop members 90, 92 are axially and radially received within second and first arcuate output openings 116, 114 respectively. Similarly, first and second arcuate output stop members 108, 110 are axially and radially received within first and second arcuate input openings 104, 106 respectively. The arcuate stop members and each corresponding arcuate opening within which the arcuate stop member is received are sized such that the angular distance of each angular opening minus the angular distance of the corresponding arcuate stop member is equal to the phase authority of eVCP 10. For example, angular distance α1′ minus angular distance α1 equals the phase authority of eVCP. Stated another way, if the phase authority for eVCP is 50 degrees, then angular distance α1′ (in degrees) minus angular distance α1 (in degrees) equals 50 degrees.
Angular distances α1, α2 of first and second arcuate input stop members 90, 92 are preferably equal and first and second arcuate input stop members 90, 92 are preferably angularly spaced in a symmetric manner. Similarly, angular distance α3′, α4′ of first and second arcuate output stop members 108, 110 are preferably equal and first and second arcuate output stop members 108, 110 are preferably angularly spaced in a symmetric manner. As can now be seen, distinct eVCPs can be provided for different engine application requiring different amounts of phase authority simply by redesigning the input stop members and the output stop members to achieve the desired phase authority.
Angular distances α3, α4 of first and second arcuate input openings 104, 106 are preferably equal and first and second arcuate input openings 104, 106 are preferably angularly spaced in a symmetric manner. Similarly, angular distance α1′, α2′ of first and second arcuate output openings 114, 116 are preferably equal and first and second arcuate output openings 114, 116 are preferably angularly spaced in a symmetric manner.
In operation, when eVCP is commanded to provide maximum valve timing advance, electric motor 14 will actuate harmonic gear drive unit 12 to rotate output hub 20 with respect to sprocket housing 40 until first and third advance stop surfaces 96, 96′ are in contact with each other (
While the embodiment described herein describes input sprocket 16 as being smaller in diameter than sprocket housing 40 and disposed axially behind sprocket housing 40, it should now be understood that the input sprocket may be radially surrounding the sprocket housing and axially aligned therewith. In this example, the back plate may be press fit into the sprocket housing rather than having a sleeve gear type joint.
While the embodiment described herein includes first and second input stop members, it should now be understood that more or fewer arcuate input stop members may be included. Similarly, more or fewer arcuate output stop members may be included.
While the embodiment described herein describes angular distances α1, α2 of first and second arcuate input stop members 90, 92 as equal and first and second arcuate input stop members 90, 92 are angularly spaced in a symmetric manner, it should now be understood that the first and second arcuate input stop members may be have unequal lengths and may also be spaced asymmetrically. This will result in the first and second arcuate output members being unequal in length and being spaced asymmetrically.
The embodiment described herein describes harmonic gear drive unit 12 as comprising outer first spline 28 which may be either a circular spline or a dynamic spline which serves as the input member; an outer second spline 30 which is the opposite (dynamic or circular) of first spline 28 and which serves as the output member and is coaxially positioned adjacent first spline 28; a flexspline 32 disposed radially inwards of both first and second splines 28, 30 and having outwardly-extending gear teeth disposed for engaging inwardly-extending gear teeth on both first and second splines 28, 30; and a wave generator 36 disposed radially inwards of and engaging flexspline 32. As described, harmonic gear drive unit 12 is a flat plate or pancake type harmonic gear drive unit as referred to in the art. However, it should now be understood that other types of harmonic gear drive units may be used in accordance with the present invention. For example, a cup type harmonic gear drive unit may be used. The cup type harmonic gear drive unit comprises a circular spline which serves as the input member; a flexspline which serves as the output member and which is disposed radially inwards of the circular spline and having outwardly-extending gear teeth disposed for engaging inwardly-extending gear teeth on the circular spline; and a wave generator disposed radially inwards of and engaging the flexspline.
While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.
