Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Reexamination Certificate
2000-03-22
2001-12-11
Walberg, Teresa (Department: 3742)
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
C123S090170, C123S090180, C123S090110
Reexamination Certificate
active
06328006
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a device for adjusting the phase angle of a camshaft of an internal combustion engine with a drive gear for driving a camshaft accommodated in a coaxial arrangement relative to the camshaft, and with an electric motor communicating with the camshaft via a Harmonic Drive gear having a roller bearing with an elliptical inner ring, an externally toothed, flexible gear arranged on said roller bearing and a rigid, internally toothed gear engaging the externally toothed gear.
To obtain optimum values for fuel consumption and exhaust emissions in different areas of the internal combustion engine's operating characteristics, the valve timing must be varied depending on different operating parameters. An elegant manner of varying the valve timing is realized by rotating the camshaft relative to its driving gear. The camshaft of an internal combustion engine is usually driven by a sprocket wheel, which is connected to the crankshaft via a drive chain, or by a drive gear configured as a pulley, which is connected to the crankshaft via a toothed belt.
DESCRIPTION OF THE PRIOR ART
In GB 2 221 513 A a camshaft adjusting mechanism is described wherein an electric motor operates a set of link arms turning the camshaft relative to its driving gear. To this purpose an actuating element carrying the pivoted arms is shifted in axial direction. This solution however involves considerable expense and play on account of the large number of bearings.
In DE 41 10 088 C1 and DE 39 29 619 A1 adjusting mechanisms are described wherein an adjusting element is provided between a member connected to the camshaft and a member connected to the drive gear, which element has two helical threads meshing with corresponding threads of the camshaft or the drive gear. By axially displacing this adjusting element, the camshaft can be rotated relative to its drive gear. Axial displacement of the adjusting element may be obtained by actuating a hydraulic plunger which is operated in dependence of the desired adjustment. The disadvantage of this solution is that the forces required can only be attained with a large hydraulic plunger necessitating considerable constructional expense. Moreover, a comparatively large quantity of oil is required for operating the plunger, which will necessitate a suitably sized pump and thus add to the engine load. As a further drawback of this known type of mechanism, adjustment of the camshaft is possible only between two extreme positions.
Electric adjusting devices also are presented in DE 41 01 676 A1 and in DE 44 06 983 C, wherein an electric motor is provided for displacing the adjusting element by means of a threaded spindle. As the adjusting element rotates essentially at camshaft speed, an axial thrust bearing must be provided between the electric motor and the adjusting element, which takes up the relative movement between the non rotating and the rotating member. In the above solution, the thrust bearing is more or less permanently subject to load throughout the entire operating period, since the torsional moments acting between drive gear and camshaft will produce a force acting on the adjusting element in axial direction. For this reason the thrust bearing is a critical component which will limit the useful life of the engine. A similar solution is disclosed in DE 33 20 835 A1, wherein the same disadvantages are encountered.
In DE 36 07 256 A a mechanism is described wherein a stepping motor is provided for adjusting the camshaft, the stepping motor being connected to both camshaft and drive gear. As the stepping motor must take up the entire driving torque for the camshaft, such a solution cannot be achieved within reasonable limits of expense.
EP 0 596 860 A discloses a device for adjusting the valve opening times in which the camshaft has a hollow configuration and comprises an inner shaft. The cams are bipartite, wherein each single cam section can be turned relative to the other by a determined angle. The rotation of the two cam sections is executed by a revolving electric motor, which is supplied via slip rings. Similar solutions are disclosed in U.S. Pat. Nos. 5,417,186 and 4,770,060.
A former suggestion of the applicant, published in EP-A 0 903 471, presents an adjusting mechanism for the phase angle of a camshaft with a planetary gear set, in which the adjustment is executed by an electric motor that is supplied with current by means of slip contacts.
In most of the solutions described above, the electromotive adjustment is made via a set of gears with a big gear reduction ratio in order to maintain the torque on the engine in an acceptable range. To transmit the force via an axially slidable element with screw-shaped teeth has the advantage that very big gear reduction ratios may obtained quite easily. The disadvantage of these solutions however is that the friction is quite high and that accordingly high forces originate in axial direction which have to be absorbed by the bearings. On the other side, the transmission of force via a planetary gear set is quite complicated and it is difficult to achieve big gear reduction ratios.
By using a so-called Harmonic Drive, a big gear reduction ratio may be obtained easily. If for example the number of teeth of the externally toothed gear is smaller by two than the number of teeth of the internally toothed gear which amount to for example 50, a step-up ratio of about 1:25 is achieved.
In the concrete embodiment of a device of the type mentioned above however and on top of the already described questions, the following groups of problems have to be overcome in construction: the bearing of the electric motor has to be as simple as possible, so that the Harmonic Drive is not submitted to inadmissible big loads. Furthermore, the bearing of the electric motor is not allowed to hinder the necessary sealing between the oil guiding sections (camshaft and set of gears) and those sections that do not guide oil. It also is necessary that the bearing of the drive gear be realized in the simplest possible way.
Another constructional aim to be achieved is to guarantee an emergency operation of the motor in case of failure of the adjusting device and generally, to restrict the adjusting range to allowable angles.
SUMMARY OF THE INVENTION
It is an object of the present invention to develop a device as described above in such a manner that the above mentioned requirements may be met with as little expenditure as possible.
According to the invention, the electric motor is provided with a housing rigidly connected to the camshaft via retaining elements which are guided in the drive gear by recesses in such a manner that the recesses delimit the allowable adjusting range of the camshaft. Thanks to the solution according to the invention, the electric motor itself does not need any bearing since it is over-mounted on the camshaft. Excessive load on the Harmonic Drive can thus reliably be prevented, since a relative movement between the different component parts of the Harmonic Drive caused by osillations or the like is virtually excluded. Moreover, there is one simple possibility to bear the drive gear. The size of the recesses automatically defines the adjusting range of the camshaft relative to the drive gear. That's why it is possible to restrict this adjusting range to allowable values. This measure also assures a defined position of the camshaft in case of failure of the adjusting device or in case of cold start. Actually, the Harmonic Drive is self-locking due to the big gear reduction ratio, but, due to the ineluctable rotational oscillations in the system, a position of the camshaft corresponding to the latest possible valve opening time is almost possible to tune the electronics of the motor in such a way that, under these conditions, acceptable emergency operation is possible.
It is particularly preferable when the electric motor is provided with a shaft that is directly connected to the elliptical inner ring of the roller bearing. It provides benefits to have the flexible
Dahbour Fadi H.
Dykema Gossett PLLC
TCG Unitech Aktiengesellschaft
Walberg Teresa
LandOfFree
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