This invention relates to an internal combustion engine and more particularly to a cam shaft drive incorporating a variable valve timing mechanism for operating the valves of such an engine.
It is well known that one of the factors that controls the performance of an internal combustion engine is the valve timing. Generally, the valves are operated by one or more cam shafts at a timed relationship to the rotation of the engine output shaft. Frequently, the intake valves are operated by the cam shaft that is different from that which operates the exhaust valves. Generally, the optimal valve timing for an engine varies, depending upon the speed and load at which the engine is operating. Thus, conventional engines having fixed valve timing arrangement generally are designed to provide a comprise between good running at low speeds and low loads and maximum engine output. Depending upon the use of the engine, the bias may be toward one or the other end of these two alternative ranges.
In order to improve performance over a wider range of engine speed and load conditions, it has been proposed to employ a variable valve timing arrangement (VVT) in the drive for the cam shafts. In this way, the timing relationship of the cam shafts can be adjusted so as to provide optimal performance for more running condition.
The variable valve timing mechanisms which have been proposed generally fall into two categories. With the first of these and the simpler arrangement, the timing of both cam shafts is generally altered in the same direction and at the same degree. This is done by interposing one variable valve timing mechanism in the timing drive between the engine output shaft and the cam shafts. This has the advantages of simplicity, lower cost and still provides greater flexibility in engine performance.
The other type of system includes a variable valve timing mechanism that is interposed between the drive for each of the cam shafts from the output shaft. This obviously doubles the number of components, including the control mechanism. It does, however, offer the possibility of a greater flexibility in overall engine performance.
It has been discovered, however, that there are a number of running conditions where the performance is optimal if both cam shafts are adjusted at the same phase angle. Other running conditions require a different adjustment between the cam shafts. Although this can be employed were it easily in an arrangement wherein there are independent variable valve timing mechanisms associated with each cam shaft, this makes the control strategy more complicated.
There has been proposed, therefore, an improved variable valve timing mechanism for an engine wherein two cam shafts may have their timing altered simultaneously or independently of each other, depending upon the running characteristics. Several embodiments of ways of accomplishing this are disclosed in the copending application of Kaoru Okui and Masahiro Uchida, entitled "Variable Valve Timing Mechanism for Engine", Ser. No. 09/471,887, filed Dec. 23, 1999, now U.S. Pat. No. 6,250,266, and assigned to the assignee hereof.
It is an object of this invention to provide an additional embodiment of cam shaft drives that permit either simultaneous adjustment of the timing of both cam shafts or independent adjustment of the timing of the cam shafts relative to each other to those disclosed in the aforenoted co-pending application.
All of the embodiments in the aforenoted co-pending application employ plural chain drives for driving two cam shafts from the crankshaft. When chain drives are employed, they generally are positioned within the engine body and receive lubricant from the engine lubricating system for their lubrication. This has some disadvantages with servicing and can present some objectionable driving noise.
It is, therefore, a still further object to this invention to provide a variable cam shaft driving mechanism wherein the timing of the cam shafts can be adjusted simultaneously or independently and wherein at least part of the drive is through a toothed belt that need not be enclosed within the body of the engine.
Where a flexible driving belt is employed, it is desirable to maintain a short length for this belt in order to reduce the effects of belt stretch or elongation and to simply the driving arrangement. It is, therefore, a still further object to this invention to provide an improved variable valve timing drive for the cam shafts of an engine where at least part of the drive is accomplished through a drive belt that is maintained with a relatively short length.
In the drive mechanism shown in the aforenoted co-pending application, the cam shaft drives are all cantilevered on the outer end of the cam shafts and at a point spaced outwardly from the forward most journal therefore. This tends to put large bending loads on the cam shafts and also on the bearing surfaces.
It is, therefore, a still further object of this invention to provide a variable valve timing cam drive arrangement of this general type wherein the loading on the cam shaft bearing is reduced.
It is a further object of this invention to provide an improved cam shaft variable valve timing mechanism wherein simultaneous adjustment of both cam shafts can be accomplished through the use of one variable valve timing mechanism and adjustment of the timing of the cam shafts relative to each other is done by a separate variable valve timing mechanism.