Camshaft Drive for an Internal Combustion Engine

This invention relates generally to a camshaft drive for an internal combustion engine and more particularly to a drive arrangement for a pair of adjacently arranged parallel camshafts.

In a drive of the type that is described in German Patent Application No. P 33 09 376.8-13, toothed gearing is used which provides a driving connection from the crankshaft to the camshaft via several intermediate stages. Such toothed gearings are required for some applications for reasons of safety (for example, aircraft engines), but cause loud noise emissions and are quite costly to manufacture. Therefore, camshafts are conventionally driven by means of toothed belts which wind around a toothed belt wheel of the crankshaft and a toothed belt wheel of the camshaft. Since this toothed belt drive arranged on the front side of the internal combustion engine is at the same time used for driving several accessory apparatuses, the toothed belt is relatively long and tends to stretch during operation. This danger exists especially in the case of high-performance engines which, during acceleration, cause such a high straining and stretching of the belt that occasionally the belt may skip a tooth. The resulting change of the control time of the valves causes increased fuel consumption and higher emissions of harmful substances.

Accordingly, one objective of the present invention is the provision of a camshaft drive that eliminates unintended changes in the timing of the valves.

Another objective of the present invention is the provision of a camshaft drive that operates at a minimal noise level.

A further objective of the present invention is the provision of a camshaft drive that permits timing changes during the operation of the internal combustion engine.

An even further objective of the present invention is the provision of a camshaft drive that can be manufactured in a cost- effective manner.

These and other objectives of the present invention are attained by the provision of a camshaft drive for first and second parallel adjacent camshafts wherein the first camshaft is driven by a toothed belt driven by the crankshaft of the internal combustion engine. The second camshaft is driven by a chain drive connected with the first camshaft. The relative rotating position of each camshaft can be changed by means of a hydraulic tensioning device engageable with the chain drive. The control time of the valves of the internal combustion engine is adjusted by changing the relative rotating position of the camshafts by means of the tensioning device.

As discussed above, the toothed belt connecting the camshaft with the crankshaft in high performance engines will routinely be stretched and strained during acceleration of the engine. The lengthening of the toothed belt results in a change of the rotating position of the camshaft driven by the toothed belt. The tensioning device of the present invention compensates for the change of the rotating position of the camshaft with respect to the crankshaft which results from this lengthening of the toothed belt. The general type of chain tensioning device employed in the present invention is disclosed in German DE-OS No. 2 249 310. However, that disclosure relates to use of such a tensioning device with a long chain between the crankshaft and the camshaft. In such an arrangement, a shifting of the tensioning element by a certain amount results in a significantly smaller adjustment at the camshaft than in the case of the shorter chain drive between the camshaft and the intermediate shaft of the present invention. Since relatively large adjustments at the camshaft can be made by relatively small shifts of the tensioning elements according to the present invention, a hydraulic tensioning device can be employed which requires very little space for operation. This space advantage is particularly noticeable in an internal combustion engine having a four valve cylinder head and two overhead camshafts.

In accordance with the present invention, one camshaft is driven by the crankshaft by means of a toothed belt and this camshaft in turn drives the other camshaft by means of a chain. The chain drive with a tensioning device may be so small that it comfortably fits into the cylinder head and may be shielded to reduce noise emissions. Also, because it may be positioned in the oil bath, it is continually protected from dirt and abrasion. Advantageously, it is contemplated that the chain drive will be located in the center of the longitudinally extending camshafts. With this arrangement, the camshafts are stressed symmetrically from the center by the torque developed. Resulting torsion is only half of what it would be if the torque was introduced conventionally on the front side of the cylinder head.

Other objects, advantages and novel features of the present invention will become readily apparent when the following detailed description of the preferred embodiment is considered in conjunction with the attached drawings.

FIG. 1 shows a frontal view of a camshaft drive of an eight- cylinder V-type engine in accordance with a preferred embodiment of the present invention; and

FIG. 2 shows a cross-section of a cylinder head having a camshaft drive in accordance with a preferred embodiment of the present invention.

A toothed belt 7 is wound around a toothed gear 1 of a camshaft 2 and the toothed gears 3 and 4 of the outer camshafts 5 and 6 of an eight cylinder V-type engine. By means of tensioning rollers 8, 9 and 10, the toothed belt 7 contacts the toothed gears with the appropriate contact area. By means of this toothed drive belt 11, arranged at the crankcase of the internal combustion engine, several accessory instruments (not shown) are driven at the same time. The camshafts 5 and 6, disposed longitudinally above the rows of cylinders, drive the two inside camshafts 15 and 16 disposed along the cylinder rows via chain drive 12. Each chain drive consist of two identical sprockets 13 and one chain 14 wound around them. A cylinder head 17, cast in one piece, is assigned to the four cylinders of each cylinder row. The cylinder head 17 is placed on the V-shaped crankcase and is tightly secured to it. In each cylinder 17, the two camshafts 5 and 15 are disposed in parallel at six bearing points. One camshaft is used for the control of the inlet valves, and the other one is used for control of the outlet valves. On the front side of head 17, the toothed gear 3 is centered and screwed to one camshaft 5. The chain drive 12 is arranged approximately midway between the center bearing points 18 and 19 of the camshafts. For receiving the chain drive 12, the cylinder head 17 at this point is slightly bulged.

By means of a hydraulic tensioning device 20 applied to the chain drive 12, chain 14 can be held under tension. At the same time, the relative rotating position of one camshaft with respect to the other camshaft can be adjusted in order to change the valve control times or their degree of contact.

The tensioning device 20 consists of a hydraulic piston 22 guided in a hydraulic housing 21. A plastic tensioning shoe 23 is fitted onto the hydraulic piston 22, with the tensioning shoe 23 resting against the loose side 24 of the chain 14. A stationary guide shoe 26 rests against the load end 25 of the chain 14. The guide shoe 26 is only used for guiding the chain. Hydraulic pressure is supplied via a pressure pipe 27 connected to the cylinder head 17. The hydraulic housing 21 is secured to a lateral extension 28 of spark plug housing 29 by which the spark plug is protected from splashing oil.

Instead of this tensioning device 20, a tensioning device 30 may also be provided as is shown at the chain drive 12 of the other row of cylinders in FIG. 1. Here tensioning shoes 33 and 34 are fitted onto the hydraulic piston 31 projecting a both its ends from the hydraulic housing 32. One of the tensioning shoes rests against the load end 25 of the chain 14 on its inside, and the other shoe rests against the loose end 24 of the chain 14 at its inside. When the hydraulic piston 31 is shifted longitudinally by pressure, the chain contacting area of the two sprockets 13 changes. One end is lengthened and the other end is shortened by the same amount so that the control times of the valves change, but the tension of the chain remains constant. Instead, the one tensioning element may also act on the inside of the chain, and the other tensioning element on the outside of the chain. This solution is recommended especially when tension rollers are used as the tensioning elements.

From the preceding description of the preferred embodiments, it is evident that the objects of the invention are attained, and although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the invention are to be limited only by the terms of the appended claims.