FIELD OF THE INVENTION
The present invention relates to an engine with a variable valve timing. In particular, the invention relates to implementing variable valve timing in an engine employing SCP camshafts, the term “SCP camshaft” being used herein to refer to a camshaft which carries two groups of cams and comprises an outer tube coupled for rotation with a first group of cams and an inner shaft rotatably supported by the outer tube and coupled for rotation with the second group of cams. The acronym “SCP” stands for “Single Cam Phaser” because such a camshaft has hitherto been used to implement variable valve timing in an engine having a single camshaft by using a phaser to rotate the outer tube relative to the inner shaft.
BACKGROUND OF THE INVENTION
Several internal combustion engines have a layout where multiple camshafts each have intake and exhaust cams along their length. Examples of such a layout can be found in the following engines:
- Pushrod V-engines, where two parallel camshafts are situated next to each other in the engine block.
- Several V-twin motorcycle engines currently use such a layout.
- DOHC (dual overhead cam) engines where the valve layout is rotated by 90° (to improve port generated swirl). Each camshaft then has intake and exhaust cams along its length.
- SOHC (single overhead cam) V-engines where a single camshaft controls all the valves on each bank.
It is desirable to be able to control the phase of the intake and the exhaust cams in such engines independently and this would be rendered possible by the use of two SCP camshafts. However, the use in such a case of two independent actuators (or phasers) to transmit torque separately from the engine crankshaft to each SCP camshaft would present problems. In particular, such a solution would prove costly to implement, because a separate set of sensors, control valves, oil feeds, and actuator parts would be required for each camshaft. There would also be added complications for the electronic engine control unit.
SUMMARY OF THE INVENTION
With a view to mitigating the foregoing disadvantages, the present invention provides an engine having a crankshaft, a first SCP camshaft, namely a camshaft carrying first and second groups of cams and comprising an outer tube coupled for rotation with the first group of cams and an inner shaft rotatable relative to the outer tube and coupled for rotation with the second group of cams, a phaser for enabling the phase of at least one of the two groups of cams on the first SCP camshaft to be varied with reference to the phase of the engine crankshaft, a second SCP camshaft having a second inner shaft and a second outer tube coupled for rotation two further groups of cams, and drive links for ensuring that each group of cams on the first SCP camshaft rotates in unison with a corresponding one of the two groups of cams on the second SCP camshaft.
The drive links ensuring that the inner shafts and the outer tubes of the two SCP camshaft rotate in unison with one another may comprise continuous belts (which term in the present context includes chains) or gear drives.
The invention allows the phase of the intake and/or exhaust cams of an engine with two SCP camshafts to be varied with reference to the phase of the crankshaft using a single phaser.
To vary the phase of both the intake and the exhaust cams relative to the engine crankshaft, it is possible either to use one twin vane-type phaser, such as described in EP 1 234 954, or to use two single vane-type phasers, one phaser acting to vary the phase of the intake valves relative to the crankshaft and the other acting to vary the phase of the exhaust cams.
The layout of the phaser or phasers is not of fundamental importance to the present invention. Thus, it is possible when using a twin vane-type phaser for it to be mounted directly on one of the SCP camshafts or for it to be mounted on the engine block and indirectly coupled to both SCP camshafts. In a similar vein, if two single phasers are used, each of them can be directly mounted on one of the two SCP camshafts or it may be mounted on the engine block and coupled indirectly to one group of cams of each of the two SCP camshafts.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 schematic shows a layout using two SCP camshafts and a single twin vane-type phaser,
FIG. 2 is a similar view to FIG. 1 showing a layout using two separate single vane-type phasers,
FIG. 3 is a partial perspective view of an embodiment applicable to a DOHC or pushrod engine in which the drive links are constituted by directly meshing gearwheels,
FIG. 4 is a section through the embodiment shown in FIG. 3,
FIGS. 5 and 6 are respectively a front and a perspective view of an embodiment applicable to an engine have two banks of cylinders each with a single overhead SCP camshaft (i.e. a SOHC V-engine), and
FIGS. 7 and 8 are schematic representations of alternative drive link layouts applicable to SOHC V-engines.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The layout in FIG. 1 gives an example of an engine assembly that uses two SCP camshafts. A twin vane-type phaser 12 driven by the engine crankshaft 10 drives the inner shaft 14a and the outer tube 14b of the first SCP camshaft which are in turn are coupled for rotation with the inner shaft 16a and outer tube 16b of the second SCP camshaft by drive links represented in the drawings by arrows. As the twin vane-type phaser 12 is itself known, e.g. from EP-A-1 234 954, it is not deemed necessary to describe its construction in detail in the context of the present invention. It suffices to understand that the twin vane-type phaser 12 can alter the phase of both the inner shafts 14a, 16a and the outer tubes 14b, 16b of the SCP camshaft relative to the engine crankshaft 10.
In the layout of FIG. 2, the twin vane-type phaser 12 is replaced by two separate single vane-type phasers 12a and 12b each of which can only alter the phase of one group of cams relative to the engine crankshaft. In other respects, the two layouts are the same.
FIGS. 1 and 2 suggest that torque is always transmitted from the phaser 12 to the first SCP camshaft 14 and that from there the torque is transmitted to the second camshaft 16. While this may be the case in some embodiments of the invention, it is not necessary the case, as will become clear from other embodiments described below. As long as the inner shafts and the outer tubes of the SCP camshaft are coupled to rotate in unison, it does not matter how torque is transmitted to them by the phaser(s). Thus the phaser may itself separately drive the two SCP camshafts, using common or separate drive links. The drive links may themselves be meshing gearwheels, chains or belts.
The embodiment of the invention shown in FIG. 3 and FIG. 4 has two assembled SCP camshafts each of which, for simplicity, is shown as having only two cams, one driven by the inner shaft and the other by the outer tube. In this case, two of the cams 17a and 17b can be formed directly on the two inner shafts 14a and 16a while the other two cams 19a and 19b can be formed on the two outer tubes 14b and 16b. It is however possible to provide multiple cams on each SCP camshaft, such that a first group will rotate with the inner shaft and the second with the outer tube. The cams in this case are formed on separate collars that are slid in sequence over the outer tube. Cams that are to rotate with the outer tube have their collars coupled to the outer tube, such as by heat shrinking, while cams that are to rotate with the inner shaft are a loose fit on the outer tube and are connected to the inner shaft by pins that pass through circumferentially elongated slots in the outer tube.
In the embodiment of FIGS. 3 and 4, the drive links coupling the inner shafts of the two SCP camshafts for rotation with one another are two meshing gearwheels 13a and 13b while two further meshing gearwheels 15a and 15b couple the two outer tubes for rotation with one another. A twin vane-type phaser 12 is shown as driving the camshaft 14, but it could clearly alternatively drive the second camshaft 16. As a further possibility two single vane-type phasers could be mounted on the two camshafts, one driving the inner shafts and the other the outer tubes.
Whereas the engine of FIGS. 3 and 4 has two camshafts arranged side by side on the same cylinder block, the remaining embodiments of the invention described below relate to an engine with two banks of cylinders, such as a V-engine, with an SCP camshaft associated with each bank of cylinders.
The embodiment of FIGS. 5 and 6 employs a twin vane-type phaser 12 that is not directly mounted on either camshaft but on the engine cylinder block. The twin vane-type phaser 12 has a driven sprocket which engages a chain 38 that passes around the crankshaft sprocket 10. The phaser has two drive sprockets engaged by two chains 32 and 34, which in FIG. 5 lie one behind the other. One chain 32 passes over sprockets on the SCP camshafts 14 and 16 which drive the inner shafts while the other chain 34 passes over sprockets which drive the outer tubes of the two SCP camshafts. The two chains 32 and 34 also pass under free-wheeling idler sprockets 36 which constrain the chains to follow a compact path and can also be used for chain tensioning.
In FIG. 7, a twin vane-type phaser 12 driven by means of a chain 40 that passes around the crankshaft sprocket 10 has two pairs of ganged drive sprockets. One pair drives the inner shafts of the two camshafts 14, 16 through two chains 42, 46 while the other pair drives the outer tubes of the two camshafts through chains 44, 48 lying directly behind the chains 42 and 46 in the drawing.
The embodiment of FIG. 8 uses two single vane-type phasers 12a and 12b that are driven by a common chain 50 that passes around the crankshaft sprocket 10. The phaser 12a drives the inner shafts of the two camshafts 14, 16 by way of a chain 52 that passes under an idler sprocket 54 while the phaser 12b drives the outer tubes of the two camshafts by way of a chain 56 that passes under an idler sprocket 58.
Though, for convenience, reference has been made above to vane-type type phasers, it should be clear that the invention can use any form of phase change mechanism, of which numerous types are disclosed in the prior art.
While the invention has been described above by reference to preferred embodiments, it will be clear to the person skilled in the art that various modifications may be made without departing from the scope of the invention as set forth in the appended claims.
Patent Application
20050279302
