The present invention relates to motor-vehicle internal combustion engines having at least one cylinder with an associated pair of intake and/or exhaust valves and a camshaft for controlling the opening and closing of both valves of the pair by means of two respective pivoted rocker fingers.
In this type of engine, in order to obtain optimum performance, it is desirable to effect the sequential cycle of the opening/closing of the two valves in a differentiated manner so that the full opening of one of the two valves follows the full opening of the other valve after a predetermined delay.
Conventionally this is achieved, as illustrated in the European patent publication EP-A-319956, with the aid of two separate cams with different profiles for controlling the two valves and/or with different shaped cam-follower profiles for the rockers of one valve and the other.
Both solutions are structurally complex in that they require the manufacture of different components.
The object of the present invention is to avoid this disadvantage by providing an internal combustion engine of the type defined above in which the differentiated opening of the two valves is achieved without the need to provide two separate cams or two rockers with different profiles.
According to the invention, this object is achieved by virtue of the fact the rockers of the two valves are identical to each other and have respective centres of pivoting disposed on opposite sides of the camshaft, and in that the two rockers are operated by a single common cam.
By virtue of this arrangement, the common cam operates the two valves differently due to the different interactions of the two rockers resulting exclusively from their opposed positioning. This effect is achieved with a number of essential components (a single cam and two identical rockers) which eliminates almost all structural complication in the engine.
The invention will now be described in detail with reference to the appended drawings, provided purely by way of non-limiting example, in which:
With reference to the drawings, part of the head of a multi-cylinder internal combustion engine for motor vehicles is generally indicated 1 and is provided in correspondence with each cylinder with a pair of intake (or exhaust) valves 2, 3 controlled by a distributor shaft 4 supported for rotation by the head 1. Each valve 2, 3 has a rod 5, 6 the top of which is operatively associated with a respective rocker 7, 8. The two rods 5, 6 also have respective associated biasing springs 9, 10, in conventional manner.
The two rockers 7, 8 are of the finger type and are formed with respective cam-follower surfaces 11, 12 which, according to the invention, cooperate with a single, common cam 13 of the distributor shaft 4. This cam 13 has an axial extent such as to ensure contact between its own outer profile and the two cam-follower surfaces 11, 12.
The two rocker fingers 7, 8 are pivotably supported at respective ends opposite the rods 2, 3 by means of articulation heads 14, 15 supported at the tops of respective hydraulic tappets 16, 17 of known type with automatic play recovery.
In accordance with the invention, the two rocker fingers 7, 8 are identical to each other and are disposed symmetrically, in mutually opposite positions with respect to the distributor shaft 4: the fulcra 14, 15 with the respective hydraulic tappets 16, 17 are in fact situated on opposite sides of the shaft 4.
Thanks to this arrangement, the interaction between the cam 13 and the two cam-follower surfaces 11, 12 of the two rockers 7, 8 effects a differentiated opening/closing cycle for the two valves 2, 3, in use. This cycle is shown schematically in the graph of Figure 3 as a function of time and of the angular position of the cam 13 during one complete revolution. The curve A represents the opening/closing graph of the valve 2 while the curve B represents the valve 3: as can be seen, the opening/closing cycle of the second is delayed relative to that of the first so that, when the valve 2 starts its reclosure travel, the valve 3 has yet to reach its fully-open position. The condition in which both valves are completely closed, is, however, reached at the same instant.