Internal-combustion engines – Poppet valve operating mechanism – With means for varying timing
Reexamination Certificate
1999-12-13
2001-09-25
Lo, Weilun (Department: 3748)
Internal-combustion engines
Poppet valve operating mechanism
With means for varying timing
C123S090480, C123S19800E
Reexamination Certificate
active
06293239
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a valve gear for charge cycle valves of internal combustion engines having a camshaft whose cam acts on a valve stem of a charge cycle valve via a bucket tappet which is guided in a cylinder head and on which an inner tappet is supported via a first pressure spring, and having a switchable coupling element, which is arranged between the bucket tappet and the circumference of the inner tappet, which bears against the valve stem, and permits force transmission from the bucket tappet to the valve stem.
The valve gears of the known type are used, on one hand, to switch off individual valves or a plurality of valves or, on the other hand, to switch over from one stroke characteristic to another in the case of the same valve. With spark-ignition engines, valves of individual cylinders are switched off in order to avoid the pumping losses at the switched-off cylinders and cooperate with a higher mean pressure in the case of the remaining, fired cylinders, as a result of which the thermal efficiency is raised. There is a corresponding reduction in the consumption and exhaust gas in the part load range.
With directly injecting diesel engines, the charge cycle causes substantially smaller losses. If, however, some of a plurality of inlet valves per cylinder can be switched off, the intake turbulence in the combustion chamber can be influenced as a function of rotational speed, and thus the combustion can be improved.
The stroke characteristic of a valve can be changed in many regards by switching over the valve, specifically both with reference to the opening stroke and with reference to the valve timing. Thus, the valve timings for two speed ranges can be optimized with the aid of two different stroke characteristics, the result being advantages as regards consumption and exhaust gas.
Numerous valve gears for charge cycle valves are known which have a device for switching off or switching over a charge cycle valve, for example DE 42 13 147 A1, DE 43 33 927 A1, DE 44 05 189 C2 and DE 44 43 101 A1. Two systems can be distinguished in principle in this case.
A first system a shown in, DE 44 43 101 A1, has, inside a tappet, a hydraulic fluid chamber which is filled with or emptied of oil in order to switch the charge cycle valve on and off, respectively. Transmission of the actuating forces requires a high oil pressure which renders a simple seal difficult and requires expensive provision of hydraulic fluid. Both the seal and the provision of hydraulic fluid call for a great deal of installation space.
In a second system, as shown in DE 44 05 189 C2, the charge cycle valves are turned on or off by self-closed, switchable coupling elements, for example pins, bolts or plates which are arranged between a bucket tappet and the charge cycle valve. The arrangement and actuation of the self-closed coupling elements likewise call for a substantial installation space. Moreover, an impermissibly high face-to-face pressure, and wear associated therewith, can arise at the coupling point when the self-closure achieved upon engagement is not complete. Furthermore, troublesome coupling noises can occur.
SUMMARY OF THE INVENTION
An object of the present invention is, in conjunction with a low overall volume, to reduce the outlay on constructing valve gears, and to improve their efficiency. This object has been achieved according to the invention by providing that a switchable clamping device is arranged as coupling element between the circumference of the inner tappet and the buck tappet.
Owing to the fact that a switchable clamping device is provided as a coupling element, there is no need to build up hydraulic pressure in order to transmit the actuating forces from the cam to the charge cycle valve. The forces for switching the clamping device are low and can be applied electrically or by a low oil pressure which can be sealed easily and be produced by an oil pump. Furthermore, there is the advantage, by comparison with self-closed coupling elements, that the clamping device operates noiselessly and smoothly. It is capable of automatically compensating manufacturing tolerances, wear and play resulting from different operating temperatures. If charge cycle valves are only to be switched on and off, an additional device for compensating valve play is unnecessary.
A simple, space-saving clamping device is achieved with a clamping ring which cooperates with the aid of its conical surface with a corresponding mating surface on the bucket tappet. The conical surfaces can also be located on the side facing the inner tappet, or on both sides, but it is generally sufficient for the clamping ring to have a conical surface on only one side.
The conical angle is dimensioned to be so small that self-locking occurs in a position of the clamping ring in which the actuating force is transmitted. As a result, large actuating forces can be transmitted to the charge cycle valve in a force-close fashion without the need to apply energy for additional holding forces. The clamping ring is held only by one or more pressure springs in a position in which the conical surfaces touch one another lightly, so that the friction forces required for self-locking become active. In order to be able to mount the clamping ring more easily, and so that its conical surface bears more effectively against the mating surface, the clamping ring is expediently divided in the axial direction in the form of sectors.
The clamping ring is subjected to an oil pressure in order to release the clamped connection. It is expedient for this purpose to make use of the oil pressure of the internal combustion engine. Since only a low pressure is required for releasing purposes, this pressure suffices as long as the cam does not actuate the bucket tappet. If the cam acts on the bucket tappet after the clamped connection is released, this tappet is displaced counter to the force of a first pressure spring in the direction of the charge cycle valve, without the latter opening.
Friction losses are produced because the bucket tappet moves in the cylinder head without actuating the charge cycle valve, and slides on the cam of the camshaft. In accordance with a further development of the invention, these losses can be avoided by giving the clamping body a second conical surface which forms an obtuse angle with the first one and is brought into action by the oil pressure counter to the force of the second pressure spring. The conical angle is selected here to be so large that no self-locking occurs. The second clamped connection holds the bucket tappet in its lower dead-center position as long as the oil pressure acts. The result is that friction losses are avoided between the bucket tappet and the cam as well as the cylinder head.
When pressure is released from the clamping ring, the second pressure spring can overcome the clamping action, which is supported by the first pressure spring, and release the clamped connection. In this case, the first pressure spring presses the bucket tappet against the base circle of the camshaft. Any valve play which may be present is thereby compensated. An additional device for compensating valve play is not required. A throttle valve and a pressure-retaining valve are expediently used to maintain a limited residual pressure in order to improve the response of the switching-off or changeover device.
The spring chambers are connected to an unpressurized space in the cylinder head so that no oil pressure can build up through leakage in the spring chambers of the first and second pressure springs.
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patent: 94 03 420 (1994-06-01), None
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Crowell & Moring LLP
Daimler-Chrysler AG
Lo Weilun
LandOfFree
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