Device for actuating two outlet valves, which are acted on via a valve bridge, of a valve-controlled internal combustion engine
A device for actuating outlet valves that are acted on via a valve bridge of a valve-controlled internal combustion engine for motor vehicles includes a first piston-cylinder unit in the force flow between a cam of a driving camshaft and one of the outlet valves. When one of the outlet valves opens under the action of exhaust-gas counterpressure, play compensation in the valve drive can be produced in order to hold the one of the outlet valves open by the first piston-cylinder unit. A second, coaxial piston-cylinder unit interacts with a positionally fixed counter bracket and controls a discharge duct out of the pressure chambers of the two piston-cylinder units. The piston of the first piston-cylinder unit is preloaded in a readjustment direction by a spring. To reliably obtain a lack of play in the valve drive in particular during engine braking operation, the readjustment spring acts, without being supported in the valve bridge, on the pistons of both piston-cylinder units.
1. A device for actuating outlet valves of a valve-controlled internal combustion engine for motor vehicles, the device being part of a valve drive between a cam of a driving camshaft and the outlet valves, the device comprising:
a valve bridge for acting on the outlet valves;
a fixed counter bracket;
a first hydraulically charged piston-cylinder unit having a piston and a pressure chamber, disposed on the valve bridge in a force flow between the cam of the driving camshaft and one of the outlet valves, and configured so that when the one of the outlet valves opens under action of exhaust-gas counter pressure, play compensation in the valve drive can be produced to hold the one of the outlet valves open;
a second piston-cylinder unit having a piston and a pressure chamber arranged on the valve bridge coaxial with the first piston-cylinder unit, the second piston-cylinder unit interacts with the fixed counter bracket to control a discharge duct out of the pressure chambers of the first piston-cylinder unit and the second piston-cylinder unit; and
a spring acting on the pistons of the first piston-cylinder unit and the second piston-cylinder unit without being supported in the valve bridge, the spring preloading the piston of the first piston-cylinder unit in a readjustment direction.
2. The device of claim 1 , wherein the chambers of the first piston-cylinder unit and the second piston-cylinder unit are connected to one another by a recess or bore in the valve bridge, the spring extending through the recess or bore such that the spring is supported only on the pistons of the first piston-cylinder unit and the second piston-cylinder unit.
3. The device of claim 2 , wherein the spring is a helical compression spring.
4. The device of claim 1 , wherein the pistons of the first piston-cylinder unit and the second piston-cylinder unit are guided in corresponding cylinder bores with limiting stops, which permit a readjustment movement of the pistons.
5. The device of claim 1 , wherein the piston of the first piston-cylinder unit is movable between a retracted position and a deployed position, the discharge duct includes pressure discharge ducts in a cylinder wall and the piston of the first piston-cylinder unit, wherein the pressure discharge ducts are open when the piston of the first piston-cylinder unit is in the retracted position and the pressure discharge ducts are closed when the piston is in the deployed position.
6. The device of claim 5 , wherein the piston of the first piston-cylinder unit is in the retracted position during engine operation and is in the deployed position during a braking operation.
7. The device of claim 1 , further comprising a hydraulic valve play compensation element in the force flow between the cam of the camshaft and the valve bridge.
8. The device of claim 7 , wherein the first piston-cylinder unit, the second piston-cylinder unit, and the hydraulic valve play compensation element are supplied with pressurized oil from a forced-circulation lubrication oil system of the internal combustion engine, the device comprising a pressure-maintaining check valve in a feed line to the first piston-cylinder unit, the second piston-cylinder unit, and the hydraulic valve play compensation element.
9. A vehicle powered by an internal combustion engine including the device of claim 1 .