The present invention relates to a valve lifting arrangement for opening an exhaust valve of a piston engine in accordance with the preamble of claim 1. The invention also concerns a method for operating an exhaust valve of a piston engine.

Nitrogen oxide (NOx) emissions of internal combustion engines are subject to continuously tightening regulations. In general, NOx emissions can be reduced by reducing temperature in the combustion chamber. An effective way to reduce NOx emissions of an internal combustion engine is to use exhaust gas recirculation (EGR), where part of the exhaust gases are directed back to the cylinders of the engine. Since the heat capacity of the recirculated exhaust gas is higher than the heat capacity of air, the same amount of energy released by combustion leads to lower temperature increase in engines with EGR. Also the lower oxygen mass inside the cylinders and reduction of combustion speed help to achieve lower temperature increase. An EGR system can be either external or internal. In an external EGR system exhaust gases are recirculated from the exhaust duct into the air intake duct. In an internal EGR system part of the exhaust gas is trapped within the cylinder or back-flow from the exhaust duct into the combustion chamber is utilized. Typically, the exhaust valves of a cylinder are opened for a short period of time after closing of the intake valves. Exhaust pressure needs to be higher than the pressure in the cylinder in order to get exhaust gases to flow into the cylinder.

There are different ways for implementing the exhaust valve opening in internal EGR systems. Japanese patent application JP 2001065320 discloses a cam that comprises a second lobe that opens the exhaust valve for EGR. The second lobe is asymmetrical enabling adjustment of the valve lift by moving the cam in the camshaft direction.

Patent application GB 2442813 A discloses a camshaft assembly comprising a fixed cam and a moveable cam lobe. The movable cam lobe can be used for an extra exhaust valve opening event for internal EGR. The movable cam lobe can be moved by hydraulic pressure.

An object of the present invention is to provide an improved valve lifting arrangement for opening an exhaust valve of a piston engine. The characterizing features of the valve lifting arrangement according to the invention are given in the characterizing part of claim 1. Another object of the invention is to provide a method for operating an exhaust valve of a piston engine, as defined in the characterizing part of the other independent claim.

The valve lifting arrangement according to the invention comprises a first exhaust cam, which is arranged to open at least one exhaust valve for allowing outflow of exhaust gas from a cylinder of the engine during the exhaust stroke, and a second exhaust cam, which can be arranged to open the exhaust valve for allowing exhaust gas recirculation during the intake and/or compression stroke.

In the method according to the invention, the exhaust valve is opened by means of a first exhaust cam for allowing outflow of exhaust gas from a cylinder of the engine during the exhaust stroke, and a second exhaust cam is used for opening the exhaust valve for allowing exhaust gas recirculation during the intake and/or compression stroke.

The valve lifting arrangement is thus provided with two separate exhaust cams. The first exhaust cam is responsible for the normal opening of the exhaust valve, and the second exhaust cam is responsible for an additional opening, which is used for the EGR. The normal opening can thus be partly or completely mechanically implemented, which makes the system reliable. Also the additional opening is accomplished at least partly mechanically. The opening and closing timing and the valve lift of the exhaust valve can be controlled by the shapes of the exhaust cams, which makes the control fast, reliable and accurate. The valve lifting arrangement can also be used for upgrading engines already in use.

According to an embodiment of the invention, the second exhaust cam is arranged to operate a piston device, which piston device is arranged to pressurize hydraulic fluid that is used for opening the exhaust valve for exhaust gas recirculation. The piston device can be arranged to protrude into a first hydraulic fluid chamber, which first hydraulic fluid chamber is in fluid communication with a second hydraulic fluid chamber, which second hydraulic fluid chamber is provided with a piston, which piston is in force transmission connection with the exhaust valve. A hydraulic force transmission from the second exhaust cam allows flexible positioning of the components of the valve lifting arrangement. It is also possible to easily add additional control functions to a partly hydraulic system.

According to an embodiment of the invention, the arrangement comprises an outlet duct, which outlet duct is connected to the first hydraulic fluid chamber and provided with a valve. The valve can be a fast acting solenoid valve. The outlet duct can be connected to a pressure accumulator. The use of a valve allows switching the EGR function on and off. If hydraulic fluid is conducted from the first hydraulic fluid chamber into the outlet duct instead of the second hydraulic fluid chamber, the exhaust valve is not opened for the EGR. If the valve is fast enough, it can also be used for controlling the opening and closing timing and lift of the exhaust valve when the exhaust valve is opened for the EGR.

According to another embodiment of the invention, there is mechanical force transmission connection between the first exhaust cam and the exhaust valve. This is a reliable solution, but the force transmission connection can also be partly hydraulic.

According to another embodiment of the invention, the first exhaust cam and the second exhaust cam are attached to a common camshaft.

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

• Fig. 1 shows schematically a valve lifting arrangement according to an embodiment of the invention,
• Fig. 2 shows the embodiment of figure 1 with a VEC-function, and
• Fig. 3 shows a valve lifting arrangement according to another embodiment of the invention.

In figure 1 is shown a valve lifting arrangement according to an embodiment of the invention. The valve lifting arrangement is used for opening an exhaust valve 5 of a piston engine. The exhaust valve 5 opens and closes fluid communication between an exhaust port and a cylinder 17 of the engine. In the figure only one exhaust valve 5 is shown, but each cylinder 17 of the engine can be provided with more than one exhaust valves 5, and the valve lifting arrangement can be used for operating all the exhaust valves 5 of one cylinder 17. The engine is a large internal combustion engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. However, the invention could also be applied to other types of engines.

The valve lifting arrangement comprises a first exhaust cam 1, which can be of conventional design. The first exhaust cam 1 comprises a base circle 1 a and a lobe 1 b extending radially away from the base circle 1 a. The first exhaust cam 1 is attached to a rotating camshaft. The valve lifting arrangement is further provided with a first cam follower 3. The first cam follower 3 comprises a cam follower wheel 3a, which is engaged with the cam profile of the first exhaust cam 1. The first cam follower 3 forms part of force transmission means, which transform the rotating motion of the first exhaust cam 1 into a linear motion and transmit it to the exhaust valve 5 at least in the opening direction of the exhaust valve 5. In the embodiment of the figure, the force transmission means further comprises a rocker arm 6, a push rod 8 and a piston 9. One end of the rocker arm 6 is arranged to push the exhaust valve 5 in its opening direction when the rocker arm 6 is turned around its shaft 7. The push rod 8 is arranged to push the other end of the rocker arm 6 for opening the exhaust valve 5. The piston 9 is arranged between the first cam follower 3 and the push rod 8 and it is thus in force transmission connection with the exhaust valve 5. When the cam follower wheel 3a is engaged with the lobe 1 b of the first exhaust cam 1, the first cam follower 3 is forced away from the rotating axis of the first exhaust cam 1. The first cam follower 3 pushes the piston 9, which in its turn moves the push rod 8. The push rod 8 turns the rocker arm 6 around the shaft 7 of the rocker arm 6, and the exhaust valve 5 is opened. When the cam follower wheel 3a returns to the base circle 1 a of the first exhaust cam 1, the exhaust valve 5 is closed. This is the normal opening and closing sequence of the exhaust valve 5, i.e. the way in which the valve lifting arrangement works when the exhaust valve 5 is opened during the exhaust stroke for allowing outflow of the exhaust gases from the cylinder 17. The valve lifting arrangement can be provided with one or more springs for facilitating the closing of the exhaust valve 5 and for keeping the first cam follower 3 engaged with the first exhaust cam 1. Part of the force transmission means between the first exhaust cam 1 and the exhaust valve 5 could also be hydraulic. For instance, the rocker arm 6 could be replaced by two hydraulic pistons.

For allowing exhaust gas recirculation, i.e. exhaust gas flow from the exhaust port into the cylinder 17 during the intake stroke and/or compression stroke, the valve lifting arrangement is provided with a second exhaust cam 2, which is separate from the first exhaust cam 1. The additional opening of the exhaust valve 5 for the EGR can begin during the intake stroke, usually after the intake valves have been closed, and the exhaust valve 5 can be closed at the beginning of the compression stroke. When the exhaust pressure is higher than the pressure in the cylinder 17, exhaust gas flows from the exhaust port into the cylinder 17. The second exhaust cam 2 can be arranged on the same camshaft as the first exhaust cam 1, or a separate camshaft can be used for rotating the second exhaust cam 2. The first exhaust cam 1 and the second exhaust cam 2 are arranged in a suitable angular position in relation to each other for achieving correct timing for the additional exhaust valve opening. The valve lifting arrangement comprises a second cam follower 4, which is provided with a cam follower wheel 4a. The cam follower wheel 4a of the second cam follower 4 is engaged with the cam profile of the second exhaust cam 2. Also the second exhaust cam 2 comprises a base circle 2a and a lobe 2b extending radially away from the base circle 2a. The lobe 2b of the second exhaust cam 2 is lower than the lobe 1 b of the first exhaust cam 1, i.e. the radius of the lobe 2b of the second exhaust cam 2 is smaller. Also, the length of the lobe 2b of the second exhaust cam 2 is shorter than the length of the lobe 1 b of the first exhaust cam 1. The lobe 2b of the second exhaust cam 2 thus extends over a smaller angle than the lobe 1 b of the first exhaust cam 1. When the cam follower wheel 4a of the second cam follower 4 is engaged with the lobe 2b of the second exhaust cam 2, the second cam follower 4 is pushed away from the rotation axis of the second exhaust cam 2.

The second cam follower 4 is arranged to move a piston device 12, which protrudes into a first hydraulic fluid chamber 13. The piston device 12 can be used for pressurizing hydraulic fluid, which is used for opening the exhaust valve 5 for exhaust gas recirculation. A hydraulic duct 11 connects the first hydraulic fluid chamber 13 to a second hydraulic fluid chamber 10, which is arranged around the piston 9, which forms part of the force transmission means between the first exhaust cam 1 and the exhaust valve 5. When hydraulic fluid is introduced from the first hydraulic fluid chamber 13 into the second hydraulic fluid chamber 10, the piston 9 is moved. The piston 9 is not fixed to the first cam follower 3, and the cam follower wheel 3a thus remains in contact with the base circle 1 a of the first exhaust cam 1. The piston 9 moves the push rod 8, which turns the rocker arm 6 opening the exhaust valve 5. When the cam follower wheel 4a of the second cam follower 4 returns to the base circle 2a of the second exhaust cam 2, the piston device 12 retracts from the first hydraulic fluid chamber 13 and hydraulic fluid is allowed to flow from the second hydraulic fluid chamber 10 back into the first hydraulic fluid chamber 13. Also the piston 9 can thus move back to its original position and the exhaust valve 5 is closed.

The valve lifting arrangement further comprises an outlet duct 18, which is connected to the first hydraulic fluid chamber 13 and to a pressure accumulator 16. In the embodiment of the figure, the outlet duct 18 is branched from the hydraulic duct 11. A valve 14 is arranged between the first hydraulic fluid chamber 13 and the pressure accumulator 16 for selectively opening and closing the fluid communication between the first hydraulic fluid chamber 13 and the pressure accumulator 16. When the valve 14 is closed, no fluid can flow from the first hydraulic fluid chamber 13 into the pressure accumulator 16. The fluid must thus flow from the first hydraulic fluid chamber 13 into the second hydraulic fluid chamber 10 when the hydraulic fluid is pressurized by the piston device 12. The exhaust valve 5 is thus opened every time the cam follower wheel 4a of the second cam follower 4 is engaged with the lobe 2b of the second cam 2. This means that the EGR function of the valve lifting arrangement is switched on when the valve 14 is closed. When the valve 14 is open, hydraulic fluid can flow from the first hydraulic fluid chamber 13 into the pressure accumulator 16 and move the piston 15 of the pressure accumulator 16. When the piston device 12 protrudes into the first hydraulic fluid chamber 13, the fluid does not flow into the second hydraulic fluid chamber 10, but it is stored in the pressure accumulator 16. The exhaust valve 5 remains closed even when the cam follower wheel 4a of the second cam follower 4 is engaged with the lobe 2b of the second exhaust cam 2. The EGR function is thus switched off when the valve 14 is open.

If the valve 14 is fast enough, it can also be used for adjusting the valve lift and the duration of the time the exhaust valve 5 remains open during the EGR opening. If the valve 14 is kept open when the cam follower wheel 4a of the second cam follower 4 enters the lobe 2b of the second exhaust cam 2, the opening of the exhaust valve 5 does not start until the valve 14 is closed. The opening of the exhaust valve 5 during the EGR operation can thus be delayed and also the maximum valve lift becomes smaller. If the valve 14 is opened when the cam follower wheel 4a of the second cam follower 4 is still engaged with the lobe 2b of the second exhaust cam 2, part of the hydraulic fluid flows from the second hydraulic fluid chamber 10 into the pressure accumulator 16 and the exhaust valve 5 is closed faster.

Figure 2 shows a valve lifting arrangement that is similar to the arrangement of figure 1. However, the valve lifting arrangement of figure 2 is provided with an additional function for variable exhaust valve closing (VEC). The VEC-function allows delaying or slowing of the closing movement of the exhaust valve 5 during the normal opening of the exhaust valve 5. The valve lifting arrangement is provided with a second piston 19, which is in force transmission connection with the exhaust valve 5. The second piston 19 is arranged in the second hydraulic fluid chamber 10 between the first cam follower 3 and the first piston 9. However, the arrangement could also be provided with a separate chamber for the second piston 19, and the second piston 19 could be arranged in a different place in the force transmission path between the first exhaust cam 1 and the exhaust valve 5. When the exhaust valve 5 opens, Hydraulic fluid can be introduced behind the second piston 19, i.e. into the space that grows when the second piston 19 moves in the opening direction of the exhaust valve 5. By limiting outflow from the space when the cam follower wheel 3a of the first cam follower 3 is on the descending slope of the lobe 1 b of the first cam 1, closing of the exhaust valve 5 can be delayed or slowed down. For instance a throttle or a valve can be used for the flow limitation. The second piston 19 is not fixed to the first cam follower 3, and the cam follower wheel 3a can thus follow the cam profile of the first exhaust cam 1 even when the VEC-function is in use.

In figure 3 is shown a valve lifting arrangement according to another embodiment of the invention. In this arrangement, the force transmission between the first exhaust cam 1 and the exhaust valve 5 is partly hydraulic. The first cam follower 3 is arranged to operate a third piston 21, which protrudes into a third hydraulic fluid chamber 20. The third hydraulic fluid chamber 20 is via a second hydraulic duct 22 in fluid communication with the second hydraulic fluid chamber 10. In this embodiment, the second hydraulic fluid chamber 10 is arranged in connection with the exhaust valve 5. The piston 9, which is used for the additional opening of the exhaust valve 5, is also used for the normal opening of the exhaust valve 5. The piston 9 is connected to the exhaust valve 5. When the cam follower wheel 3a of the first cam follower 3 becomes engaged with the lobe 1b of the first exhaust cam 1, the third piston 21 pressurizes the hydraulic fluid in the third hydraulic fluid chamber 20. The hydraulic fluid is conducted through the second hydraulic duct 22 to the second hydraulic fluid chamber 10, where the piston 9 is moved and the exhaust valve 5 opens. The exhaust valve 5 can be provided with a spring for closing the valve 5 when the cam follower wheel 3a of the first cam follower 3 returns to the base circle 1 a of the first exhaust cam 1. The additional opening of the exhaust valve 5 for the EGR works in the same way as in the embodiment of figures 1 and 2. The hydraulic duct 11 is connected to the second hydraulic duct 22 for conducting the hydraulic fluid from the first hydraulic fluid chamber 13 into the second hydraulic fluid chamber 10 or vice versa. The arrangement is further provided with an inlet duct 23, which comprises a check valve 24. Via the inlet duct 23, hydraulic fluid can be supplied into the system for compensating leakages.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims.