The present invention relates to a gas exchange valve arrangement for a supercharged piston engine in accordance with the preamble of claim 1 .

Background of the invention

Very high charge pressures are used in modern compression ignition internal combustion engines to achieve better fuel efficiency. High charge pressures are advantageously accompanied with Miller timing, where intake valves are closed well before bottom dead center during the intake stroke. Miller timing reduces cylinder temperature during combustion and helps to achieve lower NOx emissions. Since all the gas exchange valves are closed when the piston is still moving downwards, the cylinder pressure at the end of the intake phase might be significantly lower than the pressure in the air intake duct. A problem with large pressure differences over the intake valves is that standard valve springs have been designed for smaller pressure differences and the intake valves tend to open. A similar problem is encountered with the exhaust valves. Since a high charge pressure increases also the pressure in the exhaust manifold, greater force is needed for keeping the exhaust valves closed at the end of the suction stroke and at the beginning of the compression stroke. An obvious solution for solving this problem would naturally be the use of stiffer valve springs. However, this is often not possible without significant modifications to the cylinder head. In many cases it would be very difficult or even impossible to redesign the cylinder head so that it could accommodate the larger springs needed to withstand the large pressure differences over the gas exchange valves.

One option for reducing the force the valve springs need to carry is to use air springs to assist conventional mechanical springs.

Patent document US 6745738 B1 discloses a valve spring device using pres- surized gas for biasing a gas exchange valve. The device comprises a dynamic housing having a chamber into which pressurized gas can be communicated so that the dynamic housing moves away from a static housing moving the at- tached valve. The device uses a separate pressure source and control means for controlling the flow of the pressurized gas into the dynamic chamber and out of the chamber.

Patent document US 5988124 A discloses an electromagnetically actuated cyl- inder valve having pneumatic resetting springs. The valve is opened and closed by means of electromagnetic actuators. Control means are needed for operating the gas springs. The arrangement also comprises a mechanical spring for closing the valve when the gas springs are in a depressurized state.

Patent application GB 2326444 A discloses another electro-pneumatically ac- tuated gas exchange valve. The system may also employ a mechanical spring for closing the valve when the electro-pneumatic means are not activated. Also this system needs control means and a separate compressor for regulating the pressure in the gas springs.

In none of the above mentioned documents the air springs are meant for as- sisting the mechanical springs, but they are rather used for valve timing. The mechanical springs are provided for closing the valves when the air springs are depressurized. All of the disclosed systems use an external source of pressurized gas and control means for regulating gas flow into the gas springs. This makes the systems complicated and susceptible to malfunction. Most of the above mentioned problems have been solved by a gas exchange valve arrangement disclosed in WO 2012104482 A1 . In this arrangement, pressurized intake air is conducted from an intake duct into a chamber that is arranged around the valve stem. The construction is simple and reliable. However, some problems still remain. For instance, pressurized air can leak from the chamber through the gap between the valve stem and the valve guide, which disturbs lubrication.

Summary of the invention

The object of the present invention is to provide an improved gas exchange valve arrangement, which solves the above mentioned problems. The characterizing features of the gas exchange valve arrangement according to the invention are given in the characterizing part of claim 1 . The gas exchange valve arrangement according to the invention comprises a gas exchange valve having a valve head and a valve stem, a valve guide, which is arranged in a cylinder head of the engine for supporting the valve stem in radial direction, a valve spring, which is arranged around the valve stem for creating a force for closing the gas exchange valve, an air chamber, which is arranged in the cylinder head around the valve guide, a piston, which is attached to the valve stem and delimits the air chamber, and an air channel for introducing pressurized charge air from an intake duct of the engine into the air chamber. The piston is arranged around the valve guide and the piston is provided with a first seal, which is arranged between the valve guide and the inner perimeter of the piston and with a second seal, which is arranged between the outer perimeter of the piston and the wall of the air chamber.

Due to the sealing arrangement and the location of the piston, the pressurized intake air is prevented from leaking from the air chamber between the valve guide and the valve stem. The lubrication of the valve stem is thus not disturbed.

According to an embodiment of the invention, the piston comprises a hollow stem, which is arranged around the valve guide and extends away from the valve head and beyond the end of the valve guide. The hollow stem supports the piston against the valve guide. The stem of the piston can be provided with one or more openings for introducing lubricating oil inside the stem of the piston for lubricating the valve stem.

According to an embodiment of the invention, the valve stem is provided with a first wedge-shaped fixing surface, and the stem of the piston is provided with a second wedge-shaped fixing surface, which is engaged with the first fixing surface for attaching the piston to the valve stem.

According to an embodiment of the invention, the arrangement comprises a supporting ring, which is arranged above the air chamber for supporting the lower end of the valve spring. The supporting ring can be part of a supporting collar. Because of the supporting ring, the diameter of the valve spring is not dependent on the diameter of the piston. This allows the use of a large piston and greater closing force of the gas exchange valve. According to an embodiment of the invention, the air chamber is machined in the cylinder head. By making the air chamber integral with the cylinder head, the number of parts in the gas exchange valve arrangement can be reduced.

Brief description of the drawings

Embodiments of the invention are described below in more detail with reference to the accompanying drawing, which shows a cross-sectional view of a gas exchange valve arrangement according to an embodiment of the invention.

Description of embodiments of the invention

In Fig. 1 is shown a simplified cross-sectional view of part of a cylinder head 9 of a large internal combustion engine. The engine could be, for instance, a main or an auxiliary engine of a ship or a power plant engine. The engine is supercharged for achieving high intake air pressure. For instance two turbo- chargers that are connected in series can be used, each of the turbo-chargers comprising a turbine and a compressor. The engine preferably comprises a plurality of cylinders and each of the cylinders is provided with its own cylinder head 9. The cylinders can be arranged for instance in line or in a V- configuration. An intake duct 10 is arranged in the cylinder head 9 for introducing pressurized charge air into the cylinder. An exhaust duct (not shown) is arranged in the cylinder head 9 for guiding exhaust gas out of the cylinder. The intake duct 10 and the exhaust duct continue outside the cylinder head 9 and are connected to the compressors and turbines of the turbochargers, respec- tively. The cylinder head 9 is provided with intake valves 1 for opening and closing fluid communication between the intake duct 10 and the cylinder, and with exhaust valves for opening and closing fluid communication between the exhaust duct and the cylinder. Together the intake valves 1 and the exhaust valves can be referred to as gas exchange valves. Each cylinder of the engine is preferably provided with two intake valves 1 and two exhaust valves, Of course, the number of the gas exchange valves could also be different. The intake valve 1 comprises a valve head 2 and a valve stem 3. The valve stem 3 is integrated to the valve head 2 and extends away from the cylinder. The valve stem 3 is needed for moving the intake valve 1 in a reciprocating manner for opening and closing the fluid communication between the cylinder and the intake duct 10. When the intake valve 1 is closed, the valve head 2 forms a gas tight connection with a valve seat 16, which is arranged in the cylinder head 9. The cylinder head 9 is provided with a valve guide 8 for supporting the intake valve 1 in radial direction. The valve guide 8 is a cylindrical part, inside which the valve stem 3 can reciprocate. The valve guide 8 is a separate part, which is attached to the cylinder head 9 for example with a shrink fit. The valve guide 8 is provided with a protrusion 8A, which supports the valve guide 8 against the cylinder head 9 in axial direction. Since the valve guide 8 is a separate part, it can be made of a different material than the cylinder head 9. This allows the material of the valve guide 8 to be chosen so that the wear re- sistance and friction properties of the valve guide 8 are suitable for accommodating the reciprocating intake valve 1 . The intake valve 1 is provided with a valve spring 4, which creates a force with a direction away from the cylinder. The force tends to close the intake valve 1 and keep it closed. The valve spring 4 is arranged around the valve stem 3. The intake valve 1 is cam- operated, and the engine is thus provided with at least one camshaft, which comprises a cam for the intake valves 1 of each cylinder. A conventional opening mechanism with push rods and rocker arms can be arranged between the intake cam and the intake valve 1 . The opening mechanism can also comprise hydraulic components, for instance for allowing delayed intake valve closing timing.

To avoid opening of the intake valve 1 when high charge pressures are used without a need to overdimension the valve spring 4, the pressure of the intake air is used for assisting the valve spring 4. An air chamber 5 is arranged in the cylinder head 9 around the valve stem 3 and the valve guide 8. A piston 6, which is attached to the valve stem 3, is arranged in the air chamber 5. The piston 6 delimits the air chamber 5 and forms thus the upper end of the air chamber 5. The air chamber 5 is in fluid communication with the intake duct 10 via an air channel 7. The air channel 7 can be made by drilling, or it can be made when the cylinder head 9 is cast. Through the air channel 7, the pres- sure of the intake air can affect the piston 6. The pressure in the air chamber 5 pushes the piston 6 away from the cylinder, and helps thus keeping the intake valve 1 closed. The arrangement is self-adjusting - if the pressure of the intake air increases, both the force affecting the upstream side of the valve head 2 and the force affecting the piston 6 increase. By suitable dimensioning of the piston 6, the forces compensate each other at least to a great extent, and a less stiff valve spring 4 is sufficient for keeping the intake valve 1 closed. The air chamber 5 can be an integral part of the cylinder head 9. The air chamber 5 can thus be machined directly in the cylinder head 9. By making the air chamber 5 an integral part of the cylinder head 9, the number of components in the valve arrangement can be reduced. The piston 6 is arranged around the valve guide 8. The piston 6 is an annular part, of which inner perimeter faces the valve guide 8 and the outer perimeter faces the cylindrical wall of the air chamber 5. The piston 6 thus moves along the valve guide 8. The piston 6 is provided with a first seal 1 1 , which is arranged between the valve guide 8 and the inner perimeter of the piston 6, and with a second seal 12, which is arranged between the wall of the air chamber 5 and the outer perimeter of the piston 6. Because of the sealing and the location of the piston 6, the pressurized intake air is prevented from flowing from the air chamber 5 between the valve stem 3 and valve guide 8. The lubrication of the valve stem 3 is thus not disturbed. The piston 6 comprises a hollow stem 6A, which is arranged around the valve guide 8. The stem 6A of the piston 6 extends away from the valve head 2 and beyond the end of the valve guide 8. When the intake valve 1 is closed, an empty space 19 is formed around the valve stem 3 above the upper end of the valve guide 8. The stem 6A of the piston 6 is provided with openings 13. The openings 13 are above the upper end of the valve guide 8 at least when the intake valve 1 is closed. Through the openings 13, lubricating oil can be introduced inside the stem 6A of the piston 6 for lubricating the valve stem 3.

For attaching the piston 6 to the valve stem 3, the valve stem 3 is provided with a first wedge-shaped fixing surface 14. The first fixing surface 14 is attached to the valve stem 3 and tapers towards the valve head 2. The first fixing surface 14 can be, for instance, in the form of a conical sleeve or two or more wedges that are arranged around the valve stem 3. The stem 6A of the piston 6 is provided with a second wedge-shaped fixing surface 15. The second fixing surface 15 is formed on the inner perimeter of the stem 6A of the piston 6 and is an integral part of the piston 6. Also the second fixing surface 15 tapers towards the valve head 2. The second fixing surface 15 is engaged with the first fixing surface 14 for attaching the piston 6 to the intake valve 1 . A disc spring 17 is arranged around the valve stem 3 above the fixing surfaces 14, 15 for removing any unwanted play between the intake valve 1 and the piston 6. A locking ring 21 is arranged above the disc spring 17 for securing the connec- tion.

The lower end of the valve spring 4 is supported against a supporting ring 18A. The supporting ring 18A is part of a collar 18. The supporting ring 18A is supported against the cylinder head 9 above the air chamber 5. Since the valve spring 4 is supported by the collar 18, the size of the piston 6 is not dependent on the size of the valve spring 4, but the diameter of the piston 6 can be greater than the diameter of the valve spring 4. It is thus possible to use a larger piston 6. The valve spring 4 is outside the air chamber 5. Lubricating oil can thus be used for dampening vibrations of the of the valve spring 4 and for lubricating and cooling the valve spring 4. A valve rotator 20 is attached to the valve stem 3 above the first fixing surface 14. The valve rotator 20 turns the intake valve 1 slightly each time the valve 1 is actuated. The upper end of the valve spring 4 is arranged against the valve rotator 20 for pushing the intake valve 1 in the closing direction.

It will be appreciated by a person skilled in the art that the invention is not lim- ited to the embodiments described above, but may vary within the scope of the appended claims. Although the invention has been described above by referring to the intake valves, it is also applicable to exhaust valves.