Technical field of the invention

The present invention relates to a multi-cylinder piston engine in accordance with the preamble of claim 1. The engine comprises a starting arrangement and at least one cam- operated valve lifting device for each cylinder of the engine, the valve lifting device being arranged to open a gas exchange valve.

Background of the invention

Large internal combustion engines that are used for instance as main or auxiliary en- gines in ships or for producing electricity and/or heat at power plants are usually started by using pressurized air that is injected sequentially into the cylinders of the engine for rotating the crankshaft. It is desirable to minimize the consumption of starting air in order to save energy and the space needed for storing the starting air, and therefore accurate control of the air injection timing is needed.

Patent application WO 2007/003693 Al discloses a starting system for a multi-cylinder piston engine. A pipe system connects a pressure medium source to each cylinder of the engine. Each cylinder is provided with a starting air valve for introducing pressure medium into the cylinders. Each starting valve is controlled by a control valve. The starting system is provided with timing equipment comprising a control part being in mechanical connection with a rotating part of the engine. This kind of starting arrangement guarantees accurate timing of the starting air injection, but a drawback is that control air has to be brought to each control valve in a separate control air pipe. Also, different timing equipment needs to be designed for engines with different number of cylinders.

Summary of the invention

The object of the present invention is to provide an improved multi-cylinder piston engine with a starting arrangement. The engine according to the present invention is characterized by the characterizing part of claim 1. According to the present invention, the multi-cylinder piston engine comprises at least one cam-operated valve lifting device for each cylinder of the engine, the valve lifting device being arranged to open a gas exchange valve. The starting arrangement comprises a pressure medium source, at least one starting valve for introducing pressure me- dium into a cylinder of the engine, means for connecting the pressure medium source to the starting valves, and a control valve for each cylinder of engine that is provided with a starting valve for controlling the operation of the starting valve. Each control valve is arranged to be operated by a gas exchange cam of the respective cylinder. The invention has several advantages. Since the starting valves are controlled by the gas exchange cams, a separate starting cam is not needed. Identical cams can be used regardless of the cylinder number of the engine and there is no need to design different starting cams for engines with different number of cylinders. Starting air can be distributed to the cylinders in a single starting air duct instead of separate ducts for each cy- linder. This saves both space and material. The invention also enables reliable and accurate control of starting air injection.

According to an embodiment of the invention, each gas exchange cam that operates a control valve comprises a portion that is arranged below the base circle of the cam for operating the control valve. When the control valve is operated by a portion of the gas exchange cam that is under the base circle of the cam, a simple and reliable control mechanism can be achieved. Identical cams can be used for engines with different number of cylinders and no separate adjustment is needed for the starting air injection timing. According to an embodiment of the invention, each control valve is in flow communication with the pressure medium source and the respective starting valve via a control air duct.

According to an embodiment of the invention, the control valve is arranged to allow pressure medium flow to the starting valve for opening the starting valve when the valve lifting device is engaged with the portion of the gas exchange cam that is below the base circle of the cam. According to an embodiment of the invention, each control valve comprises a valve member that is movable by the valve lifting device.

According to an embodiment of the invention, the valve lifting device comprises a plunger pushing the valve member towards the gas exchange cam when the valve lifting device is engaged with the portion of the gas exchange cam that is below the base circle of the cam.

According to an embodiment of the invention, the valve member is pushed towards the valve lifting device by the pressure of the pressure medium.

According to an embodiment of the invention, the gas exchange cam is an intake cam. Starting air injection is started near top dead center and terminated close to the crank angle where the exhaust valve opens. When the intake cam is used for controlling the starting valve, the portion of the cam that is below the base circle is far from the lobe of the cam. The shape of the portion below the base circle is thus not dependent on the shape of the lobe. If the portion below the base circle is close to the lobe of the cam, there might be some limitations to the shape of the portion below the base circle. According to an embodiment of the invention, each intake cam comprises a second portion that is arranged below the base circle of the cam for controlling the operation of variable intake valve closing, and the portion that is arranged below the base circle of the cam is arranged also below the said second portion. According to an embodiment of the invention, the gas exchange cam is an exhaust cam. When the control valves are operated by the exhaust cams, the invention can be used even in two-stroke engines with cam-operated exhaust valves.

According to an embodiment of the invention, in a two-stroke engine at least three cy- linders of the engine are provided with a starting valve and in a four-stroke engine at least five cylinders of the engine are provided with a starting valve. According to an embodiment of the invention, all the cylinders of the engine are provided with a starting valve. Since identical cam profiles and valve lifting devices can be used for all the cylinders of the engine, it is practical to provide all the cylinders with starting valves.

According to an embodiment of the invention, each control valve comprises an inlet channel for introducing pressure medium into the control valve, an outlet channel for introducing the pressure medium to the starting valve for opening the starting valve, and a drain channel for releasing the pressure medium out of the control valve when the control valve is not actuated by the gas exchange cam.

According to an embodiment of the invention, the starting arrangement comprises a main starting valve for initiating starting of the engine.

Brief description of the drawings

Fig. 1 shows schematically a starting system of an internal combustion engine.

Fig. 2 shows a simplified cross-sectional view of a part of a cylinder head with gas exchange valves and a starting valve.

Figs. 3-6 show an intake cam of the engine and the control arrangement of a starting valve at different stages of operation according to an embodiment of the invention.

Fig. 7 shows an intake cam of the engine and the control arrangement of a starting valve according to another embodiment of the invention.

Fig. 8 shows schematically a starting system of an internal combustion engine according to another embodiment of the invention.

Fig. 9 shows an exhaust cam of the engine and a control arrangement of an air injection valve.

Figs. 10 shows the vertical position of the cam followers at different crank angles.

Figs. 11 shows the vertical position of the cam followers at different crank angles in an engine with VIC. Detailed description of the invention

Embodiments of the present invention are now described in more detail with reference to the accompanying drawings. Fig. 1 shows schematically a starting arrangement of a piston engine 20. In the example of Fig. 1, the engine 20 comprises six cylinders 19, but the engine 20 could comprise any reasonable number of cylinders 19 arranged for instance inline or in a V- configuration. The engine 20 is a large internal combustion engine, such as those used for producing electricity and/or heat at a power plant or as main or auxiliary engines in ships. In the embodiment shown in Fig. 1 , all the cylinders 19 are provided with means for introducing pressure medium, such as pressurized air, into the cylinders 19 for starting the engine 20. However, in an engine with a large number of cylinders it is not necessary that all the cylinders of the engine are used for starting. In case of a four-stroke engine, starting air injection into at least five of the cylinders is needed to be able to start the engine irrespective of the initial crankshaft position. In a two-stroke engine, three cylinders suffice. If the engine is provided with means for rotating the crankshaft into a better starting position, even a smaller number of cylinders with starting air injection is sufficient. However, since the starting arrangement according to the present invention enables using of identical means for injecting starting air into each cylinder 19 of the engine 20, it is practical to use all the cylinders 19 of the engine 20 for starting.

Pressurized air is stored in a starting air tank 18, from which it can be introduced into the cylinders 19 of the engine 20 in a starting air duct 26. Instead of air, also some other pressurized gas could be used. The starting air tank 18 can be filled by a compressor (not shown). Each cylinder 19 of the engine 20 that is used for starting is provided with a starting valve 10. In figure 2 is shown a starting valve 10 in connection with a cylinder head 23. The cylinder head 23 is also provided with an intake duct 32 for introducing combustion air into the respective cylinder 19, and with an exhaust duct 33 for guiding exhaust gases out of the cylinder 19. One or more intake valves 24 and one or more ex- haust valves 25 are arranged in the cylinder head 23 for opening and closing the connection between the cylinder 19 and the intake and exhaust ducts 32, 33. The operation of each starting valve 10 is controlled by a respective control valve 12. Each cylinder 19 of the engine 20 can also be provided with more than one starting valves 10. If each cy- linder 19 is provided for instance with two starting valves 10, both valves 10 can be controlled by the same control valve 12. The control valves 12 are connected to the starting air duct 26. The starting air duct 26 is provided with a main starting valve 17 for controlling when the engine 20 is started. An example of the starting valve 10 and the control valve 12 is shown as a simplified illustration in Figs. 3-6.

According to an embodiment of the present invention, the starting arrangement comprises a valve lifting device 4 and a gas exchange cam 1. The gas exchange cam 1 is shown in the figures as being rotated 90 degrees around its vertical axis to better illu- strate the cam profile 2. The cam profile 2 of the gas exchange cam 1 comprises a base circle 2a, a lobe 2b and a portion 2c that is arranged below the base circle 2a. The expression "below the base circle" means that the radius of that portion of the cam profile

2 is smaller than the radius of the base circle 2a. The radius of the portion 2c below the base circle 2a does not need to be constant over the whole portion 2c. In the embodi- ment shown in Figs. 3-6, the gas exchange cam 1 is an intake cam and the valve lifting device 4 is arranged to open an intake valve 24 of the engine 20. An identical cam lifting device 4 can be used for opening an exhaust valve 25 of the engine 20.

The cam lifting device 4 comprises a body part 5 and a reciprocating plunger 6 that is arranged inside the body part 5. The plunger 6 is arranged to lift a push rod 8. A cam follower 3 is attached to the plunger 6 with a bearing. A coil spring 7 between the body part 5 and the plunger 6 pushes the cam follower 3 against the cam profile 2. The lifting arrangement works in a conventional manner. When the cam follower 3 is on the portion 2c of the cam profile 2 that is below the base circle 2a, the plunger 6 is at its lowest position. The body part 5 is provided with a stopper surface 9, against which the push rod 8 can rest so that the position of the push rod 8 and the intake valve 24 does not change when the cam follower 3 leaves the base circle 2a and enters the portion 2c below the base circle 2a, or vice versa. When the intake cam 1 is rotated, the cam follower

3 becomes eventually engaged with the lobe 2b of the cam profile 2. The plunger 6 and the push rod 8 are moved upwards and the intake valve 24 is opened. In figure 4 is shown the situation where the intake valve 24 is fully open. Each cylinder 19 of the engine 20 is provided with a starting valve 10 comprising a valve member 22. The starting valve 10 is kept closed by a spring 11. For controlling the operation of the starting valves 10, each valve lifting device 4 is provided with a control valve 12. When the cam follower 3 is engaged with the portion 2c of the cam profile 2 that is below the base circle 2a, the plunger 6 pushes a valve member 13 inside the control valve 12 downwards, as shown in Fig. 5.

For starting the engine 20, the main starting valve 17 is opened. This allows the starting air to flow in the starting air duct 26 to the control valves 12. The pressurized air enters the control valves 12 via inlet channels 14. In those control valves 12 that are in connection with a valve lifting device 4 where the cam follower 3 is on the base circle 2a or the lobe 2b of the intake cam 1, the air pressure pushes the valve member 13 inside the control valve 12 upwards into the position shown in Fig. 4. The valve member 13 thus allows the pressurized air to flow out of the control valve 12 through a drain channel 16. The air is released from the drain channel 16 for instance into the engine room. By contrast, in those control valves 12 which are in connection with a valve lifting device 4 where the cam follower 3 is on the portion 2c below the base circle 2a of the intake cam 1, the plunger 6 pushes the valve member 13 downwards, as shown in Fig. 5. This closes the connection between the inlet channel 14 and the drain channel 16 and the starting air flows to the starting valve 10. The air is introduced through a control air inlet 27 into the space above the valve member 22 of the starting valve 10 and the air pressure pushes the valve member 22 downwards and opens the starting valve 10. The starting air can thus flow in the starting air duct 26 from the starting air tank 18 to the starting valve 10 and through a starting air inlet 28 into the cylinder 19 of the engine 20, where it pushes the piston downwards and rotates the crankshaft of the engine 20.

When the camshaft rotates further, the cam follower 3 leaves the portion 2c below the base circle 2a of the cam profile 2 of the intake cam 1 and enters again the base circle 2a. The pressure in the inlet channel 14 and the pressure created by the valve member 22 of the starting valve 10 when being pushed upwards by the spring 1 1 can thus push the valve member 13 upwards inside the control valve 12. Consequently, the connection between the inlet channel 14 and the drain channel 16 of the control valve 12 opens again, as can be seen in Fig. 6, and starting air injection into the respective cylinder 19 is terminated. Starting air is injected sequentially into the cylinders 19 of the engine 20 until the engine 20 starts. When the engine 20 starts, the main starting valve 17 can be closed. With a different cam profile 2, where the portion 2c below the base circle 2a is in a different position, those cam lifting devices 4 that are used for opening the exhaust valves 25 could also be used for controlling the starting valves 10. However, it is beneficial to use the intake cams for this purpose, since the portion 2c below the base circle 2a can be arranged far from the lobe 2b of the cam profile 2. This ensures smooth operation of the cam lifting devices 4 and the control valves 12.

In figure 7 is shown a slightly different embodiment of the invention. In this embodiment, the push rod 8 is moved by a piston 30 that is arranged between the push rod 8 and the plunger 6. A second spring 29 for eliminating clearance between the push rod 8 and the piston 30 is arranged between the piston 30 and the plunger 6. When the cam follower 3 engages with the portion 2c of the cam profile 2 that is below the base circle 2a of the cam profile 2, a gap 31 is formed between the plunger 6 and the piston 30. The second spring 29 keeps the piston 30 against the end of the push rod 8 so that no gap is formed between the push rod 8 and the piston 30 or between the upper end of the push rod 8 and the rocker arm. This ensures smooth operation of the valve lifting device 4.

In figure 10 is shown as a graph the position of the cam follower 3. The broken line shows the position of the cam follower of the exhaust valve 25 and the solid line the position of the cam follower of the intake valve 24. Also the plungers 6 follow the same curves. The push rods 8 and the valve lifts also follow the same curves with the exception that because of the stopper surface 9 in the embodiment of figures 3-6 or the piston/spring arrangement 29, 30 in the embodiment of figure 7, the vertical position of the push rods 8 cannot be negative. The area below the horizontal axis indicates how the cam follower 3 of the intake valve lifting device 4 is engaged with the portion 2c below the base circle 2a. The starting air injection takes place during this phase, which starts in the embodiments shown in the figures just before top dead center (TDC) during the compression stroke and lasts until approximately 90 degrees of crank angle after top dead center during the power stroke. In figure 11 is shown the vertical positions of the cam followers in an engine that is provided with variable intake valve closing (VIC). In this embodiment, the intake cam profile comprises a portion that is arranged below the base circle for controlling the in- take valve closing timing. Another portion for controlling the starting valves is arranged below the portion that controls the intake valve closing timing.

In figure 8 is shown another embodiment of the invention, where the starting air duct 26 and the starting air tank 18 are also used for introducing additional combustion air into the cylinders 19 of the engine 20. The term "additional combustion air" refers here to air that is introduced into the cylinders 19 of the engine 20 before ignition but after the normal air intake has ended. In a four-stroke engine this means that the additional combustion air is introduced into a cylinder 19 after closing of the intake valves of that cylinder 19. Additional combustion air can be introduced into the cylinders 19 of the en- gine 20 for reducing turbo lag in situations where the engine load suddenly increases. In the embodiment of figure 8, the engine 20 is provided with a control air duct 21 that connects the control valves 12 to the starting air tank 18. The main starting valve 17 is arranged in the control air duct 21. The starting air duct 26 is provided with a closing valve 34. Each cylinder 19 of the engine 20 is provided with a second control valve 12' for controlling the injection of the additional combustion air into the cylinders 19. The second control valves 12' are connected with a second control air duct 21 ' to the starting air tank 18. The second control air duct 21 ' is provided with an air injection control valve 17' for controlling when the injection of the additional combustion air is switched on. In the embodiment of figure 8, the starting valves 10 are used also for introducing the additional combustion air into the cylinders 19. However, also separate valves could be used. The control air duct 21 is provided with a check valve 32 downstream from each control valve 12 for preventing the additional combustion air from escaping through the respective control valve 12 when additional combustion air is introduced into the engine 20. Similarly, the second control air duct 21 ' is provided with a second check valve 32' downstream from each second control valve 12' for preventing the starting air from escaping through the second control valve 12' during starting of the engine 20. The second control valves 12' are operated by the exhaust cams of the engine 20, as shown in figure 9. The valve lifting devices 4 that are used for opening the exhaust valves 25 can be identical with the valve lifting devices 4 that are used for opening the intake valves 24. The cam profile 2' of the exhaust cam 1 ' is different from the cam pro- file 2 of the intake cam 1, but otherwise the second control valves 12' are operated in the same manner as the first control valves 12. It is beneficial to use the exhaust cams 1 ' for operating the second control valves 12' since in the exhaust cams the portion 2c' below the base circle 2' can be arranged far from the lobe 2b' of the cam profile 2'. However, it would also be possible to use the intake cams 1 for operating the second control valves 12' and the exhaust cams 1 ' for operating the first control valves 12. The second control valves 12' are identical to the first control valves 12, comprising a valve member 13', an inlet channel 14', an outlet channel 15', and a drain channel 14'.

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.