The present invention relates to a fuel injection arrangement for a cylinder of a four-stroke piston engine in accordance with claim 1. The invention also con- cerns a method of operating a piston engine as defined in the other independ ent claim.

Background of the invention

A common trend in the development of piston engines is that the ratio between the output power and the cylinder volume is continuously growing. This helps increasing the thermal efficiency of the engines. However, a consequence of this development is that modern four-stroke engines need to withstand very high thermal loads. In some cases, the thermal load of the components of combustion chambers may be a limiting factor, which prevents increasing of the output power of an engine. Alternatively, substantial modifications in the engine design may be required, for instance in the form of more effective cool ing systems, or the materials of some components need to be replaced by ma terials with good thermal resistance. This naturally increases the manufactur ing costs.

Summary of the invention

An object of the present invention is to provide an improved fuel injection ar rangement for a cylinder of a four-stroke piston engine, the cylinder being pro vided with two intake valves, which are arranged on an intake valve side of an imaginary plane that is parallel to the longitudinal direction of the cylinder and divides the cylinder into two sections, and with two exhaust valves, which are arranged on an exhaust valve side of said imaginary plane.

The characterizing features of the fuel injection arrangement according to the invention are given in claim 1. Another object of the invention is to provide an improved method of operating a piston engine. The characterizing features of the method are given in the other independent claim.

The fuel injection arrangement according to the invention comprises a first fuel injection nozzle comprising a plurality of nozzle holes, a second fuel injection nozzle comprising a plurality of nozzle holes, a first injector needle for control ling fuel injection through the first nozzle, and a second injector needle for con trolling fuel injection through the second nozzle. The total cross-sectional area of the nozzle holes of the second nozzle is greater than the total cross- sectional area of the nozzle holes of the first nozzle, the nozzle holes of the first nozzle are arranged so that a greater part of the fuel injected through the first nozzle is injected towards the exhaust valve side than towards the intake valve side of the cylinder, and the nozzle holes of the second nozzle are ar ranged so that a greater part of the fuel injected through the second nozzle is injected towards the intake valve side than towards the exhaust valve side of the cylinder.

In the method according to the invention, fuel is injected into the cylinder using only the first nozzle when the engine load is below a predetermined limit value and fuel is injected into the cylinder using at least the second nozzle when the engine load is above the predetermined limit value.

Peak temperatures are often experienced in a limited area around the exhaust valves. By means of the fuel injection arrangement and the method according to the invention, thermal loads around the exhaust valves can be reduced. Of ten the thermal loads of the rest of the combustion chamber components are well below their design limits, and therefore relatively small changes in the de sign of the fuel injectors may allow increasing of the output power without a need of redesigning other components. By injecting a major part of the fuel to the exhaust valve side at low loads, exhaust gas temperature can be in creased, which boosts the operation of turbochargers and a selective catalytic reduction system.

According to an embodiment of the invention, the nozzle holes of the first noz zle are configured so that at least 80 percent of the fuel injected through the first nozzle is injected towards the exhaust valve side of the cylinder. According to an embodiment of the invention, the nozzle holes of the second nozzle are configured so that at least 80 percent of the fuel injected through the second nozzle is injected towards the intake valve side of the cylinder.

According to an embodiment of the invention, the nozzle holes of the first noz zle are configured so that at least 10 percent of the fuel injected through the first nozzle is injected towards the intake valve side of the cylinder. According to an embodiment of the invention, the nozzle holes of the second nozzle are configured so that at least 10 percent of the fuel injected through the second nozzle is injected towards the exhaust valve side of the cylinder. By injecting part of the fuel to the other side of the cylinder, structural stresses of the injec tors can be reduced.

According to an embodiment of the invention, a greater number of the nozzle holes of the first nozzle face the exhaust valve side than the intake valve side of the cylinder.

According to an embodiment of the invention, a greater number of the nozzle holes of the second nozzle face the intake valve side than the exhaust valve side of the cylinder.

According to an embodiment of the invention, the first nozzle comprises a plu rality of nozzle holes facing the exhaust valve side of the cylinder and at least one nozzle hole facing the intake valve side of the cylinder, and the cross- sectional area of the nozzle hole facing the intake valve side is smaller than the cross-sectional area of any of the nozzle holes facing the exhaust valve side of the cylinder.

According to an embodiment of the invention, the second nozzle comprises a plurality of nozzle holes facing the intake valve side of the cylinder and at least one nozzle hole facing the exhaust valve side of the cylinder, and the cross- sectional area of the nozzle hole facing the exhaust valve side is smaller than the cross-sectional area of any of the nozzle holes facing the intake valve side of the cylinder.

According to an embodiment of the invention, the second nozzle comprises at least one nozzle hole facing the exhaust valve side and at least one nozzle hole facing the intake valve side of the of the cylinder and the angle between a radial direction of the cylinder and the injection direction of those nozzle holes of the second nozzle that face the exhaust valve side of the cylinder is greater than the angle between the radial direction of the cylinder and the injection di rection of those nozzle holes of the second nozzle that face the intake valve side of the cylinder. On the exhaust valve side of the second nozzle, fuel is thus injected more towards the piston. This helps to further decrease the peak temperatures of a cylinder head.

According to an embodiment of the invention, the first nozzle comprises at least one nozzle hole facing the intake valve side of the cylinder and the sec ond nozzle comprises at least one nozzle hole facing the exhaust valve side of the cylinder and the angle between a radial direction of the cylinder and the in jection direction of those nozzle holes of the second nozzle that face the ex haust valve side of the cylinder is different than the angle between the radial direction of the cylinder and the injection direction of those nozzle holes of the first nozzle that face the intake valve side of the cylinder. This helps to avoid colliding of the fuel jets of the first nozzle and the second nozzle.

According to an embodiment of the invention, the nozzle holes of the first noz zle are configured so that all the fuel injected through the first nozzle is injected towards the exhaust valve side of the cylinder.

According to an embodiment of the invention, the nozzle holes of the second nozzle are configured so that all the fuel injected through the second nozzle is injected towards the intake valve side of the cylinder.

According to an embodiment of the invention, all the nozzle holes of the first nozzle face the exhaust valve side of the cylinder.

According to an embodiment of the invention, all the nozzle holes of the sec ond nozzle face the intake valve side of the cylinder.

According to an embodiment of the invention, the cross-sectional profile of the first nozzle is oval-shaped in a plane that is parallel to the radial direction of the cylinder, the shorter axis of the oval being parallel to the imaginary plane divid ing the cylinder. According to an embodiment of the invention, the cross- sectional profile of the second nozzle is oval-shaped in a plane that is parallel to the radial direction of the cylinder, the shorter axis of the oval being parallel to the imaginary plane dividing the cylinder. The oval-shaped profile reduces the structural stresses of the nozzle. According to an embodiment of the invention, the first nozzle is arranged ec centrically in respect of a fuel injector body. According to an embodiment of the invention, the second nozzle is arranged eccentrically in respect of a fuel injec tor body. By arranging the nozzles eccentrically, the structural strength of the fuel injector can be increased.

According to an embodiment of the invention, the first nozzle is located on the exhaust valve side and the second nozzle is located on the intake valve side of the imaginary plane. The nozzles do not therefore disturb the fuel jets from each other. According to an embodiment of the invention, the imaginary plane divides the cylinder into two halves of equal size.

According to an embodiment of the invention, in the method the fuel is injected into the cylinder using only the second nozzle when the engine load is above the predetermined limit value. According to an embodiment of the invention, the predetermined limit value is in the range of 30-60 percent of the rated output power of the engine. If the first fuel injection nozzle is optimized for part loads, at loads above 60 percent of the rated power of the engine the fuel injection duration becomes too long, which leads to poor combustion and excessive exhaust gas temperature. According to another embodiment of the invention, the predetermined limit val ue is in the range of 50-60 percent of the rated output power of the engine. By using only the first fuel injection nozzle up to loads of 50-60 percent, NOx emissions at part loads can be effectively reduced. In addition, the second fuel injection nozzle can be better optimized for higher loads. According to an embodiment of the invention, the method comprises an oper ating mode, where fuel is injected into the cylinder using both the first nozzle and the second nozzle during a single engine cycle and the injection timings of the first nozzle and the second nozzle are mutually different. With different tim ings, colliding of the fuel jets can be avoided. Brief description of the drawings

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

Fig. 1 shows a cross-sectional view of a fuel injection unit of a piston engine, Fig. 2 shows another cross-sectional view of the fuel injection unit of Fig. 1 ,

Fig. 3 shows the fuel injection unit of Fig. 1 in connection with a cylinder of a piston engine,

Fig. 4 shows a simplified cross-sectional view of a nozzle of a fuel injector,

Fig. 5 shows another cross-sectional view of a nozzle of a fuel injector, Fig. 6 shows a cross-sectional view of another nozzle of a fuel injector,

Fig. 7 A shows a cross-sectional view of still another nozzle of a fuel injector,

Fig. 7B shows an example of fuel injector with an eccentrically arranged noz zle,

Fig. 8 shows an example of a fuel injection pattern of a large nozzle of a fuel injector,

Fig. 9 shows an example of a fuel injection pattern of a small nozzle of a fuel injector,

Fig. 10 shows another example of a fuel injection pattern of a large nozzle of a fuel injector, and Fig. 1 1 shows another example of a fuel injection pattern of a small nozzle of a fuel injector.

Description of embodiments of the invention

Figures 1 and 2 show a fuel injection unit 1 for a large internal combustion en- gine, such as an engine that is used in a ship or at a power plant. The cylinder bore of the engine is at least 150 mm. The engine is a multi-cylinder four- stroke piston engine. The engine can be operated using liquid fuel that is in jected directly into the cylinders of the engine. Each cylinder of the engine is provided with a piston, which is arranged to move in a reciprocating manner within the cylinder.

Each cylinder of the engine is provided with a similar fuel injection unit 1. In figure 3 the fuel injection unit 1 of figures 1 and 2 is shown in connection with a cylinder head 21 of an internal combustion engine. The fuel injection unit 1 of figures 1 to 3 is arranged to receive pressurized fuel directly from a fuel injec tion pump 22. The fuel injection unit 1 comprises an injector part 20, a middle part 30, a control valve 9 and a pneumatic actuator 11. The fuel injection unit 1 is connected to the fuel injection pump 22 by means of a fuel delivery duct 23. Each cylinder 33 of the engine is provided with an own fuel injection pump 22. The fuel injection pump 22 is a high-pressure pump that comprises means for controlling injection timing and the amount of the injected fuel.

The fuel injection unit 1 comprises a body 29, which can consist of several parts. Two fuel injection nozzles 3, 4 are arranged in the injector part 20 of the fuel injection unit 1. The first fuel injection nozzle 3 is intended for injecting small amounts of fuel and the second fuel injection nozzle 4 is intended for in jecting large amounts of fuel into a cylinder 33 of the engine. The same kind of fuel is injected through both fuel injection nozzles 3, 4. Both fuel injection noz zles 3, 4 are configured to inject liquid fuel, such as light fuel oil or heavy fuel oil. The first fuel injection nozzle 3 can be used for instance when the engine load is 50 percent or less of the maximum load of the engine, and the second fuel injection nozzle 4 can be used when the engine load is more than 50 per cent of the maximum load. It is also possible that both nozzles 3, 4 are used at high engine loads. Conventionally, fuel injection nozzles have been optimized for the full load, which has led to higher fuel consumption and smoke formation at part load. Also NOx, CO and hydrocarbon emissions have been higher. With two different fuel injection nozzles 3, 4, each nozzle can be optimized for dif ferent engine loads. It is thus possible to achieve high engine efficiency and low emissions at low loads, while avoiding too long fuel injection durations at higher loads.

A first fuel gallery 31 is arranged inside the body 29 of the fuel injection unit 1 upstream from the first fuel injection nozzle 3, and a second fuel gallery 32 is arranged upstream from the second fuel injection nozzle 4. The fuel galleries 31 , 32 are used for storing the fuel before injection. The fuel injection unit 1 fur ther comprises a first injector needle 7 and a second injector needle 8. The first injector needle 7 opens and closes flow communication between the first fuel gallery 31 and the first fuel injection nozzle 3. The second injector needle 8 opens and closes flow communication between the second fuel gallery 32 and the second fuel injection nozzle 4. The diameter of the first injector needle 7 is smaller than the diameter of the second injector needle 8. Therefore, also the flow area past the first injector needle 7 is smaller than the flow area past the second injector needle 8. The injector needles 7, 8 are pushed towards their closed positions by springs 27, 28. The injector needles 7, 8 are actuated by the pressure of the fuel that is supplied to the fuel injection unit 1 by the fuel in jection pump 22. The fuel injection unit 1 further comprises leakage bores 5 for draining leakages out of the injection unit 1.

For choosing through which of the fuel injection nozzles 3, 4 the fuel is injected into the cylinder 33, the fuel injection unit 1 is provided with a control valve 9. The control valve 9 comprises an inlet port that is in flow communication with a fuel inlet 2 of the fuel injection unit 1. The fuel delivery duct 23 from the fuel in jection pump 22 is connected to the fuel inlet 2 of the fuel injection unit 1. The control valve 9 further comprises two outlet ports. A first outlet port is connect ed to a first fuel duct that connects the control valve 9 to the first fuel gallery 31 , and a second outlet port is connected to a second fuel duct that connects the control valve 9 to the second fuel gallery 32. The control valve 9 comprises a valve member 10 that has at least two different positions. In a first position of the valve member 10, fuel flow from the inlet port through the first outlet port to the first fuel duct is allowed. Fuel flow from the inlet port to the second outlet port is prevented. In a second position of the valve member 10, fuel flow from the inlet port through the second fuel port to the second fuel duct is allowed. Fuel flow from the inlet port to the first outlet port is prevented. The valve member 10 can have also a third position, in which flow both to the first fuel duct and the second fuel duct is allowed. The control valve 9 comprises a spring 17 that pushes the valve member 10 towards the second position. The control valve 9 is thus biased to a position, in which fuel injection through the second fuel injection nozzle 4 is allowed.

Instead of being integrated into the fuel injection unit 1 , the control valve 9 could also be integrated into the fuel injection pump 22. In that case, separate fuel delivery ducts would be needed for the first fuel injection nozzle 3 and the second fuel injection nozzle 4. A first fuel delivery duct from the first outlet port of the control valve 9 would be connected to the first fuel duct of the fuel injec tion unit 1 and a second fuel delivery duct from the second outlet port would be connected to the second fuel duct.

For selecting the position of the control valve 9, the fuel injection unit 1 is pro vided with an actuator. In the embodiment of the figures, the actuator is a pneumatic actuator 1 1. However, also other types of actuators could be used, for instance mechanic, hydraulic or electric actuators. In the embodiment of the figures, the actuator 1 1 comprises a first piston 18 that is arranged in a cylin drical chamber 24. A second piston 12 is arranged inside the first piston 18. The second piston 12 is provided with a rod 19 that is contact with the valve member 10 of the control valve 9. When the pneumatic actuator 1 1 is not acti vated, the spring 17 of the control valve 9 pushes via the valve member 10 of the control valve 9 the rod 19 of the second piston 12 upwards. The second piston 12 pushes also the first piston 18 upwards. A spring 13 is arranged above the first piston 18 for pressing the first piston 18 downwards. The spring 13 keeps the first piston 18 in contact with the second piston 12 and the rod 19 of the second piston 12 in contact with the valve member 10 of the control valve 9 even when the pneumatic actuator 11 is not activated. When pressur ized air is introduced between the first piston 18 and the second piston 12, the second piston 12 is pushed downwards, i.e. towards the control valve 9. The rod 19 of the second piston 12 pushes the valve member 10 of the control valve 9 to the first position. This position is used when the engine is operated at low load, for instance at load that is 50 percent or less of the maximum power of the engine.

When pressurized air is introduced into the chamber 24 above the first piston 18, the first piston 18 is pushed downwards, i.e. towards the control valve 9. The first piston 18 pushes also the second piston 12 downwards, and the rod 19 of the second piston 12 pushes the valve member 10 of the control valve 9 to the third position. The third position can be used occasionally to prevent sticking of the injector needles 7, 8. For instance, when the engine is operated for a long period of time at high load, the control valve 9 can be switched to the third position for a short period of time to allow fuel injection also through the first nozzle 3. This flushing can take place according to a predetermined schedule. Figure 4 shows a simplified cross-sectional view of the second nozzle 4 seen from the direction of the cylinder head 21. Fig. 5 shows a cross-sectional side view of the second nozzle 4 of figure 4. Figure 6 shows a first nozzle 3 accord ing to another embodiment of the invention and figure 7A shows a second nozzle 4 according to another embodiment of the invention.

The second nozzle 4 comprises a plurality of nozzle holes 4a. Preferably, the second nozzle 4 comprises at least four nozzle holes 4a. In the example of fig ure 4, the second nozzle 4 comprises eight nozzle holes 4a. Through the noz zle holes 4a, fuel is injected directly into a combustion chamber of the engine. The nozzle holes 4a are distributed along the perimeter of the second nozzle 4 so that each nozzle hole 4a injects fuel to a different direction seen in a plane that is perpendicular to the axial direction of the cylinder 33. Fuel is injected through the nozzle holes 4a towards the walls of the cylinder 33. Each nozzle hole 4a is arranged at an angle a in respect of a plane 34 that is perpendicular to the axial direction 35 of the cylinder 33. The injection direction of the nozzle holes 4a is tilted towards the piston. Fuel is thus injected away from the cylin der head 21. Angle a between the fuel injection direction and the radial plane 34 does not need to be the same for each nozzle hole 4a, but the nozzle holes 4a may be in different angles in respect of the radial direction of the cylinder 33.

The first nozzle 3 is similar to the second nozzle 4. The shape of the first noz zle 3 can be even identical to the shape of the second nozzle 4. However, the total cross-sectional area of the nozzle holes 3a of the first nozzle 3 is smaller than the total cross-sectional area of the nozzle holes 4a of the second nozzle 4. The second nozzle 4 may comprise more nozzle holes 4a than the first noz zle 3 and/or the nozzle holes 4a of the second nozzle 4 may be bigger than the nozzle holes 3a of the first nozzle 3. The second nozzle 4 is thus configured to inject larger amounts of fuel than the first nozzle 3.

The nozzle holes 3a, 4a of the first nozzle 3 and the second nozzle 4 are not arranged symmetrically in respect of a plane dividing the nozzle 3, 4 in the axi al direction of the cylinder 33. The nozzle holes 4a of the second nozzle 4 are arranged so that a greater part of the fuel injected through the second nozzle 4 is injected towards an intake valve side of the cylinder 33. The nozzle holes 3a of the first nozzle 3 are arranged so that a greater part of the fuel injected through the first nozzle 3 is injected towards an exhaust valve side of the cylin- der 33. The asymmetric spray patterns of the first nozzle 3 and the second nozzle 4 can be achieved in many different ways.

Figures 8-1 1 show examples of the spray patterns of the first nozzle 3 and the second nozzle 4. Jets from the nozzle holes 4a of the second nozzle 4 are il lustrated by forms 36 with dashed lines. Similarly, also the jets from the nozzle holes 3a of the first nozzle 3 are illustrated by forms 37 with dashed lines. In some cases, the first nozzle 3 and the second nozzle 4 can be used simulta neously, but for the sake of clarity, the spray patterns of the first nozzle 3 and the second nozzle 4 are shown in separate figures. Each cylinder 33 of the en gine is provided with two intake valves 15 and two exhaust valves 16. An imag inary plane 27, which is parallel to the axial direction of the cylinder 33, divides the cylinder 33 into two sections 25, 26. The intake valves 15 are located on the intake valve side 25 of the imaginary plane 27 and the exhaust valves 16 are located on the exhaust valve side 26 of the imaginary plane 27. In the em bodiment of the figures, the imaginary plane 27 divides the cylinder 33 into two halves of equal size. However, the imaginary plane 27 does not need to be a center plane of the cylinder 33. For instance, the diameter of the intake valves 15 may be bigger than the diameter of the exhaust valves 16. The intake valve side 25 may thus be bigger than the exhaust valve side 26. The imaginary plane 27 can thus be offset from the center plane of the cylinder 33.

According to the invention, the nozzle holes 4a of the second nozzle 4 are ar ranged so that a greater part of the fuel injected through the second nozzle 4 is injected towards the intake valve side 25 than towards the exhaust valve side 26 of the cylinder 33. Hot exhaust gas is discharged through the exhaust valves 16, whereas cooler intake air is introduced into the cylinder 33 through the intake valves 15. Therefore, the exhaust valve side 26 of the cylinder 33 typically experiences higher temperatures than the intake valve side 25. By in jecting at high engine loads more fuel towards the intake valve side 25 than towards the exhaust valve side 26, temperature differences can be decreased and lower peak temperatures are experienced on the exhaust valve side 26. With relatively simple changes in the construction of the fuel injection system, thermal loads of the components of the engine can thus be reduced without a need for redesigning the cooling system of the engine.

The nozzle holes 3a of the first nozzle 3 are arranged so that a greater part of the fuel injected through the first nozzle 3 is injected towards the exhaust valve side 26 than towards the intake valve side 25 of the cylinder 33. The smaller fuel injection nozzle 3 thus injects more fuel towards the exhaust valves 16. At low engine loads, this increases the exhaust gas temperature, which boosts the operation of turbochargers and selective catalytic reduction systems.

Larger fuel injection amounts of the second nozzle 4 on the intake valve side 25 can be achieved in many different ways. For instance, a greater number of the nozzle holes 4a of the second nozzle 4 can face the intake valve side 25 than the exhaust valve side 26 of the cylinder 33, as shown in figure 8. In the embodiment of figure 8, six fuel jets 36 are directed towards the intake valve side 25. Two fuel jets 36 are directed towards the exhaust valve side 26 of the cylinder 33. Figure 4 shows a corresponding second nozzle 4. Alternatively or in addition to having more nozzle holes 4a on the intake valve side, the nozzle holes 4a facing the intake valve side 25 can be larger than the nozzle holes 4a facing the exhaust valve side 26. Each of those nozzle holes 4a of the second nozzle 4 that face the intake valve side 25 can have a greater cross-sectional area than any of those nozzle holes 4a of the second nozzle 4 that face the exhaust valve side 26 of the cylinder 33. For instance, the individual cross- sectional areas of those nozzle holes 4a of the second nozzle 4 that face the exhaust valve side 26 can be 5-20 percent smaller than the individual cross- sectional areas of those nozzle holes 4a of the second nozzle 4 that face the intake valve side 25 of the cylinder 33.

Also in the case of the first nozzle 3, the different fuel injection amounts on the intake valve side 25 and the exhaust valve side 26 can be achieved in many different ways. For instance, more nozzle holes 3a can face the exhaust valve side 26 than the intake valve side 25 and/or the nozzle holes 3a facing the ex haust valve side 26 can be larger than the nozzle holes 3a facing the intake valve side 25. Figure 9 shows an embodiment, where two nozzle holes 3a of the first nozzle face the intake valve side 25 and six nozzle holes 3a face the exhaust valve side 26. A corresponding first nozzle 3 is similar to the nozzle 4 shown in figure 4.

According to an embodiment of the invention, the nozzle holes 4a of the sec ond nozzle 4 are arranged so that at least 80 percent of the fuel injected through the second nozzle 4 is injected towards the intake valve side 25. Part of the fuel, for instance 10-20 percent, can thus be injected towards the ex- haust valve side 26. By injecting part of the fuel towards the exhaust valve side 26, structural stresses of the second nozzle 4 can be reduced.

According to an embodiment of the invention, the nozzle holes 3a of the first nozzle 3 are arranged so that at least 80 percent of the fuel injected through the first nozzle 3 is injected towards the exhaust valve side 26. Part of the fuel, for instance 10-20 percent, can thus be injected towards the intake valve side 25. By injecting part of the fuel towards the intake valve side 25, structural stresses of the first nozzle 3 can be reduced.

It is also possible that the second nozzle 4 is configured to inject all the fuel towards the intake valve side 25 of the cylinder 33, as shown in figure 10. Simi larly, the first nozzle 3 can be configured to inject all the fuel towards the ex haust valve side 26, as shown in figure 1 1. Figure 6 shows a corresponding nozzle 3. The second nozzle 4 can have a similar configuration.

In the embodiment of figures 8-1 1 , the first fuel injection nozzle 3 is located on the exhaust valve side 26 of the cylinder 33 and the second fuel injection noz zle 4 is located on the intake valve side 25 of the cylinder 33. The nozzles 3, 4 do thus not disturb the fuel jets of each other in the main fuel injection direc tions.

For further decreasing the thermal load around the exhaust valves 16, the fuel jets 36 from the second fuel injection nozzle 4 can be directed more towards the piston on the exhaust valve side 26. The angle a between the radial plane 34 and the injection direction of the nozzle holes 4a can be greater for those nozzle holes 4a that face the exhaust valve side 26 of the cylinder 33 than for those nozzle holes 4a that face the intake valve side 25. The nozzle holes 4a facing the exhaust valve side 26 are thus directed more towards the piston than the nozzle holes 4a facing the intake valve side 25. This reduces further the amount of fuel in the proximity of the exhaust valves 16. The injection di rection refers to the direction of the center axis of a spray injected through a nozzle hole 4a. The spray can be for example conical, and the injection direc tion is the direction of the center axis of the cone. As an example, the angle a between the radial plane 34 and the injection direction of those nozzle holes 4a of the second nozzle 4 that face the exhaust valve side 26 of the cylinder

33 can be at least 2 degrees greater than the angle a between the radial plane

34 and the injection direction of those nozzle holes 4a of the second nozzle 4 that face the intake valve side 25 of the cylinder 33. The difference in the an gles a of the injection directions of the intake valve side 25 and the exhaust valve side 26 can be for example in the range of 2-10 degrees. The suitable difference in the angles depends mainly on the shape of the piston top. Also the position of the nozzle 4 relative to the exhaust valves 16 can be taken into account.

In case the first nozzle 3 is provided with at least one nozzle hole 3a facing the intake valve side 25 and the second nozzle 4 is provided with at least one noz zle facing the exhaust valve side 26, the angle a between a radial direction of the cylinder 33 and the injection direction of those nozzle holes 4a of the sec ond nozzle 4 that face the exhaust valve side 26 can be different than the an gle between the radial direction of the cylinder 33 and the injection direction of those nozzle holes 3a of the first nozzle 3 that face the intake valve side 25 of the cylinder 33. This helps avoiding colliding of the fuel jets from the first noz zle 3 and the second nozzle 4.

Figure 7A shows a cross-sectional view of a second nozzle 4 according to an embodiment of the invention. In the embodiment of figure 7A, the second noz zle 4 is oval-shaped in a radial plane of the cylinder 33. The shorter axis 38 of the oval is parallel to the imaginary plane 27 dividing the cylinder 33 into the in take valve side 25 and the exhaust valve side 26. The longer axis 39 of the oval is perpendicular to the plane 27. The oval-shape reduces structural stress of the nozzle 4. The first nozzle 3 can have a similar shape.

Another way of increasing the structural strength of a fuel injector with an asymmetric spray pattern is shown in figure 7B. In figure 7B, the second noz zle 4 is arranged eccentrically in respect of a fuel injector body 40. The term fuel injector body refers here to the part of the fuel injector above the nozzle 4. The second nozzle 4 is offset from a longitudinal center axis of the fuel injector body 40 towards the main fuel injection direction of the second nozzle 4, i.e. towards the intake valve side of the fuel injector body 40. The wall thickness of the fuel injector body 40 is thus greater on the exhaust valve side of the body 40 than on the intake valve side of the body 40. The first nozzle 3 can be ar ranged in a similar way, but offset towards the exhaust valve side of the fuel in jector body. The eccentric configuration can be made by machining or by add ing material on the desired side of the fuel injector. Each nozzle 3, 4 can also be located eccentrically relative to the longitudinal center axis of the injector needle 7, 8, which controls fuel injection through the nozzle 3, 4.

In a method of operating a piston engine comprising a fuel injection arrange ment described above, fuel is injected into the cylinders 33 using only the first nozzle 3 when the engine load is below a predetermined limit value. When the engine load is above the predetermined limit value, fuel is injected into the cyl inder 33 using at least the second nozzle 4. When the engine load is above the predetermined limit value, fuel can be injected into the cylinder 33 using only the second nozzle 4. Alternatively, both the first nozzle 3 and the second noz zle 4 can be used. The limit value can be in the range of 30-60 percent of the rated output power of the engine. If the first fuel injection nozzle 3 is optimized for part loads, at loads above 60 percent of the rated power of the engine, the fuel injection duration becomes too long, which leads to poor combustion and excessive exhaust gas temperature. The limit value can be in the range of range of 50-60 percent of the rated output power of the engine. By using only the first fuel injection nozzle 3 up to loads of 50-60 percent, NOx emissions at part loads can be effectively reduced. In addition, the second fuel injection nozzle 4 can be better optimized for higher engine loads.

When both the first nozzle 3 and the second nozzle 4 are used, the injection timings of the first nozzle 3 and the second nozzle 4 can be mutually different. Colliding of the fuel jets from the first nozzle 3 and the second nozzle 4 can thus be avoided.

Different variations of the invention are possible. For instance, the first fuel in jection nozzle 3 and the second fuel injection nozzle 4 do not need to be ar ranged in the same fuel injection unit, but each cylinder 33 of the engine can be provided with two separate fuel injectors. The invention can be also applied to an engine with a common rail fuel injection system. Each cylinder 33 of the engine does thus not need to be provided with an own fuel injection pump 22, but one high-pressure pump can supply fuel to fuel injectors of several cylin ders 33. The injector needles 7, 8 can be controlled for example electrically.

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.