Technical field [001 ] The present invention relates to a fuel injector for an internal combustion piston engine according to the preamble of claim 1 . More particularly, the invention relates to a fuel injector for an internal combustion piston engine comprising an injector body provided with a fuel space in the body, a fuel inlet for admission of pressurized fuel into the fuel space, at least one nozzle orifice through which fuel may be injected into a cylinder of the engine, an injector needle arrangement, a first end of which is arranged in co-operation with the nozzle orifice for opening or closing the orifice, a spring element forcing the injector needle towards a position closing the nozzle orifice, and a piezoelectric actuator arranged to control the position of the injector needle arrangement so as to control opening and closing of a flow communication from the fuel space through the at least one nozzle orifice.

Background art [002] In diesel engines, the fuel is injected as a fine mist from the fuel injector into the cylinder of the engine such that with eddies of air is achieved a good mixture of fuel and combustion air and combustion. Fuel injected by the fuel injector as tiny droplets vaporizes quickly as combustion begins after a short ignition delay. A spring-loaded injector needle is typically used as a shut-off element in the fuel injector. The injector needle is generally guided hydraulically by the pressure of the fuel or other hydraulic fluid.

[003] Due to ever more stringent emission regulations, the emissions created by diesel engines must be decreased. It is, however, desired that the performance of the engine is kept at the same operational level or even improved. One means of achieving these goals is to adjust with a greater accuracy the amount of the fuel injection and the timing and duration of injection during the injection event. The accuracy of adjustment of fuel injection can be improved by guiding the injector needle of the fuel injector by a piezoelectric actuator.

[004] WO 2013160536 A1 is related to a fuel injector for a reciprocating engine. The fuel injector comprises an injector needle for opening and closing a flow communication between a fuel chamber and a nozzle opening, and a movable control piston, a fuel space delimited by the control piston and the injector needle, whereby fuel pressure in the fuel space is arranged to press the injector needle towards a closing position.

[005] EP 1342913 A1 discloses a fuel injector comprising a body with an injection orifice at the end of the body to deliver fuel, a moving needle so as to opening and closing a flow path via the injection orifice, a piezoelectric actuator, and a return spring. Movement amplification between the actuator and the needle is provided with a hydraulic chamber with one end closed by the moving surface of the actuator and the other end closed by a surface attached to the needle. The needle is allowed to move a distance which is greater than a movement of the actuator.

[006] JP 2010223195 A is related to a fuel injection in an internal combustion engine comprising a piezoelectric actuator. The reliability of the piezoelectric actuator is enhanced while improving energy efficiency in a direct-acting type fuel injection valve having a variable displacement magnification which is opened when the piezoelectric actuator is charged. [007] US 2008/0163852 A1 discloses an injector for fuel injection systems of internal combustion engines, in particular, direct-injecting diesel engines. The injector comprises a piezoelectric actuator contained in an injector body and acted in by a first spring mechanism so that it remains in contact with the injector body at one end and with a sleeve-like booster piston on the other end. [008] An object of the invention is to provide a fuel injector for an internal combustion piston engine in which the performance is considerably improved compared to the prior art solutions.

Disclosure of the Invention

[009] An object of the invention is substantially met by a fuel injector for an internal combustion piston engine comprising an injector body provided with a fuel space in the body, a fuel inlet for admission of pressurized fuel into the fuel space, at least one nozzle orifice through which fuel may be injected into a cylinder of the engine, an injector needle arrangement, a first end of which is arranged in co-operation with the nozzle orifice for opening or closing the orifice, a spring element forcing the injector needle towards a position closing the nozzle orifice, and a piezoelectric actuator arranged to control the position of the injector needle arrangement so as to control opening and closing of a flow communication from the fuel space through the at least one nozzle orifice. It is characteristic to the invention that the piezoelectric actuator is in direct connection with a control piston at a first end of the control piston, a second end of which control piston is arranged to extend into a cylinder space arranged into the injector body, the control piston having a first cross sectional area at its second end, a second end of the injector needle arrangement is arranged to extend from the fuel space into the cylinder space towards the control piston, the second end of the injector needle arrangement having a second cross sectional area, being smaller than the first cross sectional area, the cylinder space is arranged in flow connection with the fuel space via a gap between the second end of the injector needle arrangement extending into the cylinder space and the in- jector body so that the cylinder space is filled with the fuel, and the injector body is provided with a fuel discharge duct which opens into the cylinder space. [0010] This provides a fuel injector for an internal combustion piston engine which performance is considerably improved. The fuel injector is accurate, fast and easy to control. This type of fuel injector is a so-called direct acting. Advantageously, a minor amount of fuel may leak through the gap into the cylinder space from the fuel space and further discharg- es from the fuel injector via the discharge duct. This keeps the gap clean. The discharge duct is in flow communication to a low pressure fuel system, for example, a fuel return duct and a tank. This will prevent additional leakages in the fuel injector and enhances the durability of the fuel injector. More advantageously, the invention provides a very fast injector wherein a small movement of the piezoelectric actuator is transferred to a larger movement of the injector needle arrangement. This is due to the cylinder space which amplifies the movement of the injector needle arrangement. [001 1 ] According to another embodiment of the invention, the fuel space is continuously in flow communication with the discharge duct for discharging fuel from the fuel space via a flow path successively comprising the gap between the second end of the injector needle arrangement and the injector body, the cylinder space, a gap between the second end of control piston and the injector body, and the discharge duct. The fuel is moving via the gaps and thus substantially continuously changing in the fuel space, it keeps clean moving parts of the injector needle arrangement. This is desirable, at least with fuels which are poor in properties i.e. problematic or which are not substantially clean partially because the fuel may not be stuck in the fuel injector. There is no need for additional control oil systems for operating the fuel injector. [0012] According to another embodiment of the invention, the flow communication from the fuel space through the at least one nozzle orifice into the cylinder of the engine is openable by activating the piezoelectric actuator causing a retraction movement of the control piston from the cyl- inder space and thus moving the injector needle arrangement away from the nozzle orifice, wherein the movement of the injector needle arrangement is proportional to the ratio of the first cross sectional area of the control piston and the second cross sectional area of the injector needle arrangement.

[0013] According to a yet another embodiment of the invention, the piezoelectric actuator comprises a stack of piezoelectric elements isolated from contact with fuel in the injector body when in use. This protects the piezoelectric actuator from the fuel. In other words, the fuel space is not in flow communication with the piezoelectric actuator. The piezoelectric actuator is isolated from the discharge duct in addition to the fuel inlet.

[0014] According to an embodiment of the invention, the first end of the control piston is attached to the piezoelectric element of the piezoelectric actuator.

[0015] According to another embodiment of the invention, the spring element is arranged in a spring chamber that is in flow communication with the fuel inlet and the fuel space.

[0016] According to an embodiment of the invention, when the flow communication from the fuel space via the at least one nozzle orifice is arranged to be open, at least a portion of the second end of the injector needle arrangement extends into the cylinder space.

[0017] According to a yet another embodiment of the invention, when the flow communication from the fuel space via the at least one nozzle orifice is arranged to be open, a surface of the second end of the control piston is in contact with a surface of the second end of the injector needle arrangement.

Brief Description of Drawings

[0018] In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which

Figure 1 illustrates a fuel injection system for an internal combustion pis- ton engine according to the first embodiment of the invention, and

Figure 2 illustrates a fuel injection system for an internal combustion piston engine according to another embodiment of the invention.

Detailed Description of Drawings

[0019] Figure 1 depicts schematically a fuel injector 10 for an internal combustion piston engine comprising an injector body 12 provided with a fuel space 14 in the body 12. The fuel space 14 is in connection with a fuel inlet 16 in the body 12 and receives pressurized fuel via the fuel inlet 16 from e.g. a pressure accumulator (not shown). The fuel injector 10 is provided with at least one nozzle orifice 18 at and end of the body 12 through which fuel may be injected into a cylinder of the engine. In Fig. 1 , there are schematically shown two nozzle orifices 18. In practice, how- ever, the number of nozzle orifices may vary.

[0020] The fuel injector 10 further comprises an injector needle arrangement 22 which is provided with a first end 21 which is arranged in cooperation with the nozzle orifice 18 in the body 12 for opening or closing the orifice 18 depending on the position of the injector needle arrangement 22. The fuel injector 10 comprises a spring element 24 forcing the needle arrangement 22 towards a position closing the nozzle orifice 18, that is when the needle tip is against end of the nozzle. Typically the spring element 24 is a helical spring. Specifically, Fig. 1 illustrates schematically a situation when the flow communication is closed from the fuel space 14 via the nozzle orifice(s) 18. The spring element 24 is arranged into a spring chamber 25 and surrounding a portion of the injector needle arrangement 22. The spring chamber 25 is in direct flow communication with the fuel inlet 16 and with the fuel space 14. There is a clearance between the injector needle arrangement 22 and the injector body via which the spring chamber 25 is in flow communication with the fuel space 14. In addition to the spring element 24, also fuel pressure in the fuel space 14 is arranged to subject a force component in the direction of a longitudinal axis A of the fuel injector 10 to the injector needle arrangement 22. The actual position and movement of the needle arrangement is ruled by co-operation of the spring element and the forces subjected by the fuel pressure to the needle.

[0021 ] There is a piezoelectric actuator 26 arranged to the fuel injection to control the position of the injector needle arrangement 22 so as to control opening and closing of a flow communication from the fuel space 14 through the at least one nozzle orifice 18 into the cylinder 20 of the engine, when being installed into the engine. There is control piston arranged in connection with the piezoelectric actuator 26, which is in direct connection with the control piston 28, more specifically with a first end 29 of the control piston. A second end 27 of the control piston 28 is ar- ranged to extend into a cylinder space 30 arranged into the injector body 12. The piezoelectric actuator 26 is supported into the body so that it may apply force to the control piston 28, for example by arranging the end opposite to the control piston to be supported by the body. The control piston 28 has a first cross sectional area A1 specifically at the sec- ond end 27 thereof. In this embodiment the control piston 28 has a first diameter D1 specifically at the second end 27 thereof which second end 27 is substantially cylinder. It should be, however, noted that the form of the second end 27 of the control piston 28 is not limited only to cylinder. Thus any other shape is possible for the second end 27 of the control piston 28, for instance, a rectangular prism. The control piston 28 as well as the piezoelectric actuator 26 is movable in a direction of the longitudi- nal axis A of the fuel injector 10. Similarly, the injector needle arrangement 22 is movable in the direction of the longitudinal axis A of the fuel injector 10. Obviously, the piezoelectric actuator 26 and the injector needle arrangement 22 work in co-operation but according to the invention the movement of the injector needle arrangement 22 can even be up to five times greater than the movement of the control piston 28 which is moved by the piezoelectric actuator 26..

[0022] A second end 23 of the injector needle arrangement 22 is arranged to extend from the fuel space 14 into the cylinder space 30 towards the control piston 28. The second end 23 of the injector needle arrangement 22 has a second cross sectional area A2 that is smaller than the first cross sectional area A1 . A form of the second end 23 of the injector needle arrangement 22 is cylindrical having a second diameter D2 that is smaller than the first diameter D1 of the control piston 28. In other words, a surface 27' of the second end 27 of the control piston 28 is substantially of greater cross sectional area than a surface 23' of the second end 23 of the injector needle arrangement 22. The control piston 28 is arranged between the piezoelectric actuator 26 and the injector needle arrangement 22. It should be also noted herein that the shape of the second end 23 of the injector needle arrangement 22 is not limited only to cylindrical shapes. Thus any other shape is possible for the second end 23 of the injector needle, for instance, a rectangular prism.

[0023] The cylinder space 30 is arranged in flow connection with the fuel space 14 via a gap 32 between the second end 23 of the injector needle arrangement 22 extending into the cylinder space 30 and the injector body 12 so that the cylinder space 30 is filled with the fuel when in use. The cylinder space 30 may be called as a hydraulic volume because it amplifies a relatively small movement of the piezoelectric actuator 26, and this way also the movement of the control piston, to a larger movement of the injector needle arrangement 22. When the control piston moves a distance x it replaces a volume AV = A1 ^■ x. In this embodiment when the second end 27 of the control piston 28 is cylindrical, thus AV = (π D1 ² /4)- x. The change in volume AV in turn causes the piston arrangement to move a distance z = AV /A2. In this embodiment, when the second end 23 of the injector needle arrangement 22 is cylindrical, z = AV Ι(π D2 ² /4). In other words, the distance z can be written as z = x ^■ (A1 /A2). Thus, the movement of the injector needle arrangement 22 is proportional to the ratio of the first cross sectional area A1 of the control piston 28 and the second cross sectional area A2 of the injector needle arrangement 22. In this embodiment as the distance z can be written as z = x ^■ (D1 /D2) ² . Thus, in this embodiment, the movement of the injector needle arrangement 22 is proportional to a square of the ratio of the first diameter D1 of the control piston 28 and the second diameter D2 of the injector needle arrangement 22. [0024] The injector body is provided with the fuel discharge duct 34 which opens into the cylinder space 30. The fuel space 14 is continuously in flow communication with the discharge duct 34 for discharging fuel from the fuel space 14 via a flow path comprising the gap 32 between the second end 23 of the injector needle arrangement 22 and the injector body 12, the cylinder space 30, a gap 36 between the second end 27 of the control piston 28 and the injector body 12, and the discharge duct 34. Advantageously, due to the continuous flow communication from the fuel space 14 into the discharge duct 34, the fuel is changing in the fuel injector 10 and the fuel injector 10 is kept clean by the fuel. In other words, the fuel introduced into the fuel space 14, at least a portion of fuel is substantially continuously discharged via the discharge duct 34, also, when the flow communication from the fuel space 14 into the cylinder 20 of the engine is closed. The discharge duct 34 is in flow communication to a low pressure fuel system, for example a fuel return duct and a tank, which are not shown in figures. Preferably the discharge duct 34 opens into the cylinder space 30 at a longitudinal location nearer to the first end 29 of the control piston 28 than the second end 27 of the control piston which improves the cleaning effect described above,

[0025] The flow communication from the fuel space 14 through the at least one nozzle orifice 18 into the cylinder 20 of the engine is openable by activating the piezoelectric actuator 26 causing a retraction movement of the control piston 28 from the cylinder space 30 and thus moving the injector needle arrangement 22 away from the nozzle orifice 18, wherein according to an embodiment of the invention, the movement of the injector needle arrangement 22 is proportional to the ratio of the first cross sectional area A1 of the control piston 28 and the second cross sectional area A2 of the injector needle arrangement 22. According to an embodiment of the invention, the movement of the injector needle arrangement 22 is proportional to a square of the ratio of the first diameter D1 of the control piston 28 and the second diameter D2 of the injector needle ar- rangement 22. The piezoelectric actuator 26 and the injector needle arrangement 22 work in co-operation but the movement of the injector needle arrangement 22 is larger than the movement of the piezoelectric actuator 26. [0026] The function of the piezoelectric actuator 26 is based on the piezoelectric phenomenon. The piezoelectric actuator 26 comprises piezoelectric elements 26' which comprise piezoelectric crystals made generally from PZT ceramics. The PZT ceramics comprise lead, zirconium and titanium. The piezoelectric elements 26' are arranged one above another so as to form a so-called a piezoelectric stack. Figure 1 illustrates the piezoelectric actuator 26 that comprises the stack of piezoelectric elements isolated from contact with fuel in the injector body 12 when in use. In other words, the fuel is not allowed to flow into the piezoelectric actuator 26 from the fuel space 14. This means also that the piezoelectric actuator 26 is isolated from the discharge duct 34 and the fuel inlet 16. This protects the piezoelectric actuator 26 and the piezoelectric elements 26' from the fuel. The fuel space 14 is not in flow communication with the piezoelectric actuator 26. The first end 29 of the control piston 28 is attached to the piezoelectric element 26' of the piezoelectric actuator 26.

[0027] A length of the piezoelectric actuator 26 changes in response to an electrical field. In order to initiate the fuel injection into the cylinder 20 of the engine, the piezoelectric actuator 26 is energized. According to one embodiment of the invention, when the piezoelectric actuator 26 is energized, the piezoelectric actuator 26 is compressed or exposed to retraction that is the length of the piezoelectric elements 26' is shortened. Then the needle arrangement 22 is moving away from the closing position and the at least one nozzle orifice 18 towards the control piston 28 so as to open the flow communication from the fuel space 14 into the cylinder 20 of the engine. Figure 2 illustrates schematically a position when the flow communication from the fuel space 14 is open into the cylinder 20 of the engine via the nozzle orifice 18. Specifically, a hydraulic pressure due to the fuel flowing into the fuel space 14 and the cylinder space 30 forces the needle arrangement 22 into the position wherein the flow communication from the fuel space 14 to the cylinder 20 of the engine is open. Specifically, the cylinder space 30 amplifies the movement of the injector needle arrangement 22 towards the control piston 28.

[0028] In Figure 2, it is illustrated that the second end 23 of the injector needle arrangement 22 is not in contact with the second end 27 of the control piston 28 in the cylinder space 30. This way, a small retraction movement of the piezoelectric actuator 26 is transferred to a greater opening movement of the injector needle arrangement 22. Namely, a movement of the piezoelectric actuator 26 can be e.g. five times smaller than a movement of the injector needle arrangement 22. Particularly, at least a portion of the second end 23 of the injector needle arrangement 22 extends into the cylinder space 30 when the flow communication from the fuel space 14 is open into the cylinder 20 of the engine via the at least one nozzle orifice 18. Thus, in some embodiment, the surface 23' of the second end 23 of the injector needle arrangement 22 is not in contact with the surface 27' of the second end 27 of the control piston 28.

[0029] When to the piezoelectric actuator 26 is de-energized, the fuel in- jector needle arrangement 22 is in a closing position and thus the flow communication from the fuel space 14 to the cylinder 20 of the engine is closed as illustrated in Figure 1 . The movement of the injector needle arrangement 22 is very fast due to the fact that the cylinder space 30 is filled with the fuel and amplifies the movement of the injector needle ar- rangement 22 towards the at least one nozzle orifices 18, in Fig. 1 towards the two nozzle orifices 18. Furthermore, when the piezoelectric actuator 26 is de-energized, the length of the piezoelectric actuator grows and the movement of the piezoelectric actuator is towards the injector needle arrangement 22 when expanding. This forces the injector needle arrangement 22 to its closing position.

[0030] Even though, Figure 1 and Figure 2 illustrate an embodiment of the fuel injector 10 having only one control piston 28, according to an embodiment of the invention, it is possible to have more than one control piston element 28 in the fuel injector 10.

[0031 ] While the invention has been described herein by way of examples in connection with what are, at present, considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention, as defined in the appended claims. The details mentioned in connection with any embodi- ment above may be used in connection with another embodiment when such combination is technically feasible.