Technical field of the invention

The present invention relates to a method for determining the diameter of a nozzle seat of a fuel injector in accordance with claim 1 . The invention also concerns a test arrangement for determining the diameter of a nozzle seat of a fuel injector as defined in the other independent claim.

Background of the invention A fuel injector typically used in large piston engines, such as power plant or ship engines comprises an injector needle, which is pressed against a nozzle seat with a predetermined force. Typically a coil spring is used for keeping the injector needle against the nozzle seat. The fuel to be injected through the fuel injector is introduced into a fuel gallery inside the fuel injector. In the fuel gal- lery, the pressure of the fuel is applied to a shoulder of the injector needle. When the hydraulic force acting on the injector needle overcomes the force of the spring, the injector needle is lifted from the nozzle seat and fuel injection through the fuel injector is allowed. The pressure needed for lifting the injector needle is called nozzle opening pressure. The hydraulic force created by the fuel pressure depends on the area on which the pressure is applied. The area is determined by the difference between the diameters of the shoulder of the injector needle and the diameter of the nozzle seat. The nozzle opening pressure is an important parameter affecting the performance of an engine. Also, any leakages of fuel injectors are harmful to the engine performance. Wear of the injector needle and the nozzle seat can affect the nozzle seat diameter. For verifying the characteristics of both new and used fuel injectors, it is important to be able to accurately determine the nozzle seat diameter.

Different methods for determining the nozzle seat diameter are known. For instance, silicone compounds can be used for molding a part replicating the shape of the inside of a nozzle. The molded part can then be measured for determining the nozzle seat diameter. However, because a soft material needs to be used for making the molded part, only non-contact methods can be used for measuring the part. The part can be measured, for instance, using a profile projector to project a magnified image of the part on a screen, using a microscope with an integrated measurement system or by using a measurement device based on the use of a laser. A problem with these known measurement methods is that the nozzle seat line is often not clearly visible and therefore difficult to detect. Also, the sophisticated measurement devices are expensive. Therefore, there is a need for a simple indirect method for determining the seat line diameter.

Summary of the invention

An object of the present invention is to provide an improved method for determining the diameter of a nozzle seat of a fuel injector that comprises a nozzle body, a nozzle seat arranged in the nozzle body and an injector needle comprising a sealing surface cooperating with the nozzle seat and a pressure sur- face for exerting a hydraulic opening force on the injector needle. The characterizing features of the method according to the invention are given in claim 1 . Another object of the invention is to provide an improved test arrangement for determining the diameter of a nozzle seat of a fuel injector. The characterizing features of the test arrangement are given in the other independent claim. The method according to the invention comprises the steps of pressing the injector needle against the nozzle seat with a predetermined force, applying an increasing pressure to the pressure surface of the injector needle, monitoring the pressure applied to the pressure surface of the injector needle, monitoring the opening moment of the injector needle, and determining the pressure ap- plied to the pressure surface of the injector needle immediately before the opening moment of the injector needle.

The test arrangement according to the invention comprises means for exerting a predetermined force on the injector needle for pressing the injector needle against the nozzle seat, means for applying an increasing pressure to the pressure surface of the injector needle, means for monitoring the pressure applied to the pressure surface of the injector needle, and means for determining the opening moment of the injector needle. With the method and arrangement according to the invention, the nozzle seat diameter can be determined indirectly using relatively simple equipment. The arrangement and method are suitable for different fuel injectors. When the diameter of the pressure surface, the closing force exerted on the injector needle and the pressure needed for opening the injector needle are known, the diameter of the nozzle seat can be calculated.

According to an embodiment of the invention, the injector needle is pressed against the nozzle seat by a helical spring. With a helical spring, the injector needle can be easily preloaded with a suitable force. According to an embodiment of the invention, the pressure applied to the pressure surface of the injector needle is monitored with a piezoresistive pressure sensor. With a piezoresistive pressure sensor, the pressure can be monitored with high-frequency.

According to an embodiment of the invention, the opening moment of the injec- tor needle is determined by monitoring the pressure applied to the pressure surface of the injector needle. No additional means are thus needed for monitoring the opening moment of the injector needle.

According to an embodiment of the invention, the opening moment of the injector needle is determined using a linear displacement sensor. A linear dis- placement sensor can reliably detect lifting of the injector needle and can also be used for gathering additional information on the behavior of the fuel injector. A linear displacement sensor can be used alone or in addition to the pressure monitoring for detecting opening of the injector needle.

According to an embodiment of the invention, the force pressing the injector needle against the nozzle seat is configured to differ at most 20 percent from the closing force applied to the injector needle when the fuel injector is in use. By using a force that is approximately the same as the normal closing force of the fuel injector, the behavior of the fuel injector in the test situation is similar to the behavior when in use and the testing gives reliable results. According to an embodiment of the invention, the test arrangement comprises a pneumo-hydraulic pump for producing the pressure applied to the pressure surface of the injector needle. Brief description of the drawings

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which Fig. 1 shows a nozzle portion of a fuel injector,

Fig. 2 shows a nozzle seat of a fuel injector, and

Fig. 3 shows a test arrangement for determining the diameter of a nozzle seat of a fuel injector.

Description of embodiments of the invention

Figure 1 shows a nozzle portion of a fuel injector of a piston engine. The fuel injector can be used for injecting liquid fuel, such as light fuel oil, marine diesel oil or heavy fuel oil directly into a cylinder of the engine. The fuel injector can be used in a large internal combustion engine, such as a main or an auxiliary engine of a ship or a power plant engine. The fuel injector can be either a common rail fuel injector or a fuel injector used in connection with individual fuel injection pumps. The nozzle portion of the fuel injector comprises a nozzle body 1 . The nozzle body 1 can be an integral part of the body of the fuel injector or a separate part attached to the body of the fuel injector. The nozzle body 1 comprises a plurality of nozzle openings 2, i.e. spray holes, through which the fuel is injected into a cylinder of the engine. The tip of the nozzle portion comprises a sac volume 7. The nozzle openings 2 are arranged in the walls of the sac volume 7.

The fuel injector comprises an injector needle 5, which is used for controlling fuel injection via the fuel injector. The injector needle 5 is movable in a reciprocating manner in the longitudinal direction of the fuel injector. The nozzle portion of the fuel injector comprises a fuel inlet 3. Via the fuel inlet 3, fuel can be introduced into a fuel gallery 4. The fuel gallery 4 surrounds the injector needle 5. The nozzle body 1 comprises a nozzle seat 6. The nozzle seat 6 is a conical surface. The injector needle 5 comprises a sealing surface 8 cooperating with the nozzle seat 6. The injector needle 5 is pressed against the nozzle seat 6 by a spring (not shown). The injector needle 5 further comprises a pressure surface 9. The pressure surface 9 is a shoulder encircling the injector needle 5. The pressure surface 9 is located in the fuel gallery 4. Hydraulic force P acting on the pressure surface 9 pushes the injector needle 5 towards an open posi- tion, i.e. upwards in figure 1 . When the hydraulic force acting on the pressure surface 9 of the injector needle 5 exceeds the closing force of the spring of the fuel injector, the valve needle 5 is lifted and fuel flow into the sac volume 7 and further through the nozzle openings 2 is allowed. The area of the pressure surface 9 depends on diameter D _g , which is the largest diameter of the injector needle 5 exposed to the fuel pressure inside the nozzle portion and diameter D _s , which is the smallest diameter of the injector needle 5 exposed to fuel pressure when the injector needle 5 is in the closed position.

Figure 2 shows in more detail an example of a contact region between a nozzle seat 6 and the sealing surface 8 of an injector needle 5. The line where the sealing surface 8 of the injector needle 5 contacts the nozzle seat 6 is called here effective seat line 10. On the upstream side of the effective seat line 10, i.e. above the effective seat line 10 in figure 2, the fuel pressure acts on the injector needle 5 also when the injector needle 5 is in the closed position. On the downstream side of the effective seat line 10, the fuel pressure does not act on the injector needle 5 when the injector needle 5 is in the closed position. The diameter of the injector needle 5 at the effective seat line 10 is thus diameter D _s . The angle a between the sealing surface 8 of the injector needle 5 and the nozzle seat 6 above the effective seat line 10 is called upstream included seat angle and the angle β between the sealing surface 8 and the nozzle seat 6 be- low the effective seat line 10 is called downstream included seat angle. Wear of the sealing surface 8 and the nozzle seat 6 can affect the contact line between the sealing surface 8 and the nozzle seat 6. The location and diameter of the effective seat line 10 can thus change over time. Typically the effective seat line 10 moves downstream because of wearing of the contact surfaces of the nozzle body 1 and the injector needle 5 and the diameter D _s of the effective seat line 10 decreases. The difference between diameters D _g and D _s thus increases and the opening pressure of the fuel injector drops. Due to manufacturing tolerances, also in new fuel injectors the diameters of effective seat lines 10 can vary. Figure 3 shows a test arrangement for determining the diameter of a nozzle comprises a nozzle holder 20, to which the nozzle body 1 of the fuel injector is attached. The test arrangement comprises a helical spring 13, which is used for loading the injector needle 5 against the nozzle seat 6. The spring 13 is a calibrated spring that exerts a predetermined force on the injector needle 5. Preferably the force exerted on the injector needle 5 is approximately the same as the desired opening force of the fuel injector. The spring 13 could thus be configured to exert on the injector needle 5 a force that differs at most 20 percent from the force that is exerted on the injector needle 5 when the fuel injector is in use. For instance, the force could be equal to hydraulic force that is created by pressure of 200 to 1000 bar in the fuel gallery 4. The test arrangement further comprises a hydraulic pump 14. In the embodiment of figure 3, the hydraulic pump 14 is pneumatically driven. The pump 14 is thus a pneumo- hydraulic pump. The hydraulic pump 14 supplies pressure medium, such as hydraulic fluid from a tank 19 to the nozzle portion of the fuel injector. A fluid supply line 21 connects the hydraulic pump 14 to the nozzle portion of the fuel injector. A valve 16 is arranged between the tank 19 and the hydraulic pump 14 for controlling the flow of the hydraulic fluid to the hydraulic pump 14. A check valve 17 between the hydraulic pump 14 and the nozzle portion of the fuel injector prevents backflow from the nozzle portion to the hydraulic pump 14. Instead of a pneumo-hydraulic pump, some other type of pump could be used for supplying pressure medium to the nozzle portion of the fuel injector. The pressure medium could also be fuel.

Pressurized air for driving the hydraulic pump 14 is supplied to the hydraulic pump 14 from an air source 1 1 . A valve 12 is arranged between the air source 1 1 and the hydraulic pump 14 for opening and closing fluid communication between the air source 1 1 and the hydraulic pump 14. The pressure of the air is controlled by a pressure regulating valve 15 that is arranged between the hydraulic pump 14 and the valve 12. A pressure sensor 18 is arranged between the check valve 17 and the nozzle portion of the fuel injector. The pressure sensor 18 is a piezoresistive pressure sensor, which is capable of measuring the pressure in the fluid supply line 21 with high frequency. The pressure sensor 18 is arranged close to the nozzle portion. The pressure measured by the pressure sensor 18 is thus close to the pressure in the fuel gallery 4 of the nozzle portion of the fuel injector. For testing a nozzle portion of a fuel injector, the nozzle body 1 is attached to the nozzle holder 20. Valves 12, 16 are opened for allowing flow of pressurized air from the air source 1 1 to the hydraulic pump 14 and flow of hydraulic fluid from the tank 19 to the hydraulic pump 14. The pressure regulating valve 15 is used for slowly increasing the pressure produced by the hydraulic pump 14. The pressure in the fluid supply line 21 steadily increases. Also the pressure in the fuel gallery 4 thus increases. The pressure in the fluid supply line 21 is constantly monitored by the pressure sensor 18. The pressure in the fuel gallery 4 raises until the hydraulic force acting on the pressure surface 9 of the injector needle 5 exceeds the force exerted on the injector needle 5 by the spring 13. When the spring force is exceeded, the injector needle 5 is lifted and flow from the fuel gallery into the sac volume 7 and further through the nozzle openings 2 is allowed. The pressure in the fuel gallery 4 thus drops. The decreasing pressure can be detected in the fluid supply line 21 by the pressure sensor 18. The opening pressure of the fuel injector is the pressure in the fluid supply line 21 immediately before the pressure drop. The opening moment of the fuel injector can thus be detected by the pressure sensor. The control of the pressure regulating valve 15 is preferably automated to be able to repeat fuel injector testing always in the same way.

On the basis of the opening pressure P of the injector needle 5, the diameter of the effective seat line 10 can be calculated. The diameter D _g of the pressure surface 9 of the injector needle 5 can be easily measured and the force F _N exerted by the spring 13 on the injector needle 5 is known. The diameter D _s of the seat line 10 can thus be calculated using formula

Instead of the pressure sensor 18 or in addition to it, the opening moment of the injector needle 5 could be determined using a displacement sensor that is arranged to monitor the position of the injector needle 5. With a linear displacement sensor, the exact opening moment of the injector needle 5 can be easily determined. The pressure measured by the pressure sensor 18 at the moment when the injector needle 5 starts moving corresponds to the opening pressure of the fuel injector.

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.