Patent Application
20110126393
The present disclosure relates generally to strengthening fuel injector nozzle tips, and more specifically a process for blocking nozzle outlets as part of an autofrettage process.
Ever more stringent emissions regulations have driven the compression ignition engine industry to adopt increased fuel injection pressures. One area of concern as a consequence of increased injection pressures relates to potential fatigue in the sac region of the fuel injector nozzle tip component. The sac region is often the thinnest pressure containment metallic layer, and also defines the nozzle outlets that extend between an interior volume of the fuel injector to the combustion space of the engine. The sac region will typically cycle through extreme high pressures with each engine cycle.
One strategy believed to have promise in strengthening fuel system components is to induce compressive residual stress on the inner surface the component. While a number of different strategies are possible for inducing compressive residual stress, an autofrettage process can be effective in inducing compressive residual stress on the interior surfaces of pressure vessels. For instance, Chapter 4 from Adis Basara's PhD. dissertation, Evaluation of High Pressure Components of Fuel Injection Systems Using Speckle Interferometry (2007), teaches sealing one end of a fuel line in order to perform an autofrettage process. Thus, an effective autofrettage process for a fuel injector nozzle tip may require that the nozzle outlets be sealed during the autofrettage pressurization procedure. Because the autofrettage pressures are so high, finding a robust production strategy for nozzle tips in a factory setting can be problematic.
The present disclosure is directed toward one or more of the problems set forth above.
A method of strengthening a nozzle tip includes applying a vacuum to an interior volume of the nozzle tip. Respective plugs are suctioned over each of a plurality of nozzle outlets during the vacuum application step. Each of the plurality of nozzle outlets is then blocked with one of the respective plugs. The nozzle tip is then autofrettaged at least in part by pressurizing the interior volume with autofrettage liquid.
Autofrettage is a means to introduce residual compressive stress to the inside surface of a pressure vessel, and is known to be used in the production of high pressure fuel lines for fuel injection systems. Autofrettaging involves pressurizing the component past the yield strength of the interior material, but below the yield strength for the material closer to the outside diameter of the component. The challenge in such a high pressure hydraulic process is sealing effectively. This disclosure pertains to blocking the injector nozzle outlets for the autofrettage process. It is proposed that microspheres, slightly larger in diameter than the orifices, be sucked onto the opening of each nozzle outlet. These relatively soft microspheres would then be put under mechanical pressure, and possibly deformed, to block the nozzle outlets during the pressurization of the autofrettage process. As used in this disclosure, the term “block” means that fluid flow past the plug is sufficiently low at autofrettage pressures that the autofrettage process in the sac region of the fuel injector leaves satisfactory levels of compressive residual stress.
Referring to
Plugs 26 may be suspended and/or immersed in any suitable fluid including air or possibly autofrettage liquid. The volume of plugs may be “fluidized” to mobilize the plugs to facilitate their motion to cover each nozzle outlet. In one embodiment, plugs 26 are non-magnetic stainless steel microspheres having a diameter that is greater than the diameter of nozzle outlets 14 where the outlets open through exterior surface 16. Those skilled in the art will appreciate that the autofrettage process of the present disclosure may occur after heat treatment of the fuel injector nozzle tip 10 such that the nozzle outlets 14 open through a convex uncontrolled surface that is a portion of exterior surface 16. Although not necessary, plugs 26 may be made of a material softer than the material of the nozzle tip 10 when undergoing the procedure of the present disclosure. By sizing the microsphere plugs 26 to be greater than the diameter of nozzle outlets 14, the plugs may tend to suction over the nozzle outlets 14 when the vacuum is being applied, rather than being actually drawn into the interior of the nozzle outlets 14. It is believed that sizing the microsphere plugs 26 to have a diameter at least 20% greater than the nozzle outlets 14 but less than two times the diameter the diameter of the nozzle outlets at the exterior surface 16 may work suitably well for the procedure of the present disclosure. Those skilled in the art will appreciate that when nozzle tip 10 is being inserted into vessel 25, the nozzle tip 10 may be held and maneuvered in a suitable manner, such as by a production robotic arm, not shown. Thus, when the nozzle tip is inserted into vessel 25, fluid in and around the plugs 26 will cause individual respective plugs to be suctioned over each of the plurality of nozzle outlets 14. Depending upon circumstances, confirming that plugs have been suctioned over each of the nozzle outlets may be accomplished by, for instance, monitoring the flow of fluid into interior volume 12 and confirming that the flow rate must indicate that all of the nozzle outlets are at least partially blocked by a respective plug. Those skilled in the art will appreciate that other alternative confirmation strategies, or none at all, may also be utilized. For instance, by closely monitoring the vacuum pressure within interior volume 12 as the nozzle tip 10 is inserted into vessel 25 one might be able to confirm that all the nozzle outlets are covered.
After a respective plug 30 has been suctioned over each of the plurality of nozzle outlets 14, the nozzle tip 10 may be removed from the vessel 25 to reveal a close up view as shown in
Although there may be a respective plug suctioned over each nozzle outlet 14, the nozzle outlets may not be sealed. In addition, before pressurization for the autofrettage process commences, voids within interior volume 12 are preferably evacuated. One strategy for evacuating voids may be to maneuver the nozzle tip 10 into proximity of a base 41 of an autofrettage fixture 40 so that the individual plugs 30 are trapped between a contact surface 42 and the individual nozzle outlets 14 as shown in
When there is sufficient confidence that the voids within the interior volume 12 of nozzle tip 10 have been evacuated, the nozzle tip 10 may be fully engaged with the autofrettage fixture 40 by utilizing a clamping force 45 to press each respective plug 30 against the outer surface 16 of nozzle tip 10 at each respective nozzle outlet 14 via contact surface 42. Those skilled in the art will appreciate that the base 41 of autofrettage fixture 40 may include a cup shaped cavity that is defined by contact surface 42. Contact surface 42 may have a contour that closely matches the contours of exterior surface 16 in the vicinity of nozzle outlets 14, or may have a frustoconical shape. Each of the nozzle outlets 19 are blocked, but not necessarily sealed, by pressing the respective plugs against the nozzle outlets. When this is done, the individual plugs 30 may deform and block the nozzle outlets 14 as shown in
After the nozzle outlets 14 are blocked and any voids within the interior volume 12 have been evacuated, and after the interior volume is filled with autofrettage liquid, the autofrettage pressurization process is ready to begin. In this case, a hypothetical pressure gauge 47 is included as shown in
Next, the autofrettage pressure is relieved and the nozzle tip 10 is disengaged from base 41 of autofrettage fixture 40 as shown in
Although the autofrettage process has been discussed with regard to strengthening the sac region of the nozzle tip 10, similar compressive residual stress may be provided in the cylindrical bore leading to the sac region during the autofrettage process. In other words, plastic deformation may occur in the interior surface 15 in the cylindrical bore portion while only elastic deformation occurs in the region near the exterior surface 16, resulting in compressive residual stress in the interior portion of the nozzle tip 10. Thus the cylindrical bore portion of the nozzle tip may be strengthened in this pressure sensitive region as well.
The present disclosure finds potential application in strengthening a pressure vessel component via an autofrettage process when the component includes at least one outlet. The disclosure is specifically applicable to those pressure vessels in which the component is not modified or otherwise machined to include a seating surface around the nozzle outlet to better facilitate a conventional ball and conical seat sealing strategy. Finally, the present disclosure is specifically applicable to fuel injector nozzle tips in which a plurality of microscopic nozzle outlets are distributed at different locations and are in need of being sealed for an autofrettage process. In addition, the process of the present disclosure may be generally applicable for factory based mass processing of many nozzle tips for strengthening purposes. The present disclosure is specifically applicable to strengthening nozzle tips that must undergo reliable operation at cyclic or continuous high pressures on the order of 240 MPa or higher. Thus, the process of the present disclosure is particularly applicable to blocking uncontrolled convex surfaces through which microscopic holes open, as is typical in the case of a fuel injector nozzle outlet tip 10.
The process of the present disclosure has the advantage of allowing a nozzle tip autofrettage process without requiring extra machining steps or the like to prepare the outer surface of the nozzle tip for sealing in order to undergo the autofrettage pressurization. While the nozzle outlets could conceivably be blocked by the outer surface of the injector tip being precision ground and then precision tooling being match ground to match the tip precision outer surface, such a strategy would not likely be production robust. The present process may be robust since a relatively blind process can be utilized for plugging and sealing the individual nozzle outlets prior to autofrettage pressurization.
Although the present disclosure contemplates microspheres that are soft relative to the hard material of the nozzle tip, those skilled in the art will appreciate that other shaped plugs are also contemplated. In addition, a lack of spheroidal shapes could be compensated by a more softer plug material. With regard to extra plugs 31 that might adhere to the outer surface of the nozzle tip, those skilled in the art will appreciate that their removal may only be necessary to the extent that they are in a location that could interfere with the engagement between the nozzle tip and the autofrettage fixture in a way that could undermine the sealing ability of the plugs 30 that are at the desired locations over the nozzle outlets 14. In addition, those skilled in the art would appreciate that the brushing or blowing or otherwise removing excess plugs 31 should not be too aggressive so as to possibly dislodge properly positioned plugs 30 that are located covering a nozzle outlet 14. Although the used plugs 30 are shown in the illustrations as being blown or brushed free from the nozzle tip after the autofrettage pressurization, those skilled in the art will appreciate that a pressure differential between the interior volume 12 and the exterior of the nozzle tip 10 could also be exploited to aid in removing the used plugs 30 without departing from the present disclosure.
It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Thus, those skilled in the art will appreciate that other aspects of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims.
