Patent Application
20140097275
The present disclosure relates generally to an anti-coking strategy for nozzle spray passages of a fuel injector, and more particularly to electrolessly plated nozzle spray passages with a primarily nickel coating.
The conventional wisdom in reducing certain emissions, such as soot, can be accomplished by employing ever higher injection pressures coupled with smaller diameter nozzle spray orifices. While a number of different strategies exist for boring tiny nozzle passages in injector tip pieces, each strategy has limitations. For instance, so called electrical discharge machining (EDM) strategies reach their limit at a bore size on the order of about 100 micrometers. Smaller EDM bores show too many irregularities. But the conventional wisdom suggests that substantial improvements in reducing soot through better atomization of fuel spray can be achieved with nozzle spray passages on the order of about 50 micrometers. In response to this perceived need, Argonne National Laboratories taught a strategy for reducing an initial bore size of about 200 micrometers down to about 50 micrometers by plating the bore walls with a relatively thick coating of primarily nickel applied through known electroless plating techniques. See Fabrication of Small-Orifice Fuel Injectors for Diesel Engines, ANL Report, March 2005. Although some of the reported Argonne Laboratories results appeared promising, new problems developed as the relatively thick coating required long plating time periods, rendering the strategy difficult to imagine on an industrial scale. In addition, Argonne reported problems associated with removal of certain gaseous bi-products of the plating process from the region being plated.
Apart from finding an effective nozzle bore passage size is the problem of orifice coking over time. In general, the minute quantity of liquid diesel fuel that remains in the nozzle bore after the end of an injection event combined with the high temperatures in the engine cylinder can create the development of coking on the bore wall of a nozzle passage. While some coking development is almost inevitable, each subsequent injection event may effectively flush out the coking products from a previous injection event. However, if even a small quantity of coking material manages to remain adhered to the bore wall, it may also be inevitable that a coking build up will relentlessly occur until the nozzle passage actually becomes blocked, undermining the operation of the entire fuel system. Thus, finding an effective injection pressure strategy combined with an appropriate nozzle orifice geometry that not only reduces soot but inhibits coking build up has remained a persistent problem in the fuel injection art.
The present disclosure is directed toward one or more of the problems set forth above.
In one aspect, a tip piece of a multi-piece fuel injector body includes a unitary steel body with a centerline and an inner surface separated from an outer surface by an annular contact surface. The inner surface defines a nozzle chamber separated from a sac by a needle valve seat. A plurality of bores extend between the sac and the outer surface. Each of the bores has an average diameter defined by a bore wall. A primarily nickel coating is plated to the bore wall to define a spray passage. The coating has an average thickness that is at least one order of magnitude smaller than the average diameter.
In another aspect, a fuel injector includes a multi-piece injector body with a centerline, and includes a tip piece that is a unitary steel body with an inner surface separated from an outer surface by an annular contact surface in contact with another injector body piece. The inner surface defines a nozzle chamber separated from a sac by a needle valve seat. A plurality of bores extend between the sac and the outer surface. Each the bores has an average diameter defined by a bore wall. A primarily nickel coating is plated to the bore wall to define a spray passage, and the coating has an average thickness that is at least one order of magnitude smaller than the average diameter. A needle valve member is positioned in the injector body and is movable between a closed position in contact with the needle valve seat to block the nozzle chamber to the spray passages, and an open position out of contact with the needle valve seat to fluidly connect the nozzle chamber to the spray passages.
In still another aspect, a method of making a fuel injector includes forming a unitary body of steel to include an inner surface separated from an outer surface by an annular contact surface, with the inner surface defining a nozzle chamber separated from a sac by a needle valve seat. A plurality of bores are electrical discharge machined between the outer surface and the sac, and each of the bores has an average diameter defined by a bore wall. A primarily nickel coating is electrolessly plated to the bore wall with an average thickness that is at least one order of magnitude smaller than the average diameter.
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Recognizing that injection pressures are likely to continue rising, the present disclosure seeks to leverage at least two insights to simultaneously improve or at least maintain good spray atomization to reduce soot, while also addressing coking build up problems that appear to have accompanied current day increased injection pressures. These goals are addressed by maintaining a relatively square transition contour from the end of the spray passage to the other surface of the fuel injector tip piece, producing a smaller diameter spray passage than a conventional EDM strategy, and producing a spray passage wall chemistry and smoothness that inhibits coking build up after the fuel injector is in service. Finally, the present disclosure seeks to accomplish these improvements without adding the new gaseous discharge and other problems recognized by Argonne as a result of long plating periods to produce the rather thick coating of its spray passages.
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Because the present disclosure teaches a relatively thin coating of primarily nickel 50, the transition contour 51 from the spray passage 39 to the outer surface 31 can be magnitudinally square relative to average diameter D. As used in this disclosure, the phrase “magnitudinally square” means that the transition contour 51 has an average radius that is at least one order of magnitude smaller than the average diameter D. Those skilled in the art will appreciate that the primarily nickel coating 50 may include one or more other substances in addition to nickel. However, primarily nickel coating means that a majority of the material present in coating 50 is nickel. Other substances that may be utilized include, but are not limited to, phosphorus, cobalt and maybe even PTFE. These secondary substances may be chosen to make the surface of spray passage 39 more chemically inert to the adherence of coking products and may be chosen for their ability to produce a smoother contour that defines spray passage 39. These added substances may be co-plated with the electroless nickel at any concentration as would be deemed appropriate to one with ordinary skill in the art. The present disclosure suggests that a combination of chemical inertness and smoothness in the spray passage 39 can inhibit coking products from taking hold and then building up thereafter to potentially block a spray passage. The present disclosure recognizes that the chemical changes in residual fuel left in spray passage 39 between injection events and subjected to the heat of an engine cylinder may inherently produce some carbonizing or coking products. However, by presenting a more chemically inert and smoother spray passage surface, these inevitable coking products may be flushed out of spray passages 39 with each subsequent injection event.
The present disclosure finds potential applicability in any fuel injector. The present disclosure finds particular applicability with regard to tip pieces for use in fuel injectors that inject diesel fuel into compression ignition engines. The present disclosure might also find potential applicability to any fuel injector where there may be a desire to improve at least one of spray atomization to potentially reduce soot and prevent or inhibit coking build up that can occur after the fuel injector is put into service.
The present disclosure teaches a method of making a fuel injector that includes forming a unitary steel body 15 to include an inner surface 30 separated from an outer surface 31 by a contact surface 32. The inner surface 30 is formed to define a nozzle chamber 35 separated from a sac 36 by a needle valve seat 37. A plurality of bores 40 are electrical discharge machined between the outer surface 31 and the sac 36. Each of the bores 40 has an average diameter D defined by a bore wall 41. A primarily nickel coating 50 is electrolessly plated to the bore wall 41 with an average thickness T that is at least one order of magnitude smaller than the average diameter D. Although not necessary, any sharp peaks that are left by the electrical discharge machine process may be abraded by passing an abrasive slurry through the bores before the plating step. In addition, although not necessary, the primarily nickel coating 50 may be heat treated using a known techniques to strengthen or otherwise improve some characteristic of the primarily nickel coating 50 after the plating step. Finally, a multi-piece injector body 11 is assembled by contacting the annular contact surface 32 with another injector body piece 13. Next, a needle valve member 18 is positioned in the nozzle chamber 35 in contact with the needle valve seat 37.
The present disclosure recognizes that improved performance may be obtained in reducing soot production by utilizing higher injection pressures, slightly smaller spray passage diameters due to the plating thickness and a magnitudinally square transition contour 51 from the spray passage 39 to the outer surface 31 of tip piece 12. The chemical inertness and/or smoothness provided by the primarily nickel coating 50 is believed to inhibit adherence and build up of coking and carbonizing molecules on the spray passage surface between injection events. This may allow the spray passage to remain reliably open with a substantially unchanged spray configuration over the working life of a given fuel injector.
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.
