Abstract:
A heated fuel injector for supplying fuel to a fuel consuming device includes a fuel inlet for receiving fuel, a fuel outlet for dispensing fuel from the fuel injector, and a fuel injector body extending along an axis and fluidly connecting the fuel inlet to the fuel outlet such that fuel flows within the injector body. A cylindrical heating element radially surrounds the fuel injector body and operates to heat fuel flowing through the fuel injector body. An annular space is defined between the heating element and the fuel injector body sufficiently large to accommodate thermally caused radial differential expansion between the fuel injector body and the heating element. A conductive material fills the annular space and has a melting point sufficiently low to be a liquid as the heating element operates to thereby substantially prevent transfer of mechanical stress to the heating element due to the radial differential expansion.

Description:
TECHNICAL FIELD OF INVENTION 
     The present invention relates to fuel injectors for supplying fuel to a combustion chamber of an internal combustion engine; more particularly to such a fuel injector which is heated to elevate the temperature of the fuel; and even more particularly to such a fuel injector which uses a ceramic heating element formed as a hollow cylinder to heat the fuel injector. 
     BACKGROUND OF INVENTION 
     Fuel-injected internal combustion engines fueled by liquid fuels, such as gasoline, diesel, and by alcohols, in part or in whole, such as ethanol, methanol, and the like, are well known. Internal combustion engines typically produce power by controllably combusting a compressed fuel/air mixture in a combustion cylinder. For spark-ignited engines, both fuel and air first enter the cylinder where an ignition source, such as a spark plug, ignites the fuel/air charge, typically just before the piston in the cylinder reaches top-dead-center of its compression stroke. In a spark-ignited engine fueled by gasoline, ignition of the fuel/air charge readily occurs except at extremely low temperatures because of the relatively low flash point of gasoline. (The term “flash point” of a fuel is defined herein as the lowest temperature at which the fuel can form an ignitable mixture in air). However, in a spark-ignited engine fueled by alcohols such as ethanol, or mixtures of ethanol and gasoline having a much higher flash point, ignition of the fuel/air charge may not occur at all under cooler climate conditions. For example, ethanol has a flashpoint of about 12.8° C. Thus, starting a spark-ignited engine fueled by ethanol can be difficult or impossible under cold ambient temperature conditions experienced seasonally in many parts of the world. The problem is further exacerbated by the presence of water in such mixtures, as ethanol typically distills as a 95/5% ethanol/water azeotrope. 
     In order to enhance the cold starting capabilities of such spark-ignited engines fueled by ethanol or other blends of alcohol, it has been proposed to provide a fuel injector of the engine with a heating element which is used to elevate the temperature of the fuel that passes through the fuel injector in route to a combustion chamber of the engine where the fuel is ignited. One heating element arrangement that has been proposed is a thick-film heater that is applied directly to the outside surface of a fuel injector body of the fuel injector. The thick-film heater may be applied to the outside surface of the fuel injector body, for example, by applying an insulating dielectric layer to the outside surface of the fuel injector body, applying two electrically conductive terminals to the insulating dielectric layer, then applying a conductive resistance top layer over the insulating dielectric layer and the two terminals. When electrical power is applied to the two terminals, current flows through the conductive resistance top layer which heats up. The generated heat passes through the fuel injector body and heats the fuel that is located within the fuel injector body. However, the thick-film heater must be controlled in order prevent over-heating. The thick-film heater may be controlled by an engine control module or a stand-alone controller, for example, by open-loop or closed-loop methods. While this thick-film heater arrangement may be effective, the need to control the think film heater may add cost and complexity to the system. 
     Another heating element arrangement that has been proposed is a positive temperature coefficient (PTC) ceramic heating element that is positioned around the fuel injector body of the fuel injector. When electric power is applied to the PTC ceramic heating element it elevates in temperature and the resistance of the PTC ceramic heating element increases exponentially when its temperature exceeds a threshold temperature T REF . This increase in resistance reduces the electric current that is allowed to pass through the PTC ceramic heating element, thereby allowing the PTC ceramic heating element to cool below T REF  which allows the current to increase and again raise the temperature of the PTC ceramic heating element. This process repeats itself as long as the electric power is applied to the PTC ceramic heating element. In this way, the temperature of the PTC ceramic heating element is self-regulating, for example to a temperature range of about ±5° C. and the cost and complexity of controlling the temperature used in the previously described thick-film heater arrangement is avoided. The self-regulating temperature occurs at the Curie temperature of the PTC ceramic heating element. The Curie temperature of the PTC ceramic heating element is the temperature at which a phase change in the structure occurs, thereby changing from more crystalline structure to a more amorphous structure. This change in phase is responsible for the increase in electrical resistance of the PTC ceramic heating element and is characterized by significant mechanical dimension changes measured as the coefficient of thermal expansion (CTE). The CTE of the PTC ceramic heating element is typically greatest above the Curie temperature. 
     Japanese patent application publication number JP 2003-13822A describes a fuel injector with one arrangement for a ceramic heating element which is formed as a hollow cylinder and press fit closely over the metal fuel injector body. The close press fit of the cylindrical ceramic heating element over the fuel injector body mechanically stresses the ceramic heating element when the metal body that it surrounds expands preferentially with rising temperature, which may cause the ceramic heating element to crack. Providing a sufficiently wide annular clearance between the ceramic element and the fuel injector body that it surrounds to accommodate the differential thermal expansion severely reduces the thermal conductivity, as does any dead air space. Adding known thermally conductive materials in the annular space, such as solder or conductive adhesives, improves conductivity, but effectively reintroduces the effect of a close press-fit. 
     U.S. Pat. No. 6,578,775 to Hakao describes a fuel injector with another arrangement for a ceramic heating element, obviously a response to the problems outlined above. Hakao describes a pair of arc-shaped ceramic heating elements that are pressed onto the outer periphery of the fuel injector body by a resilient clip or heater holder. By, in effect, pre-breaking the cylindrical ceramic piece into a pair of arc-shaped ceramic heating elements, the risk of cracking the ceramic heating elements present in JP 2003-13822A as described earlier is mitigated. However, the effectiveness of the ceramic heater arrangement of Hakao is reduced because the entire perimeter of the fuel injector body is not heated and the complexity of the heating arrangement is increased by the additional electrical terminals that are needed in order to apply electric power to each ceramic heating element, as well as the resilient press-fit mechanism. 
     What is needed is a heated fuel injector which minimizes or eliminates one or more of the shortcomings as set forth above. 
     SUMMARY OF THE INVENTION 
     Briefly described, a heated fuel injector is provided for supplying fuel to a fuel consuming device. The heated fuel injector includes a fuel inlet for receiving fuel, a fuel outlet for dispensing fuel from the fuel injector, and a fuel injector body extending along an axis and fluidly connecting the fuel inlet to the fuel outlet such that fuel flows within the injector body. A cylindrical heating element radially surrounds the fuel injector body and operates to heat fuel flowing through the fuel injector body over a range spanning a colder temperature to a hotter temperature. An annular space is defined between the heating element and the fuel injector body sufficiently large to accommodate thermally caused radial differential expansion between the fuel injector body and the heating element. A conductive but compliant material fills the annular space and has a melting point sufficiently low to be a liquid as the heating element operates to thereby substantially prevent transfer of mechanical stress to the heating element due to the radial differential expansion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       This invention will be further described with reference to the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a fuel injector in accordance with the present invention; 
         FIG. 2  is an enlarged portion of the fuel injector of  FIG. 1 ; and 
         FIG. 3  is an isometric view of a resistive heating element of the fuel injector of  FIGS. 1 and 2 . 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The examples set out herein illustrate various possible embodiments of the invention, including one preferred embodiment, but should not to be construed to limit the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF INVENTION 
     Referring to  FIG. 1  a cross-sectional view of a fuel injector  10  is shown in accordance with the present invention for controlling delivery of fuel from a fuel source (not shown) to a fuel consuming device (not shown), for example, a combustion chamber of an internal combustion engine. Fuel injector  10  is provided with a fuel inlet  12  for introducing fuel from the fuel source into fuel injector  10 . Fuel injector  10  is also provided with a fuel outlet  14  for dispensing fuel from fuel injector  10  to the fuel consuming device. A fuel injector body  16  of fuel injector  10  defines at least in part a flow path from fuel inlet  12  to fuel outlet  14  and extends along a fuel injector axis A. Fuel injector body  16  is preferably a metallic material, for example, stainless steel. A valve assembly which is coaxial to fuel injector body  16  includes a pintle shaft  18  and a valve  20 . Valve  20  is attached to an end of pintle shaft  18  facing toward fuel outlet  14  for selectively sealing against a valve seat  22 . At least a portion of pintle shaft  18  may be hollow as shown. Therefore, fuel may enter fuel injector body  16  from fuel inlet  12  through cross-holes  24  in pintle shaft  18 . The valve assembly is positioned within fuel injector body  16  such that a reciprocating axial movement of pintle shaft  18  is enabled by actuation of a solenoid  26 . Pintle shaft  18  is moved axially toward solenoid  26  when an electric current is applied to solenoid  26 , thereby lifting valve  20  from valve seat  22  and allowing fuel to flow from fuel inlet  12  to fuel outlet  14 . Conversely, a return spring  28  urges pintle shaft  18  axially away from solenoid  26  until valve  20  seals against valve seat  22  when no electric current is applied to solenoid  26 , thereby stopping the flow of fuel from fuel inlet  12  to fuel outlet  14 . 
     With continued reference to  FIG. 1  and with additional reference to  FIGS. 2 and 3 , a resistive heating element  30  is provided in order to heat fuel within fuel injector body  16 . Resistive heating element  30  is a hollow cylinder sized to provide an annular space radially between fuel injector body  16  and resistive heating element  30 . The annular space may have a radial dimension, for example only, of about 0.2 mm to about 1.0 mm., but in any event should be sufficient to accommodate differential thermal expansion between the fuel injector body  16  and the resistive heating element  30 , and thereby prevent a preferentially expanding fuel injector body  16  from pressing out against and stressing the heating element  30 . Resistive heating element  30  includes a first electrical terminal  32  in electrical communication with an inside surface of resistive heating element  30  and a second electrical terminal  34  in electrical communication with an outside surface of resistive heating element  30 . Resistive heating element  30  may be made of a ceramic PTC material which is self-regulating to a predetermined temperature, for example about 120° C., such that when first electrical terminal  32  and second electrical terminal  34  are connected to an electric power source (not shown) and an electric current is supplied thereto, resistive heating element  30  is heated to the predetermined temperature. A plastic overmold  36  is formed over fuel injector body  16 , solenoid  26 , resistive heating element  30 , and other components of fuel injector  10  to form the exterior shell of fuel injector  10 . Overmold  36  may be formed by injecting a liquid plastic material into a mold (not shown) containing fuel injector body  16 , solenoid  26 , resistive heating element  30 , and other components of fuel injector  10 . The liquid plastic material is allowed to cool and solidify before being removed from the mold. 
     In order to effectively transfer heat from resistive heating element  30  to the fuel within fuel injector body  16 , the annular space between fuel injector body  16  and resistive heating element  30  is occupied by a substantially compliant and high thermal conductivity material, which may be a metallic material specifically illustrated as a solder  38 . A suitable solder  38  fills and spans the annular space from the inside circumference of resistive heating element  30  to the outside circumference of fuel injector body  16 , but may not totally fill the entire axial extent of the annular space under all operational circumstances. In this way, heat produced by resistive heating element  30  is efficiently transferred to fuel within fuel injector body  16  by conduction through solder  38  and fuel injector body  16 . 
     Since fuel injector body  16  is made of a metallic material, fuel injector body  16  may expand at a greater rate than resistive heating element  30  which is made of a ceramic material when resistive heating element  30  is activated because metallic materials typically have a higher coefficient of thermal expansion than ceramic materials. Consequently, fuel injector body  16  may expand radially outward toward resistive heating element  30  when fuel injector body  16  and resistive heating element  30  are raised in temperature. In order to allow fuel injector body  16  to expand radially outward toward resistive heating element  30  without applying a radial outward force to resistive heating element  30 , solder  38  is selected to have a melting point sufficiently low to melt sufficiently soon in the heating process to liquefy before substantial differential expansion occurs. The melting point of solder  38  is below the Curie point of resistive heating element  30  and preferably below 100° C., more preferably below 50° C., even more preferably below 25° C., and still even more preferably below 10° C. Solder  38  may be, for example only, Indalloy® 46L available from Indium Corporation® which is composed of by mass percentage 61.0% Ga, 25.0% In, 13.0% Sn and 1.0% Zn and has a melting point of about 7° C. The low melting point of solder  38  allows solder  38  to change to a liquid at a low temperature, thereby allowing fuel injector body  16  to expand radially outward toward resistive heating element  30  as the temperature of fuel injector body  16  increases freely, pushing the liquefied solder  38  axially upwardly, but not pushing the heating element  30  radially outwardly. In this way, solder  38  continually remains in direct thermal contact with both fuel injector body  16  and resistive heating element  30  over the operating range of fuel injector  10  without placing substantial stress on resistive heating element  30 . 
     The cold temperature volume of solder  38  is chosen so as to leave some axial space between its top edge and the top edge of heating element  30 . When solder  38  is in liquid form and fuel injector body  16  expands radially outward toward resistive heating element  30 , both the squeezing action and the heat expansion of the solder  38  may cause the column of solder  38  in liquid form to rise. Accordingly, an annular expansion volume  40  is provided above the axially upper boundary of solder  38 , to accommodate that expansion and rise. Expansion volume  40  may be vented to the atmosphere through a vent passage  42  (illustrated as phantom lines) in overmold  36  in order to prevent expansion volume  40  from being over pressurized. It should be noted, however, that this process may reverse itself somewhat as the ceramic heating element  30  reaches its Currie temperature, where it may begin to expand radially away from the injector body  16 . In that case, the column of solder  38  can sink back down, remaining compliant and conductive, and depressurizing the space  40 . In each particular case, empirical testing can find the right initial fill of solder  38  that will accommodate the entire heating process. 
     Solder  38  may be applied to the annular space between fuel injector body  16  and resistive heating element  30  during manufacture of fuel injector  10  by various methods. In one method, solder  38  may be applied as a solder paste to either the outer perimeter of fuel injector body  16  or the inner perimeter of resistive heating element  30  prior to resistive heating element  30  being positioned to surround fuel injector body  16 . In another method, solder  38  may be flowed as a liquid into the annular space between fuel injector body  16  and resistive heating element  30 . 
     In order to retain solder  38  within the annular space between fuel injector body  16  and resistive heating element  30  during manufacture and to prevent overmold  36  from intruding into the annular space between fuel injector body  16  and resistive heating element  30  when overmold  36  is formed, a lower seal  44  may be positioned at the end of resistive heating element  30  that is proximal to valve seat  22 . Lower seal  44  blocks the lower end of the annular space between fuel injector body  16  and resistive heating element  30 . Lower seal  44  is preferably a resilient and compliant material that is able to flex with the expansion and contraction of fuel injector body  16  and resistive heating element  30 . Lower seal  44  may be, for example only, an adhesive. Similarly, an upper seal  46  may be positioned at the end of resistive heating element  30  that is opposite of lower seal  44 . Upper seal  46  blocks the upper end of the annular space between fuel injector body  16  and resistive heating element  30 . Upper seal  46  is preferably a resilient and compliant material that is able to flex with the expansion and contraction of fuel injector body  16  and resistive heating element  30 . Upper seal  46  may be, for example only, an adhesive. Lower seal  44  and upper seal  46  may also be used to maintain resistive heating element  30  in a coaxial relationship with fuel injector body  16  during manufacturing of fuel injector  10 . 
     In order prevent electrical shorting of first electrical terminal  32  which is in electrical communication with the inside surface of resistive heating element  30 , the portion of first electrical terminal  32  which may come into contact with solder  38  may be covered with a coating  48  to electrically isolate first terminal from solder  38 . Coating  48  may be, for example only, a non-electrically conductive epoxy material. 
     While the high thermal conductivity material within the annular space between fuel injector body  16  and resistive heating element  30  has been illustrated as solder  38 , it should be understood that other metallic and non-metallic materials such as oils or waxes that have a sufficiently low melting point to liquefy within the annular space between fuel injector body  16  and resistive heating element  30  as resistive heating element  30  operates may be used, thereby substantially preventing transfer of mechanical stress to resistive heating element  30  due radial differential expansion between fuel injector body  16  and resistive heating element  30 . 
     While this invention has been described in terms of preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.