Abstract:
A heated fuel injector includes a heated body, liquid fuel flowing through a fuel passage within the body, and a member that increases heat transfer from the heated body to the fuel within the fuel passage. The thermal efficiency of the fuel injector is increased separately or in combination by diverting the fuel flow along an inner circumferential contour of the heated body, by limiting the volume of fuel bypassing the heated inner surface of the body, by redirecting heat from the body to unheated portions of the fuel flow field within the fuel passage, and by increasing the available contact surface area for heat transfer. Improved heat transfer from the heated body to the fuel is achieved by integrating features that increase the contact surface area into the inside surface of the body or by positioning an insulating or a thermally conductive spacer within the fuel passage.

Description:
TECHNICAL FIELD 
     The present invention relates to internal combustion engines; more particularly, to means for vaporizing liquid fuels; and most particularly, to an apparatus and method for effectively and evenly heating fuel within a fuel injector for consumption by the engine. 
     BACKGROUND OF THE 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 many geographic areas, it is highly desirable to provide some means for enhancing the cold starting capabilities of such spark-ignited engines fueled by ethanol or other blends of alcohol. There are currently several approaches to aid cold starting of such engines in cold ambients. For example, some engines are equipped with an auxiliary gasoline injection system for injecting gasoline into the fuel/air charge in cold ambient conditions. The use of such auxiliary system adds cost to the vehicle and to the operation of the vehicle and may increase the maintenance required for the engine. 
     Another approach to aid starting of spark-ignited engines, in cold ambient conditions, fueled by ethanol or other blends of alcohol is to pre-heat the fuel before being ignited in the combustion chamber. One such method is to provide a heat source, such as a thick film heater element, on the outside surface of a fuel injector body proximate the injector tip to pre-heat the fuel. The key to implementing this method is having sufficient heater power and heater surface area to transfer heat to the fuel. When electric current is passed through the electrically resistive material, heat is exchanged from the injector body to the fuel within the injector. 
     The amount of heat exchanged to the fuel within the injector is directly dependent on the heated surface area contacted by the fuel. Accordingly, it is advantageous to maximize the surface area contacted by the fuel. However, if the surface area of the heater is increased by increasing its diameter, the outside surface of the injector body needs to be increased too, which leads to an increased overall mass of the body. Notwithstanding the weight and size penalty associated therewith, if the overall mass of the body is increased, then the initial time delay to heat the fuel will also increase because the mass of the body has to be heated before its surface will heat the fuel. 
     Also, since a larger diameter fuel injector body causes an increased internal fluid volume, and the fuel itself is a relatively poor heat conductor, the larger volume of fluid does not transfer the heat well from the fluid near the heater surface area to the rest of the fluid. Moreover, the hollow valve assembly of prior art injectors allows fuel to pass through it, preventing the fuel passing through the valve assembly from picking up heat from the walls of the heated injector body. 
     Further, in prior art injectors, the heater is typically applied to the outside surface of the injector body, which is typically made of stainless steel. The heater is further typically overmolded with a plastic material in order to offer environmental protection to the electrical circuit. Stainless steel is known to be a poor heat conductor and, even when using a relatively thin injector body, most of the energy delivered by the heater is transferred to the external plastic overmold. Since the heat diffusivity of ethanol is very low, on the order of about 27 times below the one of stainless steel, this condition is worsened with the use of ethanol fuels. 
     What is needed in the art is a method to overcome the low heat diffusivity of ethanol and to increase the thermal efficiency of a heated fuel injector. 
     It is a principal object of the present invention to increase the area of the heated surface in contact with fuel flowing through the fuel injector to overcome the low heat diffusivity of ethanol fuels. 
     It is a further object of the invention to improve the heat transfer from the heated injector body to the fuel. 
     SUMMARY OF THE INVENTION 
     Briefly described, the thermal efficiency of a heated fuel injector is increased separately or in combination by directing the fuel flow along an inner circumferential contour of a heated injector body, by limiting the volume of fluid bypassing the heated inner surface of the injector body, by redirecting heat from the heated injector body to typically unheated portions of the fuel flow field within the fuel passage of the injector body, and by increasing the available contact surface area for heat transfer. Improved heat transfer from the heated injector body to the fuel flowing through the injector body is realized by integrating surface enlarging features into the inside surface of an injector body or by positioning an insulating spacer or a thermally conductive spacer within the fuel passage of the heated injector body. The thermally insulating spacer functions as a flow diverter and may be combined with an enlarged contact surface area and/or a plug that prevents fuel from flowing through a hollow pintle shaft. The thermally conductive internal spacer functions as a heat exchanger, has a relatively large surface area in contact with fuel flowing through the fuel injector, a relatively small mass, and maintains a tight fit with the internal surface of the heated fuel injector body for optimal heat transfer, which enables the heat to be readily transferred to the thermally conductive spacer. 
     In one aspect of the invention, the thermally insulating spacer is assembled within a heated body of the fuel injector surrounding a valve assembly that is free to move through a center opening of the spacer but without contacting the inner surface of the injector body. The spacer includes diversion slots to direct fuel away from the pintle valve and towards the inner surface of the heated injector body. By taking up some of the internal volume of the injector, the amount of fuel bypassing the heated surface at a time is limited and reduced compared to the fuel flow without an internal spacer and, as a result, the fuel flowing in the space between spacer and heater body is heated more evenly. 
     In addition to the thermally insulating spacer, a plug may be inserted in the hollow valve shaft preventing cold fuel from entering and flowing through the shaft. The combination of the flow diverter and the plug restricts cold fuel from flowing through the valve assembly enabling cold fuel, such as ethanol-fuel, to be heated more effectively within the fuel injector. 
     In another aspect of the invention, the area of the heated surface in contact with the fuel flowing through the injector body is increased by incorporating a variety of features, for example, a single helical channel, multiple helical channels, or an array of projecting pins, into the inside surface of the fuel injector body. The flow vortex created by these features during fuel flow increases the heat transfer to the fuel. Additionally, the increased surface area increases heat transferred from the heater to the fuel. The heated surface enlarging features may also be formed as a separate insert that is assembled into the injector body during injector manufacture. The features may be made of a heat conductive material, such as copper, aluminum, nickel, or other material compatible with the fuel used and suitable for efficient manufacturing. The enlarged internal surface area of the injector body may be used in conjunction with the non-conductive spacer as described above. 
     In still another aspect of the invention, the spacer may be made of a thermally conductive material and designed to contact the inner surface of the heated injector body in a thermally conductive manner to increase the thermal efficiency of the heated fuel injector. The thermally conductive internal spacer redirects heat energy from the heated injector body to otherwise unheated portions of the fuel flow field of the injector. Different materials optimized for the injector body and for the thermally conductive internal spacer can be used. In this manner, the spacer may be formed of a thermally conductive material, such as copper, while the body may be formed of stainless steel for structural purposes. 
     It may still further be possible to design the thermally conductive spacer to fill the space between the valve shaft and the inner surface of the heated injector body completely and to manufacture the spacer from a porous metal, such as open cell foam. The porous material permits the flow of fuel through it and increases the contact surface area for optimal heat transfer. 
     Furthermore, the thermally conductive spacer may be a ribbon fin heat exchanger positioned within the fuel passage of the fuel injector for transferring the heat from the heated injector body to the fuel. The ribbon fins may be formed, for example, from thin metal sheeting. This thin metal may be formed into a multitude of shapes to maximize the surface area and to optimize fuel flow. The outside of the ribbon fin may be formed into a cylinder and fixed in a thermally conductive manner, for example by brazing, to the inner surface of the injector body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawing, in which: 
         FIG. 1  is a cross-sectional view of a fuel injector with a thermally non-conductive spacer assembled, in accordance with a first embodiment of the invention; 
         FIG. 2  is an exploded view of the fuel injector shown in  FIG. 1 ; 
         FIG. 3A  is an isometric view of the thermally non-conductive spacer, in accordance with the first embodiment of the invention; 
         FIG. 3B  is a cross-sectional view of the thermally non-conductive spacer shown in  FIG. 3A ; 
         FIG. 4A  is a cross-sectional view of a fuel injector including a body having a single helical channel, in accordance with a second embodiment of the invention; 
         FIG. 4B  is an isometric cross-sectional view of a single helical channel fuel injector body, in accordance with the second embodiment of the invention; 
         FIG. 5A  is a cross-sectional view of a fuel injector including a body having multiple helical channels, in accordance with the second embodiment of the invention; 
         FIG. 5B  is a cross-sectional view of a fuel injector body including multiple helical channels, in accordance with the second embodiment of the invention; 
         FIG. 6A  is a cross-sectional view of a fuel injector including a body having an array of pins, in accordance with the second embodiment of the invention; 
         FIG. 6B  is an isometric cross-sectional view of a fuel injector body including an array of pins, in accordance with the second embodiment of the invention; 
         FIG. 7  is a cross-sectional view of a fuel injector including a thermally conductive porous metal spacer, in accordance with the third embodiment of the invention; 
         FIG. 8  is a cross-sectional view of the thermally conductive porous metal spacer shown in  FIG. 7 ; 
         FIG. 9  is a view of another thermally conductive spacer, in accordance with a third embodiment of the invention; 
         FIG. 10  is an exploded view of a heated fuel injector body-ribbon fin heat exchanger assembly, in accordance with the third embodiment of the present invention; and 
         FIG. 11  is an isometric cross-sectional top view of the heated fuel injector body-ribbon fin heat exchanger assembly, in accordance with the third embodiment of the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate various possible embodiments of the invention, including one preferred embodiment in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 and 2 , a fuel injector  100  includes a spacer  120  having a low thermal conductivity, assembled within a body  108  of injector  100  in accordance with a first embodiment of the invention. Injector  100  may be a fuel injector for port injection as illustrated or a fuel injector for direct injection of fuel. The fuel flowing through fuel injector  100  from a fuel inlet  104  to a fuel outlet  106  may be any type of liquid fuel, for example, an ethanol based fuel, gasoline, or diesel. 
     Body  108  of fuel injector  100  has a heater element  110  applied to an outside surface  142  of the body for transferring heat to the body by the heater element. Heater element  110  may be, for example, a thick film heater printed on the outside surface  142  of body  108 . An overmold  112  or other type of protection covers body  108  and heater element  110 . Fuel passage  102  is defined by an inside surface  144  of body  108  and an outside surface  119  of spacer  120 . A valve assembly includes pintle shaft  114  and a valve  116 . Valve  116  is attached to an end of pintle shaft  114  facing fuel outlet  106  for sealing against a valve seat  118 . At least a portion of pintle shaft  114  may be hollow as shown in  FIGS. 1 and 2 . Therefore, fuel may enter passage  102  from fuel inlet  104  through cross-hole  115  in pintle shaft  114 . The valve assembly is positioned within body  108  such that a reciprocating axial movement of pintle shaft  114  is enabled by actuation of solenoid  121 , as known in the art. 
     Low thermal conductivity spacer  120 , shown in detail in  FIGS. 3A and 3B , has a generally cylindrical shape and extends axially for a length  122 . Spacer  120  includes an axially extending center hole  124  defined by an inner diameter  126 . Spacer  120  further includes an outer diameter  128 , at least one slot  130  at an upper end which faces fuel inlet  104 , and at least one slot  131  at a lower end, which faces fuel outlet  106 . Slots  130  and  131  divert fuel flow towards outer diameter  128  and heated body  108 , then toward seat  118 , respectively. Preferably, as shown in  FIGS. 2 and 3 , slots  130  and  131  are equally distributed along the circumferential contour at each end. One or more slots  130  positioned at the upper end of spacer  120  are positioned such that fuel exiting through cross-hole  115  is directed to flow into fuel passage  102  between inside surface  144  of body  108  and outer diameter  128  of spacer  120 . One or more slots  131  positioned at the lower end of spacer  120  direct the heated fuel toward valve  116  and valve seat  118  and through fuel outlet  106 . Spacer  120  is preferably formed from a material that has relatively low heat conductibility such as, for example, a phenolic resin, to limit heat transfer from the heated fuel to spacer  120 . 
     A plug  132  may be disposed in pintle shaft  114  downstream of and preferably in close proximity to cross-hole  115 . The combination of plug  132  and spacer  120  forces a substantial amount of the fuel to come in contact with inside surface  144  of body  108  where it is readily heated. Plug  132  prevents unheated fuel from entering a lower part of the hollow pintle shaft  114  and ensures that substantially all of the fuel flowing through injector  100  is diverted towards inside surface  144  of heated body  108 . Internal space  134  of pintle shaft  114  below plug  132  is sealed by plug  132  and is typically filled with air. 
     Inner diameter  126  of spacer  120  is adapted to allow unrestricted reciprocating axial movement of pintle shaft  114 . Inner diameter  126  is further adapted to allow minimal fuel flow through a clearance between pintle shaft  114  and spacer  120  without causing a significant drag on the moving pintle shaft. Outer diameter  128  of spacer  120  is adapted to provide a narrowed fuel passage  102  between heated body  108  and spacer  120 . By doing so, the fuel volume within heated body  108  is reduced in order to heat the fuel flowing through body  108  more evenly and more effectively. Inner diameter  126  and outer diameter  128  of spacer  120  may be optimized for a specific application depending on parameters such as fuel viscosity and heating characteristics, fuel flow rate, and a desired temperature of the fuel. 
     In operation, fuel enters inlet  104  and flows through cross-hole  115  in pintle shaft  114  where it is directed by slots  130  to flow along fuel passage  102 . The fuel makes contact with inside surface  144  of body  108 , which is heated by heater element  110 , and with surface  119  of spacer  120  which limits the transfer of heat from the heated fuel to pintle valve  114 . Heated fuel then flows though slots  131  toward valve  116  and valve seat  118 . 
     Referring to  FIGS. 4  (A and B) through  6  (A and B), exemplary fuel injectors  200 ,  300  and  400 , and fuel injector bodies  208 ,  308  and  408 , including features such as single threads  246 , multiple helical threads  346  and an array of projecting pins  546  are shown. These features provide an increased heated surface area coming in contact with fuel in accordance with a second embodiment of the invention. By increasing the inner surface area of the heated injector body, the efficiency of heat transfer from the heated body to a fuel having a low heat diffusivity, such as ethanol based fuels, may be increased. 
     Referring to  FIGS. 4A and 4B , a fuel injector  200  including a body  208  having an outside surface  142  and an inside surface  244  is shown. (Note: features identical with those in fuel injector  100  carry the same numbers; features analogous but not identical carry the same numbers but in the  200  series.) A single groove formed as a helical channel  246  is included on inside surface  244  of body  208 . Heater element  110  is applied to outside surface  142  for transferring heat to body  208 . Heat is then transferred from body  208  to the fuel flowing through helical channel  246  of fuel passage  202 . 
     Helical channel  246  may be formed directly within inside surface  244  and, therefore, may be integral with body  208  or may be formed as a separate piece, such as an insert, that is assembled within body  208  in a thermally conductive manner. Single helical channel  246  not only increases the surface area of inside surface  244  of body  208  but also, by narrowing the flow path, creates a flow vortex which increases the amount of heat transferred to the fuel by the heated body. 
     In addition to increasing the surface area of inside surface  244  by single helical channel  246 , spacer  120  of low thermal conductivity may be conjunctively used in body  208  to surround pintle shaft  114 , to limit the transfer of heat from the fuel to the pintle shaft as described above. Plug  132  may also be inserted into pintle shaft  114  to further improve the fuel heat efficiency as described above. 
     Referring to  FIGS. 5A and 5B , a fuel injector  300  having a body  308  that has multiple helical channels  346  included at an inside surface  344  is shown. (Note: features identical with those in fuel injector  100  carry the same numbers; features analogous but not identical carry the same numbers but in the  300  series.) Multiple helical channels may be wound in the same direction as shown or may be wound in opposite directions (not shown). Multiple helical channels  346  may be formed directly on inside surface  344  and, therefore, may be integral with body  308  or may be formed as a separate piece, such as an insert, that is assembled within body  308  in a thermally conductive manner. Multiple helical channels  346  increase the surface area of inside surface  344  of body  308  and create a flow vortex which increases the amount of heat transferred to the fuel by the heated body. In addition to increasing the heated contact surface area by multiple helical channels  246 , spacer  120  of low thermal conductivity may be used to limit the transfer of heat from the fuel to the pintle shaft  114  as described above. Plug  132  may also be inserted into pintle shaft  114  to further improve the fuel heat efficiency as described above. 
     Referring to  FIGS. 6A and 6B , a fuel injector  500  including a body  508  having an array of projecting pins  546  on its inside surface  544  is shown. (Note: features identical with those in first embodiment fuel injector  100  carry the same numbers; features analogous but not identical carry the same numbers but in the  500  series.) Pins  546  may be of varied heights, such as shown, or of identical heights and may be dispersed in any pattern to optimize fuel flow and heat transfer. Pins  546  extend radially from inside surface  544  of body  508  into fuel passage  102  thereby increasing the surface area of inside surface  544  that comes in contact with the fuel. Pins  546  may be formed directly on inside surface  544  and, therefore, may be integral with body  508  or may be formed as a separate piece that is inserted into body  508  in a thermally conductive manner. Pins  546  not only increase the surface area of inside surface  544  of body  508  but also create a flow vortex which increases the amount of heat transferred to the fuel. 
     The features for increasing the heated surface area contacted by the fuel as described above, such as single helical channel  246 , multiple helical channels  346 , and pins  546 , are preferably made of a material having a relatively good heat conductivity, such as, for example, copper, aluminum, nickel, or other materials compatible with the type of fuel used and suitable for efficient manufacturing. 
     Referring to  FIGS. 7 through 11 , spacers  620 ,  720 , and  820  are assembled within a heated body of a fuel injector, such as body  608  of fuel injector  600  as shown in  FIG. 8  in accordance with a third embodiment of the invention. Thermally conductive spacers  620 ,  720 , and  820  are in direct thermal contact with the heated body (see, for example, contact points  749  of feature  748  with body  708  in  FIG. 9 ), and are utilized as heat exchangers conducting heat to the fuel as well. By adapting thermally conductive spacers  620 ,  720 , and  820  to be in direct thermal contact with the heated body, the available surface area for heat transfer to the fuel can be substantially increased and heat energy can be redirected to otherwise unheated portions of the flow field of the fuel injector. As a result, the thermal efficiency of the heated fuel injector can be improved. 
     Spacers  620 ,  720 , and  820  may be formed of a material different from the material of the heated body. This allows greater latitude for selecting one material best for the injector body and another material best suited for the heat transfer characteristics of the spacer. For example, the body of a fuel injector is typically made of stainless steel for its inherent corrosion resistance. By designing a spacer to be comprised of a thermally conductive material, such as copper, aluminum, or nickel, for example, superior heat transfer may be realized without compromising the structural benefits of a stainless steel body. By assembling the spacer into the body with a tight thermally conductive press fit, the undesirable welding together of dissimilar materials can be avoided. 
     Specifically referring to  FIGS. 7 and 8 , a fuel injector  600  including a thermally conductive porous metal spacer  620  assembled within a heated body  608  of fuel injector  600  in accordance with the third embodiment of the invention is shown. (Note: features identical with those in first embodiment fuel injector  100  carry the same numbers; features analogous but not identical carry the same numbers but in the  600  series.) Porous metal spacer  620  has a generally cylindrical shape and extends axially preferably over the entire length of the heated portion of heated body  608 . Porous metal spacer  620  further includes a center hole  624  designed to surround pintle shaft  114  such that unrestricted reciprocating axial movement of pintle shaft  114  within spacer  620  is enabled. Center hole  624  is designed to allow minimal fuel flow through a clearance between pintle shaft  114  and spacer  620  without causing a significant drag on the moving pintle shaft  114 . An outer diameter  628  of spacer  620  is adapted such that spacer  620  fills the entire fuel passage  602  between pintle shaft  114  and heated injector body  608 . When inserted in body  608 , the outer circumferential contour of spacer  620  contacts the inside surface  644  of body  608  in a thermally conducting matter. The porous metal spacer  630  may be formed of open cell foam such as, for example, by mixing powdered metal with a powdered organic compound, pressing the mixture in a mold, and sintering to volatize the organic material while melting some grains of metal to adjacent ones, which results in a sponge like structure. 
     In operation, fuel from inlet  104  enters the heated porous metal spacer  620  through cross-hole  115  of pintle shaft  114  and flows through the porous structure of spacer  620  towards valve seat  118 . The porous structure of spacer  620  slows the rate of fuel flow through the spacer  620  and provides a relatively large heated contact surface area. Therefore, the amount of heat transferred to the fuel from the heated body  608  and heated spacer  620  is substantially increased. The efficiency of heat transfer may further be improved by inserting plug  132  in pintle shaft  114 , as described above. 
     Referring to  FIG. 9 , a thermally conductive spacer  720  for assembly within a heated body of a fuel injector is illustrated in accordance with the third embodiment of the invention. Thermally conductive spacer  720  may be assembled in heated body  608  of fuel injector  600  instead of porous metal spacer  620 , as shown in  FIG. 8 . 
     Thermally conductive spacer  720  has a generally cylindrical shape including radially extending features  748  formed as a single helix and extends axially preferably over the entire length of the heated portion of a heated body  708 . Spacer  720  further includes a center hole  724  designed to surround pintle shaft  114  such that unrestricted reciprocating axial movement of pintle shaft  114  within spacer  720  is enabled. Center hole  724  is designed to allow minimal fuel flow through a clearance between pintle shaft  114  and spacer  720  without causing a significant drag on the moving pintle shaft  114 . 
     Radially extending features  748  are adapted to contact an inside surface of a heated injector body  708  at contact points  749 , in a thermally conducting matter. As a result, spacer  720  is heated through heat transfer from the heated body. Features  748  extend within the fuel passage  702 , thereby heating the fuel more evenly and more effectively. Radially extending features  748  may be formed as a helix wound around a core  750 , as shown in  FIG. 9 . By forming features  748  as a helix, the dwell time that the fuel is held near the heated surfaces of the body and spacer  720  is increased. 
     Other configurations of features  748  are possible such as, for example, a double helix wound in the same or opposite directions. 
     Referring to  FIGS. 10 and 11 , a ribbon fin heat exchanger  820  utilized as a thermally conductive spacer for assembly within a heated body  808  in accordance with the third embodiment of the invention is shown. (Note: features identical with those in fuel injector  100  carry the same numbers; features analogous but not identical carry the same numbers but in the  800  series.) Ribbon fin heat exchanger  820  may be, for example, assembled in fuel injector  600  instead of porous metal spacer  620 , as shown in  FIG. 8 . Body  808  may be heated by a heater element  810  applied to an outside surface  842  of body  808 . 
     Ribbon fin heat exchanger  820  has a generally cylindrical shape and extends axially preferably over the entire length of the heated portion of heated body  808 . The formed ribbon fin may be of thin metal sheeting. The metal sheeting may be formed into a multitude of shapes to maximize the surface area of ribbon fin heat exchanger  820  and is not limited to the serpentine shape illustrated in  FIGS. 10 and 11 . The ribbon fin is formed into a cylinder having an outer diameter  828  adapted to closely fit into the heated section of body  808 . By forming the ribbon fin into a cylinder a center hole  824  is created that surrounds a pintle shaft  114 , such that unrestricted reciprocating axial movement of pintle shaft  114  within heat exchanger  820  is enabled. Ribbon fin heat exchanger  820  is assembled within heated body  808  such that an outer circumferential contour of ribbon fin heat exchanger  820  is in thermally conductive contact with an inside surface  844  of body  808 . Ribbon fin heat exchanger  820  may be assembled within heated body  808 , for example, by press fitting or by a brazing process. By making a thermally conductive contact between ribbon fin heat exchanger  820  and heated body  808 , heat is transferred from body  808  to the heat exchanger  820  increasing the available heated surface area in which fuel flowing through fuel passage makes contact. Ribbon fin heat exchanger  820  may be optimized in accordance with a specific application to provide a desired fuel flow, the largest possible surface area available for heat exchange, the smallest possible mass, and the best thermal conductivity through the entire structure. The efficiency of heat transfer of ribbon fin heat exchanger  820  may further be improved by inserting plug  132  in pintle shaft  114 , as described above. 
     While the first, second, and third embodiment of the invention have been described as being advantageous for application in a heated fuel injector to increase the thermal efficiency of such heater fuel injector, the thermally non-conductive spacer ( FIGS. 1-3 ), the enlarged inside surface area of the heated body ( FIGS. 4-6 ), and the thermally conductive spacer ( FIGS. 7-11 ) in accordance with the several embodiments of the invention may be advantageous for any application where a fluid flowing through a passage formed within the body needs to be heated from the outside of such body. 
     It should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described, including but not limited to other configurations, materials, and locations of vaporization elements. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.