Abstract:
A solenoid operated gaseous fuel injector includes a pole positioned axially movable within a fuel tube, a retaining ring axially retaining a first end of the pole, a spring element positioned in contact with a second end of said pole, and an armature transmitting a force onto said pole at impact. The gaseous fuel injector operates to effectively attenuate and dissipate armature impact force onto the pole. Accordingly, impact energy attenuation is attained while cold temperature stiction of the moving parts is avoided.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to fuel injection systems of internal combustion engines utilizing alternative fuels; more particularly, to solenoid actuated fuel injectors for gaseous fuels; and most particularly, to a compressed natural gas fuel injector including a compliant pole and a method for attenuation of mechanical energy within a gaseous fuel injector. 
       BACKGROUND OF THE INVENTION 
       [0002]    Escalating gasoline cost and ever-tightening emission regulations have instigated tremendous interest in compressed natural gas (CNG) as a viable fuel for automotive applications. Natural gas is an inherently clean-burning fuel that is domestically available, is drawn from gas wells or in conjunction with crude oil production. CNG is made by compressing natural gas, which is mostly composed of methane. CNG offers several tangible advantages over common liquid fuels, such as gasoline or diesel, in terms of cost/BTU (British thermal unit) and carbon dioxide emissions. Operating savings of 50% or more of an engine fueled by CNG is possible compared to a typical gasoline engine, particularly at today&#39;s gasoline cost. Carbon dioxide reductions of more than 30% are attainable over gasoline and other currently used liquid fuels. Tests have also shown that a reduction in carbon monoxide, nitrogen oxide emissions, and non-methane hydrocarbon emissions and elimination of evaporative emissions is possible through the use of CNG instead of gasoline. Still further, fewer toxic and carcinogenic pollutants and little or no particulate matter may be emitted when CNG is used as fuel. Therefore, a dramatic increase in the CNG vehicle world volume can be expected over the next several years. 
         [0003]    While the use of CNG as an alternative fuel is desirable, CNG poses some unique challenges in automotive applications, particularly regarding fuel injector durability. Being a gaseous fuel, CNG lacks beneficial damping and lubricating properties inherent to liquid fuels. Consequently, impact velocity and energy go largely undissipated, which results in the generation of high and concentrated impact stresses, objectionable operating noise, and excessive performance deterioration of the injector contact surfaces from wear. For example, imparted stresses often result in severe deformation, particularly of the pole-armature interface of solenoid actuated CNG injectors. Such deformation has been observed to cause anomalous performance and intolerable flow shifts of a CNG injector. 
         [0004]    Currently, injectors derived directly from liquid fuel injectors are typically used in CNG internal combustion engines. Appreciable problems have been experienced with such derivative injectors based on significantly shortened durability and excessive noise compared to their operation with liquid fuels. Some current CNG fuel injector designs have attempted to address these concerns through the employment of soft elastomeric interfaces in the valve mechanism. However, these methods have proven to be largely ineffective, as they have shown susceptibility to cold temperature stiction and long-term deterioration from repeated impact cycling. Cold temperature stiction, a resultant of emulsified residual moisture, compressor oil, and other contaminants severely impair operation of the injector at temperatures below about 10° C. Consequently, applicability of such fuel injectors may be very limited and may preclude widespread usage in many climates. 
         [0005]    What is needed in the art is a viable solution to minimize the undesirable attributes associated with CNG fuel, particularly regarding the fuel injector. 
         [0006]    It is a principal object of the present invention to provide a fuel injector suitable for use with CNG that displays energy attenuating and dissipating properties, is not susceptible to cold temperature stiction, and offers long-term wear resistance. 
       SUMMARY OF THE INVENTION 
       [0007]    Briefly described, a gaseous fuel injector has compliance incorporated into the pole design. Such compliance is attained by designing the pole to be a close-tolerance slip-fit to the fuel tube, thereby permitting a controlled amount of relative axial motion with a relatively small radial degree of freedom of the pole within the fuel tube, instead of being welded to the fuel tube as typical for prior art designs. The pole is held in position, axially, by a retainer near the pole&#39;s frontal face adjacent to the armature such that a prescribed overlap of the retainer by the pole&#39;s frontal face is realized. Such overlap may be tailored to reasonably maximize the impulse at impact of the pole with the armature, to minimize transmitted force, and to reduce impact stress. At the distal pole end, a spring element loads the pole against the retainer toward the armature. A pre-load of the spring element against the pole is selected to dissipate the impact energy of the armature striking the pole at the pole/armature interface. 
         [0008]    The gaseous fuel injector in accordance with the invention operates to effectively attenuate and dissipate armature impact force onto the pole. The inherent restoring rate of the spring element controllably decelerates the moving mass of the armature, thereby diminishing the impact force. Inherent damping resulting from the spring-mass arrangement allows the pole and armature to quickly come to rest after impact. 
         [0009]    The spring element in accordance with the invention may be of any design and of any flexible elastic material capable of storing mechanical energy such as, for example, a coil spring, a wave washer, or an elastomeric ring/spring. 
         [0010]    Impact energy attenuation of an injector, in accordance with the invention, is attained while critical metallic impacting interfaces of the armature and the pole are retained. As a result, cold temperature stiction can be avoided. Furthermore, since metals are dimensionally more stable than polymer materials, critical clearances between the moving parts of the injector can be precisely maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0012]      FIG. 1  is a cross-sectional view of a prior art gasoline fuel injector; 
           [0013]      FIG. 2  is a cross-sectional view of a gaseous fuel injector employing a first type of a spring element in accordance with the invention; and 
           [0014]      FIG. 3  is a cross-sectional view of the gaseous fuel injector employing a second type of the spring element in accordance with the invention. 
       
    
    
       [0015]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates preferred embodiments of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Referring to  FIG. 1 , a typical prior art gasoline fuel injector  100  consists of two primary sub-assemblies; an actuator sub-assembly  110  and a metering sub-assembly  120 . Actuator sub-assembly  110  includes a self-contained solenoid  112 , which is slipped over a fuel tube  114  and welded to a metering body  128 . A pole  116  is secured to fuel tube  114 , for example by welding, typically in an area proximate to the center of solenoid  112  and particularly at the longitudinal axis of a coil  118 , to maximize flux linkage and force. As a result, pole  116  is in a fixed connection with fuel tube  114 . Pole  116  is preferably made from a material within the steel family that has relatively good magnetic properties, as known in the art. Metering sub-assembly  120  is assembled within metering body  128  and includes a valve shaft  122  having an armature  124  attached at one end and a valve  126  at the opposite end. An axially gap also referred to as valve lift  130  is set to define the total length of the axial travel/lift of metering sub-assembly  120  in response to a voltage applied to solenoid  112 , whereupon a magnetic field results that creates attractive forces between pole  116  and armature  124 . The lift  130  of metering sub-assembly  120  coupled with an applied pressure differential across valve  126  results in a desired fuel flow past a valve seat  132  and through a discharge orifice  134 . The size of pole  116  and of armature  124  is typically limited by the available force and the engine space. Armature  124  is preferably formed from stainless steel. 
         [0017]    Typical automotive internal combustion engines desirably operate stoichiometrically near a mass air/fuel ratio of about 14.7/1. Since the density of a gaseous fuel, such as CNG, is significantly less than that of gasoline or other typically used liquid fuels, operating pressure and valve lift  130  must be significantly higher for a gaseous injector to deliver a similar amount of mass fuel in a similar time duration as a liquid fuel injector. While the flow area proximate to discharge orifice  134  could be increased, this is typically avoided due to an increase of parasitic forces proportional to the square of the seal diameter. As a result, to deliver an equal amount of fuel mass, typically valve lift  130  is increased. However, since an equal amount of fuel mass must be delivered to the engine within the same time duration when using a gaseous fuel, armature velocities must be significantly greater for a gaseous fuel injector compared to a similar capacity liquid fuel injector. Thus, since kinetic energy is proportional to the velocity squared of a moving object, a substantial increase in impact energy in a gaseous fuel injector results. Since an armature and a coil of a gaseous fuel injector typically would be limited to similar size and material restrictions as a typical prior art liquid fuel injector, an increase of the impact force of the armature on the pole occurs. Consequently, repetitive impact cycling may result in surface distortion and deterioration and/or ultimate breakage at the armature/pole interface, as a form of energy dissipation. Therefore, a gaseous fuel injector in accordance with the invention is proposed where the mass impact energy is managed by providing energy attenuation and energy dissipating possibilities that enable long-term wear resistance without introducing susceptibility to cold temperature stiction. 
         [0018]    Referring to  FIGS. 2 and 3 , a fuel injector  200  for metering a gaseous fuel, such as CNG, into a combustion chamber of an internal combustion engine is fundamentally similar in configuration to the prior art gasoline fuel injector  100  as shown in  FIG. 1 . Accordingly, features identical with those in prior art gasoline fuel injector  100  carry the same numbers; features analogous but not identical carry the same numbers but in the 200 series. 
         [0019]    Gaseous fuel injector  200  has compliance incorporated into the design of a pole  216 . Such compliance is attained by eliminating the fixed connection between pole  216  and fuel tube  214 , and by enabling pole  216  to axially move within fuel tube  214 . Axially movement of pole  216  within fuel tube  214  is enabled, for example, by designing the outer circumferential contour of pole  216  to have a close-tolerance slip fit relative to the inner circumferential contour of fuel tube  214 , where pole  216  has a relatively small radial degree of freedom. The clearance between the outer circumferential contour of pole  216  and the inner circumferential contour of fuel tube  214  may be, for example, similar to the clearance between pole  116  and fuel tube  114  of fuel injector  100  (as shown in  FIG. 1 ) prior to the welding process. 
         [0020]    Instead of providing a fixed connection between pole  216  and fuel tube  214 , pole  216  is axially retained in one direction within fuel tube  214  by a stop such as retaining ring  240 . Retaining ring  240  fixed in place to fuel tube  214 , for example by a welding process. Retaining ring  240  is preferably assembled within fuel tube  214  after a valve lift  130  has been set. Pole  216  extends axially from a first end  242  proximate armature  224  to a second end  244  opposite the first end. Pole  216  includes a step  246  at first end  242  adapted to mate with retaining ring  240 . The axial dimensions of step  246  and retaining ring  240  are designed such that a nose  243  of first end  242  of pole  216  extends through retaining ring  240  when in a retained position thereby forming a prescribed overlap  248  at first end  242 . Proximate to second end  244  of pole  216 , a spring element  250  is placed within fuel tube  214  with one end of the spring element in contact with pole  216 . The other end of spring element  250  may be axially positioned by a spring retainer  252  fixed to fuel tube  214 , for example by welding and/or press fitting. The axial position of spring retainer  252  relative to the fuel tube sets the preload of spring element  250  on pole  216  to achieve a desired attenuation of the upwards movement of pole  216  and force dissipation. Spring element  250  may be, for example a typical coil spring  254 , as shown in  FIG. 1  or a wave washer  256 , as shown in  FIG. 2 . All though not illustrated, spring element  250  may also be an elastomeric ring or any other device suitable for attenuation of upward movement of pole  216 . 
         [0021]    By permitting a controlled amount of resistive axial movement of pole  216  within fuel tube  214 , the impact force of armature  224  on pole  216  can be effectively attenuated and dissipated. The inherent restoring rate of spring element  250  decelerates the moving mass of armature  224  by increasing the time of contact between armature  224  and pole  216  upon impact, thereby diminishing the impact force. After making contact with pole  216 , armature  224  and pole  216  continue to move upward in tandem against the force of spring element  250  until armature  224  is stopped by retaining ring  240 . Pole  216  may continue to move against spring element  250 . Once pole  216  is contacted at first end  242  by upward moving armature  224 , kinetic energy is transferred from armature  224  to pole  216 . Inherent energy damping by the moving pole causes armature  224  to come to a relatively quick but not sudden stop. Therefore, the length of overlap  248  is selected to optimize both the damping effect of the axially movable pole and the opening time of the injector. 
         [0022]    As can be seen by comparing gaseous fuel injector  200  as shown in  FIGS. 2 and 3  with prior art liquid fuel injector  100  as shown in  FIG. 1 , the novel elements in accordance with the invention, such as axially movable pole  216 , retaining ring  240 , spring element  250 , and spring retainer  252 , may be readily retrofitted into an existing prior art fuel injector with little or no modification to injector components. By forming armature  224  and pole  216  of injector  200  from metallic materials similarly used for armature  124  and pole  116  of prior art gasoline fuel injector  100 , cold temperature stiction caused by the use of polymers as interfacing materials, can be avoided. Moreover, better dimensional stability enjoyed by the metallic materials may be realized. 
         [0023]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. 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.