Patent Publication Number: US-7721716-B1

Title: High pressure piezoelectric fuel injector

Description:
RELATED APPLICATIONS 
   This application claims the benefit under 35 USC 119(e) of provisional application Ser. No. 61/081,174, Titled “Fuel Injector”, filed Jul. 16, 2008 by Harwood. All of the above listed U.S. Patent and Patent Applications are hereby incorporated herein by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention pertains generally to the field of internal combustion engines, more particularly to the field of fuel injection systems for internal combustion engines. 
   BACKGROUND OF THE INVENTION 
   Typical injectors for a Diesel engine operate in conjunction with a heavy, high pressure pump to operate the injector. The systems are well suited to the large diesel engines in trucking, automotive and marine service, however the systems scale poorly for smaller engines or where light weight is needed as in aircraft applications. As engine size decreases, the injectors and injector pump do not scale proportionately. The engine ends up with a significant fraction of the total weight invested in the injection system. Thus, there is a need for simple light weight injector systems and pump systems for small and light weight applications. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Briefly, the present invention relates to a combined injector and fuel pump suitable for high pressure direct injection of heavy fuels into Diesel engines, in particular small light weight Diesel engines as may be used in small aircraft. The injector utilizes a piezoelectric actuator driving a piston assembly comprising an inlet reed check valve disposed thereon. Fuel enters an inlet port coupled to an inlet chamber on a first side of the piston. Piezoelectric actuator contraction transfers fuel from the inlet chamber through the reed valve to the pressurization chamber on a second side of the piston. Piezoelectric actuator expansion drives the piston to pressurize the fuel in the pressurization chamber, which forces open a conical annular valve and nozzle assembly injecting a finely atomized mist of fuel into the cylinder. 
   In one aspect of the invention, the injector is adapted to receive fuel at low pressure, including gravity feed pressures. 
   In another aspect the injector may be adapted to deliver fuel by direct injection into a cylinder at high pressure during a combustion interval. 
   In another embodiment, the injector may be adapted to accurately deliver very low quantities of fuel per stroke. 
   In another aspect of the invention, the output valve and injector spray nozzle features are integrated into the same structure and utilize the same components. 
   In another aspect of the invention, the injector may direct the spray pattern at a thirty degree angle with respect to a plane perpendicular to the injector axis. 
   In a further feature, the output valve/injector nozzle may have adjustable spring tension. 
   In a further feature of the invention, the nozzle generates fine atomization without requiring protrusions into the combustion chamber that tend to collect carbon deposits. 
   In a further feature, the nozzle presents a substantially flush and rugged face to the combustion chamber for minimum combustion gas flow disturbance and minimum deposit buildup. 
   In a further feature of the invention, the injector spray nozzle comprises a flexible metal cap having a conical face matching a conical face of the nozzle portion of the injector housing and filling the depression in the injector reed valve, bringing the injector exposure to a substantially continuous level with the cylinder head surface. The injector directly injects fuel at a desired angle into the cylinder, avoiding protrusions within the cylinder subject to carbon deposit buildup. 
   In a further aspect of the invention, the actuator length dimension is coupled to the piston to move the piston to compress a volume of fuel to cause injection. In one embodiment, the width dimension is decoupled from the fluid by a close fitting piston or by O-rings or other sealants. 
   In a further aspect of the invention, the actuator is coupled to the piston by an axial coupling having rotational decoupling to minimize torque transmitted to the actuator, for example, a flexible coupling, a spherical dome coupling, a contact coupling. The coupling may be spring loaded to provide return motion. 
   In a further embodiment, the input reed valve seat includes small holes for fuel transfer. The holes should be small enough so that full pressure on the reed does not flex the reed enough across the span of the hole under maximum peak pressure to cause long term fatigue concerns in the reed. Standard stress strain analysis may be used to determine the strain, which is then compared with known fatigue properties for the reed material. 
   These and further benefits and features of the present invention are herein described in detail with reference to exemplary embodiments in accordance with the invention. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
       FIG. 1A  illustrates a cross section view of an exemplary high pressure piezoelectric actuated impulse pump and fuel injector in accordance with the present invention. 
       FIG. 1B  illustrates a perspective view of the injector of  FIG. 1A . 
       FIG. 2  illustrates a detail cross section view of the lower portion of  FIG. 1A   
       FIG. 3A  illustrates a cross section view of an alternative exemplary high pressure piezoelectric actuated impulse pump and fuel injector in accordance with the present invention. 
       FIG. 3B  illustrates a perspective view of the injector of  FIG. 3A . 
       FIG. 4  illustrates a detail cross section view of the lower portion of  FIG. 3A . 
       FIG. 5  shows the relationship between the maximum injection pressure and volume for exemplary fuel injectors in accordance with the present invention. 
       FIG. 6  illustrates the high pressure piezoelectric fuel injector of  FIG. 1A  including alternative features. 
       FIG. 7  is a block diagram representing an exemplary drive system for the injector of the present invention. 
       FIG. 8  illustrates an exemplary drive pulse for an actuator in accordance with the present invention. 
       FIG. 9A ,  FIG. 9B , and  FIG. 9C  illustrate an exemplary reed assembly comprising six reeds in accordance with the present invention. 
       FIG. 10  illustrates a magnified view of a portion of  FIG. 1A  showing detail of the reed injector structure. 
       FIG. 11  is a bottom view of the assembly of  FIG. 10 . 
       FIG. 12  shows a further magnification of a portion of  FIG. 10  showing in greater detail the arrangement of the components if the nozzle. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
   The injector of the present invention eliminates the need for large, heavy high-pressure fuel pumps while maintaining the fine atomization consistent with the needs of state-of-the-art direct fuel injection systems. The high pressure necessary for the fine atomization is produced by a piezoelectric actuator driven piston. Piezoelectric actuators are found to be exceptionally well suited for very small heavy fuel (VSHF) engine injectors. Piezoelectric actuators may also be referred to as piezoelectric transducers, or PZT&#39;s. While the actuation distance of piezoelectric actuators is often small (10-100 micrometers (μm)), the injection volume of injectors designed for very small (i.e. ˜20 cubic centimeters (cc)) engines is also very small 1 to 2 cubic millimeters (1-2 mm 3 ) per stroke at maximum power output. In addition, the piezoelectric actuator is adapted to produce relatively large forces in a compact package, and consequently, are able to create high pressures on the order of three thousand psi (200 bar) (1 bar=100 kPa) consistent with the needs of a Diesel engine. Exemplary piezo actuators may P-841.20 manufactured by Physik Instrumente. The present invention eliminates the need for a separate high pressure pump by the use of piezoelectric actuators as a driver for a compact high pressure impulse pump integrated with an injector nozzle assembly. 
   The present invention is an enabling technology for small engines burning heavy fuels. A plunger pressurization mechanism is built into the injector itself eliminating the high-pressure fuel pump typical of most diesel injection systems, while maintaining the atomization consistent with state-of-art injectors. A piezoelectric actuator is used to both provide a compact pressurization mechanism and rapid, precision control of the injection pulse to ensure that the proper amount of fuel is injected at the proper time. 
   Two exemplary embodiments are shown in the figures. The first embodiment shown in  FIG. 1A-1B  and  FIG. 2  illustrates a reed valve injection nozzle combination. The second embodiment, shown in  FIGS. 3A-3B  and  FIG. 4  illustrates a poppet valve injector nozzle combination. The detailed embodiments will now be described with respect to the drawings. 
     FIG. 1A  illustrates a cross section view of an exemplary high pressure piezoelectric actuated impulse pump and fuel injector in accordance with the present invention, and  FIG. 1B  illustrates a perspective view of the injector of  FIG. 1A . Referring to  FIG. 1A  and  FIG. 1B , the fuel injector comprises a piezoelectric actuator  101  driving a piston  102  to pressurize fuel in a pressurization chamber  112 , forcing the fuel through a reed valve nozzle assembly  103  to be injected into a cylinder. The piezo actuator  101  and piston  102  are fitted within a bore within housing. The housing may be constructed of several casings as is convenient for assembly or repair. As shown in  FIG. 1A  the housing comprises a main casing  108  having an input port  111  and a precision bore closely matching the piston  102  while allowing free movement of the piston  102 . The main casing  108  is fitted with an end cap  105  having a threaded attachment. The end cap  105  secures a mounting plate  109  attached to the piezo actuator  101  against the upper end of the main casing  108 . The main casing  108 , or alternatively, the end cap  105  may include a cable  113  for electrical connection to the piezo actuator  101 . On the lower end, the main casing  108  is threadably attached to a nozzle casing  107  carrying the nozzle assembly. 
     FIG. 1  illustrates a single input port in accordance with one embodiment of the invention. Alternatively, the input chamber may have two ports, one on each side of the main body, for flow through capability to aid in purging air in the input chamber to prime the injector. As a further alternative, the injector system may include a low pressure pump to keep the injector supplied with fuel. In a further alternative, the injector system may include an intermediate pressure pump to permit the use of a stiffer spring constant on the input reed valve. 
     FIG. 2  illustrates a detail cross section of the lower portion of  FIG. 1A . Referring to  FIG. 2 , the piston  102  operates within a matching bore in the main casing  108 . The piston  102  is aligned with the input port  111  to allow fuel to pass to the input chamber  202  above the bottom face of the piston  102 . The piston  102  has a reed valve  104  attached to the pressure face (bottom face) of the piston. The piston has through holes  218  around the periphery of the piston  102  to allow the fuel to pass from the input chamber  202  through the piston  102 , between the piston face and the reed valve  104  and into a pressurization chamber  204  below the piston  102 . The reed valve  104  is held by a reed clamp  106 . The reed valve  104  presents a very light captive force holding the reed  104  in contact with the face of the piston  102 . The light captive force permits opening of the reed valve by a slight pressure difference between the input pressure and the pressure of the pressurization chamber. When injecting fuel, however, the reed valve has to withstand pressure differences of up to 3000 psi, (200 bar) or more, has to operate in tens of microseconds and has to have a near zero on to off state displacement because of the very small movement of the piezo actuator. 
   The piston is preferably a strong, tough, light, corrosion resistant material. Depending on pressure required, steel, stainless steel, titanium, and even aluminum alloys or other materials may be found suitable. As shown in  FIG. 1A , the piston is a precision fit to the bore and operates without rings or seals. A precision fit of, for example, 0.001 inch (0.025 mm), or less relative to the diameter is desirable. Alternatively, O-rings or other sealant techniques may be applied. In particular, an O-ring may be placed above the input port between the piston and casing at location  115  indicated in  FIG. 1A  or a similar location in  FIG. 3A . The space  116  between the actuator and casing is preferably maintained free of fuel and preferably contains air to prevent interference with width variations in the actuator that may be associated with length variations used to drive the piston. To prevent gradual filling with fuel, the space  116  may be vented to drain any fuel leakage into space  116 . 
   The lower casing  107  is alternatively referred to as the nozzle casing  107  as this casing includes the nozzle assembly. The nozzle assembly comprises the nozzle casing  107  having a main bore  214  extending to a nozzle bulkhead  216 . The bore  214  and nozzle casing  107  are shown longer than necessary in  FIG. 2 . The extra length may accommodate installation of a pressure sensor or other feature as desired. In an alternative embodiment the bore and nozzle casing may be reduced in length as much as practical. The nozzle bulkhead  216  is bored with feeder holes  210  to the nozzle structure. The nozzle structure comprises an annular conical face  212  machined into the nozzle casing  107 . A matching conical reed assembly  103  fits inside the recess and a matching conical section holder  110  (alternatively referred to as a reed cap  110 ) fills the void and protects the reed valve  103  from damage from the combustion chamber. The holder is attached by a threaded attachment  208  or alternative attachment means as are known in the art. The conical face  212  may have an angle of preferably from 15 to 45 degrees, more preferably 30 degrees from a plane perpendicular to the injector axis  218 . In operation, high pressure fuel lifts the reed valve  103  and flows between the reed valve  103  and nozzle conical face  212  and then is injected into the combustion chamber. The high velocity, thin section flow between the matched surfaces  103  and  212  results in very fine atomization of the fuel. The Sauter Mean Diameter (SMD) of the fuel droplets is calculated to be on the order of tens of micrometers. 
   While there are many competing correlations for SMD, one correlation available in literature is provided below. 
   
     
       
         
           SMD 
           = 
           
             .0217 
             ⁢ 
             
                 
             
             ⁢ 
             
               
                 
                   
                     
                       
                         
                           
                             D 
                             ⁡ 
                             
                               [ 
                               Re 
                               ] 
                             
                           
                           0.25 
                         
                         ⁡ 
                         
                           [ 
                           We 
                           ] 
                         
                       
                       
                         - 
                         0.32 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           μ 
                           l 
                         
                         
                           μ 
                           g 
                         
                       
                       ] 
                     
                   
                   0.37 
                 
                 ⁡ 
                 
                   [ 
                   
                     
                       ρ 
                       l 
                     
                     
                       ρ 
                       g 
                     
                   
                   ] 
                 
               
               0.32 
             
           
         
       
     
   
   where, 
                                  D   is the diameter of the orifice in meters       Re   is the Reynolds number       We   is the Weber number       μ l     is the absolute viscosity of the fuel in Newton - seconds per square           meter       μ g     is the absolute viscosity of the gas in Newton - seconds per square           meter       ρ l     is the density of the liquid in kilograms per cubic meter       ρ g     is the density of the gas in kilograms per cubic meter                    
Using exemplary values:
 
           SMD   =     .0217   ⁢                     (     50.8   ×     10     -   6         )     ⁡     [   3152   ]       0.25     ⁡     [   12508   ]         -   0.32       ⁡     [       1.2   ×     10     -   3           1.8   ⁢           ⁢   e   ×     10     -   5           ]       0.37     ⁡     [     804   1.22     ]       0.32             
SMD=15.7 μm
 
   In operation, in accordance with one exemplary embodiment, the drive circuit for the piezo actuator is initially at zero volts with the actuator at rest. The input chamber and pressurization chamber are filled with fuel at equilibrium pressure between the input chamber and pressurization chamber and the reed valve is closed. When an injection is initiated, an electrical drive pulse is sent to the actuator causing the actuator to expand. The expansion is small, but very rapid. Typical piezo devices may expand by 1/1000 of the length at maximum drive voltage. Thus, a piezo may expand on the order of, for example, 100 microns (0.1 millimeter) in, for example, 100 microseconds. The pulse is generated as a function of the rising slope of the drive pulse together with the response of the actuator and associated mechanics. The injection may be complete in, for example, 100 microseconds. The drive pulse may continue to hold the drive voltage high as the injection completes. The pulse may be complete in, for example, 100 microseconds and the piezo driver then drops the voltage to the piezo driver according to a desired voltage drop profile. Since the piezo driver has less tensile strength than compressive strength, it is desirable to reduce the voltage at a slower rate than the expansion rate to minimize tensile stress on the actuator. The relaxation of the actuator generates a relative vacuum in the pressurization chamber which opens the input reed valve and allows the fuel to refill the pressurization chamber for a return to the initial at rest conditions. Alternative electrical drive states may include a positive and negative voltage state for compression and expansion or other drive states as appropriate for the chosen piezoelectric material and configuration. 
   Referring to  FIG. 2 , beginning with the actuator relaxed at the end of the recharge phase, the reed valve is closed and the piston is moved upward by, for example, 100 microns ready for an injection pulse. When the injection pulse is triggered, the actuator expands by 100 microns pushing the piston down and pressurizing the fuel in the pressurization chamber. The high pressure closes the input reed valve tightly and holds the valve closed. The pressurized fuel flows through the passages in the nozzle bulkhead and presses on the nozzle reed valve to open the nozzle reed valve and flow guided by the conical reed valve and seat to be injected into the cylinder according to the angle of the reed valve and seat. The injection reed valve is a relatively stiff high pressure reed valve that opens very little under the injection pressure to keep the gap between the reed valve and the seat very small forcing the fuel to greatly accelerate through narrow dimensions to generate a fine mist upon injection into the cylinder. 
   At the end of the 100 microsecond injection pulse phase, the injection reed valve closes. The drive voltage then decays, allowing the piezo actuator to return to the relaxed length. As the piston moves upward, the input reed valve opens due to partial vacuum in the compression chamber combined with any pressure available in the input chamber. Fuel then flows to fill the pressurization chamber until equilibrium is established, at which point, spring forces in the reed valve close the reed valve and the process repeats again for the next injection pulse. 
   In a further advantage of the position of the reed valve on the piston, the reed valve is positioned so that the inertia of the reed valve works to enhance the operation of the reed valve. As the piston accelerates downward to compress the compression volume  112 , the inertia of the mass of the reed valve presses the reed valve against the piston, closing and sealing the reed valve. Thus, the inertia of the reed valve works to enhance the closing pressure provided by the back pressure of the pressurized volume  112 . When the piston accelerates upward, the inertia of the reed valve acts to open the reed valve, enhancing the action provided by the pressure differential between the input chamber and pressurization chamber and increasing the fuel flow into the pressurization chamber. 
     FIG. 3A  illustrates a cross section view of an alternative exemplary high pressure piezoelectric actuated impulse pump and fuel injector in accordance with the present invention, and  FIG. 3B  illustrates a perspective view of the injector of  FIG. 3A . 
     FIG. 4  illustrates a cross section view of the lower portion of the injector of  FIG. 3A . Referring to  FIG. 3A ,  FIG. 3B  and  FIG. 4 , the injector of  FIG. 3A  is similar to the injector of  FIG. 1  in that the piezoelectric actuator drives a piston. The piston  102  has a reed valve  104  located thereon for reducing back flow during the pulse operation. The piston  102  pressurizes the fuel in a pressurization chamber  112  and the pressurized fuel forces open a nozzle valve  302 . The nozzle valve  302  is integrated with the injector spray nozzle  308  to cooperatively deliver the atomized fuel in a desired pattern in response to the fuel pressure spike from the piezo actuator. Detail differences can be found in the piston  102  and actuator  101 . A significant difference is to be found in the injector valve. The injector of  FIG. 3A  has a poppet valve  302  with a spring  304  return and support insert  306 . Like the reed injector valve  308  of  FIG. 1A , however, the poppet valve  302  also has conical valve surface and seat to produce a fine mist and direct the mist in a particular pattern. The valve stem  302  has a conical surface mating with a conical seat in the housing  107 . The valve is opened just sufficiently to release the desired fuel amount. The fuel is accelerated through the narrow passages at the valve exit to produce the fine mist injection. One advantage of the poppet valve embodiment is the small area of the valve exposed to the harsh combustion chamber environment and the small area subject to back pressure effects from the combustion chamber. 
   Injection Pressure 
   The injection pressure is a primary sizing requirement for direct fuel injection (DFI) systems, as is injection volume. Given that the maximum actuation distance, D xactuator , for a given actuator is fixed, the maximum injection pressure also is an inverse function of the maximum injection volume, V max  due to the elasticity of the actuator. 
   
     
       
         
           
             
               
                 
                   p 
                   injector_actuator 
                 
                 = 
                 
                   
                     F 
                     actuator 
                   
                   
                     A 
                     actuator 
                   
                 
               
             
           
           
             
               
                 
                   p 
                   injector_actuator 
                 
                 = 
                 
                   
                     F 
                     actuator 
                   
                   
                     ( 
                     
                       
                         V 
                         max 
                       
                       ⁢ 
                       
                         / 
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         x 
                         actuator 
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
   
     FIG. 5  shows the relationship between the maximum injection pressure and volume for exemplary fuel injectors in accordance with the present invention. Referring to  FIG. 5 , the solid line  502  uses a commercially available piezoelectric actuator. The dashed line  504  reflects a higher force actuator that is within the current technology limits. Injection volumes  506  and  508  represent two exemplary designs presently contemplated. While piezoelectric actuators are available that can produce even higher pressures, reducing the injection pressures minimizes the size of the actuator and eases performance tolerances. 
   The maximum injection pressure of the exemplary embodiment is 638 psi. However, if needed, injection pressures could be increased to 4000 psi and potentially approach 10,000 psi. At such high pressure, the lower injection volume per injection may be compensated by scheduling multiple injections per engine revolution. The pressures shown in  FIG. 5  are significantly greater than the 15-30 psi injection systems found in automotive port fuel injection systems and other small engine fuel injection systems. While piezoelectric actuators are available that can produce even higher pressures, the reduced injection pressure simplifies the design. 
     FIG. 6  illustrates the high pressure piezoelectric fuel injector of  FIG. 1A  including alternative features. Referring to  FIG. 6 , the actuator  101  operates the piston  102  as in  FIG. 1A .  FIG. 6  further illustrates the sealing of the space  116  between the actuator  101  and the casing  108 . Depending on the actuator  101 , driving the actuator  101  to change the length of the actuator may also change the width of the actuator. If fluid fills the space  116  between the actuator  101  and casing  108 , then the change in width will couple to the casing. The coupling will influence the drive impedance and loading on the actuator and thus may influence the length change or length change rate effected for a given drive pulse. Thus, it is desirable for the side space  116  loading to be constant, i.e. not to have a variable amount of fluid. In one embodiment, the side space is allowed to fill with fuel. In a preferred embodiment as shown in  FIG. 6 , the side space is an air space. A sealing means is shown as an O-ring  115  to prevent the fill of the side space  116  with fuel and to decouple width changes of the actuator  101  from the compression volume  112 . The side space  116  may also include a vent  602  do drain any fuel that may leak into the side space  116 , especially as the injector ages. In an alternative embodiment (not shown), the side space  116  may be continuous with the compression volume  112  and the injector may or may not have a piston  102 . In the continuous embodiment, the compression volume would be responsive to the bulk expansion of the actuator and not so much to the length. In the preferred embodiment of  FIG. 6 , the compression volume is responsive to the length changes and decoupled from the width changes. Coupling the compression volume to the length changes of the actuator permits greater actuation volume for a given actuator because typically the width dimension decreases as the length dimension increases, reducing the bulk volume change. Further, the use of the piston  102  and placing the reed valve  104  on the piston  102  allows the compression volume  112  to be made as small as practical. Preferably, the largest dimension of the compression volume should be less than % wave at the highest principle frequency component of the drive waveform, i.e., associated with the rise time of the pulse. Keeping the compression volume small helps reduce acoustic bounces for more consistent injection volumes. 
     FIG. 6  shows an O-ring  604  for further sealing between the input chamber  202  and the compression chamber  112 . Any leakage during the compression interval takes away from potential delivered injection volume.  FIG. 6  also shows a dome coupling  606  between the actuator  101  and piston  102 . The dome provides axial coupling without coupling rotationally, either around the actuator length axis or a perpendicular (tilt) axis, thus minimizing any tension stress that may lead to cracks and failure of the actuator. A Bellville spring  608  with flow through holes is provided to keep the piston in contact with the dome. 
     FIG. 7  is a block diagram representing an exemplary drive system for the injector of the present invention. Referring to  FIG. 7 , an electronic computer unit (ECU)  708  receives timing information  706  relating to crank shaft angle and stroke for each cylinder. The computer  708  may use clock timing information  710  to interpolate between crank shaft angle events and to develop RPM information as needed by a timing algorithm. The computer then calculates the desired pulse timing in accordance with the timing algorithm and generates a pulse waveform. The pulse waveform is then amplified by amplifier  704  and delivered as a drive pulse to each injector actuator  101 . 
     FIG. 8  illustrates an exemplary drive pulse for an actuator in accordance with the present invention. Referring to  FIG. 8 , the drive pulse for a single injection comprises a positive pulse having a rising edge  802 , a peak hold period  804 , and a falling edge  806 . The rising edge  802  has a rise time reflecting the time to achieve a percentage, for example 90% of the peak. The extension of the actuator  101  may follow the rising edge of the drive pulse with some delay according to the elasticity of the actuator and the mechanical load (including, among other things, the piston  102 , pressurization chamber  112 , and injection valve  104 .) During the rising edge portion  802 , the actuator compresses the fuel and the injection valve opens. As the voltage approaches the peak, the rate of rise slows and gradually transitions to a steady level  804  for a period of time. The actuator finishes extension during this time, and the fuel is injected. As fuel is injected, the pressure drops and the injection valve closes. The drive voltage then transitions to the falling edge  806 , during which the actuator contracts to the relaxation state, the input reed valve opens and fuel is admitted to the pressurization chamber. The falling edge  806  may be slower than the rising edge  802  and the transitions from rising edge to peak hold and from peak hold to falling edge may be rounded to reduce tension stress in the actuator. Alternatively, or in combination, the actuator may be constructed with a mechanical (spring loaded) compressive preload to reduce tension stress. The graph of  FIG. 8  is somewhat idealistic to illustrate the principles. In practice, overshoots and ringing may be typically found in an actual voltage plot. The specific voltages and associated currents depend on the actuator design. An actuator may be fabricated of a stack of actuator components wired in parallel for a lower voltage, higher current embodiment. Typically the amount of fuel injected may be varied by varying the peak voltage of the drive pulse up to a maximum allowable for the actuator. If more fuel is needed, a larger actuator may be provided, or alternatively, multiple injections per stroke may be provided. The typical repetition rate is a function of the rotation rate of the engine. Typical small engines may run at 200 to 10,000 revolutions per minute (RPM) with one injection for each two revolutions. 
   Injector Reed Valve 
     FIGS. 9A-12  illustrate further details of the exemplary reed valve injection nozzle of  FIG. 2  in accordance with the present invention. 
     FIG. 9A ,  FIG. 9B , and  FIG. 9C  illustrate an exemplary reed assembly comprising six reeds in accordance with the present invention.  FIG. 9A  is an isometric view.  FIG. 9B  is a bottom view.  FIG. 9   c  is a side view. Referring to  FIGS. 9A-9C , the exemplary reed assembly is made out of blue tempered 1095.004 inch thick spring steel sheet. In  FIG. 9A , the reed valve “fingers” are 0.024 inches by 0.100 inches. Six fingers extend from a central hub that is 0.056 inches radius. 
     FIG. 10  illustrates a magnified view of a portion of  FIG. 1A  showing detail of the reed injector structure.  FIG. 10  shows the nozzle casing  107 , reed assembly  103  (comprising reed fingers  902  and hub  904 ), and reed cap  110 . Within the nozzle casing  107  is shown the pressurization chamber  112 , and leading from the pressurization chamber are the nozzle feed bores  210  followed by the injection holes  211 . The reed valve is formed by the reed fingers  902  seating against the nozzle casing  107  at the injection holes  211 . The reed cap  110  limits the travel of the reed fingers  902  away from the seat and establishes a narrow channel between each reed finger  902  and the injector casing  107  for rapid acceleration of the fuel to produce fine atomization. 
     FIG. 11  is a bottom view of the assembly of  FIG. 10 .  FIG. 11  shows the alignment of the various elements shown in  FIG. 10 . Shown are the positions of the reed fingers  902 , injector holes  211 , nozzle feed bores  210 , reed cap outer diameter  110 , and injector conical cavity  212 . 
     FIG. 12  shows a further magnification of a portion of  FIG. 10  showing in greater detail the arrangement of the components if the nozzle. Referring to  FIG. 12 , the positions of the nozzle casing  107 , reed finger  902  and reed cap  110 . Shown is the conical seat  212  for the reed fingers and a gap  1202  formed when the fuel forces the reed valve open and forces the reed  902  against the reed cap  110 . Note that the reed cap  110  presents a limiting surface for the movement of the reed  902  that is parallel to the deflected position of the reed. Note that all six reeds may conform to the single conical surface of the reed cap. 
   The exemplary injection holes are 0.016 inches in diameter. The gap between the valve seat and cap is 0.008 inches to allow a 0.004 inch maximum movement of the reed. Using the cap to restrict the movement, both improves the injector valve response time at the end of the injection pulse and also controls the effective nozzle diameter and thus improves atomization. The cap  110  also protects the reed  902  from combustion chamber pressure and temperature. 
   While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.