Solenoid controlled variable pressure injector

Abstract

A unit fuel injector adapted to receive fuel from a fuel supply at relatively low pressure and adapted to inject fuel at relatively high pressure into the combustion chamber of an internal combustion engine is provided, comprising an injector body having a first internal bore and an injector orifice and a plunger mounted for reciprocating movement within the first internal bore to define a variable volume fuel pressurization chamber including a cam actuated upper plunger portion and a lower plunger portion mounted in the first internal bore between the variable volume fuel pressurization chamber and the upper plunger portion. While the upper plunger portion is in its retracted position, low pressure fuel from the fuel supply is supplied to the variable volume fuel pressurization chamber. A spring is positioned within the first internal bore to bias the upper and lower plunger portions apart to thereby allow for variation of the volume of fuel which flows into the variable volume fuel pressurization chamber during each cycle of injection operation in dependence on the pressure of the fuel from the fuel supply. A valve assembly including a valve element mounted for reciprocating movement within a second internal bore controls the flow of fuel from the variable volume fuel pressurization chamber to the injector orifice. The valve assembly allows fuel to be discharged through the injector orifice only during the time when the upper plunger portion is in its fully advanced position so that injection pressure is independent of the velocity at which the upper plunger portion moves between its retracted and advanced position.

What is claimed is:  
     
       1. A unit fuel injector adapted to receive fuel from a fuel supply at relatively low pressure and adapted to inject fuel at relatively high pressure into the combustion chamber of an internal combustion engine, comprising: 
       (a) an injector body containing a first internal bore and an injector orifice;  
       (b) a plunger mounted for reciprocating movement within said first internal bore to define a variable volume fuel pressurization chamber into which fuel is received at low pressure from the fuel supply and from which fuel is discharged periodically at relatively high pressure for injection through said injector orifice into the combustion chamber; and  
       (c) injection pressure control means for causing fuel to be injected through the injector orifice at a pressure level which is dependent on variation in the level of pressure of the fuel received from the fuel supply and which is independent of changes in the velocity of the reciprocating movement of said plunger.  
     
     
       2. A unit fuel injector as defined in claim  1  for use in an internal combustion engine having a cam for operating said unit injector, wherein said plunger includes an upper plunger portion which is adapted to reciprocate between advanced and retracted position in response to the rotation of the cam and a lower plunger portion mounted in said first internal bore between said variable volume fuel pressurization chamber and said upper plunger portion. 
     
     
       3. A unit fuel injector as defined in claim  2 , wherein said injection pressure control means includes 
       (a) supply means for directing low pressure fuel from the fuel supply into said variable volume fuel pressurization chamber when said upper plunger portion is in said retracted position; and  
       (b) biasing means for biasing said plunger portions apart to thereby vary the volume of fuel which flows into said variable volume fuel pressurization chamber during each cycle of injection operation in dependence on the pressure of the fuel from the fuel supply.  
     
     
       4. A unit fuel injector as defined in claim  3 , wherein said upper plunger portion includes a first reduced diameter portion extending toward said lower plunger portion and said lower plunger portion includes a second reduced diameter portion extending toward said upper plunger portion, said reduced diameter portions being positioned to engage during the downward movement of said upper plunger portion to define the minimum effective length of said plunger. 
     
     
       5. A unit fuel injector as defined in claim  4 , wherein said biasing means includes a coil spring surrounding said upper and lower plunger reduced diameter portions. 
     
     
       6. A unit fuel injector as defined in claim  5 , further including injection timing control means for causing fuel injection during each cycle of injection operation to occur only during the time when said upper plunger portion is in its fully advanced position so that injection pressure is independent of the velocity at which said upper plunger portion moves between its retracted and advanced position. 
     
     
       7. A unit fuel injector as defined in claim  6 , wherein said injection timing control means is responsive to an electrical control signal which is adapted to control both the timing and quantity of fuel injected on a cycle-to-cycle basis. 
     
     
       8. A unit fuel injector as defined in claim  7 , wherein said injection timing control means includes a valve assembly for controlling the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice. 
     
     
       9. A unit fuel injector as defined in claim  8 , wherein said injector body contains a transfer passage for fluid communication between said variable volume fuel pressurization chamber and said injector orifice and wherein said valve assembly includes a valve element reciprocating between: 
       (a) a first position blocking the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice during movement of said upper plunger portion from its retracted to its advanced position to pressurize the fuel trapped in said fuel pressurization chamber to a relatively high pressure level; and  
       (b) a second position permitting the relatively high pressure fuel to flow from said variable volume fuel pressurization chamber through said injector orifice for discharge into the combustion chamber while said upper plunger portion is held in its advanced position thereby decoupling movement of said upper plunger portion from the discharge of fuel into the combustion chamber.  
     
     
       10. A unit fuel injector as defined in claim  9 , wherein said valve assembly includes: 
       a spring means for biasing said valve element from said first position toward said second position; and  
       an electronically actuated solenoid for moving said valve element to said first position and for maintaining said valve element in said first position upon receipt of an electrical control signal.  
     
     
       11. A unit fuel injector as defined in claim  10 , further including check valve means for allowing fuel to flow into said variable volume fuel pressurization chamber when the pressure level within said pressurization chamber is below the pressure level of the fuel supply and for preventing reverse flow of fuel through said supply means when the pressure level in said pressurization chamber is above the pressure level of the fuel supply. 
     
     
       12. A unit fuel injector as defined in claim  11 , wherein said valve element includes a hollow sleeve. 
     
     
       13. A unit fuel injector as defined in claim  12 , wherein the hollow interior of said sleeve forms a portion of said transfer passage. 
     
     
       14. A unit injector as defined in claim  13 , wherein said valve assembly includes a first valve seat adjacent the upper end of said hollow sleeve, said first valve seat being engaged by the upper end of said hollow sleeve when said valve element is in its first position. 
     
     
       15. A unit injector as defined in claim  14 , wherein said valve assembly includes a second valve seat adjacent the lower end of said hollow sleeve, said second valve seat being engaged by the lower end of said hollow sleeve when said valve element is in its second position. 
     
     
       16. A unit injector as defined in claim  15 , wherein said hollow sleeve is co-axially mounted within said injector body with respect to said plunger. 
     
     
       17. A unit injector as defined in claim  16 , wherein said solenoid is concentrically mounted about said hollow sleeve 
     
     
       18. A unit fuel injector as defined in claim  15 , wherein said injector body includes a drain passage for discharging fuel from said unit injector and wherein said valve element in its first position forms an opening for permitting the flow of fuel from a tip valve chamber, defined by a pressure actuated tip valve mounted for reciprocating movement in a third internal bore formed in said injector body for allowing the flow of fuel through said injector orifice to said combustion chamber only when the fuel pressure exceeds a predetermined level, into said drain passage and in its second position blocks the flow of fuel from said tip valve chamber into said drain passage. 
     
     
       19. A unit fuel injector as defined in claim  2 , wherein the reciprocating movement of said plunger is dependent upon the profile of said cam, said cam profile including a plunger advancement segment for controlling the velocity of upper plunger section advancement and an advanced dwell segment for holding the upper plunger section in its advanced position 
     
     
       20. A unit fuel injector as defined in claim  19 , wherein said plunger advancement segment is shaped to extend over a relatively long portion of the cam circumference to cause the upper plunger portion velocity to be relatively low. 
     
     
       21. A unit fuel injector as defined in claim  20 , wherein said plunger advancement segment extends over at least 30° of the circumference of the cam. 
     
     
       22. A unit fuel injector as defined in claim  21 , wherein said advanced dwell segment follows said plunger advancement segment and is shaped to hold the upper plunger portion in its advanced position during the injection event thereby decoupling upper plunger portion advancement from the injection event. 
     
     
       23. A unit fuel injector as defined in claim  2 , further including a drive train for converting rotational movement of said cam into reciprocating movement of the upper plunger portion depending on the profile of said cam and for returning to the cam during each cycle the energy stored in the fuel remaining in the variable volume fuel pressurization chamber following each injection event due to the elastic compressibility of the fuel. 
     
     
       24. A unit fuel injector as defined in claim  3 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is less than 5000 cubic millimeters. 
     
     
       25. A unit fuel injector as defined in claim  3 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is in the range of 3000 to 5000 cubic millimeters. 
     
     
       26. A unit fuel injector as defined in claim  3  wherein the load applied by said cam to said upper plunger portion is equal to or less than 3000 pounds. 
     
     
       27. A unit fuel injector as defined in claim  3  wherein the upper plunger portion travel is in the range of 0.20 to 0.35 inches. 
     
     
       28. A unit fuel injector adapted to receive fuel from a fuel supply at relatively low pressure and adapted to inject fuel at relatively high pressure into the combustion chamber of an internal combustion engine, comprising: 
       (a) an injector body having a first internal bore and an injector orifice;  
       (b) a plunger mounted for reciprocating movement within said first internal bore to define a variable volume fuel pressurization chamber into which fuel is received at low pressure from the fuel supply and from which fuel is discharged periodically at relatively high pressure for injection through said injector orifice into the combustion chamber; and  
       (c) injection pressure control means for controlling the volume of low pressure fuel supplied to the variable volume fuel pressurization chamber during each cycle to vary independent of engine speed the pressure at which fuel is subsequently injected into the combustion chamber within the same cycle of injection operation  
     
     
       29. A unit fuel injector as defined in claim  28 , for use in an internal combustion engine having a cam for operating said unit injector, wherein said plunger includes an upper plunger portion which is adapted to reciprocate between advanced and retracted positions in response to the rotation of the cam and a lower plunger portion mounted in said first internal bore between said variable volume fuel pressurization chamber and said upper plunger portion. 
     
     
       30. A unit fuel injector as defined in claim  29 , wherein said injection pressure control means includes: 
       (a) supply means for directing low pressure fuel from the fuel supply into said variable volume fuel pressurization chamber when said upper plunger portion is in said retracted position; and  
       (b) biasing means for biasing said plunger portions apart to thereby vary the volume of fuel which flows into said variable volume fuel pressurization chamber during each cycle of injection operation in dependence on the pressure of the fuel from the fuel supply.  
     
     
       31. A unit fuel injector as defined in claim  30 , wherein said upper plunger portion includes a first reduced diameter portion extending toward said lower plunger portion and said lower plunger portion includes a second reduced diameter portion extending toward said upper plunger portion, said reduced diameter portions being positioned to engage during the downward movement of said upper plunger portion to define the minimum effective length of said plunger. 
     
     
       32. A unit fuel injector as defined in claim  31 , further including injection timing control means for causing fuel injection during each cycle of injection operation to occur only during the time when said upper plunger portion is in its fully advanced position so that injection pressure is independent of the velocity at which said upper plunger portion moves between its retracted and advanced position. 
     
     
       33. A unit fuel injector as defined in claim  32 , wherein said injection timing control means is responsive to an electrical control signal which is adapted to control both the timing and quantity of fuel injected on a cycle-to-cycle basis. 
     
     
       34. A unit fuel injector as defined in claim  33 , wherein said injection timing control means includes a valve assembly for controlling the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice. 
     
     
       35. A unit fuel injector as defined in claim  34 , wherein said injector body contains a transfer passage for fluid communication between said variable volume fuel pressurization chamber and said injector orifice wherein said valve assembly includes a valve element reciprocating between: 
       (a) a first position blocking the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice during movement of said upper plunger portion from its retracted to its advanced position to pressurize the fuel trapped in said fuel pressurization chamber to a relatively high pressure level; and  
       (b) a second position permitting the relatively high pressure fuel to flow from said variable volume fuel pressurization chamber through said injector orifice for discharge into the combustion chamber while said upper plunger portion is held in its advanced position thereby decoupling movement of said upper plunger portion from the discharge of fuel into the combustion chamber.  
     
     
       36. A unit fuel injector as defined in claim  35 , wherein said valve assembly includes: 
       a spring means for biasing said valve element from said first position toward said second position; and  
       an electronically actuated solenoid for moving said valve element to said first position and for maintaining said valve element in said first position upon receipt of an electrical control signal  
     
     
       37. A unit fuel injector as defined in claim  36 , further including check valve means for allowing fuel to flow into said variable volume fuel pressurization chamber when the pressure level within said pressurization chamber is below the pressure level of the fuel supply and for preventing reverse flow of fuel through said supply means when the pressure level in said pressurization chamber is above the pressure level of the fuel supply. 
     
     
       38. A unit fuel injector as defined in claim  37 , further including a pressure actuated tip valve for allowing the flow of fuel through said injector orifice to said combustion chamber only when the fuel pressure exceeds a predetermined level. 
     
     
       39. A unit fuel injector as defined in claim  29 , wherein the reciprocating movement of said plunger is dependent upon the profile of said cam, said cam profile including a plunger advancement segment for controlling the velocity of upper plunger section advancement and an advanced dwell segment for holding the upper plunger section in its advanced position. 
     
     
       40. Unit fuel injector as defined in claim  39 , wherein said plunger advancement segment is shaped to extend over a relatively long portion of the cam circumference to cause the upper plunger portion velocity to be relatively low. 
     
     
       41. A unit fuel injector as defined in claim  40 , wherein said plunger advancement segment extends over at least 30° of the circumference of the cam. 
     
     
       42. A unit fuel injector as defined in claim  41 , wherein said advanced dwell segment follows said plunger advancement segment and is shaped to hold the upper plunger portion in its advanced position during the injection event thereby decoupling upper plunger portion advancement from the injection event. 
     
     
       43. A unit fuel injector as defined in claim  29 , further including a drive train for converting rotational movement of said cam into reciprocating movement of the upper plunger portion depending on the profile of said cam and for returning to the cam during each cycle the energy stored in the fuel remaining in the variable volume fuel pressurization chamber following each injection event due to the elastic compressibility of the fuel. 
     
     
       44. A unit fuel injector as defined in claim  28 , wherein said injection pressure control means controls the pressure at which fuel is injected through the injector orifice dependent on variation in the level of pressure of the fuel received from the fuel supply and independent of changes in the velocity of the reciprocating movement of said plunger. 
     
     
       45. A unit fuel injector as defined in claim  30 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is less than 5000 cubic millimeters. 
     
     
       46. A unit fuel injector as defined in claim  30 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is in the range of 3000 to 5000 cubic millimeters. 
     
     
       47. A unit fuel injector as defined in claim  30 , wherein the load applied by said cam to said upper plunger portion is equal to or less than 3000 pounds. 
     
     
       48. A unit fuel injector as defined in claim  30 , wherein the upper plunger portion travel is in the range of 0.20 to 0.35 inches. 
     
     
       49. A unit fuel injector adapted to receive fuel from a fuel supply at relatively low pressure and adapted to inject fuel at relatively high pressure into the combustion chamber of an internal combustion engine, comprising: 
       (a) an injector body having a first internal bore and an injector orifice;  
       (b) a plunger mounted for reciprocating movement within said bore to define a variable volume fuel pressurization chamber into which fuel is received at low pressure from the fuel supply for elastic compression by said plunger and from which fuel is discharged periodically at relatively high pressure for injection through said injector orifice into the combustion chamber; and  
       (c) means for utilizing the pressure of the fuel remaining in said variable volume fuel pressurization chamber as a result of the energy stored in the fuel due to the elastic compressibility of the fuel following each injection event to assist in retraction of said plunger.  
     
     
       50. A unit fuel injector as defined in claim  48   49 , for use in an internal combustion engine having a cam and cam shaft for operating said unit injector, wherein said plunger includes an upper plunger portion which is adapted to reciprocate between advanced and retracted positions in response to the rotation of the cam and a lower plunger portion mounted in said first internal bore between said variable volume fuel pressurization chamber and said upper plunger portion; wherein said means for utilizing acts on said lower plunger portion. 
     
     
       51. A unit fuel injector as defined in claim  50 , wherein said means for returning during each cycle the energy stored in the fuel remaining in the variable volume fuel pressurization chamber includes a drive train for linking the reciprocating movement of said upper plunger portion to the rotational movement of said cam to thereby allow for the transfer of the energy stored in the fuel pressurization chamber due to the elastic compressibility of the fuel through the drive train to the cam. 
     
     
       52. A unit fuel injector as defined in claim  51 , further including injection pressure control means for causing fuel to be injected through the injector orifice at a pressure level which is dependent on variation in the level of pressure of the fuel received from the fuel supply and which is independent of changes in the velocity of the reciprocating movement of said plunger. 
     
     
       53. A unit fuel injector as defined in claim  52 , wherein said injection pressure control means includes: 
       (a) supply means for directing fuel from the fuel supply into said variable volume chamber when said upper plunger portion is in said retracted position; and  
       (b) biasing means for biasing said plunger portions apart to thereby vary the volume of fuel in which flows into said variable volume fuel pressurization chamber during each cycle of injection operation in dependence on the pressure of the fuel from the fuel supply.  
     
     
       54. A unit fuel injector as defined in claim  53 , wherein said upper plunger portion includes a first reduced diameter portion extending toward said lower plunger portion and said lower plunger portion includes a second reduced diameter portion extending toward said upper plunger portion, said reduced diameter portions being positioned to engage during the downward movement of said upper plunger portion to define the minimum effective length of said plunger. 
     
     
       55. A unit fuel injector as defined in claim  54 , further including injection timing control means for causing fuel injection during each cycle of injection operation to occur only during the time when said upper plunger portion is in its fully advanced position so that injection pressure is independent of the velocity at which said upper plunger portion moves between its retracted and advanced position. 
     
     
       56. A unit fuel injector as defined in claim  55 , wherein said injection timing control means is responsive to an electrical control signal which is adapted to control both the timing and quantity of fuel injected on a cycle-to-cycle basis. 
     
     
       57. A unit fuel injector as defined in claim  56 , wherein said injection timing control means includes a valve assembly for controlling the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice. 
     
     
       58. A unit fuel injector as defined in claim  57 , wherein said injector body contains a transfer passage for fluid communication between said variable volume fuel pressurization chamber and said injector orifice and wherein said valve assembly includes a valve element reciprocating between: 
       (a) a first position blocking the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice during movement of said upper plunger portion from its retracted to its advanced position to pressurize the fuel trapped in said fuel pressurization chamber to a relatively high pressure level; and  
       (b) a second position permitting the relatively high pressure fuel to flow from said variable volume fuel pressurization chamber through said injector orifice for discharge into the combustion chamber while said upper plunger portion is held in its advanced position thereby decoupling movement of said upper plunger portion from the discharge of fuel into the combustion chamber.  
     
     
       59. A unit fuel injector as defined in claim  58 , wherein said valve assembly includes: 
       a spring means for biasing said valve element from said first position toward said second position; and  
       an electronically actuated solenoid for moving said valve element to said first position and for maintaining said valve element in said first position upon receipt of an electrical control signal.  
     
     
       60. A unit fuel injector as defined in claim  59 , further including check valve means for allowing fuel to flow into said variable volume fuel pressurization chamber when the pressure level within said pressurization chamber is below the pressure level of the fuel supply and for preventing reverse flow of fuel through said supply means when the pressure level in said pressurization chamber is above the pressure level of the fuel supply. 
     
     
       61. A unit fuel injector as defined in claim  60 , further including a pressure actuated tip valve for allowing the flow of fuel through said injector orifice to said combustion chamber only when the fuel pressure exceeds a predetermined level. 
     
     
       62. A unit fuel injector as defined in claim  50 , wherein the reciprocating movement of said plunger is dependent upon the profile of said cam, said cam profile including a plunger advancement segment for controlling the velocity of upper plunger section advancement and an advanced dwell segment for holding the upper plunger section in its advanced position. 
     
     
       63. A unit fuel injector as defined in claim  62 , wherein said plunger advancement segment is shaped to . extend over a relatively long portion of the cam circumference to cause the upper plunger portion velocity to be relatively low. 
     
     
       64. A unit fuel injector as defined in claim  63 , wherein said plunger advancement segment extends over at least 30° of the circumference of the cam. 
     
     
       65. A unit fuel injector as defined in claim  64 , wherein said advanced dwell segment follows said plunger advancement segment and is shaped to hold the upper plunger portion in its advanced position during the injection event thereby decoupling upper plunger portion advancement from the injection event. 
     
     
       66. A unit fuel injector as defined in claim  53 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is less than 5000 cubic millimeters. 
     
     
       67. A unit fuel injector as defined in claim  53 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is in the range of 3000 to 5000 cubic millimeters. 
     
     
       68. A unit fuel injector as defined in claim  53 , wherein the load applied by said cam to said upper plunger portion is equal to or less than 3000 pounds. 
     
     
       69. A unit fuel injector as defined in claim  53 , wherein the upper plunger portion travel is in the range of 0.20 to 0.35 inches. 
     
     
       70. A unit fuel injector adapted to receive fuel from a fuel supply at relatively low pressure and adapted to inject fuel at relatively high pressure into the combustion chamber of an internal combustion engine, comprising: 
       (a) an injector body having a first internal bore and an injector orifice;  
       (b) a plunger mounted for reciprocating movement within said first internal bore to define a variable volume fuel pressurization chamber into which fuel is received at low pressure from the fuel supply and from which fuel is discharge periodically at relatively high pressure for injection through said injector orifice into the combustion chamber; and  
       (c) injection pressure control means for responding to a hydraulic control signal defined by the pressure level of the fuel received from the fuel supply for varying injection pressure during each cycle of injection operation substantially independent of engine speed over substantially the entire range of engine operating speeds.  
     
     
       71. A unit fuel injector adapted to receive fuel from a fuel supply at relatively low pressure and adapted to inject fuel at relatively high pressure into the combustion chamber of an internal combustion engine, comprising: 
       (a) an injector body having a first internal bore and an injector orifice;  
       (b) a plunger mounted for reciprocating movement with said first internal bore to define a variable volume fuel pressurization chamber into which fuel is received at low pressure form the fuel supply and from which fuel is discharged periodically at relatively high pressure for injection through said injector orifice into the combustion chamber; and  
       (c) injection pressure control means for responding to a hydraulic control signal for varying injection pressure substantially independent of engine speed;  
       for use in an internal combustion engine having a cam for operating said unit injector, wherein said plunger includes an upper plunger portion which is adapted to reciprocate between advanced and retracted positions in response to the rotation of the cam and a lower plunger portion mounted in said first internal bore between said variable volume fuel pressurization chamber and said upper plunger portion. 
     
     
       72. A unit fuel injector as defined in claim  71 , wherein said injection pressure control means includes: 
       (a) supply means for directing low pressure fuel from the fuel supply into said variable volume fuel pressurization chamber at a predetermined pressure level defining said hydraulic control signal when said upper plunger portion is in said retracted position; and  
       (b) biasing means for biasing said plunger portions apart to thereby vary the volume of fuel which flows into said variable volume fuel pressurization chamber during each cycle of injection operation in dependence on said hydraulic control signal.  
     
     
       73. A unit fuel injector as defined in claim  72 , wherein said upper plunger portion includes a first reduced diameter portion extending toward said lower plunger portion and said lower plunger portion includes a second reduced diameter portion extending toward said upper plunger portion, said reduced diameter portions being positioned to engage during the downward movement of said upper plunger portion to define the minimum effective length of said plunger. 
     
     
       74. A unit fuel injector as defined in claim  73 , further including injection timing control means for responding to an electrical control signal for varying the timing of fuel injection during each cycle of injection operation causing fuel injection during each cycle of injection operation to occur only during the time when said upper plunger portion is in its fully advanced position so that injection pressure is independent of the velocity at which said upper plunger portion moves between its retracted and advanced position. 
     
     
       75. A unit fuel injector as defined in claim  74 , wherein said injection timing control means is responsive to an electrical control signal which is adapted to control both the timing and quantity of fuel injected on a cycle-to-cycle basis. 
     
     
       76. A unit fuel injector as defined in claim  75 , wherein said injection timing control means includes a valve assembly for controlling the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice. 
     
     
       77. A unit fuel injector as defined in claim  76 , wherein said injector body contains a transfer passage for fluid communication between said variable volume fuel pressurization chamber and said injector orifice and wherein said valve assembly includes a valve element reciprocating between: 
       (a) a first position blocking the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice during movement of said upper plunger portion from its retracted to its advanced position to pressurize the fuel trapped in said fuel pressurization chamber to a relatively high pressure level; and  
       (b) a second position permitting the relatively high pressure fuel to flow from said variable volume fuel pressurization chamber through said injector orifice for discharge into the combustion chamber While said upper plunger portion is held in its advanced position thereby decoupling movement of said upper plunger portion from the discharge of fuel into the combustion chamber.  
     
     
       78. A unit fuel injector as defined in claim  77 , wherein said valve assembly includes: 
       a spring means for biasing said valve element from said first position toward said second position; and  
       an electronically actuated solenoid for moving said valve element to said first position and for maintaining said valve element in said first position upon receipt of an electrical control signal.  
     
     
       79. A unit fuel injector as defined in claim  78 , further including check valve means for allowing fuel to flow into said variable volume fuel pressurization chamber when the pressure level within said pressurization chamber is below the pressure level of the fuel supply and for preventing reverse flow of fuel through said supply means When the pressure level in said pressurization chamber is above the pressure level of the fuel supply. 
     
     
       80. A unit fuel injector as defined in claim  79 , further including a pressure actuated tip valve for allowing the flow of fuel through said injector orifice to said combustion chamber only when the fuel pressure exceeds a predetermined level. 
     
     
       81. A unit fuel injector as defined in claim  71 , wherein the reciprocating movement of said plunger is dependent upon the profile of said cam, said cam profile including a plunger advancement segment for controlling the velocity of upper plunger section advancement and an advanced dwell segment for holding the upper plunger section in its advanced position. 
     
     
       82. A unit fuel injector as defined in claim  81 , wherein said plunger advancement segment is shaped to extend over a relatively long portion of the cam circumference to cause the upper plunger portion velocity to be relatively low. 
     
     
       83. A unit fuel injector as defined in claim  82 , wherein said plunger advancement segment extends over at least 30° of the circumference of the cam. 
     
     
       84. A unit fuel injector as defined in claim  83 , wherein said advanced dwell segment follows said plunger advancement segment and is shaped to hold the upper plunger portion in its advanced position during the injection event thereby decoupling upper plunger portion advancement from the injection event. 
     
     
       85. A unit fuel injector as defined in claim  71 , further including a drive train for converting rotational movement of said cam into reciprocating movement of the upper plunger portion depending on the profile of said cam and for returning to the cam during each cycle the energy stored in the fuel remaining in the variable volume fuel pressurization chamber following each injection event due to the elastic compressibility of the fuel. 
     
     
       86. A unit fuel injector as defined in claim  72 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is less than 5000 cubic millimeters. 
     
     
       87. A unit fuel injector as defined in claim  72 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is in the range of 3000 to 5000 cubic millimeters. 
     
     
       88. A unit fuel injector as defined in claim  72 , wherein the load applied by said cam to said upper plunger portion is equal to or less than 3000 pounds. 
     
     
       89. A unit fuel injector as defined in claim  72 , wherein the upper plunger portion travel is in the range of 0.20 to 0.35 inches. 
     
     
       90. A unit fuel injector adapted to receive fuel from a fuel supply at relatively low pressure and adapted to inject fuel at relatively high pressure into the combustion chamber of an internal combustion engine, comprising: 
       (a) an injector body having a first internal bore and an injector orifice;  
       (b) a plunger mounted for reciprocating movement within said first internal bore to define a variable volume fuel pressurization chamber into which fuel is received at low pressure from the fuel supply and from which fuel is discharged periodically at relatively high pressure for injection through said injector orifice into the combustion chamber;  
       (c) injection pressure control means for responding to a hydraulic control signal defined by the pressure level of the fuel received from the fuel supply for varying injection pressure substantially independent of engine speed over substantially the entire range of engine generator speeds; and  
       (d) injection timing control means for responding to a second control signal for varying the timing of fuel injection during each cycle of injector operation.  
     
     
       91. A unit fuel injector as defined in claim  90 , for use in an internal combustion engine having a cam for operating said unit injector, wherein said plunger includes an upper plunger portion which is adapted to reciprocate between advanced and retracted positions in response to the rotation of the cam and a lower plunger portion mounted in said first internal bore between said variable volume fuel pressurization chamber and said upper plunger portion. 
     
     
       92. A unit fuel injector as defined in claim  91 , wherein said injection pressure control means includes: 
       (a) supply means for directing low pressure fuel from the fuel supply into said variable volume fuel pressurization chamber at a predetermined pressure level defining said second control signal when said upper plunger portion is in said retracted position; and  
       (b) biasing means for biasing said plunger portions apart to thereby vary the volume of fuel which flows into said variable volume fuel pressurization chamber during each cycle of injection operation in dependence on said second control signal.  
     
     
       93. A unit fuel injector as defined in claim  92 , wherein said upper plunger portion includes a first reduced diameter portion extending toward said lower plunger portion and said lower plunger portion includes a second reduced diameter portion extending toward said upper plunger portion, said reduced diameter portions being positioned to engage during the downward movement of said upper plunger portion to define the minimum effective length of said plunger. 
     
     
       94. A unit fuel injector as defined in claim  93 , wherein said injection timing control means includes a valve assembly for controlling the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice in response to said second control signal. 
     
     
       95. A unit fuel injector as defined in claim  94 , wherein said injector body contains a transfer passage for fluid communication between said variable volume fuel pressurization chamber and said injector orifice and wherein said valve assembly includes a valve element reciprocating between: 
       (a) a first position blocking the flow of fuel from said variable volume fuel pressurization chamber to said injector orifice during movement of said upper plunger portion from its retracted to its advanced position to pressurize the fuel trapped in said fuel pressurization chamber to a relatively high pressure level; and  
       (b) a second position permitting the relatively high pressure fuel to flow from said variable volume fuel pressurization chamber through said injector orifice for discharge into the combustion chamber while said upper plunger portion is held in its advanced position in response to said second control signal thereby decoupling movement of said upper plunger portion from the discharge of fuel into the combustion chamber.  
     
     
       96. A unit fuel injector as defined in claim  95 , wherein said valve assembly includes: 
       a spring means for biasing said valve element from said first position toward said second position; and  
       an electronically actuated solenoid for moving said valve element to said first position and for maintaining said valve element in said first position upon receipt of said second control signal.  
     
     
       97. A unit fuel injector as defined in claim  96 , further including check valve means for allowing fuel to flow into said variable volume fuel pressurization chamber when the pressure level within said pressurization chamber is below the pressure level of the fuel supply and for preventing reverse flow of fuel through said supply means when the pressure level in said pressurization chamber is above the pressure level of the fuel supply. 
     
     
       98. A unit fuel injector as defined in claim  97 , further including a pressure actuated tip valve for allowing the flow of fuel through said injector orifice to said combustion chamber only when the fuel pressure exceeds a predetermined level. 
     
     
       99. A unit fuel injector as defined in claim  90 , wherein said first control signal is a hydraulic control signal and said second control signal is an electrical control signal. 
     
     
       100. A unit fuel injector as defined in claim  91 , wherein the reciprocating movement of said plunger is dependent upon the profile of said cam, said cam profile including a plunger advancement segment for controlling the velocity of upper plunger section advancement and an advanced dwell segment for holding the upper plunger section in its advanced position. 
     
     
       101. A unit fuel injector as defined in claim  99 , wherein said plunger advancement segment is shaped to extend over a relatively long portion of the cam circumference to cause the upper plunger portion velocity to be relatively low. 
     
     
       102. A unit fuel injector as defined in claim  101 , wherein said plunger advancement segment extends over at least 30° of the circumference of the cam. 
     
     
       103. A unit fuel injector as defined in claim  102 , wherein said advanced dwell segment follows said plunger advancement segment and is shaped to hold the upper plunger portion in its advanced position during the injection event thereby decoupling upper plunger portion advancement from the injection event. 
     
     
       104. A unit fuel injector as defined in claim  91 , further including a drive train for converting rotational movement of said cam into reciprocating movement of the upper plunger portion depending on the profile of said cam and for returning to the cam during each cycle the energy stored in the fuel remaining in the variable volume fuel pressurization chamber following each injection event due to the elastic compressibility of the fuel. 
     
     
       105. A unit fuel injector as defined in claim  92 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is less than 5000 cubic millimeters. 
     
     
       106. A unit fuel injector as defined in claim  92 , wherein the maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is in the range of 3000 to 5000 cubic millimeters. 
     
     
       107. A unit fuel injector as defined in claim  92 , wherein the load applied by said cam to said upper plunger portion is equal to or less than 3000 pounds. 
     
     
       108. A unit fuel injector as defined in claim  92 , wherein the upper plunger portion travel is in the range of 0.20 to 0.35 inches. 
     
     
       109. A unit injector as defined in claim  49 , further including an injection timing control valve for causing fuel injection during each cycle of injection operation, said injection timing control valve responsive to an electrical control signal which is adapted to control both the timing and quantity of fuel injected on a cycle- by - cycle basis.    
     
     
       110. A unit injector as defined in claim  109 , wherein the unit fuel injector further includes a drain passage, said injection timing control valve is movable between a first position blocking high pressure fuel flow to said drain passage and a second position permitting high pressure fuel flow to said drain passage.  
     
     
       111. A unit injector as defined in claim  49 , wherein said means for utilizing the pressure of the fuel to assist in the retraction of the plunger, acts on said plunger for returning during each cycle the energy stored in the fuel to the engine.  
     
     
       112. A unit injector as defined in claim  111 , wherein the engine includes a cam and cam shaft for operating the unit injector, said means for utilizing the pressure of the fuel to assist in the retraction of the plunger and for returning during each cycle the energy stored in the fuel to the engine including a drive train for linking the reciprocating movement of the plunger to the rotational movement of the cam to thereby allow for the transfer of the energy stored in the fuel pressurization chamber due to the elastic compressibility of the fuel through the drive train to the cam.  
     
     
       113. A unit injector as defined in claim  49 , further including a pressure actuated tip valve for allowing the flow of fuel through said injector orifice to said combustion chamber only when the fuel pressure exceeds a predetermined level.  
     
     
       114. A unit injector as defined in claim  49 , wherein a maximum volume of fuel trapped in said fuel pressurization chamber in any cycle is less than  5000  cubic millimeters.  
     
     
       115. A unit injector as defined in claim  49 , wherein a load applied by a cam to said plunger is equal to or less than  3000  pounds.