Abstract:
In dual fuel engines, it is desirable to have injection of two distinct quantities of liquid fuel. Engines with these operating requirements have typically used two separate fuel injectors, or two separate nozzle assemblies, to satisfy this need. However, these systems can be rather complex and difficult to control. In addition, engineers have learned that fuel injectors having direct control have better performance which can increase engine efficiency. Therefore, the present invention addresses the needs of engines desiring injection of two distinct quantities of fuel, such as dual fuel engines, by utilizing a direct control fuel injector having a dual concentric check having separate orifices for the pilot and main injections.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to electrically controlled fuel injectors and, more particularly, to fuel injectors with a directly controlled dual concentric check valve.  
         BACKGROUND  
         [0002]    In a dual fuel engine, a fuel injector is used to inject liquid fuel, such as diesel distillate, into the engine cylinder, and a second system is responsible for delivering a second type of fuel, such as natural gas. For such dual fuel engines it is desirable to be able to inject two distinct quantities of liquid fuel. A small pilot injection of diesel fuel is used to assist in ignition of a main charge of gaseous fuel when the engine is operating in dual fuel mode. However, when gaseous fuel is unavailable, or for some other reason diesel-only operation of the engine is desired, a larger injection of only diesel is made.  
           [0003]    In the past, it would have been necessary to use two separate fuel injectors, or at least two separate nozzle assemblies in an engine with these operating requirements. One nozzle would have been necessary for the small initial pilot injection, and a second nozzle would have been necessary for the larger diesel-only injection. Such systems tend to be complex and difficult to control. It is thus desirable to create a system capable of fulfilling the dual fuel injection requirements with a single injector.  
           [0004]    A dual concentric check design is known in the art. Lauren W. Burnett invented one example of such an injector in 1989 that could be used to inject both liquid fuel and slurry fuel through concentric nozzle outlets. It is shown in U.S. Pat. No. 4,856,713. However, this injector is not directly controlled, and designed for a liquid or slurry mixture injection rather than for use in a dual fuel engine.  
           [0005]    A typical diesel engine must operate with a broad range of fuel quantities and operating speeds. The necessary precision of injection timing, injection duration, and the provision of sufficient pressure are difficult to accomplish with a single fuel injector. Direct control allows for better performance by enabling the precise control of injection timing and duration. As a result, the engine operates more efficiently and the fuel burns more completely, producing lower emissions.  
           [0006]    The present invention is directed to overcoming one or more of the problems and disadvantages set forth above.  
         SUMMARY OF THE INVENTION  
         [0007]    A directly controlled dual concentric check fuel injector has an injector body that defines a nozzle chamber, a check control chamber, and a plurality of nozzle outlets. A dual concentric check assembly is at least partially positioned in the injector body, and has a closing hydraulic surface exposed to fluid pressure in the check control chamber. The dual concentric check assembly is movable between a first position in which the nozzle outlets are blocked, a second position in which a first portion of the nozzle outlets are open, and a third position in which a second portion of the nozzle outlets are open.  
           [0008]    In another aspect, a method of injecting fuel includes a step of providing a fuel injector with a plurality of nozzle outlets and a directly controlled dual concentric check assembly. The check assembly has a closing hydraulic surface exposed to fluid pressure in a check control chamber. The check control chamber is connected to a low pressure passage. The check assembly is moved to a configuration in which at least a portion of the nozzle outlets are open. Finally, the check control chamber is connected to a high pressure passage.  
           [0009]    In still another aspect, the dual fuel engine includes a plurality of directly controlled dual concentric check fuel injectors attached to an engine housing. Each of these injectors has a dual concentric check assembly at least partially positioned in the injector body. Each of the assemblies has a closing hydraulic surface that is exposed to fluid pressure in a check control chamber. The dual concentric check assembly is movable between a first configuration in which the nozzle outlets are blocked, a second configuration in which a first portion of the nozzle outlets are open, and a third configuration in which all of the nozzle outlets are open. A source of liquid fuel is fluidly connected to the fuel injectors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a partial diagrammatic representation of a dual fuel engine that includes a dual concentric direct operated check fuel injector according to the present invention;  
         [0011]    [0011]FIG. 2 is a diagrammatic sectioned side view of the preferred embodiment of the fuel injector from FIG. 1;  
         [0012]    [0012]FIG. 3 is a diagrammatic representation of a pump-line-nozzle fuel injection system that includes another embodiment of a dual concentric direct operated check fuel injector according to the present invention; and  
         [0013]    [0013]FIG. 4 is a diagrammatic sectioned side view of the fuel injector of FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0014]    The present invention combines the high efficiency of direct control with a single fuel injector capable of delivering two distinct quantities of fuel. It employs a dual concentric check assembly  72  operated with an electrically actuated direct control valve assembly.  
         [0015]    The injector has dual-check nozzles with separate orifices for pilot and main injection that are operated by electronic actuator control. The outer check has a relatively low valve opening pressure and controls a set of orifices with a smaller flow area. The inner check has a relatively high valve opening pressure and controls a set of spray orifices with a relatively large flow area. Combined with a standard unit pump or high pressure fuel common rail, the dual concentric check design provides a fuel injection system capable of higher initial injection pressures. The result is an improvement of the combustion bum quality, especially at part engine loads. To achieve improved delivery ratios between the fuel delivery from the outer check outlets alone, and the fuel delivery with both checks open, electronic direct control is necessary.  
         [0016]    Referring to FIG. 1, there is shown a system level diagram of a dual fuel engine  10  application of a dual concentric direct operated fuel injector  16  according to the present invention. Either version of the fuel injector described herein can be utilized in dual fuel engine  10 , which would employ a plurality of such injectors attached to an engine housing. The method of supplying pressurized fuel within engine  10  may be with a high pressure common rail  26 , but it might also be accomplished with a plurality of electronic unit pumps connected to each injector or any other suitable method known in the art. Engine  10  also has a source of gaseous fuel fluidly connected to the engine housing. Engine  10  is capable of operation with a combination of liquid and gaseous fuel or liquid fuel alone. Engine  10  has a fuel injector  16  according to the present invention whose tip  18  protrudes into cylinder  12 . Liquid fuel from liquid fuel supply  26  is supplied to injector  16  via liquid fuel supply line  28 . The injection of liquid fuel into cylinder  12  is controlled by an electrical actuator  20  attached to injector  16 . Electrical actuator  20  is controlled with electronic control module  22  via communication line  24  in a conventional manner. When electrical actuator  20  is energized, liquid fuel is injected into cylinder  12  via injector tip  18 .  
         [0017]    When dual fuel operation is desired, gaseous fuel from gaseous fuel supply  30  is supplied via gaseous fuel supply line  32  to cylinder  12 , controlled by control valve  36 . The opening and closing of control valve  36  is achieved with electronic actuator  38 , itself controlled in operation by electronic control module  22  via communication line  40  in a conventional manner.  
         [0018]    The combustion of liquid and/or gaseous fuel in cylinder  12  creates force that acts on piston  14 . In dual fuel mode, the compression ignition of the diesel fuel pilot injection ignites the gaseous fuel from the main injection. This provides the force that acts on piston  14 . In diesel-only mode, the compression ignition of the liquid diesel fuel alone creates the force that acts on piston  14 . A check valve  34  positioned within gaseous supply line  32  prevents the leaking of pressure out of cylinder  12  through gaseous supply line  32  during combustion.  
         [0019]    Referring to FIG. 2, there is shown a diagrammatic sectioned side view of a direct control dual concentric check fuel injector  16  according to the present invention. The injector shown in FIG. 2 is the preferred embodiment of the present invention and may employ either a high pressure fuel common rail system or an electronic unit pump as the means of pressurizing fuel. Fuel injector  16  consists of an injector body  50  made up of various components attached to one another in a manner well known in the art, and a number of movable parts positioned in the manner they would be at the initiation of an injection event. As discussed with regard to FIG. 1, liquid fuel source  26 , either a high pressure fuel common rail or an electronic unit pump, supplies pressurized fuel to injector  16  through liquid fuel supply line  28 . The pressurized fuel enters injector  16  through fuel inlet  51 , defined by injector body  50 , and is supplied thenceforth to a pressure communication passage  56  (high pressure passage) and a nozzle supply passage  52 , both defined by injector body  50 . Pressure communication passage  56  is in constant fluid communication with a control volume  69 , defined by injector body  50 . The present invention uses pressurized fuel as the control hydraulic fluid, though it should be appreciated that engine oil, transmission, power steering, brake, coolant, or some other suitable engine fluid might be used.  
         [0020]    Fuel injector  16  is controlled in operation by a control valve assembly  60 , preferably attached to and located within the injector itself. Control valve assembly  60  has an electrical actuator  20  that is preferably a solenoid. It should be appreciated, however, that another suitable device such as a piezoelectric actuator might be used. Solenoid  64  has a coil  66 , and an armature  67 , which is attached to a control valve member  62 . An electrical connector  21  connects solenoid  64  with the control module  22 . A biasing spring  65  biases armature  67  and solenoid  64  toward their upward position closing low pressure seat  70 .  
         [0021]    Control valve member  62  has been shown as a poppet valve, though it should be appreciated that another suitable valve type, such as a spool, might be used. Control valve member  62  is movable within injector body  50  between an upward (off) position in which it closes low pressure seat  70  and a downward (on) position in which it closes high pressure seat  71 .  
         [0022]    When solenoid  64  is de-energized and control valve member  62  is in its upward position closing low pressure seat  70 , control volume  69  provides fluid communication between pressure communication passage  56  and needle (check) control chamber  57 . The high pressure fluid supplied to needle control chamber  57  exerts a downward force on closing hydraulic surface  85  of piston  84 . When solenoid  64  is energized, and control valve member  62  is in its downward position closing high pressure seat  71 , internal passage  63  provides fluid communication between needle control chamber  57  and vent passage  58 . Vent passage  58  is relatively low pressure and connects to a vent outlet  59  defined by injector body  50 . The up or down state of control valve member  62  thus determines whether there is hydraulic pressure acting on the hydraulic surface of piston  84  and, as discussed below, whether the dual check nozzle outlets  54  and  55  are open or closed.  
         [0023]    Within injector body  50 , piston  84  abuts an outer check coupler  83  that in turn attaches to outer check extension  82 . Outer check extension  82  abuts an outer check needle member  81 . The outer check  80  is comprised of piston  84 , outer check coupler  83 , outer check extension  82 , and outer check needle member  81 . Outer check  80  moves up and down within injector body  50  to open and close outlets  54 . An outer check biasing spring  89  exerts a downward force on outer check coupler  83 , biasing the assembly toward a down position in which outer check needle member  81  is held to close outer check seat  73 . Seated thusly, outer check  80  closes a first set of nozzle outlets  54  distributed radially around a centerline  19  and fluidly isolates inner check  90  from nozzle chamber  53 . Thus, when electrical actuator  20  is de-energized, and closing hydraulic surface  85  of piston  84  is exposed to high pressure fuel, the entire outer check assembly  80  is biased downward against outer check seat  73 , closing the outer check nozzle outlets  54 .  
         [0024]    Housed in part within outer check extension  82  and in part within outer check needle member  81  is an inner check  90  which is mechanically biased downward by an inner check biasing spring  95  to close inner check seat  74 . Inner check  90  thus holds closed the inner check nozzle outlets  55  that are distributed radially around the centerline  19  of injector  16 . Inner check  90  moves up and down within a center passageway  96  defined in part by outer check needle member  81  and in part by outer check extension  82 . Inner check  90  is preferably guided in its movement by a matched clearance with outer check needle member  81 .  
         [0025]    As described above, high pressure fuel is continuously supplied to nozzle chamber  53  via nozzle supply passage  52 . Inside nozzle chamber  53 , the high pressure fuel exerts an upward force on the outer check opening hydraulic surfaces  86 . When outer check  80  opens, inner check  90  becomes fluidly connected to nozzle chamber  53  and high pressure fuel can act on the inner check opening hydraulic surfaces  91 .  
         [0026]    The area of piston closing hydraulic surface  85  and the strength of outer check biasing spring  89  are preferably such that outer check nozzle outlet  54  is held closed in spite of the constant opening hydraulic force on its opening hydraulic surfaces  86  when pressure communication passage  56  is open to needle control chamber  57 . Those skilled in the art will appreciate that opening hydraulic surfaces  86  are preferably sized such that outer check  80  will open when needle control chamber  57  is fluidly connected to low pressure passage  58 . The outer check valve opening pressure (VOP) is defined by the pressure in low pressure passage  58 , the area of piston hydraulic surface  85 , the strength of biasing spring  89 , and the effective area of opening hydraulic surfaces  86 .  
         [0027]    The valve opening pressure of inner check nozzle outlet  55  is defined by the strength of inner check biasing spring  95  and the size of inner check opening hydraulic surfaces  91 . In the preferred embodiment, the relevant parts of the injector are sized such that the outer check VOP is relatively low as compared to the inner check VOP. Additionally, the flow area of outer check nozzle outlets  54  is preferably significantly less than the flow area of inner check nozzle outlets  55 . However, different application of the invention might call for a different flow area relationship.  
         [0028]    Referring to FIG. 3, there is shown a diagrammatic representation of a pump-line-nozzle system  110  that has a dual concentric check DOC controlled fuel injector  116  according to another embodiment of the present invention. In contrast to the high pressure fuel common rail system, the system pictured in FIG. 3 employs an electronic unit pump  126  for pressurizing and supplying fuel to injector  116  via liquid fuel supply line  128 . Adjusting timing and duration of the electronic unit pump  126  controls peak fuel pressure in high pressure line  128 , that is, low pressure prevails in high pressure line  128  between injection events.  
         [0029]    Referring to FIG. 4, there is shown a diagrammatic sectioned side view of a dual concentric check DOC controlled fuel injector  116  according to the embodiment of the present invention shown in FIG. 3. Injector  116  operates in a similar manner to the preferred embodiment of the present invention, but with several significant differences. Like injector  16 , injector  116  is supplied with high pressure fuel via a supply line  128 . In contrast to injector  16 , the control valve assembly  160  employs a controlled leakage strategy to control pressure. After the fuel enters injector body  150  through fuel inlet  151 , the high pressure fuel travels through nozzle supply passage  152  to a nozzle chamber  153 , and through pressure communication passage  156  to a control valve assembly  160 . Control valve assembly  160  consists of an electrical actuating device  164  and a valve member  162  that moves up and down within injector body  150 . Electrical actuator  164  consists of a coil  166  and an armature  167  that is attached to control valve member  162 . Electrical actuator  164  is preferably a solenoid, but like injector  16  another suitable device might be used.  
         [0030]    When actuator  164  is de-energized, armature  167  and thereby control valve member  162  are biased downward by biasing spring  165 . Control valve member  162  is positioned partly within the top of piston  184 . Fuel in check control chamber  157  can flow past high pressure seat  171  and around control valve member  162  to act on closing hydraulic surface  185  of piston  184 . The fuel then drains through leak passage  178  and out vent outlet  195 . It should be appreciated that the inside diameter  177  of piston  184  and the outside diameter  169  of control valve member  162  should be sized such that pressurized fuel can flow around control valve member  162  and to leak passage  178 . However, if the clearance between inside diameter  177  and outside diameter  169  is too large, fuel will leak past control valve member  162  at an unacceptably high rate. If the clearance is too small, fuel cannot flow through fast enough and the pressure drop in check control chamber  157  can be too delayed and unpredictable to allow accurate timing of injection. The hydraulic force acting on closing hydraulic surface  185  of piston  184  biases piston  184  downward. As a result, outer check  180  holds outer check nozzle outlet  154  closed in a manner similar to that employed in injector  16 . Inner check  190  operates in much the same way that inner check  90  does in injector  16 . Outer check  180 &#39;s VOP is defined by the pressure in leak passage  178 , the strength of biasing spring  189 , the area of piston closing hydraulic surface  185 , and the area of outer check  180 &#39;s opening hydraulic surface.  
         [0031]    It should be appreciated that the relative area of piston closing hydraulic surface  185 , the strength of outer check biasing spring  189 , and the area of outer check opening hydraulic surfaces  186  are preferably such that outer check  180  will open when control valve member  162  closes high pressure seat  171 . However, outer check  180  preferably remains closed when control valve member  162  is in its down position, and high pressure seat  171  is open. Inner check  190 &#39;s VOP is defined by the area of inner check opening hydraulic surfaces  191  and the strength of inner check biasing spring  195 . It should be appreciated that the area of inner check opening hydraulic surfaces  191  and the strength of biasing spring  195  are preferably such that inner check  190 &#39;s VOP is less than the VOP of outer check  180 .  
         [0032]    When electrical actuator  164  is energized, control valve member  162  moves toward its upward position. When control valve member  162  reaches the upper limit of its travel, it closes high pressure seat  171  and thereby blocks fluid communication between pressure communication passage  156  and control chamber  157 . As a result, the hydraulic pressure on closing hydraulic surface  185  of piston  184  drops dramatically due to the controlled leakage through vent passage  178 . Consequently, piston  184  exerts very little downward force on outer check  180 . In the preferred embodiment, pressurized fuel acting on the outer check opening hydraulic surfaces  186  can force the outer check  180  to open outer check nozzle outlets  154  and inject fuel into the combustion space when high pressure seat  171  is closed, but not when seat  171  is open. This embodiment of the present invention might in theory be used with either a common rail or unit pump hydraulic system.  
         [0033]    However, the controlled leakage embodiment presents significant problems when used with a common rail system. Continuous leakage of fuel makes the maintenance of sufficient pressure in the common rail problematic and the wastage of energy unacceptable. Therefore, this second embodiment of the present invention should preferably employ an electronically controlled unit pump or some other periodic pressurizing device as the means of pressurizing fuel.  
       INDUSTRIAL APPLICABILITY  
       [0034]    In the preferred embodiment of the present invention, prior to an injection event, solenoid  64  is de-energized and control valve member  62  is biased toward its up (off) position by the force of biasing spring  65 . In this state, control valve member  62  closes low pressure seat  70 . Control volume  69  fluidly connects needle control chamber  57  and pressure communication passage  56 . Thus, high pressure prevails in needle control chamber  57  and acts on piston closing hydraulic surface  85 . The high pressure acting on piston closing hydraulic surface  85  and the force of spring  89  bias outer check  80  downward against outer check seat  73  to close outer check nozzle outlets  54 . When outer check  80  is held against outer check seat  73 , inner check  90  is fluidly isolated from nozzle chamber  53 , and is held closed by the force of inner check biasing spring  95 . Nozzle chamber  53  is always supplied with high pressure fuel via nozzle supply passage  52 , regardless of the state of control valve assembly  60 .  
         [0035]    When an injection event is desired, solenoid  64  is energized whereby control valve assembly  60  and control valve member  62  begin to move downward, opening low pressure seat  70 . When low pressure seat  70  is opened, internal passage  63  provides fluid communication between control volume  69  and vent passage  58 . At the instant that low pressure seat  70  is opened, both pressure communication passage  56  and needle control chamber  57  are exposed to relatively low pressure. However, when control valve member  62  reaches the downward limit of its travel, it closes high pressure seat  71 . It should be appreciated that the distance control valve member  62  must travel to close high pressure seat  71  is relatively small, and the assembly travels relatively quickly.  
         [0036]    When high pressure seat  71  is closed, the internal passage  63  of control valve member  62  ceases to provide fluid communication between control volume  69  and vent passage  58 , but continues to fluidly connect needle control chamber  57  with vent passage  58 . As a result, the hydraulic pressure in needle control chamber  57  drops dramatically. There is no longer substantial hydraulic pressure acting on closing hydraulic surface  85  and only the force of biasing spring  89  acts to push outer check  80  downward.  
         [0037]    Recall that nozzle chamber  53  is always exposed to high pressure fuel via nozzle supply line  52 , and thus high pressure fuel is continuously acting on the outer check opening hydraulic surfaces  86 . The force on outer check opening hydraulic surfaces  86  thus pushes outer check  80  up away from seat  73 , and high pressure fuel in nozzle chamber  53  sprays out outer check nozzle outlets  54  into the combustion space. When outer check  80  opens, the opening hydraulic surfaces of inner check  90  become exposed to the high pressure fuel in nozzle chamber  53 . Whether there is sufficient pressure to overcome the VOP of inner check  90  is controlled by adjusting the pressure of the fuel supplied by the common rail or electronic unit pump.  
         [0038]    Recall that the valve opening pressure (VOP) of outer check  80  is defined by the strength of biasing spring  89  and the size of outer check opening hydraulic surfaces  86 . Similarly, the VOP of inner check  90  is defined by the strength of inner check biasing spring  95  and the size of inner check opening hydraulic surfaces  91 . In dual fuel mode operation, where injection of only a relatively small amount of liquid fuel is desired, the fuel pressure should be set such that it is sufficient to overcome the VOP of the outer check, but insufficient to overcome the VOP of the inner check. In this manner, the quantity of liquid fuel injected is relatively small given the relatively small flow area of the outer check nozzle outlets  54 . For a single fuel application the fuel pressure should be adjusted such that it is sufficient to overcome the VOP of both the outer check  80  and the inner check  90 . With the pressure adjusted accordingly, an injection event will occur whereby both checks are opened and fuel sprays through the relatively large combined flow areas of nozzle outlets  54  and  55 .  
         [0039]    Shortly before the desired amount of fuel has been injected, the current to solenoid  64  is shut off. Control valve member  62  opens high pressure seat  71  and begins to move back toward its upward (off) position. When high pressure seat  71  is opened, needle control chamber  57  is once again exposed to high pressure fuel from pressure communication passage  56  via control volume  69 . With the assistance of biasing spring  89 , the high pressure in needle control chamber  57  exerts downward force on piston closing hydraulic surface  85 , and pushes the entire outer check  80  down against outer check seat  73 . This closes outer check nozzle outlets  54 , and fluidly isolates the inner check opening hydraulic surfaces  91  from nozzle chamber  53 . Consequently, inner check biasing spring  95  forces inner check  90  down against seat  73  to close inner check nozzle outlets  55 , and the injection of fuel through both sets of nozzle outlets ceases.  
         [0040]    The second embodiment of the present invention, shown in FIG. 4, also allows direct control of the injection event. This version does not have a low pressure seat, but instead employs a controlled leakage to provide low pressure in control chamber  157  when injection is desired. Between injection events, the electrical actuating device is de-energized, and control valve assembly  160  is in its downward position. High pressure seat  171  is open, and high pressure fuel flows around control valve member  162  to act on piston  184 &#39;s closing hydraulic surface  185 . The hydraulic pressure assists in holding the outer check closed in a very similar manner to that employed in injector  16 . In this de-energized state, the pressurized fuel flows out leak passage  178  at a relatively constant rate.  
         [0041]    When an injection event is desired, electrical current is supplied to actuating device  164 , and the control valve assembly begins to move upward. When control valve member  162  reaches the upper end of its travel, it closes high pressure seat  171 . As a result, high pressure fuel is blocked from flowing past control valve member  162  and acting on piston closing hydraulic surface  185 . The controlled leakage out passage  195  allows the pressure in control chamber  157  to drop. In a manner similar to injector  16 , the high pressure fuel in nozzle chamber  153  may act to open outer check  180  and/or inner check  190 , depending on whether single fuel or dual fuel operation of the engine is desired.  
         [0042]    Shortly before the desired amount of fuel has been injected, electrical actuating device  164  is de-energized. Control valve member  162  begins to move to its downward position, and opens high pressure seat  171 . As high pressure seat  171  opens, pressurized fuel begins to flow around control valve member  162 , and hydraulic pressure once again acts on piston closing hydraulic surface  185 . The resultant closing of outer check  180  and  190  takes place in a very similar way to the preferred embodiment of the present invention.  
         [0043]    The present invention allows precise control of fuel injection in an engine that operates on a single liquid fuel or a combination of liquid and gaseous fuel. By simply varying the pressure at which the fuel is supplied to the injector, a relatively small or a relatively large quantity of fuel can be injected. This system is advantageous because it allows the high efficiency of direct control to be combined with the versatility of a dual fuel engine. Direct control allows the injection of two discrete quantities of fuel. Furthermore, the present invention can be operated with one of two different fuel pressurization systems, adding further versatility.  
         [0044]    It should be understood that the present description is for illustrative purposes only and is not intended to limit the scope of the present invention in any way. Although the invention was described in the context of a dual fuel engine, other engines could benefit from the present invention. For example, a dual concentric check might be employed where injection of different quantities of fuel is desired for reasons other than dual fuel operation, like different engine operating speeds or varying engine loads. Those skilled in the art will recognize that direct control of the present invention could allow for split injections. Alternately energizing and de-energizing the electrical actuator could allow for a variety of injection schemes. Small point injections might be made sequentially or might be alternated with larger main injections, depending on the operating conditions. Thus, those skilled in the art will appreciate that various modifications could be made to either of the described embodiments without departing from the intended scope of the present invention.  
         [0045]    Other aspects and features of the present invention can be obtained from a study of the drawings, the disclosure, and the appended claims.