Abstract:
In one class of fuel injection systems, the individual fuel injectors cycle between high and low pressure during and between injection sequences in a given engine cycle. The fuel injectors may be hydraulically actuated, mechanically actuated, and possibly include common rail injectors equipped with an admission valve that enable the fuel injectors to cycle between high and low pressures. Many of these fuel injection systems also include a directly controlled nozzle valve that can apply or relieve pressure on a closing hydraulic surface associated with the nozzle valve. The nozzle valve is typically spring-biased and therefore has a pre-defined valve opening pressure that defines at what fuel pressure the nozzle valve will open when pressure is relieved on its closing hydraulic surface. While these fuel injection systems can produce a wide variety of rate shapes and injection sequences, generally, an injection sequence of particular interest is one that includes a relatively small volume pilot injection followed quickly in time by a relatively large volume main injection. In order to make the accuracy of the pilot injection more consistent, the nozzle valve is held closed while fuel pressure in the fuel injector builds and surpasses the valve opening pressure of the nozzle valve. This strategy helps to alleviate sensitivity of the pilot injection volume to inherent variability factors, such as geometrical tolerances, within and between fuel injectors.

Description:
RELATION TO OTHER PATENT APPLICATION 
   This application is a continuation in part of co-pending patent application Ser. No. 10/637,452, filed Aug. 8, 2003, entitled Hydraulic Fuel Injection System With Independently Operable Direct Control Needle Valve. 

   TECHNICAL FIELD 
   The present disclosure relates generally to pilot plus main fuel injection sequences, and more particularly to a strategy for improving accuracy in a pilot injection for fuel injectors that cycle between high and low pressure during each engine cycle. 
   BACKGROUND 
   Over the years, engineers have come to recognize that some undesirable emissions can be substantially reduced using particular injection sequences and/or rate shapes at particular engine operating conditions. For instance, engineers have come to recognize that at some engine operating conditions, it is desirable to deliver fuel to the engine cylinder in a so called pilot plus main injection sequence. By injecting a relatively small pilot amount of fuel and then following the same with the main injection event containing the bulk of the fuel for that cylinder, it has been found that the resulting combustion is improved relative to a similar injection quantity injected all at once. In other words, at least one of NOx, unburned hydrocarbons and particulates are reduced when utilizing a pilot plus main injection sequence at certain engine operating conditions. 
   While it may be known that pilot plus main injection sequences are desirable at certain engine operating conditions, it has proven problematic to consistently and accurately control the relatively small pilot injection. Not only do realistic geometrical tolerances and other factors cause a plurality of otherwise identical fuel injectors to behave somewhat differently when supplied with identical control signals, a given injector may also not produce consistent injection results based upon receiving identical control signals over a plurality of engine cycles. If the injector&#39;s behavior deviates too substantially from an expected injection sequence, the goal of lower undesirable emissions from the engine may not be consistently achieved. 
   The present disclosure is directed to one or more of the problems set forth above. 
   SUMMARY OF THE DISCLOSURE 
   In one aspect, a method of injecting fuel includes a step of raising fuel pressure in a fuel injector at least in part with a pressure control valve. A nozzle valve is opened for a pilot injection after fuel pressure is above a valve opening pressure for the nozzle valve. This is accomplished at least in part by actuating a needle control valve in a first direction. The nozzle valve is then closed while fuel pressure is maintained above the valve opening pressure, at least in part by actuating the needle control valve in a second direction. Next, the nozzle valve is reopened for a main injection while fuel pressure is maintained above the valve opening pressure, at least in part by actuating the needle control valve back in its first direction. Finally, fuel pressure in the fuel injector is reduced. 
   In another aspect, a method of improving accuracy of a pilot injection in a pilot plus main injection sequence includes a step of holding the nozzle valve closed while fuel pressure surpasses a valve opening pressure for the nozzle valve. The nozzle valve is then opened at least in part by either energizing or deenergizing an electrical actuator. The nozzle valve is enclosed to end the pilot injection event at least in part by the other of energizing and de-energizing the electrical actuator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a hydraulically actuated fuel injector according to one aspect of the disclosure; 
       FIG. 2  is a schematic illustration of a mechanically actuated fuel injector according to another aspect of the disclosure; 
       FIG. 3  is a graph of fuel injection rate verses time for a pilot plus main injection sequence according to another aspect of the present disclosure; and 
       FIGS. 4   a-d  are graphs of pressure control valve actuator signal, needle control valve actuator signal, sleeve pressure and injection flow rate verses time for an example pilot plus main injection sequence. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , example fuel injection systems  12  and  112  are illustrated in a schematic form. Fuel injection system  12  is a hydraulically actuated pressure intensified fuel injector that includes a direct control needle valve  60 . Fuel injection system  112  is a mechanically actuated fuel injector that also includes a direct control needle valve  160 . Both fuel injection systems include a separate pressure control valve. Thus, during each engine cycle, each fuel injector will cycle between high and low pressure states during and between injection sequences, respectively. Thus, the present disclosure is applicable to any fuel injection system that cycles between high and low pressure, and includes an electrical actuator associated with a direct control needle valve. Apart from the fuel injection systems illustrated, the present disclosure might also find potential application to common rail fuel injectors that are equipped with both an admission valve (pressure control valve) and a separate direct control needle valve. Although these different fuel injection systems operate differently, they are all designed to have the capability of producing a wide variety of different injection sequences and injection rate shapes in order to have the flexibility to produce injection profiles at different engine operating conditions to reduce undesirable emissions, which include NOx, unburned hydrocarbons and particulates. Although there are a wide variety of injection sequences and rate shapes, the present disclosure is primarily concerned with injection sequences that include a so called pilot plus main injection, in which a relatively small volume pilot injection is followed after a brief dwell period with a relatively large volume main injection, as shown in FIG.  3 . 
   Referring specifically to  FIG. 1 , fuel injection system  12  includes a fuel injector  14  mounted in an engine  10  for a direct injection into a cylinder  11 . Although only one injector  14  is shown. Those skilled in the art will appreciate that a separate injector would be associated with each engine cylinder. Fuel injector  14  includes an oil inlet  32  connected to a source of high pressure oil  18 , which can be common to a plurality of fuel injectors. After performing work in injector  14 , the oil is returned for recirculation to a low pressure reservoir  22  via an oil drain outlet  33 . The flow of oil into and out of fuel injector  14  is controlled by a pressure control valve  30  that is operably coupled to an electrical actuator  31 , which can be a solenoid, a piezo electric bender, a piezo stack or any other suitable electrical actuator. When electrical actuator  31  is de-energized, intensifier passage  36  is fluidly connected to drain passage  35  such that intensifier piston  40  will retract toward its upper position to expel used oil from the fuel injector  14  via a return spring (not shown). When electrical actuator  31  is energized, high pressure passage  34  is connected to intensifier passage  36  to allow high pressure oil to act on the top of intensifier piston  40  to drive it and plunger  41  downward to pressurize fuel in fuel pressurization chamber  42  for an injection sequence. Between injection events, when plunger  41  and intensifier piston  40  are retracting, low pressure fuel is drawn from a source  20  via a low pressure fuel inlet  43  into fuel pressurization chamber  42 . Reverse flow of fuel out of inlet  43  is prevented by a check valve  48  in a conventional manner. 
   Fuel injector  14  includes a direct control needle valve  60  that controls the opening and closing of nozzle outlet set  49 . In particular, direct control needle valve  60  includes a needle portion  61  that is biased downward toward a closed position by a biasing spring  64  in a conventional manner. Direct control needle valve  60  also includes a closing hydraulic surface  63  exposed to fluid pressure in a pressure communication passage  56 . A needle control valve  50  is operable to fluidly connect pressure control passage  56  either to a low pressure return line  45  or to fuel pressurization chamber  42  in a conventional manner. An electrical actuator  51 , which can be a solenoid, a piezo or any other suitable electrical actuator, is operably coupled to move needle control valve between these two positions. However, needle control valve  50  is preferably normally biased, such as via a spring, to a position that fluidly connects pressure control passage  56  to low pressure drain line  45  when electrical actuator  51  is de-energized. 
   When pressure communication passage  56  is connected to low pressure return line  45 , and fuel pressure in nozzle supply passage  44  acting on lifting hydraulic surface  62  of needle portion  61  is above a valve opening pressure, needle portion  61  will lift against the action of spring  64  to open nozzle outlet set  49 . When electrical actuator  51  is energized and pressure communication passage  56  is connected to fuel pressurization chamber  42 , fluid pressure acting on closing hydraulic surface  63  will cause direct control needle nozzle valve  60  to either stay in or move toward its downward closed position to close nozzle outlets  49 . Thus, needle control valve  50  allows for the nozzle outlets  49  to be opened at or above the valve opening pressure for the direct control needle valve  60 , which is defined by the relationship between the fuel pressures, the effective area of lifting hydraulic surface  62  and the pre-load of biasing spring  64  in a manner well known in the art. Electrical actuators  31  and  51  are independently controlled via an electronic control module  16  in a conventional manner. 
   Referring now to  FIG. 2 , a cam actuated fuel injection system  112  includes an individual fuel injector  114  positioned in each engine cylinder  111  of engine  110 . When cam  118  rotates, it causes a tappet  140  and a plunger  141  to move downward to pressurize fuel in a fuel pressurization chamber  142 . If a spill valve (pressure control valve)  130  is open, the fuel is merely displaced at a low pressure via spill passage  135  and drain outlet  133  to a low pressure reservoir  120 . However, if electrical actuator  131  is energized to close spill valve  130 , fuel pressure can build within fuel injector  114  to injection pressures. Pressurized fuel from fuel pressurization chamber  142  is supplied to the nozzle via a nozzle supply  144 , and is sprayed into engine cylinder  111  when nozzle outlets  149  are open. The opening and closing of nozzle outlet set  149  is controlled by a direct control needle valve  160  via a needle control valve  150 , which is operably coupled to an electrical actuator  151 . When in its first position, needle control valve  150  fluidly connects a pressure control passage  156  to low pressure reservoir  120  via return line  145 . When in this position, a fuel pressure acting on opening hydraulic surface  162  that is above a valve opening pressure, needle valve member  161  will lift upward toward its open position to allow fuel to spray out of nozzle outlet set  149 . As is well known in the art, the valve opening pressure for direct control needle valve  160  is a function of the effective area of lifting hydraulic surface  162 , the spring pre-load of spring  164  and the fluid pressures in the system. Direct control needle valve  160  also includes a closing hydraulic surface  163  that is exposed to fluid pressure in pressure communication passage  156 . However, when needle control valve  150  connects pressure control passage  156  to low pressure return line  145 , the needle portion  161  will lift against the action of spring  164  when fuel pressure acting on opening hydraulic surface  162  is above a valve opening pressure (VOP). When needle control valve  150  is moved to its second position, pressure control passage  156  becomes fluidly connected to fuel pressurization chamber  142 . The effective area of closing hydraulic surface  163  is such that needle portion  161  will stay in, or move toward, its downward closed position when needle control valve  150  fluidly connects pressure control passage  156  to fuel pressurization chamber  142 . The movement of needle control valve  150  is controlled by an electrical actuator  151 . Those skilled in the art will appreciate that needle control valve  150  can be arranged such that electrical actuator  151  needs to be de-energized to allow injection to occur, or be arranged such that electrical actuator  151  needs to be energized in order for an injection event to occur. Either arrangement is compatible with the present disclosure. As in the previous fuel injection system, the electrical actuators  131  and  151  are independently controlled and energized in a conventional manner by an electronic control module  116 . 
   INDUSTRIAL APPLICABILITY 
   The present disclosure find potential application to any fuel injector that cycles between high and low pressure via a pressure control valve during an engine cycle. Although the illustrated examples show an electronically controlled pressure control valve, the present disclosure might also find application to pressure control valves that are mechanically actuated. The present disclosure also applies to such pressure controlled fuel injectors that include a direct control needle valve that allows the nozzle valve to be held closed even when fuel pressure within the fuel injector is above the valve opening pressure of the nozzle valve. Thus, the present disclosure might find potential application to mechanically actuate a pressure control valves, such as a fuel injection system that utilizes a flow distributor to sequentially connect different fuel pressures to a source of high pressure oil rather than one that utilizes an electronically controlled pressure control valve for each individual fuel injector. In addition, the present disclosure might find potential application to a common rail fuel injection system wherein each fuel injector is cycled through high and low pressure via an admission valve that is opened and closed for each injection cycle. The admission valve would preferably be operated via a separate electronic actuator. 
   Although the illustrated examples show a needle control valve that is a three-way valve that either connects the pressure communication passage to low pressure to high pressure, other needle control valves could be compatible with the present disclosure. For instance, a needle control valve that opens and closes the pressure communication passage to drain in order to allow injection would also be compatible. In this alternative, the pressure control passage is always fluidly connected to the fuel pressurization chamber, but by locating flow restrictions at selected locations that are known in the art, the single two-way needle control valve on the drain side can effectively control the pressure in the volume acting on the closing hydraulic surface of the direct control nozzle valve. Thus, the disclosure is not limited to three way needle control valves, but also encompasses any strategy and structure for direct control needle valves that effectively apply and relieve pressure on a closing hydraulic surface of the nozzle valve. The present disclosure is not so much interested in how the fuel is pressurized for an injection sequence, but rather that it is pressurized and de-pressurized during each engine cycle. Although fuel injection systems according to the present disclosure can normally produce a relatively wide variety of flow rate shapes and injection sequences, the present disclosure is primarily concerned with injection sequences that include a relatively small pilot injection followed quickly by a relatively large main injection. Such an injection sequence has proven to have the ability to reduce undesirable admissions at certain engine operating conditions. Such an injection sequence is shown for example in  FIG. 3  where multiple successive injection sequences  90  are graphed to show the repeatability of the small pilot injection  91  with the relatively large main injection  92 . The problem addressed in the present disclosure is not so much how to create a pilot plus main injection sequence, but rather how to produce such a sequence wherein the pilot injection volume is repeatable, consistent and accurately controlled. 
   Referring now to  FIGS. 4   a-d , an example pilot plus main injection sequence for each of the fuel injection systems  12  and  112  is illustrated. Between injection events, fuel pressure within the individual injectors is relatively low. At some time T 1 , as the timing of a desired injection event approaches, the pressure control actuator  31 ,  131  is energized to a pull-in current to allow fuel pressure to begin to build in the individual fuel injectors. In the case of fuel injector  14 , this allows high pressure oil to begin acting on intensifier piston  40  to begin moving plunger  41  downward to pressurize fuel in fuel pressurization chamber  42 . In the case of fuel injector  114 , the energizing electrical actuator  131  closes spill pressure control valve  130  to allow fuel pressure in fuel pressurization chamber  142  and nozzle supply passage  144  to build in injection levels.  FIG. 4   c  shows the fuel pressure in the individual fuel injectors growing after some brief delay from the timing T 1 . At a timing T 2 , the needle control valve actuator  51 ,  151  is briefly energized to initiate the pilot injection event  91 . The needle control valve actuator  51 ,  151  is then de-energized a short time later at time T 3 . The duration between time T 2  and T 3  is determined to produce a relatively small pilot injection quantity  91 . However, it is important to note that the pilot injection  91  occurs when fuel pressure is relatively high and well above the valve opening pressure (VOP) of the direct control needle valve  60 ,  160 . By initiating the pilot injection event when fuel pressure is relatively high, the pilot injection event is desensitized from small variabilities that inevitably exist between different fuel injectors as to their specific valve opening pressure, when that fuel injector would achieve that valve opening pressure after operating its respective pressure control valve, and different issues, such as friction, regarding the rate at which the nozzle valve opens when fuel pressure exceeds the valve opening pressure. Instead, the present disclosure seeks to initiate the pilot injection event when fuel pressure is somewhat or substantially above the valve opening pressure but far before reaching the full pressure so that, not only does the individual fuel injector behave consistent with itself, but behaves in a manner more consistent with other identical fuel injectors, that may be located in the same engine. 
   A short time after T 3 , the needle control valve actuator  51 ,  151  is again re-energized at time T 4  to initiate the main injection event  92 . At a time T 5 , the pressure control actuator  31 ,  131  is dropped to a hold in current, that maintains the valve in its position without an unnecessary expenditure of energy. Likewise, at time T 6 , the needle control valve actuator  51 ,  151  is dropped to a hold in current level for the remaining duration of the main injection event  92 . At time T 7 , the electrical actuators are de-energized to end the main injection event. 
   The conventional wisdom has long held that accurate injection of relatively small amounts of fuel should be done at lower pressures in order to expand the duration over which the small injection takes place. The present disclosure, however, defies the conventional wisdom by seeking to inject small pilot injections at higher pressures for briefer periods of time. Thus, the conventional wisdom would suggest that the electrical actuators of the individual fuel injector should be energized at relative timings such that the pilot injection event occurs while pressure is increasing such that the nozzle valve merely opens when the fuel pressure overcomes the valve opening pressure of the nozzle valve. While such a strategy at first glance appears sound, achieving consistent results has proven problematic due to a variety of known and possibly unknown factors. The present disclosure desensitizes the injector performance to many of these factors by holding the nozzle valve closed until the fuel pressure is substantially above the valve opening pressure of the nozzle valve by utilizing the needle control valve to maintain high pressure on the closing hydraulic surface of the nozzle valve while fuel pressure is increasing within the fuel injector. 
   It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present disclosure in any way. Thus, those skilled in the art will appreciate that other aspects, objects, and advantages of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims.