Patent Publication Number: US-7707993-B2

Title: Electronic pressure relief in a mechanically actuated fuel injector

Description:
TECHNICAL FIELD 
   The present disclosure relates generally to mechanically actuated electronically controlled fuel injection systems, and more particularly to a strategy for electronically moderating fuel pressure, such as to achieve small close coupled post injections. 
   BACKGROUND 
   Mechanically actuated electronically controlled unit injectors (MEUI) have seen great success in compression ignition engines for many years. In recent years, MEUI injectors have acquired additional control capabilities via a first electrical actuator associated with a spill valve and a second electrical actuator associated with a direct operated nozzle check valve. MEUI fuel injectors are actuated via rotation of a cam, which is typically driven via appropriate gear linkage to an engine&#39;s crankshaft. Fuel pressure in the fuel injector will generally remain low between injection events. As the cam lobe begins to move the plunger, fuel is initially displaced at low pressure to a drain via the spill valve for recirculation. When it is desired to increase pressure in the fuel injector to injection pressure levels, the first electrical actuator is energized to close the spill valve. When this is done, pressure quickly begins to rise in the fuel injector because the fuel pressurization chamber becomes a closed volume when the spill valve closes. Fuel injection commences by energizing a second electrical actuator to relieve pressure on a closing hydraulic surface associated with the direct operated nozzle check valve. The nozzle check valve can be opened and closed any number of times to create an injection sequence consisting of a plurality of injection events by relieving and then re-applying pressure onto the closing hydraulic surface of the nozzle check valve. These multiple injection sequences have been developed as one strategy for burning the fuel in a manner that reduces the production of undesirable emissions, such as NOx, unburnt hydrocarbons and particulate matter, in order to avoid over reliance on an exhaust aftertreatment system. 
   One multiple injection sequence that has shown the ability to reduce undesirable emissions includes a relatively large main injection followed closely by a small post injection. Because the nozzle check valve must inherently be briefly closed between the main injection event and the post-injection event, pressure in the fuel injector may surge due to the continued downward motion of the plunger in response to continued cam rotation. Thus, if the dwell between the main injection event and the post-injection event is too long, the increased pressure in the fuel injector will undermine the ability to controllably produce small post injection quantities. In other words, the longer the dwell, the larger the post injection quantity becomes. Thus, the inherent structure and functioning of MEUI injectors makes it difficult to control fuel pressure during an injection sequence because the fuel pressure is primarily dictated by plunger speed (engine speed) and the flow area of the nozzle outlets, if they are open. Again, when the nozzle outlets are closed, the fuel has nowhere to go and pressure surges within the fuel injector. As expected, this pressure surging problem can become more pronounced at higher engine speeds and loads, which may be the operational state at which a closely coupled small post injection is most desirable. 
   The present disclosure is directed to overcoming one or more of the problems set forth above. 
   SUMMARY 
   In one aspect, a method of operating a fuel injector includes closing a spilled valve while a plunger of the fuel injector is moving in response to rotation of a cam. Fuel pressure in the fuel injector is moderated above a valve opening pressure of a nozzle check valve, while the plunger is moving and the nozzle check valve is closed, by opening the spill valve. After initiating the moderating step, the nozzle check valve is opened. 
   In another aspect, a fuel system includes at least one cam actuated fuel injector with a plunger operably coupled to a rotating cam. The fuel injector includes at least one electrical actuator operably coupled to a spill valve and a nozzle check valve with a closing hydraulic surface exposed to fluid pressure and a needle control chamber. An electronic controller is in communication with the at least one electrical actuator, and includes a pressure moderating algorithm programmed for execution by a processor. The pressure moderating algorithm is operable to generate control signals to the at least one electrical actuator for moderating fuel pressure above a valve opening pressure of the nozzle check valve, while the plunger is moving and the nozzle check valve is closed. The spill valve opens and closes responsive to the control signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side sectioned diagrammatic view of a fuel injector according to one aspect of the present disclosure; and 
       FIGS. 2A-F  represent graphs of a first electrical actuator control signal, spill valve position, a second electrical actuator control signal, needle control chamber pressure, injection pressure, and injection rate, respectively, versus time for an example main plus post injection sequence according to the present disclosure, and with a comparison to the prior art. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a fuel system  5  includes a mechanical electronic unit injector  10  that is actuated via rotation of a cam  9  and controlled by an electronic controller  6 . Fuel injector  10  includes a first electrical actuator  21  operably coupled to a spill valve  22 , and a second electrical actuator  31  operably coupled to control pressure in a needle controlled chamber  33  via a needle control valve  30 . The first and the second electrical actuators  21  and  31  respectively, are energized and de-energized via control signals communicated from electronic controller  6  via communication lines  7  and  8 , which may be wireless. Fuel injector  10  includes an injector body  11  made up of a plurality of components that together define several fluid passageways and chambers. In particular, a fuel pressurization chamber  17  is defined by injector body  11  and a cam driven plunger  15 . When plunger  15  is driven downward due to rotation of cam  9 , fuel is displaced into a spill passage  20 , past spill valve  22 , and out a drain passage (not shown) fluidly connected to fuel supply/return opening  13 . When first electrical actuator  21  is energized, a spill valve member  25  is moved with an armature  23  until a valve surface  26  comes in contact with an annular valve seat  29  to close spill passage  20 . When this occurs, fuel pressure in fuel pressurization chamber  17  increases, as well as a fuel pressure in nozzle chamber  19  via the fluid connection provided by fuel passage  18 . Spill valve member  25  is normally biased to a fully open position where end  28  comes in contact with a stop surface via a compression biasing spring  36 . Spill valve member  25  is attached to move with armature  23  via fastener  24 . Biasing spring  36  also serves to bias the needle control valve  30  to a configuration in contact with flat seat  37  in order to fluidly connect needle controlled chamber  33  to pressure communication passage  35 , which is fluidly connected to fuel passage  18 . 
   Pressure in needle control chamber  33  acts upon a closing hydraulic surface  34  associated with nozzle check valve  32 . As long as pressure in needle control chamber  33  is high, nozzle check valve  32  will remain in a closed position blocking nozzle outlets  12 . When second electrical actuator  31  is energized, needle control valve  30  moves to a position in contact with conical seat  38  to block pressure communication passage  35 , and instead fluidly connects needle control chamber  33  to low pressure fuel supply/return opening  13  via a passage not shown. When pressure in needle control chamber  33  is low and pressure in nozzle chamber  19  is above a valve opening pressure of the nozzle check valve  32 , the nozzle check valve  32  will lift to an open position to allow fuel to spray through nozzle outlets  12 , in a conventional manner. 
   Although not necessary, spill valve member  25  may have a fluid flow force imbalance shape. A fluid flow force imbalance refers to a phenomenon that may occur due to fluid pressure forces acting upon spill valve member  25  as fuel flows along its outer surface and past seat  29 . An imbalance refers to a geometrical structure that causes a net flow force in a closing direction, especially when spill valve member  25  is partially open and fluid flow speeds are high in the vicinity of annulus  27  that terminates in valve surface  26 . In a typical flow imbalance situation, the speed of fuel flow adjacent one wall of an annulus will be greater than the other wall, resulting in a stagnation pressure greater at the wall where fluid is moving slower, which in turn results in a net fluid force in that direction acting on the spill valve member. In the present case, that fluid flow force imbalance may occur at the wall of the annulus  27  remote from valve surface  26 , resulting in a fluid flow force imbalance that tends to push the spill valve member  25  back toward its closed position. Although some versions of a fuel injector  10  according to the present disclosure may utilize a spill valve member  25  with a fluid flow force imbalance shape, spill valve geometries having fluid flow force characteristics and geometries with an imbalance tending to urge the spill valve toward an open position are also within the intended scope of the present disclosure. Spill valve member geometry that exhibits neutral fluid flow force behavior is also within the intended scope of the present disclosure. 
   In one aspect, fluid flow force imbalances can be exploited to utilize the spill valve  22  as a pressure control mechanism that permits the spill valve to be just barely cracked open to relieve some pressure, and then rely upon a force imbalance to quickly reclose the spill valve before too much fuel pressure is lost. For instance, current to electrical actuator  21  may be reduced sufficiently that spring  36  begins moving spill valve member off its seat, but the flow that is initiated causes the flow force imbalance force to dominate, resulting in the spill valve member returning to its seat. This re-closes the valve and stops the flow imbalance force. As long as the reduced current to the actuator is maintained, the spill valve member will chatter at its seat cracking open and then re-closing, and repeating this cycle many times. However, even without exploiting a flow force imbalance, the spill valve  22  may have the ability to be cracked open and then quickly re-closed simply through control signals to first electrical actuator  21 . For instance, briefly reducing an energization level or completely de-energizing electrical actuator  21  for a brief instant and then resuming the energization level may allow the spill valve member to briefly move off its seat  29  to moderate pressure in nozzle chamber  19  and fuel pressurization chamber  17  via fuel passage  18  and spill passage  20 , while still maintaining fuel pressure above the valve opening pressure of the nozzle check valve  32 . 
   As stated above, if a multiple injection sequence is desired, the nozzle check valve  32  must briefly be closed between injection events. While closed and with plunger  15  continuing to move, the fuel has no place to go and pressure may rapidly increase. The present disclosure contemplates an electronic strategy for moderating that pressure surge between injection events by briefly cracking the spill valve  22  open during the dwell between the two injection events. In this way, when it comes time to re-open the nozzle check valve for the post-injection event, pressure in fuel injector  10  will be lower than it otherwise would be, allowing for a smaller and more desirable quantity post injection event. In addition, through suitable control signals to first electrical actuator  21 , the spill valve  22  maybe cracked open one, two or more times in order to increase the dwell to some desired dwell duration without allowing fuel pressure to drop below a valve opening pressure of nozzle check valve  32 . The valve opening pressure for nozzle check valve  32  refers to what pressure must be in nozzle chamber  19  in order to lift nozzle check valve  32  toward an open position against the action of a biasing spring when pressure in needle controlled chamber  33  is low. 
   INDUSTRIAL APPLICABILITY 
   The present disclosure finds potential application to any fuel system that utilizes mechanically actuated electronically controlled fuel injectors that include at least one electrical actuator operably coupled to a spill valve and a nozzle check valve. Although both the spill valve and the nozzle check valve may be controlled with a single electrical actuator within the intended scope of the present disclosure, a typical fuel injector according to the present disclosure will include a first electrical actuator associated with the spill valve and a second electrical actuator associated with the nozzle check valve. Any electrical actuator may be compatible with the fuel injectors of the present disclosure, including solenoid actuators as illustrated, but also other electrical actuators including piezo actuators. The present disclosure finds particular suitability in compression ignition engines that benefit from an ability to produce injection sequences that include a relatively large main injection followed by a closely coupled small post-injection, especially at higher speeds and loads in order to reduce undesirable emissions at the time of combustion rather than relying upon after-treatment systems. The present disclosure also recognizes that every fuel injector exhibits a minimum controllable injection event duration, below which behavior of the injector becomes less predictable and more varied. 
   Referring now to  FIGS. 2A-F , an injection sequence  50  that includes a large main injection  51  and a closely coupled small post injection  52  is shown in  FIG. 2F . Also shown is a similar result with a large post injection  53  according to the prior art using the same fuel injector  10 . Any injection sequence generally begins when the lobe of cam  9  starts to move plunger  15 . As plunger  15  begins moving, first electrical actuator  21  is energized to a pull-in current  64  to close spill valve  22 . As cam  9  continues to rotate, pressure in nozzle chamber  19  begins to ramp up as per pressure increase  45  shown in  FIG. 2E . The closure of spill valve  22  is reflected in  FIG. 2B  by the movement of spill valve member  25  from its fully open position  60  to its closed position  61 . At this time, second electrical actuator  31  remains de-energized to facilitate a fluid connection via pressure communication passage  35  to needle control chamber  33  so that the pressure therein tracks closely with the pressure increase  45  as shown in  FIG. 2D . After spill valve member  25  comes to rest at its closed position, the current or control signal to electrical actuator  21  may be dropped to a hold in level  65  ( FIG. 2A ) that is sufficient to hold spill valve member  25  in its fully closed position  61  as shown in  FIG. 2B . When it comes time to initiate the main injection event  51 , second electrical actuator  31  is energized to a pull in current level  70  ( FIG. 2C ) that moves needle control valve  30  to a position in contact with conical seat  38  to close pressure communication passage  35 , but opens needle control chamber  33  to a low pressure drain passage (not shown). This causes pressure to quickly drop as shown in low-pressure region  80  ( FIG. 2D ) of needle control chamber  33 . Because pressure in nozzle chamber  19  is above the valve opening pressure (VOP) as shown in  FIG. 2E , nozzle check valve  32  will lift, and fuel will commence to spray out of nozzle outlet  12  for main injection event  51 . As with first electrical actuator  21 , second electrical actuator  31  may have its control signal dropped to a low or hold in current level  71  after the needle control valve  30  has come to rest as shown in  FIG. 2G . The main injection event  51  may be terminated by de-energizing second electrical actuator  31  to increase pressure in needle control chamber  33  as shown at  81  by reclosing flat seat  37 . This results in the abrupt closure of nozzle check valve  32  to end injection through nozzle outlets  12 . 
   In the predecessor fuel injector, pressure would begin to increase as per pressure surge  47  during the dwell between main injection event  51  and the post injection event  53  as shown in dotted lines in  FIG. 2E . Thus, in the predecessor injector, fuel pressure at the time of post-injection event  53  is relatively high due to pressure surge  47  resulting in a larger than desirable post injection quantity  53 . The present disclosure addresses this problem by briefly cracking open spill valve  22  as shown in  FIG. 2B  at location  62  where spill valve member  25  is moved just off of seat  29  to moderate pressure in nozzle chamber  19  as shown in  FIG. 2E , while maintaining that pressure above the valve opening pressure of nozzle check valve  32 .  FIGS. 2A and 2B  also show in dotted lines an example case where the spill valve member  25  exhibits the force imbalance phenomenon discussed earlier. In particular, current is dropped to a lower level  68  which allows biasing spring  36  to move valve member  25  off its seat  29 , but the resulting flow induces a return force that re-closes the seat  29 . This behavior repeats as per the dotted line saw tooth shaped chatter  63  shown in  FIG. 2B . By moderating the pressure surge  47  while maintaining the pressure above the valve opening pressure of nozzle check valve  32 , little to no delay will occur when the nozzle check valve  32  is again reopened after time dwell D to perform a small post injection event  52  as shown in  FIG. 2F . The small post injection event  52  is accomplished by re-energizing the second electrical actuator  31  as shown at  72  to drop pressure in needle control chamber  33  as shown at region  82  of  FIG. 2D  after initiating the pressure moderating step. Thereafter, second electrical actuator  31  is de-energized to again increase pressure in needle control chamber  33  to end the injection sequence  50 . Those skilled in the art will appreciate that the injection event could also conceivably be ended by the lobe of cam  9  passing its peak, or by opening spill valve  22  to relieve pressure in fuel injector  10  to below the valve closing pressure sufficient to maintain nozzle check valve  32  in its open position. The valve closing pressure and the valve opening pressure may be similar in magnitude. 
   The present disclosure has the advantage of achieving small post injections  52  following relatively large main injections  50  with substantial control over the duration of the dwell between injection events in order to achieve better emissions without any changes to existing hardware. Moreover, the strategy of the present disclosure may achieve even more controllable results by possibly exploiting the use of a spill valve member  25  with a fluid flow force imbalance shape in order to exploit fluid forces to quickly re-close the spill valve  22  rather than over reliance on quicker acting electrical actuators to re-pull the spill valve member  25  to its closed position. 
   The events illustrated in  FIGS. 2A-F  are accomplished by an electronic controller executing a fuel injection control algorithm programmed for execution by a processor of the controller  6 . The fuel injection control algorithm includes many features known in the art, but also includes a pressure moderating algorithm operable to generate control signals to the at least one electrical actuator  21  and  31  for moderating fuel pressure above a valve opening pressure of the nozzle check valve  32  while the plunger  15  is moving and the nozzle check valve  32  is closed. The fuel injection control algorithm might employ the pressure moderating algorithm at any suitable time when plunger  15  is moving and nozzle check valve  32  is closed, but may more specifically execute the pressure moderating algorithm when the fuel injection control algorithm is executing a multiple injection algorithm programmed for execution by the processor. For instance, the multiple injection algorithm may be operable to generate control signals to the first and second electrical actuators  21  and  31 , respectively, for injecting fuel in a plurality of injection events of an injection sequence  50 , which may include a large main injection  51  followed by a small closely coupled post injection  52 . In such a case, the pressure moderating algorithm could be considered a portion of a multiple injection algorithm, which in turn would be a portion of an overall fuel injection control algorithm being periodically executed by electronic controller  6 . As discussed earlier, when the pressure moderating algorithm is being executed, the spill valve  22  will partially open  62  but not reach a fully opened position  60  in order to moderate, but not completely dump, the fuel pressure in fuel injector  10 . Depending upon the action of spill valve  22 , the pressure moderating aspect of the present disclosure could be achieved by either decreasing the energization level of first electrical actuator  31  or completely de-energizing electrical actuator  21 , and then increasing the energization level responsive to the execution of the pressure moderating algorithm by the processor of electronic controller  6 . Depending upon the sophistication level of fuel system  10 , electronic controller  6  might also include a dwell determination algorithm programmed for execution by the processor of electronic controller  6 . The dwell determination algorithm would be operable to determine a desired dwell between two injection events. In turn, control signals generated by execution of the pressure moderating algorithm would be responsive to the desired dwell. In extreme cases, the spill valve  22  may be cracked open more than one time in order to achieve longer dwell times than that in the illustrated example injection sequence  50 . 
   Although the present disclosure has been illustrated in the context of moderating fuel pressure between injection events for an injection sequence that includes a large main injection followed by a small post injection, it is foreseeable that the same techniques could be utilized to moderate fuel pressure in the fuel injector at any time that the plunger  15  is moving and the nozzle check valve  32  is closed. Furthermore, it is conceivable that with suitable calibration, the concepts of the present disclosure may actually be exploited to control the magnitude of the injection pressure, and hence injection rate, beyond merely moderating against a pressure surge situation as previously described. For instance, the teachings of the present disclosure might even be utilized to reduce a injection rate during part or all of an injection event to occur at a controlled lower pressure that still is above the valve opening pressure of the nozzle check valve  32 . The moderating pressure technique of the present disclosure can be utilized to relax the ever increasing demands for faster and faster electrical actuators associated with both the spill valve  22  and the nozzle check valve  32 . Thus, the nozzle check valve  32  may be re-opened while the spill valve  22  is cracked open or after it has reclosed without departing from the present disclosure. 
   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 of the disclosure can be obtained from a study of the drawings, the disclosure and the appended claims