Abstract:
An efficient control wave form is utilized to actuate the solenoids of a fuel system to reduce boost power/energy consumption. The solenoid is initially energized by applying a boost voltage from an electronic controller across a solenoid coil circuit. The electronic controller monitors the current level in the solenoid coil circuit, and changes to a reduced battery voltage when the current level in the solenoid coil circuit reaches a predetermined trigger current. The controller then maintains a pull-in current based upon battery voltage for a pull-in duration that initiates movement of the solenoid armature from an initial air gap position toward a final air gap position. After the pull-in duration, the current level is dropped to a hold in level for the remaining duration of the actuation event. The solenoid may be used for fuel injector control and/or pump control, such as to control fuel injection and pumping events respectively.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to electronically controlled fuel systems, and more particularly to an efficient wave form for controlling the operation of solenoids in fuel injectors and/or pumps of a fuel system. 
     BACKGROUND 
     Today&#39;s electronically controlled fuel systems typically include numerous electrical actuators whose activation is controlled by an electronic controller. For instance, fuel injectors may include one or more electrical actuators to control injection timing and/or injection quantity. In common rail fuel systems, an electronically controlled pump or other actuator may control pressure in a common rail that supplies pressurized fuel to a bank of fuel injectors. While both piezo and solenoids are known for use as electrical actuators in fuel systems, solenoids continue to dominate in most applications. Over the years, there has been a continuous effort to improve actuator performance through various solenoid design strategies, pressure control strategies, mass property improvements, control wave forms and other considerations in an effort to improve consistency, robustness and speed, as well as other performance characteristics. 
     Co-owned U.S. Pat. No. 4,922,878 teaches a typical wave form control strategy for energizing a solenoid of a fuel injector to perform an injection event. The &#39;878 patent teaches an electronic controller that has the ability to briefly apply a substantially higher voltage to the solenoid circuit to initiate movement of an armature of the solenoid to commence an injection event. For instance, this higher voltage may be supplied by capacitors that are continuously charged from system voltage “battery” between injection events. In order to hasten the time delay between initially applying a voltage to the solenoid circuit and the time at which the armature actually starts moving, the conventional wisdom has been to maintain the elevated voltage across solenoid circuit until the solenoid armature begins moving from its initial air gap position toward its final air gap position. During this initial period, current in the solenoid circuit is controlled to have a saw tooth pattern by the electronic controller maintaining current between a minimum and a maximum current by opening the circuit when the maximum circuit is reached, then closing the circuit at the minimum current, and repeating this process during what is commonly referred to as the pull-in duration. At the end of the pull-in duration, the controller may then drop to a battery voltage and a lower tier average current since less energy is needed to continue movement of the armature, and maybe even less energy needed to hold the armature at its final air gap position. These lower tiered current levels after the pull-in duration are often referred to as hold-in current levels. 
     As is well known in the art, movement of the solenoid armature changes a pressure configuration within the fuel injector causing a fuel injection event to occur. When it comes time to end the injection event, the circuit is opened, current decays and a bias (e.g. spring) moves the armature back toward its initial air gap position to again change a pressure condition within the fuel injector and end the injection event. While this type of wave form control strategy has worked well for many years, there are continued efforts being made to decrease hardware requirements and reduce power/energy requirements without compromising performance. 
     The present disclosure is directed toward one or more of the problems set forth above. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect, a method of operating a fuel system for an engine includes energizing a solenoid of the fuel system and then later deenergizing the solenoid. The energizing step includes applying a boost voltage from an electronic controller across a solenoid coil circuit, and changing from the boost voltage to a reduced voltage responsive to a current in the solenoid coil circuit reaching a trigger current. 
     In another aspect, an electronic controller for a fuel system of an engine includes a processor, a memory in communication with the processor, a solenoid coil circuit port, a battery port and a driver circuit that includes a boost power source. A solenoid actuation algorithm that is stored on the memory and executable by the processor is configured to electrically connect the solenoid coil circuit port to the driver circuit to provide a boost voltage, then electrically disconnect the solenoid coil circuit port from the driver circuit responsive to a current through the solenoid coil circuit port reaching a trigger current, and then electrically connecting the solenoid coil circuit port to the battery port. 
     In still another aspect, a method of operating a solenoid of a fuel injector for an engine includes applying a boost voltage from an electronic controller across a solenoid coil circuit. The solenoid coil current is compared to a predetermined trigger current. Voltage in the solenoid coil circuit is changed from the boost voltage to a reduced voltage responsive to a current in the solenoid coil circuit reaching the trigger current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a fuel system for an engine according to the present disclosure; 
         FIG. 2  is a logic flow diagram of a solenoid actuation algorithm according to another aspect of the present disclosure; 
         FIG. 3  is an overlay of voltage, solenoid coil current and armature position versus time utilizing a control wave form according to the present disclosure; and 
         FIG. 4  is a graph similar to that of  FIG. 3  except showing a comparison of two wave forms according to the present disclosure with different trigger currents and different pull-in current levels. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a fuel system  10  for a compression ignition engine includes a common rail  12  that supplies pressurized fuel to individual fuel injectors  15  via individual branch passages  16 . A high pressure pump  13  is supplied with fuel from tank  14  via a low pressure supply line  19 . An outlet from high pressure pump  13  is fluidly connected to common rail  12  via a high pressure supply line  18 . Although only one is shown, each fuel injector  15  is fluidly connected to tank  14  by a low pressure return line. Each fuel injector  15  includes a solenoid  20  that may be energized via a voltage applied by electronic controller  30  to individual solenoid coil circuit  22 . The solenoid coil circuits  22  are electrically connected to respective solenoid circuit ports  38  of electronic controller  30 . The output from pump  13 , and hence the pressure in common rail  12  is also controlled by electronic controller  30  via a pump circuit  26  that is electrically connected to controller  30  at pump circuit port  39 . High pressure pump  13  may or may not be of a type that has a control feature that may benefit from the efficient wave form of the present disclosure. For instance, pump  13  may be an inlet throttle metered pump those inlet flow area is controlled by an electronic controller that may not benefit from the efficient wave form of the present disclosure. On the otherhand, high pressure pump  13  may be an outlet metered pump those outlet is controlled by one or more solenoid actuated spill valves known in the art that could benefit from the efficient wave form of the present disclosure. Thus, electronic controller  30  controls injection pressure via output for pump  13 , and controls injection timing via the energization and deenergization of individual solenoids  20  to control the opening and closing of nozzle outlets for an injection event. 
     Electronic controller  30  is of a well known structure, in that it includes a processor  31  that is configured to execute programmable code stored on memory  32 . Electronic controller  30  also includes a driver circuit  33  that includes a boost power source  34 , and electronic controller  30  is also electrically connected to a battery  50  via battery port  36 . When executing code stored on memory  32 , processor  31  can electrically connect solenoid circuit ports  38  and/or pump circuit port  39  to either driver circuit  33  for an elevated boost voltage, or electrically connect the same to battery  50  for a reduced voltage on the respective solenoid coil circuits  22  and/or pump circuit  26 . Boost power source  34  may include one or more capacitors that may be continuously charged with electrical energy from battery  50 , but are capable of being discharged through driver circuit  33  to provide an elevated boost voltage that may be many times greater than the voltage associated with battery  50 . For instance, battery voltage may be on the order of 12 volts, whereas the boost voltage may be on the order of 100 volts. Although the boost voltage will always be greater than the battery voltage, those skilled in the art will appreciate that the magnitude of the boost voltage is a matter of design choice taking into account known considerations including cost and performance, among other considerations. Although the present disclosure is illustrated in the context of a common rail fuel system for a compression ignition engine, those skilled in the art will appreciate that the concepts of the present disclosure may also apply to any electronically controlled fuel system (e.g. cam actuated fuel injectors) for any type of engine (e.g., spark ignited, gaseous fuel, heavy fuel, etc.) 
     Those skilled in the art will appreciate that solenoids utilized in both fuel injectors and pumps for electronically controlled fuel systems include well known features in common to all. For instance, a solenoid includes a stationary stator that assists in channeling magnetic flux generated by a solenoid coil to move an armature from an initial air gap position to a final air gap position. For instance, fuel injectors  15  might be equipped with a direct operated check in which the armature movement serves to allow a coupled valve member to move to connect and disconnect a pressure control chamber to drain to allow a needle valve member to open and close to perform an injection event in a well known manner. On the otherhand, in the case of pump, the movement of the solenoid armature may close a spill valve associated with the pumping chamber to displace a controlled fraction of a pump displacement to the high pressure common rail while spilling another fraction of the displacement back to tank at a low pressure. 
       FIG. 2  shows an example solenoid actuation algorithm  80  that could be encoded and stored on memory  32  for execution by processor  31  to control the action of solenoids  20  of the individual fuel injectors  15 . Nevertheless, a similar solenoid actuation algorithm could also be suitable for controlling pump events in certain electronically controlled pumps known in the art that utilize one or more solenoids to control their operation. Those skilled in the art will appreciate that the general goal of any solenoid in its control features are to move the armature quickly and efficiently from its initial air gap position to its final air gap position to meet the performance requirements of the fuel system while doing so with hardware of an acceptable cost and acceptable power/energy requirements. 
     The present disclosure recognizes that acceptable performance that meets the rigorous demands of today&#39;s fuel systems can be achieved with a lesser hardware requirement associated with the driver circuit  33  utilizing the efficient control wave form of the present disclosure. The present disclosure teaches the use of a relatively brief but high boost voltage to initiate current in the solenoid coil and then drop to battery voltage and modulate to maintain a high current level in the solenoid during the so called pull-in duration. The high boost current value should speed up the force rise at the beginning of an event to overcome other forces, such as a spring pre-load. Quickly dropping to battery voltage may lead to a relatively slower start of motion for the armature, but the higher current level achieved with battery voltage can result in armature travel times comparable to prior art wave forms that rely upon boost voltage during the entire pull in duration. The present disclosure recognizes that a major cost and performance driver is the power/energy demands of the driver circuit  33  and its associated boost power source  34 . Whereas the power/energy drawn directly from battery  50  during a majority of a solenoid actuation event is of little concern. The wave form of the present disclosure relies upon substantially less boost power/energy than that associated with the prior art, and also eliminates so called current chops to control current while the boost voltage is applied. 
     Referring now to  FIGS. 2 and 3 , an example solenoid actuation event is graphed in  FIG. 3  as per the actuation algorithm  80  of  FIG. 2 . The process starts at Start  81  and leads to a query  82  as to whether it is time to initiate a start of injection or other solenoid actuation event (pumping event in the case of a pump). If not, the logic circles back to repeat the query at a subsequent clock time for processor  31 . When it is time to start the injection event, electronic controller electrically connects a solenoid coil circuit  22  of one of the fuel injectors  15  to driver circuit  33  to apply a boost voltage  60  at the relevant solenoid coil port  38  as per box  84 . Electronic controller  30  is configured to monitor the solenoid coil current allowing for the query  85 . The monitored solenoid coil current is compared to a predetermined trigger current  65  at query  85 . If the current level  64  has not yet reached trigger current  65 , boost voltage is maintained for another time increment of processor  31 . When the answer to query  85  is yes, the logic then advances to box  86  where the solenoid coil circuit  22  is disconnected from driver circuit  33 . As expected, solenoid circuit current  64  will abruptly start decaying from the peak current associated with trigger current  65 . Next, the electronic controller  30  connects the solenoid coil circuit  22  to battery voltage  62  to complete a change from boost voltage  60  to battery voltage  62 . In the case of the wave form shown in  FIG. 3 , this connection occurs when the monitored solenoid coil current  64  reaches a minimum pull-in current  67 . On the otherhand, if the average pull-in current is set higher than trigger current  65 , the disconnection from driver circuit  33  and the subsequent connection to battery might be facilitated as quickly as possible to lessen the interference that might be caused by eddy currents in driving the current level in the solenoid circuit upward on battery voltage to a desired pull-in current level. It is important to note that at this point in the actuation event, the armature is still at its initial air gap position  23 . After the solenoid coil circuit  22  is connected to battery voltage  62 , the logic query  88  is whether the end of the pull-in duration  71  has been reached. If not, the logic advances to a subsequent query  89  to determine whether the monitored solenoid current  64  has reached the predetermined maximum pull-in current  66 . If so, the solenoid coil circuit is disconnected from battery voltage at box  90 . If not, the solenoid coil circuit remains connected to battery voltage at box  87 . After the solenoid coil circuit  22  is disconnected from battery voltage at box  90 , the logic advances to query  91  to determined whether the monitored solenoid coil current  64  has dropped down to the minimum pull-in current  67 . If so, the electronic controller  30  reconnects the solenoid coil circuit  22  to battery voltage at box  87 . This process continues during the pull-in duration  71  producing the recognized current chop profile in which the solenoid coil current  64  is mentioned to oscillate between a predetermined minimum pull-in current  67  and a maximum pull-in current  66 , that together result in an average desired pull-in current. One feature that will always appear in an efficient wave form of the present disclosure is that the current chop during the pull-in period will occur on battery voltage and not during the boost period  70  at boost voltage  60  as in prior art control wave forms. It is likely that sometime during the pull-in duration  71  that the armature will begin moving from its initial air gap position  23  toward its final air gap position  24 . 
     When query  88  determines that the end of the pull-in duration  71  has been achieved, the hold-in duration  72  is initiated by disconnecting solenoid coil circuit  22  from battery voltage at box  92 . When this occurs, the solenoid coil current  64  predictably decays as shown in  FIG. 3 . At query  93 , electronic controller  30  determines whether the end of the actuation event has been achieved. If not, the logic advances to query  94  where the monitored solenoid coil current is compared to a minimum hold-in current  69 . When the solenoid coil current level  64  has decayed to the minimum hold-in current  69 , the solenoid coil circuit  22  is reconnected to battery voltage  62  at box  95 . Next, at query  96 , the monitored solenoid coil current  64  is compared to the maximum hold-in current  68 . When the maximum hold-in current is reached, the solenoid coil circuit  22  is again disconnected from battery voltage  62  at box  92 . The hold-in duration  72  continues with the solenoid coil current  64  maintaining oscillation between the maximum hold-in current  68  and the minimum hold-in current  69  until the query  93  determines that the end of the solenoid actuation event has arrived. Thereafter, the solenoid is deenergized by opening the solenoid coil circuit  22  and disconnecting the same from battery voltage  52  to end the event at  97 . When this occurs, a spring or some other bias will push the armature from its final air gap position  24  back to its initial air gap position  23  to prepare for a subsequent actuation event. Although the wave form of the present disclosure is illustrated as including only one hold-in current level tier, two or more lower hold-in current levels during the hold-in duration  72  would also fall within the scope of the present disclosure. 
     Referring now to  FIG. 4 , two more wave forms according to the present disclosure are compared with the solid line indicating a scenario when the trigger current  65  is set to be lower than the average solenoid coil current  64  during the pull-in duration, and the dotted line showing the scenario when the trigger current  65  is set equal to the average pull-in solenoid coil current  64 . As expected, the duration of the boost voltage  60  is slightly longer with the higher trigger current  65 . However, when one actually calculates the energy required from the boost power source  34  during the boost period  70 , as much as one third less energy is required from the boost power source  34  when the pull-in solenoid current  64  on battery voltage is set higher than the trigger current  65  as per the solid line relative to that of the dotted line wave form. This substantial reduction in power/energy requirement is achieved with only a slight additional delay of when the armature starts moving from its initial air gap position  23  toward its final air gap position  24 . Thus, depending upon the specific geometry of the application, the materials utilized, the number of turns in the solenoid coil, the battery voltage, etc., engineers might choose to set the average pull-in current solenoid current  64  on battery voltage to be higher than the trigger current  65  with only a small degradation in performance, but with a substantial savings in energy required and hence hardware required by the boost power source  34 . 
     INDUSTRIAL APPLICABILITY 
     The efficient wave form for actuating solenoids according to the present disclosure finds general applicability in any high speed application that utilizes a solenoid. The present disclosure finds specific application in fuel systems generally, and especially to solenoids utilized to control fuel injection events in fuel injectors and possibly pumping events, such as in some high pressure pumps associated with common rail systems. 
     Although the disclosed strategy is taught as the electronic controller  30  monitoring a solenoid current level  64  in comparing the same to a trigger current  65 , those skilled in the art will appreciate that the wave form of the present disclosure could be carried out by monitoring a duration of the boost voltage  60  only, with or without accompanying monitoring of the solenoid current level  64 . In other words, lab experiments could correlate a boost period duration  70  with a trigger current level  65  so that duration could be monitored in place of current level and achieve similar results. However, in all cases of the present disclosure, their should be no current chop during the boost period  70  while operating on boost voltage  60  from the driver circuit  33 . Instead, all of the current chop associated with the solenoid control wave form of the present disclosure occurs on battery voltage  60 . The waveform of the present disclosure allows for comparable performance with regard to solenoid actuation, but achieves this comparable performance with a substantial lesser expenditure of power/energy during the boost period  70 . As such, the wave form of the present disclosure relaxes demands upon the hardware associated with the boost power source  34  and the drive circuit  33  to potentially reduce costs while achieving performance levels comparable to the prior art wave forms. 
     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.