Abstract:
Prior to the operation of a solenoid type of fuel injector, a DC voltage is applied across the injector to create a current through the injector that is below the activation current of the injector. A capacitor is then placed in series with the injector and the flyback energy from the injector transfers a charge onto the capacitor. When the injector current drops to a predetermined level, the capacitor is removed from the circuit and isolated. This process is repeated until a minimum charge is on the capacitor. By placing the capacitor charge onto the injector at the time that the injector is to be activated, the opening response of the injector is improved. By applying the charge on the capacitor to the injector in a manner to neutralize the eddy currents when the voltage across the injector is removed, the closing response is improved.

Description:
TECHNICAL FIELD 
   The present invention relates to the art of the electronic control of the solenoid in a fuel injector in an internal combustion engine. 
   BACKGROUND OF THE INVENTION 
   The accurate control of the activation and deactivation of solenoids in fuel injectors in internal combustion engines is of importance since the operational characteristics of the fuel injector affect the efficiency of the engine. While fuel injectors have traditionally been driven by the battery voltage in a vehicle, a higher voltage has been used in the prior art to improve the rise time characteristics of the current through a fuel injector. Still, it is desirable to further improve the performance of a fuel injector. 
   Therefore, it is a primary object of the invention to improve the performance of a fuel injector. 
   SUMMARY OF THE INVENTION 
   Briefly described, a method of operating a solenoid includes applying a voltage across the solenoid so that a current of a first magnitude flows through the solenoid. The voltage across the solenoid is stopped and the flyback energy in the solenoid is routed to a capacitor such that charge is transferred to the capacitor until the current through the solenoid falls to a second magnitude. The voltage is reapplied at the same time that the capacitor is isolated from the solenoid until the current through the solenoid again reaches the first magnitude at which time the voltage is interrupted and the flyback energy is used to further charge the capacitor. The voltage on the capacitor is applied across the solenoid such that the current through the solenoid reaches a third magnitude. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram of a fuel injector control circuit according to the present invention; 
       FIG. 2  is a graphical representation of the voltage at one terminal of an injector and the current through the injector driven by a prior art injector driver; 
       FIG. 3  is a graphical representation of the voltage at one terminal of an injector and the current through the injector using the driver circuit of  FIG. 1  in a first method of operation; 
       FIG. 4  is a graphical representation of the voltage at one terminal of an injector and the current through the injector using the driver circuit of  FIG. 1  in a second method of operation; 
       FIG. 5  is a schematic diagram of the circuit of  FIG. 1  modified by the addition of an external voltage source; 
       FIG. 6  is a graphical representation of the voltage at one terminal of an injector and the current through the injector using the driver circuit of  FIG. 1  in a third method of operation; and 
       FIG. 7  is the schematic diagram of the circuit of  FIG. 1  modified by the removal of two of the diodes. 
   

   It will be appreciated that for purposes of clarity and where deemed appropriate, reference numerals have often been repeated in the figures to indicate corresponding features, and that the various elements in the drawings have not necessarily been drawn to scale in order to better show the features of the invention. 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic diagram of a fuel injector control circuit  10  according to the present invention. The diagram  10  shows a first solenoid, such as a fuel injector,  12 , labeled “Solenoid  1 ” in  FIG. 1 , and a second solenoid, such as a fuel injector,  14 , labeled “Solenoid  2 .” Battery voltage  16 , labeled “Battery Supply Voltage,” placed in parallel with a voltage stabilizing capacitor  18 , is coupled through the anode-to-cathode junction of a diode  20  and an n-channel transistor  22 , labeled “Hi-Side,” to a node  24 . Node  24  is connected to the upper terminals of the injectors  12  and  14 , and coupled to chassis ground through the anode-to-cathode junction of another diode  26  and another n-channel transistor  28 , labeled “Reverse Ground Path.” A third diode  30 , labeled “Recirculation Diode,” couples node  24 , connected to the cathode of the diode  30 , to chassis ground. 
   The lower terminal of injector  12  at a node  32  is coupled through another n-channel transistor  34 , labeled “Lo-Side  1 ,” to a node  36  which, in turn, is coupled to chassis ground through a solenoid current sensing resistor  38 , labeled “Solenoid Current Sense.” Voltage amplifier  40  provides an output signal at terminal  42  indicative of the current through the current sensing resistor  38 . Node  32  is also coupled through the anode-to-cathode junction of a diode  46 , that is in parallel with the drain and source of a p-channel transistor  48 , labeled “Reverse  1 ,” to a node  50  that, in turn, is coupled through a storage capacitor  52 , labeled “Storage Capacitor,” an n-channel transistor  54 , labeled “Charge Capacitor Enable,” and a charge current sensing resistor  56 , labeled “Charge Current Sense,” to chassis ground. Voltage amplifier  58  provides a signal at terminal  60  indicative of the current through the charge current sensing resistor  56 . A third voltage amplifier  62 , having one input connected to node  50  and the other input connected to chassis ground, provides an output signal at terminal  64  indicative of the voltage at node  50 . 
   The lower terminal of injector  14  is coupled through another n-channel transistor  44 , labeled “Lo-Side  2 ,” to the node  36 . The lower terminal of injector  14  is also coupled through the anode-to-cathode junction of a diode  66 , that is in parallel with the drain and source of a p-channel transistor  68 , labeled “Reverse  2 ,” to the node  50 . The node  50  is coupled through a p-channel transistor  70 , labeled “Boost,” and the anode-to-cathode junction of a diode  72  to the junction of the diode  20  and the n-channel transistor  22 . Diodes  46  and  66  are used because they have better forward bias and switching characteristics than the intrinsic diodes of the transistors  48  and  68 , but could be eliminated if the intrinsic diodes of the transistors  48  and  68  have acceptable forward bias and switching characteristics. 
   An external high voltage can be connected at terminal  74 , labeled “External Charge Supply,” which, in turn, is coupled to node  50  through the anode-to-cathode junction of a diode  76 . 
   Transistor  34  has its drain coupled to its gate by the series combination of a cathode-to-anode junction of a zener diode  78  and an anode-to-cathode junction of a diode  80 . The gate of transistor  34  is driven by a FET driver circuit  82 . Similarly, n-channel transistor  44  has its drain coupled to its gate by the series combination of a cathode-to-anode junction of a zener diode  84  and an anode-to-cathode junction of a diode  86 , and the gate of transistor  44  is driven by a FET driver circuit  88 . 
   It will be understood that the circuit  10  of  FIG. 1  is arranged to drive the two injectors  12  and  14  in the same manner but not at the same time. Although two injectors are shown in  FIG. 1 , any number of injectors can be included in the circuit  10  of  FIG. 1 . 
     FIG. 2  is a graphical representation  90  of the voltage  92  at node  32  and the current  94  through the injector  12  driven by a prior art injector driver. As can be seen in  FIG. 2 , the initiation of an injector command  96  is coincident with the initiation of a peak mode phase  98  and causes the current  94  through the injector  12  to rise to a desired peak current  100  in approximately 330 μs. When the peak mode  98  ends, a hold mode phase  102  begins and stays active until the end of the injector command  96 . During the hold mode  102 , the injector current  94  is lower than during the peak mode  98 , but at a level to hold the armature in the solenoid in the injector  12  in the fuel delivery position after the peak mode  98  operation has caused the injector current  94  to rise high enough to move the solenoid armature into the fuel delivery position. 
   These waveforms could be produced by the circuit  10  of  FIG. 1  by disabling all of the transistors except transistors  22  and  34 . Transistor  22  would be selectively enabled to increase the current through the injector  12  and would be disabled to allow the injector  12  current to fall, and transistor  34  would be on throughout the duration of the injector command  96 . The current through the injector  12  would be sensed by the current sensing resistor  38  and amplifier  40 . When a predetermined peak current is detected, during both the peak mode  98  and the hold mode  102 , transistor  22  would be turned off and the current through the injector  12  would be routed through the diode  30  and the transistor  34  to thereby effectively short circuit the terminals of the injector  12 . Similarly, when the injector current  94  would have decayed to a predetermined lower current, the transistor  22  would be enabled again. 
     FIG. 3  is a graphical representation  110  of the voltage  112  at node  32  and the current  114  through the injector  12  using the driver circuit  10  of  FIG. 1  in a first method of operation according to the present invention. In the first method of operation as shown in  FIG. 3 , at the same time as the initiation of the injector command  96 , a charge mode phase  116  is initiated. In the charge mode phase  116 , transistors  22  and  54  remain conductive and transistor  34  is initially conductive to allow current to build up in the injector  12 . When a pre-determined peak current  117  is detected using the current sensing resistor  38  and voltage amplifier  40 , transistor  34  is turned off and the flyback energy from the injector  12  is captured by the storage capacitor  52  with the injector  12  current flowing through the diode  46 , storage capacitor  52 , transistor  54 , and charge current sensing resistor  56 . Once the current through the charge current sensing resistor  56  has dropped to a second lower level  120 , transistor  34  is turned back on and the cycle is repeated. The RMS current  118  during the charge mode  116  is less than the current necessary to move the pintle or armature in the solenoid of the injector  12 . This method essentially uses the injector  12  in a voltage boost mode configuration. The voltage  112  in  FIG. 3  is at zero volts when transistor  34  is conductive (when the injector current  114  is increasing) and becomes the voltage level  122 , which is a diode drop above the voltage at node  50 , when transistor  34  is nonconductive. Zener diode  78  determines the upper limit of the voltage on node  32  to avoid overstressing the transistor  34 . This upper limit in the preferred embodiment is about 50 volts. Although the duration of the charge mode  116  is usually set to last a predetermined time, with the peak mode phase  98  and a current boost mode phase  126  beginning at the termination of the charge mode  116 , the voltage amplifier  62  can be used to terminate the charge mode operation once a desired voltage at node  50  has been reached. If the charge mode  116  duration is determined by the output of the voltage amplifier  62 , the peak mode  98  and boost mode  126  could be delayed in order to deliver fuel to the engine at the proper time. 
   In the boost mode  126 , transistors  22 ,  34 ,  54 , and  70  are conductive to apply the voltage present at node  50  (approximately 50 volts in the preferred embodiment) across the injector  12 . Placing this capacitor voltage across the injector  12  sharply decreases the rise time in the peak mode phase  98  of operation from approximately the 336 μs of  FIG. 2  to approximately 104 μs as shown in  FIG. 3 . At the end of the boost mode  126 , which occurs sometime after the peak operating current  128  of the injector  12  has been reached, the transistors  70  and  54  are turned off. The operation of the circuit  10  after the end of the boost mode phase  126  is the same as the operation of the circuit  10  described above with respect to  FIG. 2 . 
     FIG. 4  is a graphical representation  130  of the voltage  132  at node  32  and the current  134  through the injector  12  using the driver circuit of  FIG. 1  in a second method of operation according to the present invention. The second method differs from the first method of  FIG. 3  in that the charge built up on the storage capacitor  52  is not applied to the injector  12  at the beginning of the peak mode  98 , but rather the voltage on the storage capacitor  52  is applied shortly after the end of the injector command  96  in a direction to reverse the voltage across the injector  12  and quickly collapse the magnetic field and eddy currents in the injector  12 . This results in improved injector closing response. More specifically, the charge mode  116  is the same as described above for  FIG. 3 , and the peak mode  98  and hold mode  102  are the same as described above for  FIG. 2 . At the termination of the injector command  96 , a delay  136  is provided to allow the injector current  134  to decay to zero amps when the flyback voltage across the injector  12  quickly reduces the injector current  134 . At the end of the delay  136 , a reverse mode phase  138  begins by enabling transistors  48 ,  28  and  54  to apply the reverse voltage to the injector  12 . The duration of the reverse mode  138  is a predetermined time. The rise time of the injector current  134  is improved from 336 μs of  FIG. 2  to 156 μs in  FIG. 4  due to the reduction in the eddy currents in the injector  12  during the charge mode  116 . This reduction is most beneficial if the peak mode  98  begins at the end of the charge mode  116 . 
     FIG. 5  is  FIG. 1  with the addition of an external voltage supply  142 . The external voltage supply  142  is applied to node  50  through the anode-to-cathode junction of a diode  76 . The transistor  54  is conductive in this third method of operation and the storage capacitor  52  operates as a voltage stabilizing capacitor. 
     FIG. 6  is a graphical representation  150  of the voltage  152  at node  32  and the current  154  through the injector  12  using the driver circuit of  FIG. 5  in a third method of operation according to the present invention. In the third method of operation, an external voltage supply  142  is applied to terminal  74 . Since the external voltage supply  142  is applied to node  50 , there is no need for a charge mode  116 , and both the boost mode  126  and reverse mode  138  can be used since external voltage supply  142  does not lose charge as does the storage capacitor  52  when current is drawn from node  50 . 
     FIG. 7  is the driver circuit  10  of  FIG. 1  with the diodes  26  and  30  removed. The transistor  28  would then be enabled at the appropriate times to provide a current path to chassis ground when either diode  26  or diode  30  were to be conductive in the operation of the driver circuit  10  of  FIG. 1 . 
   While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.