Abstract:
Lift pump guard for a pump operable by an electrical motor. The lift pump guard monitors variable parameters and actuates a relay to electrically terminate operation of the pump motor when the pump is dry running. Also a method of protecting a pump from dry running compares a measured parameter to a predetermined value and terminates operation based on the comparison.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a lift or transfer pump for supplying fuel from a tank to an internal combustion engine. More particularly, the invention relates to a device and method to prevent dry running of a lift pump. 
     Internal combustion engine powered devices typically include a lift pump to transfer fuel from a remote tank to the engine fuel supply. The engine fuel supply may comprise a carburetor, a fuel injection system, or a high pressure fuel injection pump. While lift pumps may be mechanically driven, electric motor powered lift pumps have become common place. These electric powered lift pumps are energized to operate whenever the engine electrical system is in a start or run condition. The lift pump, or a downstream portion of the fuel system, will include a pressure switch or a pressure regulator and return line to prevent “dead heading” of the lift pump. Thus, it is possible for the fuel pump to continue operating when the ignition is in the on condition but the engine is not operating. 
     The ability of the lift pump to operate when the engine is not also operating leads to a number of potential problems. In the case of an ignition switch inadvertently left in the on position with the engine not operating, the lift pump will operate and drain the battery. Further, continuous operation of the lift pump circulates fuel through the fuel system and back to the tank leading to fuel heating. More seriously, if the fuel tank empties so that the lift pump runs dry, the continuously operating lift pump motor can overheat. In severe cases of dry running, the motor commutator melts, requiring costly and time consuming replacement of the lift pump before the vehicle can be restarted. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the invention in a preferred form is an electric lift pump guard which prevents unintended dry running of the lift pump. In one embodiment, the lift pump guard monitors current flowing through the electric motor powering the lift pump. This is accomplished by monitoring the voltage drop across a precision value sensing resistor within the lift pump guard circuit. This voltage is compared with a standard or threshold voltage. Under normal conditions, when the lift pump is pumping, the measured voltage is greater than the threshold voltage. When the lift pump motor is operating but no fuel is being pumped, the measured voltage falls below the threshold voltage creating a low voltage condition. After a predetermined time the low voltage condition will open the normally closed connection of a relay, thereby disconnecting the electric motor of the lift pump from the power supply circuit. The relay remains in the off condition until it is reset. Resetting of the relay is accomplished by interrupting the power supply to the lift pump guard. 
     In an alternative embodiment of a lift pump guard for a diesel engine, the pump guard monitors the voltage at an electric shutoff (ESO) solenoid of a high pressure fuel injection pump as well as at an oil pressure switch. The voltages are compared to preestablished values. As long as one of the measured voltages is above the preestablished value, the normally open connection of a relay is held closed, thereby energizing the lift pump electric motor. When the engine runs out of fuel it stalls, the voltage values are below the predetermined level and the relay will de-energize, terminating electricity to the lift pump motor. This embodiment preferably includes a momentary contact switch which can be used to manually activate the lift pump to allow purging of air from the fuel system. 
     Preferably, the lift pump guard in any embodiment is designed so that it may comprise a sealed housing. Wiring leads or harnesses extend from the housing and via detachable electrical connectors allow the lift pump guard to be added to existing or new electrical systems quickly and inexpensively. 
     An object of the invention is to provide an automatic shutoff device for a lift pump of a fuel supply system. 
     Another object of the invention is to provide an automatic shutoff device to prevent dry running of a lift pump which device is compact and can be efficiently incorporated into presently existing wiring harnesses or electrical connections. 
     A further object of the invention is to provide a new and improved method for monitoring and controlling a lift pump to prevent dry running. 
     Other objects and advantages of the invention will become apparent from the specification and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings, in which: 
     FIG. 1 is an electrical schematic diagram of an embodiment of a lift pump guard; 
     FIGS. 2 a  and  2   b  are overhead and side views, respectively, of the lift pump guard of FIG. 1 in a sealed enclosure; and 
     FIG. 3 is an electrical schematic diagram of an alternative embodiment of a lift pump guard; and 
     FIG. 4 is a schematic diagram of a fuel system incorporating one embodiment of a lift pump guard. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings, wherein like numerals represent like parts throughout the several figures, a preferred lift pump guard in accordance with the present invention is generally designated by the numerals  10  or  100 . The lift pump guard  10 ,  100 , is designed for lift pumps  12  powered by direct current electrical motors  14 . While the following description refers to the use of a lift pump guard  10 ,  100  in a vehicular environment, it should be understood that the lift pump guard  10 ,  100  may also be employed in any environment where there is a concern about dry running of a pump  12 , such as, for example, stationary power generators powered by internal combustion engines. 
     The lift pump guard will be described with respect to a 12 volt direct current (DC) power supply which is commonly used in marine and automotive applications of internal combustion engines. The 12 VDC is commonly stored in a battery and/or generated by an alternator and rectifier attached to the electrical system of the internal combustion engine. 
     Typically, the engine electrical wiring harness portion leading to the lift pump  12  is selectively energized, that is, that portion  16  of the wiring harness is only energized when the ignition is in the start or run position. When the ignition is in the off position, no electrical current is supplied to that portion  16  of the wiring harness. As shown in FIG. 1, the inventive lift pump guard  10 is disposed between the engine wiring harness portion  16  and the lift pump  12 . The use of detachable electrical connectors Y 1 , Y 2  allows the lift pump guard  10  to be quickly and easily connected to the lift pump  12  and wiring harness portion  16 . 
     A normally closed relay K 1  in the pump guard  10  connects one leg of the electrical harness portion  16  to the lift pump motor  14 . Thus, when the ignition is switched from an off position to a start or run position, the lift pump motor  14  is immediately energized through the pump guard  10  for fuel transfer. 
     The lift pump guard  10  monitors current flowing through the lift pump motor  14  by measuring the voltage drop across precision value sense resistor R 1  at different lift pump operating conditions. Preferably, resistor R 1  has a tolerance of 1% or better to ensure accurate measurement by the pump guard  10 . When the lift pump  12  is transferring fuel from a fuel tank  24  to a high pressure fuel injection pump  26  or an engine  28  (shown in FIG.  4 ), or running “wet”, the pump motor  14  will be under a load and, for example, draw approximately 2.5 amperes. Under these conditions resistor R 1 , and thereby input pin  2  of comparator U 1 , will be at a voltage of 250 mV. The input voltage at input pin  2  exceeds the 200 mV at input pin  3  which is set by resistors R 3  and R 4  functioning as a voltage divider. As long as the voltage at input pin  2  is greater than that of input pin  3 , output pin  1  of comparator U 1  will remain in a low state. When output pin  1  is in a low state, input pin  5  of comparator U 1  will be at a voltage less than input pin  6 . Therefore, output pin  7  of comparator U 1  will be in a low state, transistor Q 1  will be in an off state and relay K 1  remains de-energized and in the normally closed condition. The lift pump motor  14  is accordingly supplied power from the electrical harness  16  through the normally closed connection of relay K 1 . 
     When the fuel tank  24  empties so that the lift pump  12  does not transfer fuel and therefore is running “dry”, the motor  14  is under a lessened load and, for example, will draw approximately 1.5 amperes. Under these conditions, resistor R 1 , and thereby input pin  2  of comparator U 1 , will be at a voltage of 150 mV. This voltage is below the 200 mV on input pin  3  and therefore output pin  1  becomes energized. The voltage from output pin  1  charges capacitor C 2  through resistor R 6 , maintaining a low voltage on pin  5  as long as capacitor C 2  is charging. After a predetermined time capacitor C 2  becomes charged, and the voltage on pin  5  of comparator U 1  rises above 9 volts. When the voltage on input pin  5  exceeds the voltage on input pin  6 , output pin  7  of comparator U 1  becomes energized. The voltage supplied by output pin  7  causes transistor Q 1  to turn on, thereby energizing relay K 1 . In the energized condition, the normally closed relay connection opens, stopping the flow of electric current from the engine harness  16  to the lift pump motor  14 . Relay K 1  remains energized until power to the engine harness portion  16  is interrupted, as by placing the ignition in the off position. 
     The RC circuit combination of resistor R 6 , capacitor C 2  and the predetermined input voltage at pin  6  creates a delay of approximately 1 minute between the time the lift pump  12  starts to run dry and the time at which the relay K 1  connection opens, shutting off the lift pump  12 . This delay is short enough to prevent damage to the fuel pump  12  but long enough to accommodate minor fluctuations in current drawn by motor  14 . Naturally, other combinations may be used to provide delays with different time periods. 
     While the lift pump guard as described comprises a comparator U 1 , an operation amplifier (not shown) may be substituted. Resistor R 2  and capacitor C 1  are used in the lift pump guard  10  to reduce any ripple produced by the motor  14 . Resistor R 5  provides a discharge path for capacitor C 2 . The normally open connection of relay K 1  is used to lower the voltage on pin  6  of comparator U 1  when the relay is switched, thereby minimizing chattering of the relay. Capacitors C 3  and C 4  function to prevent the lift pump motor  14  from turning off when the lift pump guard  10  is exposed to external electromagnetic radiation. 
     As shown in FIGS. 2 a  and  2   b , the lift pump guard  10  may further comprise a housing or compact enclosure  20  which, for example, may be approximately 26 mm by 38 mm by 64 mm. The enclosure  20  may be environmentally sealed and include wire leads  22  connecting the pump guard  10  to detachable wiring harness connectors Y 1 , Y 2 . Each connector Y 1 , Y 2  is preferably configured or polarized to prevent incorrect electrical connection of the lift pump guard  10  between the wiring harness  16  and the lift pump motor  14 . 
     The above embodiment is described in the context of a lift pump  12  having a 12 volt direct current motor  14  operating with a normal current draw of approximately 2.5 amps and a dry running current draw of approximately 1.5 amps. It should be understood the invention encompasses use with other lift pumps with motors of different operating voltages and having different current draws and different threshold settings. 
     A second embodiment of a lift pump guard  100  is shown in FIG.  3 . This embodiment is especially adapted for use with diesel engines having a solenoid activated electric shutoff (ESO)  120  and oil pressure switch  122 . Typically, the electric shutoff  120  will be found connected to a high pressure fuel injection pump  26  (see FIG. 4) to stop the flow of fuel to a diesel engine to thereby force engine shutdown. The oil pressure switch  122  will be operably connected to the engine lubrication system. The lift pump guard  100  is electrically disposed between the lift pump  12  and an engine wiring harness portion, shown in FIG. 3 as individual connections. The wiring harness portion includes connections to an electrical power source, shown as a battery  124  in FIG. 3, an electrical ground, and terminals on the oil pressure switch  122  and electric shut off  120 . When a diesel engine is started, the oil pressure in the lubrication system increases over a short time period, typically about 30 seconds. At a predetermined pressure, the normally closed oil pressure switch  122  opens. Until the oil pressure switch  122  opens, there is no voltage available at the switch terminal. 
     The ESO solenoid  120  is energized to start the engine and must remain energized while the engine is operating. At engine startup, the ESO solenoid terminal supplies 11 volts to pin  105  of comparator U 101 . Resistors R 103  and R 104 , functioning as a voltage divider, supply 1.0 volt to input pins  102  and  106 . The 11 volt input at pin  105  is greater than the 1.0 volt threshold at pin  106  and allows a high voltage at output pin  107  of device U 101 . The high voltage at output pin  107 , acting through diode D 103  and resistor R 106 , turns on transistor Q 101 , which energizes relay K 101 , closing the normally open connection of the relay. Energized relay K 101  completes the electrical circuit between the lift pump motor  14  and the vehicle harness portion, thereby energizing the pump  12  for transfer of fuel from the tank  24  to the high pressure fuel injection pump  26 . 
     During the first 30 seconds after engine startup, capacitor C 101  charges and the initial voltage of 11 volts at input pin  105  decays below 1.0 volt. Below 1.0 volt, the voltage on input pin  105  is insufficient to maintain comparator U 101  output pin  107  in the energized state. 
     However, before the voltage at pin  105  has fallen below the threshold, the oil pressure switch  122  opens, placing a voltage at pin  103  above the comparator U 101  threshold set by pin  102 . The high voltage on pin  103  allows a high voltage at output pin  101 . The high voltage at pin  101 , acting through diode D 102  and resistor R 106  maintains transistor Q 101  in the on state, thereby maintaining relay K 101  and lift pump motor  14  in an energized state. 
     Thus at startup, pin  105  is above the comparator U 101  threshold, but pin  103  is below the threshold. Within 30 seconds after engine startup, pin  105  is below the threshold, but pin  103  is above the threshold. As long as one of input pins  105  and  103  is energized above the threshold, one of output pins  101  and  107  will remain high, relay K 101  will remain energized and lift pump motor  14  will continue to operate. 
     When the engine runs out of fuel the engine will stall and the oil pressure in the lubrication system will drop. Below the predetermined oil pressure, switch  122  will become closed so that input at pin  103  is at a low level. The voltage at input pin  105  previously decayed below the threshold level. The low voltage at input pins  103  and  105  causes output pins  101  and  107  of device U 101  to become low, which then turns off transistor Q 101  thereby de-energizing relay K 101  and opening the electrical connection between the lift pump motor  14  and the wiring harness portion. Since the lift pump guard  100  will switch the lift pump motor  14  off after 30 seconds if the engine is not operating, this embodiment performs the additional function of preventing battery  124  rundown in situations wherein the ignition is left on but the engine is not operating. 
     In fuel systems for diesel powered engines, any air introduced into the fuel system during the changing of a fuel filter must be purged. While a delay of 1 minute before pump shutoff will usually be adequate for purging air from a diesel engine fuel system, the delay of 30 seconds in the above described embodiment is likely to be unacceptable. For this reason, the lift pump guard  100  preferably incorporates a manually activated air purging circuit. The air purging circuit incorporates a momentary contact switch S 100 . Switch S 100  when depressed bypasses comparator U 101  and transistor Q 101 , thereby energizing relay K 101  to start the lift pump motor  14 . As soon as the switch is released, the contact is broken and the lift pump is de-energized. 
     Diode D 101  functions to protect comparator U 101  input and capacitor C 101  from excessive voltage, high voltage spikes or reverse polarities which may be generated by the ESO solenoid during shut off. Resistor R 101  functions to maintain comparator U 101  input pin  103  high in the event an oil pressure indicator light burns out. Resistor R 101  also functions to prevent voltage “float” to input pin  103  caused by the mechanical oil pressure switch transitioning from an open to closed condition and vice versa. Diodes D 102  and D 103  prevent cross voltage on output pins  101  and  107 . Capacitor C 102  functions to filter RF noise. Naturally, an operation amplifier (not shown) may be used in place of the comparator U 101 . 
     While the above embodiment is described in the context of a lift pump  12  having a 12 volt direct current motor  14 , it should be understood the invention encompasses use with lift pumps of different operating voltages having different threshold settings. While not shown, the lift pump guard  100  is preferably enclosed in an environmentally sealed housing and electrically connected with detachable wiring harness connectors in a manner similar to that shown in FIGS. 2 a  and  2   b.    
     While preferred embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and the scope of the present invention.