Abstract:
A fuel control system for delivering gaseous fuel from a source to a gas-operated engine that includes a normally open electronic control valve connected between the fuel source and the engine, and responsive to electronic valve control signals for variably closing connection between the source and the engine. An electronic control unit supplies the electronic valve control signals responsive to engine operation. The electronic control unit includes facility for responding to termination of engine operation by automatically generating a valve control signal to hold the valve fully closed for a preselected time duration. This preselected time duration preferably includes a fixed minimum time duration and a user-programmable additional valve closure time duration.

Description:
The present invention is directed to gas-operated internal combustion engines, such as liquid petroleum (LP) and natural gas engines. The invention is more specifically directed to a system and method for controlling delivery of gaseous fuel from a source to a gas-operated engine. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     In conventional gas-operated engines, gaseous fuel, such as LP fuel or natural gas, is fed from a source under pressure through a pressure regulator/vaporizer to a mixer or carburetor. The pressure regulator/vaporizer functions to feed gas vapor at constant pressure to the mixer. The mixer functions to mix the gaseous fuel with air, and to feed the mixture to the engine intake manifold for transmission to the cylinder intake ports. It is important in fuel delivery systems of this character to provide a mechanism for terminating fuel flow when the engine ceases operation to help insure that fuel vapor will not flow to the hot engine and potentially cause a combustible mixture to form in the exhaust manifold. 
     It is a general object of the present invention to provide a fuel control system and method for gas-operated engines in which fuel flow is affirmatively terminated when engine operation ceases. Another object of the present invention is to provide a fuel control system and method for gas-operated engines that obtain improved control of air/fuel ratio during all phases of engine operation. 
     A fuel control system for delivering gaseous fuel from a source to a gas-operated engine in accordance with one aspect of the present invention includes a normally open electronic control valve for connection between the fuel source and the engine, and responsive to electronic valve control signals for variably closing connection between the source and the engine. An electronic control unit, for supplying the electronic signals responsive to engine operation, includes facility responsive to termination of engine operation for automatically generating a valve control signal to hold the valve fully closed for a preselected time duration. This preselected time duration preferably includes a fixed minimum time duration and a user-programmable additional valve closure time duration. 
     In accordance with another aspect of the present invention, the system includes an oxygen sensor for operative coupling to the engine to supply a signal indicative of air/fuel ratio around a stoichiometric value, and at least one additional sensor for supplying an electrical sensor signal as a function of engine operation. The electronic control unit includes an electronic memory in which a table of base valve control signals is stored. The control unit is responsive to electrical signals from the at least one additional sensor for obtaining a corresponding base control signal from the table. The control unit is also responsive to a signal from the oxygen sensor for modifying the base control signals as a function of air/fuel ratio at the engine, and applying the modified control signals to the electronic control valve. The control unit is also responsive to absence of a usable signal from the oxygen sensor for providing the valve control signal in a predetermined manner independent of the table. In the preferred embodiment of the invention, the electronic control unit is responsive to absence of a usable signal from the oxygen sensor upon initial start-up, when the engine has not been operating for a sufficient time for the oxygen sensor to reach operating temperature, and in the event of malfunction or severed connection to the sensor. The oxygen sensor in the preferred embodiment of the invention supplies an electrical signal that toggles or changes state at the stoichiometric value of the air/fuel ratio. Malfunction at or a severed connection to the cable is sensed as a failure of the sensor output signal to toggle for a preselected time duration, which is selectively programmable by a user. In this event, a control signal is applied to the valve to place or “park” the valve at the most recent position at which the oxygen sensor toggled from lean to rich. At start-up, the valve is positioned or “parked” at a preselected percentage of a fully opened condition, which again is preferably programmable by a user. 
     In the preferred embodiment of the invention, several parameters of valve control are selectively programmable by a user, including rate of change of the control signal as the signal from the oxygen sensor varies or toggles around the stoichiometric value. Rate of change may be selectively programmed in the preferred embodiment of the invention at differing rates for when the signal from the oxygen sensor varies from rich to lean and from lean to rich. In this way, the system may be biased toward either rich or lean operation. Other valve control parameters that are selectively programmable by the user include speed of valve movement, valve damping, the maximum closed and minimum open positions, and filtration of the input signal from the engine sensor, which preferably comprises a manifold air pressure sensor. In accordance with another feature or aspect of the present invention, the electronic control unit is coupled to a status light on an operator panel, and is programmed to flash the status light at intervals and durations indicative of differing operating conditions at the engine. All features or aspects of the invention may be implemented separately from, or more preferably in combination with, each other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with additional objects, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a fuel delivery system for a gas-operated engine in accordance with one presently preferred embodiment of the invention; and 
     FIG. 2 is a partially sectioned elevational view of the fuel delivery control valve in the system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a fuel delivery system  10  that includes an engine  12  having an engine intake manifold  14  and an exhaust line  16 . A tank  18  contains fuel, such as liquid petroleum fuel, under pressure. The outlet of tank  18  is connected to a solenoid-operated flow control valve  24  through a high-pressure lock-off valve  20  and a vaporizer/pressure regulator  22 . The outlet of valve  24  is connected to a mixer/carburetor  26 , in which the fuel is mixed with intake air and fed to engine intake manifold  14 . Thus, fuel from tank  18  is vaporized at  22  and fed at regulated pressure to flow control valve  24 . Fuel from valve  24  is mixed with air at  26 , and fed to engine intake manifold  14  for combustion at engine  12 . 
     An electronic control unit or ECU  28  receives an input signal from an exhaust oxygen sensor  30  mounted on exhaust line  16 . A manifold air pressure sensor  32  is responsive to pressure within manifold  14  for supplying a corresponding signal to ECU  28 . An engine speed sensor  34  is responsive to the speed of operation at engine  12  for supplying a corresponding signal to ECU  28 . ECU  28  receives input information from an operator input unit  36 , such as a suitably programmed personal computer. ECU  28  provides control signals to valve  24  as a pulse-width modulated output to the solenoid of the control valve. ECU  28  is also connected to one or more lamps  38 , such as a check-engine light at the vehicle dashboard, for indicating engine operating status and fault conditions to an operator. 
     FIG. 2 illustrates control valve  24  in greater detail. A solenoid coil  40  is mounted on one end of a valve body  42 . Solenoid coil  40  has a connector  44  for connection to ECU  28  (FIG.  1 ). A valve disk  46  is carried by the armature  48  of solenoid coil  40 , and is normally biased by a conical coil spring  50  away from a valve seat  52 . Thus, valve  24  is a normally open valve, which means that fuel may normally flow from an inlet  54  to an outlet  56  in valve body  42  in the absence of suitable control signals to solenoid  40 . Pulse-width modulated control signals to solenoid  40  from ECU  28  selectively moves valve disk  46  toward seat  52  against the force of spring  50  for modulating flow of fuel to the engine. O-rings  58 ,  60  are provided for sealing valve body  42 . 
     In accordance with the preferred embodiment of the invention, base valve control signals are stored in electronic memory within ECU  28 , which preferably is a suitably programmed microprocessor-based ECU. These base valve control signals preferably are stored in the form of a look-up table as a function of manifold air pressure from sensor  32  and engine speed from sensor  34 . In a presently preferred embodiment of the invention, the fuel map table comprises an eight-by-eight table having sixty-four entries, typically between about 12% and 90% open at the valve. Thus, for a given engine speed and manifold air pressure, there is a corresponding base valve control signal in the stored electronic table that seeks to achieve stoichiometric operation at the engine. These base control signals are calibrated and stored in any suitable manner, such as that disclosed in U.S. Pat. No. 5,091,858 assigned to the assignee hereof. Thus, for a given manifold air pressure and engine speed, ECU  28  can obtain a base control signal from the look-up table, which provides a base pulse width for application to control valve  24 . In the event of a change in engine operating conditions, such as an increase in engine demand and a corresponding change in manifold air pressure, ECU  28  obtains a corresponding new base valve control signal from the look-up table. Other engine condition sensors, such as an engine temperature sensor and a throttle position sensor, can be employed for selectively modifying the base control signal, as disclosed in above-noted U.S. Pat. No. 5,091,858, the disclosure of which is incorporated herein by reference for purposes of background. 
     Closed-loop fuel control is obtained in accordance with one aspect of the present invention by means of oxygen sensor  30  coupled to engine exhaust  16 . Oxygen sensor  30  preferably is of conventional construction, providing a bistable electrical output signal to ECU  28  that toggles or changes state as the air/fuel ratio passes through the stoichiometric level. Thus, the output of oxygen sensor  30  will be at one voltage level when the air/fuel ratio is above the stoichiometric level, and will be at a second voltage level when the air/fuel ratio is below the stoichiometric level. The output of the oxygen sensor is monitored by ECU  28  to detect a change of state between rich and lean conditions. If a lean condition is detected, the base valve control signal from the look-up table is modified within ECU  28  so that valve  24  is moved to a more open position to allow more fuel to mix with the intake air. If a rich condition is detected, the base valve control signal from the look-up table is sufficient so that the valve is moved toward a more closed position, decreasing the amount of fuel added to the intake air. The output of sensor  30  is monitored at preselected intervals, and valve  24  is then moved incrementally to force the bistable output of sensor  30  to change state or toggle. This incremental trimming of the base valve control signal continues indefinitely until a changed input from sensor  32  or  34  causes a new base control signal to be obtained. 
     One important feature of the present invention is to provide default settings for the valve control signal when the oxygen sensor output does not toggle state, such as when the engine has not been running for a time sufficient for the oxygen sensor to reach operating temperature, or when there is a malfunction at or severed connection to the sensor. At start-up, valve  24  will be set or “parked” at a percentage of the fully closed condition that is user programmable, as will be described. This is usually around 45% to 50% closed. During all other scenarios in which the oxygen sensor does not change state, ECU  28  detects an apparent malfunction at the sensor, and the valve is positioned or “parked” at the most recent position at which the oxygen sensor toggled from lean to rich. If the valve has already been moved to a maximum distance (either open or closed) permitted by the corresponding user-programmed parameter as will be described, the system is run open-loop, and a base valve control signal obtained from the look-up table in ECU  28  is used without modification for valve control purposes. In all movements, if the output of the oxygen sensor fails to toggle at a previous toggle point, rate of movement at the valve is increased. When oxygen sensor toggling then resumes, the rate of movement is decreased for direction reversal. 
     In accordance with another feature of the present invention, ECU  28  may be connected to an operator input device  36 , such as a suitably programmed personal computer, for selectively setting and varying various control parameters. These control parameters preferably include: 
     (1) Valve movement speed, which determines how fast the valve moves in making its initial adjustment to the value obtained from the fuel map in the look-up table. This value represents the number of counts that are added to or subtracted from the “on” time for the pulse-width modulated control value obtained from the look-up table. The output of oxygen sensor  30  determines the direction of movement. 
     (2) Damping value, which controls the time interval between each update of the pulse-width modulated control signal. This helps account for the fact that the oxygen sensor does not instantly detect changes in air/fuel mixture. 
     (3) Trim region around the point obtained from the look-up table, which controls the permitted range of trimming of the table base value. 
     (4) Maximum closed percent, which sets the maximum permitted closed percent at the valve. A typical value may be sixty percent closed. 
     (5) Minimum closed percentage at the valve, which may be on the order of twelve percent. 
     (6) Default percent closed on engine start-up. Until the engine temperature increases to operating temperature, fuel table readings and oxygen sensor output are not used. A typical value may be forty-five percent. 
     (7) A trim point bias toward either lean or rich operation. This feature takes advantage of the time lag in the output of the oxygen sensor to changes from rich to lean or lean to rich at the fuel input. To bias the air/fuel ration to a richer level than the stoichiometric level, the valve opening rate in response to a rich-to-lean toggle at the oxygen sensor is set greater than the valve closing rate in response to a lean-to-rich toggle at the oxygen sensor. This causes the valve to overshoot the stoichiometric level by opening slightly farther than required. Since the valve closing rate is less, the valve will spend more time in the rich region. This overshoot is controlled by this control parameter, called “trim bias.” The bias may also be set to a lean condition, or to zero. Other parameters that may be set by the user include filtration of the manifold air pressure sensor output, and time delay before failure of the oxygen sensor output to toggle will indicate a fault condition. 
     In accordance with another feature of the invention, the user may program ECU  28  to hold valve  24  closed for a programmed timed duration after operation is terminated at engine  12 . This helps prevent gaseous fuel collected in vaporizer  12  and the fuel lines from migrating through mixer  26  into manifold  14  and into the hot exhaust manifold. ECU  28  preferably is programmed to hold valve  24  closed for a minimum time duration after the engine terminates operation, such as a time duration of five seconds. This time duration may be selectively increased by user programming, with a typical programmed parameter value being thirty seconds, thereby holding valve  24  closed for a total of thirty-five seconds after the engine terminates operation. 
     In accordance with yet another feature of the present invention, ECU  28  is programmed to flash light  38  at intervals and durations indicative of differing operating conditions at engine  12 . The following table is illustrative, with “long” flashes being on the order of about one second in duration, and “short” flashes being on the order of about 0.5 seconds in duration. There is approximately a two second interval between flash codes: 
     
       
         
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Diagnostic Codes 
               
             
          
           
               
                   
                   
                   
                 General 
                   
               
               
                 Device 
                 Open 
                 Shorted 
                 Failure 
                 OK 
               
               
                   
               
               
                 ECU Processor 
                 N/A 
                 N/A 
                 N/A 
                 10 short 
               
               
                 Battery 
                 N/A 
                 N/A 
                 1 long 1 short 
                 N/A 
               
               
                 O 2  Sensor 
                 N/A 
                 N/A 
                 1 long 2 short 
                 N/A 
               
               
                 MAP Sensor 
                 1 long, 3 short 
                 1 long, 4 short 
                 N/A 
                 N/A 
               
               
                 Water Temp Sensor 
                 1 long, 5 short 
                 1 long, 6 short 
                 N/A 
                 N/A 
               
               
                 Air Temp (EFI) 
                 1 long, 7 short 
                 1 long, 8 short 
                 N/A 
                 N/A 
               
               
                 Governor Actuator 
                 2 long, 1 short 
                 2 long, 2 short 
                 N/A 
                 N/A 
               
               
                 IAC Actuator 
                 2 long, 3 short 
                 2 long, 4 short 
                 N/A 
                 N/A 
               
               
                 OETune/Low Press Valve 
                 3 long, 1 short 
                 3 long, 2 short 
                 N/A 
                 N/A 
               
               
                 Lock-off Valve 
                 3 long, 3 short 
                 3 long, 4 short 
                 N/A 
                 N/A 
               
               
                   
               
             
          
         
       
     
     There have therefore been disclosed a system and method for delivering gaseous fuel to a gas-operated engine that fully satisfy all of the objects and aims previously set forth. The presently preferred embodiment of the invention has been disclosed, together with a number of possible modifications and variations. Other modifications and variations will readily suggest themselves to persons skilled in the art. The invention is intended to encompass all such modifications and variations as fall within the spirit and broad scope of the appended claims.