Abstract:
A fuel pressure control for an alternate-fuel engine utilizes adaptively learned corrections for fuel injection pulsewidth to dynamically adjust a base fuel pressure control signal. The resulting fuel pressure control can thus be characterized as open-loop with closed-loop correction based on air/fuel ratio error. The control dynamically adjusts the fuel pressure in a manner to optimize the air/fuel ratio control instead of controlling to a predetermined pressure, and fuel pressure measurement is not required.

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel control for an internal combustion engine designed to operate with a fuel other than gasoline, and more particularly to a fuel pressure control for such an engine. 
     BACKGROUND OF THE INVENTION 
     In general, an alternate-fuel engine is a spark-ignition internal combustion engine designed to operate with a fuel other than gasoline, and encompasses both single fuel engines and so-called dual-fuel engines. Regardless of the type of fuel, the overall objective of the fuel control is to maintain a desired air/fuel ratio for purposes of meeting fuel economy and emission control targets. However, the fuel control strategies vary to some degree depending on the type of fuel being utilized. For example, precise fuel pressure control is much more important with alternate fuels such as compressed natural gas (CNG) or liquid propane (LP) than with gasoline. Accordingly, most alternate-fuel engines utilize closed-loop fuel pressure control, with one or more sensors for precisely measuring the fuel pressure and a pressure adjustment mechanism for maintaining the measured fuel pressure at a fixed or predetermined value. The fuel pressure adjustment mechanism may be either an adjustable fuel pressure regulator or an adjustable speed fuel pump. However, it is difficult to schedule the desired fuel pressure for optimal air/fuel ratio control, and the required fuel pressure sensors significantly increase the fuel system cost. Accordingly, what is needed is an improved fuel pressure control that provides optimal air/fuel ratio control, and that does not require precise fuel pressure measurement. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved fuel pressure control for an alternate-fuel engine, wherein adaptively learned corrections for fuel injection pulsewidth based on air/fuel ratio sensing are also utilized to dynamically adjust a base fuel pressure control signal. The fuel pressure control may therefore be characterized as an open-loop control with a closed-loop correction term based on air/fuel ratio error. The control dynamically adjusts the fuel pressure in a manner to optimize the air/fuel ratio control instead of controlling to a predetermined pressure, and fuel pressure measurement is not required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a motor vehicle engine and fuel control including a microprocessor-based engine control module (ECM) programmed to operate in accordance with this invention. 
     FIG. 2 is a block diagram depicting a control carried out by the ECM of FIG. 1 according to this invention. 
     FIGS. 3-6 are flow diagrams representing a software routine executed by the ECM of FIG. 1 according to this invention. FIG. 3 is a main flow diagram for fuel control, FIG. 4 details a portion of the flow diagram of FIG. 3 pertaining to closed-loop adjustment of a block learn fuel control table, FIG. 5 details a portion of the flow diagram of FIG. 3 pertaining to updating a fuel injection pulsewidth, and FIG. 6 details a portion of the flow diagram of FIG. 3 pertaining to updating fuel pressure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the reference numeral  10  generally designates a motor vehicle powerplant including an alternate-fuel internal combustion engine  12  having an output shaft  14  connected to drive the vehicle, and a microprocessor-based engine control module (ECM)  16  for carrying out the fuel control of this invention. The engine  12  includes an intake air control valve  18  mounted in an intake passage  19 , a fuel system including a plurality of fuel injectors  20  coupled to a fuel rail (FR)  21  for combining fuel with the intake air for delivery to the engine cylinders for combustion therein, an exhaust manifold  22  and pipe  23  for receiving exhaust gasses from the cylinders, a three-way catalytic converter (CAT)  24  for minimizing certain exhaust gas emissions, and a tail pipe  26 . The fuel system additionally includes a motor driven fuel pump  28  disposed in a fuel tank or reservoir  30  for supplying pressurized fuel to the fuel rail (FR)  21  via pressure line  32 , and optionally, a fuel pressure regulator  34  disposed in fuel return line  36  for regulating the fuel pressure in fuel rail  21  by returning a controlled portion of the fuel in pressure line  32  to the fuel tank  30 . 
     The ECM  16  controls both the fuel pressure and the duration of fuel injection based on various environmental and engine operating parameters including the engine speed (ES) on line  38 , the manifold absolute air pressure (MAP) on line  40  and a measure of the exhaust gas oxygen (O 2 ) on line  42 . The engine speed ES is measured by a conventional speed sensor  44  disposed in proximity to the engine output shaft or flywheel  44 , the manifold absolute pressure MAP is measured by a conventional pressure sensor  46  coupled to the engine intake manifold, and the exhaust gas oxygen (O 2 ) is measured by a conventional oxygen sensor  48  of the switching or wide-range type. 
     The fuel pressure can be controlled by adjusting either the operating speed (capacity) of fuel pump  28  or a control pressure within the optional pressure regulator  34 . In fuel pump control applications, the ECM  16  generates a fuel pump capacity command (FPCC) for fuel pump  28  on line  50 ; in pressure regulator control applications, the ECM  16  generates a fuel pressure regulator command (FPRC) for pressure regulator  34  on line  52 , and the fuel pump capacity command FPCC may be a fixed value for operating the pump  28  at a substantially constant speed. In both cases, the control signal may be characterized as pulse-width-modulation (PWM) duty cycle signal. 
     The duration of fuel injection is specified by a fuel pulsewidth signal (FPW) generated by ECM  16  on line  54 . The FPW signal is applied to fuel injection control unit (FIC)  56  which activates the fuel injectors  20  in synchronism with engine rotation in a conventional manner. 
     Referring to the block diagram of FIG. 2, the fuel control method carried out by ECM  16  is represented by the blocks  70 ,  72 ,  74 ,  76 ,  78 ,  80 ,  82  and  84 . The block  70  represents a calibrated look-up table of fuel injector base pulsewidth (BPW) as a function of ES and MAP. The same two input parameters (ES and MAP) are applied to three other look-up tables represented by the blocks  72 ,  74  and  76 . In applications where the fuel pump  28  is used to control fuel pressure, the block  72  represents a calibrated table of PWM values for controlling the pump speed based on ES and MAP. In applications where the fuel pressure regulator  34  is used to control fuel pressure, the block  74  represents a calibrated table of PWM values for controlling a regulator bias pressure based on ES and MAP. The block  76 , customarily referred to as a block learning memory or BLM, represents a table of adaptive fuel corrections for various combinations of ES and MAP. The adaptive logic block (ADAPT)  78  determines the adaptive fuel corrections during engine operation based on the deviation of the actual air/fuel ratio (as determined from the O 2  signal on line  42 ) from the desired value, and supplies the corrections to block  76  via line  86 . Thus, the block  76  is implemented with an electrically alterable non-volatile memory device such as an EEPROM or a Flash Memory. The adaptive fuel corrections are configured as multipliers, and are applied along with a closed-loop term to the base fuel pulsewidth BPW in conventional fuel control applications. Thus, FIG. 2 includes a closed-loop (CL) block  80  for developing a closed-loop correction (using a PID or similar formulation), and a multiplier  82  for applying the closed-loop term of block  80  and the adaptive fuel correction of block  76  to the base pulsewidth BPW to form the fuel pulsewidth FPW. 
     According to the present invention, the adaptive fuel pressure corrections stored in the BLM  76  are additionally used to adjust the fuel pressure control signal FPCC or FPRC. In applications where the fuel pump  28  is used to control fuel pressure, the multiplier  84  combines the calibrated fuel pump control signal from block  72  with the adaptive fuel pressure correction from BLM  76  to form the fuel pump capacity command FPCC on line  50 . In applications where the fuel pressure regulator  34  is used to control fuel pressure, the multiplier  84  combines the calibrated fuel pressure regulator control signal from block  74  with the adaptive fuel pressure correction from BLM  76  to form the fuel pressure regulator command FPRC on line  52 . Applying the adaptive fuel pressure corrections from BLM  76  to the calibrated fuel pressure control signal serves to dynamically adjust the fuel pressure in a manner to optimize the air/fuel ratio control, and coordinates the fuel pressure control with the fuel injection duration control. 
     The flow diagrams of FIGS. 3-6 represent a software routine executed by ECM  16  for carrying out the above described control. FIG. 3 is a main flow diagram for the fuel control, and includes the blocks  92 ,  94 ,  96  and  98  which are successively executed as shown. At block  92 , the ECM  16  retrieves the current BLM correction based on ES and MAP. At block  94  the closed-loop terms are updated, as detailed by the flow diagram of FIG.  4 . At block  96  the injector pulsewidth control is updated, as detailed by the flow diagram of FIG.  5 . And at block  98  the fuel pressure control is updated, as detailed by the flow diagram of FIG.  6 . 
     Referring to FIG. 4, the block  100  is first executed to compute a proportional-integral-differential (PID) closed-loop term based on the deviation of the detected air/fuel ratio from the desired value. The blocks  102  and  104  then compare the integral term INT of the PID computation to calibrated thresholds Klow and Khigh. If INT is less than Klow or greater than Khigh, the block  106  is executed to increment the BLM Update Timer; otherwise, the block  108  resets the BLM Update Timer to zero. Once the BLM Update Timer exceeds an update threshold Kupdate, the block  110  is answered in the affirmative, and the blocks  111 ,  112 ,  114  and  116  are executed to reset the BLM Update Timer to zero and to update the current BLM cell. If the air/fuel ratio is too rich, as determined at block  112 , the block  114  decreases the current BLM cell to reduce the amount of injected fuel; otherwise, the air/fuel ratio is too lean, and the block  116  increases the current BLM cell to increase the amount of injected fuel. 
     Referring to FIG. 5, the blocks  120 ,  122  and  124  are executed in sequence as shown to update the fuel injection pulsewidth. The block  120  determines the base pulsewidth BPW by table look-up as described in reference to block  70  of FIG.  2 . The blocks  122  and  124  then respectively apply the closed-loop and BLM correction values to BPW to form the fuel pulsewidth FPW. 
     Referring to FIG. 6, the block  130  is initially executed to determine if the fuel pressure is being controlled by means of an adjustable pressure regulator such as the regulator  34  of FIG.  1 . If not, the blocks  132  and  134  are executed to determine a base PWM duty cycle for the motor-driven fuel pump  28 , and to apply the adaptive BLM fuel correction to the base value to form the fuel pump capacity command FPCC. If block  130  is answered in the affirmative, the blocks  136  and  138  are executed to determine a base PWM duty cycle for the pressure regulator  34 , and to apply the adaptive BLM fuel correction to the base value to form the fuel pressure regulator command FPRC. 
     In summary, the present invention provides an effective and low-cost fuel control method for an alternate-fuel internal combustion engine. The air/fuel ratio control performance is improved compared to systems that attempt to maintain the fuel pressure at a constant or predetermined value, and the expense associated with accurately measuring the fuel pressure is eliminated. While the control of this invention has been described in reference to the illustrated embodiment, it is anticipated that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, the ECM  16  could be programmed in the case of a dual fuel engine to utilize the disclosed fuel pressure control only when operation with an alternate fuel is detected. In this regard, it should be understood that control methods incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.