Abstract:
An internal combustion engine fuel injector pulsewidth is calculated as a function of desired fuel mass and injector pressure. Accounting for variations in injection pressures provides improved accuracy of fuel mass injection.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electronic controls for an internal combustion engine. 
     2. Prior Art 
     In known production implementations, fuel delivery systems have typically used a mechanical fuel pressure regulator to control to a nominal fuel injection pressure. Fuel not ingested by the engine was returned to the fuel tank (see FIG. 1). With this type of fueling system, the instantaneous pressure across the fuel injectors (Δp inj ) was not known exactly, nor was it adjustable during operation. Therefore, fueling calculation done in an electronic engine control may have used a fixed nominal curve relating the desired fuel to be injected (m inj ) to a corresponding injection pulsewidth (PW inj ) that tells the time the injector is to be commanded open. An example of this type of piece-wise linear fuel injector flow curve is shown in FIG. 2, at a fixed injection pressure. 
     Current production often modifies fuel injector pulsewidths, but strictly as a function of the desired fuel mass to be injected. There are also Hot Injector COMPensation (HICOMP) strategies, but these are ad hoc and may not use a fuel rail temperature sensor. Neither of these account or allow for varying injection pressures. 
     SUMMARY OF THE INVENTION 
     With the advent of returnless fuel delivery systems (no fuel returned to the tank), a sensor to measure ΔP inj  was needed to help replace the function of the mechanical pressure regulator (see FIG. 3). Furthermore, a sensor to measure the temperature of the fuel within the fuel rail (T fr ) was needed since ΔP inj  is commanded to be higher with temperature to minimize fuel vaporization in the rail. Beyond simply using the information provided by the pressure sensor to help maintain Δp inj  to a desired value, it may be used to modify the calculation of the PW inj  for the following two reasons. First, since maintaining the exact pressure in a returnless fuel delivery system with a pump controller is not possible, transient pressure errors may be accounted for by using the actual Δp inj  in the PW inj  calculation. Second, since the Δp inj  desired across the injectors may not be constant, fuel metering accuracy may still be maintained using the same idea; account for the actual Δ inj  in the PW inj  calculation. 
     The invention includes a method to adjust injector pulsewidth, PW inj , to account for the instantaneous Δp inj , in order to maximize fuel metering accuracy. This Δp inj  can be measured using a differential pressure sensor mounted between the fuel rail and the intake manifold. This method can also account for the temperature of the fuel injector body which may be approximated as T fr . As T fr  and injector tip temperatures vary, so do the flow characteristics of the injectors. Thus, in accordance with an embodiment of this invention, internal combustion engine fuel injector pulsewidths, to deliver the desired fuel mass, are calculated as a function of injector pressure. Fuel rail temperature may also be used. The purpose is to keep fuel injection flows accurate regardless of variations in injection pressure and/or fuel injection temperature. Thus, this invention provides more accurate fuel metering. 
     Use of this invention provides additional control in providing the desired amount of fuel into the engine. Not only is the fuel pump controller attempting to control Δp inj  to the desired value, any transient pressure errors are compensated for by the invention in the calculation of the PW inj . 
     Further, in certain applications, it may be desirable to change the Δp inj  during operation to optimize injection characteristics (variable pressure injection) and the method of this invention facilitates this. In order to execute a variable pressure injection scheme accurately, the injector flow curves must change to account for the desired Δp inj  operating point being changed. So, the same algorithm (the invention) used to account for modest transient pressure variations (typically unintended variations around the nominal Δp inj ) can also be used for large, intended, long lasting pressure variations. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block diagram of a fuel delivery system using a mechanical pressure regulator and a return line to the fuel tank in accordance with the prior art; 
     FIG. 2 is a graphical representation of the injector open time versus desired fuel mass to be flowed in accordance with the prior art; 
     FIG. 3 is a schematic of a fuel delivery system without any return flow to the fuel tank in accordance with an embodiment of this invention; 
     FIG. 4 is a block diagram wherein the fuel injector flow curve is a function of the instantaneous injection pressure and the fuel rail temperature, in accordance with an embodiment of this invention; 
     FIG. 5 is a fuel injector flow curve of injector open time versus the desired fuel mass to be flowed in accordance with an embodiment of this invention; 
     FIG. 6 is a block diagram implementing the curve of FIG. 5 using the block diagram of FIG. 4 in accordance with an embodiment of this invention; and 
     FIG. 7 is a block diagram of an implementation using algebraic parameterization or equation put in the form of the block diagram shown in FIG. 4 in accordance with an embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 3, a fuel tank 300 includes a fuel pump 301 to pump fuel from fuel tank 300 through a fuel line 302 to a fuel rail 303. Injectors 304A, 304B, 304C, and 304D are coupled to fuel rail 303 and provide for injection of fuel into an engine 305. A fuel temperature sensor 306 is coupled to fuel rail 303. A differential pressure sensor 307 is coupled between fuel rail 303 and engine 305. Differential pressure sensor 307 measures the actual injector pressure by looking at the pressure across the injector. A control unit 308 receives input signals from fuel temperature sensor 306 and differential pressure sensor 307 and provides output signals to fuel injectors 304A, 304B, 304C, 304D to control fuel pulsewidth and to pump 301 to control pump duty cycle and fuel pressure. Control unit 308 is typically a microprocessor with stored processing information as further discussed below. 
     The invention may be represented by the block diagram in FIG. 4. First, in Block 1, the characteristics of the injector&#39;s flow curve are kept as a function of Δp inj  and T fr . The output of Block 1 (the flow curve characteristics) modify Block 2. Block 2 is the relationship which tells what PW inj  is required to meter out a desired m inj . 
     One possible implementation of the invention of FIG. 4 may be seen in FIGS. 5 and 6. Block 2 of FIG. 6, the flow relationship of the fuel injector, is a piece-wise linear curve shown in more detail in FIG. 5. This curve may be completely described by four terms of parameters: The x-axis intercept (X int ), the breakpoint (X bkpt ), the slope along the lower portion (Δ low ), and the slope along the higher portion (Δ high ). Block 1 of FIG. 4 becomes four relationship (f 1 , f 2 , f 3  and, f 4 ) that determine the four fuel-injector curve parameters given Δp inj  and T fr . 
     A second possible implementation of the invention, shown in FIG. 4, can be seen in FIG. 7. Here Block 2 of FIG. 7 would be a smooth curve (no discontinuities as with the piece-wise linear curve). This is a more accurate representation of injector operation than the piece-wise linear embodiment. The curve again relates PW inj  to the desired m inj  to be metered. This curve may be an algebraic parameterization of an equation, such as that in Eq. 1, where the coefficients are functions of Δp inj  and T fr . 
     
         PW.sub.inj =. . . +a.sub.-2 (Δp.sub.inj,T.sub.fr)m.sub.inj.sup.-2 +a.sub.-1 (Δp.sub.inj,T.sub.fr)m.sub.inj.sup.-1 +a.sub.0 =a.sub.1 (Δp.sub.inj,T.sub.fr)m.sub.inj +a.sub.2 (Δp.sub.inj,T.sub.fr)m.sub.inj.sup.2 +              (Eq. 1) 
    
     Block 1 of FIG. 7 has as inputs Δp inj  and T fr , and as outputs the &#34;a&#34; coefficients to define the flow relationship in Block 2 mapping the desired m inj  to the PW inj  that should be commanded. The function f of Block 1 in FIG. 7 are preselected fixed functions. 
     For any given pair of Δp inj  and T fr  values, the &#34;a&#34; coefficients will be fixed, yielding a smooth non linear mathematical relationship between m inj  and PW inj . But when Δp inj  and T fr  move to different values, so does the set of &#34;a&#34; coefficients. 
     By running various fuel flow bench tests on a given fuel injector, several sets of &#34;a&#34; coefficient values may be determined by regressing the data for each Δp inj , T fr  pair. The regression would yield the set of &#34;a&#34; coefficients whose resulting curve best matched the curve in the actual flow bench data. 
     With the various sets of requested &#34;a&#34; coefficients in hand, each coefficient itself may be regressed as a function of Δp inj  and T fr . This results in the functions shown in Block 1 of FIG. 7. 
     Many other implementations of this invention are possible, but they all adjust the PW inj  not only as a function of desired mini, but also a function of injector pressure. Further, if desired, fuel injector temperature may also be used to compensate PW inj . 
     Various modifications and variations will no doubt occur to those skilled in the art to which this invention pertains. Such variations which basically rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention.