Patent Publication Number: US-10323612-B2

Title: Methods and systems for dual fuel injection

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to U.S. Provisional Patent Application No. 62/175,059, entitled “Methods and Systems for Dual Fuel Injection,” filed on Jun. 12, 2015, the entire contents of which are hereby incorporated by reference for all purposes. 
    
    
     FIELD 
     The present description relates to systems and methods for adjusting operation of an internal combustion engine that includes high pressure port and direct fuel injectors. 
     BACKGROUND AND SUMMARY 
     Direct fuel injection (DI) systems provide some advantages over port fuel injection systems. For example, direct fuel injection systems may improve cylinder charge cooling so that engine cylinders may operate at higher compression ratios without incurring undesirable engine knock. However, direct fuel injectors may not be able to provide a desired amount of fuel to a cylinder at higher engine speeds and loads because the amount of time a cylinder stroke takes is shortened so that there may not be sufficient time to inject a desired amount of fuel. Consequently, the engine may develop less power than is desired at higher engine speeds and loads. In addition, direct injection systems may be more prone to particulate matter emissions. 
     In an effort to reduce the particulate matter emissions and fuel dilution in oil, very high pressure direct injection systems have been developed. For example, while nominal direct injection maximum pressures are in the range of 150 bar, the higher pressure DI systems may operate in the range of 250-800 bar using a high pressure piston pump that is mechanically driven by the engine via a camshaft. In engines configured with dual injection systems, that is engines enabled with both direct and port fuel injectors, pressurized fuel from the fuel tank may be supplied to both the direct injection high pressure fuel pump (HPFP) as well as the port injection fuel rail. In order to reduce hardware complexity, the fuel may be supplied to the port injection fuel rail either through the HPFP, or may be branched off before the pump, thereby reducing the need for a dedicated pump for the port injection fuel rail. 
     However, one issue with such dual fuel injection system configurations is that fuel pulsations from the high pressure fuel pump may enter the port injection fuel rail. This is due to the sinusoidal fuel pressure generated at the high pressure fuel pump due to the pump being driven by the engine via a camshaft (and cam lobes). The pulsations may worsen when the HPFP is not flowing any fuel into the direct injection fuel rail (such as when direct injection is disabled) due to the pump returning all of the ingested volume back into the low pressure region of the fuel system. The pulsations in the port injection fuel rail can lead to larger discrepancies between the value of rested fuel in the port injection fuel rail as compared to value of fuel injected from the port injection fuel rail. As such, this can result in large fueling errors. 
     In one example, the above issue may be at least partly addressed by a method for an engine, comprising: pressurizing fuel in a port injection fuel rail via an engine camshaft driven high pressure fuel pump; and injecting a port fuel injection with a timing balanced around an average pressure-crossing of port fuel injection pressure. In this way, fueling errors due to fuel pump induced pressure fluctuations in the port injection fuel rail are reduced. 
     As one example, an engine system may include an engine-driven high pressure fuel pump supplying fuel to each of a port and direct injection fuel rail. The fuel pump may be a piston pump coupled to the engine via each of a camshaft and cam lobes, and due to this configuration, the fuel pressure in the fuel pump may vary in a sinusoidal manner. This may in turn cause sinusoidal fluctuations in a fuel pressure in the port injection fuel rail. An engine controller may estimate the pressure in the port injection fuel rail based on the pressure at the fuel pump, and further based on an engine speed dependent fuel pulse delay. The controller may estimate the timing (with respect to engine position) of local maxima and local minima in the waveform of the port injection fuel rail pressure, and accordingly determine the position of zero-crossings of the waveform. An initial timing and width of a port fuel injection pulse may be determined based on engine operating conditions including, for example, intake valve opening (IVO) and fuel flow velocity through the fuel system to allow for a closed intake valve injection. The timing of the port injection fuel pulse may then be moved to coincide with the timing of a first zero-crossing in the advanced direction. In addition, the initial pulse width of the port injection fuel pulse may be adjusted based on the adjusted timing to compensate for any difference in fuel puddle dynamics. 
     The technical effect of centering a port fuel injection pulse around a zero-crossing of a fuel rail pressure waveform is that under-average pressure changes can be cancelled out by over-average pressure changes. By moving the middle of injection angle of the port injection fuel pulse to coincide with an average pressure at the first zero-crossing in the advanced direction, closed intake valve port fuel injection can be maintained while port injection fuel rail pressure fluctuations induced by sinusoidal changes in a fuel pump pressure are substantially removed. By relying on the average port injection fuel rail pressure, the need for fast fuel pressure sampling is reduced. In addition, pressure fluctuations can be addressed without requiring additional pressure dampeners, check valves, or orifices. Overall, metering of fuel from the port injection fuel rail is improved while removing the need for a dedicated fuel line for the port injection fuel system. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts an example embodiment of a cylinder of an internal combustion engine. 
         FIG. 2  schematically depicts an example embodiment of a fuel system configured for high pressure port injection and high pressure direct injection that may be used with the engine of  FIG. 1 . 
         FIG. 3  depicts alternate embodiments of a dual fuel injection system that may be used with the engine of  FIG. 1 . 
         FIG. 4  shows a flow chart of a method for adjusting a timing of a port fuel injection pulse based on an average pressure-crossing of a port injection fuel rail pressure. 
         FIG. 5  shows an example of aligning fuel injection pulse timing with a zero-crossing of a port injection fuel rail pressure waveform. 
         FIG. 6A  depicts an example relationship between port injection fuel rail pressure and fuel pressure at a high pressure fuel pump delivering fuel to the port injection fuel rail. 
         FIG. 6B  depicts an example relationship between fuel pulse delay and engine speed. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description provides information regarding a high pressure fuel pump and a system for reducing high pressure pump-induced pressure fluctuations at a port injection fuel rail. An example embodiment of a cylinder in an internal combustion engine is given in  FIG. 1  while  FIGS. 2-3  depict example fuel systems that may be used with the engine of  FIG. 1 . A controller may be configured to perform a control routine, such as the example routine of  FIG. 4 , to reposition a port injection fuel pulse so as to align a center of the fuel pulse with an average port injection fuel rail pressure. The port injection fuel rail pressure may be estimated based on fuel pump pressure and engine speed ( FIGS. 6A-6B ). An example repositioning of a port fuel injection pulse is shown at  FIG. 5 . 
     Regarding terminology used throughout this detailed description, a high pressure pump, or direct injection pump, may be abbreviated as a DI or HP pump. Similarly, a low pressure pump, or lift pump, may be abbreviated as a LP pump. Port fuel injection may be abbreviated as PFI while direct injection may be abbreviated as DI. Also, fuel rail pressure, or the value of pressure of fuel within a fuel rail, may be abbreviated as FRP. Also, the mechanically operated inlet check valve for controlling fuel flow into the HP pump may also be referred to as the spill valve. As discussed in more detail below, an HP pump that relies on mechanical pressure regulation without use of an electronically-controlled inlet valve may be referred to as a mechanically-controlled HP pump, or HP pump with mechanically-regulated pressure. Mechanically-controlled HP pumps, while not using electronically-controlled inlet valves for regulating a volume of fuel pumped, may provide one or more discrete pressures based on electronic selection. 
       FIG. 1  depicts an example of a combustion chamber or cylinder of internal combustion engine  10 . Engine  10  may be controlled at least partially by a control system including controller  12  and by input from a vehicle operator  130  via an input device  132 . In this example, input device  132  includes an accelerator pedal and a pedal position sensor  134  for generating a proportional pedal position signal PP. Cylinder (herein also “combustion chamber”)  14  of engine  10  may include combustion chamber walls  136  with piston  138  positioned therein. Piston  138  may be coupled to crankshaft  140  so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft  140  may be coupled to at least one drive wheel of the passenger vehicle via a transmission system. Further, a starter motor (not shown) may be coupled to crankshaft  140  via a flywheel to enable a starting operation of engine  10 . 
     Cylinder  14  can receive intake air via a series of intake air passages  142 ,  144 , and  146 . Intake air passage  146  can communicate with other cylinders of engine  10  in addition to cylinder  14 . In some examples, one or more of the intake passages may include a boosting device such as a turbocharger or a supercharger. For example,  FIG. 1  shows engine  10  configured with a turbocharger including a compressor  174  arranged between intake passages  142  and  144 , and an exhaust turbine  176  arranged along exhaust passage  148 . Compressor  174  may be at least partially powered by exhaust turbine  176  via a shaft  180  where the boosting device is configured as a turbocharger. However, in other examples, such as where engine  10  is provided with a supercharger, exhaust turbine  176  may be optionally omitted, where compressor  174  may be powered by mechanical input from a motor or the engine. A throttle  162  including a throttle plate  164  may be provided along an intake passage of the engine for varying the flow rate and/or pressure of intake air provided to the engine cylinders. For example, throttle  162  may be positioned downstream of compressor  174  as shown in  FIG. 1 , or alternatively may be provided upstream of compressor  174 . 
     Exhaust passage  148  can receive exhaust gases from other cylinders of engine  10  in addition to cylinder  14 . Exhaust gas sensor  128  is shown coupled to exhaust passage  148  upstream of emission control device  178 . Sensor  128  may be selected from among various suitable sensors for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor, for example. Emission control device  178  may be a three way catalyst (TWC), NOx trap, various other emission control devices, or combinations thereof. 
     Each cylinder of engine  10  may include one or more intake valves and one or more exhaust valves. For example, cylinder  14  is shown including at least one intake poppet valve  150  and at least one exhaust poppet valve  156  located at an upper region of cylinder  14 . In some examples, each cylinder of engine  10 , including cylinder  14 , may include at least two intake poppet valves and at least two exhaust poppet valves located at an upper region of the cylinder. 
     Intake valve  150  may be controlled by controller  12  via actuator  152 . Similarly, exhaust valve  156  may be controlled by controller  12  via actuator  154 . During some conditions, controller  12  may vary the signals provided to actuators  152  and  154  to control the opening and closing of the respective intake and exhaust valves. The position of intake valve  150  and exhaust valve  156  may be determined by respective valve position sensors (not shown). The valve actuators may be of the electric valve actuation type or cam actuation type, or a combination thereof. The intake and exhaust valve timing may be controlled concurrently or any of a possibility of variable intake cam timing, variable exhaust cam timing, dual independent variable cam timing or fixed cam timing may be used. Each cam actuation system may include one or more cams and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT) and/or variable valve lift (VVL) systems that may be operated by controller  12  to vary valve operation. For example, cylinder  14  may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT. In other examples, the intake and exhaust valves may be controlled by a common valve actuator or actuation system, or a variable valve timing actuator or actuation system. 
     Cylinder  14  can have a compression ratio, which is the ratio of volumes when piston  138  is at bottom center to top center. In one example, the compression ratio is in the range of 9:1 to 10:1. However, in some examples where different fuels are used, the compression ratio may be increased. This may happen, for example, when higher octane fuels or fuels with higher latent enthalpy of vaporization are used. The compression ratio may also be increased if direct injection is used due to its effect on engine knock. 
     In some examples, each cylinder of engine  10  may include a spark plug  192  for initiating combustion. Ignition system  190  can provide an ignition spark to combustion chamber  14  via spark plug  192  in response to spark advance signal SA from controller  12 , under select operating modes. However, in some embodiments, spark plug  192  may be omitted, such as where engine  10  may initiate combustion by auto-ignition or by injection of fuel as may be the case with some diesel engines. 
     In some examples, each cylinder of engine  10  may be configured with one or more fuel injectors for providing fuel thereto. As a non-limiting example, cylinder  14  is shown including two fuel injectors  166  and  170 . Fuel injectors  166  and  170  may be configured to deliver fuel received from fuel system  8 . As elaborated with reference to  FIGS. 2 and 3 , fuel system  8  may include one or more fuel tanks, fuel pumps, and fuel rails. Fuel injector  166  is shown coupled directly to cylinder  14  for injecting fuel directly therein in proportion to the pulse width of signal FPW- 1  received from controller  12  via electronic driver  168 . In this manner, fuel injector  166  provides what is known as direct injection (hereafter referred to as “DI”) of fuel into combustion cylinder  14 . While  FIG. 1  shows injector  166  positioned to one side of cylinder  14 , it may alternatively be located overhead of the piston, such as near the position of spark plug  192 . Such a position may improve mixing and combustion when operating the engine with an alcohol-based fuel due to the lower volatility of some alcohol-based fuels. Alternatively, the injector may be located overhead and near the intake valve to improve mixing. Fuel may be delivered to fuel injector  166  from a fuel tank of fuel system  8  via a high pressure fuel pump, and a fuel rail. Further, the fuel tank may have a pressure transducer providing a signal to controller  12 . 
     Fuel injector  170  is shown arranged in intake passage  146 , rather than in cylinder  14 , in a configuration that provides what is known as port injection of fuel (hereafter referred to as “PFI”) into the intake port upstream of cylinder  14 . Fuel injector  170  may inject fuel, received from fuel system  8 , in proportion to the pulse width of signal FPW- 2  received from controller  12  via electronic driver  171 . Note that a single driver  168  or  171  may be used for both fuel injection systems, or multiple drivers, for example driver  168  for fuel injector  166  and driver  171  for fuel injector  170 , may be used, as depicted. 
     In an alternate example, each of fuel injectors  166  and  170  may be configured as direct fuel injectors for injecting fuel directly into cylinder  14 . In still another example, each of fuel injectors  166  and  170  may be configured as port fuel injectors for injecting fuel upstream of intake valve  150 . In yet other examples, cylinder  14  may include only a single fuel injector that is configured to receive different fuels from the fuel systems in varying relative amounts as a fuel mixture, and is further configured to inject this fuel mixture either directly into the cylinder as a direct fuel injector or upstream of the intake valves as a port fuel injector. As such, it should be appreciated that the fuel systems described herein should not be limited by the particular fuel injector configurations described herein by way of example. 
     Fuel may be delivered by both injectors to the cylinder during a single cycle of the cylinder. For example, each injector may deliver a portion of a total fuel injection that is combusted in cylinder  14 . Further, the distribution and/or relative amount of fuel delivered from each injector may vary with operating conditions, such as engine load, knock, and exhaust temperature, such as described herein below. The port injected fuel may be delivered during an open intake valve event, closed intake valve event (e.g., substantially before the intake stroke), as well as during both open and closed intake valve operation. Similarly, directly injected fuel may be delivered during an intake stroke, as well as partly during a previous exhaust stroke, during the intake stroke, and partly during the compression stroke, for example. As such, even for a single combustion event, injected fuel may be injected at different timings from the port and direct injector. Furthermore, for a single combustion event, multiple injections of the delivered fuel may be performed per cycle. The multiple injections may be performed during the compression stroke, intake stroke, or any appropriate combination thereof. 
     As described above,  FIG. 1  shows only one cylinder of a multi-cylinder engine. As such, each cylinder may similarly include its own set of intake/exhaust valves, fuel injector(s), spark plug, etc. It will be appreciated that engine  10  may include any suitable number of cylinders, including 2, 3, 4, 5, 6, 8, 10, 12, or more cylinders. Further, each of these cylinders can include some or all of the various components described and depicted by  FIG. 1  with reference to cylinder  14 . 
     Fuel injectors  166  and  170  may have different characteristics. These include differences in size, for example, one injector may have a larger injection hole than the other. Other differences include, but are not limited to, different spray angles, different operating temperatures, different targeting, different injection timing, different spray characteristics, different locations etc. Moreover, depending on the distribution ratio of injected fuel among injectors  170  and  166 , different effects may be achieved. 
     Fuel tanks in fuel system  8  may hold fuels of different fuel types, such as fuels with different fuel qualities and different fuel compositions. The differences may include different alcohol content, different water content, different octane, different heats of vaporization, different fuel blends, and/or combinations thereof etc. One example of fuels with different heats of vaporization could include gasoline as a first fuel type with a lower heat of vaporization and ethanol as a second fuel type with a greater heat of vaporization. In another example, the engine may use gasoline as a first fuel type and an alcohol containing fuel blend such as E85 (which is approximately 85% ethanol and 15% gasoline) or M85 (which is approximately 85% methanol and 15% gasoline) as a second fuel type. Other feasible substances include water, methanol, a mixture of alcohol and water, a mixture of water and methanol, a mixture of alcohols, etc. 
     In still another example, both fuels may be alcohol blends with varying alcohol composition wherein the first fuel type may be a gasoline alcohol blend with a lower concentration of alcohol, such as E10 (which is approximately 10% ethanol), while the second fuel type may be a gasoline alcohol blend with a greater concentration of alcohol, such as E85 (which is approximately 85% ethanol). Additionally, the first and second fuels may also differ in other fuel qualities such as a difference in temperature, viscosity, octane number, etc. Moreover, fuel characteristics of one or both fuel tanks may vary frequently, for example, due to day to day variations in tank refilling. 
     Controller  12  is shown in  FIG. 1  as a microcomputer, including microprocessor unit  106 , input/output ports  108 , an electronic storage medium for executable programs and calibration values shown as non-transitory read only memory chip  110  in this particular example for storing executable instructions, random access memory  112 , keep alive memory  114 , and a data bus. Controller  12  may receive various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor  122 ; engine coolant temperature (ECT) from temperature sensor  116  coupled to cooling sleeve  118 ; a profile ignition pickup signal (PIP) from Hall effect sensor  120  (or other type) coupled to crankshaft  140 ; throttle position (TP) from a throttle position sensor; and absolute manifold pressure signal (MAP) from sensor  124 . Engine speed signal, RPM, may be generated by controller  12  from signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum, or pressure, in the intake manifold. The controller  12  receives signals from the various sensors of  FIG. 1  (and  FIG. 2 ) and employs the various actuators of  FIG. 1  (and  FIG. 2 ) to adjust engine operation based on the received signals and instructions stored on a memory of the controller 
       FIG. 2  schematically depicts an example embodiment  200  of a fuel system, such as fuel system  8  of  FIG. 1 . Fuel system  200  may be operated to deliver fuel to an engine, such as engine  10  of  FIG. 1 . Fuel system  200  may be operated by a controller to perform some or all of the operations described with reference to the process flows of  FIGS. 4 and 6A-6B . 
     Fuel system  200  includes a fuel storage tank  210  for storing the fuel on-board the vehicle, a lower pressure fuel pump (LPP)  212  (herein also referred to as fuel lift pump  212 ), and a higher pressure fuel pump (HPP)  214  (herein also referred to as fuel injection pump  214 ). Fuel may be provided to fuel tank  210  via fuel filling passage  204 . In one example, LPP  212  may be an electrically-powered lower pressure fuel pump disposed at least partially within fuel tank  210 . LPP  212  may be operated by a controller  222  (e.g., controller  12  of  FIG. 1 ) to provide fuel to HPP  214  via fuel passage  218 . LPP  212  can be configured as what may be referred to as a fuel lift pump. As one example, LPP  212  may be a turbine (e.g., centrifugal) pump including an electric (e.g., DC) pump motor, whereby the pressure increase across the pump and/or the volumetric flow rate through the pump may be controlled by varying the electrical power provided to the pump motor, thereby increasing or decreasing the motor speed. For example, as the controller reduces the electrical power that is provided to lift pump  212 , the volumetric flow rate and/or pressure increase across the lift pump may be reduced. The volumetric flow rate and/or pressure increase across the pump may be increased by increasing the electrical power that is provided to lift pump  212 . As one example, the electrical power supplied to the lower pressure pump motor can be obtained from an alternator or other energy storage device on-board the vehicle (not shown), whereby the control system can control the electrical load that is used to power the lower pressure pump. Thus, by varying the voltage and/or current provided to the lower pressure fuel pump, the flow rate and pressure of the fuel provided at the inlet of the higher pressure fuel pump  214  is adjusted. 
     LPP  212  may be fluidly coupled to a filter  217 , which may remove small impurities contained in the fuel that could potentially damage fuel handling components. A check valve  213 , which may facilitate fuel delivery and maintain fuel line pressure, may be positioned fluidly upstream of filter  217 . With check valve  213  upstream of the filter  217 , the compliance of low-pressure passage  218  may be increased since the filter may be physically large in volume. Furthermore, a pressure relief valve  219  may be employed to limit the fuel pressure in low-pressure passage  218  (e.g., the output from lift pump  212 ). Relief valve  219  may include a ball and spring mechanism that seats and seals at a specified pressure differential, for example. The pressure differential set-point at which relief valve  219  may be configured to open may assume various suitable values; as a non-limiting example the set-point may be 6.4 bar or 5 bar (g). An orifice  223  may be utilized to allow for air and/or fuel vapor to bleed out of the lift pump  212 . This bleed at  223  may also be used to power a jet pump used to transfer fuel from one location to another within the tank  210 . In one example, an orifice check valve (not shown) may be placed in series with orifice  223 . In some embodiments, fuel system  8  may include one or more (e.g., a series) of check valves fluidly coupled to low-pressure fuel pump  212  to impede fuel from leaking back upstream of the valves. In this context, upstream flow refers to fuel flow traveling from fuel rails  250 ,  260  towards LPP  212  while downstream flow refers to the nominal fuel flow direction from the LPP towards the HPP  214  and thereon to the fuel rails. 
     Fuel lifted by LPP  212  may be supplied at a lower pressure into a fuel passage  218  leading to an inlet  203  of HPP  214 . HPP  214  may then deliver fuel into a first fuel rail  250  coupled to one or more fuel injectors of a first group of direct injectors  252  (herein also referred to as a first injector group). Fuel lifted by the LPP  212  may also be supplied to a second fuel rail  260  coupled to one or more fuel injectors of a second group of port injectors  262  (herein also referred to as a second injector group). As elaborated below, HPP  214  may be operated to raise the pressure of fuel delivered to each of the first and second fuel rail above the lift pump pressure, with the first fuel rail coupled to the direct injector group operating with a variable high pressure while the second fuel rail coupled to the port injector group operates with a fixed high pressure. As a result, high pressure port and direct injection may be enabled. The high pressure fuel pump is coupled downstream of the low pressure lift pump with no additional pump positioned in between the high pressure fuel pump and the low pressure lift pump. 
     While each of first fuel rail  250  and second fuel rail  260  are shown dispensing fuel to four fuel injectors of the respective injector group  252 ,  262 , it will be appreciated that each fuel rail  250 ,  260  may dispense fuel to any suitable number of fuel injectors. As one example, first fuel rail  250  may dispense fuel to one fuel injector of first injector group  252  for each cylinder of the engine while second fuel rail  260  may dispense fuel to one fuel injector of second injector group  262  for each cylinder of the engine. Controller  222  can individually actuate each of the port injectors  262  via a port injection driver  237  and actuate each of the direct injectors  252  via a direct injection driver  238 . The controller  222 , the drivers  237 ,  238  and other suitable engine system controllers can comprise a control system. While the drivers  237 ,  238  are shown external to the controller  222 , it should be appreciated that in other examples, the controller  222  can include the drivers  237 ,  238  or can be configured to provide the functionality of the drivers  237 ,  238 . Controller  222  may include additional components not shown, such as those included in controller  12  of  FIG. 1 . 
     HPP  214  may be an engine-driven, positive-displacement pump. As one non-limiting example, HPP  214  may be a BOSCH HDP5 HIGH PRESSURE PUMP, which utilizes a solenoid activated control valve (e.g., fuel volume regulator, magnetic solenoid valve, etc.)  236  to vary the effective pump volume of each pump stroke. The outlet check valve of HPP is mechanically controlled and not electronically controlled by an external controller. HPP  214  may be mechanically driven by the engine in contrast to the motor driven LPP  212 . HPP  214  includes a pump piston  228 , a pump compression chamber  205  (herein also referred to as compression chamber), and a step-room  227 . Pump piston  228  receives a mechanical input from the engine crank shaft or cam shaft via cam  230 , thereby operating the HPP according to the principle of a cam-driven single-cylinder pump. A sensor (not shown in  FIG. 2 ) may be positioned near cam  230  to enable determination of the angular position of the cam (e.g., between 0 and 360 degrees), which may be relayed to controller  222 . 
     Fuel system  200  may optionally further include accumulator  215 . When included, accumulator  215  may be positioned downstream of lower pressure fuel pump  212  and upstream of higher pressure fuel pump  214 , and may be configured to hold a volume of fuel that reduces the rate of fuel pressure increase or decrease between fuel pumps  212  and  214 . For example, accumulator  215  may be coupled in fuel passage  218 , as shown, or in a bypass passage  211  coupling fuel passage  218  to the step-room  227  of HPP  214 . The volume of accumulator  215  may be sized such that the engine can operate at idle conditions for a predetermined period of time between operating intervals of lower pressure fuel pump  212 . For example, accumulator  215  can be sized such that when the engine idles, it takes one or more minutes to deplete pressure in the accumulator to a level at which higher pressure fuel pump  214  is incapable of maintaining a sufficiently high fuel pressure for fuel injectors  252 ,  262 . Accumulator  215  may thus enable an intermittent operation mode (or pulsed mode) of lower pressure fuel pump  212 . By reducing the frequency of LPP operation, power consumption is reduced. In other embodiments, accumulator  215  may inherently exist in the compliance of fuel filter  217  and fuel passage  218 , and thus may not exist as a distinct element. 
     A lift pump fuel pressure sensor  231  may be positioned along fuel passage  218  between lift pump  212  and higher pressure fuel pump  214 . In this configuration, readings from sensor  231  may be interpreted as indications of the fuel pressure of lift pump  212  (e.g., the outlet fuel pressure of the lift pump) and/or of the inlet pressure of higher pressure fuel pump. Readings from sensor  231  may be used to assess the operation of various components in fuel system  200 , to determine whether sufficient fuel pressure is provided to higher pressure fuel pump  214  so that the higher pressure fuel pump ingests liquid fuel and not fuel vapor, and/or to minimize the average electrical power supplied to lift pump  212 . While lift pump fuel pressure sensor  231  is shown as being positioned downstream of accumulator  215 , in other embodiments the sensor may be positioned upstream of the accumulator. 
     First fuel rail  250  includes a first fuel rail pressure sensor  248  for providing an indication of direct injection fuel rail pressure to the controller  222 . Likewise, second fuel rail  260  includes a second fuel rail pressure sensor  258  for providing an indication of port injection fuel rail pressure to the controller  222 . An engine speed sensor  233  can be used to provide an indication of engine speed to the controller  222 . The indication of engine speed can be used to identify the speed of higher pressure fuel pump  214 , since the pump  214  is mechanically driven by the engine  202 , for example, via the crankshaft or camshaft. 
     First fuel rail  250  is coupled to an outlet  208  of HPP  214  along fuel passage  278 . In comparison, second fuel rail  260  is coupled to an inlet  203  of HPP  214  via fuel passage  288 . A check valve and a pressure relief valve may be positioned between the outlet  208  of the HPP  214  and the first fuel rail. In addition, pressure relief valve  272 , arranged parallel to check valve  274  in bypass passage  279 , may limit the pressure in fuel passage  278 , downstream of HPP  214  and upstream of first fuel rail  250 . For example, pressure relief valve  272  may limit the pressure in fuel passage  278  to 200 bar. As such, pressure relief valve  272  may limit the pressure that would otherwise be generated in fuel passage  278  if control valve  236  were (intentionally or unintentionally) open and while high pressure fuel pump  214  were pumping. 
     One or more check valves and pressure relief valves may also be coupled to fuel passage  218 , downstream of LPP  212  and upstream of HPP  214 . For example, check valve  234  may be provided in fuel passage  218  to reduce or prevent back-flow of fuel from high pressure pump  214  to low pressure pump  212  and fuel tank  210 . In addition, pressure relief valve  232  may be provided in a bypass passage, positioned parallel to check valve  234 . Pressure relief valve  232  may limit the pressure to its left to 10 bar higher than the pressure at sensor  231 . 
     Controller  222  may be configured to regulate fuel flow into HPP  214  through control valve  236  by energizing or de-energizing the solenoid valve (based on the solenoid valve configuration) in synchronism with the driving cam. Accordingly, the solenoid activated control valve  236  may be operated in a first mode where the valve  236  is positioned within HPP inlet  203  to limit (e.g., inhibit) the amount of fuel traveling through the solenoid activated control valve  236 . Depending on the timing of the solenoid valve actuation, the volume transferred to the fuel rail  250  is varied. The solenoid valve may also be operated in a second mode where the solenoid activated control valve  236  is effectively disabled and fuel can travel upstream and downstream of the valve, and in and out of HPP  214 . 
     As such, solenoid activated control valve  236  may be configured to regulate the mass (or volume) of fuel compressed into the direct injection fuel pump. In one example, controller  222  may adjust a closing timing of the solenoid pressure control check valve to regulate the mass of fuel compressed. For example, a late pressure control valve closing may reduce the amount of fuel mass ingested into compression chamber  205 . The solenoid activated check valve opening and closing timings may be coordinated with respect to stroke timings of the direct injection fuel pump. 
     Pressure relief valve  232  allows fuel flow out of solenoid activated control valve  236  toward the LPP  212  when pressure between pressure relief valve  232  and solenoid operated control valve  236  is greater than a predetermined pressure (e.g., 10 bar). When solenoid operated control valve  236  is deactivated (e.g., not electrically energized), solenoid operated control valve operates in a pass-through mode and pressure relief valve  232  regulates pressure in compression chamber  205  to the single pressure relief set-point of pressure relief valve  232  (e.g., 10 bar above the pressure at sensor  231 ). Regulating the pressure in compression chamber  205  allows a pressure differential to form from the piston top to the piston bottom. The pressure in step-room  227  is at the pressure of the outlet of the low pressure pump (e.g., 5 bar) while the pressure at piston top is at pressure relief valve regulation pressure (e.g., 15 bar). The pressure differential allows fuel to seep from the piston top to the piston bottom through the clearance between the piston and the pump cylinder wall, thereby lubricating HPP  214 . 
     Piston  228  reciprocates up and down. HPP  214  is in a compression stroke when piston  228  is traveling in a direction that reduces the volume of compression chamber  205 . HPP  214  is in a suction stroke when piston  228  is traveling in a direction that increases the volume of compression chamber  205 . 
     A forward flow outlet check valve  274  may be coupled downstream of an outlet  208  of the compression chamber  205 . Outlet check valve  274  opens to allow fuel to flow from the high pressure pump outlet  208  into a fuel rail only when a pressure at the outlet of direct injection fuel pump  214  (e.g., a compression chamber outlet pressure) is higher than the fuel rail pressure. Thus, during conditions when direct injection fuel pump operation is not requested, controller  222  may deactivate solenoid activated control valve  236  and pressure relief valve  232  regulates pressure in compression chamber  205  to a single substantially constant pressure during most of the compression stroke. On the intake stroke the pressure in compression chamber  205  drops to a pressure near the pressure of the lift pump ( 212 ). Lubrication of DI pump  214  may occur when the pressure in compression chamber  205  exceeds the pressure in step-room  227 . This difference in pressures may also contribute to pump lubrication when controller  222  deactivates solenoid activated control valve  236 . One result of this regulation method is that the fuel rail is regulated to a minimum pressure, approximately the pressure relief of pressure relief valve  232 . Thus, if pressure relief valve  232  has a pressure relief setting of 10 bar, the fuel rail pressure becomes 15 bar because this 10 bar adds to the 5 bar of lift pump pressure. Specifically, the fuel pressure in compression chamber  205  is regulated during the compression stroke of direct injection fuel pump  214 . Thus, during at least the compression stroke of direct injection fuel pump  214 , lubrication is provided to the pump. When direct fuel injection pump enters a suction stroke, fuel pressure in the compression chamber may be reduced while still some level of lubrication may be provided as long as the pressure differential remains. Another pressure relief valve  272  may be placed in parallel with check valve  274 . Pressure relief valve  272  allows fuel flow out of the DI fuel rail  250  toward pump outlet  208  when the fuel rail pressure is greater than a predetermined pressure. 
     As such, while the direct injection fuel pump is reciprocating, the flow of fuel between the piston and bore ensures sufficient pump lubrication and cooling. 
     The lift pump may be transiently operated in a pulsed mode where the lift pump operation is adjusted based on a pressure estimated at the outlet of the lift pump and inlet of the high pressure pump. In particular, responsive to high pressure pump inlet pressure falling below a fuel vapor pressure, the lift pump may be operated until the inlet pressure is at or above the fuel vapor pressure. This reduces the risk of the high pressure fuel pump ingesting fuel vapors (instead of fuel) and ensuing engine stall events. 
     It is noted here that the high pressure pump  214  of  FIG. 2  is presented as an illustrative example of one possible configuration for a high pressure pump. Components shown in  FIG. 2  may be removed and/or changed while additional components not presently shown may be added to pump  214  while still maintaining the ability to deliver high-pressure fuel to a direct injection fuel rail and a port injection fuel rail. 
     Solenoid activated control valve  236  may also be operated to direct fuel back-flow from the high pressure pump to one of pressure relief valve  232  and accumulator  215 . For example, control valve  236  may be operated to generate and store fuel pressure in accumulator  215  for later use. One use of accumulator  215  is to absorb fuel volume flow that results from the opening of compression pressure relief valve  232 . Accumulator  227  sources fuel as check valve  234  opens during the intake stroke of pump  214 . Another use of accumulator  215  is to absorb/source the volume changes in the step room  227 . Yet another use of accumulator  215  is to allow intermittent operation of lift pump  212  to gain an average pump input power reduction over continuous operation. 
     While the first direct injection fuel rail  250  is coupled to the outlet  208  of HPP  214  (and not to the inlet of HPP  214 ), second port injection fuel rail  260  is coupled to the inlet  203  of HPP  214  (and not to the outlet of HPP  214 ). Although inlets, outlets, and the like relative to compression chamber  205  are described herein, it may be appreciated that there may be a single conduit into compression chamber  205 . The single conduit may serve as inlet and outlet. In particular, second fuel rail  260  is coupled to HPP inlet  203  at a location upstream of solenoid activated control valve  236  and downstream of check valve  234  and pressure relief valve  232 . Further, no additional pump may be required between lift pump  212  and the port injection fuel rail  260 . As elaborated below, the specific configuration of the fuel system with the port injection fuel rail coupled to the inlet of the high pressure pump via a pressure relief valve and a check valve enables the pressure at the second fuel rail to be raised via the high pressure pump to a fixed default pressure that is above the default pressure of the lift pump. That is, the fixed high pressure at the port injection fuel rail is derived from the high pressure piston pump. 
     When the high pressure pump  214  is not reciprocating, such as at key-up before cranking, check valve  244  allows the second fuel rail to fill at 5 bar. As the pump chamber displacement becomes smaller due to the piston moving upward, the fuel flows in one of two directions. If the spill valve  236  is closed, the fuel goes into the high pressure fuel rail  250 . If the spill valve  236  is open, the fuel goes either into the low pressure fuel rail  250  or through the compression relief valve  232 . In this way, the high pressure fuel pump is operated to deliver fuel at a variable high pressure (such as between 15-200 bar) to the direct fuel injectors  252  via the first fuel rail  250  while also delivering fuel at a fixed high pressure (such as at 15 bar) to the port fuel injectors  262  via the second fuel rail  260 . The variable pressure may include a minimum pressure that is at the fixed pressure (as in the system of  FIG. 2 ). In the configuration depicted at  FIG. 2 , the fixed pressure of the port injection fuel rail is the same as the minimum pressure for the direct injection fuel rail, both being higher than the default pressure of the lift pump. Herein, the fuel delivery from the high pressure pump is controlled via the upstream (solenoid activated) control valve and further via the various check valve and pressure relief valves coupled to the inlet of the high pressure pump. By adjusting operation of the solenoid activated control valve, the fuel pressure at the first fuel rail is raised from the fixed pressure to the variable pressure while maintaining the fixed pressure at the second fuel rail. Valves  244  and  242  work in conjunction to keep the low pressure fuel rail  260  pressurized to 15 bar during the pump inlet stroke. Pressure relief valve  242  simply limits the pressure that can build in fuel rail  250  due to thermal expansion of fuel. A typical pressure relief setting may be 20 bar. 
     Controller  12  can also control the operation of each of fuel pumps  212 , and  214  to adjust an amount, pressure, flow rate, etc., of a fuel delivered to the engine. As one example, controller  12  can vary a pressure setting, a pump stroke amount, a pump duty cycle command and/or fuel flow rate of the fuel pumps to deliver fuel to different locations of the fuel system. A driver (not shown) electronically coupled to controller  222  may be used to send a control signal to the low pressure pump, as required, to adjust the output (e.g. speed) of the low pressure pump. 
     The embodiment depicted at  FIG. 2 , and also at embodiment  300  of  FIG. 3 , show a first fuel system configuration wherein fuel is supplied to the port injection fuel rail from the fuel tank by branching off before the high pressure direct injection fuel pump (HPFP). It will be appreciated, however, that in alternate embodiments, such as shown at embodiment  350  of  FIG. 3 , fuel may be supplied to the port injection fuel rail from the fuel tank via the high pressure direct injection fuel pump. 
     In earlier engine fuel system configurations (e.g., prior art), a dedicated low pressure pump was used for pressurizing the port injection fuel rail, the low pressure pump distinct from the high pressure pump used for pressurizing the direct injection fuel rail. The present configurations, depicted at  FIGS. 2-3 , allow for hardware reduction by using the same pump to pressurize both fuel rails. However in both configurations, fuel flow through the HPFP can cause fuel pulsations to enter the port injection fuel rail. This is due to the high pressure piston pump being driven by an engine camshaft, resulting in a defined number of pulses being experienced at the HPFP, and thereby in the port injection fuel rail, on each engine rotation (e.g., 3 pulses every 270 degrees on a 4-cylinder in-line engine). The port injection fuel rail pulsations may be exacerbated during conditions when the high pressure fuel pump is not supplying fuel to the direct injection fuel rail (such as when no direct injection of fuel is requested) and when fuel is only being supplied to the port injection fuel rail (such as when only port injection of fuel is requested). This is due to the direct injection fuel rail returning all of the ingested volume of fuel back to the lower pressure system. These pulsations in the port injection fuel rail can lead to significant fueling errors. 
     As elaborated herein, the fueling errors may be reduced by adjusting the timing of a port injection fuel pulse. The delivery of the port injection pulse may be moved to coincide with an average pressure-crossing of the waveform (e.g., sinusoidal curve) of the port injection fuel rail&#39;s pressure (as shown at  FIGS. 4-6B ). In this way, fueling errors due to positive pressure excursions are cancelled by fueling errors due to negative pressure excursions, improving port fuel injection metering. 
     Turning now to method  400 , an example method is shown for pressurizing fuel in a port injection fuel rail via an engine camshaft driven high pressure fuel pump and injecting a port fuel injection with a timing balanced around an average pressure-crossing of port fuel injection pressure. Instructions for carrying out method  400  and the rest of the methods included herein may be executed by a controller based on instructions stored on a memory of the controller and in conjunction with signals received from sensors of the engine system, such as the sensors described above with reference to  FIGS. 1-3 . The controller may employ engine actuators of the engine system to adjust engine operation, according to the methods described below. 
     At  402 , the method includes estimating and/or measuring engine operating conditions. The parameters may include, for example, engine speed, driver torque demand, fuel rail pressure, engine temperature, ambient conditions, etc. At  404 , the method includes determining a fuel injection profile based on the estimated engine operating conditions. The determined fuel injection profile may include an amount of fuel to be delivered via port injection (a port injection fuel pulse) and an amount of fuel to be delivered via direct injection (a direct injection fuel pulse). As an example, a desired fuel mass may be determined based on driver demand. Based on the desired fuel mass (for port and direct injection), and further based on fuel rail pressure, corresponding fuel pulses (for port and direct injection) may be calculated. 
     At  406 , it may be confirmed that port injection was requested. If at least some port injection was requested, the method proceeds to  408 . If no port injection is requested, and only direct injection (DI) is requested, the method moves to  424 . Therein, only the direct injection fuel rail is pressurized via a high pressure fuel pump that is coupled to the engine and driven via the engine camshaft. Specifically, an output of the high pressure fuel pump is adjusted to provide the desired fuel rail pressure at the direct injection fuel rail. At  426 , the method further includes calculating a DI fuel pulse width and timing based on the desired DI fuel mass. At  428 , the method includes delivering the desired DI fuel mass by operating the high pressure fuel pump and injecting fuel via the direct injector with the determined DI fuel pulse width and timing. 
     If at least some port fuel injection (PFI) was requested at  406 , the routine includes pressurizing fuel in the port injection fuel rail via the camshaft-driven high pressure fuel pump. This includes conditions where only port fuel injection is requested as well as conditions where port and direct fuel injection are both requested. Specifically, an output of the high pressure fuel pump is adjusted to provide the desired fuel rail pressure at the port injection fuel rail. At  410 , a PFI fuel injection pulse width is calculated based on engine operating conditions, such as desired fuel mass and PFI fuel rail pressure. In one example, the port injection fuel rail pressure may be estimated by a pressure sensor coupled to the PFI fuel rail. In an alternate example, the port injection fuel rail pressure may be estimated based on each of fuel pressure at the high pressure fuel pump and a fuel pulse delay, the delay based on engine speed, as elaborated herein at  FIGS. 6A-6B . 
     Specifically, map  600  of  FIG. 6A  shows a waveform of fuel pressure at the HPFP at plot  602  (dashed line, FP_HPFP). The fuel pressure at the HPFP may be measured by a pressure sensor coupled to the HPFP. A fuel pulse delay  604  may be applied to the fuel pressure at the HPFP and used to estimate the fuel pressure in the port injection fuel rail, depicted at plot  606  (solid line, FP_PFIFR). The fuel pulse delay represents a delay in the travel of a pulse of fuel from the HPFP into the PFI fuel rail. In the depicted example, the delay is represented as a duration elapsed from detection of a local maxima of a wave cycle in the fuel pressure at the HPFP to detection of the same local maxima in the fuel pressure of the PFI fuel rail. As such, the delay is based on engine speed, their relationship depicted at plot  650  of  FIG. 6B . In particular, the delay is increased as the engine speed increases. 
     Returning to  FIG. 4 , at  412 , the controller may calculate an initial timing for the port fuel injection based on the engine operating conditions and the fuel pulse width. The initial timing may be based on the velocity of the fuel travelling to the fuel rail from the HPFP, as well as a timing (or engine position) corresponding to intake valve opening. For example, the controller may calculate an initial end of injection angle for the PFI fuel pulse based on the fuel pulse width and the desired timing. In one example, the initial timing may correspond to a timing that allows for closed intake valve injection of the determined fuel mass. The initial timing may correspond to an engine position and may include a defined number of crank angle degrees. At  414 , the method includes calculating a middle of injection angle for the PFI fuel pulse based on the initial end of injection angle timing and the fuel pulse width. 
     As such, the initial timing corresponding to closed intake valve injection may occur at any position of the waveform of the port injection fuel pressure, such as at or near a local maxima or a local minima. However, such locations may result in pressure fluctuations and thereby fueling errors. As elaborated herein, the controller may be configured to adjust the delivery of the port injection fuel pulse to inject the port fuel injection with an updated timing that is balanced around an average pressure-crossing of the estimated port injection fuel pressure. This allows fueling errors to be reduced. 
     Specifically, at  416 , the method includes identifying a nearest average pressure crossing of port injection fuel pressure in the advanced direction. The average pressure-crossing may include an average pressure between a local maxima and a local minima (for a cycle of the pressure waveform) of port injection fuel pressure. In other words, for a sinusoidal pressure waveform, the average pressure-crossing may correspond to a timing where above-average pressure is cancelled by under-average pressure. The average pressure-crossing timing may be with reference to an engine position and may include a defined number of engine crank angle degrees. In one example, the average pressure-crossing includes a zero-crossing of port fuel injection pressure. As such, for every cycle of the waveform, there are two average pressure-crossings (or zero-crossings). The controller may identify and select a first average pressure-crossing in the advanced direction even if a second average pressure-crossing in the retarded direction is closer. By selecting the first average pressure-crossing in the advanced direction, closed intake valve injection of fuel can be maintained. 
     At  418 , the method includes moving delivery of the port fuel injection pulse from the initial timing corresponding to closed intake valve injection to the first average pressure crossing in the advanced direction. In other words, delivery of the port fuel injection pulse is not moved to a second average pressure crossing in the retarded direction, even if a distance between the initial timing and the second average pressure crossing is smaller than the distance between the initial timing and the first average pressure crossing. The moving specifically includes aligning the middle of injection angle of the port injection fuel pulse (as determined for the initial timing) with the first average pressure crossing in the advanced direction. 
     At  420 , the method includes adjusting intake port fuel puddle model dynamics based on the moving. In one example, due to the advancing of the timing, the adjusting may be performed to account for increased vaporization of fuel in the intake port due to a longer duration of the intake port fuel puddle sitting on the intake valve or on valve walls. Accordingly, the adjusting may include updating the fuel pulse width and moving an end of injection angle of the port injection fuel pulse based on the aligning of the middle of injection angle and the adjusted intake port fuel puddle model dynamics. As an example, to account for the increased vaporization of fuel in the intake port, the fuel pulse width may be shortened by maintaining the middle of injection angle at the average pressure-crossing while advancing the end of injection angle. A trimming factor may be determined based on the updated fuel pulse width relative to the initial fuel pulse width (as determined at  410 ), and the trimming factor may be applied to the end of injection angle. 
     It will be appreciated that injecting the port fuel injection with a timing balanced around an average pressure-crossing of port fuel injection pressure may include injecting each fuel injection pulse in a selected engine speed-load region with the timing balanced around the average pressure-crossing, while injecting each fuel injection pulse outside the selected engine speed-load region with a timing based on intake valve opening. Herein, the timing based on intake valve opening may include a timing offset from the average pressure-crossing (e.g., offset from the average pressure-crossing and towards the local maxima or a local minima of a cycle of the pressure waveform). 
     At  422 , the method includes delivering or injecting fuel via the port injector according to the updated fuel pulse timing, and updated fuel pulse width, where applicable. If direct fuel injection was also requested along with the port fuel injection, the method may further include determining the pulse width add timing of the DI fuel pulse, and also delivering fuel via the direct injector according to the determined DI fuel pulse profile. 
     In this way, port fueling errors induced by pressure fluctuations at the HPFP are reduced. An example delivery of fuel via port fuel injection at an average pressure-crossing of port fuel injection pressure is now discussed with reference to  FIG. 5 . 
     Map  500  of  FIG. 5  depicts a port injection fuel rail pressure at plot  502 , and a port fuel injector duty cycle (PDI_DutyCycle) at plot  520 . All plots are shown over time, depicted herein in terms of engine position in crank angle degrees (CAD). 
     As shown by the sinusoidal waveform of plot  502 , the port injection fuel rail pressure may periodically fluctuate between a local maxima  504  and a local minima  506 . A statistical average of the local maxima and the local minima is determined as the average pressure (P_average), shown here as a dashed line. The average pressure-crossings (herein also referred to as a zero-crossing) of the port injection fuel rail pressure, representing positions of the waveform that overlap with the average pressure, are represented by solid dots  508 . As such, for each waveform cycle  505  (from one local minima to a subsequent local minima, as depicted, or from one local maxima to a subsequent local maxima), there may be two average pressure crossings  508  including one average pressure crossing on the ascending limb of the waveform (also referred to herein as an upward average pressure-crossing or upward zero-crossing) and one average pressure crossing on the decending limb of the waveform (also referred to herein as an downward average pressure-crossing or downward zero-crossing). It will be appreciated that while the waveform of  FIG. 5  shows symmetric waves of equal intensity and a fixed frequency, in alternate examples, the waveform may be asymmetric such that the local maxima, minima, and average pressures for the waveform of each cycle are different from those of another cycle. 
     In the depicted example, a first port injection fuel pulse PW 0  is determined initially for port injection of fuel in a first cylinder, and a second port injection fuel pulse PW 1  is determined initially for port injection of fuel in a second cylinder, the second cylinder firing immediately after the first cylinder. First fuel pulse PW 0  may have an initial pulse width W 0  and an initial timing  511  corresponding to a position at or around the local maxima. Second fuel pulse PW 1  may have an initial pulse width W 1  and an initial timing  513  corresponding to a position at or around the local minima. 
     To reduce fueling errors induced by the sinusoidal fuel pressure change, the duty cycle of the first port injection fuel pulse PW 0  is adjusted to move the timing to be balanced around a first average pressure-crossing in the advanced direction relative to initial timing  511 . Specifically, a middle of injection angle of first fuel pulse PW 0  is moved from initial timing  511  and repositioned to be aligned with a first average pressure-crossing  508   a  in the advanced direction. Herein, the first average pressure-crossing in the advanced direction is an upward pressure-crossing. Thus, initial first fuel pulse PW 0  (dotted line) is repositioned, as shown by arrow  510 , to updated first fuel pulse PW 0 ′ (solid line). As a result of the repositioning, fueling errors caused by over-average pressure estimation can be cancelled by fueling errors caused by under average-pressure estimation (as shown by straight lines in PW 0 ′ that are centered around  508   a ). The repositioning is performed without the need for additional adjustments to the fuel pulse width. Thus, after the repositioning, updated first fuel pulse PW 0 ′ has the same pulse width w 1  as initial first fuel pulse PW 0 . 
     Also to reduce fueling errors induced by the sinusoidal fuel pressure change, the duty cycle of the second port injection fuel pulse PW 1  is adjusted to move the timing to be balanced around a first average pressure-crossing in the advanced direction relative to initial timing  513 . Specifically, a middle of injection angle of second fuel pulse PW 1  is moved from initial timing  513  and repositioned to be aligned with a first average pressure-crossing  508   b  in the advanced direction. Herein, the first average pressure-crossing  508   b  in the advanced direction is a downward pressure-crossing. Herein, even though the initial timing  513  is significantly closer to second (upward) average pressure-crossing  508   c , due to second average pressure-crossing  508   c  being in a retarded direction relative to initial timing  513 , it is not selected. This allows closed intake valve port injection of fuel to be maintained. Instead, initial second fuel pulse PW 1  (dotted line) is repositioned, as shown by arrow  512 , to updated second fuel pulse PW 1 ′ (solid line). As a result of the repositioning, fueling errors caused by over-average pressure estimation can be cancelled by fueling errors caused by under average-pressure estimation (as shown by straight lines in PW 1 ′ that are centered around  508   b ). The repositioning is performed with the need for additional adjustments to the fuel pulse width. Specifically, to compensate for the additional duration that the fuel sits at or near the closed intake valve, and the resultant increase in fuel vapor generation, after the repositioning, updated second fuel pulse PW 1 ′ has a smaller pulse width w 2 ′ as compared to the pulse width w 2  of initial second fuel pulse PW 1 . Herein, this is achieved by aligning the middle of injection angle of PW 1  with average-pressure crossing  508   b  and then advancing the end of injection angle of PW 1 ′ towards average-pressure crossing  508   b.    
     One example method for an engine comprises: pressurizing fuel in a port injection fuel rail via an engine camshaft driven high pressure fuel pump; and injecting a port fuel injection with a timing balanced around an average pressure-crossing of port fuel injection pressure. In the preceding example, additionally or optionally, the average pressure crossing includes a zero-crossing of port fuel injection pressure. In any or all of the preceding examples, additionally or optionally, the average pressure crossing of port fuel injection pressure includes an average pressure between a local maxima and a local minima of port fuel injection pressure. In any or all of the preceding examples, additionally or optionally, the injecting includes moving delivery of a port injection fuel pulse from an initial timing corresponding to closed intake valve injection to a first average pressure crossing in an advanced direction. In any or all of the preceding examples, additionally or optionally, the injecting further includes not moving delivery of the port injection fuel pulse to a second average pressure crossing in a retarded direction. In any or all of the preceding examples, additionally or optionally, the moving includes aligning a middle of injection angle of the port injection fuel pulse with the first average pressure crossing in the advanced direction. In any or all of the preceding examples, additionally or optionally, the method further comprises adjusting intake port fuel puddle model dynamics based on the moving. In any or all of the preceding examples, additionally or optionally, the method further comprises moving an end of injection angle of the port injection fuel pulse based on the aligning of the middle of injection angle and the adjusted intake port fuel puddle model dynamics. In any or all of the preceding examples, additionally or optionally, the injecting includes injecting each fuel injection pulse in a selected engine speed-load region with the timing balanced around the average pressure-crossing, the method further comprising, injecting each fuel injection pulse outside the selected engine speed-load region with a timing based on intake valve opening, the timing based on intake valve opening including a timing offset from the average pressure-crossing. In any or all of the preceding examples, additionally or optionally, the port fuel injection pressure is estimated based on each of fuel pressure at the high pressure fuel pump and a fuel pulse delay, the delay based on engine speed, the delay increased as the engine speed increases. 
     Another example method for an engine comprises: pressurizing fuel in a port injection fuel rail via an engine camshaft driven high pressure fuel pump; and moving a port injection fuel pulse from an initial timing based on intake valve opening to a final timing based on an estimated fuel pressure of the port injection fuel rail. In the preceding example, additionally or optionally, the estimated fuel pressure of the port injection fuel rail is based on a measured fuel pressure of the high pressure fuel pump. In any or all of the preceding examples, additionally or optionally, the high pressure fuel pump is a piston pump, and the estimated fuel pressure of the port injection fuel rail has a waveform, wherein a local maxima of the waveform is based on a fuel pressure of the high pressure fuel pump measured when the piston is at top dead center and a local minima of the waveform is based on the fuel pressure of the high pressure fuel pump measured when the piston is at bottom dead center. In any or all of the preceding examples, additionally or optionally, the estimated fuel pressure of the port injection fuel rail is further based on a fuel pulse delay between fuel at the high pressure fuel pump and fuel at the port injection fuel rail, the fuel pulse delay based on engine speed, the delay increased as the engine speed increases. In any or all of the preceding examples, additionally or optionally, the moving includes moving to a final timing that coincides with a first zero-crossing of the waveform in an advanced direction, and not to a second zero-crossing of the waveform in a retarded direction, the first and second zero-crossings corresponding to a mid-point between the local maxima and the local minima of the waveform. In any or all of the preceding examples, additionally or optionally, during a first condition, the first zero-crossing is an upward zero-crossing across the waveform while the second zero-crossing is a downward zero-crossing across the waveform, while during a second condition, the first zero-crossing is a downward zero-crossing across the waveform while the second zero-crossing is an upward zero-crossing across the waveform. In any or all of the preceding examples, additionally or optionally, the moving includes aligning a middle of injection angle of the port injection fuel pulse with the first zero-crossing. In any or all of the preceding examples, additionally or optionally, the moving further includes adjusting a beginning and end angle of the fuel pulse based on the aligning of the middle of injection angle. In any or all of the preceding examples, additionally or optionally, the method further comprises updating modeled puddle dynamics of an intake port fuel puddle based on the moving, and adjusting a pulse width of the port injection fuel pulse based on the updated model, the pulse width adjusted by adjusting an end of injection angle of the port injection fuel pulse. In any or all of the preceding examples, additionally or optionally, each of the initial and final timing include engine crank angle degrees (that is, they are related to an engine position). In any or all of the preceding examples, additionally or optionally, the method further comprises operating a port fuel injector to deliver the port injection fuel pulse at the final timing, and if direct injection is requested, pressurizing fuel in a direct injection fuel rail via the engine camshaft driven high pressure fuel pump. 
     Another example method for an engine comprises: pressurizing each of a port injection fuel rail and a direct injection fuel rail via an engine-driven high pressure piston fuel pump; and advancing a timing of a port injection fuel pulse based on a fuel pressure at the pump when the piston is at top dead center and a fuel pressure at the pump when the piston is at bottom dead center. In the preceding example, additionally or optionally, the advancing includes estimating a local maxima for a given pressure cycle in the port injection fuel rail based on a fuel pressure in the fuel pump when the piston is at TDC; estimating a local minima for the given pressure cycle in the port injection fuel rail based on the fuel pressure in the fuel pump when the piston is at BDC; estimating an average pressure in the port injection fuel rail based on the local maxima and local minima; and advancing the timing of an injection pulse to coincide with a timing of the average pressure. 
     In a further representation, an engine fuel system comprises: an engine; a first fuel rail coupled to a direct injector; a second fuel rail coupled to a port injector; a high pressure mechanical fuel pump driven by the engine via a camshaft, the fuel pump delivering fuel to each of the first and second fuel rails, the first fuel rail coupled to an outlet of the high pressure fuel pump, the second fuel rail coupled to an inlet of the high pressure fuel pump; a pressure sensor coupled to the fuel pump for estimating a fuel pressure at the fuel pump; an engine speed sensor; and a controller. The controller may be configured with computer readable instructions stored on non-transitory memory for: pressurizing the port injection fuel rail via the high pressure fuel pump; calculating fuel pressure in the second fuel rail based on the estimated fuel pressure at the fuel pump and estimated engine speed; determining an initial port fuel injection profile including an initial injection timing based on engine operating conditions; calculating each of a local maxima, a local minima, and an average pressure-crossing for one or more pressure cycles in the second fuel rail around the initial injection timing; and delivering fuel via the port injector at a final timing balanced around an average pressure-crossing advanced from the initial injection timing. 
     In another representation, a method for an engine comprises: pressurizing a port injection fuel rail and a direct injection fuel rail via an engine-driven high pressure piston fuel pump; estimating a local maxima for a given pressure cycle in the port injection fuel rail based on a fuel pressure in the fuel pump when the piston is at TDC; estimating a local minima for the given pressure cycle in the port injection fuel rail based on the fuel pressure in the fuel pump when the piston is at BDC; estimating an average pressure in the port injection fuel rail based on the local maxima and local minima; and moving a timing of an injection pulse to coincide with a timing of the average pressure. In the above embodiment, the moving includes advancing the timing of the injection pulse to coincide with a timing of the average pressure. 
     In this way, a PFI fuel pulse center is adjusted around an average pressure-crossing (e.g., a zero-crossing) of a fuel rail pressure to reduce metering errors. By aligning the middle of injection angle of the port injection fuel pulse to coincide with a first average pressure-crossing in the advanced direction, closed intake valve port fuel injection is enabled with fueling errors caused by over-pressure estimation to be cancelled by fueling errors caused by under-pressure estimation. By aligning the fuel pulse center with a position where the port injection fuel rail pressure fluctuates the least, the need for fast fuel pressure sampling while also reducing the dependence on hardware or software related to pressure dampening. Overall, metering of fuel from the port injection fuel rail is improved. By reducing fueling errors in fuel systems where a single high pressure pump can advantageously pressurize both direct and port injection rails, component reduction benefits are achieved without incurring a loss in fueling accuracy. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and may be carried out by the control system including the controller in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system, where the described actions are carried out by executing the instructions in a system including the various engine hardware components in combination with the electronic controller. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.