Patent Publication Number: US-9422898-B2

Title: Direct injection fuel pump

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application No. 61/763,881 filed on Feb. 12, 2013, the entire contents of which are incorporated herein by reference for all purposes. 
    
    
     BACKGROUND AND SUMMARY 
     A vehicle&#39;s fuel systems may supply fuel to an engine in varying amounts during the course of vehicle operation. During some conditions, fuel is not injected to the engine but fuel pressure in a fuel rail supplying fuel to the engine is maintained so that combustion can be reinitiated. For example, during vehicle deceleration fuel flow to one or more engine cylinders may be stopped by deactivating fuel injectors. If the engine torque demand is increased after fuel flow to the one or more cylinders ceases, fuel injection is reactivated and the engine resumes providing positive torque to the vehicle driveline. However, if the engine is supplied fuel via direct fuel injectors and a high pressure fuel pump, the high pressure pump may degrade when fuel flow through the high pressure pump is stopped while the fuel injectors are deactivated. Specifically, the lubrication and cooling of the pump may be reduced while the high pressure pump is not operated, thereby leading to pump degradation. 
     The inventors herein have recognized the above-mentioned issue may be at least partly addressed by a method of operating a direct injection fuel pump, comprising: regulating a pressure in a compression chamber of the direct injection fuel pump to a single pressure during a direct injection fuel pump compression stroke, the pressure greater than an the pressure on the low pressure side of the piston. This pressure may be the output pressure of a low pressure pump supplying fuel to the direct injection fuel pump. 
     By regulating pressure in the compression chamber of a direct injection fuel pump it may be possible to lubricate the direct injection fuel pump&#39;s cylinder and piston when flow out of the direct injection fuel pump to fuel injectors is stopped. Specifically, a fuel pressure differential across the direct injection fuel pump&#39;s piston may be provided that allows fuel to flow into the piston/bore clearance and lubricate an area. Further, pressure in the compression chamber is less than pressure in the fuel rail so there is no flow from the direct injection fuel pump to the fuel rail. In this way, the piston may continue to reciprocate within the direct injection fuel pump with a low rate of degradation and without supplying fuel to the engine. 
     The present description may provide several advantages. Specifically, the approach may improve fuel pump lubrication and reduce fuel pump degradation. Additionally, pressure in the compression chamber can be regulated to a higher pressure than low pressure fuel pump pressure so that engine operation may be improved during conditions of direct injection fuel pump degradation. Further, the approach may be applied at low cost and complexity. Further still, the approach may reduce fuel pump noise since a solenoid activated check valve at an inlet of the direct injection fuel pump may be deactivated when fuel flow to the engine is stopped. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     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  shows an example of a cylinder of an internal combustion engine; 
         FIG. 2  shows an example of a fuel system that may be used with the engine of  FIG. 1 ; 
         FIG. 3  shows another example of a fuel system that may be used with the engine of  FIG. 1 ; 
         FIG. 4  shows an example of a high pressure direct injection fuel pump of the fuel system of  FIGS. 2 and 3 ; 
         FIG. 5  shows another example of a high pressure direct injection fuel pump of the fuel system in  FIGS. 2 and 3 ; 
         FIGS. 6-8  show example high pressure direct injection fuel pump operating sequences; 
         FIG. 9  shows an example flow chart of a method for operating a high pressure direct injection fuel pump; 
         FIG. 10  shows an alternative example fuel system that may be used with the engine of  FIG. 1 ; and 
         FIG. 11  shows an alternative example high pressure direct injection fuel pump of the fuel system of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure relates to methods and systems for operating a direct injection fuel pump, such as the system of  FIGS. 2 and 3 . The fuel system may be configured to deliver one or more different fuel types to a combustion engine, such as the engine of  FIG. 1 . Alternatively, the fuel system may supply a single type of fuel as shown in the system of  FIG. 3 . A direct injection fuel pump with integrated pressure relief and check valves as shown in  FIG. 4  may be incorporated into the systems of  FIGS. 2 and 3 . Alternatively, the pressure relief valves and check valves may be external to the direct injection fuel pump. In some examples, the direct injection fuel pump may further include an accumulator as shown in  FIG. 5  to further enhance direct injection fuel pump operation. The direct injection fuel pumps may operate as shown if  FIGS. 6-8  when fuel is not being supplied to the engine while the engine is rotating.  FIG. 9  shows a method for operating a direct injection fuel pump in the systems of  FIGS. 2 and 3  to provide the sequences shown in  FIGS. 7 and 8 . 
       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. 
       FIG. 2  schematically depicts an example fuel system  8  of  FIG. 1 . Fuel system  8  may be operated to deliver fuel to an engine, such as engine  10  of  FIG. 1 . Fuel system  8  may be operated by a controller to perform some or all of the operations described with reference to the process flow of  FIG. 9 . 
     Fuel system  8  can provide fuel to an engine from one or more different fuel sources. As a non-limiting example, a first fuel tank  202  and a second fuel tank  212  may be provided. While fuel tanks  202  and  212  are described in the context of discrete vessels for storing fuel, it should be appreciated that these fuel tanks may instead be configured as a single fuel tank having separate fuel storage regions that are separated by a wall or other suitable membrane. Further still, in some embodiments, this membrane may be configured to selectively transfer select components of a fuel between the two or more fuel storage regions, thereby enabling a fuel mixture to be at least partially separated by the membrane into a first fuel type at the first fuel storage region and a second fuel type at the second fuel storage region. 
     In some examples, first fuel tank  202  may store fuel of a first fuel type while second fuel tank  212  may store fuel of a second fuel type, wherein the first and second fuel types are of differing composition. As a non-limiting example, the second fuel type contained in second fuel tank  212  may include a higher concentration of one or more components that provide the second fuel type with a greater relative knock suppressant capability than the first fuel. 
     By way of example, the first fuel and the second fuel may each include one or more hydrocarbon components, but the second fuel may also include a higher concentration of an alcohol component than the first fuel. Under some conditions, this alcohol component can provide knock suppression to the engine when delivered in a suitable amount relative to the first fuel, and may include any suitable alcohol such as ethanol, methanol, etc. Since alcohol can provide greater knock suppression than some hydrocarbon based fuels, such as gasoline and diesel, due to the increased latent heat of vaporization and charge cooling capacity of the alcohol, a fuel containing a higher concentration of an alcohol component can be selectively used to provide increased resistance to engine knock during select operating conditions. 
     As another example, the alcohol (e.g. methanol, ethanol) may have water added to it. As such, water reduces the alcohol fuel&#39;s flammability giving an increased flexibility in storing the fuel. Additionally, the water content&#39;s heat of vaporization enhances the ability of the alcohol fuel to act as a knock suppressant. Further still, the water content can reduce the fuel&#39;s overall cost. 
     As a specific non-limiting example, the first fuel type in the first fuel tank may include gasoline and the second fuel type in the second fuel tank may include ethanol. As another non-limiting example, the first fuel type may include gasoline and the second fuel type may include a mixture of gasoline and ethanol. In still other examples, the first fuel type and the second fuel type may each include gasoline and ethanol, whereby the second fuel type includes a higher concentration of the ethanol component than the first fuel (e.g., E10 as the first fuel type and E85 as the second fuel type). As yet another example, the second fuel type may have a relatively higher octane rating than the first fuel type, thereby making the second fuel a more effective knock suppressant than the first fuel. It should be appreciated that these examples should be considered non-limiting as other suitable fuels may be used that have relatively different knock suppression characteristics. In still other examples, each of the first and second fuel tanks may store the same fuel. While the depicted example illustrates two fuel tanks with two different fuel types, it will be appreciated that in alternate embodiments, only a single fuel tank with a single type of fuel may be present. 
     Fuel tanks  202  and  212  may differ in their fuel storage capacities. In the depicted example, where second fuel tank  212  stores a fuel with a higher knock suppressant capability, second fuel tank  212  may have a smaller fuel storage capacity than first fuel tank  202 . However, it should be appreciated that in alternate embodiments, fuel tanks  202  and  212  may have the same fuel storage capacity. 
     Fuel may be provided to fuel tanks  202  and  212  via respective fuel filling passages  204  and  214 . In one example, where the fuel tanks store different fuel types, fuel filling passages  204  and  214  may include fuel identification markings for identifying the type of fuel that is to be provided to the corresponding fuel tank. 
     A first low pressure fuel pump (LPP)  208  in communication with first fuel tank  202  may be operated to supply the first type of fuel from the first fuel tank  202  to a first group of port injectors  242 , via a first fuel passage  230 . In one example, first fuel pump  208  may be an electrically-powered lower pressure fuel pump disposed at least partially within first fuel tank  202 . Fuel lifted by first fuel pump  208  may be supplied at a lower pressure into a first fuel rail  240  coupled to one or more fuel injectors of first group of port injectors  242  (herein also referred to as first injector group). While first fuel rail  240  is shown dispensing fuel to four fuel injectors of first injector group  242 , it will be appreciated that first fuel rail  240  may dispense fuel to any suitable number of fuel injectors. As one example, first fuel rail  240  may dispense fuel to one fuel injector of first injector group  242  for each cylinder of the engine. Note that in other examples, first fuel passage  230  may provide fuel to the fuel injectors of first injector group  242  via two or more fuel rails. For example, where the engine cylinders are configured in a V-type configuration, two fuel rails may be used to distribute fuel from the first fuel passage to each of the fuel injectors of the first injector group. 
     Direct injection fuel pump  228  that is included in second fuel passage  232  and may be supplied fuel via LPP  208  or LPP  218 . In one example, direct injection fuel pump  228  may be a mechanically-powered positive-displacement pump. Direct injection fuel pump  228  may be in communication with a group of direct injectors  252  via a second fuel rail  250 , and the group of port injectors  242  via a solenoid valve  236 . Thus, lower pressure fuel lifted by first fuel pump  208  may be further pressurized by direct injection fuel pump  228  so as to supply higher pressure fuel for direct injection to second fuel rail  250  coupled to one or more direct fuel injectors  252  (herein also referred to as second injector group). In some examples, a fuel filter (not shown) may be disposed upstream of direct injection fuel pump  228  to remove particulates from the fuel. Further, in some examples a fuel pressure accumulator (not shown) may be coupled downstream of the fuel filter, between the low pressure pump and the high pressure pump. 
     A second low pressure fuel pump  218  in communication with second fuel tank  212  may be operated to supply the second type of fuel from the second fuel tank  202  to the direct injectors  252 , via the second fuel passage  232 . In this way, second fuel passage  232  fluidly couples each of the first fuel tank and the second fuel tank to the group of direct injectors. In one example, third fuel pump  218  may also be an electrically-powered low pressure fuel pump (LPP), disposed at least partially within second fuel tank  212 . Thus, lower pressure fuel lifted by low pressure fuel pump  218  may be further pressurized by higher pressure fuel pump  228  so as to supply higher pressure fuel for direct injection to second fuel rail  250  coupled to one or more direct fuel injectors. In one example, second low pressure fuel pump  218  and direct injection fuel pump  228  can be operated to provide the second fuel type at a higher fuel pressure to second fuel rail  250  than the fuel pressure of the first fuel type that is provided to first fuel rail  240  by first low pressure fuel pump  208 . 
     Fluid communication between first fuel passage  230  and second fuel passage  232  may be achieved through first and second bypass passages  224  and  234 . Specifically, first bypass passage  224  may couple first fuel passage  230  to second fuel passage  232  upstream of direct injection fuel pump  228 , while second bypass passage  234  may couple first fuel passage  230  to second fuel passage  232  downstream of direct injection fuel pump  228 . One or more pressure relief valves may be included in the fuel passages and/or bypass passages to resist or inhibit fuel flow back into the fuel storage tanks. For example, a first pressure relief valve  226  may be provided in first bypass passage  224  to reduce or prevent back flow of fuel from second fuel passage  232  to first fuel passage  230  and first fuel tank  202 . A second pressure relief valve  222  may be provided in second fuel passage  232  to reduce or prevent back flow of fuel from the first or second fuel passages into second fuel tank  212 . In one example, lower pressure pumps  208  and  218  may have pressure relief valves integrated into the pumps. The integrated pressure relief valves may limit the pressure in the respective lift pump fuel lines. For example, a pressure relief valve integrated in first fuel pump  208  may limit the pressure that would otherwise be generated in first fuel rail  240  if solenoid valve  236  were (intentionally or unintentionally) open and while direct injection fuel pump  228  were pumping. 
     In some examples, the first and/or second bypass passages may also be used to transfer fuel between fuel tanks  202  and  212 . Fuel transfer may be facilitated by the inclusion of additional check valves, pressure relief valves, solenoid valves, and/or pumps in the first or second bypass passage, for example, solenoid valve  236 . In still other examples, one of the fuel storage tanks may be arranged at a higher elevation than the other fuel storage tank, whereby fuel may be transferred from the higher fuel storage tank to the lower fuel storage tank via one or more of the bypass passages. In this way, fuel may be transferred between fuel storage tanks by gravity without necessarily requiring a fuel pump to facilitate the fuel transfer. 
     The various components of fuel system  8  communicate with an engine control system, such as controller  12 . For example, controller  12  may receive an indication of operating conditions from various sensors associated with fuel system  8  in addition to the sensors previously described with reference to  FIG. 1 . The various inputs may include, for example, an indication of an amount of fuel stored in each of fuel storage tanks  202  and  212  via fuel level sensors  206  and  216 , respectively. Controller  12  may also receive an indication of fuel composition from one or more fuel composition sensors, in addition to, or as an alternative to, an indication of a fuel composition that is inferred from an exhaust gas sensor (such as sensor  126  of  FIG. 1 ). For example, an indication of fuel composition of fuel stored in fuel storage tanks  202  and  212  may be provided by fuel composition sensors  210  and  220 , respectively. Additionally or alternatively, one or more fuel composition sensors may be provided at any suitable location along the fuel passages between the fuel storage tanks and their respective fuel injector groups. For example, fuel composition sensor  238  may be provided at first fuel rail  240  or along first fuel passage  230 , and/or fuel composition sensor  248  may be provided at second fuel rail  250  or along second fuel passage  232 . As a non-limiting example, the fuel composition sensors can provide controller  12  with an indication of a concentration of a knock suppressing component contained in the fuel or an indication of an octane rating of the fuel. For example, one or more of the fuel composition sensors may provide an indication of an alcohol content of the fuel. 
     Note that the relative location of the fuel composition sensors within the fuel delivery system can provide different advantages. For example, sensors  238  and  248 , arranged at the fuel rails or along the fuel passages coupling the fuel injectors with one or more fuel storage tanks, can provide an indication of a resulting fuel composition where two or more different fuels are combined before being delivered to the engine. In contrast, sensors  210  and  220  may provide an indication of the fuel composition at the fuel storage tanks, which may differ from the composition of the fuel actually delivered to the engine. 
     Controller  12  can also control the operation of each of fuel pumps  208 ,  218 , and  228  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  12  may be used to send a control signal to each of the low pressure pumps, as required, to adjust the output (e.g. speed) of the respective low pressure pump. The amount of first or second fuel type that is delivered to the group of direct injectors via the direct injection pump may be adjusted by adjusting and coordinating the output of the first or second LPP and the direct injection pump. For example, the lower pressure fuel pump and the higher pressure fuel pump may be operated to maintain a prescribed fuel rail pressure. A fuel rail pressure sensor coupled to the second fuel rail may be configured to provide an estimate of the fuel pressure available at the group of direct injectors. Then, based on a difference between the estimated rail pressure and a desired rail pressure, the pump outputs may be adjusted. In one example, where the high pressure fuel pump is a volumetric displacement fuel pump, the controller may adjust a flow control valve of the high pressure pump to vary the effective pump volume of each pump stroke. 
     As such, while the direct injection fuel pump is operating, flow of fuel there-though ensures sufficient pump lubrication and cooling. However, during conditions when direct injection fuel pump operation is not requested, such as when no direct injection of fuel is requested, and/or when the fuel level in the second fuel tank  212  is below a threshold (that is, there is not enough knock-suppressing fuel available), the direct injection fuel pump may not be sufficiently lubricated if fuel flow through the pump is discontinued. 
     Referring now to  FIG. 3 , is shows a second example fuel system for supplying fuel to engine  10  of  FIG. 1 . Many devices and/or components in the fuel system of  FIG. 3  are the same as devices and/or components shown in  FIG. 2 . Therefore, for the sake of brevity, devices and components of the fuel system of  FIG. 2 , and that are included in the fuel system of  FIG. 3 , are labeled the same and the description of these devices and components is omitted in the description of  FIG. 3 . 
     The fuel system of  FIG. 3  supplies fuel from a single fuel tank to direct injectors  252  and port injectors  242 . However, in other examples, fuel may be supplied only to direct injectors  252  and port injectors  242  may be omitted. In this example system, low pressure fuel pump  208  supplies fuel to direct injection fuel pump  228  via fuel passage  302 . Controller  12  adjusts the output of direct injection fuel pump  228  via adjusting a flow control valve of direct injection pump  228 . Direct injection pump may stop providing fuel to fuel rail  250  during selected conditions such as during vehicle deceleration or while the vehicle is traveling downhill. Further, during vehicle deceleration or while the vehicle is traveling downhill, one or more direct fuel injectors  252  may be deactivated. 
       FIG. 4  shows first example direct injection fuel pump  228  show in the systems of  FIGS. 2 and 3 . Inlet  403  of direct injection fuel pump compression chamber  408  is supplied fuel via a low pressure fuel pump as shown in  FIGS. 2 and 3 . The fuel may be pressurized upon its passage through direct injection fuel pump  228  and supplied to a fuel rail through pump outlet  404 . In the depicted example, direct injection pump  228  may be a mechanically-driven displacement pump that includes a pump piston  406  and piston rod  420 , a pump compression chamber  408  (herein also referred to as compression chamber), and a step-room  418 . Piston  406  includes a top  405  and a bottom  407 . The step-room and compression chamber may include cavities positioned on opposing sides of the pump piston. In one example, engine controller  12  may be configured to drive the piston  406  in direct injection pump  228  by driving cam  410 . Cam  410  includes four lobes and completes one rotation for every two engine crankshaft rotations. 
     A solenoid activated inlet check valve  412  may be coupled to pump inlet  403 . Controller  12  may be configured to regulate fuel flow through inlet check valve  412  by energizing or de-energizing the solenoid valve (based on the solenoid valve configuration) in synchronism with the driving cam. Accordingly, solenoid activated inlet check valve  412  may be operated in two modes. In a first mode, solenoid activated check valve  412  is positioned within inlet  403  to limit (e.g. inhibit) the amount of fuel traveling upstream of the solenoid activated check valve  412 . In comparison, in the second mode, solenoid activated check valve  412  is effectively disabled and fuel can travel upstream and downstream of inlet check valve. 
     As such, solenoid activated check valve  412  may be configured to regulate the mass of fuel compressed into the direct injection fuel pump. In one example, controller  12  may adjust a closing timing of the solenoid activated check valve to regulate the mass of fuel compressed. For example, a late inlet check valve closing may reduce the amount of fuel mass ingested into the compression chamber  408 . The solenoid activated check valve opening and closing timings may be coordinated with respect to stroke timings of the direct injection fuel pump. By continuously throttling the flow into the direct injection fuel pump from the low pressure fuel pump, fuel may be ingested into the direct injection fuel pump without requiring metering of the fuel mass. 
     Pump inlet  499  allows fuel to check valve  402  and pressure relief valve  401 . Check valve  402  is positioned upstream of solenoid activated check valve  412  along passage  435 . Check valve  402  is biased to prevent fuel flow out of solenoid activated check valve  412  and pump inlet  499 . Check valve  402  allows flow from the low pressure fuel pump to solenoid activated check valve  412 . Check valve  402  is coupled in parallel with pressure relief valve  401 . Pressure relief valve  401  allows fuel flow out of solenoid activated check valve  412  toward the low pressure fuel pump when pressure between pressure relief valve  401  and solenoid operated check valve  412  is greater than a predetermined pressure (e.g., 10 bar). When solenoid operated check valve  412  is deactivated (e.g., not electrically energized), solenoid operated check valve operates in a pass-through mode and pressure relief valve  401  regulates pressure in compression chamber  408  to the single pressure relief setting of pressure relief valve  401  (e.g., 15 bar). Regulating the pressure in compression chamber  408  allows a pressure differential to form from piston top  405  to piston bottom  407 . The pressure in step-room  418  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 piston top  405  to piston bottom  407  through the clearance between piston  406  and pump cylinder wall  450 , thereby lubricating direct injection fuel pump  228 . 
     Piston  406  reciprocates up and down. Direct fuel injection pump  228  is in a compression stroke when piston  406  is traveling in a direction that reduces the volume of compression chamber  408 . Direct fuel injection pump  228  is in a suction stroke when piston  406  is traveling in a direction that increases the volume of compression chamber  408 . 
     A forward flow outlet check valve  416  may be coupled downstream of an outlet  404  of the compression chamber  408 . Outlet check valve  416  opens to allow fuel to flow from the compression chamber outlet  404  into a fuel rail only when a pressure at the outlet of direct injection fuel pump  228  (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  12  may deactivate solenoid activated inlet check valve  412  and pressure relief valve  401  regulates pressure in compression chamber to a single substantially constant (e.g., regulation pressure±0.5 bar) pressure. Controller  12  simply deactivates solenoid activated check valve  412  to lubricate direct injection fuel pump  228 . One result of this regulation method is that the fuel rail is regulated to approximately the pressure relief of  402 . Thus, if valve  402  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  408  is regulated during the compression stroke of direct injection fuel pump  228 . Thus, during at least the compression stroke of direct injection fuel pump  228 , 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. 
     Now turning to  FIG. 5 , another example direct injection fuel pump  228  is shown. Many devices and/or components in the direct injection fuel pump of  FIG. 5  are the same as devices and/or components shown in  FIG. 4 . Therefore, for the sake of brevity, devices and components of the direct fuel injection pump of  FIG. 4 , and that are included in the direct injection fuel pump of  FIG. 5 , are labeled the same and the description of these devices and components is omitted in the description of  FIG. 5   
     Direct injection fuel pump  228  includes an accumulator  502  positioned along pump passage  435  between solenoid activated check valve  412  and pressure relief valve  401 . In one example, accumulator  502  is a 15 bar accumulator. Thus, accumulator  502  is designed to be active in a pressure range that straddles the pressure relief valve  401 . Accumulator  502  stores fuel when piston  406  is in a compression stroke and releases fuel when piston is in a suction stroke. Consequently, a pressure differential from piston top  405  to piston bottom  407  exits during compression and suction strokes of direct fuel injection pump  228 . Further, when rod is in communication with the position providing least lift from cam  410 , the pressure differential is the substantially the same as when direct fuel injection pump  228  is on a compression stroke. Pressure relief valve  401  and accumulator  502  store and release fuel from compression chamber  408  when solenoid activated check valve is deactivated. 
     Referring now to  FIG. 6 , an example of prior art direct injection fuel pump operating sequence is shown. The sequence illustrates direct injection fuel pump operation when fuel flow out of the direct injection fuel pump to the direct injection fuel rail is ceased. 
     The first plot from the top of  FIG. 6  shows direct injection fuel pump cam lift versus time. The Y axis represents direct injection fuel pump cam lift. The X axis represents time and time increases from the left side of  FIG. 6  to the right side of  FIG. 6 . Cam lift is increases during a compression stroke for 100 crankshaft degrees. Cam lift decreases during the suction stroke for 80 crankshaft degrees. 
     The second plot from the top of  FIG. 6  shows direct injection fuel pump compression chamber pressure versus time. The Y axis represents direct injection fuel pump compression chamber pressure. The X axis represents time and time increases from the left side of  FIG. 6  to the right side of  FIG. 6 . Horizontal line  602  represents low pressure pump output pressure at the direct injection fuel pump compression chamber when the low pressure pump is operating, the solenoid activated check valve is in a pass-through state, and there is no net fuel flow to the fuel rail. 
     Vertical markers T 1 -T 4  indicate time of interest during the direct injection fuel pump operating sequence. Time T 1  represents start of first direct injection fuel pump compression stroke. Time T 2  represents end of first direct injection fuel pump compression stroke and beginning of direct injection fuel pump suction stroke. Time T 3  represents end of first direct injection fuel pump suction stroke and beginning of a second compression stroke. Time T 4  represents the end of the second direct injection fuel pump compression stroke. 
       FIG. 6  shows that direct injection fuel pump compression chamber pressure is near low pressure fuel pump output pressure during first and second compression strokes as well as during first and second suction strokes. The solenoid activated check valve is operated in a pass through state so that the direct injection fuel pump does not pump fuel to the fuel rail. Fuel pressure at in the step-chamber is at low pressure fuel pump outlet pressure. Thus, little if any direct injection fuel pump lubrication is provided. 
     Referring now to  FIG. 7 , an example direct injection fuel pump operating sequence of the fuel pump shown in  FIG. 4  is shown. The sequence illustrates direct injection fuel pump operation when fuel flow out of the direct injection fuel pump to the direct injection fuel rail is ceased. 
     The first plot from the top of  FIG. 7  shows direct injection fuel pump cam lift versus time. The Y axis represents direct injection fuel pump cam lift. The X axis represents time and time increases from the left side of  FIG. 7  to the right side of  FIG. 7 . 
     The second plot from the top of  FIG. 7  shows direct injection fuel pump compression chamber pressure versus time. The Y axis represents direct injection fuel pump compression chamber pressure. The X axis represents time and time increases from the left side of  FIG. 7  to the right side of  FIG. 7 . Horizontal line  702  represents low pressure pump output pressure Horizontal line  704  represents the pressure relief valve  401  of  FIG. 4  is set to regulate. 
     Vertical markers T 10 -T 13  indicate time of interest during the direct injection fuel pump operating sequence. Time T 10  represents start of first direct injection fuel pump compression stroke. Time T 11  represents end of first direct injection fuel pump compression stroke and beginning of direct injection fuel pump suction stroke. Time T 12  represents end of first direct injection fuel pump suction stroke and start of a second compression stroke. Time T 13  represents end of the second direct injection fuel pump compression stroke. 
       FIG. 7  shows that direct injection fuel pump compression chamber pressure increases during the first and second compression strokes. Pressure in the step-chamber (not shown) is at low pressure fuel pump output pressure during first and second compression strokes as well as during first and second suction strokes. Consequently, a pressure difference develops between the piston top and bottom allowing fuel to squeeze between the piston and the compression chamber walls lubricating the pump. The pressure difference decreases during the first suction stroke. Consequently, a reduced amount of lubrication may be provided during the suction stroke. Further, when cam lift is zero and the cam base circle is in mechanical communication with the piston, pressure in the compression chamber is reduced to pressure output of the low pressure pump supplying fuel to the direct injection fuel pump. The solenoid activated check valve is operated in a pass through state so that the direct injection fuel pump does not pump fuel to the fuel rail. Thus, during the compression stroke and part of the suction stroke, pressure in the direct injection fuel pump compression chamber is greater than low pressure pump outlet pressure. Consequently, direct injection fuel pump lubrication is increased as compared to the prior art. 
     Referring now to  FIG. 8 , an example direct injection fuel pump operating sequence of the fuel pump shown in  FIG. 5  is shown. The sequence illustrates direct injection fuel pump operation when fuel flow out of the direct injection fuel pump to the direct injection fuel rail is ceased. 
     The first plot from the top of  FIG. 8  shows direct injection fuel pump cam lift versus time. The Y axis represents direct injection fuel pump cam lift. The X axis represents time and time increases from the left side of  FIG. 8  to the right side of  FIG. 8 . 
     The second plot from the top of  FIG. 8  shows direct injection fuel pump compression chamber pressure versus time. The Y axis represents direct injection fuel pump compression chamber pressure. The X axis represents time and time increases from the left side of  FIG. 8  to the right side of  FIG. 8 . Horizontal line  802  represents low pressure pump output pressure 
     Vertical markers T 20 -T 23  indicate time of interest during the direct injection fuel pump operating sequence. Time T 20  represents start of first direct injection fuel pump compression stroke. Time T 21  represents end of first direct injection fuel pump compression stroke and beginning of direct injection fuel pump suction stroke. Time T 22  represents end of first direct injection fuel pump suction stroke and start of a second compression stroke. Time T 23  represents end of the second direct injection fuel pump compression stroke. 
       FIG. 8  shows that direct injection fuel pump compression chamber pressure is elevated during the first and second compression strokes and during the first suction stroke. Thus, the pressure in the direct injection fuel pump compression chamber is substantially constant at a pressure greater than low pressure pump output pressure. The direct injection fuel pump pressure is at the constant elevated pressure after a first compression stroke of the direct injection fuel pump after the solenoid operated check valve is placed in a pass through mode. Consequently, a pressure difference develops between the piston top and bottom allowing fuel to squeeze between the piston and the compression chamber walls lubricating the pump. Accumulator  502  in  FIG. 5  allows pressure in the compression chamber to stay substantially constant during the pump&#39;s suction stroke. 
     While this lube strategy cures an issue of lubrication ceasing when the DI system was in disuse, the lubrication that occurs in  FIGS. 7 and 8  can even give better lubrication than if only a small fraction the pump&#39;s full displacement is being pumped out to the fuel rail. 
     Another feature is that in  FIG. 8 , since accumulator pressure is being used to “push down” the piston, the system conserves more energy than it would if controlled as is shown in  FIG. 7 . 
     Referring now to  FIG. 9  a method for operating a direct injection fuel pump is shown. The method of  FIG. 9  may be stored as executable instructions in non-transitory memory of controller  12  shown in  FIGS. 1-5 . The method of  FIG. 9  may provide the sequences shown in  FIGS. 7 and 8 . 
     At  902 , method  900  determines operating conditions. Operating conditions may include but are not limited to engine speed, engine load, vehicle speed, brake pedal position, engine temperature, ambient air temperature, and fuel rail pressure. Method  900  proceeds to  904  after operating conditions are determined. 
     At  904 , method  900  judges whether or not the fuel system is a direct injection system only. If method  900  judges that there are no port injectors and the system is direct injection only, the answer is yes and method  900  proceeds to  906 . Otherwise, the answer is no and method  900  proceeds to  908 . 
     At  906 , method  900  judges whether or not the piston in the direct injection fuel pump is reciprocating while less than a threshold amount of fuel is flowing into the direct injection fuel rail from the direct injection fuel pump. In one example, the threshold amount of fuel is zero. In another example, the threshold amount of fuel is an amount of fuel less than an amount of fuel to idle the engine. If method  900  judges that the piston in the direct injection fuel pump is reciprocating and less than a threshold amount of fuel is flowing into the direct injection fuel rail from the direct injection fuel pump, the answer is yes and method  900  proceeds to  918 . Otherwise, the answer is no and method  900  proceeds to exit. 
     At  908 , method  900  determines an amount of fuel to deliver to the engine via the direct injectors and an amount of fuel to deliver to the engine via the port fuel injectors. In one example, the amount of fuel to be delivered via port and direct injectors is empirically determined and stored in two tables or functions, one table for port injection amount and one table for direct injection amount. The two tables are indexed via engine speed and load. The tables output an amount of fuel to inject to engine cylinders each cylinder cycle. Method  900  proceeds to  910  after determining the amounts of fuel to directly inject and port inject. 
     At  910 , whether or not to deliver fuel to the engine via port and direct injectors or solely via direct injectors. In one example, method  900  judges whether or not to deliver fuel to the engine via port and direct injectors or solely via direct injectors based on output from tables at  908 . If method  900  judges to deliver fuel to the engine via port and direct injectors or solely via direct injectors, the answer is yes and method  900  proceeds to  912 . Otherwise, the answer is no and fuel is not injected via direct injectors while the engine is rotating and the direct injection fuel pump piston is reciprocating. Method  900  proceeds to  914  when the answer is no. 
     At  912 , method  900  adjusts the duty cycle of a signal supplied to the solenoid activated check valve  412  in  FIGS. 4 and 5  to adjust flow through the direct injection fuel pump so as to provide the amount of fuel desired to be directly injected and to provide the desired fuel pressure in the direct injection fuel rail. The solenoid activated check valve duty cycle controls how much of the pump&#39;s actual displacement is being engaged to pump fuel. In one example, the duty cycle is increased to increase flow through the direct injection fuel pump and to the direct injection fuel rail. If the fuel system includes a single low pressure fuel pump, the low pressure fuel pump command is adjusted in response to the amount of fuel to be delivered to the engine. For example, low pressure fuel pump output is increased as the amount of fuel injected to the engine is increased. If the fuel system includes two low pressure fuel pumps, the first low pressure fuel pump output is adjusted in response to the amount of fuel injected by the port fuel injectors. The second low pressure fuel pump output is adjusted in response to the amount of fuel injected by the direct fuel injectors. Fuel is then supplied to the engine via the port and direct fuel injectors. Method  900  proceeds to exit after the direct and low pressure pumps are adjusted. 
     At  914 , method  900  judges whether or not to deliver fuel to the engine via port injectors. In one example, method  900  judges to deliver fuel to the engine via only port injectors based on the output of the two tables at  908 . If the direct fuel injection amount is zero or less than a threshold amount of fuel necessary for the engine to operate at idle speed and port injection is requested, method  900  proceeds to  916 . Otherwise, port fuel injection and direct fuel injection are not requested and method  900  proceeds to  918 . Port fuel injection and direct fuel injection may not be requested during low engine load conditions such as when the vehicle is decelerating or traveling downhill. 
     At  916 , method  900  adjusts low pressure fuel pump output. If the fuel system includes only a single low pressure fuel pump, the low pressure fuel pump output is adjusted in response to the amount of port fuel injected and the desired port injector fuel rail pressure. If the fuel system includes two low pressure fuel pumps, the first low pressure fuel pump output is adjusted in response to the amount of fuel injected by the port fuel injectors and the port injector fuel rail pressure. The second low pressure fuel pump output is adjusted in response to fuel pressure in a passage that provides fluidic communication between the low pressure fuel pump and the direct injection fuel pump. In particular, the low pressure pump command is adjusted in response to fuel pressure between the low pressure fuel pump and the direct injection fuel pump. Fuel is then injected to the engine via the port fuel injectors and not via the direct fuel injectors. 
     At  918 , method  900  judges whether or not to supply direct injection fuel pump full cam stroke (e.g., compression stroke and suction stroke, and in some examples while the piston is in communication with a cam&#39;s base circle) fuel pump lubrication. In one example, method  900  judges whether or not to supply direct injection fuel pump full cam stroke lubrication based on whether or not accumulator  502  of  FIG. 5  is included in the direct injection fuel pump or fuel system. If the accumulator is present and fuel flow from the direct injection fuel pump is less than a threshold fuel flow rate, the answer is yes and method  900  proceeds to  920 . Otherwise, the answer is no and method  900  proceeds to  922 . 
     At  920 , method  900  regulates fuel pressure in the direct injection fuel pump compression chamber via a pressure relief valve  401  and accumulator  502  as shown in  FIG. 5 , although other regulation schemes are also envisioned. The fuel pressure in the compression chamber is regulated to a single pressure that is greater than pressure output of the low pressure fuel pump that is supplying fuel to the direct injection fuel pump. By regulating pressure in the compression chamber a pressure differential between the direct injection fuel pump piston&#39;s top and bottom develops and fuel flow from the piston top to bottom provides lubrication to the direct injection fuel pump. At the same time, fuel flow out of the direct injection fuel pump to the direct injection fuel rail is stopped because pressure in the direct fuel injection fuel rail is greater than direct injection fuel pump output pressure. Consequently, the direct fuel injection pump is lubricated without raising direct injection fuel rail pressure. Additionally, direct injection fuel pump lubrication is provided when fuel flow through the direct fuel injectors is stopped. In this way, the direct injection fuel pump may be lubricated while direct fuel injection fuel pump output to the fuel rail is zero or less than a threshold fuel flow rate. Method  900  proceeds to exit after full cam stroke lubrication begins. 
     At  922 , method  900  judges whether or not to supply direct injection fuel pump half cam stroke (e.g., compression stroke) fuel pump lubrication. In one example, method  900  judges whether or not to supply direct injection fuel pump full cam stroke lubrication based on whether or not pressure relief valve  401  of  FIG. 4  is included in the direct injection fuel pump or fuel system. If the pressure relief valve is present and fuel flow from the direct injection fuel pump is less than a threshold fuel flow rate, the answer is yes and method  900  proceeds to  924 . Otherwise, the answer is no and method  900  proceeds to  930 . 
     At  930 , method  900  opens the solenoid activated check valve  412  shown in  FIGS. 4 and 5  to allow the check valve to operate as a pass through device. The direct injection fuel pump does not develop fuel pressure at outlet  404  when the solenoid activated check valve is operated in a pass through mode. Consequently, the direct injection fuel rail pressure does not increase; however, the direct injection fuel pump may be operated in this state for a limited amount of time to limit direct injection fuel pump degradation. Method  900  proceeds to exit after the solenoid activated check valve is operated in a pass through mode. 
     At  924 , method  900  regulates fuel pressure in the direct injection fuel pump compression chamber via a pressure relief valve  401  as shown in  FIG. 4 , although other regulation schemes are also envisioned. The fuel pressure in the compression chamber is regulated to a single pressure during the pump&#39;s compression stroke that is greater than pressure output of the low pressure fuel pump that is supplying fuel to the direct injection fuel pump. By regulating pressure in the compression chamber a pressure differential between the direct injection fuel pump piston&#39;s top and bottom develops and fuel flow from the piston top to bottom provides lubrication to the direct injection fuel pump. At the same time, fuel flow out of the direct injection fuel pump to the direct injection fuel rail is stopped because pressure in the direct fuel injection fuel rail is greater than direct injection fuel pump output pressure. Consequently, the direct fuel injection pump is lubricated without raising direct injection fuel rail pressure. Additionally, direct injection fuel pump lubrication is provided when fuel flow through the direct fuel injectors is stopped. In this way, the direct injection fuel pump may be lubricated while direct fuel injection fuel pump output to the fuel rail is zero or less than a threshold fuel flow rate. Method  900  proceeds to exit after half cam stroke lubrication begins. 
     Referring now to  FIG. 10 , is shows a second example fuel system for supplying fuel to engine  10  of  FIG. 1 . Many devices and/or components in the fuel system of  FIG. 10  are the same as devices and/or components shown in  FIG. 2 . Therefore, for the sake of brevity, devices and components of the fuel system of  FIG. 2 , and that are included in the fuel system of  FIG. 10 , are labeled the same and the description of these devices and components is omitted in the description of  FIG. 10 . 
     The fuel system of  FIG. 10  shows fuel passage  1002  leading from fuel pump  228  to port fuel injection rail  240  and fuel injectors  242 . Fuel passage  1002  allows fuel to come in contact with both the step room and pump&#39;s compression chamber. The fuel then may pick up heat and exit to the PI fuel system as shown. That fuel enters and exits the high pressure pump; however, the fuel enters and exits at lift pump pressure (e.g., the same pressure as output by low pressure fuel pump  208 ). 
       FIG. 11  shows another example direct injection fuel pump  228  is shown. Many devices and/or components in the direct injection fuel pump of  FIG. 11  are the same as devices and/or components shown in  FIG. 4 . Therefore, for the sake of brevity, devices and components of the direct fuel injection pump of  FIG. 4 , and that are included in the direct injection fuel pump of  FIG. 11 , are labeled the same and the description of these devices and components is omitted in the description of  FIG. 11 . 
     The fuel pump of  FIG. 11  includes fuel passage  1002  which allows fuel to come into contact with step room  418  and pump compression chamber  408  before proceeding to port fuel injectors. By allowing fuel to come into contact with portions of high pressure fuel pump  228 , it may be possible to cool high pressure fuel pump  228  and improve fuel atomization. 
     Thus, either example pump shown in  FIG. 4, 5 , or  11  may be selected and fuel rail pressure greater than lift pump pressure may be provided via engaging the solenoid operated check valve. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. 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 acts, operations, 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 acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system. 
     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.