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Methods and systems are provided for operating an engine fuel system including a low pressure pump and a high pressure pump. During conditions when direct injection is not requested, a fuel rail pressure is maintained by the low pressure pump and fuel is port injected. Further, a stroke amount of th

Methods and systems are provided for operating an engine fuel system including a low pressure pump and a high pressure pump. During conditions when direct injection is not requested, a fuel rail pressure is maintained by the low pressure pump and fuel is port injected. Further, a stroke amount of the high pressure pump is adjusted to maintain an outlet pressure of the high pressure pump just below the fuel rail pressure. By maintaining fuel flow within the high pressure pump when high pressure pump operation is not required, and without flowing fuel from the high pressure pump outlet into the fuel rail, the high pressure pump may be cooled and lubricated without affecting the fuel rail pressure.

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1. A method of operating a fuel system including a first fuel pump coupled upstream of a second, positive-displacement fuel pump, comprising, when second fuel pump operation is not requested, operating the first fuel pump to maintain a fuel rail pressure at a fuel rail while supplying fuel to an eng

1. A method of operating a fuel system including a first fuel pump coupled upstream of a second, positive-displacement fuel pump, comprising, when second fuel pump operation is not requested, operating the first fuel pump to maintain a fuel rail pressure at a fuel rail while supplying fuel to an engine cylinder through an injector; andadjusting a flow control valve to vary an effective pump volume of each pump stroke of the second fuel pump to maintain an elevated pressure in a pump chamber of the second fuel pump below the fuel rail pressure and without affecting fuel rail pressure. 2. The method of claim 1, wherein the first fuel pump is a lower pressure fuel pump and the second fuel pump is a higher pressure fuel pump. 3. The method of claim 2, wherein the first fuel pump is electrically-powered and the second fuel pump is mechanically-powered. 4. The method of claim 1, wherein adjusting the flow control valve pump includes adjusting a stroke amount of the second fuel pump to a greater and lesser stroke amount even when second fuel pump operation is not requested. 5. The method of claim 4, wherein the stroke amount of the second fuel pump is adjusted responsive to fuel rail pressure, without flowing fuel from a pump chamber outlet of the second fuel pump to the fuel rail. 6. The method of claim 1, wherein adjusting the flow control valve includes intermittently operating the second fuel pump. 7. The method of claim 1, wherein direct fuel injection is deactivated, and wherein adjusting the flow control valve includes operating the second fuel pump at a first stroke amount while maintaining a pump chamber outlet pressure of the second fuel pump below the fuel rail pressure, and then intermittently increasing second fuel pump operation to a second, higher stroke amount until fuel is pushed from the pump chamber outlet of the second fuel pump into the fuel rail. 8. The method of claim 7, wherein intermittently increasing second fuel pump operation includes intermittently increasing the stroke amount of the second fuel pump until fuel rail pressure responds to the increased second fuel pump operation, and then decreasing operation of the second fuel pump so that the fuel rail pressure no longer responds to operation of the second fuel pump. 9. The method of claim 8, further comprising, after fuel is pushed out, immediately decreasing second fuel pump operation to below the second stroke amount. 10. The method of claim 1, wherein the fuel system further includes a fuel tank coupled to the first and second fuel pumps, and wherein second fuel pump operation is not requested when a fuel level in the fuel tank is below a threshold level. 11. A method of operating a fuel system including a low pressure fuel pump (LPP) coupled upstream of a high pressure fuel pump (HPP), comprising, when HPP operation is not requested, operating the LPP to maintain a fuel rail pressure in a fuel rail while injecting fuel into an engine cylinder;ingesting fuel into the HPP and adjusting a stroke amount of the HPP responsive to the fuel rail pressure to maintain a pump chamber outlet pressure of the HPP within a threshold pressure, but below the fuel rail pressure; andintermittently pulsing the HPP stroke amount to intermittently flow fuel from an outlet of the HPP into the fuel rail. 12. The method of claim 11, wherein intermittently pulsing the HPP stroke amount includes intermittently pulsing the HPP pump chamber outlet pressure to correspondingly pulse the fuel rail pressure. 13. The method of claim 11, wherein the intermittent pulsing includes: intermittently increasing the HPP stroke amount to flow fuel from the pump chamber outlet of the HPP into the fuel rail; andafter fuel flow into the fuel rail, immediately decreasing the HPP stroke amount to below the increased HPP stroke amount. 14. The method of claim 11, wherein the fuel system includes a fuel tank and wherein the LPP is configured to supply fuel from the fuel tank to a first group of port injectors in the engine cylinder, and wherein the HPP is configured to receive fuel from the fuel tank via the LPP, and supply the received fuel to a second group of direct injectors in the engine cylinder. 15. The method of claim 14, wherein HPP operation is not requested when a fuel level in the fuel tank is below a threshold. 16. The method of claim 11, further comprising, when HPP operation is requested, adjusting operation of the LPP and the HPP to maintain the fuel rail pressure in the fuel rail while injecting fuel into the engine cylinder. 17. The method of claim 16, further comprising, adjusting operation of the LPP in response to pressure fluctuations at an inlet of the HPP. 18. A fuel system for an internal combustion engine, comprising: a first fuel tank storing a first fuel type;a group of port fuel injectors in communication with a group of cylinders;a group of direct injectors in communication with the group of cylinders;a first low pressure fuel pump coupled to the first fuel tank and positioned upstream of a second high pressure fuel pump, the first fuel pump in communication with the group of port fuel injectors via a first fuel rail, the second high pressure fuel pump in communication with the group of port fuel injectors via a solenoid valve, the second high pressure fuel pump further communicating with the group of direct injectors via a second fuel rail; anda control system configured with instructions for: when second high pressure fuel pump operation is not requested, operating the first low pressure fuel pump to maintain a first fuel rail pressure in the first fuel rail while supplying the first fuel type to the group of cylinders through the group of port fuel injectors; andingesting fuel into the second fuel pump and adjusting operation of the second high pressure fuel pump to maintain an elevated pressure in a pump chamber of the second high pressure fuel pump below the first fuel rail pressure and without affecting fuel rail pressure. 19. The system of claim 18, wherein adjusting operation of the second fuel pump to maintain an elevated pressure in the pump chamber of the second high pressure fuel pump below the first fuel rail pressure includes, adjusting a stroke amount of the second high pressure fuel pump responsive to the first fuel rail pressure to maintain the pump chamber pressure of the second high pressure fuel pump below the first fuel rail pressure;intermittently increasing the stroke amount to flow fuel from an outlet of the second high pressure fuel pump into the first fuel rail; andafter fuel flow into the first fuel rail, immediately decreasing the stroke amount to below the increased stroke amount. 20. The system of claim 19, further comprising a second fuel tank storing a second fuel type, and a third low pressure pump coupled to the second fuel tank, the third low pressure pump also positioned upstream of the second high pressure fuel pump, wherein second high pressure fuel pump operation is not requested when a fuel level in the second fuel tank is below a threshold level. 21. The system of claim 20, further comprising, when second high pressure fuel pump operation is requested, adjusting operation of the second high pressure fuel pump and at least one of the first and third fuel pumps to maintain a second fuel rail pressure in the second fuel rail while supplying at least one of the first and second fuel types to the group of cylinders through the group of direct injectors.