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A fuel system includes a plurality of fuel injectors each defining a nozzle supply passage, a nozzle outlet and a low pressure space. The fuel system includes a plurality of mechanically actuated pressure intensifiers each including a tappet and being positioned partially within one of the fuel inje

A fuel system includes a plurality of fuel injectors each defining a nozzle supply passage, a nozzle outlet and a low pressure space. The fuel system includes a plurality of mechanically actuated pressure intensifiers each including a tappet and being positioned partially within one of the fuel injectors, and a common rail fluidly connecting with each of the fuel injectors. Each of the fuel injectors further includes an injection pressure control mechanism having an injection pressure control valve. Each injection pressure control valve blocks the corresponding pressure intensifier from the common rail and fluidly connects the pressure intensifier with the low pressure space at a first position, and fluidly connects the pressure intensifier with the common rail and blocks the pressure intensifier from the low pressure space at a second position. Injecting fuel via operating the fuel system may include operating the fuel system in a low leakage mode where the pressure intensifier displaces fuel at a low pressure, between high pressure injections.

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1. A method of operating a fuel system for an internal combustion engine comprising the steps of: injecting fuel into an engine cylinder at a medium pressure level at least in part by fluidly connecting a nozzle outlet of a fuel injector with a common rail containing the fuel at the medium pressure

1. A method of operating a fuel system for an internal combustion engine comprising the steps of: injecting fuel into an engine cylinder at a medium pressure level at least in part by fluidly connecting a nozzle outlet of a fuel injector with a common rail containing the fuel at the medium pressure level;increasing a pressure of fuel in a plunger cavity of the fuel injector from a low pressure level existing in the plunger cavity during the medium pressure injection to the medium pressure level by fluidly connecting the plunger cavity with the common rail;increasing a pressure of fuel in the plunger cavity from the medium pressure level to a high pressure level by moving a tappet of a mechanically actuated pressure intensifier in response to rotation of a cam;injecting fuel at the high pressure level into the engine cylinder at least in part by fluidly connecting the nozzle outlet with the plunger cavity; andoperating the fuel system in a low leakage mode subsequent to the injection of fuel at the high pressure level at least in part via a step of returning a pressure of fuel in the plunger cavity from the high pressure level to the low pressure level. 2. The method of claim 1 wherein the step of operating the fuel system in a low leakage mode further includes the steps of supplying fuel into the plunger cavity from a low pressure space at least in part by moving the tappet in a first direction in response to rotation of the cam, and displacing fuel from the plunger cavity to the low pressure space at least in part by moving the tappet in a second direction in response to further rotation of the cam. 3. The method of claim 2 wherein the step of increasing the pressure of fuel from a low pressure level further includes a step of moving a poppet valve from a first pressure control position at which the poppet valve contacts a first seat to a second pressure control position at which the poppet valve contacts a second seat, and wherein the step of returning a pressure further includes a step of returning the poppet valve from the second position to the first position. 4. The method of claim 3 wherein the step of moving the poppet valve further includes energizing a first electrical actuator coupled with the poppet valve, and wherein each of the steps of injecting fuel further includes energizing a second electrical actuator coupled with a needle control valve for a direct control needle check of the fuel injector. 5. The method of claim 4 wherein the needle control valve includes a second poppet valve, and wherein each of the steps of injecting fuel further includes moving the second poppet valve from a first injection control position contacting a third seat to a second injection control position contacting a fourth seat. 6. The method of claim 4 wherein the step of injecting fuel at the high pressure level further includes the steps of: moving the direct control needle check from a closed position blocking the nozzle outlet from a nozzle supply passage to an open position via a pressure of fuel in the nozzle supply passage acting on an opening hydraulic surface of the needle check; andmoving the direct control needle check from the open position to the closed position via a pressure of fuel in a control passage acting on a closing hydraulic surface of the needle check, the pressure level of fuel in the control passage being equal to the high pressure level. 7. The method of claim 6 wherein the step of increasing a pressure of fuel from the low pressure level further includes increasing the pressure level of fuel in the plunger cavity while a plunger of the mechanically actuated pressure intensifier is stationary or retracting. 8. The method of claim 7 further comprising the steps of: blocking the plunger cavity from the nozzle supply passage with a one-way valve during the step of increasing a pressure level of fuel from the low pressure level; andblocking the plunger cavity from the nozzle supply passage with the one-way valve during the step of injecting fuel at the medium pressure level;wherein the step of injecting fuel at the high pressure level further includes moving the one-way valve from a first one-way valve position blocking the plunger cavity from the nozzle supply passage to a second one-way valve position at which the plunger cavity is fluidly connected with the nozzle supply passage. 9. The method of claim 8 further comprising a step of blocking the common rail from the plunger cavity with a second one-way valve during the step of injecting fuel at the high pressure level. 10. The method of claim 1 wherein: the step of injecting fuel at the high pressure level includes injecting fuel a first time in an engine cycle while autoignition conditions exist in the engine cylinder; andthe method further including a step of injecting fuel a second time in the engine cycle while autoignition conditions exist, and the step of injecting fuel a second time includes injecting fuel at the medium pressure level. 11. The method of claim 10 wherein: injecting fuel a first time includes injecting a relatively greater quantity of fuel; andthe step of injecting fuel a second time includes injecting a relatively smaller quantity of fuel in an expansion portion of the engine cycle. 12. A fuel injector comprising: an injector body defining a nozzle supply passage, a nozzle outlet connecting with the nozzle supply passage, a control passage and a low pressure space, the injector body further defining at least one fuel inlet connecting with the nozzle supply passage, a plunger cavity and a pressure intensification passage connecting the plunger cavity with the nozzle supply passage within the injector body;a direct control needle check positioned within the injector body and movable between a closed position blocking the nozzle outlet from the nozzle supply passage and an open position, the direct control needle check having an opening hydraulic surface exposed to a fluid pressure in the nozzle supply passage and a closing hydraulic surface exposed to a fluid pressure in the control passage;a check control valve movable between a first injection control position at which the control passage is blocked from the low pressure space and a second injection control position at which the control passage is open to the low pressure space;a mechanically actuated pressure intensifier positioned partially within the injector body, the mechanically actuated pressure intensifier including a tappet and a plunger configured to move between a first plunger position and an advanced plunger position within the plunger cavity, in response to rotation of a cam;a one-way valve positioned fluidly between the pressure intensification passage and the nozzle supply passage and permitting fluid flow from the plunger cavity to the nozzle supply passage; andan injection pressure control mechanism having a first pressure control configuration and a second pressure control configuration;wherein in the first pressure control configuration the injection pressure control mechanism blocks the plunger cavity from the at least one fuel inlet, fluidly connects the at least one fuel inlet with the nozzle outlet, and fluidly connects the plunger cavity with the low pressure space; andwherein in the second pressure control configuration the injection pressure control mechanism fluidly connects the plunger cavity with the at least one fuel inlet and blocks the plunger cavity from the low pressure space. 13. The fuel injector of claim 12 wherein the injection pressure control mechanism includes a poppet valve and the injector body defines a first seat and a second seat, and wherein the first pressure control configuration includes a first poppet valve position at which the poppet valve contacts the first seat and the second pressure control configuration includes a second poppet valve position at which the poppet valve contacts the second seat. 14. The fuel injector of claim 13 further comprising a first electrical actuator coupled with the poppet valve, and a second electrical actuator coupled with the check control valve. 15. The fuel injector of claim 14 wherein the injector body defines a third seat and a fourth seat, the check control valve including a second poppet valve contacting the third seat at the first control valve position and contacting the fourth seat at the second control valve position. 16. The fuel injector of claim 12 wherein: the injector body defines a high pressure supply passage fluidly connecting the at least one high pressure inlet with the nozzle supply passage, and the injector body further defines a bidirectional passage fluidly connecting with the pressure intensification passage; andthe injection pressure control mechanism blocks the bidirectional passage from the at least one high pressure inlet and fluidly connects the bidirectional passage with the low pressure space in the first pressure control configuration, and the injection pressure control mechanism fluidly connects the bidirectional passage with the at least one high pressure inlet and blocks the bidirectional passage from the low pressure space in the second pressure control configuration. 17. The fuel injector of claim 16 further comprising a second one-way valve positioned fluidly between the at least one high pressure inlet and the bidirectional passage and permitting fluid flow from the at least one high pressure inlet to the bidirectional passage. 18. A fuel system for an internal combustion engine comprising: a plurality of fuel injectors, each of the fuel injectors including an injector body defining a nozzle supply passage, a nozzle outlet and a fuel inlet each connecting with the nozzle supply passage, a low pressure space, and a plunger cavity;a plurality of one-way valves each positioned fluidly between one of the plunger cavities and the nozzle supply passage in the corresponding injector body and permitting fluid flow from the plunger cavity to the nozzle supply passage;a plurality of mechanically actuated pressure intensifiers each including a tappet and being positioned partially within the plunger cavity of one of the injector bodies;a common rail fluidly connecting with the fuel inlet of each of the fuel injectors and containing a fuel at a medium pressure level; andeach of the fuel injectors further having an injection pressure control mechanism which includes an injection pressure control valve movable between a first pressure control position and a second pressure control position;wherein each of the injection pressure control valves blocks the corresponding pressure intensifier from the common rail and fluidly connects the pressure intensifier with the low pressure space at the first pressure control position, such that fuel within the plunger cavity is at a low pressure level and an injection of the fuel from the corresponding nozzle outlet occurs at the medium pressure level at the first pressure control position; andwherein each of the injection pressure control valves fluidly connects the pressure intensifier with the common rail and blocks the pressure intensifier from the low pressure space at the second pressure control position, such that fuel within the plunger cavity is at a high pressure level and an injection of the fuel via the corresponding nozzle outlet occurs at the high pressure level at the second pressure control position. 19. The fuel system of claim 18 further comprising: a second plurality of one-way valves each being disposed fluidly between the common rail and one of the pressure intensifiers and permitting fuel flow from the common rail to the one of the pressure intensifiers. 20. The fuel system of claim 18 wherein each of the injection pressure control mechanisms includes an electrical actuator, and each of the injection pressure control valves includes a poppet valve coupled with the electrical actuator and being movable between the first pressure control position and the second pressure control position by energizing the electrical actuator, each of the injector bodies further defining a first seat and a second seat, the poppet valve contacting the first seat at the first pressure control position and contacting the second seat at the second pressure control position. 21. The fuel system of claim 19 wherein: each of the fuel injectors further includes a direct control needle check, a second electrical actuator and a check control valve which includes a second poppet valve coupled with the second electrical actuator and being movable between a first control valve position and a second control valve position by energizing the second electrical actuator; andeach of the injector bodies further defines a third seat, a fourth seat and a control passage, the second poppet valve contacting the third seat and blocking the control passage from the low pressure space at the first control valve position, and the second poppet valve contacting the fourth seat and fluidly connecting the control passage with the low pressure space at the second control valve position.