Dual fuel engine system with fuel actuation unit pump and method

Abstract

A dual fuel engine system (10) includes an engine (12) and a fuel system (34) including a pressurized fuel reservoir (42), a first fuel pump (44), and a plurality of second fuel pumps (52) each having an actuating fluid inlet (54) fluidly connected to at least one of the first fuel pump or the pressurized fuel reservoir. The plurality of second fuel pumps may pressurize or depressurize the pressure of the second fuel relative to the pressure of the first fuel used to actuate the second fuel pumps in order to achieve an increased flow rate of the second fuel. Related apparatus and methods are also disclosed.

Description

Technical Field

[0001] This disclosure generally relates to a dual-fuel engine system, and more specifically, to a dual-fuel engine system that employs a first fuel pump to pressurize a first fuel reservoir and actuate a plurality of second fuel pumps. Background Technology

[0002] Dual-fuel internal combustion engine systems are well-known and increasingly adopted worldwide, with applications ranging from power generation to the operation of vehicles and industrial equipment. Engineers have discovered many different strategies for advantageously utilizing the combustion and emission characteristics of different types of fuels simultaneously or in different engine cycles within the engine cylinders. In one exemplary dual-fuel system, a relatively small amount of pilot-injected diesel fuel is compressed and ignited in the cylinder, and used to trigger the ignition of a larger main charge of a gaseous fuel such as natural gas. Other known strategies attempt to utilize two different types of liquid fuels, such as diesel fuel and alcohol fuels (such as methanol). Factors driving the ongoing research and development of dual-fuel engine systems include the desire to reduce certain types of emissions, fuel economy, and cost.

[0003] A significant drawback of some dual-fuel engine implementations is the need for equipment to supply, contain, pressurize, and manage two different types of fuel. In typical examples, two separate and independent pumping systems are required for the two different fuel types, each including a low-pressure delivery pump and a high-pressure pump typically driven by the engine gear train. Injection pressures in dual-fuel applications can be very high, requiring relatively large and heavy-duty pumps that can occupy considerable space and increase costs. A known dual-fuel system is described in U.S. Patent No. 9,664,122B2 to Coldren et al. Summary of the Invention

[0004] In one aspect, a dual-fuel engine system includes: an engine having a plurality of cylinders formed therein; and a fuel system including at least one fuel injector associated with each of the plurality of cylinders, and each of the at least one fuel injector having a first fuel outlet group and a second fuel outlet group. The fuel system further includes a pressurized fuel reservoir, a first fuel pump having a first fuel pump outlet fluidly connected to the pressurized fuel reservoir, and a plurality of high-pressure first fuel conduits, each extending from the pressurized fuel reservoir to one of the first fuel outlet groups in the first fuel outlet group. The fuel system further includes: a plurality of second fuel pumps, each having an actuation fluid inlet and a second fuel pump outlet; a plurality of low-pressure second fuel conduits, each extending to one of the plurality of second fuel pumps; and a plurality of high-pressure second fuel conduits, each fluidly connecting one of the second fuel pump outlets to at least one of the corresponding second fuel outlet groups. The actuation fluid inlet of each of the plurality of second fuel pumps is fluidly connected to the outlet of the first fuel pump or at least one of the pressurized fuel reservoirs.

[0005] In another aspect, a method of operating a dual-fuel system includes feeding a pressurized first fuel from a first fuel pump to a pressurized fuel reservoir, and feeding the pressurized first fuel to actuation fluid inlets of a plurality of second fuel pumps. The method further includes: actuating the plurality of second fuel pumps via the pressurized first fuel to pressurize a second fuel; injecting the pressurized first fuel from a first fuel outlet group fluidly connected to the pressurized fuel reservoir into cylinders in an engine; and injecting pressurized second fuel from a second fuel outlet group fluidly connected to one of the plurality of second fuel pumps into the cylinders.

[0006] In another aspect, a dual-fuel system includes a plurality of fuel injectors, each of the plurality of fuel injectors including at least one of a first fuel outlet group and a second fuel outlet group, and the number of the plurality of fuel injectors is equal to the number of the first fuel outlet groups and the number of the second fuel outlet groups in the dual-fuel system. The dual-fuel system also includes a pressurized fuel reservoir, a first fuel pump having a first fuel pump outlet fluidly connected to the pressurized fuel reservoir, and a plurality of high-pressure first fuel conduits, each extending from the pressurized fuel reservoir to one of the first fuel outlet groups. The dual-fuel system further includes: a plurality of second fuel pumps, each having an actuation fluid inlet and a second fuel pump outlet; a plurality of low-pressure second fuel conduits, each extending to one of the plurality of second fuel pumps; and a plurality of high-pressure second fuel conduits, each fluidly connecting one of the plurality of second fuel pump outlets to at least one of the corresponding second fuel outlet groups. The dual-fuel system further includes a plurality of actuation fluid conduits, each of which extends from the first fuel pump or the pressurized fuel reservoir to the actuation fluid inlet of one of the plurality of second fuel pumps. Attached Figure Description

[0007] Figure 1 This is a schematic view of a dual-fuel engine system according to one embodiment;

[0008] Figure 2 Is it like this? Figure 1 A cross-sectional side view of a portion of the dual-fuel engine system in the diagram;

[0009] Figure 3 It is used in, for example Figure 1 and 2 A cross-sectional side view of the pump unit used in the dual-fuel engine system; and

[0010] Figure 4 This is a schematic view of a dual-fuel system according to another embodiment. Detailed Implementation

[0011] refer to Figure 1This illustration shows a dual-fuel internal combustion engine system 10 according to one embodiment. The engine system 10 includes an internal combustion engine 12 having a cylinder block 14 and a cylinder head 16 attached to the cylinder block 14. A plurality of cylinders 18 are formed in the cylinder block 14. A plurality of pistons 20 are positioned within the cylinders 18 and are movable between bottom dead center and top dead center positions to rotate a crankshaft 22. The cylinders 18 can include any number of any suitable arrangement (e.g., inline, V-type, or others). As further discussed herein, the pistons 20 are typically movable within the cylinders 18 to increase the pressure of the fluid therein to the auto-ignition threshold for compressing and igniting liquid fuel. The engine system 10 can be used in a variety of applications, including for operating generators, powering pumps or compressors or transmissions in mobile machinery such as marine vessels, etc.

[0012] Engine system 10 also includes an intake inlet 24 configured to feed intake air to compressor 26 in turbocharger 28. Exhaust from engine 12 operates turbine 30 of turbocharger 28. Pressurized intake air is fed to intake manifold 32 and distributed to cylinders 18 in a generally conventional manner. Engine system 10 will typically operate in a four-stroke engine cycle and includes suitable valves and exhaust system equipment for controlling the feeding of intake air into and from cylinders 18. Engine system 10 also includes a fuel system 34 having at least one fuel injector 92 associated with each corresponding cylinder in cylinders 18. Each of the at least one fuel injector 92 (sometimes referred to in the singular below) has a first fuel outlet group 38 and a second fuel outlet group 40. Embodiments are envisioned where a single fuel injector includes dual concentric outlet groups, and where the respective outlet groups are positioned side-by-side, such as... Figure 1 The embodiment shown is illustrated. In other embodiments, each cylinder in cylinder 18 may be associated with two separate fuel injectors, each including one of a first outlet group and a second fuel outlet group. As further discussed herein, each of the first fuel outlet group 38 and the second fuel outlet group 40 is positioned within one cylinder of cylinder 18 for direct injection of the first fuel and the second fuel, respectively.

[0013] Fuel system 34 also includes a pressurized fuel reservoir 42 and a first fuel pump 44 having a first fuel pump outlet 46 fluidly connected to the pressurized fuel reservoir 42. The pressurized fuel reservoir 42 may include a so-called common rail configured to supply pressurized fuel to a plurality of fuel injectors in the fuel system, potentially all fuel injectors. In some embodiments, the first fuel pump 44 may be driven from the engine gear train and receives a feed of low-pressure first fuel from a first fuel supply source 50 by means of a fuel delivery pump 47. The first fuel supply source 50 may contain a compression-ignition liquid fuel, such as diesel distillate fuel. Other suitable compression-ignition fuels may include high-octane fuels with cetane enhancers. Fuel system 34 also includes a plurality of high-pressure first fuel conduits 48, each extending from the pressurized fuel reservoir 42 to a first fuel outlet group 38 of a corresponding first fuel outlet group.

[0014] The fuel system 34 also includes a plurality of second fuel pumps 52, each having an actuating fluid inlet 54 and a second fuel pump outlet 56. The second fuel pumps 52 can be understood as hydraulic unit pumps specifically coupled to one or sometimes multiple cylinders 18. The second fuel pumps 52 may be mounted in a housing, manifold, or frame, or in some embodiments may be directly coupled to individual fuel injectors. A plurality of low-pressure second fuel conduits 60 each extend to one of the second fuel pumps 52. The fuel system 34 also includes a second fuel supply source 53 and a delivery pump 55 configured to feed second fuel from the second fuel supply source 53 to each corresponding fuel inlet 58 via the low-pressure second fuel conduits 60. The second fuel supply source 53 may contain alcohol fuels such as methanol, or various blends including methanol (methanol is typically but not necessarily dominant). In other cases, different liquid fuels, such as gasoline, naphtha, or other fuels, may be used in the second fuel supply source 53. The fuel system 34 also includes a plurality of high-pressure second fuel conduits 62, each of which fluidly connects one of the second fuel pump outlets 56 to at least one of the respective second fuel outlet groups 40. In some embodiments, the high-pressure second fuel conduits 62 may extend to the second fuel outlet groups 40, or may fluidly connect one of the second fuel pump outlets 56 to one or more of the second fuel outlet groups 40 by means of a pressurized fuel reservoir or a common rail. An actuation fluid inlet 54 of each of the plurality of second fuel pumps 52 is fluidly connected to at least one of the first fuel pump outlets 46 or the pressurized fuel reservoir 42. In the illustrated embodiment, the high-pressure actuation fluid conduit 64 is fluidly connected directly from the pressurized fuel reservoir 42 to each of the respective actuation fluid inlets 54. A low-pressure return line 57 for delivering used actuated fuel is fluidly connected from at least one of the second fuel pumps 52 to the fuel supply source 50.

[0015] Follow us now Figure 2The fuel system 34 also includes a plurality of fuel injection nozzle assemblies 90 in a plurality of fuel injectors 92. Each cylinder 18 in the engine system 10 may be associated with one fuel injector operable to selectively inject both a first fuel and a second fuel, or with two separate fuel injectors operable to selectively inject the first fuel and the second fuel. Each cylinder 18 in the engine 12 will be associated with at least one fuel injection nozzle assembly 90, which includes a first fuel outlet group 38, a first injection valve 96 movable to open and close a corresponding first fuel outlet group 38 to a pressurized fuel reservoir 42, a second fuel outlet group 40, and a second injection valve 106 movable to open and close a second fuel outlet group 40 to one of the second fuel pump outlets 56.

[0016] In the illustrated embodiment, the fuel injector 92 includes a first fuel inlet 98 that receives a feed of first fuel from a pressurized fuel reservoir 42, and a second fuel inlet 112 that receives a feed of second fuel from one of the second fuel pumps 52. A fuel chamber 102 is formed in the fuel injector 92 and is fluidly connected to the first fuel inlet 98 and, when the first injection valve 96 is open, fluidly connected to the first fuel outlet assembly 38. A second fuel chamber 114 is fluidly connected to the fuel inlet 112 and, when the second injection valve 106 is open, fluidly connected to the second fuel outlet assembly 40. The first fuel injection valve 96 includes a hydraulic control surface 100 exposed to the fluid pressure of the pressurized fuel reservoir 42. The second injection valve 106 also includes a hydraulic control surface 108 exposed to the fluid pressure of the pressurized fuel reservoir 42. In this way, it should be understood that each of the first fuel injection valve 96 and the second fuel injection valve 106 is controlled at least in part based on the fuel pressure of the fuel contained in the pressurized fuel reservoir 42, which typically comprises diesel fuel. The fuel injector 92 also includes an injection control valve assembly 110. The injection control valve assembly 110 is electrically actuated and may include two solenoid actuators to individually and independently change the shut-off hydraulic pressure on hydraulic control surfaces 100 and 108 to control the start of injection timing, the end of injection timing, injection quantity, and other potential characteristics of fuel injection.

[0017] Recall that the first fuel may be a diesel distillate fuel, and the second fuel may be an alcohol fuel, such as methanol. Diesel fuel has a higher energy density than methanol, and for a given engine power output, methanol typically requires a relatively larger fuel injection quantity. Engine system 10 can operate in diesel-only mode or pilot-ignition dual-fuel mode. In diesel-only mode, the engine power output demand is met by compression ignition combustion of diesel fuel alone. Operating in this mode involves using a diesel-only injection valve 96 to inject diesel fuel alone. In dual-fuel mode, a relatively small pilot-injection diesel fuel is delivered to cylinder 18 using injection valve 106 to compress and ignite a larger charge or larger quantity of methanol. Fuel injectors 92 are typically designed such that each respective first fuel outlet group 38 in the corresponding nozzle assembly 90, together with the first injection valve 96, defines a smaller nozzle steady-state flow, and each respective second fuel outlet group 40, together with the second injection valve 106, defines a larger nozzle steady-state flow. The steady-state flow, including the nozzle steady-state flow, is a well-known property of fuel injectors and refers to the flow rate that can be expected under given equivalent conditions. Therefore, for a given fuel pressure and a given opening time, a larger amount of fuel can be expected to be injected through the second fuel outlet group 38, which has a larger steady-state flow than the first fuel outlet group 40. The individual fuel spray outlets or orifices in the second fuel outlet group 40 may be larger in size and / or number than the individual outlets in the first fuel outlet group 38.

[0018] Figure 3An exemplary second fuel pump in the second fuel pump 52 is shown in more detail, comprising a pump housing 126 and a pressurized fuel outlet passage 130. The pump housing forms a fuel inlet passage 128 extending from a second fuel inlet 58 to a pumping chamber 36, and the pressurized fuel outlet passage extends from the pumping chamber 36 to a second fuel pump outlet 56 and thereby to a second fuel outlet group 40. As described above, within the scope of this disclosure, the pressurized fuel outlet flow from the second fuel pump 52 may extend to a single nozzle outlet group, to multiple nozzle outlet groups, or to a pressurized fuel reservoir that then feeds pressurized fuel to the nozzle outlet group for injection. The pump housing 126 also forms an actuated fluid inlet passage 132 extending from an actuated fluid inlet 54, and an actuated fluid discharge passage 134. The pump 52 also includes a flow control valve assembly 136. The flow control valve assembly 136 includes an electric actuator 138, such as a solenoid actuator, coupled to an armature 141, which in turn is coupled to an electrically actuated flow control valve 142. The electrically actuated flow control valve 142 can be fluidly positioned between an actuated fluid inlet 54 and an actuated surface 141. Energizing the flow control valve assembly 136 causes the flow control valve 142 to open a valve seat 144 against the bias force of a return spring 146. Opening the valve seat 144 allows a pressurized actuated fluid flow, including a pressurized first fuel, to flow through the actuated fluid inlet passage 132 and act on the actuated surface 141, thereby driving the pumping element 140 downward during the pumping stroke to pressurize the second fuel in the pumping chamber 36.

[0019] As described above, at least in part based on the lower energy density of the second fuel, the flow rate of the second fuel is ideally greater than that of the first fuel. To enable the flow rate of the second fuel from the second fuel pump 52 to be greater than that of the first fuel serving as the actuating fluid, the pumping element 140 of pump 52 may include a reverse booster plunger, wherein an actuating surface 141 has a first area, and a pumping surface 145, positioned opposite the actuating surface and exposed to the pumping chamber 36, has a second area larger than the first area. In other embodiments, the pumped second fuel may be pressurized rather than reverse-pressurized or depressurized. The pumped second fuel with depressurized pressure may be a fuel with an energy density greater than that of the first fuel used as the actuating fluid. In other cases, the first and second fuels may have similar or substantially equal energy densities, in which case pressurization may not be used at all, and the flow rate ratio of the first fuel used for actuation to the pumped second fuel may be approximately 1:1.

[0020] The pumping element 140 may include a two-piece plunger having: a first plunger 148 having an actuating surface 141; and a separate second plunger 150 having a pumping surface 145. A single plunger having corresponding surfaces that are typically different in area may also be used. In this way, a relatively small flow rate and high pressure of the first fuel results in a relatively large flow rate and low pressure of the second fuel. An annular space 152 may be formed in the pump housing 126 and extend around the second plunger 150 for lubrication and collection of fuel escaping through the gap between the second plunger 150 and the pump housing 126. The annular space 152 may be fluidly connected to the inlet passage 128 as shown. The pumping element 140 moves during the pumping stroke (in... Figure 4 (The downward movement shown in the diagram) can occur against the bias of the return spring 154. A properly arranged check valve (unnumbered) can be fluidly positioned between the pumping chamber 36 and the inlet passage 128 and outlet passage 130, respectively.

[0021] Now for reference Figure 4 The diagram illustrates a dual-fuel system 234 according to another embodiment, which includes multiple fuel injectors 236 associated with multiple cylinders 218 in a dual-fuel engine system. The fuel injectors 236 may be configured similarly to the fuel injector 92 discussed above, including a dual concentric outlet group, a side-by-side outlet group, or possibly a separate fuel injector for two separate fuels. The dual-fuel system 234 also includes a first fuel supply source 250 containing compressed ignition liquid fuel, a fuel delivery pump 247, and a high-pressure pump 244 configured to pressurize the first fuel and supply it to a pressurized fuel reservoir 242. The dual-fuel system 234 also includes a second fuel supply source 253, a delivery pump 255, and multiple second fuel pumps 252, the second fuel supply source being similar to the fuel supply source 53 discussed above, containing a second liquid fuel or potentially a gaseous fuel. However, in the above discussion and... Figure 1 In the embodiment shown, each second fuel pump 52 is associated with a fuel injector. Figure 4 In one embodiment, each second fuel pump 252 is associated with a plurality of fuel injectors (two in the case shown).

[0022] Therefore, it should be understood that, Figure 4 In this embodiment, the high-pressure second fuel conduit can be understood as a group of multiple second fuel outlets extending to the dual-fuel system 234. In other words, in Figure 4In one embodiment, instead of a single unit pump for each cylinder, each second fuel pump pressurizes the second fuel to deliver it to multiple fuel injectors in individual cylinders. Other arrangements, as discussed herein, may include second fuel pumps associated with more than two fuel injectors and more than two cylinders or pressurized fuel reservoirs. The number of fuel injectors in any embodiment will generally be equal to the number of the first fuel outlet groups and equal to the number of the second fuel outlet groups in a dual-fuel system.

[0023] Industrial applicability

[0024] As discussed above, engine system 10 can operate in several different modes, including a dual-fuel mode, in which a relatively small amount of pilot-injected first fuel is compressed and ignited to ignite a larger amount of second fuel injected into cylinder 18. It is still desirable for engine system 10 to operate in a single-fuel (diesel only) mode. For this reason, the first fuel pump 44 is typically configured with sufficient capacity to operate engine system 10 solely on diesel fuel across the entire speed and load range. This means that, sometimes and including in dual-fuel mode, the first fuel pump 44 has additional capacity, meaning that the first fuel pump 44 can operate to pressurize the first fuel in pressurized fuel reservoir 42 as a relatively small injection to ignite the second fuel, and also supply the first fuel to actuate multiple second fuel pumps 52 to pressurize the second fuel for injection.

[0025] Refer to the attached diagram for general details, but focus on... Figure 1According to embodiments, operating the dual-fuel system in a dual-fuel mode may include feeding pressurized first fuel from a first fuel pump 44 to a pressurized fuel reservoir 42, and feeding pressurized first fuel to an actuation fluid inlet 54 of a plurality of second fuel pumps 52. Operating the dual-fuel system according to this disclosure also includes actuating the plurality of second fuel pumps 52 via pressurized first fuel to pressurize a second fuel. Pressurized first fuel is injected from a first fuel outlet group 38 fluidly connected to the pressurized fuel reservoir 42 into cylinders 18 in the engine 12. Pressurized second fuel is injected from a second fuel outlet group 40 into cylinders 18, each of the second fuel outlet groups being fluidly connected to one of the plurality of second fuel pumps 52. As discussed above, in the illustrated embodiment, pressurized first fuel is fed directly from the pressurized fuel reservoir 42 to the actuation fluid inlet 54. In other embodiments, different piping strategies can be used to feed the first fuel at pump outlet pressure to the actuation fluid inlet 54, such as directly from the pump outlet of the first fuel pump 44 or even selectively from at least one and potentially both of the pressurized fuel reservoir 42 and the pump outlet of the first fuel pump. The pressurized first fuel can be compressed and ignited in each cylinder, thereby causing the ignition of the injected second fuel. The injection of the first fuel can occur just before the top dead center position of the corresponding piston at the end of the compression stroke, wherein the injection of the second fuel typically occurs just after the injection of the first fuel, but the timing of the respective injections overlaps within the scope of this disclosure. Other combinations of the first and second fuels, and the order in which they are introduced into the cylinder, are also within the scope of this disclosure. For example, a small amount of the second fuel can be injected just before the injection of the first fuel, followed by a larger amount of the second fuel. Such strategies, or other strategies, can optimize engine performance and emissions under certain operating conditions.

[0026] This specification is for illustrative purposes only and should not be construed as limiting the scope of this disclosure in any way. Therefore, those skilled in the art will understand that various modifications can be made to the embodiments currently disclosed without departing from the full and reasonable scope and spirit of this disclosure. Other aspects, features, and advantages will become apparent from the accompanying drawings and appended claims. As used herein, the articles “a” and “an” are intended to include one or more items and may be used interchangeably with “one or more”. The term “one” or similar language is used when intended to refer to only one item. Furthermore, as used herein, the terms “has,” “have,” “having,” etc., are intended to be open-ended terms. Additionally, the phrase “based on” is intended to mean “at least partially based on” unless otherwise expressly stated.

Claims

1. A dual-fuel engine system (10), comprising: An engine (12) comprising a plurality of cylinders (18) formed therein. The fuel system (34) includes at least one fuel injector associated with each of the plurality of cylinders, and each of the at least one fuel injector has a first fuel outlet group (38) and a second fuel outlet group (40). The fuel system also includes a pressurized fuel reservoir (42), a first fuel pump (44) having a first fuel pump outlet (46) fluidly connected to the pressurized fuel reservoir, and a plurality of high-pressure first fuel conduits (48) each extending from the pressurized fuel reservoir to one of the first fuel outlet groups of the first fuel outlet group. The fuel system further includes: a plurality of second fuel pumps (52), each having an actuation fluid inlet (54) and a second fuel pump outlet (56); a plurality of low-pressure second fuel conduits (60), each extending to one of the plurality of second fuel pumps; and a plurality of high-pressure second fuel conduits (62), each fluidly connecting one of the second fuel pump outlets to at least one of the respective second fuel outlet groups; and The actuation fluid inlet of each of the plurality of second fuel pumps is fluidly connected to the outlet of the first fuel pump or at least one of the pressurized fuel reservoirs.

2. The engine system according to claim 1, wherein: Each of the first fuel outlet group and the second fuel outlet group is located in one of the plurality of cylinders; Each of at least one fuel injector includes a first fuel injection valve (96) movable to open and close a corresponding first fuel outlet group, and a second fuel injection valve (106) movable to open and close a corresponding second fuel outlet group. Each of the first fuel injection valve and the second fuel injection valve includes a hydraulic control surface (100, 108) exposed to the fluid pressure of the pressurized fuel reservoir.

3. The engine system according to claim 1 or 2, wherein each of the high-pressure second fuel conduits extends to a plurality of second fuel outlet groups.

4. The engine system according to any one of claims 1-3, wherein each of the plurality of second fuel pumps comprises a hydraulically actuated pumping element (140) having an actuated surface (141) exposed to the fluid pressure of a corresponding actuated fluid inlet.

5. The engine system according to claim 4, wherein: Each hydraulically actuated pumping element includes a plunger (140) having a pumping surface (145) opposite to the corresponding actuating surface, and an electrically actuated flow control valve (142) is fluidly positioned between the corresponding actuating fluid inlet and the actuating surface; and Each of the hydraulically actuated pumping elements includes a booster or a pressure reducer (140), and the actuating surface has a first area, and the pumping surface has a second area different from the first area.

6. The engine system according to any one of claims 1-5, further comprising a first fuel supply source (50) containing compressed ignition liquid fuel and fluidly connected to the first fuel pump and a second fuel supply source (53) containing a second fuel, wherein the engine further comprises a plurality of pistons (20) movable within the plurality of cylinders to increase the pressure therein to the auto-ignition threshold of the compressed ignition liquid fuel.

7. A method of operating a dual-fuel system (34), comprising: Pressurized first fuel is fed from the first fuel pump (44) to the pressurized fuel reservoir (42); The pressurized first fuel is fed into the actuation fluid inlet (54) of a plurality of second fuel pumps (52); The plurality of second fuel pumps are actuated via the pressurized first fuel to pressurize the second fuel; The pressurized first fuel is injected from a first fuel outlet assembly (38) fluidly connected to the pressurized fuel reservoir into the cylinders of the engine; as well as A pressurized second fuel is injected into the cylinder from a second fuel outlet group (40) of one of the plurality of second fuel pumps, which is fluidly connected to the second fuel pump.

8. The method of claim 7, wherein feeding the pressurized first fuel to the actuation fluid inlet comprises feeding the pressurized first fuel from the pressurized fuel reservoir to the actuation fluid inlet.

9. The method according to claim 7 or 8, further comprising pressurizing or depressurizing the pressure of the second fuel in the plurality of second fuel pumps relative to the pressure of the pressurized first fuel.

10. The method according to any one of claims 7-9, wherein: Injecting the pressurized first fuel includes injecting a pilot amount of the first fuel, and injecting the pressurized second fuel includes injecting a larger amount of the pressurized second fuel; The method further includes compressing and igniting a first fuel in the cylinder, and igniting a second fuel in the cylinder via compressing and igniting the first fuel; and The pressurized first fuel comprises a high cetane number fuel, and the pressurized second fuel comprises a low cetane number fuel.

11. The method of claim 9, wherein pressurizing or depressurizing the second fuel comprises depressurizing the pressure by actuating a plunger (140) in each of the plurality of second fuel pumps, the plunger having an actuation surface (141) exposed to the fluid pressure of the first pressurized fuel and a pumping surface (145) exposed to the pumping chamber (36) containing the second fuel.

12. A dual-fuel system (34), comprising: A plurality of fuel injectors (36), each of the plurality of fuel injectors including at least one of a first fuel outlet group (38) and a second fuel outlet group (40), and the number of the plurality of fuel injectors is equal to the number of the first fuel outlet group and the number of the second fuel outlet group in the dual-fuel system; Pressurized fuel storage unit (42); A first fuel pump (44) having a first fuel pump outlet (46) fluidly connected to the pressurized fuel reservoir, and a plurality of high-pressure first fuel conduits (48) each extending from the pressurized fuel reservoir to one of the first fuel outlet groups of the first fuel outlet group. A plurality of second fuel pumps (52), each having an actuation fluid inlet (54) and a second fuel pump outlet (56); a plurality of low-pressure second fuel conduits (60), each of the plurality of low-pressure second fuel conduits extending to one of the plurality of second fuel pumps; and a plurality of high-pressure second fuel conduits (62), each of the plurality of high-pressure second fuel conduits fluidly connecting one of the plurality of second fuel pump outlets to at least one of the respective second fuel outlet groups; as well as A plurality of actuation fluid conduits (60), each of the plurality of actuation fluid conduits extending from the first fuel pump or the pressurized fuel reservoir to the actuation fluid inlet of one of the plurality of second fuel pumps.

13. The dual-fuel system of claim 12, wherein each of the plurality of second fuel pumps comprises a booster (140) having an actuation surface (141) exposed to fluid pressure at a corresponding actuation fluid inlet and a pumping surface (145) opposite the actuation surface.

14. The dual-fuel system of claim 13, wherein the booster includes a reverse booster (140), and wherein the actuating surface has a first area, and the pumping surface has a second area greater than the first area.

15. The dual-fuel system according to any one of claims 12-14, wherein each of the plurality of fuel injectors includes a first injection valve (96) movable to open and close the first fuel outlet group and a second injection valve (106) movable to open and close the second fuel outlet group, and wherein each of the first injection valve and the second injection valve includes a hydraulic control surface (100, 108) exposed to the fluid pressure of the pressurized fuel reservoir.

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