Engine parasitic loading strategy using fuel pressurization

Abstract

Operating the engine system includes cold starting the engine, closing an overflow valve to pressurize fuel in a plurality of plunger cavities, opening injection valves in some of a plurality of fuel injectors to inject fuel into an ignition cylinder in an engine cycle, and opening relief valves in some of the plurality of fuel injectors while injection valves in the plurality of fuel injectors remain closed to discharge fuel to a lower pressure space in the engine cycle. Fuel in the fuel injector that remains closed pressurizes the parasitic engine to increase the amount of fuel combustion, thereby accelerating warm-up and limiting misfire. Related devices and control logic are also disclosed.

Description

Technical Field

[0001] The present invention generally relates to operating an engine system, and more particularly to parasitic loading of an engine via fuel pressurization without injection during cold start conditions. Background Art

[0002] Internal combustion engines are used worldwide for a variety of purposes, from operating a driveline in a vehicle to powering pumps, compressors, and generators. For heavy-duty applications, compression ignition diesel engines are typically employed. Substantially all internal combustion engines operate in some form of a basic cycle of controlled combustion of fuel in a cylinder to produce a rapid increase in temperature and pressure that drives a piston coupled to a rotatable crankshaft.

[0003] When an engine is started to initiate the cycle, it is generally necessary to rotate components in the engine via an external mechanism such as a starter motor. Combined with the necessary actuation of the moving components in the engine is the necessity to initiate the combustion process. In many cases, particularly for compression ignition engines, when the engine is cold, fuel cannot simply be delivered into the engine and ignited with sufficient reliability. For example, the cold metal surfaces forming the engine cylinders can have a tendency to extinguish nascent combustion flames. Over the years, various strategies have been proposed to achieve initial ignition of the fuel in the cylinder and subsequent maintenance of combustion. An exemplary engine starting operation mode is set forth in U.S. Patent No. 7,201,127B2 to Rockwell et al. Ample opportunities exist in the art for improved and / or alternative strategies. Summary of the Invention

[0004] In one aspect, a method of operating an engine system includes: cold starting an engine; moving a plurality of plungers between a forward position and a retracted position in a plurality of plunger chambers; and closing a plurality of overflow valves, each overflow valve being fluidly positioned between one of the plurality of plunger chambers and a low pressure space to pressurize fuel in each of the plurality of plunger chambers. The method further includes opening a first injection valve in a first fuel injector to inject fuel pressurized by a first one of the plurality of plungers into an ignition cylinder of the engine during an engine cycle, and opening one of the plurality of overflow valves in a second fuel injector while a second injection valve in the second fuel injector remains closed to discharge fuel pressurized by a second one of the plurality of plungers into a low pressure space in the engine cycle. The method further includes parasitic loading the engine by pressurizing fuel with a second one of the plurality of plungers.

[0005] In another aspect, an engine system includes: an engine having an engine housing with a plurality of cylinders therein; and a fuel system having a first fuel injector and a second fuel injector, each of the first fuel injector and the second fuel injector including a plunger located in a plunger chamber, an injection valve, and an electro-actuated overflow valve. The fuel system defines a low-pressure space. The engine system further includes a parasitic load control unit configured to command closing of the overflow valve in the first fuel injector to pressurize the fuel in the corresponding plunger chamber and command closing of the overflow valve in the second fuel injector to pressurize the fuel in the corresponding plunger chamber. The parasitic load control unit is further configured to command opening of the injection valve in the first fuel injector during an engine cycle to inject the pressurized fuel from the corresponding plunger chamber into a firing cylinder of the plurality of cylinders in the engine, and command opening of the overflow valve in the second fuel injector during the engine cycle while the injection valve in the second fuel injector remains closed to discharge the pressurized fuel from the corresponding plunger chamber into the low-pressure space.

[0006] In yet another aspect, a fuel system includes a parasitic load control unit configured to be in control communication with each of a plurality of electro-actuated overflow valves and a plurality of injection valves in a plurality of fuel injectors in the fuel system. The additional load control unit is further configured to command closing of the plurality of overflow valves to pressurize the fuel in the plurality of plunger chambers of the plurality of fuel injectors and command opening of the injection valves of a firing group of the plurality of fuel injectors to inject the pressurized fuel into a plurality of firing cylinders in an engine. The parasitic load control unit is further configured to command opening of the overflow valves of a non-firing group of the plurality of fuel injectors while the corresponding injection valves remain closed to parasitic load the engine during cold start. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a schematic diagram of an engine system according to one embodiment;

[0008] Figure 2 is a cross-sectional side schematic diagram of a fuel system according to one embodiment;

[0009] Figure 3 is a graph of fuel system events and states during crankshaft angle timing;

[0010] Figure 4 is a graph of valve displacement and fuel pressure versus time; and

[0011] Figure 5 is a flow chart showing an example method and logic flow according to one embodiment. DETAILED DESCRIPTION

[0012] SeeFigure 1 , shows an internal combustion engine system 10 according to one embodiment. The engine system 10 includes an engine 12 having an engine housing 14, the engine housing 14 including a plurality of combustion cylinders 16 formed therein. A plurality of pistons 18 are movable within the cylinders 16 between a top dead center position and a bottom dead center position. The pistons 18 are connected to a crankshaft 20 in a generally conventional manner. The cylinders 16 may include any number greater than one and may be in any suitable arrangement, such as an in-line pattern, a V-pattern, or another. The cylinders 16 may be physically and / or functionally arranged into a first cylinder bank and a second cylinder bank. In one embodiment, the first cylinder bank and the second cylinder bank may be arranged on opposite sides of a V-pattern. The first and second cylinder banks may also each include several first cylinders and several second cylinders in an in-line configuration, alternating cylinders in an in-line configuration, or another arrangement. The engine 12 may include a four-stroke compression ignition engine operating on a suitable compression ignition liquid fuel such as diesel distillate fuel. In other implementations, the engine 12 may be a spark ignition engine, a pre-chamber ignition engine, a pilot fuel ignition engine in a dual fuel strategy, or other engines. The engine 12 is operable to drive a load, such as a powertrain, a pump, a compressor, or others in a vehicle.

[0013] The engine system 10 also includes a camshaft 24 having a plurality of cams 26, the camshaft 24 being rotatable at engine half speed by a drive mechanism (not shown) coupled to the crankshaft 20 in a generally conventional manner. The engine system 10 also includes a fuel system 28. The fuel system 28 may include a pump 30 configured to deliver fuel, such as diesel fuel, from a fuel supply 32 to a plurality of fuel injectors 36 through a fuel supply conduit 34. The fuel injectors 36 may be supported in an engine head 22 attached to the engine housing or cylinder block 14. In one implementation, the fuel supply conduit 34 extends through the engine head 22 to provide a low-pressure fuel supply to each fuel injector 36 simultaneously. Other embodiments may include top feeding of fuel to the individual fuel injectors, or some other fuel supply strategy. In some embodiments, a drain or return conduit (not shown) may return the drained fuel from the engine head 22 to the fuel supply 32. Each fuel injector 36 includes a tappet 38 that directly contacts one of the cams 26 or directly contacts one of the cams 26 through a rocker arm (not shown).

[0014] For the purposes of this specification, the fuel system 28 can be understood to include a first fuel injector 36 and a second fuel injector 36, which can be any two fuel injectors in the fuel system 28, and as further discussed herein, the fuel system 28 includes a first fuel injector in a first set of fuel injectors 36 and a second fuel injector in a second set of fuel injectors 36. The first set can sometimes be an ignition set and sometimes a non-ignition set. The second set can likewise sometimes be an ignition set and sometimes a non-ignition set. The first set can include fuel injectors associated with ignition cylinders in a first set of cylinders, and the second set can include non-ignition cylinders in a second set of fuel injectors. The listed sets and ignition and non-ignition functions can be switched during service, such as during a cold start, as further discussed herein.

[0015] Each of the plurality of fuel injectors 36 (sometimes referred to in the singular) is interchangeable for use in the fuel system 28. The fuel injector 36 includes a plunger 40 within a plunger chamber 42. The plunger 40 can be coupled to a tappet 38 and can move in response to rotation of an associated cam 26 between a forward position and a retracted position within the respective plunger chamber 42. In other embodiments, the plunger and plunger chamber can be located external to the fuel injector. Additionally, an embodiment is contemplated where one plunger pressurizes fuel for a plurality of fuel injectors.

[0016] The fuel injector 36 further includes an injection valve 44. The injection valve 44 is movable to controllably initiate, stop, and potentially alter the fuel injection rate profile of fuel injected into a corresponding one of the cylinders 16. The fuel injector 36 further includes an injection control valve 46. As used herein, an "injection valve" refers to a valve that controls fuel injection. Thus, for the purposes of this specification, an outlet check valve or a needle valve (substantially as shown for injection valve 34 in Figure 1 or an injection control valve that controls the corresponding external check valve or needle valve can be considered an injection valve.

[0017] The fuel injector 36 further includes an electric overflow valve 48, which is also further discussed herein. The fuel system 28 further defines a low-pressure space 50. As used herein, a low-pressure space refers to a physical cavity, conduit, passageway, etc. that has a low pressure at least sometimes relative to the higher pressure within the fuel injector. Thus, the low-pressure space 50 can include internal fuel supply conduits, drain conduits within the engine cylinder head 22, or some other cavity, void, etc. within a single fuel injector.

[0018] The engine system 10 further includes a control system 52. The control system 52 can include various sensors, electro-actuators, and other electrical or electronic components within or associated with the fuel system 28. As Figure 1As shown, the control system 52 includes a crankshaft angle timing sensor 54 that is operable to generate data representative of the crankshaft angle timing of the engine 12, which is used in the control system 52 to perform the control aspects of the present invention at a desired engine crankshaft angle timing, which is also further discussed herein. The control system 52 also includes a fuel supply control unit 56. The fuel supply control unit 56 is hereinafter described as a parasitic load control unit 56 that is configured to selectively parasitically load the engine system 10 so as to, for example, accelerate engine warm-up and / or suppress misfire during cold start. For example, the parasitic load may act before and until the engine system 10 reaches a low idle engine speed.

[0019] The parasitic load control unit 56 may include any suitable programmable logic unit, such as a microprocessor or a microcontroller, and a computer-readable volatile or non-volatile memory, such as RAM, ROM, flash memory, or any one or more of many other memories that store computer-executable program instructions, maps, tables, etc. The features and functions of the parasitic load control unit 56 are further discussed below.

[0020] Now also referring to Figure 2 , additional features of the fuel injector 36 are shown in more detail. The fuel injector 36 includes an injector housing 60 having a housing 62 that can be positioned within the engine cylinder head 22 such that the fuel injector 36 includes a direct injector that extends into a respective one of the cylinders 16 for direct injection of fuel. The housing 62 includes at least one fuel inlet 64 formed therein that receives a low-pressure fuel supply through the fuel supply conduit 34. The plunger 40 is also shown within the injector housing 60 and is at least partially movable within the plunger chamber 42 to a retracted position to draw fuel into the plunger chamber 42 via the fuel inlet 64 and movable to a forward position to pressurize the fuel for injection or to discharge the fuel back to the low-pressure space 50 through the fuel inlet 64. As described above, the fuel injector 36 includes an electro-actuated overflow valve 48. In some embodiments, the overflow valve may be within the injector housing 60 or potentially located externally. The overflow valve 48 is movable between a fully open position and a fully closed position. In the fully open position, the reciprocating movement of the plunger 40 passively exchanges fuel with the low-pressure space 50. In the fully closed position, the plunger chamber 42 is blocked from the low-pressure space 50 and the plunger 40 advances to pressurize the fuel and supply the fuel to the nozzle outlet 68 through the nozzle passage 66.

[0021] The injection valve 44 is movable to open and close the nozzle outlet 68, thereby controlling fuel injection. The fuel injector 36 also includes a control chamber 70 fluidly connected to the nozzle passage 66. When there is high pressure in the nozzle passage 66, as long as the control valve 46 is closed, a closing hydraulic pressure can be applied to the injection valve 44 in the hydraulic control chamber 70. When the control valve 46 is open, the control chamber 70 will be connected to the low-pressure space 50, allowing the high pressure from the nozzle passage 66 to push open the injection valve 44. Each of the relief valve 48, the injection valve 44, and the control valve 46 can be of known design. A biasing spring 72 is operatively positioned between the relief valve 48 and the control valve 46. The biasing spring 72 can bias the control valve 46 toward the closed position and bias the relief valve 48 toward the open position. Separate springs can be used in some embodiments. Energizing the solenoid actuator for the control valve 46 causes the control valve 46 to open against the closing bias of the biasing spring 72. Energizing the solenoid actuator for the relief valve 48 causes the relief valve 48 to close against the opening bias of the biasing spring 72.

[0022] Figure 2 The features of the control system 52 are also shown, including normal operation software or control logic 74, and cold start software or control logic 76. The parasitic load control unit 56 can be part of the fuel supply control unit as described above, which operates the fuel system 28 under all conditions including "normal" conditions as well as cold start conditions. The cold start conditions contemplated herein refer to the starting conditions where the engine system 10 transitions from non-operating or off to operating or on. Thus, cold start does not necessarily require the engine system 10 to be cold, but is initiated by, for example, turning the ignition key or actuating the start button after it has been off.

[0023] As described above, cold starting an internal combustion engine and in particular starting a compression ignition diesel engine can be associated with certain challenges. One challenge that has been observed over the years is the possibility of at least some cylinders not firing. Non-firing cylinders increase the time it takes for the engine system to warm up and reach a stable operating state (e.g., low idle engine speed), and potentially cause other problems related to combustion control and / or emissions. In many applications, it may be desirable to minimize the starting time, and thus it is often desirable to transition the engine to a stable operating state, such as low idle engine speed, as quickly as possible. It has been found that parasitic loading of the engine system 10 during cold start can temporarily increase the amount of fuel burned in each cylinder by adding load to the engine 12, to increase the temperature more quickly and suppress non-firing. The control system 52 can generally be configured in accordance with these and other objectives.

[0024] To this end, the parasitic load control unit 56 can be configured to command the closing of the overflow valve 48 in the first fuel injector 36 to pressurize the fuel in the corresponding plunger chamber 42, and to command the closing of the overflow valve 48 in the second fuel injector 36 to pressurize the fuel in the corresponding plunger chamber 42. The parasitic load control unit 56 can also be configured to command the opening of the injection valve 44 in the first fuel injector 36 during an engine cycle to inject the pressurized fuel from the corresponding plunger chamber 42 into the firing cylinders of the plurality of cylinders 16 in the engine 12. The parasitic load control unit 56 can further be configured to command the opening of the overflow valve 48 in the second fuel injector 36 during an engine cycle while the injection valve 44 in the second fuel injector 36 remains closed so as to discharge the pressurized fuel from the corresponding plunger chamber 42 to the low-pressure space 50.

[0025] The foregoing description has focused on a situation where one fuel injector is used to inject fuel into the firing cylinders, where the fuel is burned by rotating the crankshaft 20 to generate the output power of the engine 12, while the other fuel injector pressurizes the fuel but discharges or vents the fuel pressure back to the low-pressure space 50. In this way, the engine 12 can be considered to need to perform the work of pressurizing fuel for two fuel injections, but only realizes the combustion energy output from one of these fuel pressurizations.

[0026] As a result, the engine system 10 is parasitically loaded by the non-firing cylinders, requiring the firing cylinders to increase the fuel injection amount and the fuel combustion amount to meet the loading requirements of the engine 12. In a practical implementation, the control system 52 will typically utilize a plurality of firing cylinders supplied with fuel by a plurality of fuel injectors in the firing bank, and a plurality of non-firing cylinders associated with the non-firing bank of fuel injectors, rather than only applying this practice to two fuel injectors. Accordingly, the foregoing first injection valve 44 can be one of the plurality of injection valves 44 in the plurality of fuel injectors 36 that opens during an engine cycle to inject fuel into the plurality of firing cylinders of the engine 12, and the foregoing second injection valve 44 can be one of the plurality of injection valves 44 that remains closed during the same engine cycle to discharge the fuel pressurized by the plurality of plungers 40 to the low-pressure space 50. The plurality of injection valves 44 that open to inject fuel can be located in the plurality of fuel injectors 36 associated with the plurality of firing cylinders in the first cylinder bank of the engine 12, and the plurality of injection valves 44 that remain closed can be located in the plurality of fuel injectors 36 associated with the plurality of non-firing cylinders in the second cylinder bank of the engine 12. The parasitic load control unit 56 can further be configured to switch the parasitic load of the engine 12 from the second cylinder bank to the first cylinder bank during a second engine cycle. Similarly, the parasitic load control unit 56 can be understood to be configured to parasitically load the engine 12 via the second fuel injector during a first engine cycle and via the first fuel injector during a second engine cycle.

[0027] Reference is now also made to Figure 3 which shows a graph 100 that depicts events and states in an engine cycle of an engine and a fuel system over a range of crankshaft angles shown on the X-axis. Cam speed is shown at 102. Cam displacement (CamDisplcmnt) is shown at 104. Numeral 106 shows the plunger pressure (Plunger Prsr), and 108 shows the rocker pressure (Rocker Prsr). As will be recalled,[ Figure 1 shows cam 26 contacting tappet 38. It should be understood that cam 26 can rotate in contact with a rocker arm that is sequentially connected to tappet 38.[

[0028] Reference numerals 110, 112, and 114 represent injector currents. Injector currents 110, 112, and 114 can include an electrical control current or command generated by parasitic load control unit 56 and used to energize the solenoid actuator of overflow valve 48. In one embodiment, parasitic load control unit 56 can be configured to command the re-closing and re-opening of overflow valve 48 in order to discharge pressurized fuel in multiple pulses during an engine cycle. In the illustrated embodiment, the multiple pulses include three pulses. Other embodiments can include two pulses, one pulse, or more than three pulses. Thus, according to[ Figure 3 the example shown, parasitic load control unit 56 energizes the solenoid actuator of overflow valve 48 three times individually. Reference numerals 120, 122, and 124 show the movement of overflow valve in response to pulses of injector currents 110, 112, 114. It can be observed that pulses 110, 112, and 114 are not exactly the same. Pulse 110 has a longer duration. The dwell time 118 between pulse 112 and pulse 114 can be longer than the dwell time 116 between pulse 110 and pulse 112.[

[0029] Those skilled in the art will recognize the possibility of overpressurizing a fuel injector by advancing a plunger within a plunger cavity to pressurize fuel when the overflow valve is closed and the injection valve remains closed. Parasitic load control unit 56 is operable to prevent overpressurization of a target fuel injector by controllably opening and then closing overflow valve 48 to prevent damage or performance degradation that could result from overpressurization. Overpressurization of main fuel injector 36 can be inhibited based on at least one of the pulse duration, the number of pulses, or the pulse-to-pulse dwell time of the multiple pulses. Numeral 126 shows a check movement, showing that the associated injection valve remains closed. From[ Figure 3 it can also be noted that the multiple pulses occur during the rising portion of the cam displacement profile 104 of a cam that is coupled to a tappet, and the tappet is coupled to the plunger of the fuel injector of the present invention.[

[0030] Reference is now also made to Figure 4, shows a graph 200, which shows the changes in the displacement of the overflow valve at 210 and the fuel pressure at 225 over time. From Figure 4 It can be noted that multiple pulses 220 are evident in the overflow valve displacement 210, and multiple pulses 230 are evident in the fuel pressure 225. It is contemplated that in many cases, it is desirable to fit as many fuel pressure release pulses as possible into the engine cycle, as feasible, to maximize parasitic loading while suppressing overpressure.

[0031] Industrial Applicability

[0032] Generally referring to the drawings, but now focusing on Figure 5 , shows a flowchart 300 that shows an example method and logic flow. At block 310, the engine 12 is cold-started, which means the engine 12 is turned on and the crankshaft 20 is rotated, for example, by a starter motor coupled to an associated transmission mechanism. At block 320, the camshaft 24 rotates to move the plunger 42 between the advanced position and the retracted position in each fuel injector 36. The flowchart 300 proceeds from block 320 to block 330 to command the overflow valve 48 to close at a specified timing to pressurize the fuel in the fuel injector 36. Since each cylinder 16 will have different phases of the associated components, the commanded closing of the overflow valve 48 will generally occur at different times.

[0033] The flowchart 300 proceeds from block 330 to block 340 to command the injection valve 44 of the ignition group of the injector 36 to open at a specified timing to inject fuel into the associated cylinder 16 for combustion. The flowchart 300 proceeds from block 340 to block 350 to command the overflow valve 48 of the non-ignition group of the injector 36 to open at a specified timing to parasitically load the engine 12. It should be understood that the commanded opening of the injection valve and the commanded opening of the overflow valve can have different timings or even overlapping timings and thus do not occur simultaneously.

[0034] Also as discussed herein, once a cylinder bank of the engine 12 has been used to parasitically load the engine for a predetermined time (e.g., a predetermined number of engine cycles), the control can switch to using another cylinder bank to parasitically load the engine 12 at a specified timing (e.g., multiple engine cycles). The control can also switch back from another cylinder bank to the first cylinder bank, switch to a third cylinder bank, or switch between any number of cylinders and associated fuel injectors in any mode to selectively parasitically load the engine 12. When the engine system 10 has been preheated, for example, by reaching low idle or even before reaching low idle, the logic performed for cold-start operation can exit and normal operation can continue.

[0035] This specification is for illustrative purposes only and should not be construed as limiting the scope of the invention in any way. Thus, those skilled in the art will understand that various modifications can be made to the presently disclosed embodiments without departing from the full and proper scope and spirit of the invention. Other aspects, features, and advantages will become apparent by studying the drawings and the 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." Where only one item is intended, the term "one" or similar language is used. Additionally, as used herein, the terms "has," "have," "having," etc. are intended to be open-ended terms. Further, the phrase "based on" is intended to mean "at least partially based on" unless otherwise expressly stated.

Claims

1. A method of operating an engine system, comprising: Cold start engine; moving a plurality of plungers in the plurality of plunger chambers between an advanced position and a retracted position; closing a plurality of relief valves, each relief valve being fluidly positioned between one of the plurality of plunger cavities and the low pressure space, to pressurize fuel in each of the plurality of plunger cavities; opening a first injection valve in a first fuel injector to inject fuel pressurized by a first one of the plurality of plungers into a firing cylinder of the engine during an engine cycle; opening one of the plurality of relief valves in the second fuel injector while a second injection valve in the second fuel injector remains closed to discharge fuel pressurized by a second one of the plurality of plungers into a low pressure space in the engine cycle; as well as The engine is parasitic loaded by pressurizing the fuel by a second of the plurality of plungers.

2. The method according to claim 1, wherein: The moving the plurality of plungers includes moving the plurality of plungers via a plurality of tappets coupled to a plurality of cams on a camshaft of the engine; Parasitic loading of the engine occurs during cold starting of the engine and prior to transitioning the engine to a low idle engine speed; the first injection valve being one of a plurality of injection valves of a plurality of fuel injectors that are opened to inject fuel into a plurality of firing cylinders of the engine during the engine cycle; as well as The second injection valve is one of the plurality of injection valves that is kept closed so as to discharge the fuel pressurized by the plurality of plungers to a low-pressure space in the engine cycle.

3. The method according to claim 2, wherein: the plurality of injection valves opened to inject fuel are in a plurality of direct injectors extending into the plurality of firing cylinders in a first cylinder bank in the engine; The plurality of injection valves that remain closed are located in a plurality of direct injectors that extend into a plurality of non-firing cylinders in a second cylinder bank in the engine; and The method further includes switching parasitic loading of the engine from the second cylinder bank to the first cylinder bank in a second engine cycle.

4. The method according to any of the preceding claims further comprises reclosing said one of said plurality of relief valves while said second injection valve remains closed and then reopening said one of said plurality of relief valves to discharge fuel pressurized by said second of said plurality of plungers in a plurality of pulses. The method of claim 4 , wherein the plurality of pulses comprises three pulses.

6. The method of claim 4 or 5, further comprising inhibiting over-pressurization of the second fuel injector based on at least one of a pulse duration, a number of pulses, or a pulse-to-pulse dwell time of the plurality of pulses.

7. The method of any one of claims 4-6, wherein each of the closing and reclosing of one of the plurality of relief valves comprises energizing a relief valve electrical actuator.

8. An engine system, comprising: an engine comprising an engine housing having a plurality of cylinders therein; a fuel system including a first fuel injector and a second fuel injector, the first fuel injector and the second fuel injector each having a plunger in a plunger cavity, an injection valve, and an electrically actuated relief valve, and the fuel system defining a low pressure space; A parasitic load control unit is configured to: commanding closure of the spill valve in the first fuel injector to pressurize fuel in the corresponding plunger cavity; commanding closing of a spill valve in a second fuel injector to pressurize fuel in a corresponding plunger chamber; commanding an injection valve in a first fuel injector to open during an engine cycle to inject pressurized fuel from a corresponding plunger cavity into a firing cylinder of a plurality of cylinders in the engine; as well as The spill valve in the second fuel injector is commanded to open during an engine cycle while the injection valve in the second fuel injector remains closed to discharge pressurized fuel from the corresponding plunger cavity to the low pressure space.

9. The engine system of claim 8, wherein the engine is parasitically loaded via the second fuel injector in the engine cycle, and the parasitic loading control unit is further configured to parasitically load the engine via the first fuel injector in a second engine cycle; and Wherein the parasitic loading control unit is further configured to command re-closing and re-opening of the spill valve in the second fuel injector during the engine cycle so as to discharge the pressurized fuel in a plurality of pulses.

10. The engine system of claim 9, wherein: The plurality of pulses includes three pulses, and the plurality of pulses vary with respect to at least one of a pulse duration or a pulse-to-pulse dwell time; The plurality of pulses occur during a rising portion of a cam displacement profile of a cam coupled to a tappet coupled to a plunger of the second fuel injector; and Commanding opening of the spill valve in the second fuel injector includes commanding de-energizing the spill valve, and commanding de-energizing the spill valve a plurality of times to generate a plurality of pulses.

11. A fuel system comprising: a parasitic loading control unit configured to be in control communication with each of a plurality of electrically actuated spill valves and a plurality of injection valves of a plurality of fuel injectors in the fuel system; the parasitic loading control unit being further configured to command closing of the plurality of spill valves to pressurize fuel in the plurality of plunger cavities of the plurality of fuel injectors; The parasitic loading control unit is further configured to command opening of injection valves of firing groups of the plurality of fuel injectors to inject the pressurized fuel into a plurality of firing cylinders in an engine; and The parasitic loading controller is further configured to command opening of spill valves of a non-firing group of the plurality of fuel injectors while corresponding injection valves remain closed to parasitic load the engine during a cold start.

12. The fuel system of claim 11, wherein: The parasitic loading control unit is further configured to pulse command opening of the relief valves to discharge pressurized fuel from each of the plurality of plunger chambers to the low pressure space in a plurality of pulses; The parasitic loading control unit is also configured to vary at least one of a duration, a number, or a pulse-to-pulse dwell time of the plurality of pulses from each of the plurality of plunger cavities.

13. The fuel system according to claim 11 or 12, further comprising: The plurality of relief valves each include an electric actuator and a biasing spring, and wherein each of the plurality of relief valves is movable to an intermediate position between a fully open position and a fully closed position by energizing the corresponding electric actuator against a biasing force of the biasing spring to generate the plurality of pulses from the corresponding plunger chamber; The plurality of fuel injectors, and at least one camshaft having a plurality of cams coupled to the plurality of plungers.

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