Dual-fuel common-rail systems and methods for transitioning from a diesel-only to a dual-fuel operating procedure

Abstract

Method for operating a dual-fuel engine (10) comprising the following steps: Operating a dual-fuel system in regular mode; Operating the dual-fuel system in an emergency running mode; Accumulation of liquid fuel in a gas nozzle chamber (115) of each fuel injection device (25) of the dual fuel system (20) over a multitude of engine cycles, during operation in emergency mode; Transition from emergency mode to regular mode by displacing the accumulated liquid fuel with gaseous fuel; Injection of liquid fuel from a first nozzle outlet set (103) and gaseous fuel from a second nozzle outlet set (104) into an engine cylinder (12) when the dual fuel system is operated in a regular mode; Injection of liquid fuel from a first nozzle outlet set (103), but not injection of gaseous fuel from a second nozzle outlet set (104) into an engine cylinder (12) when the dual-fuel system is operated in emergency running mode; and Injection of liquid fuel from a first nozzle outlet set (103) and injection of the accumulated liquid fuel from the second nozzle outlet set (104) into the engine cylinder (12) during the transition step.

Description

Technical area

[0001] The present disclosure relates generally to dual-fuel common rail systems, and more particularly to a method for transitioning from a diesel-only operating mode to a diesel and gas combustion operating mode. background

[0002] A relatively new class of engines attempts to utilize two different fuels to achieve the efficiencies associated with compression ignition combined with the benefits associated with the combustion of a natural gas fuel. Specifically, one type of dual-fuel engine utilizes a small pre-injection amount of liquid diesel fuel, which is compression-ignited, to in turn ignite a much larger amount of natural gas fuel in each engine cylinder. In one strategy for this type of engine, both fuels are injected directly from a single fuel injector associated with each engine cylinder. For example, US Pat. No. 7,627,416 B2 teaches a dual-fuel common rail system in which liquid diesel fuel and natural gas fuel are both injected from a single fuel injector associated with a respective engine cylinder.This document recognizes that there may be cases where the engine must operate solely on liquid diesel fuel because the natural gas fuel supply is depleted or because there may be a fault in the natural gas portion of the system. However, this document does not recognize the problems and challenges associated with transitioning or switching the dual-fuel common rail system from a diesel-only fuel supply mode back to a regular dual-fuel mode.

[0003] Furthermore, CA 2 773 651 A1 discloses a method and a device for controlling the fuel pressure in an internal combustion engine that consumes a gaseous fuel and a liquid fuel. A target value for the pressure of the gaseous fuel is determined as a function of an engine operating condition. The pressure of the liquid fuel is set based on a target value and regulated based on the pressure of the liquid fuel, so that the pressure of the gaseous fuel corresponds to the target value for the pressure of the gaseous fuel within a predetermined tolerance range.

[0004] The present disclosure addresses one or more of the foregoing problems. Summary

[0005] The object of the present invention is achieved by a method and a system according to the main claims. The subclaims relate to preferred embodiments of the invention.

[0006] A method of operating a dual-fuel engine includes operating a dual-fuel system in a regular mode and a limp-home mode. Liquid fuel accumulates in a gas nozzle chamber of each fuel injector of the dual-fuel system over a plurality of engine cycles during operation in the limp-home mode. The accumulated liquid fuel is displaced by gaseous fuel when transitioning from the limp-home mode to the regular mode. The liquid fuel is injected from a first nozzle outlet set, and the gaseous fuel is injected from a second nozzle outlet set, into an engine cylinder during operation of the dual-fuel system in the regular mode. The liquid fuel is injected from the first nozzle outlet set, but no gaseous fuel is injected from the second nozzle outlet set, into the engine cylinder during operation of the dual-fuel system in the limp-home mode.During the transition step, the liquid fuel is injected from the first nozzle outlet set and the accumulated liquid fuel is injected into the engine cylinder from the second nozzle outlet set.

[0007] According to another aspect, the dual-fuel common rail system includes a plurality of fuel injectors, each fluidly connected to both the gaseous fuel common rail and a liquid fuel common rail. The liquid fuel supply and pressure control devices are fluidly connected to the liquid fuel common rail. The gas fuel supply and Pressure control devices are fluidly connected to the common rail of the gaseous fuel. An electronic control device is in control communication with the plurality of fuel injectors, the liquid fuel supply and pressure control devices and gas fuel supply and -pressure control devices. The electronic control device has a limp-home algorithm configured to transmit liquid injection control signals to inject liquid fuel from a first nozzle outlet set, and has a regular algorithm configured to transmit liquid injection control signals and liquid fuel injection control signals, respectively, to inject liquid fuel from the first nozzle outlet set, and gas injection control signals to inject gaseous fuel from a second nozzle outlet set.

[0008] The electronic controller also includes a transition algorithm configured to generate gas injection control signals to inject the accumulated liquid fuel into a gas nozzle chamber of each of the fuel injectors from the second nozzle outlet set. Short description of the drawings Fig. 1 is a schematic view of a dual-fuel engine according to the present disclosure; Fig. 2 is a perspective view of a portion of the engine and a dual-fuel common rail system for the engine of the Fig. 1; Fig. 3 is a sectioned perspective view of a portion of the engine housing shown in Fig. 2 to reveal the structure for a fuel injector and an engine cylinder; Fig. 4 is a side sectional view through a coaxial ridge line assembly according to another aspect of the present disclosure; Fig. 5 is a front cross-sectional view of a fuel injector according to one aspect of the present disclosure; and Fig. 6 is a logic flow diagram showing a transition procedure from an emergency mode to a regular operation mode. Detailed description

[0009] Initially on Fig. Referring to Figures 1-3, a dual-fuel engine 10 includes a dual-fuel common rail system 20 mounted on an engine housing 11 that defines a plurality of engine cylinders 12. The dual-fuel common rail system 20 includes a single fuel injector 25 positioned for direct injection into each of the plurality of engine cylinders 12. A gaseous fuel common rail 21 and a liquid fuel common rail 22 are fluidly connected to each of the fuel injectors 25. The dual fuel common rail system 20 also includes gas supply and pressure control devices 16 fluidly connected to the gaseous fuel common rail 21 as well as liquid supply and pressure control devices 17 fluidly connected to the liquid fuel common rail 22.The fuel injectors 25, the gas pressure supply and control devices, and the liquid supply and pressure control devices 17 are in control communication with and controlled by an electronic engine control device 15 in a known manner. The gas supply and pressure control devices 16 may include a pressurized, refrigerated or cryogenic liquefied natural gas tank 40 having an outlet fluidly connected to a variable-flow cryogenic pump 41. The devices 16 may also include a heat exchanger 42, an accumulator 44, a gas filter 43, and a fuel conditioning module 45 that control the supply and pressure within the common rail 21 for the gaseous fuel.The liquid supply and pressure control devices 17 may include a diesel fuel tank 50, fuel filter 51, and an electronically controlled high-pressure fuel pump 52 that supplies liquid fuel to the liquid fuel common rail 22 and controls the pressure therein.

[0010] Additionally on Fig. 4, the dual-fuel common rail system 20 may include a web assembly 30 having an inner web 32 and an outer web 33 in sealing contact with a common conical seat 27 of each fuel injector 25. The blocks 31 of the coaxial web assembly 30 may be connected together in series with gaseous fuel line segments 18 and liquid fuel line segments 19 to define the gaseous fuel common rail 21 and the liquid fuel common rail 22, respectively. The last coaxial web assembly 30 in the series connection may include a set of plugs instead of the plugs shown in Fig. 2. A coaxial web assembly 30 is fluidly positioned between each of the plurality of fuel injectors 25 and both the gaseous fuel common rail 21 and the liquid fuel common rail 22.

[0011] Each coaxial web assembly 30 may include a load adjustment clamp 34 with a pivot surface 75 in contact with a block 31 at a load adjustment position 56 intersected by the axis 29 of the inner web 32. The load adjustment clamp 34 may define a fastener slot 77 and a fastener bore 76 that receive a first fastener 81 and a second fastener 80, respectively. The load adjustment clamp 34 pivots at the load adjustment position 56 in response to adjustments of the first and second fasteners 81, 80. The fasteners 80 and 81 are received in the fastener bore 54 and the fastener slot 55, respectively, of the blocks 31.

[0012] Each block 31 of each coaxial land assembly 30 defines a segment of a gaseous fuel common rail 21 oriented perpendicular to the axis 19 of the inner land 32. A gaseous fuel passage 60 opens into the gaseous fuel common rail 21 at one end and opens into the first fuel inlet 101 of the fuel injector 25 at its other end. A segment of the gaseous fuel passage 60 is located between the inner land 32 and the outer land 33. Each of the blocks 31 also defines a segment of the liquid fuel common rail 22. A liquid fuel passage 61 opens into the liquid fuel common rail 22 at one end and opens into the second fuel inlet 102 of the fuel injector 25 at its opposite end.

[0013] To capture metallic debris that is often released into the fuel stream during initial operation of the engine 10 after it is built, the coaxial fin assembly 30 may include a gaseous fuel edge filter 36 and a liquid fuel edge filter 37. In the illustrated embodiment, the liquid fuel edge filter 37 may be positioned within the inner fin 32. The gaseous fuel edge filter 36 is shown positioned within the outer fin 33. Those skilled in the art will recognize that the edge filters 36 and 37 may be arranged differently, or omitted, without departing from the scope of this disclosure.

[0014] Additionally on Fig. 5, a fuel injector 25 according to the present disclosure includes an injector body 100 defining a first nozzle outlet set 103, a second nozzle outlet set 104, and a drain outlet 105. The injector body 100 also defines a first fuel inlet 101 and a second fuel inlet 102, which in the sectional view of Fig. 4, which open through the common conical seat 27 of the fuel injector 25. Disposed within the injector body 100 are a first control chamber 106 and a second control chamber 107. A first check valve member 110 has a hydraulically closing surface 112 exposed to the fluid pressure in the first control chamber 106. The first check valve member 110 is movable between a closed position, as shown, in contact with a first seat 108 to fluidly block the first fuel inlet 101 from the first nozzle outlet set 103, and an open position out of contact with the first seat to fluidly connect the first fuel inlet 101 to the first nozzle outlet set 103 via a passage shown in the sectional view of Fig. 5 is not visible. A second check valve member 120 has a hydraulic closing surface 121 which is exposed to the fluid pressure in the second control chamber 107.

[0015] The second check valve member 120 is movable between a closed position, as shown, in contact with a second seat 113 to fluidly block the second fuel inlet 102 from the second nozzle outlet set 104, and an open position out of contact with the second seat 113 to fluidly connect the second fuel inlet 102 to the second nozzle outlet set 104 via a passage not shown in the sectional view of the Fig. 5. Thus, injection of a first fuel (e.g., natural gas) through the first nozzle outlet set 103 is enabled by movement of the first check valve member 110, while injection of the second fuel (e.g., liquid diesel) through the second nozzle outlet set 104 is enabled by movement of the second check valve member 120. Those skilled in the art will appreciate that the first and second nozzle outlet sets 103, 104 may each be considered to have six nozzle outlets arranged about respective centerlines in a manner known in the art. However, the nozzle outlet sets 103 and 104 may each have only one nozzle outlet or any number of nozzle outlets in any arrangement without departing from the scope of the present disclosure.

[0016] A first check valve member 130 is positioned within the injector body 100 and is movable along a common centerline 125 between a first position in contact with the flat seat 151, where the first control chamber 106 is fluidly blocked from the drain outlet 105, and a second position where the first control chamber 106 is fluidly connected to the drain outlet 105 via the control passage 133. When the control chamber 106 is fluidly connected to the drain outlet 105, the pressure in the first control chamber 106 drops, relieving the pressure on the hydraulic closure surface 112, allowing the first check valve member 110 to lift to enable injection of the first fuel (e.g., natural gas) through the first nozzle outlet set 103.A second control valve member 135 is positioned within the injector body 100 and is movable along the common centerline 125 between a first position in contact with the flat seat 156, in which the second control chamber 107 is fluidly blocked from the drain outlet 105, and a position out of contact with the flat seat 156, in which the second control chamber 107 is fluidly connected to the drain outlet 105. When the second control chamber 107 is fluidly connected to the drain outlet 105, the fluid pressure acting on the hydraulic closure surface 121 is relieved, allowing the second check valve member 120 to lift to an open position to enable injection of the second fuel (e.g., liquid diesel) through the second nozzle outlet set 104.

[0017] In the illustrated embodiment, the second control valve member 135 is intersected by the common centerline 125, but the first control valve member 130 defines a bore 131 therethrough that is concentric with the common centerline 125. In the illustrated fuel injector 25, the respective control valve members 130, 135 can be moved to one of the corresponding first and second positions with the first and second electrical actuators 111, 122, respectively. The control valve members 130, 135 can be biased to the other of their respective first and second positions by spring(s) 146, 147. In particular, a first armature 141 can be mounted on a plunger 145 in contact with the first control valve member 130.The first armature 141, the plunger 145, and the first control valve member 130 may be biased to the position shown in contact with the flat seat 151 by biasing the spring 146. The control valve member 130 may rotate slightly about an axis perpendicular to the common centerline 125 by action of a self-aligning feature 136 that allows the convex surface 137 to move on a concave bearing surface 138 each time the control valve member 130 contacts the flat seat 151.

[0018] In this manner, the first armature 141 may be considered operative to move the first control valve member 130, and a second armature 142 may be considered operatively coupled to move the second control valve member 135 by means of a plurality of plungers 143. A common stator 144 separates the first armature 141 from the second armature 142.

[0019] The first control valve member 130 is in contact and out of contact with the flat seat 151 at the first position and the second position, respectively. Similarly, the second control valve member 135 is in contact and out of contact with the flat seat 156 at its first position and its second position, respectively. Either one or both of the seats 151 and 156 may be conical seats. The first control valve member 130 may be coupled to move with the first armature 141 in response to de-energization of the lower coil attached to the common stator 144. When the lower coil attached to the common stator 144 is energized, the armature 141 and plunger 145 are lifted upward, allowing the high pressure in the control passage 133 to force the first control valve member 130 out of contact with the flat seat 151, thereby fluidly connecting the control chamber 106 to the drain outlet 105.The first control chamber 106 and the second control chamber 107 may always be fluidly connected to the second fuel inlet 102 via passages which are shown in the sectional view of FIG. Fig. 5 are not visible. In this way, the liquid diesel originating from the second fuel inlet 102 can be used as a control fluid to control the operation of the first check valve member 110 to enable gaseous fuel injections, and the second member 120 to enable liquid fuel injections.

[0020] A hydraulic lock seal 132 in the form of a ring, which is continuously fluidly connected to the second fuel inlet 102, may be useful to prevent the migration of gaseous fuel from the gas nozzle chamber 115 up into the control chamber 106. The gas nozzle chamber 115 is continuously fluidly connected to the first fuel inlet 101 via passages that are not in Fig. 5 are visible. Back to Fig. 4 and Fig. Referring to Figure 5, the present disclosure teaches a strategy for preventing liquid fuel from leaking from the respective fuel injectors 25 to the gaseous fuel common rail 21 under certain conditions. When the dual-fuel common rail system 20 is operating in a regular mode, the liquid fuel common rail 22 may be maintained at a moderate high pressure (e.g., perhaps 40 MPa), and the gaseous fuel common rail 21 may be maintained at a moderate low pressure (e.g., perhaps 35 MPa). This slight pressure differential is intended to prevent leakage of gaseous fuel into the liquid fuel portions of the fuel injector 25 and, consequently, into the entire dual-fuel common rail fuel system 20.The inclusion of the hydraulic lock seal 132 is another feature to prevent gaseous fuel from migrating to the liquid fuel side of the dual-fuel common rail system 20. Nevertheless, some leakage of liquid fuel to the gaseous fuel side of the system might be expected during regular operating mode, but this small amount of leakage may be allowed to allow for proper lubrication of the moving parts. For example, a small amount of liquid diesel fuel might leak from the hydraulic lock seal 132 down into the gas nozzle chamber 115 during regular operating mode. This small amount of liquid diesel might be expected to be expelled from the nozzle outlet set 103 with each gas injection event.This small amount of leaking liquid diesel may serve to assist in lubricating the guiding movement of the first check valve member 110 and the seat 108 during regular operating mode.

[0021] The dual-fuel common rail system may also have a single-fuel operating mode in which only liquid diesel fuel is used to power the engine 10. This operating mode may be referred to as a limp-home mode because this operating mode may be preferred when the supply of gaseous fuel has been depleted. When operating in limp-home mode, the electronic control device 15 may maintain the liquid fuel common rail 22 at a high pressure (e.g., around 80 MPa) while allowing the pressure in the gaseous fuel common rail 21 to drop and slowly decrease to atmospheric pressure. During limp-home mode, the engine 10 is operated as a conventional diesel engine, with liquid diesel fuel injected through the nozzle outlet set 104 in sufficient quantities and at appropriate times to ignite by compression.On the other hand, a relatively small pilot injection of diesel fluid through nozzle outlet set 104 can be expected to be compression-ignited to, in turn, ignite a larger charge of gaseous fuel injected through nozzle outlet set 103 to power engine 10 in a regular operating mode. Due to the higher pressure potential between the liquid fuel and the gaseous fuel escaping during the limp-home operating mode, more liquid fuel can be expected to leak onto the gas side of the dual-fuel common rail system 20 than would leak during a regular operating mode with a smaller pressure differential between the two fuels.Because little to no gaseous fuel is utilized during the limp-home mode of operation, and because the leakage rate of the liquid fuel to the gaseous fuel side is greater, the present disclosure teaches the inclusion of a check valve 66 or 67 to prevent the buildup of leaked liquid diesel in the gas nozzle chamber 115 from ultimately reaching and entering the gaseous fuel common rail 21. Referring particularly to FIG. Fig. 4, in a specific embodiment, a check valve 66 may be located in the passage extending from the first fuel inlet 101 to the gas nozzle chamber 115 within the single fuel injector body 100. On the other hand, Fig. 4 also shows an alternative position in which the check valve 67 may be located in the gaseous fuel passage 60, such as within the block 31. Those skilled in the art will appreciate that the check valve 66 or 67 blocks the escaped liquid fuel from migrating to the gaseous fuel common rail 21 during a limp home mode of operation, but is open and allows the free flow of the gaseous fuel to the gas nozzle chamber 115 during the regular mode of operation.

[0022] Back to Fig. 1, although not required, the dual-fuel common rail system 20 may also include an electronically controlled isolation valve 46 operatively positioned between the gaseous fuel supply and pressure control devices 16 and the gaseous fuel common rail 21. The isolation valve 46 may be mechanically biased to a closed position but movable to an open position in response to a control signal from the electronic control device 15. When the dual-fuel common rail fuel system 20 is operating in a regular mode, the electronic control device 15 may maintain the isolation valve 46 in an open position.However, in the event that the system enters a limp-home mode of operation, the electronic control device 15 may close the isolation valve 46 to fluidly isolate the gas supply and pressure control devices 16 from any escaping liquid diesel fuel that may find its way onto the gas side of the dual-fuel common rail system 20.

[0023] The present disclosure recognizes that the gaseous fuel and the liquid fuel have different energy densities and, consequently, this should be taken into account when transitioning the dual-fuel common rail system 20 from the limp home mode back to the regular operating mode. An example scenario may be a mining truck operating in a regular mode until the gaseous fuel supply is depleted, at which point the engine 10 would transition to a single-fuel, or limp home, mode. After the liquefied natural gas tank 40 is refilled, there may be a desire to transition back from the limp home mode to the regular mode to again supply the engine 10 with both liquid diesel and natural gas fuel.The present disclosure recognizes that some care should be taken to drain or expel the accumulated liquid diesel fuel in the gas nozzle chambers 115 of the individual fuel injectors 25 before commanding large gas fuel injection events. In other words, erroneously injecting large quantities of liquid diesel fuel from the gas nozzle chambers 115 during a transient could disrupt the overall fueling logic, causing an increase in engine speed, resulting in a disordered engine control response.Accordingly, the present disclosure teaches transitioning from a limp-home mode to the regular mode by initially commanding small injections of accumulated liquid diesel fuel from the gas nozzle chambers while maintaining the desired engine power and a majority of the fuel supply using the liquid diesel fuel as in the limp-home mode of operation. Upon detecting that all or most of the accumulated liquid fuel in the gas nozzle chambers 115 has been drained, the present disclosure teaches replacing the liquid fuel injection amounts with increasing amounts of gaseous fuel to continue maintaining the engine operating at a desired speed and load.As the gaseous fuel substitution strategy continues, the strategy will ultimately arrive at a ratio of gaseous fuel to liquid diesel fuel that corresponds to regular mode operation. At this point, the logic can then transition to operating according to regular mode, in which a small pilot injection diesel quantity is compression-ignited to, in turn, ignite a larger charge of injected gaseous fuel. During limp-home mode operation, the liquid fuel is expected to accumulate in the gas nozzle chambers 115 of each fuel injector 25 of the dual-fuel common rail system 20 over a plurality of engine cycles. This accumulated liquid fuel is displaced by the gaseous fuel during the transition from limp-home mode to regular mode.During the transition, liquid fuel continues to be injected from the nozzle outlet set 104 and the accumulated liquid fuel is injected into the corresponding engine cylinders through the gas nozzle outlet set 103.

[0024] The transition from limp home mode to regular mode may include a measurement to confirm that the accumulated liquid fuel has been injected before completing the transition. For example, one way to confirm that the accumulated liquid fuel has been fully injected would be to detect the injection of gaseous fuel from the gas nozzle outlet set 103. Detection of gaseous fuel injection may be determined indirectly by measuring the engine speed and / or torque responses to the combustion events in each of the engine cylinders. For example, for a given injection duration and assuming equal volumes of liquid fuel and gaseous fuel, an engine speed or torque increase might be expected when combusting liquid diesel fuel instead of gaseous fuel.Consequently, the gas injection control signals could have a duration that provides measurable differences in engine speed and / or torque, which would enable one to distinguish whether gaseous fuel or liquid fuel was injected.

[0025] In the illustrated embodiment, the first check valve member 110 and the second check valve member 120 move along corresponding lines parallel to, but spaced from, the common centerline 125. Nevertheless, those skilled in the art will recognize that the construction could vary. For example, double concentric check valve members concentric with the common centerline 125 would also be within the scope of the present disclosure. Industrial applicability

[0026] The present disclosure is broadly applicable to any engine that utilizes two fluidly distinct common rails to deliver fuel to a single fuel injector associated with each engine cylinder. The contents of the respective common rails may differ in either pressure and / or chemistry and / or phase without departing from the present disclosure. In the illustrated example, the respective common rails may differ in all three, delivering pressurized natural gas fuel or pressurized diesel fuel.liquid diesel fuel, each at different pressures. The present disclosure is also applicable to a dual-fuel common rail system with the capability of operating in a regular mode utilizing both fuels, as well as in a limp-home mode in which the engine is supplied with only one of the fuels. For example, a limp-home mode may correspond to the use of liquid diesel fuel due to the unavailability of gaseous fuel. In particular, the present disclosure is applied to a strategy for transitioning from the single-fuel limp-home mode to the regular dual-fuel mode.

[0027] Back to all of the Fig. 1-5, a method of operating a dual-fuel engine 10 begins by mounting a dual-fuel common rail system 20 to an engine housing 11. Gaseous fuel is delivered from the gaseous fuel common rail 21 to each of the plurality of fuel injectors 25 through a corresponding coaxial flywheel assembly 30. Similarly, liquid fuel is delivered from a liquid fuel common rail 22 to each of the plurality of fuel injectors 25 through like, corresponding coaxial flywheel assembly 30. When operating in the regular operating mode, gaseous fuel is injected from each fuel injector 25 into an engine cylinder 12 in response to a gaseous fuel injection signal communicated to the fuel injector 25 from the electronic engine controller 15.Liquid fuel from the fuel injector 25 is injected directly into the engine cylinder 12 from the same fuel injector 25 in response to a liquid fuel injection signal from the electronic engine control device 15.

[0028] A method of operating the dual-fuel engine 10 includes operating the dual-fuel system 20 preferably in a regular mode most of the time. Preferably, the dual-fuel common rail system 20 is operated in a limp-home mode for a short time, which may be due to a depleted gaseous fuel supply or possibly a fault in the gaseous fuel system that may cause a switch to a single-fuel mode. During regular mode operation, liquid fuel is injected into an engine cylinder 12 from the liquid nozzle outlet set 104, and gaseous fuel is injected from the gas nozzle outlet set 103.When the dual-fuel common rail system 20 is operating in the limp-home mode, liquid fuel is injected into the engine cylinder 12 from the liquid nozzle outlet set 104, but gaseous fuel is not injected from the gas nozzle outlet set 103. During limp-home mode operation, the electronic control device 15 will act to maintain a high ratio of the liquid common rail pressure to the gas common rail pressure, but this ratio is kept low when the dual-fuel common rail system 10 is operating in regular mode. During limp-home mode operation, the check valve(s) 66, 67 will effectively prevent the leaked liquid fuel from reaching the gaseous fuel common rail 21.When equipped with a shut-off valve 46, the operating method may include shutting off the gas fuel supply and pressure control devices 16 from the gaseous fuel common rail 21 in the limp-home mode, but not in the regular mode.

[0029] Now, in particular, with reference to Fig.6, an exemplary fuel supply transition control algorithm 160 is shown in accordance with the present disclosure. The algorithm starts at oval box 161 and proceeds to block 162 for limp home mode operation. At block 163, the isolation valve 46 is closed. At block 164, the liquid fuel line pressure is maintained high, such as perhaps 80 MPa. At query 166, the logic determines whether to transition to regular mode. If not, the logic loops back to block 162 to continue operation in limp home mode. If query 166 is affirmative, the logic proceeds to block 167, where the isolation valve 46 is opened. Next, the gas fuel supply and pressure control devices are activated at block 168 to begin supplying the gaseous fuel to the common rail 21.At block 169, the gaseous fuel line pressure is increased. At block 170, the liquid fuel line pressure is decreased. For example, the gaseous fuel common rail 21 pressure may be increased to or slightly above its regular operating pressure of perhaps 35 MPa. Alternatively, the liquid fuel line pressure may begin to be gradually decreased from 80 MPa to the regular mode pressure, which may be equivalent to 40 MPa. Depending on hardware or components, preferences, and other considerations, the liquid fuel line pressure may be maintained at its high level and not decreased until later in the transition. At block 171, the electronic control device 15 determines and transmits the liquid fuel injection control signals.Additionally, the logic at block 172 will determine and transmit the liquid fuel vent control signals for the gas side of the fuel system 20. For example, the electronic controller 15 may transmit a gaseous fuel injection control signal to open the gas nozzle outlet set 103 to inject a small amount of accumulated liquid diesel fuel into the gas nozzle chamber 115. The duration of these ejection injections may be relatively short, so that the injection quantity is small, but may be of sufficient duration to directly or indirectly detect whether liquid or gaseous fuel has been injected from the gas nozzle outlet sets 103.This detection may occur at block 173, such as by monitoring engine speed and / or torque in response to combustion events associated with the injection of the accumulated liquid fuel, as from block 172. At query 174, the logic asks whether the injection of the gaseous fuel has been confirmed. In other words, if detection step 173 detects that gaseous fuel is now being injected instead of the liquid diesel, query 174 will return an affirmative response. On the other hand, if the logic continues to detect liquid fuel injection of the remaining accumulated liquid diesel fuel, query 174 will return a negative response and return to block 169 to continue increasing the gaseous fuel line pressure.The portion of logic 169-174 may continue with small, short-duration injections until the logic confirms that all accumulated liquid diesel fuel has been expelled from the gas nozzle chambers 115 of all fuel injectors 25. In parallel, the engine will continue to be driven at a desired level by relatively large injections of liquid diesel fuel from the liquid nozzle outlet sets 104, as if the engine were continuing to operate essentially in limp-home mode.

[0030] While the transition may occur at a low load or perhaps even at an idle condition, the logic may be prepared to continue operation equally at slightly increased load and speed conditions. After successfully confirming that the accumulated liquid diesel fuel has been injected and that the gaseous fuel is now being injected from the gas nozzle outlet sets 103, the logic will proceed to block 175 and begin replacing the liquid fuel with the gaseous fuel necessary to maintain the desired engine speed and load. At block 176, the logic will adjust the liquid fuel pressure to a medium-high pressure, such as 40 MPa, associated with the regular operating mode.At block 177, the gaseous fuel line pressure is adjusted to a medium-low level, such as 35 MPa, associated with the regular mode. At block 178, the electronic control device 15 will determine and transmit the liquid fuel injection control signals. At block 179, the electronic control device will determine and transmit the gaseous fuel injection control signals. During this portion of the logic, and in accordance with block 175, the ratio of gaseous fuel injection to liquid fuel injection may increase in successive engine cycles, each time query 180 returns with a negative result, and the logic returns to block 175.At query 180, the logic will determine whether the replacement of the liquid fuel with the gaseous fuel has reached a level where the liquid fuel injection amount corresponds to the regular mode level. In other words, during the regular mode, only a small pilot injection amount of liquid diesel may be desired to ignite a much larger charge of gaseous fuel, which provides the majority of the engine's fuel supply during the regular operating mode. If query 180 is affirmative, the logic proceeds to block 181, and the engine is returned to normal control and is now operating in the regular mode. The engine may be considered derated when operating in the limp home mode, as full rated engine power may not be available.At block 182, the logic will maintain the liquid fuel line pressure at a medium-high level, and at block 183, the gaseous fuel line pressure will be maintained at a medium-low level. Thus, when the common rail fuel system 20 is operating in a limp home mode, the ratio of the liquid fuel common rail pressure to the gaseous fuel common rail pressure will be high. On the other hand, when the system 20 is operating in the regular mode, the ratio of the liquid common rail pressure to the gas common rail pressure will be maintained low.At block 184, the logic will determine and communicate the liquid fuel injection control signals, such as the small pilot injection quantities necessary to ignite the larger charge of gaseous fuel by compression, which will be provided in response to the determination and communication of the gaseous fuel injection control signals at block 185. The logic ends at oval box 186.

Claims

[1] A method of operating a dual-fuel engine (10) comprising the following steps: Operating a dual-fuel system in a regular mode; Operating the dual-fuel system in an emergency mode; Accumulating liquid fuel in a gas nozzle chamber (115) of each fuel injector (25) of the dual fuel system (20) over a plurality of engine cycles when operating in limp home mode; Transition from emergency mode to regular mode by displacing the accumulated liquid fuel with gaseous fuel; Injecting the liquid fuel from a first nozzle outlet set (103) and the gaseous fuel from a second nozzle outlet set (104) into an engine cylinder (12) when the dual-fuel system is operated in a regular mode; Injecting the liquid fuel from a first nozzle outlet set (103) but not injecting the gaseous fuel from a second nozzle outlet set (104) into an engine cylinder (12) when the dual-fuel system is operated in the limp-home mode; and Injecting liquid fuel from a first nozzle outlet set (103) and injecting the accumulated liquid fuel from the second nozzle outlet set (104) into the engine cylinder (12) during the transition step. [2] The method of claim 1, wherein the transitioning step comprises confirming that the accumulated liquid fuel has been injected before completing the transitioning step; wherein the confirming step comprises detecting the injection of the gaseous fuel from the second nozzle outlet set (104). [3] The method of claim 2, wherein the determining step comprises measuring the engine speed and / or engine torque response to combustion events in each of the engine cylinders (12). [4] A method according to claim 1, comprising the following steps: Maintaining a high ratio of liquid common rail pressure to gas common rail pressure when operating the dual-fuel system in limp home mode; Maintaining a low ratio of liquid common rail pressure to gas common rail pressure when operating the dual-fuel system in regular mode. [5] The method of claim 1, wherein the transition step comprises: Opening a shut-off valve (46) fluidly separating a gas fuel supply and pressure control system (16) from the gas common rail (21); and Increasing the pressure in the gas fuel common rail (21). [6] The method of claim 1, wherein the transitioning step comprises substituting the liquid fuel injection with the gaseous fuel injection for fueling each of the engine cylinders (12); wherein the substituting step comprises increasing a ratio of the injection of the gaseous fuel to the injection of the liquid fuel in successive engine cycles. [7] The method of claim 1, wherein the transition step comprises lowering the pressure in a common rail (22) for the liquid fuel. [8] Dual-fuel common rail system (20) comprising: a common rail (21) for gaseous fuel; a common rail (22) for liquid fuel; a plurality of fuel injectors (25) each fluidly connected to both the gaseous fuel common rail (21) and the liquid fuel common rail (22); a liquid fuel supply and pressure control system (17) fluidly connected to the liquid fuel common rail (22); a gas fuel supply and pressure control system (16) fluidly connected to the gaseous fuel common rail (21); an electronic control device (15) in control communication with the plurality of fuel injectors (25), the liquid fuel supply and pressure control system (17), and the gaseous fuel supply and pressure control system (16), and having a limp home algorithm configured to transmit liquid injection control signals to inject liquid fuel from a first nozzle outlet set (103) and gaseous injection control signals to inject liquid fuel from a second outlet set (104), and having a regular algorithm configured to transmit liquid injection control signals to inject liquid fuel from the first nozzle outlet set (103), and further having a transition algorithm (160) configured to generate gaseous injection control signals to reduce accumulated,injecting liquid fuel into a gas nozzle chamber (115) of each of the fuel injectors (25) from the second outlet set (104). [9] The dual fuel common rail system (20) of claim 8, wherein the limp home algorithm is configured to maintain a ratio of the liquid common rail pressure to the gas common rail pressure high; wherein the regular algorithm is configured to keep the ratio of the liquid common rail pressure to the gas common rail pressure low; wherein a check valve (66, 67) is operatively arranged to block movement of the liquid fuel from each of the plurality of fuel injectors (25) to the gas common rail (21); and wherein an electronically controlled shut-off valve (46) is operatively arranged between the gas fuel supply and pressure control system (16) and the gas common rail (21); and wherein the transition algorithm (160) is configured to generate a shutoff valve opening signal to open the shutoff valve (46) and to generate a gas line pressure control signal to increase the pressure in the common rail (21) for the gaseous fuel. [10] The dual-fuel common rail system (20) of claim 9, wherein the transition algorithm (160) is also configured to change successive liquid injection control signals and gas injection control signals to replace the injection of the liquid fuel with the injection of the gaseous fuel for each of the fuel injectors (25); wherein the transition algorithm (160) is also configured to change successive liquid injection control signals and gas injection control signals to increase a ratio of gaseous fuel injection to liquid fuel injection in successive engine cycles; wherein the transition algorithm (160) is also configured to generate a liquid rail pressure control signal to reduce the pressure in the common rail (22) for the liquid fuel; and wherein the transition algorithm (160) is also configured to detect an injection of gaseous fuel from the second nozzle outlet set (104) for each of the fuel injectors (25).

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