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Title:
METHOD AND APPARATUS FOR CONTROLLING A DUAL FUEL ENGINE BETWEEN OPERATING MODES
Document Type and Number:
WIPO Patent Application WO/2016/095044
Kind Code:
A1
Abstract:
During transition between operating modes on a dual fuel engine, when the engine switches from operating with one fuel to operating with another fuel, there has been the problem of maintaining engine torque within a predetermined range. An improved method for operating an internal combustion engine comprises selecting between operating in a liquid fuel mode with a liquid fuel and a dual fuel mode with a liquid fuel and a gaseous fuel; adjusting an air system to set-points associated with one of the dual fuel mode when transitioning to the dual fuel mode and the liquid fuel mode when transitioning to the liquid fuel mode, as a function of engine operating conditions; and adjusting at least one of liquid fuel injection quantity and liquid fuel injection timing in response to changes in the air system when transitioning to one of the dual fuel mode and the liquid fuel mode, as a function of engine operating conditions.

Inventors:
CARVALHO, Steed (4 Homer Street, Vancouver, British Columbia V6B 0B1, V6B 0B1, CA)
LEE, Kevin Derek (1-3470 Highland Drive, Coquitlam, British Columbia V3E 0M1, V3E 0M1, CA)
GHAZI, Ahmad (308-937 West 14th Avenue, Vancouver, British Columbia V5Z 1R3, V5Z 1R3, CA)
Application Number:
CA2015/051336
Publication Date:
June 23, 2016
Filing Date:
December 16, 2015
Export Citation:
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Assignee:
WESTPORT POWER INC. (1 West 75th Avenue, Vancouver, British Columbia V6P 6G2, V6P 6G2, CA)
International Classes:
F02B45/10; F02B69/04; F02D19/06; F02D19/08; F02D19/10; F02D41/40; F02M25/00
Foreign References:
US20100332106A12010-12-30
US20040139944A12004-07-22
US20070000456A12007-01-04
US20020007816A12002-01-24
Attorney, Agent or Firm:
WESTPORT POWER INC. (1 West 75th Avenue, Vancouver, British Columbia V6P 6G2, V6P 6G2, CA)
Download PDF:
Claims:
What is claimed is:

1. A method for operating an internal combustion engine comprising: selecting between operating in a liquid fuel mode with a liquid fuel and a dual fuel mode with the liquid fuel and a gaseous fuel; adjusting an air system to set-points associated with one of the dual fuel mode when transitioning to the dual fuel mode and the liquid fuel mode when transitioning to the liquid fuel mode, as a function of engine operating conditions; and adjusting at least one of liquid fuel injection quantity and liquid fuel injection timing in response to changes in the air system when transitioning to one of the dual fuel mode and the liquid fuel mode, as a function of engine operating conditions.

2. The method of claim 1, when transitioning to the dual fuel mode, further comprising: injecting the gaseous fuel as a function of engine operating conditions after the air system has been adjusted to set-points associated with the dual fuel mode; and adjusting liquid fuel injection quantity and timing as functions of engine operating conditions to dual fuel set-points.

3. The method of claim 2, wherein after the air system has been adjusted to dual fuel set-points, gaseous fuel injection quantity and liquid fuel injection quantity are adjusted in one engine cycle to dual fuel set-points as a function of engine operating conditions.

4. The method of claims 1, 2 or 3 when in the liquid fuel mode, further comprising: checking predetermined enabling conditions for operating in the dual fuel mode; and transitioning to the dual fuel mode when the predetermined enabling conditions are met.

5. The method of claim 4, wherein the predetermined enabling conditions comprise at least one of: gaseous fuel storage pressure above a predetermined minimum storage pressure; liquefied gaseous fuel level above a predetermined minimum level; gaseous fuel rail pressure above a predetermined minimum rail pressure; engine diagnostics indicating a gaseous fuel system is operational; and an operating point of the internal combustion engine is within a dual fuel operating region.

6. The method of claim 5, wherein the dual fuel operating region is defined by at least one of a predetermined minimum engine torque and a predetermined minimum engine speed.

7. The method of any one of claims 1 to 6, when in the dual fuel mode, further comprising: checking predetermined enabling conditions for operating in the dual fuel mode; and transitioning to the liquid fuel mode when the predetermined enabling conditions are not met.

8. The method of any one of claims 1 to 7, when transitioning to the liquid fuel mode, further comprising: - 17 - stopping gaseous fuel injection simultaneously with adjusting the air system to the set-points associated with liquid fuel mode; and adjusting liquid fuel injection quantity and timing to reduce torque disturbances.

9. The method of any one of claims 1 to 8, further comprising adjusting at least one of a throttle, a wastegate valve, an exhaust gas recirculation valve and a turbo air bypass valve when adjusting the air system.

10. The method of any one of claims 1 to 8, further comprising adjusting in parallel at least two of a throttle, a wastegate valve, an exhaust gas recirculation valve and a turbo air bypass valve when adjusting the air system. 11. The method of any one of claims 1 to 8, further comprising adjusting sequentially at least two of a throttle, a wastegate valve, an exhaust gas recirculation valve and a turbo air bypass valve when adjusting the air system.

12. The method of claim 9, wherein the at least one of the throttle, the wastegate valve, the exhaust gas recirculation valve and the turbo air bypass valve can be adjusted between a fully closed position and a fully open position.

13. The method of any one of claims 1 to 12, wherein an air/fuel ratio in liquid fuel mode is between a range of 1.5 and 3.0 and an air/fuel ratio in dual fuel mode is between a range of 1.0 and 1.15.

14. The method of any one of claims 1 to 13, wherein the liquid fuel comprises diesei. 15. The method of any one of claims 1 to 14, wherein the gaseous fuel comprises methane.

16. An apparatus for an internal combustion engine comprising: a combustion chamber; a liquid fuel supply; - 18 - a direct fuel injector in fluid communication with the liquid fuel supply and directly introducing liquid fuel into the combustion chamber; a gaseous fuel supply; a gaseous fuel injector in fluid communication with the gaseous fuel supply and introducing gaseous fuel into the combustion chamber; an air system in fluid communication with the combustion chamber and comprising at least one of a throttle, a wastegate valve, an exhaust gas recirculation valve and a turbo air bypass valve; and a controller operatively connected with the direct fuel injector, the gaseous fuel injector and the at least one of the throttle, the wastegate valve, the exhaust gas recirculation valve and the turbo air bypass valve and programmed to: operate selectively in a liquid fuel mode and a dual fuel mode; adjust an air system to set-points associated with one of the dual fuel mode when transitioning to the dual fuel mode and the liquid fuel mode when transitioning to the liquid fuel mode, as a function of engine operating conditions; and adjust at least one of liquid fuel injection quantity and liquid fuel injection timing in response to changes in the air system when transitioning to one of the dual fuel mode and the liquid fuel mode to reduce torque disturbances, as a function of engine operating conditions.

17. The apparatus of claim 16, wherein the gaseous fuel injector injects gaseous fuel into an intake port associated with the combustion chamber.

18. The apparatus of claims 16 or 17, wherein, when transitioning to the dual fuel mode, the controller is further programmed to: actuate the gaseous fuel injector to inject the gaseous fuel as a function of engine operating conditions after the air system has been adjusted to set-points associated with the dual fuel mode; and adjust liquid fuel injection quantity and timing as functions of engine operating conditions to dual fuel set-points. 19. The apparatus of claims 16, 17 or 18, wherein, when transitioning to the liquid fuel mode, the controller is further programmed to: stop actuating the gaseous fuel injector simultaneously with adjusting the air system to the set-points associated with liquid fuel mode; and actuate the liquid fuel injector to adjust liquid fuel injection quantity and timing to reduce torque disturbances.

20. The apparatus of claims 16, 17, 18 or 1 , wherein the controller is further programmed to: check predetermined enabling conditions for operating in the dual fuel mode; and transition to the dual fuel mode when the predetermined enabling conditions are met and to the liquid fuel mode when the predetermined enabling conditions are not met.

Description:
METHOD AND APPARATUS FOR CONTROLLING A DUAL

FUEL ENGINE BETWEEN OPERATING MODES

Field of the Invention

[0001] The present application relates to controlling the transition between fuelling modes of a dual fuel internal combustion engine.

Background of the Invention

[0002] Dual fuel engines are internal combustion engines that are simultaneously fuelled with two fuels in most operating conditions. An example of a dual fuel engine is one that is fuelled with natural gas serving as a main fuel and diesel serving as a pilot fuel that is compression ignited to ignite the main fuel. On average, measured on an energy basis when operating in a dual fuel mode, more main fuel is consumed compared to the pilot fuel. It is known to operate such a dual fuel engine in different fuelling modes, including a diesel mode when the engine is fuelled only with diesel fuel. In both modes the engine can operate over a range of engine operating conditions including high load.

[0003] The combustion performance of natural gas is different than diesel for a variety of reasons. The manner in which the two fuels are introduced into the combustion chamber and how they mix with air are different since natural gas is a compressible fluid where diesel is an incompressible liquid. When diesel fuel is injected it absorbs heat within the combustion chamber and evaporates, lowering the overall temperature of the charge in the combustion chamber. Natural gas and diesel have different heating values. A different quantity by mass of natural gas needs to be burned in the combustion chamber compared to diesel to obtain the same amount of heat from combustion. For equivalent amounts of fuel by energy, the heat release rate of an engine burning natural gas is different than an engine burning diesel, such that the rate of cylinder pressure increase during combustion varies between these fuels. Different air/fuel ratios (lambda) are employed when fuelling with natural gas compared to fuelling with diesel to obtain equivalent brake mean effective pressure (BMEP) with comparable emissions. Due to the many factors that affect combustion performance, it is a challenge to switch from operating on diesel to operating on natural gas without the driver noticing any disturbances in torque and without significant changes in emission levels.

[0004] United States Patent No. 7,913,673, issued March 29, 2011 to Vanderslice et al., discloses a method of controlling liquid fuel delivery during transition between modes for a dual fuel engine. Referring to FIG. 4 in Vanderslice, a process is illustrated for transitioning from a diesel only mode to a diesel-pilot ignited, natural gas fuelling mode, simply referred to as pilot mode. Curves 100, 102 represent the quantity of diesel fuel and natural gas respectively being supplied to the engine as a function of time. Diesel fuel is initially supplied at a quantity QDIESEL DEM required for the prevailing speed and load conditions, and the transition to pilot mode occurs at time Tl when the gaseous fuel supply quantity is increased immediately from zero to the final quantity QGAS DEM required for prevailing speed and load conditions. While the gas supply upstream of the inlet of air intake manifold 34 increases essentially immediately to QGASDEM > gas lambda in the cylinders 121-126 does not decrease immediately due to the fact that it takes some time for the introduced gas to reach the cylinders. The delay period (T2-T1) depends on the instantaneous speed and load conditions, and on the physical geometry of the engine. In addition, the delay period will depend on the location in the engine's firing sequence when gas switchover occurs. Vanderslice determines the delay period empirically for a full set of engine operating conditions, including a full set of speed and load conditions. After the gas reaches the cylinders at time T3, the commanded quantity of diesel is decreased in a calibrated number of steps to a pilot quantity QDIESEL PILO T- With this method the overall quantity of fuel on an energy basis in the cylinders can increase during transition, resulting in perceptible torque disturbances. In the technique of Vanderslice, it is difficult to control the overall air/fuel ratio and the overall quantity of fuel on an energy basis within the engine cylinders during transition between modes, making it difficult to control torque and emissions during transitions. [0005] The state of the art is lacking in techniques for improving transitions between operating modes in dual fuel engines. The present method and apparatus provide a technique for improving emissions and reducing torque disturbances during transitions between operating modes in dual fuel internal combustion engines.

Summary of the Invention [0006] An improved method for operating an internal combustion engine comprises selecting between operating in a liquid fuel mode with a liquid fuel and a dual fuel mode with a liquid fuel and a gaseous fuel; adjusting an air system to set- points associated with one of the dual fuel mode when fransitioning to the dual fuel mode and the liquid fuel mode when transitioning to the liquid fuel mode, as a function of engine operating conditions; and adjusting at least one of liquid fuel injection quantity and liquid fuel injection timing in response to changes in the air system when transitioning to one of the dual fuel mode and the liquid fuel mode, as a function of engine operating conditions.

[0007] When in the liquid fuel mode, the technique further comprises checking predetermined enabling conditions for operating in the dual fuel mode; and transitioning to the dual fuel mode when the predetermined enabling conditions are met. Predetermined enabling conditions comprise at least one of gaseous fuel storage pressure above a predetermined minimum storage pressure; liquefied gaseous fuel level above a predetermined minimum level; gaseous fuel rail pressure above a predetermined minimum rail pressure; engine diagnostics indicating a gaseous fuel system is operational; and an operating point of the internal combustion engine is within a dual fuel operating region. The dual fuel operating region can be defined by at least one of a predetermined minimum engine torque and a predetermined minimum engine speed. When transitioning to the dual fuel mode, the technique further comprises injecting the gaseous fuel as a function of engine operating conditions after the air system has been adjusted to set-points associated with the dual fuel mode; and adjusting liquid fuel injection quantity and timing as functions of engine operating conditions to dual fuel set-points. Preferably, after the air system has been adjusted to dual fuel set-points, gaseous fuel injection quantity and liquid fuel injection quantity are adjusted in one engine cycle to dual fuel set-points as a function of engine operating conditions.

[0008] When in the dual fuel mode, the technique further comprises checking predetermined enabling conditions for operating in the dual fuel mode; and transitioning to the liquid fuel mode when the predetermined enabling conditions are not met. When transitioning to the liquid fuel mode, the technique further comprises stopping gaseous fuel injection simultaneously with adjusting the air system to the set-points associated with liquid fuel mode; and adjusting liquid fuel injection quantity and timing to reduce torque disturbances. [0009] The air system is adjusted by adjusting at least one of a throttle, a wastegate valve, an exhaust gas recirculation valve and a turbo air bypass valve. The at least one of the throttle, the wastegate valve, the exhaust gas recirculation valve and the turbo air bypass valve can be adjusted in parallel. Alternatively, the at least one of the throttle, the wastegate valve, the exhaust gas recirculation valve and the turbo air bypass valve can be adjusted sequentially. The at least one of the throttle, the wastegate valve, the exhaust gas recirculation valve and the turbo air bypass valve can be adjusted between a fully closed position and a fully open position, and anywhere inbetween.

[0010] In the liquid fuel mode, an air/fuel ratio is between a range of 1.5 and 3.0, and in the dual fuel mode, the air/fuel ratio is between a range of 1.0 and 1.15. In an exemplary embodiment, the liquid fuel comprises diesel and the gaseous fuel comprises methane.

[0011] An improved apparatus for an internal combustion engine comprises a combustion chamber; a liquid fuel supply and a gaseous fuel supply, There is a direct fuel injector in fluid communication with the liquid fuel supply and directly introducing liquid fuel into the combustion chamber; and a gaseous fuel injector in fluid communication with the gaseous fuel supply and introducing gaseous fuel into the combustion chamber. An air system is in fluid communication with the combustion chamber and comprises at least one of a throttle, a wastegate valve, an exhaust gas recirculation valve and a turbo air bypass valve. A controller is operatively connected with the direct fuel injector, the gaseous fuel injector and the at least one of the throttle, the wastegate valve, the exhaust gas recirculation valve and the turbo air bypass valve. The controller is programmed to operate selectively in a liquid fuel mode and a dual fuel mode; adjust an air system to set-points associated with one of the dual fuel mode when transitioning to the dual fuel mode and the liquid fuel mode when transitioning to the liquid fuel mode, as a function of engine operating conditions; and adjust at least one of liquid fuel injection quantity and liquid fuel injection timing in response to changes in the air system when transitioning to one of the dual fuel mode and the liquid fuel mode to reduce torque disturbances, as a function of engine operating conditions.

[0012] When transitioning to the dual fuel mode, the controller is further programmed to actuate the gaseous fuel injector to inject the gaseous fuel as a function of engine operating conditions after the air system has been adjusted to set- points associated with the dual fuel mode; and adjust liquid fuel injection quantity and timing as functions of engine operating conditions to dual fuel set-points.

[0013] When transitioning to the liquid fuel mode, the controller is further programmed to stop actuating the gaseous fuel injector simultaneously with adjusting the air system to the set-points associated with liquid fuel mode; and actuate the liquid fuel injector to adjust liquid fuel injection quantity and timing to reduce torque disturbances.

[0014] In an exemplary embodiment, the gaseous fuel injector injects gaseous fuel into an intake port associated with the combustion chamber. In other embodiments, the gaseous fuel injector can be configured to inject gaseous fuel directly into the combustion chamber, either early or late in the compression stroke. In yet another embodiment, the liquid fuel injector and the gaseous fuel injector can be integrated side-by-side into a common injector body, or can be integrated as a concentric needle fuel injector, as is known to those familiar with the technology. The controller is further programmed to check predetermined enabling conditions for operating in the dual fuel mode; and transition to the dual fuel mode when the predetermined enabling conditions are met and to the liquid fuel mode when the predetermined enabling conditions are not met.

Brief Description of the Drawings [0015] FIG. 1 is a schematic view of an internal combustion engine according to one embodiment.

[0016] FIG. 2 is flowchart view of a transition technique between a liquid fuel mode and a dual fuel mode for the internal combustion engine of FIG. 1.

[0017] FIG. 3 is flowchart view of a transition technique between a dual fuel mode and a liquid fuel mode for the internal combustion engine of FIG. 1.

[0018] FIG. 4 is a graphical view of an engine map for the internal combustion engine of FIG. 1.

Detailed Description of Preferred Embodiment(s) [0019] Referring to the schematic view of FIG. 1, dual fuel engine 10 is illustrated according to one embodiment for which the technique of transitioning between operating modes described herein can be performed. This embodiment is illustrative of an application of the transition technique and is by no way limiting. Liquid fuel supply 20 delivers liquid fuel to liquid fuel rail 30 at a pressure suitable for late cycle direct injection into combustion chambers 50a through 50f (collectively 50a-f). Direct injectors 40a through 40f (collectively 40a-f) are in fluid communication with the liquid fuel rail and introduce liquid fuel into respective combustion chambers 50a-f when commanded by electronic controller 60. Liquid fuel system 55 comprises liquid fuel supply 20, liquid fuel rail 30 and direct injectors 40a-f. Gaseous fuel supply 70 delivers gaseous fuel to gaseous fuel rail 80 at a pressure suitable for introduction into respective intake ports of combustion chambers 50a-f. In an exemplary embodiment gaseous fuel is stored as a compressed gas in pressurized storage vessels, or cylinders. In other embodiments the gaseous fuel can be stored in liquefied form in a cryogenic storage vessel. Port injectors 90a through 90f (collectively 90a-f) are in fluid communication with the gaseous fuel rail and introduce gaseous fuel upstream of intake valves associated with respective combustion chambers 50a-f when commanded by electronic controller 60. In other embodiments a single point injection system or mixer can be employed instead of port injectors 90a-f, or alternatively gaseous fuel can be directly introduced into combustion chambers 50a-f by way of direct gaseous fuel injectors. Temperature sensor 82 and pressure sensor 84 send signals to electronic controller 60 representative of gaseous fuel temperature and pressure respectively in gaseous fuel rail 80, such that gaseous fuel density can be determined to more accurately meter gaseous fuel through port injectors 90a-f. Alternatively, in other embodiments a sensor comprising both a temperature sensor and pressure sensor can be employed. Gaseous fuel system 95 comprises gaseous fuel supply 70, gaseous fuel rail 80 and port injectors 90a-f. In the illustrated embodiment there are six cylinders; however this is merely an application example, and in other embodiments there can be a different number of cylinders.

[0020] In an exemplary embodiment the liquid fuel is diesel and the gaseous fuel is natural gas. In a liquid mode of operation only liquid fuel is burned in combustion chambers 50a-f, and electronic controller 60 commands direct injectors 40a-f to inject fuel accordingly. As used herein liquid fuel mode is used synonymously with diesel mode, which refers to the preferred embodiment when the liquid fuel is diesel fuel. In a dual fuel mode of operation, a main quantity of gaseous fuel (for example natural gas) and a pilot quantity of liquid fuel (for example diesel) are burned in combustion chambers 50a-f, and electronic controller 60 commands port injectors 90a-f and direct injectors 40a-f to inject fuel accordingly. The pilot quantity of diesel is compression ignited and is employed to ignite the main quantity of natural gas, which provides the majority of power generated by engine 10. In preferred embodiments, under most operating conditions, the pilot quantity of diesel is between 1% and 30%, and preferably between 1% and 10%, of total fuel on an energy basis burned by dual fuel engine 10 in dual fuel mode.

[0021] Intake air is received in intake manifold 100 through throttle 110. In the diesel mode of operation engine 10 operates with a lean air/fuel ratio (lambda) whereby throttle 110 is normally fully open. In the dual fuel mode engine 10 operates at or around stoichiometric air/fuel ratios as a function of engine operating conditions such that throttle 110 is in a range of positions from nearly closed to fully open depending on the engine operating conditions. To improve power, turbocharger system 120 can boost the pressure in intake manifold 100 above ambient pressure to increase the mass of oxygen delivered to combustion chambers 50a-f during intake strokes. Pressurized exhaust gases from exhaust manifold 130 cause turbine 140 to spin that in turn drives compressor 150 to compress intake air to a desired level of boost. Compressed intake air leaving the outlet of compressor 150 is elevated in temperature as a result of compression, compared to fresh intake air, and is communicated through charge air cooler 155 where it is cooled. Turbo air bypass (TAB) valve 160 can be actuated by electronic controller 60 between a closed position and an open position, and anywhere therebetween, to communicate intake air upstream of throttle 110 back to the inlet of compressor 150 to reduce the likelihood of compressor surge that could be caused by backfiow coming from throttle 110 during transient conditions. Similarly, wastegate valve 170 can be actuated by electronic controller 60 between a closed position and an opened position, and to anywhere in between these positions, to allow at least a portion of exhaust gases to bypass turbine 140.

[0022] During certain engine operating conditions, a portion of exhaust gases can be communicated to intake manifold 100 through exhaust gas recirculation (EGR) system 180. Controller 60 commands EGR valve 190 between a closed position and an open position, and anywhere therebetween, to control the EGR mass flow rate through EGR system 180, for a given back pressure in exhaust manifold 130. EGR cooler 200 reduces the temperature of exhaust gasses to protect the intake manifold and to lower in-cylinder temperatures. In the illustrated embodiment at least a portion of exhaust gases from all cylinders 50a-f can be recirculated to intake manifold 100. In other embodiments any other EGR architecture can be employed, such as those that dedicate one or more cylinders, or one or more exhaust ports, to EGR. Exhaust gases not passing through EGR system 180 are communicated through turbine 140 and/or wastegate valve 170 to engine after-treatment system 210. As used herein air system 195 comprises throttle 110, wastegate valve 170, EGR valve 190 and TAB valve 160, and each one of these valve components can be in a closed, opened and a range of partially opened positions. Engine 10 comprises other conventional components which are not illustrated. Engine 10 can be for a vehicle, and can also be employed in marine, locomotive, mine haul, power generation or stationary applications. [0023] Electronic controller 60 receives sensor information from conventional sensors employed in internal combustion engines, generally represented by input 220, and commands conventional actuators, generally represented by output 230. The actuators comprise those that actuate throttle 110, TAB valve 160, wastegate valve 170, EGR valve 190, in addition to other actuators. The controller also commands liquid fuel supply 20 and direct injectors 90a-f, and gaseous fuel supply 70 and port injectors 40a-f to introduce fuel into combustion chambers 50a-f. For clarity not all electrical connections with the controller are illustrated in FIG. 1. Electronic controller 60 can comprise both hardware and software components. The hardware components can comprise digital and/or analog components. In the illustrated embodiment electronic controller 60 is a computer comprising a processor and memories, including one or more permanent memories, such as FLASH, EEPROM and a hard disk, and a temporary memory, such as SRAM or DRAM, for storing and executing a program. In another preferred embodiment electronic controller 60 is an engine control unit (ECU) for engine 10. In still another preferred embodiment, controller 60 can comprise two or more controllers, such as a base engine control unit for controlling liquid fuel system 55 and a gaseous fuel controller for controlling gaseous fuel system 95, and all such controllers can comprise communication links therebetween for exchanging information, such as commands and status information. As used herein, controller 60 is also referred to as 'the controller'. [0024] Referring now to FIG. 2, a method of transitioning between diesel mode and dual fuel mode is described. The method is programmed into and carried out by electronic controller 60. In step 300, the initial condition, engine 10 is operating in diesel mode. Before transitioning to dual fuel mode gaseous fuel system 95 must be operational and engine operation in dual fuel mode must be possible under the given engine operating conditions. Predetermined enabling conditions are checked in step 310 before the transition begins. There are a variety of predetermined enabling conditions including gaseous fuel storage pressure, liquefied gaseous fuel level, gaseous fuel rail pressure, engine diagnostics and the current operating point of engine 10 in the engine map. Gaseous fuel storage pressure is an indication of the amount of gaseous fuel that is currently stored in gaseous fuel supply 70 when the gaseous fuel is stored as a compressed gas, such as compressed natural gas (CNG). If gaseous fuel storage pressure is below a predetermined minimum storage pressure then gaseous fuel supply 70 cannot supply gaseous fuel at the required pressure for engine 10 to operate, and/or does not have an adequate supply of gaseous fuel for the engine to operate in dual fuel mode for a predetermined amount of time. Similarly, if gaseous fuel rail pressure is below a predetermined minimum rail pressure then engine 10 cannot operate under a full range of engine operating conditions. It is possible to enter dual fuel mode with the performance of engine 10 derated due to gaseous fuel rail pressure or gaseous fuel storage pressure conditions, but this is generally not preferable. When the gaseous fuel is stored in liquefied form in a cryogenic storage vessel, the level of the liquefied gaseous fuel in the cryogenic storage vessel is an indication of the amount of gaseous fuel that is currently stored. If liquefied gaseous fuel level in the cryogenic storage vessel is below a predetermined minimum level then gaseous fuel supply 70 cannot supply gaseous fuel at the required pressure of engine 10 to operate for the predetermined amount of time. Any engine diagnostics that indicate the gaseous fuel system is not operating within normal operational parameters can prevent a transition to dual fuel mode. These diagnostics can provide information related to gaseous fuel storage or rail pressure conditions, in addition to the operational status of components within the gaseous fuel system. With reference to FIG. 4, engine 10 preferably operates in dual fuel mode in region 25 of engine map 15, which is defined by a predetermined minimum torque T m j n and a predetermined minimum engine speed n m j n ; however, it is possible that engine 10 can operate in dual fuel mode at any operating point in engine map 15. For commanded engine torques below T m j n and engine speeds below n m j n the emissions generated if operating in dual fuel mode are above predetermined maximum emission levels at least for one constituent of the emissions. Additionally, for commanded torques below T m i ft the injection accuracy of port injectors 90a-f is below a predetermined minimum injection accuracy such that gaseous fuel mass flow cannot be accurately controlled during gaseous fuel injection. The transition to dual fuel mode begins when all the enabling conditions are met. [0025] In step 320, the first step in the transition to dual fuel mode, controller 60 gradually moves air system 195 from the set-points employed in diesel mode to the set-points employed in dual fuel mode, under the current engine operating conditions. The air/fuel ratio in diesel mode is lean compared to the air/fuel ratio employed in dual fuel mode, and air system 195 is adjusted to change the air/fuel ratio in this manner. In an exemplaiy embodiment, the air/fuel ratio in diesel mode is within a range of 1.5 and 3.0, and the air/fuel ratio in dual fuel mode is within a range of 1 and 1.15. For each operating point in map 15 (seen in FIG. 4) throttle 110, wastegate valve 170, EGR valve 190 and TAB valve 160 (the air system) each have respective set-points for each mode of operation (diesel mode, dual fuel mode), and during transient operating conditions these components can be adjusted to maintain predetermined engine performance parameters. With respect to the throttle (and similarly for the other air system components), a set-point refers to the amount the throttle is opened (or closed) relative to a fully closed and fully open position. The set-points for each component of air system 195 can be adjusted simultaneously (in parallel), at the same time, or sequentially (in series), one after the other. For example, to avoid a big sudden change in pressure across compressor 150, the wastegate valve can be moved to or towards an opened position first to reduce the speed of turbine 140 and therefore the outlet pressure of the compressor, to avoid surge, before adjusting throttle 110 to reduce the intake mass air flow. While air system 195 is being adjusted, the quantity of diesel fuel injected and the timing at which it is injected is adjusted to compensate for changes in torque. By moving the air system to its dual fuel set-points, (global) lambda is decreased by changing the quantity of air and not the fuelling quantity. The partial pressure of oxygen in the intake charge in the combustion chamber decreases, causing the ignition delay of diesel to increase, delaying start of combustion timing and as a result the diesel burns later, producing less torque. During the transition, diesel fuelling quantity is increased and advanced to compensate for the decrease in torque caused by the changing air system, resulting in reduced torque disturbances.

[0026] Controller 60 waits for air system 195 to be adjusted to the dual fuel mode set-points in step 330. The controller can determine that air system 195 has reached the dual fuel set-points in a variety of ways. For example, controller 60 can determine the air/fuel ratio, by measuring it directly or indirectly, and compare this value to a predetermined air/fuel ratio in dual fuel mode, as a function of engine operating conditions. Alternatively, the controller can determine intake mass air flow and EGR mass flow, by measuring them directly or indirectly, and compare these values to predetermined intake mass air flow and EGR mass flow respectively in dual fuel mode, as a function of engine operating conditions. Once air system 195 is confirmed to be in the dual fuel mode gaseous fuel injection begins and diesel fuel injection is adjusted for pilot injection in step 340. Gaseous fuel injection quantity and timing, and diesel fuel injection quantity and timing, are each set to their respective dual fuel set-points to achieve the commanded torque and emissions targets. Both gaseous fuel and diesel fuel injection quantity and timing can be adjusted simultaneously in one engine cycle, or they can be respectively stepped in a number of engine cycles. Preferably they are adjusted in one cycle. However, in some embodiments it may not be possible to guarantee that both gaseous fuel and diesel fuel injection quantity and timing can be simultaneously adjusted in the same engine cycle such that it is possible for there to be a dangerous over-fuelling condition in the event when gaseous fuelling quantity is increased before diesel fuelling quantity is decreased. In these situations the engine can be protected by stepping the fueling adjustments in smaller discrete amounts. After gaseous and diesel fuelling quantities and timing have been adjusted to dual fuel set-points, engine 10 is in dual fuel mode, represented as state 350 in FIG. 2. By adjusting air system 195 before adjusting liquid and gaseous fuelling to dual fuel mode set-points, there is reduced torque disturbance and speed fluctuation compared to when fuel is adjusted before air system 195 during transitions. As soon as the transition from diesel mode to dual fuel mode is completed, the emissions are immediately at least as good as or better than when in diesel mode.

[0027] Referring now to FIG. 3, a method of transitioning from dual fuel mode to diesel mode is described. The method is programmed into and carried out by electronic controller 60. In step 400, the initial condition, engine 10 is operating in dual fuel mode. Dual fuel mode is the preferred mode of operation, and engine 10 operates in diesel mode if the conditions for operating in dual fuel mode are not met. In the event that the enabling conditions for operating in dual fuel mode are not met while in dual fuel mode, a transition back to diesel mode begins. The enabling conditions for dual fuel mode were heretofore described, and can comprise insufficient gaseous fuel storage pressure or rail pressure, engine diagnostics indicating a condition with gaseous fuel system 95 preventing dual fuel mode, and the operating point of engine 10 moving out of region 25 in engine map 15 (seen in FIG. 4). While in diesel mode the enabling conditions are continuously checked in step 410, and when these conditions are not met the method proceeds to step 420 where the transition back to diesel mode begins.

[0028] In step 420, the first step in the transition to diesel mode, controller 60 moves air system 195 from the set-points employed in dual fuel mode to the set-points employed in diesel mode, under the current engine operating conditions. Simultaneously, gaseous fuel injection is stopped and diesel fuel injection quantity and timing are adjusted to compensate for the efficiency difference to reduce fluctuations of commanded torque. The transition back to diesel mode reverses the steps done to transition from diesel mode to dual fuel mode. The air/fuel ratio in dual fuel mode is lower compared to the air/fuel ratio employed in diesel mode, and air system 195 is adjusted to increase the air/fuel ratio when transitioning to diesel mode. The set-points for each component in air system 195 can be adjusted simultaneously (in parallel), at the same time, or sequentially (in series), one after the other. While air system 195 is being adjusted, the quantity of diesel fuel injected and the timing at which it is injected is adjusted, as the air system changes, to reduce torque fluctuations. Controller 60 waits for air system 195 to be adjusted to diesel mode set- points in step 430. The controller can determine that the air system has reached the diesel set-points in corresponding ways that the controller determines the air system is in dual fuel mode. Once air system 195 is confirmed to be in diesel mode, diesel fuel injection quantity and timing are set to their respective diesel mode set-points to achieve the commanded torque and emissions targets, after which engine 10 is in diesel mode, represented as state 440 in FIG. 3.

[0029] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.