Patent Publication Number: US-9410490-B2

Title: Fuel selection system and method for dual fuel engines

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to dual fuel internal combustion engines, and more particularly is concerned with systems and methods for selecting fuel for operation of a dual fuel internal combustion engine. 
     BACKGROUND 
     A dual fuel engine is an engine that includes a first fuel source that is utilized as the sole fuel source during certain operating conditions and a second fuel source that is integrated with the first fuel source at other operating conditions. In certain applications, the first fuel source is a diesel fuel and the second fuel source is natural gas. The diesel fuel provides, in some cases, the initial, low load levels of operation and is used for ignition for the natural gas at higher load operations. The substitution of natural gas for diesel fuel improves high load performance and emissions reduction, particularly when the engine is employed at locations where natural gas is abundant or available at low cost. 
     When the engine is operating in dual fuel mode, natural gas is introduced into the intake system. The air-to-natural gas mixture from the intake is drawn into the cylinder, just as it would be in a spark-ignited engine, but typically with a leaner air-to-fuel ratio. Near the end of the compression stroke, diesel fuel is injected, just as it would be in a traditional diesel engine. The diesel fuel ignites, and the diesel combustion causes the natural gas to burn. The dual fuel engine combusts a mixture of air and fuel in the cylinders to produce drive torque. A dual fuel engine can operate either entirely on diesel fuel or on the substitution mixture of diesel and natural gas, but generally cannot operate on natural gas alone except where auxiliary spark equipment is provided to the cylinder. 
     Dual fuel engines may encounter conditions during operation where fuelling in a dual fuel mode provides operating conditions that are not desirable. Such conditions may relate to, for example, the availability or quality of the natural gas, the operating parameters of the engine, and the operating parameters of the exhaust aftertreatment system. Thus, there remains a need for additional improvements in systems and methods for dual fuel engine operation and control. 
     SUMMARY 
     Unique systems, apparatus and methods are disclosed for fuel selection in operation of dual fuel engines. In one embodiment, a diesel only fuelling mode of operation of the engine occurs in response to a selection by an operator or service technician of the diesel only fuelling mode. In other embodiments, selection of the diesel only fuelling mode is made automatically and/or in response to at least one of a gaseous fuel supply deficiency indication, a gaseous fuel quality deficiency indication, an engine protection limit violation, and/or an aftertreatment component management requirement. 
     In some embodiments, the gaseous fuel supply deficiency indication is provided in response to at least one of a gaseous fuel supply pressure being less than a lower threshold amount, the gaseous fuel flow rate being less than a commanded gaseous fuel flow rate by more than a threshold amount, and the diesel fuelling substitution rate exceeding an upper threshold amount in response to a torque request of the engine that is fuelled at a commanded natural gas flow rate expected to satisfy the torque request. In other embodiments, the gaseous fuel quality deficiency indication is provided in response to a quality of the gaseous fuel being less than a gaseous fuel quality threshold. In yet other embodiments, the engine protection limit violation is provided in response to a high temperature limit of one or more fuel injectors being exceeded, an engine knock amount exceeding a knock limit, and/or an intake manifold pressure exceeding an upper pressure limit due to, for example, a backfire condition, a high pressure condition of the gaseous fuel supply, a failure of a gaseous fuel supply control valve, and/or failure of the gaseous fuel supply control system. In another embodiment, the aftertreatment component management requirement is provided in response to an indication of deposits of, for example, urea, sulfur, particulates and/or soot on one or more components of the aftertreatment system exceeding an accumulation limit. 
     This summary is provided to introduce a selection of concepts that are further described below in the illustrative embodiments. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a portion of an internal combustion engine system with a dual fuel system. 
         FIG. 2  is another schematic illustration of a part of the internal combustion engine system of  FIG. 1  showing further embodiments of the dual fuel system. 
         FIG. 3  is a schematic of a controller configured to control operation of a dual fuel engine. 
         FIG. 4  is a flow diagram of a procedure for controlling operation of a dual fuel engine. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, any alterations and further modifications in the illustrated embodiments, and any further applications of the principles of the invention as illustrated therein as would normally occur to one skilled in the art to which the invention relates are contemplated herein. 
     With reference to  FIGS. 1 and 2 , an internal combustion engine system  20  is illustrated in schematic form. A fueling system  21  is also shown in schematic form that is operable with internal combustion engine system  20  to provide fueling for engine  30  from a first fuel source  102  and a second fuel source  104 . Internal combustion engine system  20  includes engine  30  connected with an intake system  22  for providing a charge flow to engine  30  and an exhaust system  24  for output of exhaust gases. In certain embodiments, the engine  30  includes a lean combustion engine such as a diesel cycle engine that uses a primary fuel that is a liquid fuel such as diesel fuel and a secondary fuel that is a gaseous fuel such as natural gas. As used herein, gaseous fuel includes, but is not limited to, bio-gas, methane, propane, ethanol, producer gas, field gas, liquefied natural gas, compressed natural gas, or landfill gas. In the illustrated embodiment, the engine  30  includes six cylinders  31   a - 31   f  in an in-line arrangement. However, the number of cylinders (collectively referred to as cylinders  31 ) may be any number, and the arrangement of cylinders  31  may be any arrangement, and is not limited to the number and arrangement shown in  FIG. 1 . 
     Engine  30  includes an engine block  70  that at least partially defines the cylinders  31 . A plurality of pistons (not shown) may be slidably disposed within respective cylinders  31  to reciprocate between a top-dead-center position and a bottom-dead-center position. Each of the cylinders  31 , its respective piston, and the cylinder head form a combustion chamber. In the illustrated embodiment, engine  30  includes six such combustion chambers. However, it is contemplated that engine  30  may include a greater or lesser number of cylinders and combustion chambers and that cylinders and combustion chambers may be disposed in an “in-line” configuration, a “V” configuration, or in any other suitable configuration. 
     In one embodiment, engine  30  is a four stroke engine. That is, for each complete engine cycle (i.e., for every two full crankshaft rotations), each piston of each cylinder  31  moves through an intake stroke, a compression stroke, a combustion or power stroke, and an exhaust stroke. Thus, during each complete cycle for the depicted six cylinder engine, there are six strokes during which air is drawn into individual combustion chambers from intake supply conduit  26  and six strokes during which exhaust gas is supplied to exhaust manifold  32 . 
     The engine  30  includes cylinders  31  connected to the intake system  22  to receive a charge flow and connected to exhaust system  24  to release exhaust gases produced by combustion of the primary and/or secondary fuels. Exhaust system  24  may provide exhaust gases to a turbocharger  46 , although a turbocharger is not required. In still other embodiments, multiple turbochargers are included to provide high pressure and low pressure turbocharging stages that compress the intake flow. 
     Furthermore, exhaust system  24  can be connected to intake system  22  with one or both of a high pressure exhaust gas recirculation (EGR) system  51  and a low pressure EGR system  60 . EGR systems  51 ,  60  may include a cooler  52 ,  62  and bypass  54 ,  64 , respectively. In other embodiments, one or both of EGR systems  51 ,  60  are not provided. When provided, EGR system(s)  51 ,  60  provide exhaust gas recirculation to engine  30  in certain operating conditions. In any EGR arrangement during at least certain operating conditions, at least a portion the exhaust output of cylinder(s)  31  is recirculated to the engine intake system  22 . In the high pressure EGR system  51 , the exhaust gas from the cylinder(s)  31  takes off from exhaust system  24  upstream of turbine  48  of turbocharger  46  and combines with intake flow at a position downstream of compressor  50  of turbocharger  46  and upstream of an intake manifold  28  of engine  30 . In the low pressure EGR system  60 , the exhaust gas from the cylinder(s)  31   a - 31   f  takes off from exhaust system  24  downstream of turbine  48  of turbocharger  46  and combines with intake flow at a position upstream of compressor  50  of turbocharger  46 . The recirculated exhaust gas may combine with the intake gases in a mixer (not shown) of intake system  22  or by any other arrangement. In certain embodiments, the recirculated exhaust gas returns to the intake manifold  28  directly. 
     Intake system  22  includes one or more inlet supply conduits  26  connected to an engine intake manifold  28 , which distributes the charge flow to cylinders  31  of engine  30 . Exhaust system  24  is also coupled to engine  30  with an engine exhaust manifold  32 . Exhaust system  24  includes an exhaust conduit  34  extending from exhaust manifold  32  to an exhaust valve. In the illustrated embodiment, exhaust conduit  34  extends to turbine  48  of turbocharger  46 . Turbine  48  includes a valve such as controllable wastegate  71  or other suitable bypass that is operable to selectively bypass at least a portion of the exhaust flow from turbine  48  to reduce boost pressure and engine torque under certain operating conditions. In another embodiment, turbine  48  is a variable geometry turbine with a size-controllable inlet opening. In another embodiment, the exhaust valve is an exhaust throttle that can be closed or opened. 
     An aftertreatment system  90  can be connected with an exhaust outlet conduit  68 . The aftertreatment system  90  may include, for example, one or more oxidation devices (DOC), particulate removing devices (DPF, CDPF), constituent absorbers or reducers (SCR, AMOX, LNT), reductant systems, and other components if desired. 
     In one embodiment, exhaust conduit  34  is flow coupled to exhaust manifold  32 , and may also include one or more intermediate flow passages, conduits or other structures. Exhaust conduit  34  extends to turbine  48  of turbocharger  46 . Turbocharger  46  may be any suitable turbocharger known in the art, including variable-geometry turbine turbochargers and waste-gated turbochargers. Turbocharger  46  may also include multiple turbochargers. Turbine  48  is connected via a shaft  49  to compressor  50  that is flow coupled to inlet supply conduit  26 . 
     Compressor  50  receives fresh air flow from intake air supply conduit  23 . Second fuel source  104  may also be flow coupled at or upstream of the inlet to compressor  50 . Second fuel source  104  may also be connected to intake air supply conduit downstream of compressor  50 . Intake system  22  further includes a compressor bypass  72  that connects a downstream or outlet side of compressor  50  to an upstream or inlet side of compressor  50 . Compressor bypass  72  includes a control valve  74  that is selectively opened to allow charge flow to be returned to the inlet side of compressor  50  to reduce compressor surge under certain operating conditions, such as when an intake throttle  76  is closed. Inlet supply conduit  26  may include a charge air cooler  36  downstream from compressor  50  and intake throttle  76 . In another embodiment, a charge air cooler  36  is located in the intake system  22  upstream of intake throttle  76 . Charge air cooler  36  may be disposed within inlet air supply conduit  26  between engine  30  and compressor  50 , and embody, for example, an air-to-air heat exchanger, an air-to-liquid heat exchanger, or a combination of both to facilitate the transfer of thermal energy to or from the flow directed to engine  30 . 
     In operation of internal combustion engine system  20 , fresh air is supplied through inlet air supply conduit  23 . The fresh air flow or combined flows can be filtered, unfiltered, and/or conditioned in any known manner, either before or after mixing with the EGR flow from EGR systems  51 ,  60  when provided. The intake system  22  may include components configured to facilitate or control introduction of the charge flow to engine  30 , and may include intake throttle  76 , one or more compressors  50 , and charge air cooler  36 . The intake throttle  76  may be connected upstream or downstream of compressor  50  via a fluid passage and configured to regulate a flow of atmospheric air and/or combined air/EGR flow to engine  30 . Compressor  50  may be a fixed or variable geometry compressor configured to receive air or air and fuel mixture from fuel source  104  and compress the air or combined flow to a predetermined pressure level before engine  30 . The charge flow is pressurized with compressor  50  and sent through charge air cooler  36  and supplied to engine  30  through intake supply conduit  26  to engine intake manifold  28 . 
     With further reference to  FIG. 2 , fuel system  21  is configured to provide dual fuelling of engine  30  from first and second fuel sources  102 ,  104  or fuelling from only first fuel source  102 . First fuel source  102  is configured to provide a flow of a first fuel to cylinders  31  with one or more injectors at or near each cylinder  31 . Second fuel source  104  is connected to intake system  22  with a mixer or connection at or adjacent an inlet of compressor  50 . In certain embodiments, the cylinders  31  each include at least one direct injector for delivering fuel to the combustion chamber thereof from a primary fuel source, such as first fuel source  102 . In addition, at least one of a port injector at each cylinder or a mixer at an inlet of compressor  50  can be provided for delivery or induction of fuel from the second fuel source  104  with the charge flow delivered to cylinders  31 . 
     The fueling from the first fuel source  102  is controlled to provide the sole fueling at certain operating conditions of engine  30 , and fueling from the second fuel source  104  is provided to substitute for fueling from the first fuel source  102  at other operating conditions to provide a dual flow of fuel to engine  30 . In embodiments where the first fuel source  102  is diesel fuel and the second fuel source  104  is gaseous fuel such as natural gas, a control system including controller  200  is configured to control the flow of liquid diesel fuel from first source  102  and the flow of gaseous fuel from second source  104  in accordance with engine speed, engine loads, intake manifold pressures, and fuel pressures, for example. In addition, fuel system  21  includes a fuel source selector  140  that allows an operator or service technician to manually select a diesel fuel only fuelling mode  142  or a dual fuel fuelling mode  144 . Fuel source selector  140  can be a discrete switch with two positions, an analog input device, or a datalink input that receives a fuel source selection from a communication device. 
     A direct injector, as utilized herein, includes any fuel injection device that injects fuel directly into the cylinder volume, and is capable of delivering fuel into the cylinder volume when the intake valve(s) and exhaust valve(s) are closed. The direct injector may be structured to inject fuel at the top of the cylinder or laterally of the cylinder. In certain embodiments, the direct injector may be structured to inject fuel into a combustion pre-chamber. Each cylinder  31 , such as the illustrated cylinders  31   a - d  in  FIG. 2  (cylinders  31   e  and  31   f  omitted for brevity, it being understood that any cylinder arrangement and number as discussed herein is contemplated) may include one or more direct injectors  116   a - 116   d , respectively. The direct injectors  116   a - 116   d  may be the primary fueling device for first fuel source  102  for the cylinders  31   a - 31   d.    
     A port injector, as utilized herein, includes any fuel injection device that injects fuel outside the engine cylinder in the intake manifold to form the air-fuel mixture. The port injector injects the fuel towards the intake valve. During the intake stroke, the downwards moving piston draws in the air/fuel mixture past the open intake valve and into the combustion chamber. Each cylinder  31  may include one or more port injectors  118   a - 118   d , respectively. In one embodiment, the port injectors  118   a - 118   d  may be the primary fueling device for second fuel source  104  to the cylinders  31   a - 31   d . In another embodiment, the second fuel source  104  can be connected to intake system  22  with a mixer  115  at a natural gas connection  114  upstream of intake manifold  28 , such as at the inlet of or upstream of compressor  50 . 
     In certain embodiments, each cylinder  31  includes at least one direct injector that is capable of providing all of the designated primary fueling amount from first fuel source  102  for the cylinders  31  at any operating condition. Second fuel source  104  provides a flow of a second fuel to each cylinder  31  through a port injector or a natural gas connection upstream of intake manifold  28  to provide a second fuel flow to the cylinders  31  to achieve desired operational outcomes, such as improved efficiency, improved fuel economy, improved high load operation, and other outcomes. In certain conditions in which nominal operations of engine  30  call for fuelling from first fuel source  102  and second fuel source  104 , an override of nominal conditions is provided by selecting a diesel only fuelling mode in response to an operator or service technician input or by the occurrence of an automatic override condition, as discussed further below. 
     One embodiment of system  20  includes fuel system  21  with at least one fuel source  102  to provide a primary fuel flow to all the cylinders  31  and a second fuel source  104  that provides a second fuel flow to all the cylinders  31  in addition to the primary fuel flow under certain operating conditions. First fuel source  102  includes a first fuel pump  105  that is connected to controller  200 , and the second fuel source  104  includes, in one embodiment, a second fuel pump  106  that is connected to controller  200 . Alternatively, second fuel source  104  is a source of pressurized gaseous fuel. Each of the cylinders  31  includes an injector, such as direct injectors  116   a - 116   d  associated with each of the illustrated cylinders  31   a - 31   d  of  FIG. 2 . Direct injectors  116   a - 116   d  are electrically connected with controller  200  to receive fueling commands that provide a fuel flow to the respective cylinder  31  in accordance with a fuel command determined according to engine operating conditions and operator demand by reference to fueling maps, control algorithms, or other fueling rate/amount determination source stored in controller  200 . First fuel pump  105  is connected to each of the direct injectors  116   a - 116   d  with a first fuel line  109 . First fuel pump  105  is operable to provide a first fuel flow from first fuel source  102  to each of the cylinders  31   a - 31   d  in a rate, amount and timing determined by controller  200  that achieves a desired torque and exhaust output from cylinders  31 . 
     Second fuel pump  106  is connected to the inlet of compressor  50  with natural gas connection  114  with a second fuel line  108  or is connected with a fuel line (not shown) connected to port injectors  118 . A shutoff valve  112  can be provided in fuel line  108  and/or at one or more other locations in fuel system  21  that is connected to controller  200 . Second fuel pump  106  is operable to provide a second fuel flow from second fuel source  104  in an amount determined by controller  200  that achieves a desired torque and exhaust output from cylinders  31 . A flow control valve  117  can be provided to control the flow of gaseous fuel to engine  30  from second fuel source  104 . In another embodiment, second fuel pump  106  is omitted and fuel is supplied to connection  114  or port injectors  118  under pressure from a pressurized second fuel source  104 . 
     Controller  200  can be connected to actuators, switches, or other devices associated with fuel pumps  105 ,  106 , shutoff valve  112 , flow control valve  117 , intake throttle  76 , compressor bypass valve  74 , wastegate  71  and/or injectors  116 ,  118  and configured to provide control commands thereto that regulate the amount, timing and duration of the flows of the first and second fuels to cylinders  31 , the charge flow, and the exhaust flow to provide the desired torque and exhaust output. In addition, controller  200  can be connected to first fuel source  102 , second fuel source  104 , engine  30 , and aftertreatment system  90  and configured to detect a dual fuel override condition in which a diesel only fuelling mode is automatically selected or selected by the operator or service technician to supply fuelling solely from first fuel source  102  in an amount, timing and duration that provides the desired torque and exhaust output. A dual fuel override conditions can be determined by, for example, a gaseous fuel gas supply deficiency, a gaseous fuel quality deficiency, an engine protection limit violation, and/or an aftertreatment system management requirement. 
     As discussed above, the positioning of each of shutoff valve  112 , flow control valve  117 , intake throttle  76 , compressor bypass valve  74 , wastegate  71  and/or injectors  116 ,  118  and the on/off status of fuel pumps  105 ,  106  can be controlled via control commands from controller  200 . In addition controller  200  is configured to receive operating parameters such as first fuel source conditions  202 , second fuel source conditions  204 , engine operating conditions  206 , and aftertreatment system operating conditions  208 . 
     First fuel source conditions  202 , as discussed further below, can include one or more of the commanded fuelling amount from first fuel source  102 , the actual fuelling amount from first fuel source  102 , the pressure, timing, and duration of the fuelling amount, and other parameters. Second fuel source conditions  204  can include one or more of the gas quality of second fuel source  104 , the gas supply pressure of second fuel source  104 , and the actual and commanded gas flow rates of second fuel source  104  in response to a fuelling command. Engine operating conditions  206  can include one or more of a torque request, engine speed, engine output torque, an intake manifold pressure, a fuel injector tip temperature, knock conditions, and any other engine temperature and/or pressure conditions. Aftertreatment system operating conditions  208  can include, for example, a urea deposit accumulation condition, a particulate matter accumulation condition, a sulfur deposit accumulation condition, and an aftertreatment component regeneration condition. 
     In certain embodiments of the systems disclosed herein, controller  200  is structured to perform certain operations to control engine operations and fuelling of cylinders  31  with fueling system  21  to provide the desired speed and torque outputs from first fuel source  102  only, or from both first and second fuel sources  102 ,  104 . In certain embodiments, the controller  200  forms a portion of a processing subsystem including one or more computing devices having memory, processing, and communication hardware. The controller  200  may be a single device or a distributed device, and the functions of the controller  200  may be performed by hardware or software. The controller  200  may be included within, partially included within, or completely separated from an engine controller (not shown). The controller  200  is in communication with any sensor or actuator throughout the systems disclosed herein, including through direct communication, communication over a datalink, and/or through communication with other controllers or portions of the processing subsystem that provide sensor and/or actuator information to the controller  200 . A sensor, as used herein, may be a physical sensor or a virtual sensor in which the operating condition or output is determined from one or more other sensors and operating parameters. 
     Certain operations described herein include operations to determine one or more parameters. Determining, as utilized herein, includes receiving values by any method known in the art, including at least receiving values from a datalink or network communication, receiving an electronic signal (e.g. a voltage, frequency, current, or PWM signal) indicative of the value, receiving a software parameter indicative of the value, reading the value from a memory location on a non-transient computer readable storage medium, receiving the value as a run-time parameter by any means known in the art, and/or by receiving a value by which the parameter can be calculated, and/or by referencing a default value that is the parameter value. 
     Referring to  FIG. 3 , controller  200  includes a gaseous fuel supply module  210 , a gaseous fuel quality module  212 , an engine protection limit module  214 , an aftertreatment (AT) system management module  216 , and a fuel selection module  218 . Controller  200  also receives inputs of various operating parameters associated with engine  30 . For example, controller  200  can receive inputs of an operator torque request  220  from, for example, an accelerator position alone or in combination with other load requesting devices connected to the engine  30 . Controller  200  can also receive an engine speed input  222  from, for example, an engine speed sensor, and an engine output torque input  224  from, for example, cylinder pressure measurements or output torque of a crankshaft. 
     Gaseous fuel supply module  210  further receives an input of a gaseous fuel supply pressure  226  and an input of a gaseous fuel flow rate  228 . Gaseous fuel supply module  210  also receives or determines a gaseous fuel low pressure threshold  230 , a diesel fuel substitution rate upper threshold  232 , and a commanded gaseous fuel flow rate  234 . Gaseous fuel supply module  210  is further configured to determine a gaseous fuel supply deficiency  236  in response to a gaseous fuel supply deviation condition. A gaseous fuel supply deviation condition can be determined in response to gaseous fuel supply pressure  226  being less than a gaseous fuel low pressure threshold  230 , indicating that the supply pressure of the gaseous fuel is not capable of satisfying operating requirements for fuelling with gaseous fuel. A gaseous fuel supply deviation condition can be determined in response to gaseous fuel flow rate  228  being less than a commanded gaseous fuel flow rate  234  by more than a threshold amount, indicating that the available flow rate of the gaseous fuel supply is not capable of satisfying operating requirements for fuelling with gaseous fuel. A gaseous fuel supply deviation condition can also be determined if the actual substitution rate of diesel fuel exceeds the diesel fuel substitution rate threshold  232 , indicating that the gaseous fuel supply is not capable of satisfying operating requirements for substitution of gaseous fuel for diesel fuel. 
     The gaseous fuel supply deficiency  236  can be provided as an input to fuel source override module  238 . The fuel source override module  238  indicates that override of dual fuelling operation is required or desired and, in one embodiment, provides an override indication to fuel source selection module  218 . Fuel source selection module  218  can be configured to automatically override the dual fuelling mode of operation with a fuel source command  240  that provides fuelling to engine  30  only from first fuel source  102 . In one embodiment, fuel selection module  218  provides a fuel source indicator  242  to, for example, the operator or service technician, indicating that fuelling is being provided from first fuel source  102  only in a diesel only fuelling mode. In a further embodiment, the fuel source indicator  242  can be provided from fuel source override module  238  without an automatic override of the dual fuelling mode of operation, and the override occurs by fuel source indicator  242  prompting the operator or service technician to select a diesel only fuelling mode of operation through fuelling source selector  140  providing a diesel only fuelling mode selection  142  to fuel selection module  218 . 
     Gaseous fuel quality module  212  further receives an input of a gaseous fuel quality  244  from, for example, one or more sensors or operating conditions that provide an indication of the quality of the gaseous fuel from second fuel source  104 . Any suitable indicator of gaseous fuel quality can be provided, such as a heating value, a combustion parameter, a composition value, the absence, presence or amount of one or more constituents, or any other indicator of quality. Gaseous quality module  212  also receives or determines a gaseous fuel quality threshold  246 , and is further configured to determine a gaseous fuel quality deficiency  248  in response to a gaseous fuel quality deviation condition when the quality of the gaseous fuel is less than the quality threshold  246 , indicating that the gaseous fuel is not of sufficient quality to satisfy operating requirements for dual fuel operation. The gaseous fuel quality deficiency  248  can be provided as an input to fuel source override module  238 , which provides an output to prompt selection of a diesel fuelling only mode of operation, or directs fuel selection module  218  to automatically select a diesel only fuelling mode. Fuel selection module  218  outputs a fuel source command  240  that provides fuelling to engine  30  from first fuel source  102  only and an indicator  242  that fuelling is being provided from first fuel source only. Alternatively, fuel selection module  218  awaits an input from fuel source selector  140  in response to an output from fuel source indicator  242  indicating a diesel only fuelling mode, or waits a period of time for the operator or service technician to take another action, before switching to a diesel only fuelling mode. 
     Engine protection limit module  214  receives inputs of engine operating conditions, such as injector tip temperature  250 , intake manifold pressure  252 , and knock conditions  254 . Engine protection limit module  214  further receives or interprets engine protection limits  256  for one or more of these operating conditions, and outputs an engine protection limit violation  258  in response to one or more of these conditions exceeding the protection limits  256 . Engine protection limit violation  258  is received by fuel source override module  238 , which can provide an output to fuel source indicator  242  indicating that selection of a diesel only fuelling mode is desirable, and/or an output to fuel selection module  218  to select a diesel only fuelling mode of operation with fuel source command  240 , either automatically or in response to fuel source selector  140 . 
     AT system management module  216  receives inputs of AT system operating conditions, such as AT system accumulation conditions  260 . AT system management module  218  can also receive inputs such as an AT system failure condition  261 . For example, condition  261  can include an oxidation catalyst failure to recover from a low or no non-methane hydrocarbon (NHMC) conversion condition, indicating an oxidation catalyst failure. AT system management module  218  may further receive or interpret AT system accumulation limits  262  for one or more of these accumulation conditions  260 , and outputs an AT system management requirement  264  in response to one or more of these conditions  260  exceeding the accumulation limits  262  or in response to an AT system failure condition  261 . AT system management requirement  264  is received by fuel source override module  238 , which can provide an output to fuel source indicator  242  indicating that selection of a diesel only fuelling mode is desirable, and/or an output to fuel selection module  218  to select a diesel only fuelling mode of operation with fuel source command  240 . 
     The schematic flow description which follows provides an illustrative embodiment of a method for selection of a diesel only fuelling mode of operation in a dual fuel internal combustion engine system  20 . Operations illustrated are understood to be exemplary only, and operations may be combined or divided, and added or removed, as well as re-ordered in whole or part, unless stated explicitly to the contrary herein. Certain operations illustrated may be implemented by a computer such as controller  200  executing a computer program product on a non-transient computer readable storage medium, where the computer program product comprises instructions causing the computer to execute one or more of the operations, or to issue commands to other devices to execute one or more of the operations. 
     In  FIG. 4 , one embodiment of a flow diagram for operating engine  30  with dual fuel system  21  is disclosed. Procedure  300  starts at  302  upon, for example, starting of engine  30 . At operation  304  the engine  30  is operated in a dual fuel mode of operation in which fuel is provided from first fuel source  102  and second fuel source  104 . Procedure  300  continues at operation  306  to detect a dual fuelling mode override condition. The dual fuelling mode override condition can be detected in response to a diesel only fuelling mode selection  308  from, for example, fuel selector device  140 . The dual fuel override condition can also be selected in response to, for example, a gaseous fuel supply deficiency  310 , a gaseous fuel quality deficiency  312 , an engine protection limit violation  314 , and/or an aftertreatment system management requirement  316 . Upon detection of dual fuelling override condition, procedure  300  continues at operation  318  to operate engine  30  in a diesel only fuelling mode at operation  318 . Procedure  300  ends at  320 , or returns to operation  304  when engine  30  is operated in a dual fuelling mode of operation. 
     Various aspects of the systems and methods disclosed herein are contemplated. For example, one aspect relates to a method that includes operating an internal combustion engine system that includes an intake connected to an engine with at least one cylinder and at least two fuel sources operably connected to the engine to provide a diesel fuel and a gaseous fuel to the at least one cylinder to produce a torque output in a dual fuel mode of operation. The intake is coupled to the at least one cylinder to provide a charge flow from the intake to the at least one cylinder, and the internal combustion engine system further includes an exhaust with an aftertreatment system connected to the at least one cylinder. The method includes selecting a diesel only fuelling mode of operation of the engine in response to a dual fuel override condition occurring during the dual fuel mode of operation resulting from an input to a fuel source selector, a determination of at least one of a gaseous fuel supply deficiency associated with the gaseous fuel, a determination of a gaseous fuel quality deficiency associated with the gaseous fuel, an engine protection limit violation associated with the engine, and a determination of an aftertreatment system management requirement associated with the aftertreatment system. The method also includes operating the internal combustion engine by providing diesel fuel only to the at least one cylinder in response to selecting the diesel only fuelling mode. 
     In one embodiment, the at least one cylinder includes a plurality of cylinders. In another embodiment, selecting the diesel only fuelling mode of operation includes receiving an input from at least one of a discrete switch, an analog input, and a datalink input. 
     In another embodiment, the gaseous fuel supply deficiency includes at least one of a gaseous fuel supply pressure being less than a low pressure threshold, an actual gaseous fuel flow rate being less than a commanded gaseous fuel flow rate by more than a threshold amount, and a diesel fuel substitution rate exceeding an upper threshold amount to satisfy a torque output request at the commanded gaseous fuel flow for the torque output request. In yet another embodiment, the gaseous fuel quality deficiency includes a quality of the gaseous fuel being less than a gaseous fuel quality threshold. 
     In another embodiment, the engine protection limit violation includes at least one of a fuel injector tip exceeding a high temperature limit, an engine knock amount exceeding a knock limit, a combustion misfire condition, and an intake manifold pressure exceeding an upper pressure limit. In a refinement of this embodiment, the intake manifold pressure upper pressure limit is exceeded in response to at least one of a backfire condition, a high pressure condition of the gaseous fuel in the intake, a failure of a gaseous fuel supply control valve, and a failure of the gaseous fuel supply control system. 
     In another embodiment, the aftertreatment component management requirement is determined in response to at least one of a urea accumulation, a sulfur accumulation, a particulate accumulation, and a soot accumulation in the exhaust system exceeding an accumulation limit. 
     In accordance with another aspect, a system includes an internal combustion engine including a plurality of cylinders, an exhaust system configured to receive exhaust from the plurality of cylinders that includes at least one aftertreatment component, and an intake system configured to direct a charge flow to the plurality of cylinders and a compressor for compressing the charge flow. The system also includes a fuel system with a first fuel source operable to provide a liquid fuel to the plurality of cylinders and a second fuel source operable to provide a gaseous fuel upstream of the cylinders in the charge flow in addition to the liquid fuel in a dual fuel mode of operation. The system also includes a controller connected to a plurality of sensors associated with the engine, the at least one aftertreatment component, the first fuel source, and the second fuel source. The controller is configured to determine a dual fuel override condition in response to at least one of an input to a fuel source selector, a gaseous fuel supply deficiency, a gaseous fuel quality deficiency, an engine protection limit violation, and an aftertreatment system management requirement. The controller is further configured to control the first fuel source and the second fuel source so that only liquid fuel is provided for operation of the engine in response to the dual fuel override condition. 
     In one embodiment, the liquid fuel is diesel fuel and the gaseous fuel is natural gas. In another embodiment, the fuel source selector is in communication with the controller that is manipulatable to select a first fuelling mode where fuelling to the engine is provided only from the first fuel source and a second fuelling mode where fuelling is provided in the dual fuel mode of operation. In another embodiment, the system includes a fuel source indicator operable to indicate the dual fuel override condition. In another embodiment, the controller is configured automatically control the first fuel source and the second fuel source so that only liquid fuel is provided for operation of the engine in response to the dual fuel override condition. 
     In a further embodiment of the system, the plurality of sensors include a pressure sensor operable to provide an output indicating a pressure of the gaseous fuel at the second fuel source, a flow sensor operable to provide an output indicating a flow rate of the gaseous fuel to the engine, and a first fuel source sensor operable to provide an output indicating a substitution rate of the first fuel for the second fuel in the dual fuel mode of operation. The controller is configured to determine the gaseous fuel supply deficiency in response to at least one of the pressure of the gaseous fuel being less than a low pressure threshold, the fuel flow rate of the gaseous fuel being less than a commanded gaseous fuel flow rate by more than a threshold amount, and the substitution rate of the first fuel exceeding an upper threshold amount to satisfy a torque request at the commanded gaseous fuel flow rate for the torque request. 
     In another embodiment of the system, the plurality of sensors include a quality sensor operable to provide an output indicating a quality of the gaseous fuel of the second fuel source and the controller is configured to determine the gaseous fuel quality deficiency in response to the quality of the gaseous fuel being less than a gaseous fuel quality threshold. 
     In another embodiment of the system, the plurality of sensors include a temperature sensor operable to provide an output indicating a temperature of fuel injectors connected with the plurality of cylinders, a pressure sensor operable to provide an output indicating a pressure of an intake manifold of the intake system, and a knock sensor operable to provide an output of a knock condition of the engine. The controller is configured to determine the engine protection limit violation in response to at least one of the temperature of the fuel injectors exceeding a high temperature limit, the knock condition of the engine exceeding a knock limit, and the pressure of the intake manifold exceeding an upper pressure limit. 
     In another embodiment of the system, the plurality of sensors include at least one sensor associated with the at least one aftertreatment component operable to provide an output indicating an accumulation condition of the at least one aftertreatment component. The controller is configured to determine the aftertreatment component management requirement in response to at least one of the accumulation condition exceeding an accumulation limit and a failure condition of an aftertreatment system component. 
     According to yet another aspect, an apparatus includes a controller connected to a plurality of sensors associated with an internal combustion engine system. The internal combustion engine system includes an intake, an engine, an aftertreatment system, a first fuel source operable to provide a diesel fuel to the engine, and a second fuel source operable to provide a gaseous fuel to the engine in addition to the diesel fuel in a dual fuel mode of operation. The controller includes a fuel source override module configured to determine a dual fuel override condition in response at least one of an input to a fuel source selector, a gaseous fuel supply deficiency associated with the gaseous fuel, a gaseous fuel quality deficiency associated with the gaseous fuel, an engine protection limit violation associated with the engine, and an aftertreatment system management requirement associated with the aftertreatment system. The controller also includes a fuel selection module configured to provide a diesel only fuelling command that provides fuelling only from the first fuel source while prohibiting fuelling from the second fuel source in response to the dual fuel override condition. 
     In one embodiment, the fuel source override module is configured to provide an output of the dual fuel override condition to a fuel source indicator, and the fuel selection module is configured to provide the diesel only fuelling command in response to the input from the fuel source selector. 
     In another embodiment, the controller includes a gaseous fuel supply module configured to determine the gaseous fuel supply deficiency in response to at least one of a supply pressure of the gaseous fuel being less than a low pressure threshold, a flow rate of the gaseous fuel being less than a commanded flow rate by more than a threshold amount, and a diesel fuel substitution rate exceeding an upper threshold amount to satisfy a torque output request at the commanded gaseous fuel flow for the torque output request. The controller also includes a gaseous fuel quality module configured to determine the gaseous fuel quality deficiency in response to a quality of the gaseous fuel being less than a gaseous fuel quality threshold, an engine protection limit module configured to determine the engine protection limit violation in response to one or more operating conditions of the engine exceeding a protection limit, and an aftertreatment system management module configured to determine the aftertreatment system management requirement in response to at least one of a failure of an aftertreatment system component and an accumulation condition of the aftertreatment system exceeding an accumulation limit for at least one of urea, sulfur, particulates and soot. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only certain exemplary embodiments have been shown and described. Those skilled in the art will appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. 
     In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary.