Abstract:
An improved fuel system for a dual-fuel internal combustion engine. During normal operation, the primary fuel passes through a pressure regulator before arriving at a primary fuel rail. Further, pressurized secondary fuel is delivered to both a secondary fuel rail and to the pressure regulator for purposes of regulating the output pressure of the primary fuel that is delivered to the primary fuel rail. When the pressure at the primary fuel supply or between the primary fuel supply and the pressure regulator drops below a minimum operating pressure, the pressure regulator is isolated from the secondary fuel and the secondary fuel continues to be pressurized until it reaches a suitable pressure for operating in a limp mode. Then, the secondary fuel, which is pressurized to greater than a normal operating pressure, may be injected at the higher pressure for improved performance of the engine in the limp mode.

Description:
BACKGROUND 
     This disclosure relates to high-pressure direct-injection (HPDI) fuel systems designed to inject a primary fuel, such as natural gas, and a pilot fuel, such as diesel, into a combustion chamber, for example a cylinder of a reciprocating internal combustion (IC) engine. More specifically, this disclosure describes an HPDI system that features a “limp mode” when the supply of natural gas is depleted or otherwise interrupted. Still more specifically, this disclosure describes a control system and method for generating and isolating high-pressure pilot fuel in an HPDI system when operating in a limp mode. 
     Heavy-duty IC engines that run on natural gas instead of diesel are desirable because natural gas has cost advantages and produces fewer emission products compared to diesel. Engines that burn natural gas may be spark-ignited or compression-ignited. Spark-ignited engines are available, but spark-ignited engines that run on natural gas have reduced efficiencies and lower torques at low speeds when compared with traditional diesel engines. However, spark-ignited engines that run on natural gas are commonly used for transit buses, delivery vehicles, shuttles, street sweepers and other applications that do not require high torque and low speeds. 
     Compression-ignited engines that burn natural gas also burn some diesel as a pilot fuel and are therefore referred to as dual-fuel engines. Fuel systems for dual-fuel compression-ignited engines come in two types: substitution systems and high-pressure direct injection (HPDI) systems. Substitution systems simply add natural gas to the intake air stream and decrease the amount of diesel fuel, thereby “substituting” a percentage of the diesel with natural gas. Drawbacks of substitution systems include reduced power output because introducing natural gas into the intake air system reduces the amount of air drawn into the engine, and the amount of natural gas that can be substituted is limited by engine knock limits to avoid premature detonation of a premixed charge of natural gas and air. Engines equipped with substitution systems will run either on natural gas and substantial amounts of diesel or pure diesel (with no natural gas), but will not run primarily on natural gas. The substitution percentage of diesel with natural gas typically ranges between 0 and 65%, dependent upon the load and operating conditions. While substitution systems account for the majority of diesel/natural gas engines in use today, the inability to run on natural gas without substantial amounts of diesel renders substitution systems less environmentally friendly than HPDI systems. Further, substitution systems do not provide the fuel cost savings provided by HPDI systems when natural gas is less expensive than diesel fuel. 
     HPDI systems burn primarily natural gas with a small amount of diesel as a pilot fuel. The diesel is injected into the cylinder just prior to the injection of high-pressure natural gas to provide the ignition. Typically, the diesel amounts to less than 10% of the combusted fuel and therefore the emission reduction is substantial. Engines equipped with HPDI fuel systems offer power, torque and efficiency similar to that of traditional diesel engines. Further, a traditional diesel engine may be converted to an HPDI engine by replacing the diesel fuel system with an HPDI fuel system. 
     Current HPDI systems may run on diesel only, for those situations where the natural gas supply is depleted or natural gas is otherwise not available, or for extremely cold starts when the engine is too cold to effectively vaporize the natural gas stored in liquefied form, as disclosed in CA 2849623. During normal operations, when diesel is used as a pilot fuel, the diesel and natural gas are pressurized to a normal system pressure of about 30 MPa. However, when the engine is operating in a limp mode (also known as a limp-home mode, run-on diesel (ROD) mode and diesel only mode (DOM)), injecting diesel at the normal operating pressure provides only about 10% of the engine power. Operating at such a reduced power is very disadvantageous for some applications, such as mine haul trucks, where substantial power may be needed to move a truck off the haul road. Further, trucks with HPDI systems may need to travel substantial distances or climb steep inclines to reach a site where the natural gas supply can be replenished. 
     A solution to this problem would be to increase the pressure of the diesel from the normal operating pressure of about 30 MPa to a higher pressure of about 100 MPa while operating in the limp mode, but certain components of an HPDI system, such as the pressure regulator, cannot withstand the excessive force imbalance between the normal operating pressure and the high pressure needed to run on diesel only. Thus, a need exists for an HPDI fuel system and method that delivers high-pressure diesel (or high-pressure secondary fuel) to the engine when the engine is operating in a limp mode without compromising components needed to operate the engine in a normal operating mode. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect, a fuel system for an internal combustion engine includes a primary fuel supply and a secondary fuel supply. The primary fuel supply may be in communication with a primary chamber of a pressure regulator. The primary chamber of the pressure regulator may be in communication with a primary fuel rail. The primary fuel supply may also be in communication with a primary pressure sensor. The primary pressure sensor may be linked to a controller. The secondary fuel supply may be in communication with a secondary fuel pump. The secondary fuel pump may be in communication with a secondary fuel isolation valve and a secondary fuel rail. The secondary fuel pump may be linked to the controller and may be in communication with a secondary pressure sensor disposed downstream of the secondary fuel pump. The secondary fuel isolation valve may be in selective communication with a secondary chamber of the pressure regulator. The secondary fuel isolation valve may be linked to the controller. The secondary fuel isolation valve has a normal operating position where the secondary fuel pump is in communication with the secondary chamber through the secondary fuel isolation valve. The secondary fuel isolation valve also has a limp mode position where the secondary fuel isolation valve isolates the secondary fuel pump from the secondary chamber. The controller may be configured to command the secondary fuel pump to deliver secondary fuel to the secondary fuel isolation valve and the secondary fuel rail at a first pressure and to shift the secondary fuel isolation valve to its normal operating position when the primary pressure sensor detects that a pressure of the primary fuel supply is above a predetermined minimum operating pressure. The controller may also be configured to command the secondary fuel pump to deliver secondary fuel to the secondary fuel rail at a second pressure and to shift the secondary fuel isolation valve to its limp mode position when the primary pressure sensor detects that the pressure of the primary fuel supply is below the predetermined minimum operating pressure. Further, the second pressure may be greater than the first pressure. 
     In another aspect, a high-pressure direct-injection (HPDI) fuel system includes a primary fuel supply and a secondary fuel supply. The primary fuel supply may be in communication with a primary chamber of a pressure regulator. The primary chamber of the pressure regulator may be in communication with a primary fuel rail. The primary fuel supply may be in communication with a primary pressure sensor. The primary sensor pressure may be linked to a controller. The secondary fuel supply may be in communication with a secondary fuel pump. The secondary fuel pump may be in communication with a secondary fuel isolation valve and a secondary fuel rail. The secondary fuel isolation valve may be in selective communication with a secondary chamber of the pressure regulator. The secondary chamber may be isolated from the primary chamber and the pressure regulator includes a control member that may be moveable in response to changes in pressure in the secondary chamber. The secondary fuel isolation valve and the secondary fuel pump are linked to the controller. The controller may be configured to maintain the secondary fuel isolation valve in a normal operating position where the secondary fuel isolation valve provides communication between the secondary fuel pump and the secondary chamber when the pressure of the primary fuel supply is above a predetermined minimum operating pressure. Further, the controller may be further configured to command the secondary fuel pump to deliver secondary fuel to the secondary fuel isolation valve and the secondary fuel rail at a first pressure when the pressure of the primary fuel supply is above the predetermined minimum operating pressure. However, the controller may be configured to maintain the secondary fuel isolation valve in a limp mode position with the secondary fuel isolation valve isolating the secondary fuel pump from the secondary chamber and the controller may be further configured to command the secondary fuel pump to deliver secondary fuel to the secondary fuel rail at a second pressure that exceeds the first pressure when the pressure of the primary fuel supply falls below the predetermined minimum operating pressure. 
     In yet another aspect, a method for isolating high-pressure diesel in a high-pressure direct-injection (HPDI) fuel system includes providing a natural gas supply and providing a diesel supply that is connected to a pump. The method further includes sensing a pressure of the natural gas supply and, if the pressure of the natural gas supply is above a predetermined minimum operating pressure, the method includes providing communication between the natural gas supply and a first chamber of a pressure regulator. The method further includes operating the pump to output diesel at a desired normal operating pressure and providing communication between the pump and a secondary chamber of the pressure regulator. Further, if the pressure of the natural gas supply is below the predetermined minimum operating pressure, the method includes operating the pump to output diesel at a desired limp mode pressure that is greater than the normal operating pressure and isolating the pump from the secondary chamber of the pressure regulator. 
     Other advantages and features will be apparent from the following detailed description when read in conjunction with the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the disclosed methods and apparatuses, reference should be made to the embodiments illustrated in greater detail in the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a fuel system for an internal combustion engine, wherein the system is configured to operate in a normal operating mode. 
         FIG. 2  is another schematic illustration of the fuel system shown in  FIG. 1 , wherein the fuel system is configured to operate in a limp mode. 
         FIG. 3  is a flow chart of a method for operating the fuel system of  FIGS. 1 and 2  in both the normal operating and limp modes. 
     
    
    
     It should be understood, of course, that this disclosure is not limited to the particular embodiments illustrated herein. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a fuel system  10  supplies fuel to an engine  11 . The fuel system  10  includes a primary fuel supply  12  and a secondary fuel supply  13 . The primary fuel may be natural gas while the secondary fuel may be diesel, although other primary and secondary fuels may be employed, as will be apparent to those skilled in the art. The primary fuel supply  12  may be in communication with a primary fuel isolation valve  14 . If the primary fuel is natural gas, the natural gas may be vaporized through a heat exchanger  20  before flowing through the primary fuel isolation valve  14 . As shown in  FIGS. 1 and 2 , the primary fuel isolation valve  14  may be a normally-closed directional control valve with two ports and two finite positions. However, other types of valves may be used for the primary fuel isolation valve  14 , as will be apparent to those skilled in the art. In the position shown in  FIG. 1  the primary fuel isolation valve  14  has been shifted to an open position thereby providing communication between the primary fuel supply  12  and the pressure regulator  15 . The primary fuel isolation valve  14  may include a solenoid  16  that may be linked to a controller  17 . The controller  17  may be configured to send a signal to the solenoid  16  to shift the primary fuel isolation valve  14  to the open position as shown in  FIG. 1 . The open position of the primary fuel isolation valve  14  as shown in  FIG. 1  is a normal operating position as the primary fuel supply  12  is in communication with a primary fuel rail  18  by way of the primary fuel isolation valve  14 , the pressure regulator  15  and the check valve  19 . The primary fuel supply  12  may also be equipped with or in communication with a primary pressure sensor  22 . While the primary pressure sensor  22  is located in the primary fuel supply  12  in  FIGS. 1 and 2 , the primary pressure sensor  22  may be located downstream of the primary fuel supply  12  as well. The primary pressure sensor  22  may also be linked to the controller  17 . The primary fuel rail  18  may be in communication with one or more fuel injector valves  23 . The one or more fuel injector valves  23  are in communication with the engine  11 . 
     Turning to the secondary fuel, the secondary fuel supply  13  may be in communication with a secondary fuel pump  24 . The secondary fuel pump  24  may be a common rail pump and the secondary fuel pump  24  may be a unidirectional variable displacement pump as indicated in  FIGS. 1 and 2 , although other pumps may be employed as well. The secondary fuel pump  24  may also be linked to the controller  17 . The secondary fuel pump  24  may be in communication with both a secondary fuel isolation valve  25  and a secondary fuel rail  26 . A secondary pressure sensor  27  may be disposed downstream of the secondary fuel pump  24  and the secondary pressure sensor  27  may also be linked to the controller  17 . As shown in  FIGS. 1 and 2 , the secondary fuel isolation valve  25  may be a normally closed directional control valve with three ports and two finite positions, although other types of valves may be employed, as will be apparent to those skilled in the art. The secondary fuel isolation valve  25  may be solenoid-activated and may include a solenoid  28  that may also be linked to the controller  17 . In the position shown in  FIG. 1 , the solenoid  28  has received a command from the controller  17  to shift the secondary fuel isolation valve  25  to the open position, or to the position shown in  FIG. 1 , where the secondary fuel pump  24  is in communication with both the secondary fuel rail  26  and the pressure regulator  15  by way of the secondary fuel isolation valve  25 . While  FIGS. 1 and 2  illustrate the primary fuel isolation valve  14  and the secondary fuel isolation valve  25  as normally-closed valves, one skilled in the art will realize that one or both could be normally-open valves as well. 
     In  FIG. 1 , the primary fuel isolation valve  14  and the secondary fuel isolation valve  25  are in their normal operating positions, where the primary fuel isolation valve  14  provides communication between the primary fuel supply  12  and the pressure regulator  15  and the secondary fuel isolation valve  25  provides communication between the secondary fuel pump  24  and the pressure regulator  15 . More specifically, the primary fuel isolation valve  14  provides communication between the primary fuel supply  12  and a primary chamber  32  of the pressure regulator  15 . Further, secondary fuel from the secondary fuel supply  13  may be delivered by the secondary fuel pump  24  through the secondary fuel isolation valve  25  to a secondary chamber  33  of the pressure regulator  15 . The pressure regulator  15  may also include a control member  34  that may be responsive to changes of pressure in the secondary chamber  33 . 
     In the normal operating mode illustrated in  FIG. 1 , primary fuel from the primary fuel supply  12  passes through the primary fuel isolation valve  14 , through the pressure regulator  15 , through the check valve  19  and to the primary fuel rail  18  before being delivered to the fuel injector valve  23 . Secondary fuel, on the other hand, may be delivered from the secondary fuel supply  13  by the secondary fuel pump  24  to both the secondary fuel isolation valve  25  and the secondary fuel rail  26 . The secondary fuel, pressurized by the secondary fuel pump  24 , may be delivered to the secondary chamber  33  of the pressure regulator  15  by way of the open secondary fuel isolation valve  25  for purposes of regulating the pressure of the primary fuel in the conduit  35 , which connects the pressure regulator  15  to the primary fuel rail  18 . Further, in  FIG. 1 , the secondary fuel pump  24  also delivers secondary fuel to the secondary fuel rail  26  for use as pilot fuel. In  FIG. 1 , in the normal operating mode, the controller  17  sends one or more commands to the secondary fuel pump  24  to deliver secondary fuel to the conduit  36  at a specific normal operating pressure or a desired normal operating pressure range. Typically, the normal operating pressure for the secondary fuel, which may be diesel, may be about 30 MPa, although the normal operating pressures for different dual-fuel systems may vary. Thus, in one example, in the normal operating mode for the fuel system  10  as illustrated in  FIG. 1 , the secondary fuel pump  24  delivers secondary fuel from the secondary fuel supply  13  to downstream components such as the conduit  36 , the conduit  37 , the secondary fuel rail  26 , and the conduit  38  and the secondary chamber  33  of the pressure regulator  15  at a normal operating pressure of about 30 MPa. 
     However, in the event the supply of primary fuel in the primary fuel supply  12  becomes depleted or otherwise loses pressure, the fuel system  10  operates in a limp mode as illustrated in  FIG. 2 . Turning to  FIG. 2 , the controller  17  has sent commands to the primary fuel isolation valve  14  to shift and/or maintain the primary fuel isolation valve  14  in the closed position thereby isolating the primary fuel supply  12  from the pressure regulator  15 . The primary fuel isolation valve  14  may be optional because, as shown in  FIG. 2 , the controller  17  has sent a command to the secondary fuel isolation valve  25  to shift and/or maintain the secondary fuel isolation valve  25  in the closed position thereby isolating the secondary fuel pump  24  from the secondary chamber  33  of the pressure regulator  15 . With no pressurized secondary fuel being delivered to the secondary chamber  33  of the pressure regulator  15 , the pressure regulator  15  shuts off communication between the primary fuel supply  12  and the conduit  35 . Thus, as one skilled in the art will appreciate, the pressure regulator  15  may be used to shut off flow of primary fuel to the conduit  35  when the flow of secondary fuel to the secondary chamber  33  may also be shut off. Further, in the position shown in  FIG. 2 , the secondary fuel isolation valve  25  provides a drain from the secondary chamber  33  through the conduit  38  to the conduit  41  and back to the secondary fuel supply  13 . In the limp mode illustrated in  FIG. 2 , secondary fuel may be delivered from the secondary fuel supply  13  by the secondary fuel pump  24  to the conduit  36  and to the secondary fuel rail  26 . Little or no primary fuel is delivered to the conduit  35  and the primary fuel rail  18  during the limp mode illustrated in  FIG. 2  and accordingly,  FIG. 2  reflects conditions where the engine  11  is running or combusting secondary fuel only. For diesel/natural gas systems, the limp mode may also be referred to as a run-on diesel (ROD) mode or a diesel-only mode (DOM). 
     When operating in a limp mode, unless the pressure of the secondary fuel is substantially increased above the normal operating pressure, the engine  11  can only generate a small percentage of the normal power output of the engine  11 . To increase the power output of the engine  11  when running only on the secondary fuel, the controller  17  sends one or more commands to the secondary fuel pump  24  to increase the pressure of the secondary fuel in the conduit  36  that is delivered to the secondary fuel rail  26  to a secondary fuel-only operating pressure, which may be substantially higher than the normal operating pressure of 30 MPa. For example, in a fuel system  10  that employs natural gas as the primary fuel and diesel as the pilot fuel, a preferred diesel pressure in a limp mode may be as high as 100 MPa. Because such a high pressure could damage the pressure regulator  15 , the controller  17  has also sent one or more commands to the solenoid  28  of the secondary fuel isolation valve  25  to shift or maintain the secondary fuel isolation valve  25  in the closed position shown in  FIG. 2 . In the position shown in  FIG. 2 , the secondary chamber  33  of the pressure regulator  15  is isolated from the pressurized secondary fuel in the conduit  36  and/or in the secondary fuel rail  26 . Thus, the pressure regulator  15  is not subjected to the substantial force imbalances between secondary fuel at the normal operating pressure and at the limp mode operating pressure. By not subjecting the pressure regulator  15  to high-pressure secondary fuel, the pressure regulator  15  will last longer, require less maintenance and have a reduced failure rate. 
     A method for isolating high-pressure secondary fuel such as high-pressure diesel in an HPDI fuel system is illustrated in  FIG. 3 . At step  50 , the controller receives a pressure signal from the primary pressure sensor  22  and, at step  51 , the controller compares the signal received from the primary pressure sensor  22  against a predetermined minimum operating pressure. If the pressure of the primary fuel (P PRIM ) is greater than the minimum operating pressure for the primary fuel (P MIN ), then the controller  17  shifts or maintains the primary fuel isolation valve  14  in an open position at step  52 . The controller  17  also sends a command to the secondary fuel pump  24  at step  53  to pressurize the secondary fuel from the secondary fuel supply  13  to a normal operating pressure (P OP ) of about 30 MPa. At step  54 , the controller  17  sends a signal to the solenoid  28  to open the secondary fuel isolation valve  25 . Subsequently, an inject command can be sent to the fuel injector valve  23  at step  55 . Returning to step  51 , if the pressure of the primary fuel (P PRIM ) is not greater than the minimum operating pressure required for an injection of the primary fuel (P MIN ), the controller  17  sends a signal to the solenoid  16  to close the primary fuel isolation valve  14  at step  56  and further sends a signal to the solenoid  28  to close the secondary fuel isolation valve  25  at step  57 . The controller  17  may then send a signal to the secondary fuel pump  24  to pressurize the secondary fuel at step  58  to an appropriate limp mode pressure (P LIMP ) of about 100 MPa, or a pressure substantially higher than a normal operating pressure of 30 MPa. Subsequently, the controller  17  may send an inject command to the fuel injector valve  23  at step  59 . 
     INDUSTRIAL APPLICABILITY 
     The fuel system  10  and method described above enhances the capability of the fuel system  10  to operate in a limp mode. Specifically, the fuel system  10  can increase the pressure of the secondary fuel to a pressure that may be substantially greater than the normal operating pressure of about 30 MPa. Further, the fuel system  10  and the described method can accomplish this without exposing the pressure regulator  15  to the higher pressure of the secondary fuel in the limp mode. In essence, the secondary fuel isolation valve  25  acts to protect the pressure regulator  15  from the high pressures needed to effectively run the engine  11  on the secondary fuel only. For example, in a fuel system  10  that is an HPDI system, where natural gas is provided as the primary fuel and diesel is provided as the secondary fuel, a normal operating pressure when injecting both natural gas and diesel may be about 30 MPa. However, to effectively run the engine  11  in a limp mode on diesel only, the injection pressure of the diesel may preferably be about 100 MPa, or a pressure that is substantially higher than the normal operating pressure. Such a high pressure can cause the pressure regulator  15  to malfunction or possibly fail. By employing the secondary fuel isolation valve  25 , the pressure regulator  15  is protected from high-pressure secondary fuel when the fuel system  10  is operating in the limp mode. Accordingly, the fuel system  10  and method disclosed herein improve the performance of the engine  11  in the limp mode and enhances the reliability and useful lifespan of the pressure regulator  15 . 
     While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.