Patent Publication Number: US-2012042961-A1

Title: Method and system for water drainage in fuel system

Description:
FIELD 
     Certain embodiments of the subject matter disclosed herein relate to systems and methods for an off-highway vehicle including a fuel system. 
     BACKGROUND 
     Water may become intermixed with diesel fuel or other fuels in several ways, including purposeful mixing, condensation of humid air during transportation from refineries or other stations to end-distribution holding tanks, by leakage through faulty valves, pipes, or vents, and by careless handling. Water in fuel can cause fuel injector nozzle and pump corrosion, microorganism growth, and fuel filter plugging with materials resulting from the corrosion or microbial growth. In cold climates, ice formation in fuels containing water may cause fuel line and filter plugging degradation. Thus, various approaches are available to separate water from diesel fuel. 
     In one example, an off-highway vehicle, such as a locomotive or a mining truck may include a fuel-water separator for separating water from the fuel, and a purge tank for storing the separated water. The purge tank is then periodically inspected and emptied. 
     The inventors herein have recognized some shortcomings in such systems. For example, the required inspection interval for the purge tank may be more often than a regularly scheduled maintenance period. As such, the additional inspections for the purge tank can significantly increase maintenance costs of the vehicle. On the other hand, simply enlarging the purge tank to enable longer intervals between inspection leads to other disadvantages related to fuel system packaging, etc. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Methods and systems are provided for operating an off-highway vehicle including an engine and a fuel system. In one embodiment, a water drainage system for a fuel system comprises a fuel tank, a fuel-water separator, a drain valve, and a purge tank. The fuel-water separator is in fluid communication with the fuel tank. The drain valve is in fluid communication with the fuel-water separator. The purge tank is in fluid communication with the drain valve and the fuel tank. The purge tank may be enclosed within the fuel tank. The water drainage system for the fuel system further comprises a separator water sensor, a purge tank water sensor, and a purge port. The separator water sensor may be operably disposed in the fuel-water separator for detecting the presence of water. The purge tank water sensor may be operably coupled to the purge tank for detecting the presence of water. The purge port is in fluid communication with the purge tank for removing fluid from the purge tank. Thus, the water drainage system may operate with little or no manual intervention between scheduled maintenances of the off-highway vehicle. 
     This brief description is provided to introduce a selection of concepts in a simplified form that are further described herein. This brief description is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. Also, the inventor herein has recognized any identified issues and corresponding solutions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below: 
         FIG. 1  shows an example embodiment of a diesel-electric locomotive including a fuel system and an engine. 
         FIG. 2  shows an example embodiment of a fuel system comprising a fuel tank including an exterior wall and an interior wall. 
         FIG. 3  shows an intersection of the interior wall with the external wall of the embodiment of the fuel tank from  FIG. 2 . 
         FIG. 4  shows an example embodiment of a method of operating an engine. 
         FIG. 5  shows a high level flow chart of an embodiment of a method of operating a vehicle system including an engine and a fuel system as in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Off-highway vehicles, such as mining trucks or the example embodiment of a locomotive in  FIG. 1 , may include an engine supplied by a fuel system with a fuel tank. Fuel in the fuel tank may be intermixed with water and it may be desirable for the fuel system to separate the water and the fuel. An example embodiment of a fuel system, as illustrated in  FIG. 2 , may include a fuel tank, a fuel-water separator, and a purge tank enclosed in the fuel tank. In one embodiment, the fuel tank may include an exterior wall and an interior wall that may intersect with the exterior wall. The interior wall may be shared between the fuel tank and the purge tank.  FIG. 3  shows an intersection of the interior wall with the exterior wall.  FIGS. 4 and 5  show example embodiments of methods of operating a vehicle system, such as the locomotive in  FIG. 1 , supplied with fuel from a fuel system, such as the fuel system of  FIG. 2 . In this manner, water and fuel may be separated by a fuel system supplying an engine of an off-highway vehicle. 
       FIG. 1  is a block diagram of an example vehicle or vehicle system, herein depicted as locomotive  100 , configured to run on track  104 . In one example, locomotive  100  may be a diesel electric vehicle operating with a diesel engine  106  supplied with diesel fuel by a fuel system  105  located within a main engine housing  102 . In other non-limiting embodiments, engine  106  may combust fuel including gasoline, kerosene, biodiesel, or other petroleum distillates of similar density. Fuel system  105 , as further elaborated herein, includes a fuel-water separator for separating water out of the mixture of fuel and entrained water, or wet fuel, stored in a fuel tank. Thus, fuel with little or no water, or dry fuel, may be delivered to engine  106  and the separated water may be delivered to and stored in a purge tank of the fuel system. The fuel tank may be structurally enhanced to resist punctures and deformation. Straps and/or a protective cage may secure the fuel tank to main engine housing  102 . The purge tank may be similarly structurally enhanced, or alternatively, the purge tank may be contained in the fuel tank so that the structural enhancements of the fuel tank may also benefit the purge tank. 
     Locomotive operating crew and electronic components involved in locomotive systems control and management, for example controller  110 , may be housed within a locomotive cab  108 . In one example, controller  110  may include a computer control system. The locomotive control system may further comprise computer readable storage media including code for enabling an on-board monitoring of locomotive operation. Controller  110 , overseeing locomotive systems control and management, may be configured to receive signals from a variety of sensors, as further elaborated herein, in order to estimate locomotive operating parameters. For example, controller  110  may estimate geographic coordinates of locomotive  100  using signals from a Global Positioning System (GPS) radio receiver  140 . Controller  110  may be further linked to display  112 , such as a diagnostic interface display, providing a user interface to the locomotive operating crew. Controller  110  may control the engine  106 , in response to operator input, by sending a command to various engine control hardware components such as inverters  118 , alternator  116 , relays, fuel injectors, fuel pumps (not shown in  FIG. 1 ), etc. For example, the operator may select a power output for the locomotive by operating a throttle control  114 . 
     Controller  110  and/or a locomotive operator may communicate with a control center via radio  142 . As non-limiting examples, radio  142  may include a VHF radio, a cell radio, an 802.11 radio, and combinations thereof. The locomotive operator may communicate with the control center by sending and receiving voice and/or text messages via radio  142 . Additionally, controller  110  may communicate with the control center by sending and receiving data messages. For example, controller  110  may transmit maintenance data and/or engine operational status to the control center via radio  142 . 
     Engine  106  may be started with an engine starting system. In one example, a generator start may be performed wherein the electrical energy produced by a generator or alternator  116  (“ALT”) may be used to start engine  106 . Alternatively, the engine starting system may comprise a motor, such as an electric starter motor, or a compressed air motor, for example. It will also be appreciated that the engine may be started using energy in a battery system, or other appropriate energy sources. 
     The diesel engine  106  generates a torque that is transmitted to an alternator  116  along a drive shaft (not shown). The generated torque is used by alternator  116  to generate electricity for subsequent propagation of the vehicle. The electrical power may be transmitted along an electrical bus  117  to a variety of downstream electrical components. Based on the nature of the generated electrical output, the electrical bus may be a direct current (DC) bus (as depicted) or an alternating current (AC) bus. 
     Alternator  116  may be connected in series to one, or more, rectifiers (not shown) that convert the alternator&#39;s electrical output to DC electrical power prior to transmission along the DC bus  117 . Based on the configuration of a downstream electrical component receiving power from the DC bus, one or more inverters  118  (“INV”) may be configured to invert the electrical power from the electrical bus prior to supplying electrical power to the downstream component. In one embodiment of locomotive  100 , a single inverter  118  may supply AC electrical power from a DC electrical bus to a plurality of components. In an alternate embodiment, each of a plurality of distinct inverters may supply electrical power to a distinct component. 
     A traction motor  120 , mounted on a truck  122  below the main engine housing  102 , may receive electrical power from alternator  116  through the DC bus  117  to provide traction power to propel the locomotive. As described herein, traction motor  120  may be an AC motor. Accordingly, an inverter paired with the traction motor may convert the DC input to an appropriate AC input, such as a three-phase AC input, for subsequent use by the traction motor. In alternate embodiments, traction motor  120  may be a DC motor directly employing the output of the alternator  116  after rectification and transmission along the DC bus  117 . One example locomotive configuration includes one inverter/traction motor pair per wheel-axle  124 . As depicted herein, six pairs of inverter/traction motors are shown for each of six pairs of wheel-axle of the locomotive. Traction motor  120  may also be configured to act as a generator providing dynamic braking to brake locomotive  100 . In particular, during dynamic braking, the traction motor may provide torque in a direction that is opposite from the rolling direction, thereby generating electricity that is dissipated as heat by a grid of resistors  126  connected to the electrical bus. In one example, the grid includes stacks of resistive elements connected in series directly to the electrical bus. The stacks of resistive elements may be positioned proximate to the ceiling of main engine housing  102  in order to facilitate air cooling and heat dissipation from the grid. 
     Air brakes (not shown) making use of compressed air may be used by locomotive  100  as part of a vehicle braking system. The compressed air may be generated from intake air by compressor  128  (“COMP”). A multitude of motor driven airflow devices may be operated for temperature control of locomotive components. The airflow devices may include, but are not limited to, blowers, radiators, and fans. A variety of blowers  130  may be provided for the forced-air cooling of various electrical components. For example, a traction motor blower to cool traction motor  120  during periods of heavy work. Engine temperature is maintained in part by a radiator  132  (“RAD”). A cooling system comprising a water-based coolant may optionally be used in conjunction with the radiator  132  to provide additional cooling of the engine. The hot water-based coolant from the engine may also be used to heat fuel in fuel system  105 . 
     An on-board electrical energy storage device, represented by battery  134  (“BATT”) in this example, may also be linked to DC bus  117 . A DC-DC converter (not shown) may be configured between DC bus  117  and battery  134  to allow the high voltage of the DC bus (for example in the range of 1000V) to be stepped down appropriately for use by the battery (for example in the range of 12-75V). In the case of a hybrid locomotive, the on-board electrical energy storage device may be in the form of high voltage batteries, such that the placement of an intermediate DC-DC converter may not be necessitated. The battery may be charged by running engine  106 . The electrical energy stored in the battery may be used during a stand-by mode of engine operation, or when the engine is shut down, to operate various electronic components such as lights, on-board monitoring systems, microprocessors, displays, climate controls, and the like. Battery  134  may also be used to provide an initial charge to start-up engine  106  from a shut-down condition. In alternate embodiments, the electrical energy storage device may be a super-capacitor, for example. 
     Locomotive  100  may be coupled to a vehicle, such as another locomotive or a railroad car, with a coupling device, such as coupler  150 . Locomotive  100  may include one or more couplers to couple with one or more vehicles in a series of vehicles. In one example, a first locomotive may be connected to a second locomotive with coupler  150 . A controller in the first locomotive, such as controller  110 , may be configured to receive and transmit information to a controller in the second locomotive. The information may include the position or order of a series of locomotives, for example. As non-limiting examples, the information may be transmitted by radio  142  over a wireless network or an electrical cable connecting each locomotive. In this manner, a locomotive may communicate information such as engine and/or vehicle operating conditions to one or more other locomotives. 
     Returning to fuel system  105 ,  FIG. 2  illustrates an example embodiment of fuel system  105 . Fuel system  105  comprises a fuel tank  210 , a fuel-water separator  220 , a drain valve  230 , and a purge tank  240 . Fuel-water separator  220  is in fluid communication with fuel tank  210 . In one embodiment, fuel entrained with water is pumped from fuel tank  210  by a pump  250  to fuel-water separator  220 . An optional fuel heater  252  may be interposed between fuel tank  210  and fuel-water separator  220 . In an alternate embodiment, fuel heater  252  may be coupled to fuel tank  210 . In one embodiment, fuel heater  252  may transfer thermal energy from the cooling system to the fuel. For example, thermal energy from hot water-based coolant may be used to heat fuel in fuel system  105 . Controller  110  may be used to control operation of pump  250  and heater  252 . 
     Fuel-water separator  220  receives a mixture of fuel and water from fuel tank  210  and separates the mixture into dry fuel and purge liquid. The purge liquid may include fuel, water, or a water-fuel emulsion. The dry fuel may be delivered to engine  106 . In one embodiment, fuel-water separator  220  is in fluid communication with a fuel pressure regulating valve  260  which is in fluid communication with engine  106 . Thus, fuel may flow from an outlet port of fuel-water separator  220  through pressure regulating valve  260  to engine  106 . Fuel pressure regulating valve  260  may include a check valve with a set point pressure less than or equal to a peak fuel pressure of engine  106 . If the fuel pressure of fuel pressure regulating valve  260  is less than the set point pressure, fuel pressure regulating valve  260  may remain closed and all fuel from fuel-water separator  220  may be delivered to engine  106 . However, if the fuel pressure of fuel pressure regulating valve  260  is greater than or equal to the set point pressure, fuel pressure regulating valve  260  may open and some fuel from fuel-water separator  220  may be diverted away from engine  106 . Opening pressure regulating valve  260  may reduce the fuel pressure of fuel being delivered to engine  106  so that fuel pressure may be maintained at less than or equal to the peak fuel pressure of engine  106 . In one embodiment, pressure regulating valve  260  may return fuel to fuel tank  210  when pressure regulating valve  260  is open. In one embodiment, the fuel pressure to engine  106  may be measured with a pressure sensor  262  and the pressure may be communicated to controller  110 . 
     Fuel-water separator  220  may include a separator water sensor  270  operably disposed in fuel-water separator  220  for detecting the presence of water. Fuel-water separator  220  is a vessel, having an interior volume, which is capable of holding liquids (e.g. fuel and/or water) in a generally leak proof and watertight manner. Separator water sensor  270  may be positioned in the interior of fuel-water separator  220 . Although referred to as a “water” sensor, separator water sensor  270  is more specifically a water-in-fuel sensor, that is, a sensor configured and able to detect water in the presence of fuel. Separator water sensor  270  is electrically connected to controller  110 , and outputs a signal to controller  110  for indicating whether water is present at the sensor tip or other active sensor portion of separator water sensor  270  where water is detected. Separator water sensor  270  is considered as being dry if no water is detected at the sensing tip; exposure to air or liquid fuel (without water present) would be considered dry conditions. 
     Fuel-water separator  220  is in fluid communication with drain valve  230  which is in fluid communication with purge tank  240 . Drain valve  230  may receive the purge liquid from an outlet port of fuel-water separator  220 . Drain valve  230  may include a check valve with a set point pressure less than the set point pressure of the fuel pressure regulating valve. Additionally, drain valve  230  may have a set point pressure greater than a priming pressure of engine  106 . In one embodiment, the set point pressure of drain valve  230  may be less than half of the set point pressure of fuel pressure regulating valve  260 . In another embodiment, the set point pressure of drain valve  230  may be between ten percent and fifty percent of the set point pressure of fuel pressure regulating valve  260 . When fuel pressure is less than the set point pressure of drain valve  230  (e.g. drain valve  230  is closed), the purge liquid may not flow from fuel-water separator  220  and fuel pressure may increase faster than if drain valve  230  were open. When fuel pressure is greater than or equal to the set point pressure of drain valve  230  (e.g. drain valve  230  is open), the purge liquid may flow from fuel-water separator  220  to purge tank  240 . 
     Drain valve  230  may further include an orifice for limiting flow from fuel-water separator  220  to purge tank  240 . The size of the orifice may control a maximum flow rate through the orifice and drain valve  230 . For example, increasing the size of the orifice may increase flow through drain valve  230  and decrease fuel pressure. Alternatively, decreasing the size of the orifice may decrease flow through the orifice and drain valve  230  and increase fuel pressure. 
     Purge tank  240  is in fluid communication with drain valve  230  and fuel tank  210 . In one embodiment, the purge liquid may flow from drain valve  230  through a duct  232  with an outlet near a bottom  242  of purge tank  240 . For example, a lateral plane  243  may be defined as a plane cutting horizontally across purge tank  240  when purge tank  240  is positioned in its designated orientation for normal use. Near the bottom  242  of purge tank  240  may be defined as below lateral plane  243 . The purge liquid is received near the bottom  242  of purge tank  240 . The purge liquid may include a mixture of fuel and water which may be separated in purge tank  240 . For example, water may have a greater density than fuel and so water may preferentially sink toward the bottom  242  of purge tank  240  and fuel may preferentially rise toward a top  244  of purge tank  240 . In one example, near the top  244  of purge tank  240  may be defined as a lateral plane  245  cutting horizontally across purge tank  240 , parallel with lateral plane  243 . In one embodiment, purge tank  240  may be enclosed in fuel tank  210  and purge tank  240  may include one or more holes  246  near the top  244  of purge tank  240 . Liquid may flow from purge tank  240  through one or more holes  246  into fuel tank  210 . When fuel is less dense than water, the fuel may flow through the one or more holes  246  near the top  244  of purge tank  240  and water may be stored near the bottom  242  of purge tank  240 . As water flows into purge tank  240  the level of the water may rise from the bottom  242  toward the top  244  of purge tank  240 . An area of the one or more holes  246  may be greater than or equal to an area of the orifice of drain valve  230 . In other words, the total area of all of the one or more holes  246  may be greater than or equal to an area of the orifice of drain valve  230 . Thus, a maximum flow rate through the one or more holes  246  may be greater than or equal to a maximum flow rate through the orifice of drain valve  230 . In an alternate embodiment, the area of each one or more holes  246  may be greater than or equal to an area of the orifice of drain valve  230 . 
     An interior volume of purge tank  240  may be large enough for locomotive  100  to operate for an extended period without filling purge tank  240  with water. In one embodiment, the volume of purge tank  240  may be greater than or equal to the volume of water to be extracted from fuel when locomotive  100  is operated under typical or worst-case conditions between scheduled maintenance periods, such as a period of 180 days. For example, the volume of purge tank  240  may be sized according to average fuel consumption (e.g. miles per gallon) of locomotive  100 , an average distance to be travelled by locomotive  100 , and an average water content of fuel. In another example, the volume of purge tank  240  may be sized according to worst-case fuel consumption of locomotive  100 , a worst-case distance to be travelled, and a worst-case water content of fuel. In this manner, purge tank  240  may not fill up with water between scheduled maintenance periods of locomotive  100 . However, some conditions may lead to purge tank  240  filling with water before the maintenance period. For example, out of specification fuel (e.g. fuel with a water concentration in excess of the specified amount), water leaking into fuel system  105 , and increased fuel consumption (e.g. burning more fuel and extracting more water) may result in purge tank  240  filling more quickly than expected. 
     Thus, a purge tank water sensor  280  may be operably coupled to purge tank  240  for detecting when purge tank  240  is at or near its water holding capacity. Specifically, purge tank water sensor  280  may be operably coupled to purge tank  240  for detecting the presence of water in fuel. Similar to separator water sensor  270 , purge tank water sensor  280  is considered as being dry if no water is detected at the sensing tip; exposure to air or liquid fuel (without water present) would be considered dry conditions. If purge tank water sensor  280  is mounted at a pre-determined height above the bottom  242  of purge tank  240 , a threshold volume of water in purge tank  240  may be determined by calculating the volume of the water column that rises to the height of purge tank water sensor  280 . In one embodiment, purge tank water sensor  280  may be operably coupled to purge tank  240  above lateral plane  243 . In other words, purge tank water sensor  280  may be mounted above the outlet for receiving purge liquid. In another embodiment, purge tank water sensor  280  may be operably coupled to purge tank  240  above lateral plane  243  and below lateral plane  245 . In other words, purge tank water sensor  280  may be mounted above the outlet for receiving purge liquid and below the one or more holes  246  of purge tank  240 . Mounting purge tank water sensor  280  nearer the top  244  of purge tank  240  may allow more water to be held in purge tank  240  than if purge tank water sensor  280  is mounted nearer the bottom  242  of purge tank  240 . Thus, purge tank water sensor  280  may be mounted above a mid-point of purge tank  240 . Purge tank water sensor  280  is electrically connected to controller  110  and outputs a signal to controller  110  for indicating whether water is present at the sensor tip or other active sensor portion of purge tank water sensor  280  where water is detected. In other words, purge tank water sensor  280  may indicate to controller  110  when water in purge tank  240  exceeds a threshold amount of water which may be near the water holding capacity of purge tank  240 . 
     As further elaborated herein, the output signals from separator water sensor  270  and purge tank water sensor  280  may be processed by controller  110  for the technical effect of controlling engine  106  and fuel system  105 . In one embodiment, controller  110  includes a processor  201  and a computer readable medium, such as memory  202 . Instructions configured to execute on processor  201  may be encoded and stored in memory  202 . For example, instructions may be configured to detect if water stored in purge tank  240  exceeds a threshold amount via purge tank water sensor  280 . As another example, instructions may be configured to detect if water exceeds a threshold amount of water in fuel-water separator  220  via separator water sensor  270 . Further examples of instructions that may be encoded in controller  110  are described with regard to the methods of  FIGS. 4-5 , which may be routines carried out by controller  110 . 
     During maintenance, water may be removed from purge tank  240  via a purge port  290  in fluid communication with purge tank  240 . In one embodiment, purge port  290  may include a suction line having an inlet near the bottom  242  of purge tank  240 . In this manner, water near the bottom  242  of purge tank  240  may be removed before fuel and/or water near the top  244  of purge tank  240 . Purge port  290  may be different from duct  232  to enable water to be removed from purge tank  240  without disconnecting duct  232  from drain valve  230 . During maintenance, purge port  290  may be connected to an inlet of a fuel polishing cart  292  (“FUEL POLISHER”) and a fill port  294  of fuel tank  210  may be connected to an outlet of fuel polishing cart  292 . Fuel polishing cart  292  may pump liquid (e.g. water and/or fuel) from purge tank  240  via purge port  290 , filter (e.g. polish) the liquid, and return dry fuel to fuel tank  210  via fill port  294 . In this manner, water may be removed from purge tank  240  without removing purge tank  240  from fuel tank  210 . In an alternate embodiment, locomotive  100  may include fuel polishing cart  292  and liquid from purge tank  240  may be filtered when locomotive  100  is idle, for example. 
     Purge tank  240  may be enclosed within fuel tank  210 . In one embodiment, fuel tank  210  may include one or more exterior walls, such as exterior wall  212 , and one or more interior walls, such as interior wall  214 . The one or more exterior walls may enclose the volume of fuel tank  210  and the one or more interior walls may form one or more compartments within fuel tank  210 . For example, one compartment may form purge tank  240 . In other words, purge tank  240  may share one or more walls with fuel tank  210 . For example, wall  214  may be an interior wall of fuel tank  210  and a wall of purge tank  240 , and wall  212  may be an exterior wall of fuel tank  210  and a wall of purge tank  240 . The one or more interior walls may include one or more holes  246  extending through the one or more interior walls for fluid to flow between purge tank  240  and fuel tank  210 . 
       FIG. 3  shows an example embodiment of an intersection of interior wall  214  with external wall  212  of fuel tank  210 . Fuel tank  210  may be structurally enhanced to resist punctures and deformation. In one embodiment, external walls of fuel tank  210  may be constructed of heavy-gauge steel. Increasing the thickness of the external walls may increase the resistance to deformation and/or puncturing. However, increasing the thickness of the external walls may also increase the weight of locomotive  100  which may result in higher fuel consumption. It may also be desirable for purge tank  240  to resist deformation and punctures. Enclosing purge tank  240  within the one or more thick external walls of fuel tank  210  may protect purge tank  240  from deformation and/or punctures. Thus, internal walls of purge tank  240  (and fuel tank  210 ) may be thinner than external walls of fuel tank  210 . In one embodiment, a thickness  310  of external wall  212  may be greater than twice as thick as a thickness  320  of internal wall  214 . In an alternate embodiment, thickness  310  of external wall  212  may be greater than five times as thick as thickness  320  of internal wall  214 . In yet another alternate embodiment, thickness  310  of external wall  212  may be less than five times as thick as thickness  320  of internal wall  214  and greater than twice as thick as thickness  320  of internal wall  214 . 
       FIG. 4  shows an example embodiment of a method  400  of operating a vehicle, such as locomotive  100 . At  410 , a first mixture of fuel and water may be pumped from a fuel tank. For example, pump  250  may pump fuel entrained with water from fuel tank  210 . In one embodiment, the fuel and water may be heated with a heater, such as heater  252 . At  420 , the first mixture of fuel and water may be separated into fuel and a second mixture of fuel and water. For example, fuel-water separator  220  may separate the fuel entrained with water into dry fuel and purge liquid. The purge liquid may include a second mixture of fuel and water, where the water is less emulsified in the fuel. 
     At  430 , the separated dry fuel may be delivered to the engine. For example, dry fuel may flow from fuel-water separator  220  through fuel pressure regulating valve  260  to engine  106 . Fuel pressure regulating valve  260  may limit the fuel pressure of the dry fuel to less than a peak fuel pressure of engine  106 . 
     At  440 , the second mixture of fuel and water may be delivered to a purge tank contained in the fuel tank. For example, the purge liquid may be delivered to purge tank  240  contained in fuel tank  210 . In one embodiment, the purge liquid may be received in purge tank  240  via an outlet of duct  232  near the bottom  242  of purge tank  240 . In one embodiment, the second mixture of fuel and water may be delivered to the purge tank if fuel pressure exceeds a priming pressure of the engine. For example, drain valve  230  may be closed when fuel pressure is less than the priming pressure of engine  106  and drain valve  230  may be open when fuel pressure is greater than or equal to the priming pressure of engine  106 . In one embodiment, the priming pressure may be between ten percent and fifty percent of the peak fuel pressure. 
     At  450 , fuel may be returned from the purge tank to the fuel tank. For example, water, having a greater density than fuel, may remain near the bottom  242  of purge tank  240  and fuel may rise to near the top  244  of purge tank  240 . When purge tank  240  is full of water and fuel, and when purge liquid enters purge tank  240  through duct  232 , fuel may flow through one or more holes  246  back to fuel tank  210 . 
     At  460 , a sensor coupled to the purge tank may indicate if water exceeds a threshold level in the purge tank. For example, purge tank water sensor  280  may indicate to controller  110  when water reaches the level of purge tank water sensor  280 . During typical operation of locomotive  100 , water may remain below the threshold level of purge tank  240 . However, out-of-specification fuel having too much water, water leaks into fuel system  105 , increased fuel consumption of locomotive  100 , or delayed maintenance may lead to water in purge tank  240  exceeding a threshold level. During maintenance of locomotive  100 , water may be removed from purge tank  240  via purge line  290 , for example. Locomotive operational data and the indication from purge tank water sensor  280  may be used to diagnose potential sources of water in purge tank  240 . For example, location data from GPS radio receiver  140  and data from a fuel level sensor may be used to record each filling location for locomotive  100 . Excessive water content, as indicated by purge tank water sensor  280 , may be correlated with the filling locations of locomotive  100  to diagnose where out-of-specification fuel may be present. As another example, water may leak from an engine component, such heater  252 , into the fuel. If purge tank water sensor  280  indicates water is present earlier than expected, then additional diagnostics may be performed to identify whether one or more engine components are faulty. 
     At  470 , a sensor operably disposed in the fuel-water separator may indicate if water exceeds a threshold level in a fuel-water separator. For example, separator water sensor  270  may indicate to controller  110  when water exceeds the threshold level in fuel-water separator  220 . In one example, water may be detected if the concentration of water in fuel being pumped from fuel tank  210  exceeds the capacity of water to be separated in fuel-water separator  220 . For example, the rate of water flowing into fuel-water separator  220  may exceed the rate of purge liquid flowing from fuel-water separator  220  through drain valve  230 . In one example, drain valve  230 , duct  232 , and/or one or more holes  246  may be clogged and the flow of purge liquid may be reduced. 
     At  480 , the engine may be stopped if water exceeds the threshold level in the fuel-water separator. For example, separator water sensor  270  may indicate to controller  110  that water exceeds the threshold level in fuel-water separator  220 , and controller  110  may stop engine  106  in response thereto. Thus, engine  106  may be protected from undesirable effects of combusting fuel mixed with water. At  490 , maintenance data including water sensor data may be transmitted via a radio. For example, a maintenance message may be transmitted via radio  142  in response to separator water sensor  270  indicating water exceeds the threshold level in fuel-water separator  220 . As another example, a status message may be transmitted via radio  142  if purge tank water sensor  280  indicates water exceeds the threshold level in purge tank  240 . In one embodiment, maintenance and/or status messages may be transmitted to a control center via a VHF or cell radio. Alternatively or additionally, maintenance and/or status messages may be transmitted to another locomotive connected to locomotive  100  by coupler  150  and linked by an 802.11 radio. 
     Accordingly, a vehicle system may include fuel system  105 , engine  106 , and controller  110 . Controller  110  may be programmed to operate the vehicle system with an embodiment of a method, such as method  500 , illustrated in  FIG. 5 . At  510 , it may be determined if fuel pressure is above a threshold. For example, fuel pressure may be measured by a sensor, such as pressure sensor  262 , and compared to a threshold pressure, such as the priming pressure of engine  106 . If fuel pressure is less than the threshold pressure, then the method may end. Otherwise, the pressure is greater than or equal to the threshold pressure and the method may continue at  520 . 
     At  520 , it may be determined if water is detected in fuel-water separator  220 . For example, separator water sensor  270  may indicate to controller  110  when water exceeds the threshold level in fuel-water separator  220 . If water exceeds the threshold level, then the method may continue at  540 . If water does not exceed the threshold level, then dry fuel may be delivered to engine  106  and the method may continue at  530 . 
     At  530 , it may be determined if water is detected in purge tank  240 . For example, purge tank water sensor  280  may indicate to controller  110  when water exceeds the threshold level in purge tank  240 . If water does not exceed the threshold level, then the purge tank is not full and the method may end. If water exceeds the threshold level, purge tank  240  may be at or near water capacity and may need to be emptied soon. The method continues at  532  if water exceeds the threshold level. 
     At  532 , an operator of locomotive  100  may be notified of a maintenance condition. Specifically, the operator may be notified that purge tank  240  is at or near water capacity and may need to be drained. In one embodiment, controller  110  may notify the operator via a visual and/or auditory signal on display  112 . Additionally, an automated message may be transmitted to a control center indicating that water in purge tank  240  exceeds the threshold level. In one example, locomotive  100  may be brought in for maintenance when water in purge tank  240  exceeds the threshold level. In another example, locomotive  100  may continue to operate if a scheduled maintenance is within a pre-determined time or mileage of locomotive  100 . The method ends after  532 . 
     At  540 , water is detected in fuel-water separator  220  and water may be delivered to engine  106  if engine  106  continues to operate. Thus, engine  106  may be stopped to prevent water from being delivered to engine  106 . The operator of locomotive  100  may be notified via a visual and/or auditory signal on display  112 . An automated message may be transmitted via radio  142  indicating that water is detected in fuel-water separator  220 . In one example, a message requesting maintenance may be transmitted to a control center via radio  142 . In another example, a status message may be transmitted to another locomotive coupled to locomotive  100  via coupler  150 . The method continues at  550 . 
     At  550 , it is determined if “return home” mode is enabled. For example, stopping engine  106  of locomotive  100  in a remote location may be undesirable since the operator of locomotive  100  may be stranded and maintenance may be more difficult in a remote location. Thus, a return home mode may be configured to restart engine  106  if dry fuel can be delivered to engine  106 . However, locomotive  100  may be connected to one or more other locomotives via couplers  150  and it may be more desirable to stop engine  106  than to risk operating engine  106  with fuel that may be mixed with water. In one embodiment, return home mode may be disabled if locomotive  100  is connected to one or more locomotives. If return home mode is not enabled, the method may end. If return home mode is enabled, the method may continue at  552 . 
     At  552 , engine  106  is stopped and fuel may be pumped by pump  250  at a reduced rate for a pre-determined filter interval. For example, drain valve  230 , duct  232 , and/or one or more holes  246  may be partially clogged which may reduce the rate of flow of purge liquid from fuel-water separator  220 . In another example, the concentration of water mixed with fuel from fuel tank  210  may exceed the concentration of water that may be separated by fuel-water separator  220  when fuel is pumped near a peak flow rate. Thus, pumping fuel at a reduced rate of flow may enable fuel-water separator  220  to separate the water and to deliver dry fuel to engine  106 . In one example, a filter interval may be selected such that flow through fuel-water separator  220  is at a steady-state operating point. The method may continue at  560 . 
     At  560 , it may be determined if water is detected in fuel-water separator  220 . For example, separator water sensor  270  may indicate to controller  110  whether water is present above a threshold amount in fuel-water separator  220 . If water is detected by separator water sensor  270 , then dry fuel cannot be delivered to engine  106  at the reduced flow rate and the method continues at  562 . At  562 , the fuel pump is stopped and then the method ends. However, if water is not detected by separator water sensor  270 , then dry fuel may be delivered to engine  106  and the method may continue at  564 . 
     At  564 , engine  106  may be started and fuel delivered to engine  106  may be governed to a rate at or below the reduced rate of flow of  552 . The operator of the locomotive may be notified that locomotive  100  may be operated at a reduced rate of fuel via a visual or auditory signal on display  112 . An automated message may be transmitted to a control center via radio  142  indicating that locomotive  100  may be returning for maintenance. In this manner, locomotive  100  may be moved from a remote location to a shop for maintenance. The method ends after  564 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable a person of ordinary skill in the relevant art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. Moreover, unless specifically stated otherwise, any use of the terms first, second, etc., do not denote any order or importance, but rather the terms first, second, etc., are used to distinguish one element from another.