Abstract:
A method for improving fuel heating is presented. The method can reduce system complexity and cost when fuel is heated within a fuel injector. In one embodiment, the method independently heats and injects fuel by changing the direction of current flow through a fuel circuit.

Description:
FIELD 
       [0001]    The present description relates to a system and method for heating fuel and controlling fuel injection of a fuel injector that operates as part of an internal combustion engine. 
       BACKGROUND 
       [0002]    Fuel vaporization tends to decrease as ambient temperature decreases. This can make engine starting more difficult at lower temperatures because reduced fuel vaporization can result in an air-fuel mixture near the engine&#39;s spark plug that is less than the fuel&#39;s lower flammability limit. Further, lower rates of fuel vaporization may make engine starting particularly difficult for certain types of fuels (e.g., ethanol). One example way to improve fuel vaporization is described in U.S. Patent Application 2005/0263136. This patent application describes placing a heating coil around the nozzle of a port fuel injector. The heating coil is supplied electrical energy through an electrical connector that attaches to an engine wiring harness. Heat produced by the heating coil is conducted through the injector to heat fuel that resides within the injector. This heating apparatus purportedly improves fuel vaporization. 
         [0003]    The above-mentioned system can also have several disadvantages. Namely, the system heats the injector through conducting heat from a source outside the injector body. Since the heat source is external to the injector, some energy intended to heat the injector is lost to heating the engine and may therefore be less efficient than is desired. In addition, the heating device requires an additional electrical connector to route power to the heating device. An additional connector increases the number of wires and connections. Therefore, system reliability may be reduced when such a system is used to increase the temperature of fuel injected to an engine. In addition, the system may be difficult to implement on direct injection engine because there may be less space available to place a heating coil around the injector nozzle. 
         [0004]    The inventors herein have recognized the above-mentioned disadvantages and have developed a method that offers substantial improvements. 
       SUMMARY 
       [0005]    One embodiment of the present description includes a system to heat and inject fuel to an internal combustion engine, the system comprising: an internal combustion engine; a fuel injector capable of delivering fuel to said internal combustion engine, said fuel injector comprising a heating element and a fuel flow control element; and a controller that supplies current to said fuel injector in a first direction to heat fuel that flows through said fuel injector, and said controller supplying current to said fuel injector in a second direction to deliver fuel to said engine without substantially heating the fuel delivered through said fuel injector. This method overcomes at least some disadvantages of the above-mentioned method. 
         [0006]    Fuel vaporization and system reliability can be improved by a system that heats fuel from within the fuel injector and that supplies fuel heating energy through the same conductors that are used to actuate the injector. In one embodiment, a system provides current in a first direction to heat fuel contained or passing through the fuel injector, and the system actuates the fuel injector by providing current in a second direction. In other words, the system controls injector heating and actuation (opening and/or closing) by controlling the direction that current is delivered to the fuel injector. This allows the system to use a single pair of wires to actuate the injector and heat fuel passing through the injector. Consequently, fewer conductors have to be provided, less electrical connections are made, and existing fuel injector connectors can be used to realize the system. Furthermore, the fuel heating and fuel injection elements can be integrated into a small package. 
         [0007]    The present description can provide several advantages. Specifically, the approach can improve system reliability, reduce the cost of heating fuel, and it can be implemented with few changes to existing fuel systems. The system can also be used on a variety of injector designs. For example, the described system can be used to heat fuel flowing through port style injectors, injectors that inject fuel directly into a cylinder, injectors having a single coil controlled pintle, and injectors that use dual coil spool valve operated injectors. 
         [0008]    The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The advantages described herein will be more fully understood by reading an example of an embodiment, referred to herein as the Detailed Description, when taken alone or with reference to the drawings, wherein: 
           [0010]      FIG. 1  is a schematic diagram of an engine configured to operate with heated fuel injectors; 
           [0011]      FIG. 2  is a flowchart of an example fuel injector; 
           [0012]      FIG. 3  is a schematic diagram of an example injector fuel heating circuit; 
           [0013]      FIG. 4  is a schematic diagram of another example injector fuel heating circuit; 
           [0014]      FIG. 5  is a schematic diagram of another example injector fuel heating circuit; 
           [0015]      FIG. 6  is a plot illustrating current control for fuel injector fuel heating; and 
           [0016]      FIG. 7  is a flow chart of an example fuel heating method. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIG. 1 , internal combustion engine  10 , comprising a plurality of cylinders, one cylinder of which is shown in  FIG. 1 , is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  40 . Combustion chamber  30  is known communicating with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Intake manifold  44  is shown communicating with optional electronic throttle  62 . 
         [0018]    Fuel is directly injected into combustion chamber  30  via fuel injector  66 . The fuel injector is an example of an electrically operable mechanical valve. Fuel injector  66  receives opening and closing signals from controller  12 . Camshaft  130  is constructed with at least one intake cam lobe profile and at least one exhaust cam lobe profile. Alternatively, the intake cam may have more than one lobe profile that may have different lift amounts, different durations, and may be phased differently (i.e., the cam lobes may vary in size and in orientation with respect to one another). In yet another alternative, the system may utilize separate intake and exhaust cams. Cam position sensor  150  provides cam position information to controller  12 . Intake valve rocker arm  56  and exhaust valve rocker arm  57  transfer valve opening force from camshaft  130  to the respective valve stems. Intake rocker arm  56  may include a lost motion member for selectively switching between lower and higher lift cam lobe profiles, if desired. Alternatively, different valvetrain actuators and designs may be used in place of the design shown (e.g., pushrod instead of over-head cam, electromechanical instead of hydro-mechanical). 
         [0019]    Fuel is delivered to fuel injector  66  by a fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). Engine  10  may be designed to operate on one or more non-limiting fuel types such as diesel, gasoline, alcohol, or propane. 
         [0020]    A distributor-less ignition system (not shown) may provide ignition spark to combustion chamber  30  via a spark plug (not shown) in response to controller  12 . Universal Exhaust Gas Oxygen (UEGO) sensor  76  is shown coupled to exhaust manifold  48  upstream of catalytic converter  70 . Two-state exhaust gas oxygen sensor  98  is shown coupled to exhaust pipe  49  downstream of catalytic converter  70 . Converter  70  may include multiple catalyst bricks, particulate filters, and/or exhaust gas trapping devices. 
         [0021]    Controller  12  is shown in  FIG. 1  as a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , read-only memory  106 , random-access memory  108 , keep-alive memory  110 , and a conventional data bus. Controller  12  is shown receiving various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling sleeve  114 ; a position sensor  119  coupled to an accelerator pedal; a measurement of engine manifold pressure (MAP) from pressure sensor  122  coupled to intake manifold  44 ; engine knock sensor (not shown); fuel type sensor (not shown); humidity from humidity sensor  38 ; a measurement (ACT) of engine air temperature or manifold temperature from temperature sensor  117 ; and an engine position sensor from a Hall effect sensor  118  sensing crankshaft  40  position. In a preferred aspect of the present description, engine position sensor  118  produces a predetermined number of equally spaced pulses every revolution of the crankshaft from which engine speed (RPM) can be determined. 
         [0022]    Controller  12  storage medium read-only memory  106  can be programmed with computer readable data representing instructions executable by processor  102  for performing the methods described below as well as other variants that are anticipated but not specifically listed. 
         [0023]    Referring now to  FIG. 2 , a schematic of an example direct injection fuel injector is shown. Fuel injector  200  is designed to inject fuel directly into a cylinder of an internal combustion engine. However, the present description is not restricted to direct injectors or to injectors having the same design as the illustrated injector. For example, the present description may be utilized on port or central fuel injectors, or it may be used with fluid (e.g., oil) assisted intensifier injectors.  FIG. 2  is not intended to limit the scope or breadth of the present description. 
         [0024]    Returning to  FIG. 2 , fuel is fed to the injector through port  201 . Pressurized liquid fuel occupies reservoirs  250  and  252  until injected to a cylinder. Needle valve  232  regulates the flow of fuel from the injector to the cylinder through nozzle  207 . The needle valve position is controlled by flowing electrical current through coil  203 . The electrical current passing through coil  203  induces a magnetic field around coil  203  that attracts armature  209  toward the coil. As armature  209  approaches coil  203 , spring  221  is compressed, and needle valve  232  lifts from the injector nozzle seat. Fuel then flows to the cylinder. 
         [0025]    Fuel in reservoirs  250  and  252  can be heated by passing current through positive temperature coefficient (PTC) ceramic heating elements  207  and  205 . Alternatively, fuel may be heated using negative temperature coefficient (NTC) heating elements if desired. The heated fuel exits the fuel injector when the armature  209  is attracted to coil  203 . 
         [0026]    Current flows to the injector from electrical connector  210  via two electrical connector pins  211 , one of which is shown. Heating elements  205  and  207  along with actuator coil  203  are electrically connected to pins  211 . Diodes  233  and  231  (or similar current direction controlling devices) are inserted in the electrical path between electrical connector  210  and devices  203 ,  207 , and  205 . Diodes  233  and  231  substantially limit the direction of current through coil  203  and heating elements  207  and  205 . That is, the diodes permit substantially full current flow (i.e., current flow is only reduced by a small voltage drop across the diode) in one direction and limit current flow in the opposite direction to a few milliamps. A few circuit examples are illustrated in  FIGS. 3 and 4 . 
         [0027]    Referring now to  FIG. 3 , an example circuit for bi-directionally controlling current to a fuel injector and heater is shown. Power supply  301  provides current to actuate and heat fuel injector components identified by region boundary  311 . The direction of current supplied to fuel injector  311  is determined by the state of switches  303 ,  307 ,  305 , and  309 . Current flow can be initiated in a first direction by closing switches  307  and  305 . Current flow in a second direction can be initiated by closing switches  303  and  309 . Switches may be of solid-state (e.g., transistors) or mechanical construction (e.g., relays). Diodes  350 ,  352 ,  354 , and  356  are included to dissipate inductive energy when switches  303 ,  307 ,  354 , and  356  are operated. 
         [0028]    Current flows through the fuel injector via pins  323  and  321 . Note that a unique feature over this design is the reduction in pin count over other fuel heating injector designs. In this example, pins  323  and  321  provide power to actuator coil  313  and heater element  317 . Operation of coil  313  and heater element  317  is determined by the direction of current flow because diodes  315  and  319  are biased in different directions. 
         [0029]    If current flows into fuel injector pin  327  from wiring harness pin  323 , and out of fuel injector pin  325  and wiring harness pin  321 , then coil  313  can operate because diode  315  is forward biased. In these conditions diode  319  is reverse biased and substantially stops current flowing to heater element  317 . 
         [0030]    If on the other hand current flows from fuel injector pin  325  to fuel injector pin  327 , heating element  317  can heat fuel because diode  319  is forward biased. Under this condition diode  315  is reverse biased and substantially limits current flow to actuator coil  313 . 
         [0031]    Thus, the circuit illustrated in  FIG. 3  provides two separate functions (actuating a fuel injector and heating fuel in the fuel injector via heating elements) by way of a single electrical connector and a single pair of electrical terminals. By simply changing current direction, the fuel injector function is completely changed. Further, the functions are virtually decoupled from each other. That is, the illustrated circuit allows the fuel injector to be actuated and inject fuel to a cylinder without substantially heating fuel in the injector (when current flows to coil  313  only a small amount of current dependant on the diode design passes diode  319  (e.g., a few milliamps) reaches heating element  317 ). Consequently, the present description provides for a fuel injector that functions to inject fuel to a cylinder and heat fuel in the injector by way of a heating element that is distinct and separate from the actuator coil. 
         [0032]    In addition, the illustrated circuit permits various levels of current to be applied to the coil or heater without causing a device to inadvertently operate. For example, 1 amp or 4 amps can be applied to the heater without causing the coil to actuate the fuel injector. This allows coil or heater operation to be adjusted based on engine operating conditions if desired. 
         [0033]    Referring now to  FIG. 4 , an alternative circuit for controlling current to a fuel injector and heater is shown. Power supply  401 , switches  403 ,  407 ,  405 , and  409  are used in the manner described in  FIG. 3  to control the direction of current flow into the fuel injector components identified by boundary  411 . Likewise similar to  FIG. 3 , diodes  450 ,  452 ,  454 , and  456  are included to dissipate inductive energy when switches  403 ,  407 ,  454 , and  456  are operated. 
         [0034]    If current flows into fuel injector pin  427  from wiring harness pin  423 , and out of fuel injector pin  425  and wiring harness pin  421 , then coil  413  can operate because no diode blocks the current flow. In these conditions diode  419  is reverse biased and substantially stops current flowing to heater element  317 . 
         [0035]    If on the other hand current flows from fuel injector pin  425  to fuel injector pin  427 , heating element  417  can heat fuel because diode  419  is forward biased. In one embodiment during these conditions, current flowing into the injector can be kept below a predetermined level at which the fuel injector actuates and injects fuel. This allows the heater to operate without actuating the fuel injector. Alternatively, if desired, current can be increased to a predetermined level at which the fuel injector is actuated and heater temperature increases. 
         [0036]    Thus, this circuit configuration allows the fuel injector to be operated independent of heater operation, or alternatively, it allows the heater to be operated while the injector is actuated. Further, when the level of current is controlled, this circuit permits the fuel injector to heat fuel in the fuel injector without actuating the fuel injector. 
         [0037]    Referring now to  FIG. 5 , an example of a fuel heating circuitry integrated into an engine controller is illustrated. Engine controller  12  is comprised of a bank of high-side drivers  505 , low-side drivers  507 , and a relay control switch (e.g., a transistor)  511 . External relay  503  is toggled between a first (lower potential) and second (higher potential) voltage depending on the state of relay control switch  511 . Alternatively, the external relay  503  may be substituted with solid-state circuitry, if desired. 
         [0038]    Circuitry of four heated fuel injectors is within boundary region  501 . This injector circuitry represents an example of circuitry for heating fuel for a four cylinder engine. Cylinder number one fuel injector circuitry is within boundary region  520 , while fuel injectors for cylinders two through four are shown in boundary regions  522 ,  524 , and  526  respectively. 
         [0039]    Relay  503  is shown connecting fuel injectors  520 ,  522 ,  524 , and  526  to a first voltage reference. Relay  503  may also connect the same fuel injectors to a second voltage reference V+. The second voltage reference is at a higher potential than the first voltage reference. The operating state of switch  511  determines whether relay  503  connects fuel injectors  520 ,  522 ,  524 , and  526  to the first or second voltage reference. 
         [0040]    High-side driver  505  is comprised of individual solid-state switches that are connected to the second voltage reference on one side of the switches and to fuel injectors  520 ,  522 ,  524 , and  526  on the other side of the switches. 
         [0041]    Low-side driver  507  is also comprised of individual solid-state switches that are connected to the first voltage reference on one side of the switches and to fuel injectors  520 ,  522 ,  524 , and  526  on the other side of the switch. 
         [0042]    Fuel is heated in the injector by controlling relay control switch  511  and high-side driver  505 . Specifically, relay control switch  511  is set to a state whereby control relay  503  connects the first voltage reference to a terminal of fuel injectors  520 ,  522 ,  524 , and  526 . In addition, switches internal to high-side injector  505  are closed such that the second voltage reference is routed to a second terminal of fuel injectors  520 ,  522 ,  524 , and  526 . Current then flows from the second voltage reference to the first voltage reference in a direction that forward biases a diode in an electrical path going to the heating element of each fuel injector. 
         [0043]    On the other hand, the fuel injector is actuated by controlling relay control switch  511  and low-side driver  507 . Specifically, relay control switch  511  is set to a state whereby control relay  503  connects the second voltage reference to a terminal of fuel injectors  520 ,  522 ,  524 , and  526 . And, switches internal to low-side injector  507  are closed such that the second voltage reference is routed to a second terminal of fuel injectors  520 ,  522 ,  524 , and  526 . Current then flows from the second voltage reference to the first voltage reference in a direction that reverse biases a diode in an electrical path going to the heating element of each fuel injector. In this way, current is allowed to flow through the injector actuator coils but is limited or blocked from passing through the injector heating element. Thus, current can be driven in one direction to actuated the fuel injector and in a different direction to heat fuel in the injector. 
         [0044]    It should be noted that when current is driven in a direction that forward biases the diodes illustrated in  FIG. 5 , the level of current can be restricted or regulated by high-side driver  505  such that the fuel injector is not actuated. 
         [0045]      FIG. 6  is plot of example current supplied to fuel injectors of a four-cylinder engine. Signals INJ 1 - 4  represent current delivered to fuel injectors. The “+” represents current being driven into a fuel injector in a second direction. The “−” represents current being driven into the fuel injector in a first direction, opposite the second direction. The location that is approximately half way between the “+” and “−” represents substantially no current flowing into the fuel injector. 
         [0046]    Engine position relative to top-dead-center compression stroke of cylinder number one is represented by the signal labeled CRK. Engine cranking and starting begins at vertical marker  601  and the sequence flows from left to right. 
         [0047]    Note that the current illustrated in  FIG. 6  is not necessarily indicative of the actual current profile. Current is illustrated in  FIG. 6  to show an example of when fuel heating may be accomplished relative to fuel injector actuation, the illustration is not meant to illustrate an actual current profile. Also note that fuel heating time may vary from that illustrated without deviating from the scope or breadth of the description. For example, the injector opening timing illustrated at  605  may be increased or decreased or changed with respect to engine position. Further, the amount of current used to open the fuel injector may be increase above the amount of current necessary to open the fuel injector. The resulting additional current can be transformed into heat to further heat the fuel within the injector. Likewise, fuel heating intervals may also vary from those illustrated. For example, between injections at  605  and  619  the heater is shown as being on for the entire interval. However, if desired, the fuel heating may take place for only a fraction of the interval. Furthermore, only a few engine cycles are shown whereas fuel heating may go on for a predetermined period of time or for a specific number of cylinder cycles that exceeds the number illustrated. 
         [0048]    Also note that the present method is capable of heating fuel over the engine operating range if desired. For example, fuel heating can be used during a start as well as during engine operation. Heating fuel during engine operation allows the present method to control the cylinder charge temperature. 
         [0049]    At  603 , current is directed into fuel injector number one in the first direction. This begins heating fuel in the fuel injector. Current flow ceases briefly in region  604 . As the engine begins to rotate, right of vertical marker  601 , the first injector actuation command is issued at  605 . This command directs current into the fuel injector in a second direction. Fuel heating resumes in region  606  when current to fuel injector number one is resumed in the first direction. Fuel is injected to cylinder number one again when current is reversed and sent into fuel injector number one at  619 . The described sequence for heating and injecting fuel at fuel injector number one continues until heating is stopped in region  621 . 
         [0050]    Fuel injector number two follows a similar sequence as fuel injector number one, but fuel heating begins at region  607 , the first injection occurs at  609  and fuel heating is stopped at  623 . The initial fuel heating at  607  is offset in time from the fuel heating in injector one at  603 . This reduces the instantaneous current draw from the vehicle power source before the engine is started. If the vehicle power source has sufficient capacity, fuel in all fuel injectors may be simultaneously heated. In still another embodiment, fuel may be heated at different times in selected groups of fuel injectors. Current at  611  and  615  represents initial fuel heating for cylinders three and four. Fuel injector current at  613  and  617  represents initial actuation current. Fuel injector current at  625  and  627  is substantially zero between injector openings because cylinder temperature has increased to a level that promotes fuel vaporization. 
         [0051]    Referring now to  FIG. 7 , a flow chart of an example fuel injector heating method is shown. 
         [0052]    Note that in at least one embodiment, current to actuate a fuel injector (actuation current) enters the fuel injector through an electrical connector having two pins and is delivered in a second direction. Current to heat fuel flowing through the fuel injector is delivered through the same electrical connector and pins but in a first direction, opposite to the second direction described to actuate the fuel injector. 
         [0053]    In step  701 , the routine determines if fuel heating has been requested. A request for fuel heating may come from an external routine or it may result from evaluating the state of sensors and systems. In one example, the states of engine temperature, time since last engine start, oil temperature, desired cylinder charge temperature, and fuel temperature are used to determine if fuel heating is desired. Further, operating conditions can be used to determine the duration of fuel heating. In one example, the fuel heating duration may be determined by retrieving empirically determined heating times from memory. Specific memory locations may be interrogated by indexing arrays that are organized by engine coolant temperature and engine oil temperature, for example. 
         [0054]    In step  703 , the fuel injector heating delivery mode is selected. The heating mode describes how and when the fuel heating is delivered to one or more fuel injectors. For example, during an engine start, heat may be delivered to fuel through a group of injectors in a sequential manner and the amount of heat delivered by each injector can be varied in response to operating conditions. 
         [0055]    In one embodiment, fuel heat delivery mode can be split into two regions. Specifically, the time before the engine is started and the time after the engine is started. Heat may be delivered to the fuel through a fuel injector before a start in a way that may be different from the way that heat is added to fuel after a start. For example, before the engine begins to rotate the heating sequence may be based on time. That is, current can be sent to heat a different individual injector every 2 seconds, for example. After the engine is started, heat may be delivered at predetermined crankshaft intervals for a predetermined time or crankshaft angle. 
         [0056]    Fuel heating by the fuel injector may be delivered to the injectors simultaneously; to groups of injectors where the injectors of a group are simultaneously heated, and where the injector groups are heated at different times; sequentially to all or a group of injectors; or in combinations of the before-mentioned ways. In one embodiment, current is supplied to two or more fuel injectors simultaneously. That is, current for injector heating the injectors is delivered at substantially the same time. Alternatively, it is also possible to deliver current to heat the injectors sequentially. For example, current for injector heating can be supplied to a first injector, stopped, supplied to a second injector, stopped, and continued in the same manner to the remaining injectors. 
         [0057]    In addition, this sequence may be repeated until operating conditions, such as time since key-on has reached a predetermined level or until engine oil temperature reaches a desired level, for example. As mentioned above, after the engine is started, the fuel injector heating may be continued or may be stopped. Further, the amount of heat transferred when the engine is stopped may be different from the amount of heat delivered after the engine is started. 
         [0058]    Engine operating conditions (e.g., engine temperature, fuel temperature, cylinder charge temperature, barometric pressure) may be used to determine when to deactivate injector heating. In addition, the fuel heating mode and the timing when heat is delivered to the fuel may also be varied as the engine begins to rotate. 
         [0059]      FIG. 6  shows one example of a fuel injector heating delivery mode that is available from the present description. Specifically, injector heating is delivered at predetermined crankshaft intervals so that the heating does not interfere with injector operation. Further, it is also possible to briefly deactivate heating to one injector if current is needed to actuate another fuel injector during the same crankshaft interval. For example, if fuel injector heating is scheduled for cylinder number four fuel injector between 540 and 0 crankshaft degrees referenced to top-dead-center of cylinder one, and fuel injection is scheduled for cylinder number one during this same interval, then the heating for cylinder four fuel injector may be deactivated while injection commands are issued to the cylinder number one fuel injector. 
         [0060]    Continuing with step  703 , the heating mode may be determined by assessing engine operating conditions, injector operating conditions including barometric pressure, humidity, cylinder charge temperature, and engine temperature. In one embodiment, the operating conditions may be used to exercise a state machine that can activate different heat delivery modes before and after starting. The selection of these heat delivery modes may be empirically determined, for example.  FIG. 6  provides a sample of the available heating modes that may be selected. The routine proceeds to step  705  after the heat delivery mode is selected. 
         [0061]    Referring now to step  705 , the fuel is heated in the injectors. In one embodiment, the fuel heating duration may be reduced or increased based on the type of fuel being heated. Specifically, in one example, a sensor can determine the concentration ethanol in a fuel line leading to the fuel injector. The fuel heating time can be varied as the concentration of ethanol increases in the fuel line. In addition, the rate that heat is delivered to the fuel can be varied as the fuel type varies (e.g., as the concentration of ethanol varies) by varying the amount of current supplied to the heating element. Further, the rate heat is transferred and/or the duration of fuel heating can be varied as the engine&#39;s or vehicle&#39;s altitude varies. Further still, the rate heat is transferred and/or the duration of fuel heating can be varied as the ambient air humidity varies and/or as engine temperature varies. 
         [0062]    In one embodiment, the heating duration and heat transfer rate are empirically determined and stored in engine controller memory for later retrieval and use. In one embodiment, the amount of fuel heating is reduced as barometric pressure is reduced (i.e., altitude increases). 
         [0063]    As noted above, the present method can also adjust fuel temperature to affect the cylinder charge temperature. In one embodiment, desired cylinder charge temperature is mapped over the engine operating ranges for a particular type of fuel (e.g., ethanol). A model infers cylinder charge temperature from intake air temperature, engine temperature, engine speed, cylinder air charge amount, fuel type, and injection timing. If the inferred cylinder charge temperature deviates from the desired cylinder charge temperature, then heat can be added to the fuel (i.e., the rate of heat addition and/or the amount of time heat is delivered to fuel) or the heater can be deactivated so that the desired temperature is reached. 
         [0064]    Thus, the present method is capable of adjusting the rate of heat transfer from a fuel injector to fuel, as well as the fuel heating duration in response to environmental and vehicle operating conditions. 
         [0065]    In one example, the amount of heat transferred over a period of time to the fuel delivered to the engine after the engine is started may be increased as compared to the amount of heat delivered to fuel before the engine is started. The present method also allows different heat transfer rates to the fuel depending on the power source. When the power to heat fuel comes from a battery, current may be a first amount. When power to heat fuel comes from an alternator, current may be a second amount, different from the first amount. 
         [0066]    As previously mentioned, the fuel may be heated by PTC or NTC devices. Further, the actuator coil may be used to heat the fuel as well. The PTC/NTC heating elements may transfer heat directly to fuel or they may transfer heat to fuel through an intermediate material by conduction. Similarly, the actuator coil may transfer heat to fuel by using the internal resistance of the fuel injector coil to heat the injector components that surround the coil. The coil heat can be transferred to the surrounding material through conduction. The coil resistance transforms the electrical energy entering the coil into thermal energy. By applying a controlled current to the fuel injector coil, the temperature of the injector coil may be regulated so that the coil transfers a desired amount of thermal energy to the surrounding injector while maintaining the temperature of the coil below a predetermined level. In one example, current supplied to the coil is regulated below a predetermined amount so that there is insufficient current to operate the injector, but enough to heat fuel within the injector. 
         [0067]    In addition, eddy current heating may also be used to heat fuel by generating a time-varying magnetic field from varying the current that flows into the actuator coil. The current may be varied in a variety of ways. For example, the current entering the coil may be increased and decreased over a specified time interval, or if the engine is rotating, the current may be increased or decreased over a specified crankshaft interval (e.g., The excitation frequency may be adjusted by a predetermined amount every 3600 crankshaft angle degrees. As the current varies, a magnetic field external to the coil varies and ferrous material in the field resists the changing magnetic field, thereby heating the ferrous material. Heat is conducted from the ferrous material to the fuel. 
         [0068]    The current flow to the fuel injector may be controlled by an H bridge that allows bi-directional current flow, or by other circuitry that provides a similar function. 
         [0069]    Also note that the fuel injection timing may be adjusted as a function of the time fuel injectors are heated or as the amount of heating energy supplied to a fuel injector varies. For example, at a constant engine speed and load, the fuel injection pulse-width may be decreased as the amount of heat energy supplied to a fuel injector increases. This feature allows an engine controller to compensate for the improved response of a heated injector. After the coils start to heat, the routine proceeds to step  707 . 
         [0070]    In step  707 , the routine determines whether or not the engine is ready to start after fuel heating has commenced. In one embodiment, if the fuel has reached a desired temperature or a time since key-on, the engine controller  12  can notify the operator that the engine is ready to start or the engine may be started in other circumstances. In other embodiments, the engine may be considered ready to start after a desired amount of heating energy has been supplied to fuel in one or more injectors. For example, the engine may be considered ready to start if a predetermined number of joules of energy have been dissipated by each fuel injector heating element. Also note that in some embodiments, the engine may be allowed to start as soon as instructed by an operator. That is, fuel heating may be initiated but the engine may be started whether or not fuel has reached a desired temperature. If this mode of operation is selected, the fuel pulse-width may be adjusted to improve starting. If the routine determines that the engine is ready to start the routine proceeds to step  711 . Otherwise, the routine returns to step  705 . 
         [0071]    In step  711 , the injectors are controlled so that the desired amount of fuel is injected to the cylinders at the desired timing. That is, current is delivered in a second direction such that it flows through the actuator coil substantially unencumbered (e.g., a small reduction in current caused by a voltage drop across a diode or similar device is anticipated and permissible). When current is directed in this manner, the fuel injectors are operated in a way that is similar to conditions when injector heating is not desired. That is, current is supplied to the injector at a crankshaft angle and desired duration that delivers the desired amount of fuel to the cylinder. 
         [0072]    In step  713 , the routine determines if fuel heating is desired while the engine is operating. If it is, the routine proceeds to step  715 . If not, the routine proceeds to exit. If no fuel heating is desired during engine operation, current flow is limited to the second direction, and the injectors are operated by the main fuel injection routine and fuel is delivered in response the engine speed, operator demand, and operating conditions. 
         [0073]    In step  715 , the fuel is heated by applying current to the fuel injector in a first direction while the injector is not actuated. That is, as described above, when current flows to the fuel injector in a second direction the injector is actuated. Current flows to the fuel injector in a first direction, different from the second direction, to heat fuel passing through the injector. Accordingly, current is repeatedly reversed as the engine operates. For example, current flows into the coil when it is delivered to the injector in a second direction. When the injector has delivered the desired amount of fuel, the current is reversed and delivered in a first direction to heat fuel passing through the injector. The heating current may be delivered to the fuel injector for the entire period between fuel injections, or it may be delivered for a fraction of the period between injections. 
         [0074]    The rate of heat delivery to the fuel and the duration heat is transferred to the fuel can be an open-loop or closed-loop control process. In one embodiment, fuel heating rate and duration are determined after assessing engine temperature, barometric pressure, and humidity. In this example, fuel heating follows a prescribed schedule that has been programmed into the engine controller. 
         [0075]    In a closed-loop embodiment, engine sensors are repeatedly monitored and used to determine operating conditions so that the heat transfer rate and duration of fuel heating can be revised as engine operating conditions vary. Specifically, the following calculations are one example method to determine the heat transfer rate. 
         [0000]      HeatCur=Basecur( N )·cur h (hum)·cur f tem(fueltemp)·curetem(engtemp)·cur f typ(ftype)·curbar(baro) 
         [0000]    Where HeatCur is the amount of current to deliver in a heating interval, where Basecur is an empirically determined base amount of current that is a function of engine speed N, where curh is a modifier that is a function of humidity hum, where curftem is a modifier that is a function of fuel temperature fueltemp, where curetemp is a modifier that is a function of engine temperature engtemp, where curftype is a modifier that is a function of fuel type ftype, and where curbar is a modifier that is a function of barometric pressure baro. 
         [0076]    The fuel heating duration can be determined from a similar function. 
         [0000]      DurCur=Basedur·dur h (hum)·dur f tem(fueltemp)·dur e tem(engtemp)·dur f typ(ftype)·durbar(baro) 
         [0000]    Where DurCur is the duration current is delivered, where Basedur is an empirically determined base duration of current, where durh is a modifier that is a function of humidity hum, where durftem is a modifier that is a function of fuel temperature fueltemp, where duretemp is a modifier that is a function of engine temperature engtemp, where durftype is a modifier that is a function of fuel type ftype, and where durbar is a modifier that is a function of barometric pressure baro. 
         [0077]    Note as mentioned above, current control can vary depending on the circuitry within the fuel injector. For example, if current is impeded in one direction through the PTC/NTC heating element, and current is not impeded through the actuator coil, it may be desirable to limit current flow to the entire fuel injector (actuator coil and heating element) so that the fuel injector does not actuate when fuel is being heated. On the other hand, if current flow can be impeded through both the actuator coil and the heating element, heating current may not have to be limited to as low of a level as if current where flowing through both the actuator coil and the heating element. 
         [0078]    While the engine is being operated, it is desirable to keep the fuel injectors delivering a commanded amount of fuel. This can be accomplished by heating the injector during the portion of a cylinder cycle where fuel is not injected. For example, the fuel injectors may be heated during the power or exhaust strokes.  FIG. 6  shows an example of heating the fuel injectors while the engine is operating. Of course, the fuel injector heating interval may be varied from that which is shown in  FIG. 6 , if desired. One convenient way to achieve heating during engine operating is to time the heating period with engine positions. That is, the heating interval can be between bottom-dead-center of the exhaust stroke and top-dead-center of the exhaust stroke of the cylinder associated with the injector being heated, for example. After the coil current sequences are determined and commanded the routine returns to step  711 . 
         [0079]    This concludes the description. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the description. For example, I3, I4, I5, V6, V8, V10, V12, and turbine engines operating on non-limiting fuel types such as ethanol, kerosene, jet fuel, gasoline, propane, proponol, diesel, or other alternative fuel configurations could use the present description to advantage.