Abstract:
A fuel injector system for raising fuel to its supercritical state and injecting the supercritical-state fuel to the combustion chamber of an internal combustion engine is disclosed. A plurality of injector embodiments provides alternative ways to heat the pressurized fuel to its supercritical state. Injection of supercritical fuel into the combustion chamber is known to improve fuel entrainment and reducing ignition delay to thereby increase combustion rate, which leads to an increase in fuel efficiency. According to some embodiments, the system provides for preventing coking that may otherwise occur in an exhaust gas heat exchanger used for preheating the high pressure fuel. In other embodiments, engine cold start assistance is provided by storing pressurized, heated fuel in an insulated container.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 61/276,135 filed 8 Sep. 2009. 
    
    
     TECHNICAL FIELD 
     The present disclosure is related to the field of internal combustion engines and more specifically to improvements in fuel injection systems employed in such engines. 
     BACKGROUND 
     Several attempts have been made to provide supercritical-state fuel into the combustion chambers of internal combustion engines to obtain greater fuel efficiency through reduced ignition delay and more complete combustion, while using the improved EGR tolerance to reduce NOx emissions. 
     Supercritical-state fluid occurs when temperature and pressure reach a point where the fluid is neither a pure gas nor a pure liquid. Above the supercritical point the supercritical-state fluid can have properties that look more like a gas than a liquid, or can have properties that look more like a liquid than a gas, depending on the compound and the temperature and pressure surrounding the compound. 
     High pressure (over the critical point) creates high density. In an internal combustion engine, high density fuel allows for the creating of sprays with high kinetic energy to form a plume that promotes entrainment and mixing with air and a more complete and fast combustion with good air utilization. 
     Phase diagrams for CO.sub.2 are shown in  FIGS. 1 and 2 . In the pressure-temperature phase diagram of  FIG. 1 , the boiling boundary line  500  separates the gas and liquid regions and ends at the critical point  502 , where the liquid and gas phases disappear to become a single supercritical phase. The triple point  506  is a temperature and pressure condition at which all three phases coexist. The density-pressure phase diagram for CO.sub.2, in  FIG. 2  allows additional observations. At well below the critical temperature, e.g. 280 K, as the pressure increases, the gas compresses and eventually (at just over 40 bar) condenses into a much denser liquid, resulting in the discontinuity in the line  512  (vertical dashed line) under the liquid-vapor dome. The result is two phases in equilibrium: a dense liquid (with the density indicated at the upper end of the dashed line)  514  and a low density gas (with the density indicated at the lower end of the dashed line)  516 . As the critical temperature is approached (curve  518  is the isotherm at 300 K), the density of the gas at equilibrium becomes denser, and the density of the liquid becomes lower. At the critical point  520 , (304.1 K and 7.38 MPa (73.8 bar)). There is no difference in density, and the 2 phases become one fluid phase. Thus, above the critical temperature, e.g., 310 K shown as line  522 , a gas cannot be liquefied by pressure. At slightly above the critical temperature (310K), in the vicinity of the critical pressure, the density line is almost vertical. A small increase in pressure causes a large increase in the density of the supercritical phase. Many other physical properties also show large gradients with pressure near the critical point, e.g. viscosity, the relative permittivity and the solvent strength, which are all closely related to the density. At higher temperatures, the fluid starts to behave like a gas, as can be seen in  FIG. 2 . For carbon dioxide at 400 K, the density increases almost linearly with pressure, line  524 . 
     In Table 1 below, it can be seen that the range of density, viscosity and diffusivity for various fluids in their gas and liquid phases have different ranges of properties when the fluids reach their supercritical states. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Density 
                 Viscosity 
                 Diffusivity 
               
               
                   
                 (kg/m 3 ) 
                 (cP) 
                 (mm 2 /s) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Gases 
                   1 
                 0.01 
                  1-10 
               
               
                   
                 Supercritical fluids 
                 100-1000 
                 0.05-0.1 
                 0.01-0.1 
               
               
                   
                 Liquids 
                 1000 
                  0.5-1.0 
                 0.001 
               
               
                   
                   
               
             
          
         
       
     
     Additionally, there is no surface tension in a supercritical-state fluid, since there is no liquid/gas boundary. A change in pressure and temperature of the fluid can allow one to “tune” the fluid to be more liquid or more gas like. Solubility tends to increase with density of the fluid when held at a constant temperature potentially making solubility another important property of supercritical state fluids. Solubility of material in fluid is another important property of supercritical-state fluids, since solubility tends to increase with density of the fluid when held at constant temperature. Since density increases with pressure, solubility increases with temperature. However, close to the critical point ( 520  in  FIG. 2 ), the density can drop sharply with a slight increase in temperature. Therefore, close to the critical temperature, solubility often drops with increasing temperature, then rises again. Supercritical-state fluids are completely miscible with each other; thus, a single phase can be guaranteed for a mixture when the critical point of the mixture is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperature and pressures of the two components. For greater accuracy, the critical point can be calculated using equations of state, such as the Peng-Robinson equation or group contribution methods. Other properties such as density can also be calculated using equations of state. 
     SUMMARY 
     The present disclosure provides a fuel injector system in which supercritical-state fuel, such as super-critical state diesel fuel, is injected into the combustion chamber of an internal combustion engine. An arrangement in which the injector is coupled to the combustion chamber so that the fuel is injected directly in the combustion chamber is typically referred to as a direct-injection system. 
     In one embodiment, the fuel used to hydraulically activate the injector is separated from supercritical-state fuel that is injected into the combustion chamber of an internal combustion engine. 
     In an embodiment, fuel is heated to the super-critical state by use of one or more glow plugs immediately preceding the injector. 
     In another embodiment, fuel is heated to be super-critical state within the injector by electrical induction. 
     In one embodiment, the supercritical-state fuel is preheated in an exhaust gas heat exchange system prior to being heated to its supercritical-state. 
     In one embodiment, electric energy is provided by an exhaust gas thermo-electric generator and the electric power heats the fuel by glow plugs or induction heating upstream of or in the injector(s) to arrive at supercritical state. 
     In one embodiment, cooling of the preheated and supercritically heated fuel is accomplished immediately following operational shut down of the internal combustion engine. 
     In one embodiment, storage of a quantity of preheated fuel is maintained immediately following operational shut down of the internal combustion engine to be available to the injectors upon the next start up of the engine. 
     Although  FIGS. 1 and 2  relate to CO 2 , similar graphs can be determined for any material, including blends such as fuels including a range of hydrocarbons. Supercritical-state conditions for typical hydrocarbon blends are achieved at or above 570 K and 50 bar pressure. Ambient temperature fuel is pressurized by the high-pressure injection pump. 
     Injectors of the present disclosure are configured to reduce heat losses and radiation by reducing metal volume heat sink, thermal insulation within Injector body, keep hydraulic amplification fuel and fuel return line cold, all by e.g. ceramic insulation. 
     In one embodiment temperature control of the exhaust gas heat exchanger is achieved through hot soak scavenging to avoid coking. 
     This disclosure involves improvements to any internal combustion engine, including spark-ignition and compression-ignition engines, as examples. One non-limiting example internal combustion engine is opposed-piston, opposed-cylinder (OPOC) engine described and claimed in U.S. Pat. Nos. 6,170,443; 7,434,550; and 7,578,267 that are incorporated herein by reference. 
     Key features of the disclosed embodiments include fuel injectors that are configured to inject fuel into the combustion chamber while in its supercritical-state. The use of supercritical-state fuel facilitates short ignition delay and fast combustion thereby avoiding emissions of unburned fuel due to quenching at cylinder walls and in combustion chamber crevices. Because the combustion rate is very fast with supercritical-state fuel, droplet diffusion combustion is substantially eliminated. Fast combustion yields a high rate of pressure rise that can cause undesirably high levels of noise, but higher thermal efficiency of the engine cycle. In conventional engines, the noise may be troublesome. However, in an OPOC engine, very little noise is transmitted outside the engine due to the lack of a cylinder head. 
     Also, advanced thermal management techniques are utilized to prevent coking during the cool-down period following engine operation. 
     A fuel injector is disclosed that can provide supercritical-state fuel to the combustion chamber of an internal combustion engine. 
     In one embodiment, the fuel injector is maintains separation between fuel used to provide hydraulic operation of the fuel injector and the supercritical-state fuel that is injected into the engine. 
     According to an embodiment of the present disclosure, a fuel injector is provided that receives supercritical-state fuel from a heat source external to the injector and isolates supercritical temperatures from the actuation mechanism of the injector. 
     In yet another embodiment of the present disclosure, a fuel injector is provided that receives fuel from a source preheated to a temperature that is less than the supercritical-state and heats the preheated fuel to a supercritical state within the injector prior to being injected into the internal combustion engine. 
     In yet another embodiment of the present disclosure, a fuel injector is provided in which fuel is heated to a supercritical state by the application of an electrical induction field. 
     In yet another embodiment of the present disclosure, a fuel injector is provided in which the fuel is heated to a supercritical state within the injector by the application of an electrical induction field where the electric power is transmitted by a transformer coil. 
     In some embodiments, the fuel injector system provides cooling of the injectors immediately following stopping the operation of the associated engine. 
     In yet other embodiments, the fuel injector system provides cooling to fuel preheating elements following stopping the operation of the associated engine. 
     In yet another embodiments of the present disclosure, a fuel injector is provided that captures and stores a quantity of preheated fuel immediately following stopping the operation of the associated engine for delivery to the injectors upon the next start up of the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is plot of temperature vs. pressure for CO 2  that illustrates the various phase boundaries, including the supercritical-state. 
         FIG. 2  is a plot of pressure vs. density for CO2 showing several isotherms to illustrate the dramatic changes in density that are available for particular temperatures in the supercritical state. 
         FIG. 3  is a conceptual illustration showing a preheating and supercritical-state heating system embodiment of the present disclosure. 
         FIG. 4  is a cross-sectional view of a fuel injector according to the present disclosure. 
         FIG. 5  is an enlarged cross-sectional view of the injector needle/nozzle end of the injector shown in  FIG. 4 . 
         FIG. 6  a cross-sectional view of another embodiment of a fuel injector utilizing induction heating according to the present disclosure. 
         FIG. 7  a cross-sectional view of another embodiment of a fuel injector of the present disclosure utilizing another configuration of induction heating and the electric power transmission through a transformer coil. 
         FIG. 8  is a schematic representation of a fuel supply/injector system. 
     
    
    
     DETAILED DESCRIPTION 
     As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. 
       FIG. 3  illustrates a fuel-injection system  100  in which fuel is raised to its supercritical-state and introduced into the combustion chambers  110  and  111  of cylinders  108  and  109  respectively, for combustion. In this embodiment, system  100  is shown associated with opposing cylinders  108  and  109  of a single OPOC engine module, as shown and described in the above “incorporated by reference” patents. 
     In this embodiment, which operates with a compression ignition diesel process, can be used with any liquid fuel in super critical phase. Each combustion chamber has a pair of fuel injectors mounted in opposition on the cylinders. Injectors  150  and  152  are mounted on cylinder  108  and injectors  151  and  153  are mounted on cylinder  109 . Each injector receives heated fuel via a high pressure line ( 180 ,  182 ,  181  and  183 ) directly from glow plug heat chambers  140 ,  142 ,  141  and  143 , respectively. In an alternative embodiment, only one injector per cylinder is provided. In yet another embodiment, a port fuel injector is provided, such as in a spark-ignition application. 
     A high pressure fuel pump  102  provides fuel to the hydraulic portions of the injectors in a conventional manner through a high pressure, low temperature common rail  104 . Line  160  provides a fuel connection from common rail  104  to the hydraulic actuation portion of injector  150 . Likewise, line  162  connects to injector  152 , line  161  connects to injector  151  and line  163  connects to injector  153 . Common rail  104  also provides fuel in fuel lines  170 ,  172 ,  171  and  173  through an exhaust gas heat exchanger  106  to glow plug heat chambers  140 ,  142 ,  141  and  143 , respectively. 
     During the time fuel is flowing through the fuel lines within the exhaust gas heat exchanger  106 , heat from exhaust gases is transferred to the fuel and serves to preheat the fuel to a temperature below that which is necessary to reach supercritical state at the internal pressure of the respective fuel lines. Preheated fuel then flows into glow plug heat chambers  140 ,  142 ,  141  and  143  as demanded through operation of the individual injectors. Each glow plug heat chamber transfers energy to the fuel to cause it to enter its supercritical state prior to entering the injector and being sprayed into the combustion chamber. Although not shown, several sensors are included to monitor the pressure and temperature of the fuel at various locations within the system to allow for adjustments, to determine the injected fuel is in its supercritical state. 
       FIGS. 4 and 5  provide cross-sectional plan views of a fuel injector embodiment. The injector  200  contains an upper body  210  having a lower external thread portion  212  and an internal set of bores  211  and  213 . Upper bore  211  is configured to allow shaft  218  of injector needle  220  to move in a longitudinal motion. Lower bore  213  is larger than and in axial communication with upper bore  211 . Lower bore  213  serves to contain a biasing spring  215  and spring flange  219  that extends laterally from shaft  218 . 
     A lower body  214  is threadably connected to upper body  210  and provides sealed support to injector needle housing  216 . Needle housing  216  contains an inner bore  223  that is in communication with and larger than a lower inner bore  225 . An actuation chamber  232  is formed in inner bore  223  and is in fluid communication with a hydraulic actuation passage  230 . Injector tip  240  extends into the combustion chamber of an engine and a plurality of nozzle apertures  244  are provided at injector tip  240 . The internal portion of bore  225  in tip  240  contains a conical or concave needle seat  242  which is configured with a circular sealing element to mate with a corresponding sealing element on the conical or convex tip  222  of injector needle  221 . 
     Injector needle  221  contains an actuation shoulder  209  adjacent actuation chamber  232  onto which hydraulic pressure acts to assist the movement of the needle. Lower down on needle  221 , an injection passage  224  is provided that runs from an opening  262  in the side wall of needle  221  to needle tip  222  and provides an opening  264  through which fuel is delivered to nozzles  244  when needle  221  is retracted. A fuel passage  260  is formed in body  210  to deliver fuel to side opening  262  of injection passage  224 . 
     A labyrinth cut  226  in injector needle  221  above the location of injection passage  224  and below actuation chamber  232  functions to insulate, by restricting the flow of heat from supercritical-state fuel present in injection passage  224  from migrating into actuation chamber  232 . Allowing the actuation fuel to flow in and out of actuation chamber  232  provides additional temperature maintenance in chamber  232 . 
     Although not shown in  FIGS. 4 and 5 , hydraulic passage  230  extends from a conventional hydraulic actuation control that provides increased pressure in passage  230  which in turn acts on shoulder  209  to assist electromagnetically actuated movement shaft  218  and needle  221  against the normally closed biasing pressure of spring  215 . 
     In operation, fuel is heated to its supercritical state, as for example in  FIG. 3 , and delivered under pressure to fuel passage  260 . Injector needle  221  is shown in both  FIGS. 4 and 5  to be in its retracted and open position so that face of needle tip  222  is spaced from needle seat  242 , allowing supercritical-state fuel to be forced through nozzles  244   a - x  and into the combustion chamber. At the end of the injection period, the hydraulic pressure in chamber  232  is reduced and the injector controller releases shaft  218  to allow needle  221  to move longitudinally towards tip  240 . By the action of biasing spring  215  on flange  219 , the face of needle tip  222  abuts needle seat  242  and nozzles  244  become closed. Supercritical-state fuel remains in injection passage  224  until the next injection cycle. 
     Another embodiment of a supercritical injector  300  is shown in  FIG. 6  that utilizes electrical induction to heat fuel within the injector prior to being injected into the combustion chamber in its supercritical state. Elements of injector  300  include an upper sleeve body  316  that is threaded or otherwise sealingly connected to a lower housing  310 , an intermediate body element  307  and a lower needle housing  317 . Upper sleeve body  316  contains a central bore  311  for supporting upper injector needle shaft  318 . A hydraulic actuation chamber  323  is below bore  311  to allow unheated fuel to be employed as hydraulic fluid. Unheated fuel is introduced into hydraulic actuation chamber  323  in a conventional manner to assist a conventional electromechanical actuator to operate the movement of injector needle  318  at predetermined portions of the injection cycle. Lower needle housing body  317  is positioned at the lower end of injector  300  and contains a heating chamber  319  that surrounds a lower portion of injector needle  320 . Heating chamber  319  receives preheated fuel from a preheating source through fuel passage  360 . (See  FIGS. 3 and 8  for examples of preheating sources.) Fuel passage  360  has an open end  362  that is in communication with heating chamber  319 . Grooves or loose spacing  325  between needle  320  and bore  324  in the lower portion of lower needle housing body  317  allow heated fuel from heating chamber  319  to enter spray nozzle portion  340  of injector  300 , when needle tip  322  is retracted during its injection cycle. 
     In this embodiment, induction heating of fuel to its supercritical state is achieved by the use of an induction coil  330  mounted within heating chamber  319  to surround needle  320 . Induction coil  330  is connected to wires  332 . When connected to an electrical source, via wires  332 , induction coil  330  generates an electrical field that induces heat in the portion of injection needle  320  that is within heating chamber  319 . Induction occurring in the range of 4 kHz has been found to provide adequate heating. Fuel within heating chamber  319  and forced alongside needle  320  towards nozzle  340  in grooves or spacing  325  is heated by its contact with the outer surface of needle  320  to its supercritical state just before it reaches spray nozzle portion  340 . 
     An insulator  321  is contained within needle  320  that is disposed within bore  309  to resist the migration of heat, from the lower part of needle  320  that is subjected to induction heating, to the upper portion. Other insulating sheaves  312 ,  313  and  314  (in one non-limiting example, ceramic) are provided between body and housing elements to help contain the heating necessary to place the fuel in its supercritical state. 
     Since the injector components are subjected to high heat during engine operation, there may be a danger of coking after the engine is stopped and the injector components are subjected to hot soak and the fuel is stationary in the injector. The embodiment of  FIG. 6  is shown to employ tubular coils  330  to allow unheated fuel to be pumped there-through when the engine is shut off. This provides an immediate cool-down effect to heating chamber  319  as well as the other injector components that are subjected to supercritical temperatures and potential coking. 
     Another embodiment of a supercritical injector  400  is shown in  FIG. 7  that utilizes electrical induction to raise the temperature of fuel within the injector higher than the supercritical temperature prior to being injected into the combustion chamber. Elements of injector  400  include an upper sleeve body  416  that is threaded or otherwise sealingly connected to a lower housing  410 , an intermediate body element  407  and a lower needle housing  417 . Upper sleeve body  416  contains a central bore  411  for supporting upper injector needle shaft  418 . A hydraulic actuation chamber  423  is below bore  411  to allow unheated fuel to be employed as hydraulic fluid. Unheated fuel is introduced into hydraulic actuation chamber  423  in a conventional manner to assist a conventional electromechanical actuator to operate the movement of injector needle shaft  418  at predetermined portions of the injection cycle. Lower needle housing body  417  is positioned at the lower end of injector  400  and contains a heating chamber  419  that surrounds a lower portion of injector needle  420 . Heating chamber  419  receives preheated fuel from a preheating source through fuel passage  460 . (See  FIGS. 3 and 8  for examples of preheating sources.) Fuel passage  460  has an open end  462  that is in communication with heating chamber  419 . Grooves or loose spacing  425  between needle  420  and bore  424  in the lower portion of lower needle housing body  417  allow heated fuel from heating chamber  419  to enter spray nozzle portion  440  of injector  400 , when needle tip  422  is retracted during its injection cycle. 
     In this embodiment induction heating of fuel to its supercritical state is achieved by the use of a primary transformer coil  450  mounted between lower housing  410  and lower needle housing body  417 . Induction coil  430  mounted within heating chamber  419  surrounds needle  420 . Primary transformer coil  450  is connected to wires  432 . When connected to an electrical source, via wires  432 , primary transformer coil  450  generates an electrical field that induces heat in the portion of injection needle  420  that is within heating chamber  419 . Induction frequency in the range of 4 kHz has been found to provide adequate heating. Primary transformer coil  450  also induces current to flow in induction coil  430  and because of impedance in induction coil  430 , provides additional heat to fuel within heating chamber  419 . Fuel within heating chamber  419  and forced alongside needle  420  towards nozzle  440  in grooves or spacing  425  is heated by its contact with the outer surface of needle  420  to its supercritical state just before it reaches spray nozzle portion  440 . 
     An insulator  421  is contained within needle  420  that is disposed within bore  409  to resist the migration of energy from the lower part of needle  420  that is subjected to induction heating to the upper portion. Other insulating sheaves  412 ,  413  and  415  (in a non-limiting example, ceramic) are provided between body and housing elements to help contain the heating necessary to place the fuel in its supercritical state. 
     The supercritical-state fuel injection system of  FIG. 8  is shown in association with an opposed-piston, opposed-cylinder engine  11  of the type shown and disclosed in the above “incorporated by reference” patents. A fuel tank  1  includes a lift pump  2  which provides fuel, under comparatively low pressure and at ambient temperature, through a fuel filter  3  to the input port of a high pressure fuel pump  4 . The fuel pump  4  provides fuel at high pressure and ambient temperature to a low temperature common rail  12  for distribution to the hydraulic portion of each fuel injector  19  (although only one injector  19  is shown, it is understood that at least one injector, port or combustion chamber mounted, is provided per cylinder). Fuel pump  4  also provides fuel at high pressure and ambient temperature to a normally closed and electrically controlled high pressure valve  5  that is in series with an insulated high pressure accumulator  6 . The fuel pump  4  further provides fuel at high pressure and ambient temperature to an exhaust gas heat exchanger  7  for preheating to a temperature that is below the temperature at which the fuel reaches its supercritical state. Excess fuel related to fuel pump  4  returns to fuel tank  1 . 
     Exhaust gas heat exchanger  7  lies in the exhaust gas path exiting the engine  11  and the turbine of a turbocharger  8 . In this example, turbocharger  8  is electrically controlled with an electric motor on its shaft between the compressor and the turbine. The preheated fuel exiting exhaust gas heat exchanger  7  is fed to a high temperature common rail  20  where it is distributed the fuel injectors such as the one shown as injector  19  where it is heated to its supercritical state for injection into the combustion chamber of engine  11 . Prior to reaching the common rail, the preheated and high pressure fuel flows through a high-pressure, insulated, latent-enthalpy, storage device  16  that is in parallel with a bypass line controlled by an electrically-controlled and normally open valve  15 . Upon leaving the parallel junction above  15  and  16 , a normally closed electrically controlled valve  17  sits in series with an insulated high pressure accumulator  18 . The unused fuel exiting high temperature common rail  20  is allowed to be bled off by an electrically controlled regulator  22  to a cooling heat exchanger  23  before is returned to tank  1 . Pressure sensor  21  is used to monitor the pressure in high temperature common rail  20  and provide information to the electronic control unit  24  (“ECU”). Similarly, pressure sensor  13  senses pressure and regulator  14  bleeds off fuel in low temperature common rail  12 . The preheated fuel exiting exhaust gas heat exchanger  7  is also fed, in parallel, to a normally-closed electrically-controlled valve  9  that is in series with a cooling heat exchanger  10 . 
     The system components shown in  FIG. 8  serve the normal function of providing hydraulic actuation fuel to fuel injector  19  and also provide preheated fuel to be injected into the combustion chamber of an internal combustion engine. This is especially important when combined with an injector or injectors of the type which raise the fuel temperature to the supercritical state. In addition, the system provides heated fuel storage for assisting in cold starting and flushing of high temperature fuel from components susceptible to coking when the engine operation is stopped. 
     During engine operation, valve  5  is initially opened to allow high pressure and ambient temperature fuel from high pressure pump to enter insulated high pressure accumulator  6  (a spring loaded piston in an insulated chamber) and to be stored therein until valve  5  is again opened, after engine shut down. At the time of engine shut down, valve  5  is again opened and the relatively cooled fuel in accumulator  6  flows through exhaust gas heat exchanger  7  and purges the heated fuel. This lowers the temperature of the fuel present in exhaust gas heat exchanger  7  below 500° C., depending on the fuel blend containing some portion of oxygenated hydrocarbons—a point where coking is not an issue. The hot fuel purged from heat exchanger  7  exits the system through opened valve  9 . 
     At the time of engine start up, it is desirable to have some degree of fuel preheating for the fuel to be placed in its supercritical state prior to injection. Achieving a supercritical state early retains the fuel efficiency of the system while keeping NOx emissions low. The system depicted in  FIG. 8  achieves that goal by using high-pressure, insulated, latent-enthalpy storage device  16  and insulated high-pressure accumulator  18 . Both components are set to receive preheated high-pressure fuel immediately upon shut down of the engine by opening valve  17  for a predetermined period of time and closing valve  17 . At the time of engine shut down, there is still some residual flow of preheated fuel in the high pressure system. Closing valve  15  causes residual fuel to flow into latent-enthalpy storage device  16  which is shown as a coil of tubing inside an insulated container. The preheated fuel remains in latent enthalpy storage device  16  until the engine is again started. Also, preheated fuel is stored in insulated high-pressure accumulator  18  during this shut down period by opening valve  17  for a predetermined period of time. 
     At the time of the next engine start up, valve  17  is again opened and prior to the high-pressure pump delivering preheated fuel to the common rail  20  and the injector  19 , the fuel then in storage device  16  and high-pressure accumulator  18  are forced towards common rail  20  and injector  19 . Whatever energy remains in the stored fuel becomes a benefit during this start up period. 
     Some components of diesel fuels are known to coke at higher temperatures. In particular, aromatics and olefins are prone to undergo chemical reactions, in the absence of oxygen, that lead to the formation of hydrocarbon components that adhere to surfaces. In particular, it is the double carbon-to-carbon bonds that are particularly reactive. After a period of time, the buildup of the coking materials can impair the performance of the injector system. 
     To limit the ability of the coking hydrocarbons from adhering to the internal surfaces of the injector, the injector may be coated with a material to limit such buildup, by interfering with the chemical reactions that form the coke and/or making the surface less hospitable to adherence. Gold, platinum, palladium, and titanium are materials that help to resist buildup of coking materials. Thus, in one embodiment, any surfaces downstream of the heater that raises fuel temperature to the supercritical state have one or more of the above-listed materials on their surface. In the case of the induction heater, the chamber in which the induction heater is located and everything downstream is coated. In the case of the glow plugs external to the injector, the chamber in which the glow plugs are located and all components downstream are coated. 
     In one embodiment, chemicals that interrupt the reaction paths leading to coking materials are provided to the fuel. Two such chemicals are hydrogen peroxide and methanol, both of which contain oxygen. By oxygenating the reactive double carbon-to-carbon bonds, the reaction mechanisms are altered thereby producing less of the coking materials. 
     Another embodiment to address the coking issue is for the injector tip to protrude into the combustion chamber, as shown in  FIG. 3 . In such an embodiment, the fuel is heated upstream of the injector tip to a temperature just below the supercritical temperature. By virtue of the tip being exposed to combustion gases, it is hotter than other portions of the injector and can act to further raise the temperature of the fuel at the tip to a temperature above the supercritical state. In one embodiment, measures are taken to insulate the injector tip from the rest of the injector, such as provided by insulators  314  and  415  in  FIGS. 6 and 7 , respectively. Referring now to  FIG. 3 , in another embodiment, an insulator  190  is provided between the injector and an orifice in the cylinder head into which it is installed. Since the cylinder head is typically water cooled, the proximity of the injector to the cylinder head may act to cool the injector if no such insulation were provided. 
     While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. Where one or more embodiments have been described as providing advantages or being preferred over other embodiments and/or over background art in regard to one or more desired characteristics, one of ordinary skill in the art will recognize that compromises may be made among various features to achieve desired system attributes, which may depend on the specific application or implementation. These attributes include, but are not limited to: efficiency, direct cost, strength, durability, life cycle cost, packaging, size, weight, serviceability, manufacturability, ease of assembly, marketability, appearance, etc. The embodiments described as being less desirable relative to other embodiments with respect to one or more characteristics are not outside the scope of the disclosure as claimed.