Abstract:
A double walled fuel line connects diesel fuel injectors in a common rail system. An inner line carries high pressure fuel from a fuel pump to the fuel injectors. Low pressure return fuel flows back through an outer fuel line which acts as a return line. The double walled line reduces the number of separate pipe connections needed in the system and provides protection against loss of high pressure fuel from the system as well as a simplified method for detecting fuel leaks. Monitoring fuel pressure in the low pressure line allows a fuel pressure sensor to detect leaks in the system by comparing fuel pressure in the low pressure line against typical line pressure. Higher than normal pressure in the low pressure line indicates a leak in the high pressure line. Lower than normal pressure indicates a leak in the low pressure return line.

Description:
TECHNICAL FIELD 
     This invention relates to fuel injection systems for diesel engines. 
     BACKGROUND OF THE INVENTION 
     Typical common rail fuel injection systems have one localized high pressure fuel pump connected by serially connected inlet and outlet lines which carry high pressure fuel between the injectors and the fuel pump. The lines connecting to the fuel pump are single or double walled depending upon the application. Double walled fuel lines are currently used to provide additional leak protection around the inner fuel line. Specifically, the inner line is used to carry pressurized fuel while the outer line is kept dry to form an extra barrier between the inner fuel line and the high temperature surfaces in an engine. 
     SUMMARY OF THE INVENTION 
     The present invention provides a common rail diesel fuel injection system. The system uses a double walled fuel line having a high pressure inner fuel tube for delivering high pressure fuel from a fuel pump to a series of fuel injectors, and a low pressure outer fuel tube for returning low pressure fuel from the fuel injectors to the fuel pump or the fuel tank. The outer line surrounds the inner line and prevents leaks in the inner line from escaping into the engine compartment. The double walled fuel line provides an additional barrier of protection between the high pressure inner line and the engine as well as a convenient path for fuel flow both to and from the injectors. 
     Sensing fuel leaks in the system may be accomplished by monitoring fuel pressure in the low pressure fuel line and comparing it against a map of predetermined normal pressures. If a leak occurs in the outer line, the pressure in the line will drop below normal because of lost fuel. If a leak occurs in the inner line, high pressure fuel will flow into the low pressure outer line, increasing fuel pressure in the outer line. Therefore, an increase or decrease in the normal return fuel pressure in the outer line will indicate that there is a leak and whether it is in the inner or the outer line. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a fuel injection system according to the present invention. 
     FIG. 2 is an axial cross-sectional view of a double walled fuel line used in the system of FIG.  1 . 
     FIG. 3 is a partial cross-sectional view showing the connection of a fuel line with an injector. 
     FIG. 4 is a fragmentary view similar to FIG. 1 but showing an external fuel return to the fuel tank. 
     FIG. 5 is a fragmentary view similar to FIG. 4 but showing an external fuel return to the fuel pump inlet. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the drawings in detail, numeral  10  generally indicates a common rail fuel injection system for a diesel engine. System  10  includes a fuel tank  12  having an outlet  14  connected to a metering valve  15  of a low pressure fuel pump  16 . Fuel pump  16  has a metering valve outlet  18  connected to an inlet  20  of a fuel filter  22 . An outlet  24  of fuel filter  22  connects to an inlet  26  of a high pressure fuel pump  27 . An accumulator  28  collects high pressure fuel from the fuel pump  27 . 
     Accumulator  28  has an outlet  30  connected to a first fuel line  32 , which connects to an inlet  34  of a fuel injector  35 . Fuel injector  35  has an outlet  36 , connected to a second fuel line  38 . Additional fuel injectors  35  and fuel lines  38  are connected in a similar manner to form a common fuel rail  44 . The last fuel injector  35  in the series has an inlet  34  connected to a fuel line, but its outlet  36  has a plug  48  to terminate the common fuel rail. 
     In accordance with the present invention, fuel line  32  is double walled as shown in FIG.  2 . Fuel line  32  includes a high pressure fuel inner tube  50  surrounded by a low pressure fuel outer tube  52  and a coupling  54  mounted on the ends of both tubes. In one embodiment, the tubes  50 ,  52  and the coupling  54  cooperate with an inner collar  56  and a support sleeve  58 . These define a central high pressure fuel passage  60  and a surrounding low pressure return fuel passage  62 , both extending to opposite ends  64  of the fuel line  32 . The fuel lines  38 , which form a common rail connecting a series of fuel injectors  35  preferably have the same construction as the fuel line  32 . 
     FIG. 3 shows in cross-section the connection of a fuel line,  32  or  38  with the inlet  34  of one of the fuel injectors  35 . Both the inlet  34  and the outlet  36  define sockets connectable with the coupling  54  of the fuel lines  32 ,  38 . Thus, the high pressure inner tube  50  directly engages an injector member  66  to connect the central high pressure fuel passage  60  with an internal passage  68  which conducts high pressure fuel through the member  66  to an outlet socket  36 . The internal passage  68  also forms a T-junction with a high pressure inlet passage  70  of the member  66  to direct high pressure fuel into the body of the injector for injection into an engine cylinder. 
     The inlet an outlet sockets  34 ,  36  with fuel lines  32  or  38  also define a low pressure return fuel passage  72  extending from within the injector  35  through the member  66  to the return fuel passage  62  of the fuel line  32 . A low pressure fuel bypass passage  74  also extends between the inlet and outlet  34 ,  36  of each fuel injector  35  to allow low pressure fuel to pass from fuel lines  38  to fuel line  32 . 
     FIG.  1 . shows the outlet  30  of the accumulator  28  connected to fuel line  32 . The outlet  30  of the accumulator  28  is similar to the outlet  36  of the fuel injector  35 , in that it conducts high pressure fuel and receives low pressure fuel in a similar manner. Thus, high pressure fuel is delivered directly into the high pressure fuel passage  60  of the high pressure inner tube  50 . Similarly, return fuel is conducted from the return fuel passage  62  of the fuel line  32  into the body of accumulator  28 . The low pressure entering the accumulator  28  is returned to the inlet  15  of the fuel pump  16  through internal low pressure fuel passages, not shown, to be recirculated into the system. Alternatively, an external low pressure fuel tube may be provided to carry low pressure fuel from the accumulator  28  to the inlet  15  of the fuel pump  16  or to the fuel tank  12  if desired. 
     A low pressure fuel sensor  76  monitors return fuel pressure in the low pressure fuel return passages. A control unit  78  connects to the fuel pressure sensor  76  to compare the monitored fuel pressure against a normal fuel pressure map to determine if a fuel leak is present in the system  10 . Alternatively, the low fuel pressure sensor  76  may be located at any convenient location in the low pressure return passage  62  of the first fuel line  32 . A high fuel pressure sensor  80  could also be provided to monitor fuel pressure in the high pressure fuel passage  60 . 
     In operation, the low pressure fuel pump  16  draws fuel from the fuel tank  12  through a fuel line  82 . The fuel pump  16  sends low pressure fuel through the fuel filter  22  into the high pressure fuel pump  27 . High pressure fuel from fuel pump  27  is pumped into the accumulator  28  where pulsations in the fuel are reduced. The high pressure fuel sensor  80  monitors the fuel pressure inside the accumulator. The high pressure fuel is then delivered through the inner tube  50  of the double walled fuel line  32  and series connected fuel lines  38  to the injectors  35 . The injectors are controlled by the control unit  78  to conventionally deliver timed charges of atomized fuel to associated engine cylinders, not shown. 
     A small amount of the high pressure fuel leaks past the injection valves, not shown, of the injectors and is returned through the internal low pressure fuel passage  72  to the low pressure return fuel passages  62  of the associated fuel lines  38 ,  32 . 
     Low pressure fuel in the return passages  62  is returned to internal passages, not shown, of the accumulator  28  where its pressure is monitored by the low pressure fuel sensor  76 . The sensor output is fed to the control unit  78  which determines if a leak is present in the system  10 . For this purpose, the indicated pressure of the low pressure return fuel is compared to a map of normal fuel pressures as a function of engine parameters. If a leak in the outer tube  52  occurs, the sensor  76  will detect a below normal fuel pressure, because fuel will escape from the outer tube  52  into its surrounding environment. If a leak in the inner tube  50  occurs, the sensor will detect above normal fuel pressure, because high pressure fuel from the inner tube  50  will escape into the low pressure outer tube  52 . Under normal operating conditions the fuel pressure in the low pressure tube  2  will remain within a normal range. 
     If a leak is detected in the system, the metering valve  15  is closed, stopping the flow of fuel into the system. The metering valve  15  can also be used to reduce the flow of fuel through the system by partially closing. 
     Referring now to FIG. 4 of the drawings, numeral  85  generally indicates a variation of the system  10  of FIG. 1 in which like numerals indicate like parts. Thus, low pressure fuel pump  16 , high pressure fuel pump  27 , fuel injectors  35 , and fuel line  32  are physically similar and operate in a similar manner to those of system  10 . 
     System  85  includes a T-junction  86  at the outlet port  30  of the accumulator  28 . A low pressure fuel return line  88  extends from the T-junction  86  directly to the fuel tank  12 . A low pressure fuel sensor  90  is connected to the low pressure fuel return line  88  between the T-junction  86  and the fuel tank  12  to monitor the return fuel pressure in line  88 . 
     In operation, all of the initial steps relating to the delivery of fuel to the injectors are the same as system  10 . System  85  differs from system  10  by returning fuel to the fuel tank  12  instead of directly recirculating the fuel back to the low pressure pump through internal passages inside the housing of the pump. The returning fuel exits the double walled fuel line  32  through the T-junction  86  and flows into the low pressure fuel return line  88 . The low pressure fuel returning to the fuel tank  12  passes through the fuel pressure sensor  90  which sends a pressure signal to control unit  78  for use in leak detection as before. The low pressure fuel then returns to the fuel tank  12  where it can be recirculated into the system. 
     Referring now to FIG. 5 of the drawings, numeral  92  generally indicates a variation of the system  85  of FIG. 4 using most of the same components as indicated by like numerals. 
     System  92  differs in that the fuel return line  88  of FIG. 4 is replaced by a low pressure fuel return line  96  extending from the T-junction  86  directly to the metering valve inlet  15  of the low pressure fuel pump  16 . A low pressure fuel sensor  90  is connected to the low pressure fuel return line  96  between the T-junction  86  and the metering valve  15  to monitor fuel pressure in line  96 . 
     Operation of the system  92  of FIG. 5 is identical to that of system  85  of FIG. 4 except for the handling of return fuel. The low pressure fuel returning from the fuel injectors  35  is directed by the T-junction  86  into the low pressure fuel return line  96 , which carries the fuel to the pump  16  metering valve inlet  15 . The returning fuel passes through the fuel pressure sensor  90  which sends a pressure signal to the control unit  78  as before. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.