Patent Publication Number: US-6668800-B2

Title: Internal combustion engine fuel injection system

Description:
The present invention relates to a fuel injection system of an internal combustion engine having at least one cylinder cooperating with a piston activated to rotate a drive shaft. More specifically, the invention relates to an injection system comprising a pump having at least one pumping element activated to pump high-pressure fuel; a rail for the fuel so pumped; and an injector for injecting a given quantity of fuel from the rail into the engine cylinder. 
     BACKGROUND OF THE INVENTION 
     In old diesel engines, the injectors are supplied directly by a high-pressure fuel pump, the delivery of which is temporarily discontinuous, timed with the engine, and cyclically constant, i.e. a pump activated in synchronism with the injectors. This type of operation poses problems in adapting delivery of the pump to draw by the injectors, in the event of sharp variations in engine speed or load. 
     In modern internal combustion injection engines, each injector draws high-pressure fuel from a so-called “common rail”, which forms a fuel reserve for the injectors and is normally supplied by a high-pressure piston pump in turn supplied with fuel from the fuel tank by a low-pressure pump. 
     In modern engines, the high-pressure pump of known injection systems has a temporarily continuous delivery not timed with the engine, i.e. is activated, for example, by a cam and therefore supplies fuel substantially continuously to the common rail, whereas the injectors are activated at a predetermined stage in the engine cylinder cycle. The fuel pressure in the common rail is controlled by a pressure regulator, but, to cater to large withdrawals of fuel, the common rail must be of considerable volume and, therefore, size. The pump must also be sized to cater to maximum fuel withdrawal by the injectors as a whole during the engine cycle, so that the volumetric efficiency of the pump is relatively poor. 
     Known common-rail injection systems therefore cannot be fitted to old engines with injectors supplied directly by the high-pressure pump, on account of the bulk of the injection system, and the temporarily discontinuous delivery of the high-pressure pump, which is therefore unsuitable for common-rail injection systems. 
     Moreover, the pressure regulator of known common-rail injection systems normally comprises a valve controlled by an electromagnet and located between the high-pressure pump and the common rail. When the valve is closed, the fuel pumped by the high-pressure pump is fed to the rail; and, when the valve is opened partly or fully, the surplus fuel pumped is drained along a drain conduit back into the tank. 
     In known technology, the pressure regulating valve is closed by the electromagnet when this is energized, and is kept open by a spring when the electromagnet is deenergized, so that the electromagnet is energized by a high current to open the valve partly to regulate the fuel pressure. Moreover, if the electromagnet fails to be energized during operation of the engine, the valve is opened fully by the spring, thus draining the common rail completely and arresting the engine. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an internal combustion engine fuel injection system, which provides for a high degree of reliability, is cheap to produce, and eliminates the aforementioned drawbacks typically associated with known injection systems. 
     According to the present invention, there is provided a fuel injection system for an internal combustion engine having at least one cylinder cooperating with a piston activated to rotate a drive shaft; said system comprising a pump having at least one pumping element activated intermittently to pump high-pressure fuel; a fuel rail communicating with a delivery conduit of said pump and for receiving the fuel so pumped; and at least one fuel injector communicating with said rail and activated to draw a given quantity of fuel from said rail and inject it into said cylinder; and said quantity varying according to the instantaneous load of said engine; characterized in that said pumping element has a delivery at least equal to the maximum draw of said injector; and said pumping element being activated in pumping phase with said injector to minimize the variations in fuel pressure in said rail. 
     More specifically, in the case of an internal combustion engine having a number of cylinders associated with a corresponding number of injectors communicating with the rail, the pumping element has a delivery at least equal to the maximum draw of each of said injectors, and is activated in pumping phase with a corresponding injector in said number. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
     FIG. 1 shows a diagram of an internal combustion engine common-rail fuel injection system in accordance with the invention; 
     FIG. 2 shows a schematic section of a first variation of a high-pressure pump for the FIG. 1 injection system; 
     FIG. 3 shows a schematic section of a further variation of the high-pressure pump for the FIG. 1 injection system; 
     FIG. 4 shows an operating graph of the injection system according to the invention; 
     FIG. 5 shows a mid-section of a fuel premetering device for the FIG. 1 system; 
     FIG. 6 shows an operating graph of the FIG. 5 premetering device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Number  1  in FIG. 1 indicates as a whole a common-rail fuel injection system of an internal combustion, e.g. diesel, engine  2  comprising a number of, e.g. four, cylinders  3  cooperating with corresponding pistons (not shown) activated to rotate a drive shaft  4  indicated by the dot-and-dash line in FIG.  1 . Drive shaft  4  is connected by a transmission device  9  to a conventional camshaft  10  controlling the intake and exhaust valves of cylinders  3 . 
     Injection system  1  comprises a number of electromagnetic injectors  5  associated with and for injecting high-pressure fuel into cylinders  3 . 
     Injectors  5  are connected to a common header or so-called common rail  6 , which is supplied with high-pressure fuel along a high-pressure delivery conduit  8  by a mechanical high-pressure pump  7 . 
     High-pressure pump  7  is in turn supplied by a low-pressure, e.g. motor-driven, pump  11 . A low-pressure delivery conduit  12  and a fuel filter  13  are located between motor-driven pump  11  and pump  7 . And motor-driven pump  11  is normally housed in the fuel tank  14 , in which a drain conduit  16  terminates to drain off the surplus fuel from motor-driven pump  11  and filter  13 . 
     A pressure regulating device  17 , for regulating the pressure in conduit  8 , is located between delivery conduit  8  of high-pressure pump  7  and drain conduit  16 , and comprises a solenoid valve defined by a valve  18  controlled by an electromagnet  19 . Valve  18  provides for feeding any surplus fuel into drain conduit  16  to maintain the required pressure in common rail  6 . Conduit  16  also feeds into tank  14  the drain fuel of injectors  5  and, via a pressure-limiting valve  21 , any surplus fuel accumulated in common rail  6 . 
     The fuel in tank  14  is at atmospheric pressure. In actual use, motor-driven pump  11  compresses the fuel to a low pressure, e.g. of about 2-3 bars; high-pressure pump  7  compresses the incoming fuel from conduit  12  to feed the fuel along conduit  8  to common rail  6  at a high pressure, e.g. of about 1500 bars; and each injector  5  injects into respective cylinder  3  a quantity of fuel ranging between a minimum and maximum value, under the control of an electronic control unit  22 , which may be defined by the usual central microprocessor control unit controlling engine  2 . 
     Control unit  22  receives signals indicating the operating conditions of engine  2 —such as the position of accelerator pedal  23 , the number of revolutions of drive shaft  4 , and the fuel pressure in common rail  6 , which are detected by corresponding sensors—and, by processing the incoming signals according to a given program, controls the instant and for how long individual injectors  5  are operated, as well as the flow of low-pressure motor-driven pump  11 . 
     According to the invention, control unit  22  controls device  17  self-adaptively, so as to premeter the fuel supplied along conduit  8  to common rail  6 . High-pressure pump  7  comprises one or more pumping elements  24 , each having a cylinder  26  and a piston  27 , which is activated by a corresponding cam  28 ,  30  (see FIGS.  2  and  3 ). Cams  28 ,  30  are carried by a drive shaft of pump  7 , which is preferably defined by an engine shaft provided for other functions. For example, the drive shaft of pump  7  may be defined by shaft  10  operating the intake and exhaust valves of cylinders  3 , or by drive shaft  4  itself. 
     Each pumping element  24  of pump  7  has a constant delivery at least equal to the maximum draw of each injector  5 ; and each cam  28 ,  30  is shaped to activate the corresponding pumping element  24  in synchronism, i.e. in pumping phase, with the corresponding injector  5 , so as to minimize the variation in fuel pressure in common rail  6 . 
     Since the fuel draw time of injectors  5  is variable, the synchronism or pumping phase of piston  27  and the corresponding injector  5  is intended in the sense that the stroke, controlled by cam  28 ,  30 , of piston  27  is performed within the operating phase of the corresponding cylinder  3  of engine  2  into which fuel is injected. Advantageously, the lifts of cam  28 ,  30  are designed to activate pumping element  24  with a phase of −50° to +20° (engine angle) with respect to the top dead center position at the compression stroke of the corresponding cylinder  3  of engine  2  into which fuel is injected by the corresponding injector  5 . 
     Device  17  premeters the fuel so that the amount of fuel supplied to conduit  8  by each pumping element  24  equals the sum of the amount of fuel to be injected by the corresponding injector  5 , the amount of fuel required to operate injector  5 , and any leakage, which varies according to the wear of injector  5 . Any surplus fuel pumped by the activated pumping element  24  is drained by device  17  into conduit  16 . 
     This therefore ensures that, following fuel injection into each cylinder  3  of engine  2 , common rail  6  is supplied with substantially the amount of fuel drawn by the corresponding injector  5 , so that, when fuel is next drawn, the fuel pressure has been restored. The volume of common rail  6  may therefore be minimized, so that injection system  1  is compact and cheap to produce, and can be designed for retrofitting, even on existing direct-injection engines, i.e. with no common rail  6 . 
     In a first variation of pump  7  for injection system  1 , each piston  27  of pump  7  is activated by a cam  28  (FIG. 2) having a lift  29  for performing a full stroke of piston  27 . In which case, each pumping element  24  is activated each time in pumping phase with an injector  5  of engine  2  (FIG.  1 ). Pump  7  may have a number of pumping elements  24  equal to the number of injectors  5 , in which case, cams  28  are timed on shaft  10  so that each pumping element  24  is activated in pumping phase with the corresponding injector  5 . 
     Alternatively, pump  7  may have a number of pumping elements  24  equal to a submultiple of the number of injectors  5 , or even only one pumping element  24 . Transmission device  9  and/or the profile of cam  28  are therefore selected to activate each pumping element  24  in pumping phase with more than one injector  5  or even all of injectors  5 . 
     In a further variation of high-pressure pump  7 , each pumping element  24  is activated by a cam  30  (FIG. 3) with a segmented profile, so as to control the stroke of the corresponding piston  27  in two or more portions. Transmission device  9  and/or the profile of cam  30  are therefore selected so that each cam  30  moves piston  27  through a portion of its stroke in pumping phase with a corresponding injector  5 . 
     More specifically, for the engine  2  with four cylinders  3  in FIG. 1, the FIG. 3 pump  7  may have two pumping elements  24 , and cam  30  of each piston  27  has a lift comprising two successive up or compression steps  31  and  32 , and only one down or intake step  33 . Each step  31  and  32  moves relative piston  27  through a corresponding portion of the compression stroke, while down step  33  controls a single intake stroke. 
     The bar graph  34  in FIG. 4 shows intermittent fuel draw from rail  6  made successively by injectors  5  of engine  2 . The dash line  35  shows the maximum pressure, controlled by valve  21 , of the fuel in rail  6 , and the continuous line  36  the actual fuel pressure in rail  6 . As shown clearly by line  36 , by virtue of being pumped in phase by pumping elements  24  of pump  7 , the fuel in rail  6  undergoes very little variation, which limited to the interval between one draw and the next by injectors  5 , and is therefore practically negligible. 
     Valve  18  of premetering device  17  is normally closed by elastic means, e.g. a spring  37  (FIG.  1 ), and electromagnet  19  is energized to open valve  18  in opposition to spring  37 . In a preferred embodiment, valve  18  comprises a hollow, substantially cylindrical valve body  38  (FIG. 5) having an axial conduit  39  connectable, in use, to high-pressure conduit  8  (FIG.  1 ), and a first cylindrical cavity  41  communicating and coaxial with conduit  39 . The lateral wall of cavity  41  has an internally threaded portion  42 ; valve body  38  also has a coaxial second cylindrical cavity  43  forming an annular shoulder  44  with cavity  41 ; and the lateral wall of cavity  43  has an externally threaded portion  45 . 
     Valve  18  also comprises a shutter defined by a ball  46 , which cooperates with a truncated-cone-shaped seat  47  of a cylindrical member  48  having a central hole  49 . Member  48  is housed inside cavity  41 , so that seat  47  communicates with axial conduit  39 , and is fixed inside cavity  41  by a threaded inner ring nut  51  having a prismatic hole  52  engaged by an Allen wrench. 
     Electromagnet  19  comprises a cylindrical core  53  made of magnetic material and which has a central hole  54 , and an annular cavity  55  housing the solenoid  56  of electromagnet  19 . Solenoid  56  activates an armature  57  made of ferromagnetic material and in the form of a disk with radial slits  58 . Armature  57  has an axial appendix or stem  59  housed in hole  52  and for engaging ball  46 . The surface of armature  57  on the opposite side to stem  59  is flat and cooperates with two polar surfaces  60  of core  53 . 
     Core  53  is forced inside a cylindrical cavity  61  of a cup-shaped body  62  comprising a lateral wall  63  with two annular grooves  64 ; an end wall  66  with an axial depression  67 ; an axial conduit  68  connected, in use, to drain conduit  16  of injection system  1 ; and an annular edge  69  on the opposite side to lateral wall  63 . 
     Cup-shaped body  62  is housed inside cavity  41  of valve body  38  with the interposition of a high-pressure fuel seal  71 , and is fixed inside cavity  41  of valve body  38  by a threaded outer ring nut  72  having a shoulder  73  engaging edge  69  of cup-shaped body  62 . A calibrated shim  74  is interposed between shoulder  44  of valve body  38  and cup-shaped body  62 , and defines the axial travel of armature  57 . 
     Spring  37  of valve  18  is a helical compression spring, and is located between depression  67  in end wall  66  and a flange  76 . Flange  76  has a pin  77  inserted inside an axial depression in armature  57 ; and a further pin  78  for guiding spring  37 . Spring  37  is calibrated to keep ball  46  in the closed position until the fuel pressure in conduit  39  reaches the maximum operating value of injection system  1 . 
     The component parts of valve  18  are assembled inside valve body  38  by first inserting cylindrical member  48  inside cavity  41 . Inserting an Allen wrench inside hole  52 , inner ring nut  51  is then screwed inside threaded portion  42  to fix member  48  firmly inside cavity  41  of valve body  38 . On one side, ball  46  and stem  59  of armature  57  are then inserted inside hole  52  in member  48 , and, on the other side, core  53  and solenoid  56  are inserted inside cup-shaped body  62 . 
     Flange  76  and spring  37  are then inserted inside hole  54  in core  53 ; shim  74  is inserted inside cavity  41  of valve body  38 ; cup-shaped body  62  with seal  71  is inserted inside cavity  41 ; and outer ring nut  72  is screwed on to threaded portion  45 , so that the edge of lateral wall  63  rests on shim  74 , and cup-shaped body  62  is fixed firmly inside cavity  41  of valve body  38 . 
     Self-adaptive premetering device  17  operates as follows. 
     Spring  37  normally keeps ball  46  in the closed position, so that none of the high-pressure fuel in conduit  39  passes through valve  18 , and all the high-pressure fuel is fed along conduit  8  to common rail  6 . When the pressure of the fuel in conduit  39  exceeds the set maximum, e.g. in the event of a fault on valve  21 , the fuel pressure overcomes spring  37  to move ball  46  into the open position, so that the surplus fuel is drained into tank  14  via hole  49  in member  48 , hole  52  in ring nut  51 , slits  58  in armature  57 , hole  54  in core  53 , conduit  68  in cup-shaped body  62 , and drain conduit  16 . 
     When the operating conditions of engine  2  call for a lower fuel pressure than the maximum to which spring  37  is set, control unit  22  operates valve  18  to premeter fuel supply to rail  6  self-adaptively. That is, depending on the operating conditions of engine  2 , unit  22  simultaneously emits a control signal for controlling the individual injector  5 , and a control signal for controlling valve  18  and which energizes solenoid  56  of electromagnet  19  with a corresponding electric current I. 
     Electromagnet  19  therefore attracts armature  57  with a force in opposition to that of spring  37  to move ball  46  into a corresponding open position, so that the amount of fuel supplied to common rail  6  at each operation of a pumping element  24  substantially equals the amount of fuel drawn by the corresponding injector  5  at the same phase, and which equals the sum of the amount of fuel injected into cylinder  3 , the amount of fuel used to operate injector  5 , and the amount of fuel leaking through the joints of the various conduits of injector  5 . 
     As is known, the most frequent variations in the flow of valve  18  are those close to the flow corresponding to the setting of spring  37 , i.e. to the set maximum fuel pressure in rail  6 , while variations in fuel flow at a fuel pressure close to drain pressure are more or less rare or useless. The excitation current of electromagnet  19  advantageously varies between zero, when ball  46  is to be kept in the closed position by spring  37 , and a maximum value Imax, when valve  18  is to be opened fully. More specifically, electromagnet  19  is energized by a current I inversely proportional to the required pressure P in conduit  8 , as shown by the continuous line in the FIG. 6 graph. Current I therefore varies between zero, to allow spring  37  to keep valve  18  fully closed so that the fuel pressure in conduit  8  is maximum, and a predetermined maximum value Imax to open valve  18  fully and reduce the fuel pressure to the atmospheric pressure in tank  14 . 
     The above control strategy of device  17  is the reverse of known pressure regulators, in which the regulating valve is closed when the electromagnet is energized, and in which the fuel pressure in conduit  8 , in fact, is substantially inversely proportional to the excitation current I of the electromagnet, as shown by the dash line in FIG.  6 . The reverse control strategy is particularly useful, since a small-volume rail  6  is subject to frequent microvariations in pressure, which can be corrected by energizing electromagnet  19  with a very low current. 
     The advantages, with respect to known injection systems, of the fuel injection systems according to the invention will be clear from the foregoing description. In particular, the volume of common rail  6  can be reduced, thus reducing the cost of the injection system; the flow of pump  7  may also be lower than that required by known technology; and the injection system may be retrofitted to any known injection engine. 
     Moreover, in the event electromagnet  19  fails to be energized, premetering device  17  ensures against any pressure drop in or fuel drainage from the common rail, so that the engine continues operating. Since variations in flow at pressures close to the setting of spring  37  are obtained with a very low current, operation of premetering device  17  is more reliable. And finally, since a low current is sufficient to control considerable forces generated by the high fuel pressure, and with respect to which the inertia and/or friction of ball  46  and armature  57  are negligible, the flow of valve  18  can be controlled extremely accurately. 
     Clearly, further changes can be made to the injection system as described herein without, however, departing from the scope of the accompanying Claims. For example, engine  2  may have only one cylinder  3 ; pump  7  may have a number of pumping elements  24  other than that indicated; cams  38  may have a segmented profile with more than two lifts; and/or more than one injector  5  may be provided for each cylinder  3 . 
     Pump  7  may be activated by a dedicated shaft, as opposed to a shaft provided for other engine functions; and the dedicated shaft may be activated by the drive shaft via a gear transmission or belt and toothed pulley transmission, or even by a respective electric motor operated in time with drive shaft  4  by control unit  22 . 
     Valve  18  may also be used as a pressure regulator in known common-rail injection systems. And spring  37  in FIG. 5 may be replaced by a Belleville washer or leaf spring, and ball  46  by a plate.