Patent Publication Number: US-9422897-B2

Title: Fuel system for an internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/261,326, filed Jun. 14, 2012, which is a 35 USC 371 application of PCT/EP 2011/059633 filed on Jun. 9, 2011, the entire contents of each of which is incorporated herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fuel system for an internal combustion engine. 
     2. Description of the Prior Art 
     German patent disclosure DE 10 2007 000 878 A1 describes a fuel system for an internal combustion engine in which the fuel can be injected both into an intake tube by means of a low-pressure injection valve and directly into a combustion chamber of the engine by means of a high-pressure injection valve. To that end, a low-pressure delivery unit delivers the fuel from a fuel tank both to the low-pressure injection valves and to a high-pressure delivery unit, which delivers the fuel onward into a high-pressure rail and from there to the high-pressure injection valves. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     The advantage of the fuel system of the invention is that the fuel, delivered by the low-pressure delivery unit, flows constantly through the high-pressure delivery unit. As a result, the mechanical parts of the high-pressure delivery unit are cooled and lubricated, even if the high-pressure delivery unit itself is delivering only little, if any, fuel to the high-pressure injection valve. The service life and operating reliability of the high-pressure delivery unit are improved as a result. In particular, the especially grave case of a delivery element of the high-pressure delivery unit that seizes from a lack of lubrication and cooling is reliably averted. Uncoupling the high-pressure delivery unit, which is technically complicated, can also be avoided; that is, the high-pressure delivery unit can always “keep running”. This is especially advantageous when the high-pressure delivery unit is driven mechanically, for instance by a camshaft of the engine. A high-pressure delivery unit that keeps running constantly furthermore has the advantage that a high pressure downstream of the high-pressure delivery unit is always available, so that upon a corresponding change of types of operation, fuel at high pressure can immediately be injected, and so that the high-pressure injection valves can be permanently subjected via the high-pressure rail to a certain pressure, as a result of which, for so-called “holding-down elements” (such as a valve spring) of the high-pressure injection valves, a more-favorable design with regard to the holding-down force can be chosen. 
     In a first preferred refinement of the fuel system of the invention, the drive region includes a recess in a housing, in which recess a drive shaft and/or at least one delivery element, in particular a delivery piston, is disposed. Because low-pressure fuel flows through it or is flushed through it, this kind of drive region is cooled and lubricated especially reliably. 
     It is also advantageous if the high-pressure delivery unit includes a quantity control valve. For instance, with such a quantity control valve, an inlet valve of the high-pressure delivery unit, whenever the latter is a piston pump, can be put into the open position in compulsory fashion. The fuel quantity to be delivered can be adjusted by way of the length of time during which the inlet valve is open during a delivery stroke of the high-pressure delivery unit. In particular whenever the inlet valve is forced constantly into the open position, or in other words whenever no fuel at all is being delivered by the high-pressure delivery unit to the high-pressure injection valve, effective cooling and lubrication of the drive region is ensured by the provision according to the invention of the flushing of the drive region of the high-pressure delivery unit. 
     It is also proposed that the low-pressure delivery unit includes an electrically driven fuel pump. With such a pump, the fuel required for lubricating and cooling the drive region of the high-pressure delivery unit can be reliably furnished. An electrically driven fuel pump of this kind can for instance be disposed directly in the fuel tank, which makes especially efficient operation possible. A typical system pressure that can be furnished by the low-pressure delivery unit is in the range of 0.05 to 0.74 MPa, and in other cases is also approximately 1.00 MPa. 
     A further advantageous embodiment of the fuel system of the invention is distinguished in that the delivery output of the electric fuel pump is variable. This makes it possible to respond not only to a varying fuel demand from the engine but also to a variable demand for lubrication and cooling of the drive region of the high-pressure delivery unit. This saves fuel, since it avoids an unnecessarily high delivery output by the electric fuel pump. 
     A low-pressure rail can be disposed fluidically between the drive region of the high-pressure delivery unit and the low-pressure injection valve. It is then possible for a plurality of low-pressure injection valves, which inject the fuel into corresponding intake tubes, for instance, of respective cylinders of the engine, to be connected to one such low-pressure rail. Such a low-pressure rail creates a buffer reservoir for the fuel, and this reservoir evens out pressure pulsations. 
     The situation is also similar for the refinement in which a high-pressure rail is disposed fluidically between the delivery region of the high-pressure delivery unit and the high-pressure injection valve. In that case, a plurality of high-pressure injection valves, which for instance inject the fuel directly into respective combustion chambers assigned to them, can be connected to the high-pressure rail. 
     Advantageously, the fuel system is embodied for operation of the internal combustion engine with CNG, LPG, and/or MPI. CNG stands for “compressed natural gas” and thus allows engine operation using natural gas. LPG stands for “liquid petrol gas”; thus the engine can then be operated with special automobile gas. MPI stands for “multipoint injection” and means that the fuel is injected at various sites of the engine, such as into the intake tube, directly into the combustion chamber, or into both the intake tube and the combustion chamber simultaneously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Below, one embodiment of the present invention is described as an example, in conjunction with the sole drawing FIGURE. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A fuel system for an internal combustion engine is identified overall in  FIG. 1  by reference numeral  10 . It includes a fuel container  12 , in which a tank unit  14  is disposed. The latter in turn includes a low-pressure delivery unit  16  in the form of an electric fuel pump. 
     The low-pressure delivery unit  16  delivers the fuel into a low-pressure fuel line  18 , in which a filter  20  is disposed. Downstream of the filter  20 , a return line  22  leads back to the low-pressure delivery unit  16 . A pressure regulating or pressure limiting valve, which adjusts the pressure in the low-pressure fuel line  18  to a certain pressure, can be disposed in the return line  22 . However, this valve is not shown. 
     The low-pressure fuel line  18  leads to a high-pressure delivery unit  24 , in the form of a piston pump mechanically driven by the engine. The high-pressure delivery unit  24  includes a drive region  26  and a delivery region  28 . The drive region  26  includes a recess, not shown, in a housing, not identified by reference numeral in  FIG. 1 , of the high-pressure delivery unit  24 , in which unit a drive shaft and a delivery element, such as a delivery piston, are disposed. The drive shaft is an eccentric shaft, for instance, which in turn is driven mechanically by the engine. This drive shaft is supported in the housing via suitable bearings. 
     The delivery region  28  includes an inlet valve, a delivery chamber, and an outlet valve (not shown in the drawing). Via the inlet valve, the fuel is aspirated from the low-pressure fuel line  18  and the drive region  26  into the delivery chamber, is compressed via the piston in the delivery chamber, and is expelled via the outlet valve into a high-pressure fuel line  30 . This latter line leads via a throttle restriction  32  to a high-pressure rail  34 , to which a plurality of high-pressure injection valves  36  are connected. 
     As noted, the low-pressure fuel line  18  leads into the drive region  26 , and in particular into the recess in the drive region  26 , in which recess the drive shaft and the delivery element are disposed. From there, not only does the fuel reach the inlet valve of the delivery region  28  of the high-pressure delivery unit  24 , but it also, via a second low-pressure fuel line  38 , reaches a low-pressure rail  40 . Four low-pressure injection valves  42  are connected to this low-pressure rail. 
     The operation of the fuel system  10  is controlled and regulated by an electronic control and regulating device  44 . For example, the control and regulating device  44  communicates with the low-pressure delivery unit  16  via a power end stage  46 , making it possible to vary the delivery output of the low-pressure delivery unit. The control and regulating device  44  furthermore controls a quantity control valve, again not shown in the drawing. This is for instance an electromagnetic actuation device, by which the inlet valve of the delivery region  28  of the high-pressure delivery unit  24  can be kept open in compulsory fashion. The delivery output of the high-pressure delivery unit  24  can be varied by way of the length of time during which the inlet valve is kept open in compulsory fashion during a delivery stroke of the high-pressure delivery unit  24 . If no fuel whatever is to be delivered by the high-pressure delivery unit  24  into the high-pressure rail  34 , then the inlet valve is forced constantly into the open position, for example. 
     In addition, a pressure limiting valve  48 , which can connect the high-pressure rail  34  to the low-pressure fuel line  18  via a return line  50 , is controlled by the control and regulating device  44 . In this way, the pressure in the high-pressure rail  34  can be lowered. The control and regulating device  44  receives signals from various sensors, for instance from a pressure sensor  52  that detects the pressure in the high-pressure rail  34  and from a pressure sensor  54  that detects the pressure in the low-pressure rail  40 . The corresponding measurement and control lines are represented in  FIG. 1  by dashed lines. 
     The fuel system  10  functions as follows: From the low-pressure delivery unit  16 , the fuel is delivered into the low-pressure fuel line  18 . From there, the fuel reaches the drive region  26  of the high-pressure delivery unit  24 , and as a result the moving parts located there are lubricated and the entire drive region  26  is cooled. From the drive region  26 , the fuel on the one hand reaches the second low-pressure fuel line  38  and from there it goes on to the low-pressure rail  40 , from which it is injected via the low-pressure injection valves  42 , for instance into intake tubes of respective cylinders of the engine. On the other hand, the fuel is also delivered from the high-pressure delivery unit  24  into the high-pressure rail  34  and via the high-pressure injection valves  36  directly into the cylinders of the engine. Because the fuel is first carried through the drive region  26  and only after that is it carried onward to the low-pressure injection valves  42 , reliable lubrication and cooling of the drive region  26  of the high-pressure delivery unit  24  is ensured, even whenever the high-pressure delivery unit  24  just at that moment, because of triggering of the quantity control valve accordingly, is not delivering any fuel at all, or is delivering only very little fuel. This is especially advantageous in MPI operation, or in other words multipoint injection operation. 
     The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.