Abstract:
A low-pressure circuit for a fuel injection system, such as a common-rail injection system, includes a predelivery pump by means of which fuel can be drawn out of the fuel tank and supplied via a fuel line to a low-pressure region of a high-pressure pump. A metering unit for flow rate regulation is located in the low-pressure region, a zero-delivery line with a zero-delivery throttle branches off downstream of the metering unit, and an overflow line with an overflow valve branches off upstream of the metering unit. There is located in the zero-delivery line a shut-off element, which can be switched between an open position and a closed position, for the selective opening or blocking of the zero-delivery line.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to patent application no. A 1011/2012, filed on Sep. 17, 2012 in Austria, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The disclosure relates to a low-pressure circuit for a fuel injection system, in particular a common-rail injection system of internal combustion engines, comprising a predelivery pump by means of which fuel can be drawn out of a fuel tank and supplied via a fuel line to a low-pressure region of a high-pressure pump, wherein a metering unit for flow rate regulation is arranged in the low-pressure region, a zero-delivery line with a zero-delivery throttle branches off downstream of the metering unit, and an overflow line with an overflow valve branches off upstream of the metering unit. 
         [0003]    A low-pressure circuit for a fuel injection system of the above-stated type emerges for example from the laid-open specification DE 199 26 308 A1. The fuel injection system comprises a high-pressure pump and, connected upstream, a predelivery pump which delivers a fuel stream out of a fuel tank via a fuel line. The fuel is supplied to a metering unit which is connected upstream of the high-pressure pump for the purpose of flow rate regulation. The metering unit permits the use of a conventional, unregulated predelivery pump. In the high-pressure pump, the delivery stream is charged with high pressure and supplied to a common distributor rail. The return line ensures that excess fuel is not unnecessarily charged with high pressure but rather can flow back directly into the tank. 
         [0004]    Furthermore, an overflow line with an overflow valve is provided, which overflow line branches off from the delivery path upstream of the metering unit. It is the task of the overflow valve to discharge the excess flow rate of the predelivery pump with respect to the respectively required high-pressure pump delivery flow rate. 
         [0005]    Furthermore, it is proposed in the laid-open specification DE 199 26 308 A1 that a zero-delivery line branches off between the metering unit and the high-pressure pump, in which zero-delivery line there is arranged a zero-delivery throttle. The zero-delivery line issues into the fuel line at the suction side of the predelivery pump. The zero-delivery line is necessary because the metering unit is generally not leak-tight even in the fully closed state. During normal operation of the engine, therefore, the leakage flow from the metering unit is discharged into the return line via the zero-delivery line and the zero-delivery throttle. Without the zero-delivery throttle, it would be the case during normal operation of the engine, and when the metering unit is in the closed state, that the leakage flow would be delivered into the high-pressure circuit, which is undesirable. 
         [0006]    From the arrangement of the zero-delivery line, however, the problem now arises that, in the case of very low engine speeds, that is to say in particular upon starting of the engine, and the associated low delivery rate of the predelivery pump, a major part of said delivery flow is conducted off directly via the zero-delivery throttle into the unpressurized return line again rather than being made available for the build-up of pressure in the high-pressure system. This can considerably hinder or even prevent the starting of the engine. There is thus a conflict of aims with regard to the realization of a zero-delivery characteristic on the one hand and the starting capability on the other hand. 
         [0007]    Taking the above-cited prior art as a starting point, it is therefore the object of the present disclosure to specify a low-pressure circuit for a fuel injection system, by means of which the described conflict of aims can be alleviated. 
       SUMMARY 
       [0008]    To achieve said object, the disclosure provides, in the case of a low-pressure circuit of the type mentioned in the introduction, that there is arranged in the zero-delivery line a shut-off element, which can be switched between an open state and a closed state, for the selective opening or blocking of the zero-delivery line. This offers the possibility of shutting off the zero-delivery line for the starting process in order that no loss flows arise in the system here. When regular operation (above idle rotational speeds) commences, however, the zero-delivery line should be opened again in order to be able to realize the zero-delivery characteristic. The shut-off element is preferably arranged downstream of the zero-delivery throttle such that the shut-off element is situated in the unpressurized region of the low-pressure circuit. 
         [0009]    The actuation of the shut-off element may be controlled in a variety of ways. For example, the shut-off element may be connected to the central engine controller, or sensors may be provided for detecting the rotational speed of the internal combustion engine, which sensors open the shut-off element when the end of the starting process is identified. A structurally particularly simple and fail-safe design is preferably attained by virtue of the fact that control means are provided for controlling the state of the shut-off element as a function of the position of a valve closing member of the overflow valve. In particular, the control means are designed to open the shut-off element when the overflow valve is opened. The overflow valve opens when a certain pressure has built up upstream of the metering unit after the starting of the engine. Thus, the position of the valve closing member of the overflow valve can be used as a control parameter for the opening of the zero-delivery line. 
         [0010]    What is particularly advantageous in this connection is a direct mechanical coupling of the valve closing member of the overflow valve to the shut-off element. The embodiment is in this case preferably such that the shut-off element is in the form of a valve with a valve closing member whose movement is coupled to the movement of the valve closing member of the overflow valve, in particular in such a way that the shut-off element is opened when the overflow valve is opened. 
         [0011]    If, in a manner corresponding to a further preferred refinement, the valve closing member of the overflow valve is spring-loaded in a closing direction, the overflow valve is first opened when a lower threshold pressure is exceeded, such that the opening of the shut-off element is also correspondingly delayed. A further delay preferably arises by virtue of the fact that the movement of the valve closing member of the shut-off element is coupled to the movement of the valve closing member of the overflow valve only after an idle stroke of the latter valve closing member has been passed through. 
         [0012]    The overflow valve is advantageously in the form of a slide valve, the valve closing member of which is formed by a displaceable piston. 
         [0013]    Within the context of the disclosure, the shut-off element may be in the form of a check valve or in the form of a slide valve. 
         [0014]    To permit retrofitting of existing assemblies, it is advantageous for the shut-off element to be structurally combined with the overflow valve. This is achieved for example in that the zero-delivery line issues into a spring chamber of the overflow valve and in that the shut-off element is connected to the spring chamber. In this context, another advantageous refinement provides that the spring chamber can be connected via a slide valve seat to a piston chamber, wherein the piston chamber is connected to the fuel return line via a bore which extends through the spring chamber and which is formed in an insert part. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    The disclosure will be explained in more detail below on the basis of exemplary embodiments schematically illustrated in the drawing, in which: 
           [0016]      FIG. 1  shows a common-rail injection system having a low-pressure circuit according to the prior art, 
           [0017]      FIG. 2  shows a common rail injection system having a low-pressure circuit according to the disclosure in a first embodiment, 
           [0018]      FIG. 3  shows a detail view of the low-pressure circuit in a second embodiment, 
           [0019]      FIG. 4  shows a detail view of the low-pressure circuit in a third embodiment, 
           [0020]      FIG. 5  shows a common-rail injection system having a low-pressure circuit according to the disclosure in a fourth embodiment, and 
           [0021]      FIG. 6  shows a detail view of the embodiment as per  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates a conventional embodiment of a low-pressure circuit of a common-rail injection system. In the standard design, the high-pressure pump  1  has fitted thereon a mechanical low-pressure predelivery pump  2  which is for example in the form of an external gear pump or internal gear pump and which is driven by the camshaft of the high-pressure pump and accordingly rotates at the same rotational speed. The predelivery pump  2  draws the fuel out of the tank  4  via a pre-filter  3  with integrated water separator, and delivers said fuel via the main filter  5  to the low-pressure region of the high-pressure pump  1 . The delivery flow rate of the predelivery pump  2  is conventionally configured to be higher than the maximum delivery flow rate of the high-pressure pump  1  so as to ensure an adequate charging flow rate to the high-pressure pump in all operating states. 
         [0023]    In the high-pressure pump  1  there is installed an overflow valve  6  which has the task of discharging the excess flow rate of the predelivery pump  2  with respect to the respectively required high-pressure pump delivery flow rate. Depending on said excess flow rate, an admission pressure is generated upstream of the high-pressure pump  1  in accordance with a pressure-flow rate characteristic curve of the overflow valve. The overflow valve is in the form of a slide valve, that is to say a discharge cross section  8  is opened up as a function of the stroke of the valve piston  7 . In the case of low predelivery pressures, such as for example during the starting of the engine, the piston of the overpressure valve  6  is not deflected owing to the preload of the spring  9  of the overpressure valve  6 , and it is thus also the case that there is no discharge flow. Here, a movement and thus an opening of the discharge cross section take place only above pressures of approximately 5 bar. In the high-pressure pump  1 , the delivery flow rate of the high-pressure pump  1  is controlled by means of a metering unit  10 . Said metering unit  10  is composed for example of a slide valve and a linear magnet. As a function of the actuation of the linear magnet, a certain throughflow cross section is opened up by means of the slide valve, and the delivery flow rate of the high-pressure pump  1  is thus set. Owing to the design as a slide valve, the metering unit  10  is not leak-tight even in the fully closed state, that is to say, in the presence of a predelivery pressure, there is a leakage flow into the high-pressure pump  1  via the slide gap. Since said leakage would inevitably cause a pressure build-up in the suction chamber  11  of the high-pressure pump  1  and thus lead to the opening of the suction valves  12  and to the delivery of said leakage into the high-pressure circuit  13 , a so-called zero-delivery throttle  14  is necessary in order to realize a delivery flow rate of zero. Through the zero-delivery throttle  14 , said leakage flow is conducted off back into the unpressurized return line  15 , and thus a pressure increase in the suction chamber  11  is prevented. 
         [0024]    Upon starting of the engine, the following situation arises: the engine rotational speeds during the engine starting process are very low, approximately 100 rpm, and the rotational speeds of the high-pressure pump  1  and of the predelivery pump  2  are also correspondingly low. At such low rotational speeds, the predelivery pump  2  exhibits very low levels of delivery efficiency owing to the clearances in the delivery toothing. In the event of starting, the metering unit  10  is fully open in order to realize a maximum delivery flow rate of the high-pressure pump  1  for the build-up of pressure in the high-pressure system. In said state, a major part of the very low delivery flow rate of the predelivery pump  2  is now conducted off directly via the zero-delivery throttle  14  back into the unpressurized return line  15 , and is thus not made available for the build-up of pressure in the high-pressure system. There is thus a conflict of aims with regard to the realization of a zero-delivery characteristic on the one hand and the starting capability on the other hand. Under unfavorable circumstances (for example low starting rotational speeds owing to low battery voltages, further reduced efficiencies of the predelivery pump owing to high temperatures, low ambient pressures owing to high altitudes, etc.), said conflict of aims is intensified yet further such that they cannot be resolved without further measures, that is to say either the starting of the engine or the zero-delivery characteristic is not possible. 
         [0025]    In the embodiment according to the disclosure as per  FIG. 2 , there is now additionally provided a check valve  16  which determines the flow through the zero-delivery line or the zero-delivery throttle  14 . The check valve  16  is in this case arranged below the overflow valve  6  and is opened mechanically by the movement of the valve piston  7  of the overflow valve  6 . The valve closing member of the check valve  16  is mechanically coupled to the valve piston  7  for this purpose. The flow passing through the zero-delivery throttle  14  is introduced into the spring chamber  17  of the overflow valve. The outlet for the introduced flow in the direction of the unpressurized return line  15  is opened and closed by means of the check valve  16  that is connected to the spring chamber  17 . In the event of starting, the predelivery pressures are lower than would be required for a movement of the valve piston  7 . At the same time, owing to the mechanical coupling of the valve piston  7  to the check valve  16 , the check valve  16  is closed and thus the flow through the zero-delivery throttle  14  is blocked. Above idle-operation rotational speeds of the engine, owing to the higher predelivery pressures, a movement of the valve piston  7  of the overflow valve  6  takes place, such that the check valve  16  is also opened and thus the flow through the zero-delivery throttle  14  is permitted. 
         [0026]      FIG. 3  shows an exemplary design embodiment of the overflow valve  6  together with check valve  16 , in which the check valve  16  is located in the bore below the overflow valve  6 . The spring  17  of the check valve  16  can be arranged in a space-saving manner within the spring  9  of the overflow valve  6 , or alternatively on the opposite outlet side  18 . Between the valve piston  7  and the spring  9  there is inserted a plate  19  which, after an idle stroke  21 , opens the valve closing member of the check valve  16  via a rod  20 . Said plate  19  is formed with a bore  22  such that the fuel in the valve piston  7  can flow in and out freely. 
         [0027]    In the modified embodiment as per  FIG. 4 , the spring  9  of the overflow valve  6  is, by contrast to the situation in  FIG. 3 , also used for closing the check valve  16 : In this design, the valve closing member  23  of the check valve  16  is connected via a rod  20  to a plate  19  which is arranged between the valve piston  7  and the spring  9 . In this way, the overflow valve  6  and the check valve  16  are combined to form a unit with only a single spring  24 . In order that said unit is fixed in the pump housing in the unpressurized state, an O-ring  25  is arranged in the check valve  16 , which O-ring also ensures the leak-tightness between the spring chamber  17  and the unpressurized return line  15 . 
         [0028]    In the embodiment as per  FIGS. 5 and 6 , the flow through the zero-delivery throttle  14  is controlled by means of a slide valve. By means of an additional insert part  26  in the spring chamber  17  of the overflow valve  6 , a slide valve seat  27  composed of the valve piston  7  and the insert part  26  is realized. The zero-delivery flow is introduced into the spring chamber  17 , and the flow is then conducted via the slide valve seat into the piston interior  28  and back into the unpressurized outlet  15  via a bore  29  in the insert part  26 . The insert part  26  is formed with longitudinal grooves  30  in the discharge region, such that the valve piston  7  is guided cleanly. In the starting situation, the valve piston  7  is not deflected, and accordingly, the slide valve seat  27  is closed and the flow through the zero-delivery throttle  14  is blocked. During normal operation, the valve piston  7  is displaced and the throughflow via the zero-delivery throttle  14  into the unpressurized return line  15  is permitted.