Abstract:
An injection system for an internal combustion engine has a prefeed pump for delivering fuel from a fuel tank, a high pressure pump for delivering the fuel to a fuel accumulator arranged downstream of the prefeed pump, an accumulator return line arranged downstream of the fuel accumulator and used to return the fuel from the fuel accumulator to the fuel tank, and a valve arranged in the accumulator return line and by which the fuel can be optionally supplied to the fuel tank or to a tank bypass line which hydraulically leads to a point between the prefeed pump and the high pressure pump. A pressure relief valve is arranged in the accumulator return line downstream of the valve and upstream of the fuel tank. The pressure relief valve is suitably designed to allow adjustment of a fuel pressure in the accumulator return line upstream of the pressure relief valve.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2009/063660 filed Oct. 19, 2009, which designates the United States of America, and claims priority to German Application No. 10 2008 055 935.0 filed Nov. 5, 2008, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to an injection system for an internal combustion engine. 
       BACKGROUND 
       [0003]    Injection systems are used to inject fuel into combustion chambers of an internal combustion engine, in particular a diesel internal combustion engine, which injection systems have been increasingly configured in the last years as what are known as “common rail” systems. In the latter, the injectors which are arranged in the combustion chambers are supplied with fuel from a common fuel accumulator, the common rail. Here, the fuel which is to be injected is present in the fuel accumulator at a pressure of up to over 2000 bar. 
         [0004]    Injection systems for internal combustion engines usually have various pumps, by means of which fuel is conveyed, in order to be introduced into combustion chambers of the internal combustion engine. Injection systems of this type for internal combustion engines make high requirements of the accuracy of the injection pressure which is required for the injection of the fuel into the combustion chambers of the internal combustion engine. 
         [0005]    This is particularly important, since more and more stringent legal regulations are being passed with regard to the permissible pollutant emissions of internal combustion engines which are arranged in motor vehicles. Said legal regulations make it necessary to take various measures, by which the pollutant emissions are reduced. Thus, for example, the formation of soot is greatly dependent on the preparation of the air/fuel mixture in the respective cylinder of the internal combustion engine. Here, it is advantageous for the reduction in the pollutant emissions if the fuel can be injected very precisely into the cylinder. 
         [0006]    EP 1 296 060 B1 has disclosed an injection system for an internal combustion engine, having a prefeed pump, by way of which fuel can be conveyed out of a fuel tank to the suction side of a high pressure pump. A high pressure pump which is connected hydraulically behind the prefeed pump then conveys fuel into a fuel accumulator, from where it can then be distributed to injectors which are coupled hydraulically to the fuel accumulator. In the case of a suitable actuation of the high pressure pump, a predefined pressure which is dependent on the operating parameters of the internal combustion engine can be reached in the fuel accumulator. 
       SUMMARY 
       [0007]    According to various embodiments, an injection system for an internal combustion engine can be provided, by way of which injection system precise and reliable metering of fuel for the internal combustion engine and a simple construction of the injection system are made possible in a simple way. 
         [0008]    According to an embodiment, an injection system for an internal combustion engine, may comprise: a prefeed pump for conveying fuel out of a fuel tank, a high pressure pump which is arranged downstream of the prefeed pump for conveying the fuel into a fuel accumulator, an accumulator return line which is arranged downstream of the fuel accumulator and by way of which fuel can be returned from the fuel accumulator into the fuel tank, and a valve which is arranged in the accumulator return line and by means of which the fuel can optionally be fed to the fuel tank or a tank bypass line which opens hydraulically between the prefeed pump and the high pressure pump, wherein a pressure limiting valve is arranged in the accumulator return line downstream of the valve and upstream of the fuel tank, and it being possible to set a fuel pressure in the accumulator return line upstream of the pressure limiting valve by a suitable configuration of the pressure limiting valve. 
         [0009]    According to a further embodiment, the pressure limiting valve can be configured in such a way that the fuel pressure in the accumulator return line upstream of the pressure limiting valve corresponds approximately to a prefeed pressure of the fuel which the prefeed pump provides on the outlet side. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Various embodiments are explained in greater detail in the following text using the diagrammatic drawing. 
           [0011]    The single FIGURE shows a block circuit diagram of an injection system for an internal combustion engine. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    According to various embodiments, an injection system for an internal combustion engine has a prefeed pump for conveying fuel out of a fuel tank, a high pressure pump which is arranged downstream of the prefeed pump for conveying the fuel into a fuel accumulator, an accumulator return line which is arranged downstream of the fuel accumulator and by way of which fuel can be returned from the fuel accumulator into the fuel tank, and a valve which is arranged in the accumulator return line and by means of which the fuel can optionally be fed to the fuel tank or a tank bypass line which opens hydraulically between the prefeed pump and the high pressure pump. A pressure limiting valve is arranged in the accumulator return line downstream of the valve and upstream of the fuel tank. A fuel pressure can be set in the accumulator return line upstream of the pressure limiting valve by a suitable configuration of the pressure limiting valve. 
         [0013]    This is advantageous, since largely constant pressure conditions can thus be achieved in the accumulator return line upstream of the valve, by means of which the fuel can optionally be fed to the fuel tank or to the tank bypass line, substantially independently of the switching position of said valve. For example, a pressure regulating valve can thus be operated with high accuracy, which valve serves to regulate the pressure in the fuel accumulator and is arranged in the accumulator return line downstream of the fuel accumulator and upstream of the valve, by means of which the fuel can optionally be fed to the fuel tank or to the tank bypass line. 
         [0014]    Said pressure regulating valve can therefore be operated on a defined characteristic curve in all relevant operating states. Simple calibration of the pressure regulating valve is therefore possible. The fuel which is discharged via the accumulator return line can optionally be mixed into the inlet of the high pressure pump or returned into the tank by means of the valve as a function of the temperature of said fuel. 
         [0015]    In one embodiment, the pressure limiting valve is configured in such a way that a fuel pressure in the accumulator return line upstream of the pressure limiting valve corresponds approximately to a prefeed pressure of the fuel which the prefeed pump provides on the outlet side. The pressure regulating valve can therefore advantageously be operated on a largely identical characteristic curve for all relevant operating states. 
         [0016]    The injection system (shown in the FIGURE) for an internal combustion engine has a fuel tank  10 , out of which fuel is conveyed by means of a prefeed pump  12 . The prefeed pump  12  is preferably configured as a vane cell pump. However, another pump type can also be used, for example a gearwheel pump or a gerotor pump for the prefeed. The prefeed pump  12  can be driven mechanically by way of a drive shaft (not shown) which is coupled fixedly to an engine shaft of the internal combustion engine. As an alternative, it is also possible, however, to use an electrically operated prefeed pump  12 , as a result of which it is possible to control the delivery performance of the prefeed pump  12  independently of the delivery performance of further pumps. In the embodiment which is shown here, the prefeed pump  12  is what is known as an in-tank pump; that is to say, it is arranged inside the fuel tank  10  and is preferably configured as an immersion pump. 
         [0017]    The prefeed pump  12  is hydraulically coupled on the outlet side to a primary pressure control valve, by which, if a predefined prefeed pressure p_V is exceeded on the output side of the prefeed pump  12 , a part of the fuel which is conveyed by the prefeed pump  12  can be returned to the intake side of the prefeed pump  12  and therefore the prefeed pressure p_V on the outlet side of the prefeed pump  12  can be kept largely constant. 
         [0018]    A high pressure pump  14  for conveying the fuel into a fuel accumulator  16  is arranged downstream of the prefeed pump. The fuel accumulator  16  is hydraulically coupled to the high pressure pump  14  via a fuel accumulator feed line  44 . The high pressure pump  14  can preferably be configured as a radial piston pump or as an in-line piston pump having a plurality of cylinder units, as are known for use in injection systems of internal combustion engines. 
         [0019]    Furthermore, the fuel accumulator  16  is hydraulically coupled via lines to an injector  18  or a plurality of injectors  18 . Each of the injectors  18  is assigned a combustion chamber of the internal combustion engine and each injector  18  can be actuated in such a way that fuel is injected into the combustion chamber. 
         [0020]    The fuel which is to be injected into combustion chambers of the internal combustion engine by means of the injectors  18  can reach a relatively high injection pressure by way of the high pressure pump  14 . 
         [0021]    Excess fuel can be returned from the injectors  18  via an injector return line  46  back to the fuel tank  10 . 
         [0022]    A volumetric flow control valve  22 , by way of which the fuel flow can be set from the prefeed pump  12  into the high pressure pump  14 , is arranged hydraulically between the prefeed pump  12  and the high pressure pump  14 . A temperature sensor  24  is arranged hydraulically between the prefeed pump  12  and the volumetric flow control valve  22 . By means of the temperature sensor  24  and a pressure sensor  25 , by way of which the fuel pressure in the fuel accumulator  16  can be determined, and optionally as a function of further input variables, the volumetric flow control valve  22  can be actuated in such a way that low pressure-side control of the fuel flow which is fed to the high pressure pump  14  is possible. 
         [0023]    The fuel accumulator  16  is connected by means of an accumulator return line  42  to a pressure regulating valve  28  which can be actuated, for example, as a function of the fuel pressure which is determined by the pressure sensor  25  in the fuel accumulator  16 . If a predefined fuel pressure is exceeded in the fuel accumulator  16 , the pressure regulating valve  28  can open and a part of the fuel which is conveyed by the high pressure pump  14  can be returned via the accumulator return line  42  back into the fuel tank  10 . 
         [0024]    A flushing line  29  which opens on the outlet side into the housing of the high pressure pump  14  branches off downstream of the prefeed pump  12 , with the result that it is possible to flush the housing of the high pressure pump  14  with fuel during operation. Cooling and lubrication of the high pressure pump  14  can therefore be brought about. The fuel which is used for flushing purposes can subsequently be returned from the housing of the high pressure pump  14  via a flushing return line  36  back into the fuel tank  10 . 
         [0025]    Furthermore, a flushing line valve and, hydraulically in series thereto, a flushing line throttle are arranged in the flushing line  29 . The flushing line throttle can restrict the fuel flow through the flushing line  29 . The fuel flow which branches off via the flushing line  29  can be released by the flushing line valve if the prefeed pressure p_V on the outlet side of the prefeed pump  12  exceeds a predefined value. In order to protect against particles which are entrained in the fuel flow or in order to separate water, a filter  38  is arranged upstream of the prefeed pump  12 . The temperature sensor  24 , by way of which a temperature can be determined which is representative of the filter  38 , is arranged in the filter  38  or in its surroundings. 
         [0026]    The flushing return line  36 , the accumulator return line  42  and the injector return line  46  are preferably routed back to the fuel tank  10 . 
         [0027]    A tank bypass line  32  branches off from the accumulator return line  42  at a branching point  40  downstream of the pressure regulating valve  28  and upstream of the fuel tank  10 . By means of a valve  30  which is arranged at the branching point  40 , the fuel which flows back from the fuel accumulator  16  can be conducted back to the filter  38  via the tank bypass line  32 , bypassing the fuel tank  10 , or further via the accumulator return line  42  to the fuel tank  10 . 
         [0028]    A pressure limiting valve  34 , by way of which a fuel pressure p_R can be set in the accumulator return line  42  upstream of the pressure limiting valve  34 , is arranged in the accumulator return line  42  downstream of the valve  30  and upstream of the fuel tank  10 . The pressure limiting valve  34  is preferably configured as a spring-loaded nonreturn valve. The pressure limiting valve  34  is preferably dimensioned in such a way that the fuel pressure p_R in the accumulator return line  42  upstream of the pressure limiting valve  34  corresponds approximately to the prefeed pressure p_V of the fuel at the outlet of the prefeed pump  12 . In the following text, the function of the injection system is to be described briefly: 
         [0029]    The prefeed pump  12  conveys fuel out of the fuel tank  10 . The fuel is at the prefeed pressure p_V at the outlet of the prefeed pump  12 . The fuel then passes via the temperature sensor  24  and the filter  38  to the volumetric flow control valve  22 . As much fuel as is required by the fuel accumulator is made available to the high pressure pump  14  by the volumetric flow control valve  22 . The fuel is delivered by means of the high pressure pump  14  via the fuel accumulator feed line  44  to the fuel accumulator  16 . From the fuel accumulator  16 , the fuel is fed to the injectors  18 , and is injected by the latter into the combustion chambers of the internal combustion engine. 
         [0030]    The fuel pressure which is required for the fuel accumulator is fixed by the pressure regulating valve  28  in the accumulator return line  42 . If the pressure rises too much in the fuel accumulator  16  or if the pressure in the fuel accumulator  16  is to be reduced in a targeted manner, fuel can be released from the fuel accumulator  16  by means of the pressure regulating valve  28  via the accumulator return line  42 . 
         [0031]    The fuel which is discharged via the accumulator return line can optionally be mixed into the feed line of the high pressure pump  14  or can be conducted back into the fuel tank  10  by means of the valve  30  as a function of the temperature of said fuel. Moreover, fuel out of the flushing return line  36  and the return line  46  from the injectors is conducted back into the fuel tank  10 . 
         [0032]    If the filter  38  is to be heated, for example when the internal combustion engine is being started up, the valve  30  assumes a position, in which the fuel flow is released from the fuel accumulator  16  via the accumulator return line  42  and the tank bypass line  48  to the fuel filter  36 , with the result that the heated fuel can pass to the filter  38 . 
         [0033]    When the valve is sufficiently warm, the valve  30  can switch over and release the fuel flow from the accumulator return line  42  via the pressure limiting valve  34  to the fuel tank  10 . 
         [0034]    If the pressure limiting valve  34  is selected in such a way that the fuel pressure p_R in the accumulator return line  42  upstream of the pressure limiting valve  34  corresponds largely to the prefeed pressure p_V of the fuel at the outlet of the prefeed pump  12 , largely constant pressure conditions can be maintained in the accumulator return line  42  upstream of the valve  30  substantially independently of the switching position of the valve  30 . Since, in this case, no substantial pressure fluctuations occur in the accumulator return line  42  upstream of the valve  30 , the pressure regulating valve  28  which serves to regulate the pressure in the fuel accumulator  16  and is arranged in the accumulator return line  42  downstream of the fuel accumulator  16  and upstream of the valve  30  can be operated on a defined characteristic curve with high accuracy in the relevant operating states. Simple and inexpensive calibration of the pressure regulating valve  28  is therefore also possible.