BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a precharging device of a hydraulic brake system of a vehicle for selective imposition of brake fluid under pressure on a brake assembly, by means of a charging unit. 
     2. Description of the Prior Art 
     Precharging devices of the type with which this invention is concerned are used for targeted braking of individual vehicle wheels in the context of an electronic stability program (ESP) for the sake of stabilizing a vehicle that is unstable and is spinning. A targeted braking of individual vehicle wheels is done by active brake pressure fixtures by means of a brake assembly in the form of a hydraulic block. To achieve effective stabilization of an unstable, spinning vehicle, relatively high dynamics must be assured by active brake pressure fixtures, so that a targeted braking of the wheels of the vehicle can be done at an operationally optimal instant and with a desired intensity. However, the dynamics of the active brake pressure fixtures is affected by several ambient conditions, especially the prevailing ambient temperature at the time and the ambient air pressure, which is also variable. A low ambient temperature and a low ambient air pressure leads to relatively poor brake pressure buildup dynamics. The brake pressure buildup dynamics are also affected in terms of their effectiveness by the composition, and in particular the water content, of the brake fluid used. To achieve the most persistently uniform, effective brake pressure buildup dynamics, which should essentially be independent of the prevailing, variable ambient conditions, in an electronic stability program (ESP) so-called “precharging” is performed, in which a brake fluid is pumped from a supply tank or from a master cylinder to a hydraulic block of the vehicle by means of a precharging device. By means of such precharging, a precharge pressure is generated on the intake side of a return pump located in the hydraulic block. In known precharging devices, such precharging is done for instance by means of a precharging pump, intended especially for the purpose, or by means of so-called “booster precharging”, that is, by means of a driver-independent actuation of a brake booster of the vehicle. Disadvantageously, the known precharging devices are relatively complicated in terms of their construction. 
     SUMMARY OF THE INVENTION 
     The precharging device of the invention is characterized in that for activating the imposition of the brake fluid, the charging unit is operatively connected to a fuel supply system of the vehicle. The precharging device offers the advantage that a pressure generating unit already existing in the vehicle and used to supply fuel to the driving engine of the vehicle can be additionally used for precharging for active braking interventions. Thus additional, separate pressure generating units needed solely for creating active brake pressure fixtures are omitted, making a compact and structurally relatively simple design of a precharging device possible. Utilizing a fuel pressure that is preferably permanently present in the fuel supply system, the charging unit can be activated in a reliable, operationally favorable way for brake fluid imposition on a given brake assembly. 
     Advantageously, the charging unit is embodied as a piston unit. A piston unit is relatively simple to make and is suitable for producing a reliable operative connection with the fuel supply system of the vehicle. Because of the design flexibility of a piston unit, an adapted, operationally favorable precharging can be generated. 
     The fuel supply system is advantageously embodied as a fuel injection device. Both modern Otto engines and modern Diesel engines have fuel injection systems in which relatively high fuel pressures prevail, regardless of the engine operating state at the time, in specially provided fuel chambers (such as a so-called “rail”, in Diesel engines). From these fuel chambers, when the applicable driving engine is in operation, a controlled imposition of fuel on the individual cylinders takes place, in which cylinders combustion occurs in a known manner. A fuel injection device is thus especially suitable, because of the fuel pressures that are typically always present during operation of the internal combustion engine, for establishing an operative connection with a charging unit, which in particular can be embodied as a piston unit. 
     In a preferred embodiment, the piston unit has a fuel piston and a brake fluid piston operatively connected to the fuel piston by means of a connecting rod, which are displaceable in an adapted cylinder by means of the fuel injection device as a function of a switchable fuel imposition. In this way, it is possible to divide the piston unit into a fuel operation region and a brake fluid operation region, and different operating pressure can prevail in the two operation regions. 
     Advantageously, the piston unit has a fuel pressure chamber, which is operatively connected by means of a switchable valve to the fuel injection device for selective fuel imposition. A switchable valve, which preferably has a plurality of different functional positions, is especially well suited to producing a reliable, flexible fuel imposition on the fuel pressure chamber and thus for effective precharging. 
     Advantageously, the piston unit is operatively connected to a fuel reservoir. The fuel reservoir is part of the fuel injection device and is preferably embodied in the form of a so-called rail. It is under a relatively high operating pressure. This operating pressure of the stored fuel serves on the one hand to provided controlled fuel imposition on the corresponding driving or internal combustion engine, and on the other, it can be used simultaneously for activating and generating a precharging operation. 
     In an alternative embodiment, the piston unit is operatively connected to a fuel pump means. The fuel pump means is likewise part of a fuel injection device and is preferably embodied as a so-called “common-rail pump”. Once again, the relatively high fuel pressure generated can be used both for imposing fuel on the applicable internal combustion engine and for performing a precharging operation. 
     Advantageously, the operational face of the fuel piston is smaller than the operational face of the brake fluid piston. By a selection of the particular size of the operational face of the fuel piston and of the brake fluid piston, it is possible to obtain a hydraulic boost (or reduction) by means of the piston unit. In this way, the relatively great differences in pressure between the fuel region and the brake fluid region of the piston unit can be compensated for relatively simply. 
     Advantageously, the piston unit has a brake fluid pressure chamber, which is operatively connected to the brake assembly and to a brake fluid tank by means of a respective supply line that has an associated check valve. The check valves serve to prevent deflection of the brake fluid piston, and thus also of the fuel piston, caused by an incident operating brake pressure during braking performed by the driver, and to assure correct filling of the brake fluid pressure chamber. 
     Advantageously, the valve is embodied as a switchable control and/or regulating valve. A switchable control and/or regulating valve is especially well suited to establishing a flexible, reliable and correct operative connection between the charging unit and the fuel supply system. 
     In a preferred embodiment, the valve has a functional position that separates the piston unit from the fuel injection device. In such a separating functional position of the valve, additional or alternative security is obtained in terms of the prevailing operating position of the fuel piston at the time, and thus also of the brake fluid piston, especially during driver-performed braking (increased driver braking pressure). 
     Preferably, the valve together with the fuel pump means and/or the fuel reservoir forms a structural unit. In this way, a compact, easily installed structural unit is obtained, and the requisite high-pressure lines for establishing an operative connection between the valve and the fuel supply system can advantageously be kept relatively short. The piston unit is preferably disposed in the engine compartment of the vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be described in further detail below in an exemplary embodiment in conjunction with an associated drawing in which the single FIGURE shows a schematic block circuit diagram of a precharging device of the invention, operatively connected to a fuel injection device of a vehicle. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawing schematically shows a piston unit, identified as a whole by reference numeral  10 , of a precharging device of a vehicle (not shown), which has a fuel piston  11  and a brake fluid piston  12  operatively connected to it by means of a connecting element  13  embodied as a connecting rod; the fuel piston and the brake fluid piston are received displaceably in an adapted cylinder  14  as indicated by the double arrow  49 . The sealing element  17  serves to separate the piston unit  10  in a fluid-tight manner into a driving region, comprising what relative to the fuel piston  11  is a front fuel pressure chamber  20  and a rear fuel pressure chamber  18 , and a useful region, comprising what relative to the brake fluid piston  12  is a front brake fluid pressure chamber  21  and a rear ventilation chamber  19 . The brake fluid pressure chamber  21  is operatively connected to a brake fluid supply tank (not shown) of the vehicle for brake fluid imposition as indicated by the arrow  47 , by means of a brake fluid line  39 , a branch  40 , a check valve  42 , and a brake fluid line  43 . From the branch  40 , a further brake fluid line  44  leads to a check valve  45 , is operatively connected to a brake fluid line  46 , leading to at least one brake assembly (not shown) of the vehicle, for imposition of brake fluid on the brake assembly within the context of a precharging operation as indicated by an arrow  48 . 
     The piston unit  10  is operatively connected, by the interposition of a valve  15 , to a fuel supply system, identified in general by reference numeral  16 , preferably in the form of a fuel injection device. A fuel supply line  27  leads from a fuel tank  28  to a fuel pump  26 , which is preferably embodied as a so-called “common-rail pump”. From the fuel pump  26 , a fuel supply line  29  leads to a branch  32 , which is operatively connected to the valve  15  by means of a fuel supply line  33 . From the valve  15 , a fuel line  25  leads to the rear fuel pressure chamber  18  of the piston unit  10 . A fuel reservoir, which preferably has the function of a so-called “common rail”, is operatively connected to the branch  32  by means of a fuel line  30 . The fuel reservoir  31  is also operatively connected to an injector unit (not shown) of the vehicle by means of fuel lines  50 . From the front fuel pressure chamber  20  of the piston unit  10 , a fuel line  24  leads to the valve  15 , which by means of a fuel return line  34 , a branch  51  and a fuel return line  52  is operatively connected to the fuel tank  28 . From the ventilation chamber  19 , a line  35  leads to a branch  57 , which communicates with the branch  51  by a return line  53 . From the branch  57 , a ventilation line  54  leads to a check valve  55 , which is operatively connected to a ventilation unit (not shown) of the vehicle by means of a ventilation line  56 . The valve  15  includes three function units  36 ,  37 ,  38 ; the function unit  36  has a barrier function, and the function units  37  and  38  have a fuel imposition function. The valve  15  is embodied as a switchable control and/or regulating valve, so that each of the fuel lines  24 ,  25 , depending on an activated function unit  36 ,  37 ,  38  of the valve  15 , can be operatively connected to the fuel supply line  33  or the fuel return line  34  (function units  37 ,  37 ) or disconnected from them (function unit  36 ). 
     In the event that precharging of the brake assembly is necessary, the valve  15  is switched such that the function unit  38  is brought into the operating position. In this way, the valve  15  assures a fuel imposition on the front fuel pressure chamber  20  by means of the fuel pump  26  and/or the fuel reservoir  31 , causing a corresponding displacement of the fuel piston  11  and thus also of the brake fluid piston  12  in the cylinder  14 . Such a displacement of the brake fluid piston  12  as indicated by the double arrow  49  (toward the right in terms of the drawing) causes a reduction of the operating volume of the brake fluid pressure chamber  21 , so that brake fluid is pumped out of that chamber through the brake fluid lines  39 ,  44  and  46  as indicated by arrow  48  to the brake assembly of the vehicle, thus bringing about a precharging operation. After the termination of the precharging phase, the valve  15  is switched such that the function unit  37  assumes the operating position, so that fuel is pumped through the fuel line  25  by the fuel pump  26  and/or the fuel reservoir  31  into the rear fuel pressure chamber  18 . This causes a corresponding displacement of the fuel piston  11  and thus also of the brake fluid piston  12  in the cylinder  14  as indicated by the double arrow (to the left in terms of the drawing), so that the operating volume of the brake fluid pressure chamber  21  is increased. To that end, brake fluid is pumped through the lines  43 ,  41 ,  39  as indicated by the arrow  47  from a brake fluid supply tank (not shown) into the brake fluid pressure chamber  21 . After the desired operating position of the piston unit  10  is reached, the valve  15  is switched such that the function unit  36  assumes the operating position, so that the valve  15  is in a blocking position (separating the piston unit  10  from the fuel supply system  16 ). 
     Upon fuel imposition on the front fuel pressure chamber  20  in accordance with the function unit  38  of the valve  15 , fuel located in the rear fuel pressure chamber  18  is returned to the fuel tank  28  through the fuel lines  25 ,  34  and  52 . Also in this operating situation, the ventilation chamber  19  is subjected to air, for instance, through the ventilation lines  56 ,  54 ,  35  as indicated by the arrow  59 , in order to assure an unhindered displacement of the brake fluid piston  12  in the cylinder  14 . Upon imposition of fuel on the rear fuel pressure chamber  18  in accordance with the function unit  37  of the valve  15 , fuel is returned from the front fuel pressure chamber  20  to the fuel tank  28  through the fuel line  24  and the fuel return lines  34 ,  52  as indicated by the arrow  60 . In addition, air located in the ventilation chamber  19 , and optionally fuel vapor if the separation of the rear fuel pressure chamber  18  from the ventilation chamber  19  by the sealing element  17  is not completely gas-tight, is returned to the fuel tank  28  as indicated by the arrow  58  through the ventilation line  35  and the return lines  53 ,  52 . 
     Since the fuel piston  11  has a front operational face  22  that is smaller than the front operational face  23  of the brake fluid piston  12 , a pressure stepping operation from higher fuel pressures to lower brake fluid pressures takes place in the piston unit  10 . Since the brake fluid pressure during a precharging operation is assessed by means of a pilot pressure sensor (not shown), this pressure can be set by means of a separate pressure regulation provided for the purpose. Thus a pressure stepping operation from 1400 bar (fuel pressure), for example, to 5 bar (brake fluid pressure) is effected by means of a separate pressure regulation. The piston unit  10  thus serves to assure the pumping of brake fluid volumes (at relatively low pressure) that are required for the precharging, while utilizing relatively small fuel volumes (at relatively high pressure). The valve  15  can be switched such that the brake fluid piston  12 , at an excessive precharging pressure, is returned as indicated by the double arrow  49  (toward the left in the drawing), while if the precharging pressure is too low, it can be advanced (toward the right in the drawing) as indicated by the double arrow  49 . In this way, a suitable operation signal of a pilot pressure sensor, which is generally available in systems with an electronic stability program (ESP), can be utilized for regulating the precharging. 
     Based on such a utilization of fuel pressure generating systems in modern vehicle engines, it is unnecessary to use additional pressure generating devices that would be needed solely for the active pressure fixtures. At the same time, relatively fast, precise precharging can be done, regardless of the ambient conditions prevailing at the time, since a relatively high fuel pressure prevails in the fuel supply system  16  at practically all times during vehicle operation. There is an additional gain in safety with respect to the electronic stability program (ESP), since because of the precise, constant precharging conditions, more-exact hydraulic pressure estimation and thus also more-exact active pressure fixtures are possible. With the omission of irritating precharging noise from a precharging pump that would otherwise have to be activated, there is an additional gain in terms of passenger comfort. 
     Besides the precharging function, still other related functions can be performed by means of the precharging device of the invention, such as the functions of a so-called “braking assistant” and/or active braking events, which can be called for by a speed control system with active brake interventions (known as “active cruise control”). It is also possible for brake fluid pressure reduction phases, for instance during an activated traction control phase, to be utilized in such a way that the volumes of brake fluid to be returned are used to act upon the brake fluid pressure chamber  21  of the piston unit  10 . 
     The piston unit  10  can be operatively connected by its brake fluid pressure chamber  21  to a hydraulic brake system of a vehicle in a manner similar to a previously known, separate precharge pump. 
     The foregoing relates to 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.
The precharging device is part of a hydraulic brake system of a vehicle and serves to impose brake fluid under pressure selectively on a brake assembly, by means of a charging unit. To activate the imposition of brake fluid, the charging unit is operatively connected to a fuel supply system. The charging unit is preferably embodied as a piston unit.