Abstract:
The invention relates to a pressure boosting system for at least one fuel injector of a high pressure injection system of an internal combustion engine, having a hydraulic pressure booster that is actuated by a control valve. The hydraulic pressure booster is configured with a pressure boosting piston, which comprises a first pressure booster piston part having a first diameter and a second pressure booster piston part having a second diameter, wherein the first diameter is greater than the second diameter. The pressure booster piston is disposed within a hydraulic reservoir chamber, onto which pressure is applied, together with the first pressure booster piston part having the greater diameter, wherein the accumulator chamber in turn is configured within a base body. The base body has a piston guide body for at least one of the pressure booster piston parts. The piston guide body is at least partially surrounded by an annular space, which is part of the hydraulic accumulator chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/054531 filed on Apr. 15, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a pressure boosting system for at least one fuel injector of an internal combustion engine, having a hydraulic pressure booster. 
     2. Description of the Prior Art 
     A fuel injection system with pressure boosting, in which one central hydraulic pressure booster is provided for all the fuel injectors, is known from European Patent Disclosure EP 1 125 046 B1. The fuel supplied by means of a high-pressure pump is delivered to a central pressure reservoir (first common rail). The central pressure booster is downstream of the central pressure reservoir in the direction in which the fuel is supplied and carries the pressure-boosted fuel to a further pressure reservoir (second common rail), from which a plurality of pressure lines, corresponding in number to the number of injectors, leads away to the individual fuel injectors. The central pressure booster described in EP 1 125 046 B1, but also the other pressure boosters known, integrated with fuel injectors (as in German Patent Disclosure DE 103 25620 A1), have a pressure booster piston, which has a first piston portion with a first pressure booster piston part having a larger diameter and a second piston portion with a second pressure booster piston part having a small diameter D 22 . The one pressure booster piston part acts upon a high-pressure chamber for pressure boosting, and the other pressure booster piston part acts upon a control chamber or differential pressure chamber that is triggerable by an on-off valve. The pressure booster piston is guided axially movably inside a base body. A pressure face, which is exposed to a work chamber that acts as a hydraulic reservoir chamber and is subjected to the system pressure of the first common rail, is associated with the pressure booster piston on the pressure booster piston part having the larger diameter, on the diametrically opposed face end. 
     A disadvantage of the known pressure boosting system is the relatively large control quantity for triggering the pressure booster. If for multiple injections of small injection quantities, a boosted injection pressure is required, then the control chamber or differential pressure chamber of the pressure booster must be relieved upon each injection. The result is a large control quantity to be diverted, which must accordingly be included in the lost quantity in the injection system. Multiple injections within the context of a cylinder stroke motion are possible chronologically only within a narrowly defined window as well, since with each triggering of the pressure booster, its differential pressure chamber must refill with fuel. Moreover, with increasing injection pressures, the lost quantity increases proportionally to the fourth power by way of the gap width in the guide of the pressure booster piston, which adversely affects the hydraulic efficiency of such fuel injection systems. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is the object of the present invention to minimize the lost quantities that occur from leakage at guide gaps, in order to increase the efficiency of the pressure boosting of the fuel injection system. 
     The hydraulic pressure booster employed has a piston guide body, embodied on the base body, for at least one of the pressure booster piston parts, which part is at least partly surrounded by an annular chamber that in turn is part of the hydraulic reservoir chamber. Thus the same pressure prevails in the annular chamber as in the hydraulic reservoir chamber. Because of the surrounding annular chamber, particularly in the pressure boosting state, a supporting pressure exerted from outside is imparted to the piston guide body, as a result of which internal piston guides open less widely or are not widened as much. Consequently, the guide gaps are reduced, and the leakage quantity is minimized. Moreover, as a result, a component load, induced in the guide body, on the differential pressure between the reservoir volume and the high-pressure volume is reduced, so that the effort and expense for high-pressure-proof design and embodiment of the entire hydraulic pressure booster can be lowered. The pressure boosting system according to the invention is moreover optimized in terms of the installation space required for individual system components. Overall, a considerable increase in the total efficiency of the pressure boosting system is achieved. 
     Advantageous refinements of the invention are possible by means of the provisions of the dependent claims. 
     In a first expedient embodiment, the first pressure booster piston part having the larger diameter D 21  acts upon the high-pressure chamber provided for the pressure boosting, and the second pressure booster piston part having the smaller diameter D 22  acts upon the control chamber, and the first pressure booster piston part having the larger diameter D 21  is adjoins the hydraulic reservoir chamber. In a variant embodiment, the high-pressure chamber is disposed inside the piston guide body. In another variant embodiment, the high-pressure chamber is defined by a spring-impinged high-pressure sleeve, which is guided axially movably on the pressure booster piston and is positioned against the piston guide body at a sealing point. The diameter of the sealing point is less than or equal to a diameter D 21  of the first pressure booster piston part. In these embodiments, the control chamber of the pressure booster is embodied inside the piston guide body and is subjected to pressure by the second pressure booster piston part having the smaller diameter D 22 . 
     A second embodiment provides for transposing the control chamber and the high-pressure chamber; in that case, the second pressure booster piston part having the smaller diameter D 22  acts upon the high-pressure chamber provided for the pressure boosting, and the first pressure booster piston part having the larger diameter D 21  acts upon the control chamber. The high-pressure chamber is embodied inside the piston guide body. The control chamber, on which the pressure booster piston part having the larger diameter D 21  acts, then adjoins the hydraulic reservoir chamber. 
     It is especially advantageous that the pressure booster is provided centrally for a plurality of fuel injectors and is disposed between a high-pressure pump and a high-pressure reservoir. Because of a modular construction of the high-pressure pump, pressure booster, high-pressure reservoir, and fuel injector, this kind of central pressure booster can be used in all known installation spaces of internal combustion engines. Because of the disposition of the central hydraulic pressure booster between the high-pressure pump and the high-pressure reservoir (common rail), the central pressure booster has to be triggered only once per injection cycle of a fuel injector. As a result, the control quantity and the leakage quantity are reduced considerably, as a function of the number of injections. Because of this circumstance, the high-pressure pump can be embodied with smaller dimensions as well, since less fuel has to be supplied, because the number of refilling phases of the control chamber of the central hydraulic pressure booster is reduced considerably. The central pressure booster can as a result be designed in terms of its high-pressure supply quantity for the maximum possible injection quantity of at least one fuel injector. 
     It is moreover expedient if the hydraulic reservoir chamber is filled directly with fuel by the high-pressure pump via a high-pressure inlet. The base body, in which the hydraulic reservoir chamber is embodied, can be constructed in one part or multiple parts. The volume of the hydraulic reservoir chamber should be designed such that the pressure drop when fuel is withdrawn is reduced, and the pressure fluctuations from pump supply are damped to an amount that is tolerable for the pressure boosting. 
     From the high-pressure chamber of the central pressure booster, at least one bore leads to at least one filling valve. The filling valve communicates in turn with the hydraulic reservoir chamber via a bore. From the reservoir chamber, at least one connecting bore leads to a valve and from there to the control chamber. From the high-pressure chamber, there is at least one hydraulic communication with a high-pressure valve, from which at least one outlet extends to the high-pressure reservoir. 
     The pressure booster piston is acted upon by a restoring spring, which returns it to its outset position so that it rests with one end against a stop. The spring force of the restoring spring is designed such that after the pressure boosting, the high-pressure piston of the central pressure booster is brought back to its outset position at the stop at sufficiently high speed. 
     At injection pressures below the maximum supply pressure of the high-pressure pump, in a first switching position of an on-off valve, the pressure in the reservoir chamber is built up further by the high-pressure pump via the inflow through check valves via the high-pressure outflow to the high-pressure reservoir. From there, the fuel reaches the fuel injectors. During this mode of operation, the pressure booster is not triggered, so that the fuel supplied by the high-pressure pump reaches the high-pressure reservoir (common rail) in the bypass mode of the pressure booster. 
     If injection pressures that are above the maximum supply pressure of the high-pressure pump are required, then the pressure booster should be triggered. To that end, the on-off valve, which is a 3/2-way valve, is put in a second switching position, actuated electrically, hydraulically, or pneumatically. In this second switching position, the control chamber of the pressure booster communicates for pressure relief with a pressure booster return via the on-off valve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in further detail below in conjunction with the drawings, in which: 
         FIG. 1  shows a system layout of a fuel injection system having a central hydraulic pressure booster; 
         FIG. 2  shows a first exemplary embodiment of a hydraulic pressure booster; 
         FIG. 3.1  shows the outset position of the hydraulic pressure booster of  FIG. 2 ; 
         FIG. 3.2  shows the pressure boosting phase of the hydraulic pressure booster of  FIG. 2 ; 
         FIG. 3.3  shows a refilling phase of the hydraulic pressure booster, proposed according to the invention, of  FIG. 2 ; 
         FIG. 3.4  shows the outset position of the hydraulic pressure booster, proposed according to the invention, of  FIG. 2 ; 
         FIG. 4  shows a second exemplary embodiment of the hydraulic pressure booster; 
         FIG. 5  shows a third exemplary embodiment of the hydraulic pressure booster; and 
         FIG. 6  shows a fourth exemplary embodiment of the hydraulic pressure booster. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The fuel injection system shown in  FIG. 1  has a modular construction of a high-pressure injection system  10 , which can be applied for instance to all the installation spaces of internal combustion engines. The high-pressure injection system  10  includes a fuel tank  12 , from which fuel is supplied via a high-pressure pump  14  and directed to a hydraulic pressure booster  16 . On the one hand, the hydraulic pressure booster  16  communicates via a pressure booster inlet  44  with the aforementioned high-pressure pump  14 , and on the other, it acts upon a high-pressure reservoir  18  (common rail). Connection lines, shown only schematically in  FIG. 1  and corresponding in number to the number of fuel injectors to be supplied with fuel at system pressure, to fuel injectors  20  are located in the high-pressure reservoir  18 . The central hydraulic pressure booster  16  thus in  FIG. 1  supplies pressure-boosted fuel to all the fuel injectors  20 . However, it is also conceivable for the hydraulic pressure booster  16  described below to be integrated in noncentralized fashion with the respective fuel injector  20 . 
     On the end toward the combustion chamber of the fuel injectors, the fuel at high pressure—indicated by the arrows—is injected into the combustion chamber of a self-igniting internal combustion engine. On the return side, at the fuel injector  20 , there is an injector return  22  into which a pressure booster return  24 , connected to an on-off valve  26 , for instance a 3/2-way valve, discharges. Both the pressure booster return  24  and the injector return  22  represent the low-pressure side of the fuel injection system as shown in  FIG. 1 , in which the diverted quantity, whether it is a control quantity or a leakage quantity, is returned to the fuel tank  12 . 
     Because of the disposition of the central pressure booster  16  between the high-pressure pump  14  and the high-pressure reservoir  18 , the pressure booster  16  has to be triggered with the on-off valve  26  only once per injection cycle of a fuel injector  20 . As a result, the control or leakage quantity is reduced considerably, as a function of the number of injections. The high-pressure pump  14  does not have to supply as much fuel and can be made smaller. The pressure booster  16  should be designed in terms of its high-pressure supply quantity for the maximum possible injection quantity of at least one of the fuel injectors  20 . 
     The hydraulic pressure booster  16  in  FIG. 2  includes a base body  30 , which may be embodied in one part or multiple parts. A hydraulic reservoir chamber  48  is integrated with the base body  30 . The hydraulic reservoir chamber  48  is acted upon by fuel from the high-pressure pump  14  via the pressure booster inlet  44 . The reservoir volume of the hydraulic reservoir chamber  48  is designed such that the pressure drop is reduced, and pressure fluctuations caused by the pumping by the high-pressure pump  14  can be damped to an amount that is tolerable for the pressure boosting. 
     The central pressure booster  16  furthermore includes a pressure booster piston  32 . It in turn includes a first piston portion with a first pressure booster piston part  54 , designed with a diameter D 21 , and a second piston portion with a second pressure booster piston part  56 , designed with a diameter D 22 . The pressure booster  16  furthermore includes a high-pressure chamber  50  for pressure boosting and a control chamber  52 , which latter can also be called a differential pressure chamber. A piston guide body  36  that is surrounded by an annular chamber  49  is embodied on the base body  30 . In the exemplary embodiment of  FIG. 2 , the first pressure booster piston part  54  having the diameter D 21  and the second pressure booster piston part  56  having the diameter D 22  are guided axially movably in the piston guide body  36 . The annular chamber  49  is part of the hydraulic reservoir chamber  48  and extends axially along the guide length for the pressure booster piston  32  inside the base body  30 . As a result, the pressure prevailing in the hydraulic reservoir chamber  48  acts from outside upon the piston guide body  36 . Compared to the boosted pressure in the high-pressure chamber  50  and to the low pressure prevailing in the control chamber  52 , the pressure that prevails in the hydraulic reservoir chamber  48  and is furnished by the high-pressure pump  14  is an average pressure, which is established upon triggering of the control chamber  52  as a result of the diversion of the control quantity via the pressure booster return  24 . 
     The pressure boosting ratio i of the pressure booster  16  in the basic sketch shown in  FIG. 2  is as follows:
 
 i=D   21   2 /( D   21   2   −D   22   2 ).
 
     In the exemplary embodiments of  FIGS. 1 and 2  as well as  5  and  6 , the pressure booster piston  32  acts upon the high-pressure chamber  50  with a first pressure face on the first pressure booster piston part  54  having the larger diameter D 21 , and upon the control chamber  52  with a second pressure face on the second pressure booster piston part  56  having the smaller diameter D 22 . In the exemplary embodiment of  FIG. 4 , the situation is reversed. There, with its first pressure face on the first pressure booster piston part  54  having the larger diameter D 21 , the pressure booster piston  32  acts upon the control chamber  52  and, with a second pressure face on the second pressure booster piston part  56  having the smaller diameter D 22 , it acts upon the high-pressure chamber  50 . 
     The pressure booster piston  32  is acted upon by a restoring spring  34 , which is braced on one end on the piston guide body  36  and on the other on a collar  33  embodied on the first pressure booster piston part  54 . The pressure booster piston  32 , restoring spring  34  and piston guide body  36  are disposed in turn in the reservoir chamber  48  in such a way that the reservoir chamber surrounds the piston guide body  36  in the region of the guide of the pressure booster piston  32 , expediently in the region of the first pressure booster piston part  54  embodied with the diameter D 21 . By this provision, the guides of the pressure booster piston  32  are acted upon by a supporting pressure from outside at the instant of the pressure boosting. This supporting pressure from outside causes the guide play, which is increased because of the pressure prevailing in the interior of the pressure booster  16 , to widen less; otherwise, the result would be an unwanted outflow of guide leakage, which in turn would adversely affect the hydraulic efficiency of the pressure booster  16 . 
     From the high-pressure chamber  50 , a high-pressure outlet  46  branches off, which extends to the high-pressure reservoir  18  (common rail). A high-pressure valve  40 , which is embodied as a check valve and prevents a return flow of fuel to the pressure booster  16 , is located in the high-pressure outlet  46 . From the high-pressure chamber  50  of the high-pressure booster  16 , a line that receives a filling valve  38 , by way of which valve the high-pressure chamber  50  is refilled with fuel via a filling line  58 , also extends from the reservoir chamber  48  to the on-off valve  26 . A further line connects a further connection of the on-off valve  26  to the control chamber  52 . In the switching position shown in  FIG. 2  for the on-off valve  26 , the refilling of the control chamber  52  after its pressure relief upon actuation of the on-off valve  26  is effected via the further line, again beginning at the reservoir chamber  48 , via the filling line  58 . 
     The restoring spring  34 , which is disposed between the guide body  36  and a collar  33  on the pressure booster piston  32 , presses the pressure booster piston  32  into its outset position, so that it rests with a stop limitation means  42  on the base body  30 . The spring force of the restoring spring  34  is designed such that the pressure booster piston  32 , after the pressure boosting, is put back in the outset position at the stop limitation means  42  at an adequately high speed. 
     At injection pressures below the maximum supply pressure of the high-pressure pump  14 , in the first switching position of the on-off valve  26  as shown in  FIGS. 1 and 2 , the pressure of the high-pressure pump  14  is supplied via the pressure booster inlet  44  into the reservoir chamber  48  and from there onward, via the high-pressure valves  38 ,  40  embodied as check valves, via the high-pressure outlet  46  to the high-pressure reservoir  18 . From there, the fuel reaches the fuel injectors  20  to be supplied with fuel that is at system pressure. The fuel compressed by the high-pressure pump  14  thus in a so-called bypass mode flows from the high-pressure pump  14  directly to the high-pressure reservoir  18  (common rail); that is, in this mode of operation, the pressure booster  16  is not active. 
     To achieve injection pressures above the maximum supply pressure of the high-pressure pump  14 , the pressure booster  16  must be triggered. To that end, the on-off valve  26  is brought electrically, hydraulically or pneumatically into a second switching position. In that switching position of the on-off valve  26 , the control chamber  52  is made to communicate with the pressure booster return  24 . Fuel flows out of the pressure-relieved control chamber  52  via the on-off valve  26  into the pressure booster return  24  and from there into the low-pressure region of the fuel injection system shown in  FIG. 1 , back into the fuel tank  12 . Because of the pressure reduction in the control chamber  52 , the pressure booster piston  32  is moved axially counter to the spring force of the restoring spring  34 , so that the first pressure booster piston part  54 , embodied with the diameter D 21 , presses into the high-pressure chamber  50  and increases the pressure there. The filling valve  38  in turn is closed in the direction of the pressure booster return  24 . If the pressure then increases in the high-pressure chamber  50  to above the pressure on the side of the high-pressure outlet  46 , the compressed fuel is pumped farther into the high-pressure reservoir  18  (common rail) by the high-pressure valve  40 . The high-pressure reservoir  18  is thus filled with the elevated pressure from the high-pressure chamber  50 . From there, the fuel injectors  20  are then acted upon by the elevated fuel pressure, so that the injection via the fuel injectors is effected at the fuel pressure that is above the supply pressure of the high-pressure pump  14 . The pressure in the high-pressure chamber  50  rises until such time as a force equilibrium is again established at the pressure booster piston  32 . 
     Upon deactivation of the on-off valve  26 , the control chamber  52  again communicates hydraulically with the reservoir chamber  48 . Because of this hydraulic communication, the pressure in the control chamber  52  rises, and the pressure booster piston  32  terminates the process of pressure boosting in accordance with the pressure boosting ratio i in the high-pressure chamber  50 . Simultaneously, the high-pressure valve  40  also closes, because of the existing pressure difference. The spring force of the restoring spring  34  now presses the pressure booster piston  32 , with the stop limitation means  42 , against the base body  30  of the pressure booster  16 . During this period of time, fuel is aspirated from the reservoir chamber  48  into the high-pressure chamber  50  via the filling valve  48 . Once the pressure booster piston  32  reaches the stop limitation means  42 , the on-off valve  26  can be triggered for renewed pressure boosting. Although renewed triggering is possible before the stop limitation means  42  is reached, it would not be appropriate because of what is then a still-indefinite restoration position of the pressure booster piston  32  having a first pressure booster piston part  54  and a second pressure booster piston part  56 . 
     The sequence of  FIGS. 3.1  through  3 . 4  shows the phases in operation of the pressure booster  16  of  FIG. 2 , namely the outset position, pressure boosting, refilling phase, and again the outset position. In  FIG. 3.1 , the reservoir chamber  48  in the base body  30  is subjected to fuel under pressure via the pressure booster inlet  44 . The pressure that prevails in the reservoir chamber  48  prevails both in the control chamber  52 , via the filling line  58 , and in the high-pressure chamber  50 , via the filling valve  38 . In the outset position shown in  FIG. 3.1 , the pressure booster  16  is not activated by the on-off valve  26 . As  FIG. 3.1  shows, because of the switching position of the on-off valve  26 , the reservoir chamber  48  and the control chamber  52  are short-circuited. 
       FIG. 3.2  shows the ensuing activation of the pressure booster  16  during a pressure boosting operation. To that end, current is supplied to the on-off valve  26 , and the control chamber  52  is made to communicate with the pressure booster return  24 , that is, the low-pressure region of the fuel injection system  10 . Because of the pressure relief of the control chamber  52 , the second pressure booster piston part  56  moves into the control chamber  52 , so that the fuel kept on hand in the high-pressure chamber  50  is compressed by further movement inward of the pressure booster piston  32 , and in particular of its first pressure booster piston part  54 . The maximum pressure prevailing in the high-pressure chamber  50  is diverted via the high-pressure valve  40  into the high-pressure outlet  46 , and from there it reaches the high-pressure reservoir  18  (common rail), not shown in  FIG. 3.2 . An outflow of fuel from the high-pressure chamber  50  is not possible counter to the direction of action of the filling valve  38 . The latter blocks in the direction of medium pressure, while the connection geometry at the on-off valve  26  shown in  FIG. 3.2  blocks in the direction of low pressure. 
       FIG. 3.3 , by comparison, shows a refilling phase of the pressure booster, in which the on-off valve  26  is switched back into its switching position shown in  FIG. 3.1 . From  FIG. 3.3 , it can be seen that the reservoir chamber  48 , via the pressure booster inlet  44 , is subjected continuously to fuel under pressure which is precompressed in accordance with the pressure level of the high-pressure pump  14 . The fuel kept on hand in the reservoir chamber  48  flows via the filling line  58  and the on-off valve  26  both to the control chamber  52 , filling it, and to the high-pressure chamber via the filling valve  38 , so the high-pressure chamber is likewise replenished with fuel. Because of the action of the restoring spring  34 , which is braced on one end on the piston guide body  36  and on the other on the collar  33  of the pressure booster piston  32 , the pressure booster piston  32 , with its first pressure booster piston part  54  and its second pressure booster piston part  56 , returns to its outset position shown in  FIG. 3.4  again, in which the stop limitation means  42  touches the inside of the base body  30 . 
     In the outset position shown in  FIG. 3.4 , the same pressure and stroke conditions prevail as have already been described in conjunction with the outset position of the pressure booster  16  shown in  FIG. 3.1 , so that further remarks on this are unnecessary. 
     In the illustration in  FIG. 4 , an embodiment with transposed control and high-pressure chambers can be seen.  FIG. 4  shows that in this embodiment, the pressure booster  16  includes the base body  30 , in which the piston guide body  36  is embodied. The reservoir chamber  48 , which via the pressure booster inlet  44  is subjected by the high-pressure pump  14  shown in  FIG. 1  to pressure that is below the maximum pressure level of the this pump, is embodied in the base body  30 . The pressure booster piston  32  is also located in the reservoir chamber  48 , and the collar  33  on which the restoring spring  34  is braced is embodied on this piston. The restoring spring  34  is braced on its other end on an annular face of the piston guide body  36 . 
     In a distinction from the embodiment of the pressure booster  16  shown in  FIG. 2 , in the embodiment of  FIG. 4  the high-pressure chamber  50  is defined by the second pressure booster piston part  56  having the small diameter D 22 , while the control chamber  52  is defined by the first pressure booster piston part  54  of the pressure booster piston  32  having the larger diameter D 21 . As a result of this change in comparison to the embodiment of  FIG. 2 , an altered pressure boosting ratio i results, in accordance with the following equation:
 
 i =( D   21   /D   22 ) 2 .
 
     In this embodiment, the number of leakage points toward the low pressure at the pressure booster piston  32  is higher. At the instant of pressure boosting, as shown in  FIG. 3.2 , there are two leakage points on the guides, from high pressure and medium pressure to return pressure level. 
     In this embodiment of the pressure booster with transposed control and pressure chambers  52  and  50 , respectively, in each case referred to the embodiment of  FIG. 2 , refilling of the control chamber  52  is effected through the reservoir chamber  48 , the filling line  58 , and the short circuit at the on-off valve  26 , while refilling of the high-pressure chamber designated by reference numeral  50  is effected through the filling valve  38  from the reservoir chamber  48 . For the sake of completeness, it should be noted that in this embodiment of the pressure booster  16  as well, the high-pressure outlet is indicated by reference numeral  46 , and the pressure booster return associated with the on-off valve  26  is identified by reference numeral  24 . 
       FIG. 5  shows a further exemplary embodiment of the pressure booster  16 , in which the high-pressure chamber  50  is defined by a high-pressure sleeve  60 . In a distinction from the embodiments of the pressure booster  16  show in  FIGS. 2 and 4 , in which the high-pressure chamber  50  is defined by the piston guide body  36 , in the embodiment of the pressure booster  16  shown in  FIG. 5  the high-pressure chamber  50  is defined by a high-pressure sleeve  60  received on the first pressure booster piston part  54 . The high-pressure sleeve  60  is acted upon by a prestressing spring  64 . This spring, like the restoring spring  34 , is braced on the collar  33  of the first pressure booster piston part  54  of the pressure booster piston  32 . By the action of the prestressing spring  64 , a bite edge of the high-pressure sleeve  60 , foaming a sealing point  62 , is positioned against the piston guide body  36 . The restoring spring  34 , which is braced on the collar  33  of the first pressure booster piston part  54 , penetrates the entire reservoir chamber  48  and is braced on the base body  30 . The second pressure booster piston part  56  of the pressure booster piston  32  protrudes into the piston guide body  36 . 
     In the exemplary embodiment shown in  FIG. 5 , the high-pressure sleeve  60  for sealing off the high-pressure chamber  50  via the sealing point  62  additionally takes on the filling function the high-pressure chamber  50 . A structural advantage of this variant is the fact that the high-pressure sleeve  60  is guided by the pressure booster piston  32 . To that end, the sealing diameter at the sealing point  62  must always be less than or at most the same size as the piston diameter of the first pressure booster piston part  54 , or in other words D 21 . So that the high-pressure sleeve  60  will always be kept in a defined outset position, it is acted upon by the prestressing spring  64 . The design of the spring force for the prestressing spring  64  should be accomplished as a function of the spring force of the restoring spring  34  and the area of the remaining annular face between the sealing point  62  and the piston diameter of the second pressure booster piston part  56 , that is, D 22 . The smaller this remaining annular area is, for the same spring force of the restoring spring  34 , the less must the spring force be that is exerted by the prestressing spring  64  on the high-pressure sleeve  60 . 
     The refilling of the control chamber  52  can be effected in this embodiment in principle via the high-pressure chamber  50 , which represents a filling line  66 , and by the use of the short-circuited position of the on-off valve  26 , as shown in  FIG. 5 . Because of the reciprocating motion of the high-pressure sleeve  60  upon refilling of the high-pressure chamber  50 , this sleeve can exert uncontrolled opening and closing motions. If suitable precautions against this are not taken, the result would be high wear at the sealing point  62  and at the guide of the pressure booster piston  32 , and this would adversely affect the function of the embodiment of the pressure booster  16 . A clean switching function is assured with a suitable adaptation of seat geometry and pressure stage. 
     For the sake of completeness, it should be noted that the high-pressure valve  40 , which in this embodiment is embodied as a check valve, is received in the high-pressure outlet  46 , which extends to the high-pressure reservoir  18 , not shown in  FIG. 5 . 
     In the further exemplary embodiment of the pressure booster  16  shown in  FIG. 6 , for defining the high-pressure chamber  50 , the high-pressure sleeve  60  is again used. It includes an outer indentation which is engaged by a stroke stop element  70  that is fixed on the piston guide body  36  and which thus defines the maximum axial stroke  68  of the high-pressure sleeve  60  relative to the piston guide body  36 . Once the high-pressure sleeve  60  has executed its maximum stroke  68 , the stroke stop element  70  limits further reciprocating motions. To that end, the stroke stop element  70  is disposed between the restoring spring  34  and the piston guide body  36 . The prestressing force of the restoring spring  34  prevents lifting of the stroke stop element  70  from its contact face on the piston guide body  36 , which guide body is part of the base body  30  in this embodiment of the pressure booster  16 . 
     So that the refilling of the high-pressure chamber  50  will not be interrupted during the stroke impact of the high-pressure sleeve  60  on the stroke stop element  70 , a bypass  72  between the reservoir chamber  48  and the work chamber of the high-pressure sleeve  60  is located on the piston guide body  36 . In the embodiment of the pressure booster  16  shown in  FIG. 6 , the connection of the reservoir chamber  48  extends to the control chamber  52 , via the on-off valve  26  embodied preferably as a 3/2-way valve. This valve closes the low-pressure-side pressure booster return  24  in the switching position shown in  FIG. 6 , and upon actuation for example of an electromagnet opens it, as a result of which the control chamber  52  is pressure-relieved, the first pressure booster piston part  54  moves into the high-pressure chamber  50 , and the fuel volume stored there presses into the high-pressure reservoir  18  (common rail) via the high-pressure valve  40  via the high-pressure outlet  46 . 
     In the embodiment of the pressure booster  16  shown in  FIG. 6  as well, the prestressing spring  64  and the restoring spring  34  are braced on the collar  33  of the first pressure booster piston part  54 . The reservoir chamber  48 , which subjects the guide body  36  to a supporting pressure exerted from outside, in order to keep the leakage quantities slight, is acted upon, analogously to the above-described embodiments of the pressure booster  16 , by the high-pressure pump  14  via the pressure booster inlet  44  (compare the illustration in  FIG. 1 ). 
     The foregoing relates to the preferred exemplary embodiments 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.