in the injection devices shown in fig1 and 2 , a pump piston 1 , 101 operates in a cylinder bore 2 , 102 of a pump housing 3 , 103 and defines a pump chamber 4 , 104 , which is defined on the other end by a free piston 5 , 105 , that is disposed , axially freely displaceably , in the cylinder bore 2 , 102 . under the free piston 5 , a pressure chamber 6 of the pump is provided , from which fuel is pumped during the compression stroke to the injection nozzle , not shown . in the first exemplary embodiment shown in fig1 the pump piston 1 is driven solely to reciprocate , while in the second exemplary embodiment shown in fig2 a rotational motion is superimposed on this reciprocating motion ; preferably , the pump piston 101 executes as many stroke per revolution as there are engine cylinders to be supplied with fuel . in the first exemplary embodiment shown in fig1 a pressure conduit 7 that leads to an injection nozzle ( not shown ) on the engine branches off from the pressure chamber 6 . a pressure valve 8 opening toward the injection nozzle is disposed in this pressure conduit 7 , having a somewhat elevated opening pressure , so that it remains closed during the fuel metering into the pressure chamber 6 , as will be described below , and opens only during the injection pumping . the pump piston 1 is driven for its reciprocating pumping motion , represented by a double arrow ii , by a drive cam 9 rotating in the direction of the arrow i , counter to the force of a restoring spring 11 . the drive cam 9 is disposed on a camshaft 12 , which preferably also has the cams with which the valves of the associated engine are driven . in each case , this camshaft 12 rotates at an rpm corresponding to the engine rpm . in the normal situation , one such unit , having a pump piston 1 , is assigned to each engine cylinder . these high - pressure pump units are associated with a low - pressure fuel system , having a feed pump 13 and a pressure maintenance valve 14 ; this system pumps fuel into a suction chamber 15 , represented by dot - dash lines , and the fuel to be metered is drawn off from it . the pressure in the suction chamber 15 , as a metering pressure , is sufficiently high to serve to provide fuel metering in the pump chamber and pressure chamber . the feed pump 13 aspirates the fuel from a fuel container 16 , into which the leakage lines , not shown in further detail , discharge . the free piston 5 in principle separates the pump chamber 4 from the pressure chamber 6 ; that is , no short - circuit line is present . fuel is metered into each of the two work chambers 4 and 6 , in each case during a different portion of the intake stroke of the pump piston 1 , via a metering device that in these examples is embodied as a magnetic valve 17 . this magnetic valve 17 is incorporated into a metering conduit 18 that discharges into an inner annular groove 19 of the cylinder bore 2 . reversed via the shaft of the pump piston 1 , this inner annular groove 19 is made to communicate with the pressure chamber 6 in the first portion of the intake stroke , which here is designated as the metering stroke , and then in a second intake stroke portion , here designated the control stroke , is made to communicate with the pump chamber 4 . while only the quantity of fuel that is later to be injected upon the later compression stroke of the pump piston 1 is metered into the pressure chamber 6 during the metering stroke with the magnetic valve 17 open , during the control stroke , via the magnetic valve 17 opened at the proper time for this , a quantity of fuel that determines the supply onset is metered into the pump chamber 4 . the free piston 5 is never driven for its pumping stroke , in which it positively displaces fuel out of the pressure chamber 6 , until any voids in the pump chamber 4 have been compensated for after the beginning of the compression stroke of the pump piston , and the remaining fuel quantity in the pump chamber 4 , as a theoretically incompressible fluid , drives the free piston 5 . the larger the control quantity stored in the pump chamber 4 ahead of time , the earlier the injection begins , for a particular metering quantity in the pressure chamber . naturally the supply onset also depends fundamentally on the quantity of fuel stored beforehand in the pressure chamber 6 for the injection . in the final analysis , the supply onset is determined by adding the two quantities , that is , by the fill ratio of the pump chamber 4 , plus the fill ratio of the pressure chamber 6 . the extreme cases are a very early supply onset at maximum injection quantity , as is the case for instance at full load , and an extremely late supply onset at a very low rpm , as is for instance the case during idling . to control the inner annular groove 19 , an outer annular groove 21 for metering fuel to the pressure chamber 6 and an annular control groove 22 that controls the communication with the pump chamber 4 are both disposed on the jacket face of the pump piston 1 . between the two annular grooves 21 and 22 , an annular shaft segment 23 is provided , which has approximately the same width as the inner annular groove 19 . the entrance to a pressure chamber inflow line 24 coincides constantly with the outer annular groove 21 ; at its other end it discharges into the pressure chamber 6 , and a check valve 25 opening toward the pressure chamber 6 is disposed in this inflow line 24 . on the other hand , the entrance to a control conduit 26 coincides constantly with the annular control groove 22 and on its other end discharges into the pump chamber 4 ; a check valve 27 opening toward the pump chamber 4 is disposed in it . upon the intake stroke of the pump piston 1 , during the metering stroke the inner annular groove 19 coincides with the outer annular groove 21 , and then immediately reverses , and during the ensuing control stroke is separated from the outer annular groove 21 and communicates with the annular control groove 22 . depending on how long the magnetic valve 17 is open during these working strokes , fuel flows during the metering stroke out of the suction chamber 15 via the metering conduit 18 and the magnetic valve 17 , the inner annular groove 19 and the outer annular groove 21 , and the pressure chamber inflow line 24 and check valve 25 into the pressure chamber 6 , and during the control stroke fuel flows from the suction chamber 15 via the metering conduit 18 , the magnetic valve 17 , the inner annular groove 19 , the annular control groove 22 , the control conduit 26 and the check valve 27 into the pump chamber 4 . the magnetic valve 17 can open and close again within either the metering stroke or the control stroke , or the motion of the pump piston 1 can be used as a control device and in each case can already be opened whenever the communication between the inner annular groove 19 and the outer annular groove 21 or the annular control groove 22 is switched over by the annular shaft segment 23 , so that the first segment of the opening period of the magnetic valve 17 is used for the metering quantity , while the second portion contrarily is used for the quantity determining the supply onset . in order that the free piston 5 will execute a controlled motion during the metering stroke , the pump chamber 4 is kept at a predetermined fuel pressure , which is lower than the metering pressure that is transmitted from the pump chamber 4 , so that during the metering of fuel into the pressure chamber 6 , the free piston 5 is displaced only in accordance with the metered quantity that is later to be injected . this fuel pressure to be brought to bear in the pump chamber will hereinafter be called the maintenance pressure . necessarily , this maintenance pressure can exist only until the end of the metering stroke in the pump chamber 4 , because after that , from the beginning of the control stroke , fuel is carried at metering pressure into the pump chamber 4 . the maintenance pressure is controlled by a pressure control valve 28 , which operates with a control slide 29 that is acted upon on one side by metering pressure from the suction chamber 15 and on the other is acted upon by the maintenance pressure and by a pressure control spring 31 . as soon as the maintenance pressure drops , the control spring 29 is displaced counter to the force of the pressure control spring 31 in accordance with the altered pressure difference between the metering pressure and the maintenance pressure , and in this process opens the control cross section 32 , until the maintenance pressure has attained the desired magnitude . to provide uniform control despite varying fuel volumes on the maintenance pressure side , a pressureless outflow conduit 34 , for instance discharging into the fuel container 16 , branches off from the maintenance pressure chamber 33 , and an outflow throttle 35 is disposed in this conduit in order to enable maintaining the maintenance pressure in the maintenance pressure chamber 33 . the control slide 9 is embodied as a stepped piston , which is guided in a corresponding stepped bore , the face end of smaller diameter of which is acted upon by metering pressure and the face end of larger diameter is acted upon by maintenance pressure , while in the region of the step an outflow chamber 36 is provided , from which the outflow conduit 34 branches off . a longitudinal blind bore 37 is also disposed in the control slide 29 ; this bore is open toward the maintenance pressure chamber 33 and the outflow throttle leads away from it to the outflow chamber 36 . branching off from the maintenance pressure chamber 33 is a maintenance pressure line 38 , which intersects the cylinder bore 2 of the pump piston 1 and during the metering stroke coincides with the control annular chamber 22 , but is separate from it during the control stroke . during the metering stroke , the maintenance pressure is accordingly carried from the maintenance pressure chamber 33 into the pump chamber 4 , via the maintenance pressure line 38 , the annular control groove 22 and the control conduit 26 as well as the check valve 27 , so that during this portion of the intake stroke maintenance pressure prevails in the pump chamber 4 , counteracting the higher metering chamber in the pressure chamber 6 . branching off from the cylinder bore 2 are a relief conduit 39 and a diversion conduit 41 , which are each opened up by the free piston 5 , close to its terminal position of the pressure stroke , i . e . the outset position of the intake stroke ; via the relief conduit 39 , which is opened by the upper control edge of the free piston 5 , the pump chamber 4 is pressure - relieved toward the maintenance chamber 33 , and via the diversion conduit 41 , the pressure chamber 6 can be opened toward the suction chamber 15 . for controlling the diversion conduit 41 , an annular groove 42 disposed on the jacket face of the free piston 5 is provided ; it communicates with the pressure chamber 6 via a transverse bore 43 and a blind bore 44 each of which extends in the free piston 5 . the magnetic valve 17 , as a fuel metering device , is triggered by an electronic control unit 45 , in order to determine the particular fuel quantity to be metering and to the pressure chamber 6 and into the pump chamber 4 , and as a result to regulate the supply onset or engine rpm . the load is supplied to this electronic control unit 45 via a gas pedal 46 ; the rpm n is fed to it via an rpm transducer 47 ; and the temperature t and a further variable s , such as an exhaust gas value or an ambient pressure value , are fed to this unit via at least two further transducers , not shown . the actual supply onset can be measured via a transducer present on the injection nozzle . of the outputs 48 of the electronic control unit 45 , one output leads to the magnetic valve 17 via an electric line 49 , as shown , while the other three outputs 48 each lead to a different unit fuel injector , each having one magnetic valve 17 , assuming a four - cylinder engine . the course of motion of the drive cam 9 is divided into three segments : a relatively long intake stroke zone iii of little slope ( intake stroke flank ), a short resting portion iv ( base circle of the cam ), and a steep pressure stroke segment v ( pressure stroke flank ). in the operating position shown , the base circle of the cam is just now acting upon the pump piston 1 , shortly before the pressure stroke flank v comes into action . in conjunction with the rotational motion of the drive cam 9 , the operation of this first exemplary embodiment shown in fig1 will now be described : in the position shown , the pump piston assumes its top dead center position , corresponding to a terminal position after its intake stroke and an outset position before its compression stroke . upon further rotation of the drive cam 9 in the direction of the arrow i , the compression stroke flank v comes into action upon the pump piston 1 and actuates it downward for its compression stroke . during a first pressure stroke segment , any possible void present in the pump chamber 4 is compensated for , after which the free piston 5 is carried along downward as well by the virtually incompressible fluid in the pump chamber and pumps the pre - stored fuel out of the pressure chamber to the engine via the pressure conduit and the pressure valve 8 . the smaller the void in the pump chamber 4 , this earlier this supply accordingly begins . toward the end of this supply stroke of the pump piston 1 and free piston 5 , the relief conduit 39 of the pump chamber 4 is opened toward the maintenance chamber 33 by the upper edge of the free piston 5 , and the annular groove 42 opens the diversion conduit 41 of the pressure chamber 46 toward the suction chamber 15 . this diversion of the pressure chamber 6 takes place shortly before the relief of the pump chamber 4 , so that a defined terminal position of the free piston 5 is provided whenever the pump piston 1 is still pumping quantities of fuel located in the pump chamber 4 via the relief conduit . after that , upon the end of the compression stroke of the pump piston 1 , the maintenance pressure is established in the pump chamber 4 , while contrarily in the pressure chamber 6 the higher metering pressure is established , as a result of which the free piston 5 is pushed against the pump piston 1 in each case . at the intake stroke flank iii of the drive cam 9 that now comes into action , the fuel metering into the two work chambers takes place . as soon as the magnetic valve 17 opens , the suction chamber 15 is made to communicate with the pressure chamber 6 in the manner described above , so that fuel flows in . the free piston 5 in this process follows along precisely with the intake stroke motion of the pump piston 1 , because during this first metering stroke the maintenance pressure chamber 33 communicates with the pump chamber 4 , and therefore only the lower maintenance pressure prevails there . as soon as the desired injection quantity has been metered into the pressure chamber 6 , the magnetic valve 17 closes , and the free piston 5 separates from the pump piston 1 , whereupon fuel continues to flow at maintenance pressure into the pump chamber 4 . accordingly in this metering stroke both work chambers 4 and 6 are always filled with fuel ; the particular fill quantity depends on how much injection fuel was metered into the pressure chamber . in the extreme case , the free piston 5 remains in contact with the pump piston 1 until the end of the metering stroke . as soon as the maintenance pressure line 38 is then disconnected from the annular groove 42 in the course of the further intake stroke , in other words at the beginning of the control stroke , the suction chamber 15 is simultaneously made to communicate with the pump chamber 4 , as long as the magnetic valve 17 is opened , so that metering pressure now prevails in the pump chamber . as soon as the fuel quantity determining the supply onset has been metered , the magnetic valve 17 closes , even though the intake stroke of the pump piston 1 continues ; in this process a void can form in the pump chamber 4 or possibly in the pressure chamber 6 . the disconnection of the maintenance line 38 from the annular groove 22 takes place only shortly after the metering conduit 18 was disconnected from the pressure chamber inflow line 24 or connected to the control conduit 26 . in any case , only a safety overlap is involved here , which assures that the free piston 5 always remains fastened between two hydraulic cushions , so that no void can be created in the pressure chamber 6 during the metering of the fuel injection quantity . in the second exemplary embodiment shown in fig2 a distributor pump is used as the basic pump ; its function , insofar as it relates to the invention , is the same as in fig1 . in principle , all of the parts that are embodied like those of the first exemplary embodiment are provided with the same reference numerals , while parts that have the same function but a different design are identified by the same number , raised by 100 . for instance , the pump piston is 101 , because for its drive it executes not only the reciprocating motion ii but also a rotating motion vi , and because in addition to the outer annular groove 21 , the ring shaft segment 23 and the annular control groove 22 , it additionally has an annular distributor groove 51 , branching off from which is a longitudinal distributor groove 52 , which is disposed on the jacket face of the piston 101 and during a rotation of the pump piston 101 opens as many pressure conduits 107 as there are cylinders of the engine to be supplied with fuel . the number of reciprocating strokes performed during one revolution of the pump piston 101 also matches this number of cylinders to be supplied . pressure valves , not shown , are disposed in the pressure conduits 107 and by way of them the fuel is pumped to the injection nozzle with the engine during the compression stroke of the pump piston 101 . to enable the fuel to flow to the annular distributor groove 51 during the compression stroke , the check valve 25 is shifted farther upward , and a connecting conduit 53 is provided by way of which , during the metering stroke , fuel can flow from the pressure chamber inflow line 124 into the pressure chamber 6 , and then during the compression stroke it can be pumped back out of the pressure chamber 6 to the annular distributor groove 51 . during the metering stroke , there is no communication between the longitudinal distributor groove 52 and one of the pressure conduit 107 , so that even though a constant communication of this connecting conduit 53 with the longitudinal distributor groove 51 exists , no incorrect change is made in the quantity of fuel metered from the suction chamber 115 via the magnetic valve 17 and later intended to be injected . in this exemplary embodiment , an outflow conduit 54 branches off from the maintenance pressure chamber 33 ; disposed in this conduit is the outflow throttle 135 , which in principle is equivalent to the arrangement in the first exemplary embodiment having the outflow chamber 36 . otherwise , in terms of its control function and above all in terms of the fastening according to the invention of the free piston 5 during the metering stroke located at the beginning of the intake stroke , this second exemplary embodiment functions precisely like the first exemplary embodiment . all the characteristics described here , recited in the following claims and shown in the drawing may be essential to the invention either individually or in an arbitrary combination with one another . the foregoing relates to a 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 .