turning now to the fuel injection system shown in fig1 combustion air flows , in the direction indicated by an arrow , into an induction manifold 1 which has a conical region 2 enclosing an air - flow measuring element 3 and thence through a connecting conduit 4 and an induction tube region 5 which encloses an arbitrarily actuatable butterfly valve 6 . the air continues to flow from the region 5 on to one or several cylinders ( not shown ) which form a part of an internal combustion engine . the air - flow measuring element 3 is embodied as a plate , disposed transverse with respect to the air flow . during operation , the measuring element 3 is displaced within the conical region 2 of the induction tube in accordance with an approximately linear function of the air quantity flowing through the induction tube . the pressure prevailing between the measuring element 3 and the butterfly valve 6 remains constant provided that the resetting force acting upon the measuring element 3 is constant and that the air pressure prevailing ahead of the measuring element 3 is also constant . the air - flow measuring element 3 directly influences a metering and quantity distribution valve assembly 7 . the movements of the measuring element 3 are transmitted by an attached lever 8 which pivots about a point 9 and during such pivotal movement a projection 10 provided thereon , as shown , actuates the movable valve element , embodied as a control slide 11 , of the metering and quantity distribution valve assembly 7 . the fuel is delivered by a fuel pump 14 , driven by an electric motor 13 , from a fuel container 15 and flows through a line 16 and a channel 17 into an annular groove 18 provided in the control slide 11 . depending upon the position of the control slide 11 , the annular groove 18 extends to a greater or lesser degree over control slots 19 each of which communicates , via channels 20 , with respective chamber 21 . each of the chambers 21 is separated from a respective chamber 23 by a respective diaphragm 22 . each of the diaphragms 22 serves as the movable valve element of a flat - seat valve embodied as a differential pressure valve 24 and is loaded in the direction of opening of the valve by a spring 25 . from chamber 21 , fuel flows through channels 26 to the individual injection valves ( not shown ), which are disposed within the induction tube in the vicinity of the engine cylinders . branching off from the line 16 , is a line 27 which includes a pressure limiting valve 28 which permits the fuel to flow back into the fuel container 15 if the pressure in the system becomes too high . the end - face of control slide 11 farthest from the lever 8 is actuated by pressurized fluid which serves as the resetting force for the air - flow measuring element 3 and which reaches the control slide through a line 29 containing a damping throttle 30 . also branching off from the line 16 is a line 32 containing , in series , a first throttle 33 , the chambers 23 of the differential pressure valve 24 , a second throttle 34 , a storage unit 35 , a third throttle 36 , and an electromagnetic valve 37 . when the electromagnetic valve 37 is opened , fuel may flow out of the control pressure circuit 32 at zero gauge pressure through a return line 38 back to the fuel container 15 . the storage unit 35 includes a storage chamber 40 which is separated by a diaphragm 41 from another chamber 43 that is open to atmospheric pressure through an opening 42 . the storage chamber 40 contains a fixed valve seat 44 from which the diaphragm may be lifted by the control pressure in the storage chamber 40 and against the force of a spring 45 . the fuel injection system shown in fig1 functions as follows : when the internal combustion engine is running , air is aspirated through the induction tube 1 , 4 and 5 , causing a certain displacement of the air - flow measuring element 3 from its normal position . corresponding to the displacement of the measuring element 3 , the lever 8 displaces the control slide 11 of the metering and quantity distribution valve assembly 7 and the control slide 11 meters out fuel which flows to the individual injection valves . the direct connection between the measuring element 3 and the control slide 11 results in a constant ratio of the air quantity to the metered out fuel quantity , so long as the characteristic operating properties of the measuring element 3 and the slide 11 are sufficiently linear , which is a normally desired goal . in order to make the fuel - air mixture richer or leaner , depending on the domain of the operational region of the internal combustion engine involved , one must be able to make a change in the proportionality between the aspirated air quantity and the metered out fuel quantity and this must be possible in dependence on motor parameters . one such parameter may be , for example , the oxygen content in the exhaust gas , monitored by means of a so - called oxygen sensor 39 located in the exhaust line of the internal combustion engine . this sensor 39 may actuate the electromagnetic valve 37 through an electronic control circuit 39 . the change of the fuel - air mixture can be made either by changing the resetting force at the measuring element 3 or else by changing the differential pressure prevailing at the metering valve apertures 18 , 19 . the differential pressure prevailing at the differential pressure valves 24 can be controlled and changed , preferably in unison , by the pressure in the control pressure circuit 32 . for this purpose , the control pressure circuit 32 contains the first throttle 33 , the chambers 23 of the differential pressure valves 24 , the second throttle 34 , the storage unit 35 , the third throttle 36 and the magnetic valve 37 , all in series . the electromagnetic valve 37 is opened by an oxygen sensor 39 , acting through the electronic control circuit 39 &# 39 ; whenever the input voltage to the circuit falls below a certain threshold voltage indicating a particular concentration of oxygen within the exhaust gas or a particular leaned - out fuel - air mixture . when the electromagnetic valve 37 is open , fuel flows from the storage chamber 40 of the storage unit 35 over the fixed valve seat 44 , the third throttle 36 and through the electromagnetic valve 37 back to the fuel container 15 . the control pressure within the storage chamber 40 decreases , causing more fuel to flow through the first throttle 33 and the second throttle 34 corresponding to the greater pressure difference . as the control pressure falls in the chambers 23 of the differential pressure valve 24 , the spring 25 opens the differential pressure valve 24 to a greater extent and a larger fuel quantity can flow from the chambers 21 through the channels 26 to the injection valves . this reduces the pressure in the chambers 21 , increasing the pressure difference prevailing at the metering valve apertures 18 , 19 resulting in an increased metered - out fuel quantity . if , on the other hand , the voltage produced by the oxygen sensor 39 exceeds a certain threshold value representing a particular enriched fuel - air mixture , then the electromagnetic valve 37 is closed by the electronic control circuit 39 &# 39 ;. the fuel flowing through the first throttle 33 and second throttle 34 now flows into the storage chamber 40 , increasing the pressure in the control pressure circuit 32 . the increase of pressure in the chambers 23 of the differential pressure valve 24 causes a reduction in the flow of fuel through the channels 26 to the injection valves and thus leads to a pressure increase in the chambers 21 . as a result , the pressure difference prevailing at the metering valve apertures 18 , 19 is decreased , causing a reduced fuel quantity to be metered out until such time as the oxygen sensor 39 again produces the signal for opening the electromagnetic valve 37 . the minimum control pressure which the storage unit 35 , when closed , maintains in the control pressure circuit 32 is so chosen that no vapor bubbles are produced during any of the operational conditions of the internal combustion engine . in order to insure that , at the operating point of the regulator , the same rate of change in the metered out fuel quantity is obtained during increasing and decreasing injection quantity , the three throttles 33 , 34 and 36 are so chosen that when a particular control pressure prevails in the storage chamber 40 and while the electromagnetic valve 37 is open , the fuel quantity flowing through the throttle 36 is twice as large as that flowing through the throttles 33 and 34 . when the minimum permissible control pressure in the storage chamber 40 is reached , the diaphragm 41 interrupts the fuel return through the fixed valve seat 44 which limits the maximum fuel quantity metered out at the metering valve apertures 18 , 19 . when the magnetic valve 37 is closed , the minimum fuel quantity metered out at the metering valve apertures 18 , 19 is obtained when the pressure in the control pressure circuit 32 is equal to the system pressure in the line 16 . in that case , the injection quantity is determined by the compression of the springs 25 in the differential pressure valves 24 . in the second exemplary embodiment of the invention , according to fig2 the differential pressure valves 24 are embodied as equal pressure valves , i . e ., the differential pressure valves are not biased in the opening direction by a supplementary spring loading . this has the advantage that , when the metering and quantity distribution valve is assembled , a precisely tuned adjustment of the individual springs is unnecessary . the minimum metered out fuel quantity may be limited , in this second exemplary embodiment , in that the third throttle 36 and the electromagnetic valve 37 are permanently bypassed by a line 47 containing a fourth throttle 48 . in that case , the control pressure in the storage chamber 40 always stays below that of the main fuel system pressure by an amount depending on the dimension of the fourth throttle 48 . the third exemplary embodiment of the invention is depicted in fig3 . this figure only shows that part of the control pressure circuit 32 which lies downstream of the chambers 23 of the differential pressure valves 24 in the fuel injection system according to fig2 . in fig3 parts which are identical to those of fig1 and 2 retain the same reference numerals . the third exemplary embodiment , according to fig3 offers the advantage that the minimum fuel quantity which is metered out is independent of the main fuel system pressure in the line 16 , not shown in fig3 . to this end , the first throttle 33 and the second throttle 34 are traversed by a calibrated flow - rate determined by a fourth throttle 48 across which a pressure control valve 50 maintains a constant pressure difference . the fourth throttle 48 and the pressure control valve 50 are connected in parallel in the pressure control circuit 32 . the pressure control valve 50 is embodied as a flat seat valve which contains a chamber 51 , separated from a chamber 52 by a diaphragm 53 that serves as the movable valve member . the chamber 51 contains a fixed valve seat 54 and a spring 55 which biases the valve in the direction of opening . the fourth throttle 48 can be disposed within the diaphragm 53 and may connect the chambers 51 and 52 of the pressure control valve 50 . the change of the flow - rate through the first throttle 33 and the second throttle 34 is obtained by placing a third throttle 36 in parallel with the pressure control valve 50 and the fourth throttle 48 and by putting the magnetic valve 37 in series with the third throttle 36 . when the pressure control valve 50 is opened , fuel may flow over the fixed valve seat 54 and through the return line 38 back to the fuel container 15 . it is to be appreciated that the foregoing descriptions and accompanying figures in the drawing relate to illustrative embodiments of an improved fuel injection system given by way of example , not by way of limitation . numerous variants and other embodiments are possible within the spirit and scope of the invention , the scope being defined in the appended claims .