referring now to fig1 an embodiment of the invention will be described . in the arrangement of fig1 fuel is sucked up from a fuel reservoir 1 by a fuel pump 2 which is driven by the output shaft of an engine , not shown , and fed under pressure into a suction chamber 4 formed within the housing 3 of the fuel injection pump . in a known manner , a fuel pressure control valve 5 controls the pressure within the suction chamber 4 in dependence on engine r . p . m . so that as the engine r . p . m . increases , so does the fuel pressure in the suction chamber 4 in a predetermined manner . a plunger 7 is slidably received within a plunger barrel 6 penetrating the pump housing 3 , for simultaneous reciprocating and rotating motion to perform the dual function of fuel pumping and distribution , as hereinafter described . more specifically , a cam plate 8 is provided integral with the plunger 7 for rotation through a driving disk , not shown , by means of the drive shaft , not shown , of the fuel injection pump driven by the engine . said cam plate 8 has a cam surface formed at equi - intervals with highs corresponding in number to the number of the cylinders of the engine . said cam plate 8 has its cam surface urged against rollers 9 retained on a roller holder , not shown , by a spring , not shown , so that the cam plate 8 and accordingly the plunger 7 is caused by rotation of said drive shaft to simultaneously rotate and reciprocate . a closing plug member 10 is threadedly fitted in the housing 3 to cooperate with the plunger barrel 6 and the top end of the plunger 7 to define a pump working chamber 11 . during the suction stroke when the plunger 7 is moved downwardly in the drawing , said pump working chamber 11 has an increasing volume . at the same time , one longitudinal groove 12 of a plurality of such grooves formed in the peripheral surface of the top end of the plunger 7 faces one end of a supply line 13 formed in the housing 3 with the other end opening in the suction chamber 4 so that fuel is sucked into the pump working chamber 11 . now , when the plunger 7 is upwardly moved through the delivery stroke , the communication between the supply line 13 and the longitudinal grooves 12 is interrupted and accordingly the fuel introduced into the working chamber 11 is delivered under pressure through a channel 14 axially extending through the central portion of the plunger 7 and longitudinal distributing grooves 15 formed in the outer periphery of the plunger 7 into outlet pressure lines 16 penetrating the barrel 6 and the pump housing 3 , and then fed through delivery valves 17 into injection nozzles 18 to be injected into the respective cylinders of the engine . only one each of the outlet pressure lines 16 , delivery valves 17 , and injection nozzles 18 is shown for the simplification of illustration . said outlet pressure lines 16 are the same in number as the number of cylinders in the engine and are arranged circumferentially of the barrel 6 and the housing 3 at equi - intervals so that fuel is injected into each of the cylinders alternately in a predetermined order in accordance with the reciprocating and rotating motion of the plunger 7 . also formed in a portion of the plunger 7 projecting into the suction chamber 4 is a transverse cut - off port 19 intersecting the channel 14 for communicating said channel 14 with the suction chamber 4 . said cut - off port 19 is adapted to be obturated by the inner peripheral surface of a fuel quantity setting sleeve 20 slidably fitted on said portion of the plunger 7 . thus , when the plunger 7 is upwardly moved to disengage the cut - off port 19 from the upper edge of the sleeve 20 to cause the port to open in the suction chamber 4 , fuel in the channel 14 flows through the cut - off port 19 into the suction chamber 4 , and accordingly the delivery of fuel into the outlet pressure lines 16 is interrupted thus to terminate the injection of delivery stroke of the plunger . said fuel quantity setting sleeve 20 is arranged in engagement with a lever 21 arranged for pivotal motion by means of an operating input mechanism for presetting a desired engine speed and a governor mechanism for detecting an actual engine speed ( neither of them is shown ) so that when the engine speed is to be set to a lower valve or when the engine is being operated at a higher speed than a preset speed , said sleeve is displaced downward in the drawing to obtain a sooner injection end during the delivery stroke , thus reducing the injection quantity . formed around the outer periphery of the barrel 6 is an annular groove intersecting said supply line 13 to define an annular chamber 22 , while the pump housing 3 is formed with a bore 23 which has an outward end thereof terminating in an outer periphery thereof and the other inward end communicating with the annular chamber 22 through a communication port 24 with a small diameter formed at the bottom end of said bore 23 . said supply line 13 , annular chamber 22 and communication port 24 constitute an actuating means for a valve hereinbelow described . a piston 25 is slidably received within said bore as a main component element of said valve means , whose top end cooperates with the bore 23 to define a chamber 26 . fitted in the outward end of said bore 23 is a closing plug 28 , with a compression spring 27 interposed between the rear end of said piston 25 and said closing plug 28 . said plug 28 has an internal passage 30 having one end opening in a chamber 29 defined by said rear end of the piston 25 and the plug 28 , and the other end connected through a spill tube 31 to a zone under a lower pressure such as the suction port of the aforementioned fuel pump 2 . a port 32 is formed within said plunger 7 with one end thereof communicating with said pump working chamber 11 and the other end terminating in an outer periphery of the plunger 7 through an annular groove 32a formed around said outer periphery of the plunger 7 , while a communication passage 33 is also formed across the barrel 6 with one end thereof terminating in an inner peripheral wall of said barrel 6 , which cooperates with said internal passage 30 and spill tube 31 to form a relief passage . these ports 32 and 33 are so located that their associated ends may register with each other at a position of the plunger 7 corresponding to the onset of fuel injection . the other end of said communication passage 33 is always maintained in communication with one end of another communication passage 34 formed in the pump housing 3 , and said port 34 has the other end terminating in the inner periphery of said bore 23 . these passages 33 , 34 , 30 and 31 cooperate to constitute a relief passage for the fuel in the pump working chamber 11 . said piston 25 also has an outer periphery thereof formed with an annular groove 35 which is so located as to register with said open end of the communication passage 34 at a predetermined position of the piston 25 within the bore 23 . the piston 25 also has the interior thereof formed with a restriction passage 36 having one end thereof radially opening in said annular groove 35 and the other end opening in said chamber 29 which is under a lower pressure . with this arrangement , since the supplied fuel pressure produced by the rotation of said fuel pump 2 is nearly zero when the engine has just started and accordingly there is substantially no fuel pressure prevailing in the chambers 22 , 26 , the piston 25 is urged by the spring 27 to be biased more leftward in the drawing than its illustrated position to keep the open end of the communication passage 34 from aligning with said annular groove 35 of the piston 25 . consequently , fuel injection takes place under a normal or usual injection pressure . when the engine is operated in a low speed condition mainly including the idling speed , the fuel being delivered through the pump has a somewhat increased pressure to accordingly increase the fuel pressure in the chambers 22 , 26 which causes the piston 25 to be rightwardly displaced from said position in engine starting to the illustrated position to allow the open end of the communication passage 34 to register with the annular groove 35 of the piston 25 . accordingly , when the plunger 7 upwardly moves into its fuel pressure feeding position , the port 32 in the plunger 7 and the communication passage 33 meet each other at a point in the onset of fuel injection during the delivery stroke so that part of the fuel being delivered from the pump working chamber 11 flows through channel 14 , port 32 , communication passages 33 and 34 , annular groove 35 of the piston and restriction passage 36 into the low - pressure chamber 29 , thus keeping the increase in the fuel pressure in the pump working chamber 11 within a certain range . as noted before , in this arrangement , it is only during part of one stroke of the plunger 7 that the annular groove 32a of the plunger 7 and the communication passage 33 are in face - to - face engagement with each other . however , since the plunger 7 has a small effective stroke in idling , said annular groove 32a and passage 33 can be maintained in such engagement throughout the entire injection period during each delivery stroke of the plunger in idling , thereby reducing the injection pressure to a substantially uniform value between individual delivery strokes . meanwhile in middle and high engine speed ranges , the pressure of fuel being delivered is increased and accordingly the piston 25 is displaced rightward of the illustrated position to again cut off the communication between the open end of the communication passage 34 and the annular groove 35 of the piston 25 , so that the fuel pressure in the pump working chamber 11 can be elevated up to a normal or usual pressure . in addition , since it is only at the beginning of the delivery stroke of the plunger 7 that the annular groove 32a in the plunger 7 and the communication passage 33 register with each other , the time when the pump working chamber 11 has a substantially increased volume owing to the presence of communication passages 33 and 34 can be limited only to the beginning of fuel injection during the delivery stroke of the plunger 7 thus enabling to obtain a generally sufficient injection pressure throughout the overall pumping operation , though the injection pressure has a sllightly slower rise time at the beginning of fuel injection of the delivery stroke . incidentally , the engine speeds at which the open end of the communication passage 34 and the annular groove 35 of the piston 25 can meet each other may be adjusted merely by inserting one or more shims 37 between the plug 28 and the spring 27 which have been adjusted in thickness or in number . alternatively , the plug 28 may be so arranged as to be adjustably displaced axially of the bore 23 by turning it . the above - described arrangement thus can achieve a lower injection pressure than a normal or usual value in the low engine speed range mainly including the idling speed , while permitting fuel injecting operation under a normal or usual injection pressure in other engine speed ranges such as engine starting or middle and high engine speed ranges . thus , by previously selecting a longer injection period for the idling operation , an injection rate characteristic as shown in solid line in fig2 can be obtained at idling ( a conventional characteristic is represented in broken line ), thus enabling a reduction in the combustion rate to minimize the combustion noise . as described in the foregoing , the arrangement according to the invention has a simple construction which is capable of minimizing the combustion noise through reduction in the injection pressure during the idling operation . it is to be understood that the foregoing description relates to a preferred embodiment of the invention and that various changes and modifications may be made in the invention without departing from the spirit and scope thereof .