Abstract:
A method of controlling operation of a portable, combustion-engined, power tool and including feeding fuel into the combustion chamber of the power tool, before ignition, several times one after another in accordance with an intermittent metering operational mode, and a power tool including a control device ( 34, 64 ) for controlling operation of the fuel feeding device ( 32, 33 ) of the power tool so that the feeding device ( 32, 33 ) feeds the fuel into the combustion chamber ( 1 ) several times in accordance with the intermittent metering operational mode.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a portable, combustion-engined power tool, in particular a setting tool for fastening elements, and including a combustion chamber, a device for feeding fuel into the combustion chamber to obtain therein a combustible air-fuel mixture, and an ignition device for igniting the air-fuel mixture that fills the combustion chamber. The present invention also relates to a method of controlling operation of such power tool. 
   2. Description of the Prior Art 
   In a power tool that is formed, e.g., as a nail setting combustion-engined tool, a drive is applied by a piston to a nail-like element for driving it in an object. An ignitable air-fuel gas mixture becomes available when the setting tool is pressed against the object. Upon actuation of a trigger or an actuation lever, an electrical spark is generated that ignites the air-fuel gas mixture, starting the combustion process. The chemically bound energy of the fuel is transformed into the physical energy by the combustion process. The combustion of the air-fuel gas mixture leads to increase of pressure which is transformed via the piston surface, the pressure acts upon, into a linear movement which drives the nail-like element into the object. 
   Thus, the drive energy for driving in a nail-like element depends from the chemically bound energy available in the combustion chamber and on its transformation in a mechanically usable thermal energy. The latter is determined by a ratio λ of a gaseous fuel to air. This ratio should always be in a region of λ=1 in all of the operational conditions of the power tool. In order to be able to use the power tool in a wide temperature region with wide variations of air density, the mass of the fuel, which is fed into the combustion chamber, should correspond to the air mass which is brought in. 
   In the presently available setting tool, the feeding of fuel is effected with one or more metering devices which meter a predetermined amount of a liquefied fuel and which are mechanically or electronically controlled. The amount of fuel is determined by a fixed geometry of the metering chamber and by the temperature of the liquefied fuel. Generally, the admixable fuel volume should remain substantially constant over the entire operational region. The injection of the liquid fuel into the combustion chamber is effected in one simple stroke before actuation of the power tool or before the start of the setting process, with a continuous ejection of the fuel from the metering chamber through the metering valve. A new injection begins only after the setting of the nail-like element has been completed. 
   Controlling the amount of the air mass brought into the combustion chamber to obtain the required ratio λ is very difficult because of the interrupted flow of air during the tool operation. Besides, the air density changes linearly with the air temperature. As a result, the fuel-air ratio λ changes even when the amount of the admixed fuel is constant. With an unfavorable fuel pressure ratios at temperatures at and below 0° C., an entire gas amount is not ejected from the metering chamber(s), which further adversely affects the fuel-air ratio λ. As a result, too little or too much of air mass is available to react with the gaseous fuel which may adversely affect the energy conversion. 
   Accordingly, the object of the invention is to provide a method of controlling a portable, combustion-engined tool which would insure a most favorable energy conversion at different operational parameters and/or environmental condition. 
   Another object of the invention is to provide a power tool in which a most favorable energy conversion can be obtained at different operational parameters of the tool and environmental conditions the power tool operates at. 
   SUMMARY OF THE INVENTION 
   These and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a method of controlling operation according to which the fuel if fed into the combustion chamber several times one after another in accordance with an intermittent metering operational mode. 
   Injection of fuel into the combustion chamber several times permits to bring the ratio λ between the gaseous fuel and the air to or close to 1 even at unfavorable operational parameters and/or environmental conditions, which insures a best possible energy conversion. 
   Dependent on the operational parameters and/or operational conditions of the power tool, an intermittent metering operational mode or a basic operational mode, at which fuel is injected into the combustion chamber only once, can be selected. The selection between the intermittent metering operational mode and the basic operational mode can be effected manually or automatically. As a fuel, e.g., a fuel gas or a liquefied fuel gas can be used. 
   As a device for fuel admixing or fuel injection, e.g., a metering head can be used. Under normal operational conditions, i.e., at a temperature of about 20° C., the metering head is actuated manually or electro-mechanically in a single step. Upon actuation of the metering head, the fuel volume in the metering chamber is fed to the power tool combustion chamber through a metering valve. When the operational conditions change, e.g., when the temperature is noticeably below 20° C., the metering head is switched, manually or automatically, to the intermittent metering operational mode. The automatic switching can be effected by using thermal or electrical sensing elements. With the intermittent metering operational mode, the metering head or another fuel feeding device is actuated several times before each ignition i.e., the fuel volume, which fills or fill the metering chamber(s) is injected through the metering valve(s) into the combustion chamber, and then the filling of the metering chamber(s) and the injection of the fuel into the combustion chamber is repeated at least one more time before actuation of the setting tool. That is the mass of the gaseous fuel is increased in the combustion chamber until a ratio λ of fuel to air is close to 1, so that an appropriate ratio is obtained even when operational conditions deviates from the normal conditions. Thereby, a most effective energy conversion is obtained under substantially all conditions. 
   The feeding of the fuel gas into the combustion chamber or the combustion chamber sections can be effected by feeding a gaseous fuel gas directly into the combustion chamber or by feeding of a gas which was liquefied before injection. Upon injection of the liquefied fuel gas into the combustion chamber, the liquefied fuel gas evaporates so that gaseous fuel gas becomes available. 
   According to a further development of the present invention, with the intermittent metering operational mode, the length of following each other metering cycles increases form cycle to cycle. This is particularly advantageous during the injection of the liquefied fuel gas because during the first cycle, the metering valve is cooled to a significant extent due to consumption of the heat during evaporation. When the metering valve is again filled with the liquefied fuel gas, the latter cannot evaporate sufficiently rapidly due to its relatively low temperature. Therefore, the metering valve should be held open for a longer time period to provide for ejection therefrom of a predetermined amount of the fuel gas. 
   A power tool according to the present invention is characterized by a control device for controlling the operation of the feeding device so that the feeding device feeds the fuel into the combustion chamber several times in accordance with the intermittent metering operational cycle. For switching between the intermittent metering operational mode and the basic operational mode, there is provided a switching device which can be actuated annually or automatically, dependent on operational parameters and/or operational conditions. 
   According to an advantageous embodiment of the present invention, the fuel feeding device has at least one metering valve with which a gaseous fuel is fed but which, however, can be used for injection of a metered amount of a liquefied fuel gas into the combustion chamber. The metering valve permits to feed into the combustion chamber a relatively precise amount of the fuel gas. Preferably, the metering valve forms part of a displaceable metering head that is controlled by a control device, with the metering valve opening or closing in accordance with the movement of the metering head. The metering valve opens when the metering head moves in a direction toward the combustion chamber and closes when the metering valve moves away from the combustion chamber. Other directions of movement of the metering head are also possible. 
   According to one embodiment of the present invention, the control device includes cam plates for displacing the metering head. The cam plates displace the metering head when the power tool is pressed against an object in which a fastening element is to be driven in. In this case, the linear press-on movement is converted into the rotational movement of the cam plates which displace the metering head. 
   According to a further development of the present invention, the control device has at least two cam plates one of which has at least two cams while the other one has only one cam. Generally, the control device can include three cam plates, one with one cam, another with two cams, and a third one with three cams. With a plate having more than two cams, the fuel can be fed not with one interval but with several intervals. An appropriate operational mode in this case is selected by switching an appropriate cam plate into contact with the metering head. 
   Naturally, the metering intervals, at the intermittent metering operational mode, can be controlled by other suitable means. Not necessarily cam plates should be used. As a further means for controlling feeding intervals, electromagnets can be used for controlling opening and closing of the metering valve(s). The signals for controlling the operation of the electromagnets can be supplied by the control device. 
   As it has already been described, the switching between the basic operational mode and the intermittent metering operational mode is effected with the switching device. The switching can be effected manually, when the user actuates the switching device with his hand for selecting an appropriate operational mode. However, switching can also be effected by using different sensor devices which generate at least one control signal for selecting the appropriate operational mode dependent on the sensed or measured operational parameters and/or environmental conditions of the power tool with a change of a respective parameter(s) or an environmental condition(s), a corresponding control signal is communicated to the switching device for a necessary switching of the operational mode. 
   The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to it constructions and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments, when read with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     In the Drawings: 
       FIG. 1  shows a longitudinal cross-sectional view of a portable combustion-engined setting tool according to the present invention with an intermittent metering operational mode in an initial position of the metering head and the device for controlling the operation of the metering head; 
       FIG. 2  shows a side view of the setting tool shown in  FIG. 1  that shows a cross-sectional view along line A—A in  FIG. 2 ; 
       FIG. 3A  shows a view similar to that of  FIG. 1  but in another position of the metering head and the metering head-controlling device; 
       FIG. 3B  shows a side view of the setting tool as shown in  FIG. 3A ; 
       FIG. 4A  shows a view similar to that of  FIG. 1  but in a further position of the metering head and the metering head-controlling device; 
       FIG. 4B  shows a side view of the setting tool as shown in  FIG. 4A ; 
       FIG. 5A  shows a view similar to that of  FIG. 1  but in a still further position of the metering head and the metering head-controlling device; 
       FIG. 5B  shows a side view of the setting tool as shown in  FIG. 5A ; 
       FIG. 6A  show a longitudinal cross-sectional view of a portable combustion-engine setting tool according to the present invention with a basic operational mode in an initial position of the metering head and the device for controlling the operation of the metering head; 
       FIG. 6B  shows a side view of the setting tool shown in  FIG. 6A  that shows a cross-sectional view along line A—A in  FIG. 6B ; 
       FIG. 7A  shows a view similar to that of  FIG. 6A  but in a different position of the metering head and the metering head-controlling device; 
       FIG. 7B  shows a side view of the setting tool shown in  FIG. 7A ; 
       FIG. 8  shows a cross-sectional view of a metering head used in the setting device according to the present invention; 
       FIG. 9  show a longitudinal cross-sectional view of a portable combustion-engine setting tool according to the present invention with intermittent metering operational mode and with an electromagnetically operated metering valve; 
       FIG. 10  shows a view similar to that of  FIG. 9  but in a different position of the metering head and the metering valve; 
       FIG. 11  shows a diagram of an electrical control cycle of the intermittent metering operational mode of the tool shown in  FIGS. 9-10 ; 
       FIG. 12  shows a diagram of an electrical control cycle of the basic operational mode of the tool shown in  FIGS. 9-10 ; and 
       FIG. 13  shows a view similar to that of  FIGS. 9-10  illustrating switching between operational modes dependent on sensor signals. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a cross-sectional view of a combustion-engined setting tool for fastening elements along line A—A in  FIG. 2  in the region of the tool combustion chamber. The setting tool shown in  FIGS. 1-2  has a combustion chamber  1  having a cylindrical side wall  2  and a bottom wall  3 . In the center of the bottom wall  3 , there is provided an opening  4 . A guide cylinder  5 , which has a cylindrical wall  6  and a bottom wall  7 , adjoins the bottom wall  3  of the combustion chamber  1  in the region of the central opening  4 . A piston  8  slidably displaces in the guide cylinder  5  in a longitudinal direction of the guide cylinder  5 . The piston  8  is formed of a piston plate  9  adjoining the combustion chamber and a piston rod  10  that is connected with the plate  9  in the center of the plate  9  and projects partially through the opening  11  in the bottom wall  7  of the guide cylinder  5 . 
     FIG. 1  shows the piston in its initial, rearward position when the setting tool is not operated. The piston plate  9  adjoins the bottom wall  3  of the combustion chamber  1  to a greater or lesser degree, and the piston rod  10  slightly projects past the bottom wall  7  of the guide cylinder  5 . There can be provided, on the outer circumference of the piston plate  9  and/or on the inner circumference of the cylindrical wall  6  of the guide cylinder  5 , sealings  12 ,  13 , respectively. The sealings  12 ,  13  seal the space on opposite sides of the piston plate  9 . 
   Within the combustion chamber  1 , there is arranged a cylindrical wall  14  which further will be referred to as a movable combustion chamber wall  14 . The movable combustion chamber wall  14  is displaceable in the longitudinal direction of the combustion chamber  1  and is provided, on its outer circumference, with a sealing  15  that seals the space on opposite sides of the movable combustion wall  14 . The movable combustion chamber wall  14  has a central opening  16  with a sealing  17  located in the inner wall of the opening  16 . 
   In the combustion chamber  1 , a separation plate  18  is arranged between the bottom wall  3  and the movable combustion chamber wall  14 . The separation plate  18  is also circular and has an outer diameter corresponding to the inner diameter of the combustion chamber  1 . The separation plate  18  is connected with a cylindrical lug  19  that projects through the central opening  16  in the movable combustion chamber wall. The length of the lug  19  is several times greater than the thickness of the movable combustion chamber wall  14 . 
   The sealing  17  closely engages the outer wall of the cylindrical lug  19 . At its free end, the cylindrical lug  19  has an annular shoulder  20 . The outer diameter of the shoulder  20  is larger than the inner diameter of the opening  16 . 
   In the non-operating position of the setting tool, the separation plate  18  lies on the bottom wall  3  of the combustion chamber  1 , and the movable combustion chamber wall  14  lies on the separation plate  18 . Starting from this position, upon pressing the setting tool against a constructional component or an object, the movable combustion chamber wall  14  moves away from the bottom wall  3 . After a certain time, the movable combustion chamber wall  14  engages the shoulder  20  of the lug  19  and entrains the separation plate  18  with it. The movable combustion chamber wall  14  is spaced from the separation plate  18  and forms therewith a so-called fore-chamber  21  that is a section of the combustion chamber  1 . With further movement of the movable combustion chamber wall  14 , another chamber section is formed between the separation plate  18  and the bottom wall  3  of the combustion chamber  1 . This chamber section represents a main chamber  22 . 
   In the interior of the lug  19 , there is arranged an ignition device  23 . When a combustible air-fuel gas mixture, which fills the fore-chamber  21  and the main chamber  22 , is ignited with the ignition device  23 , the air-fuel gas mixture first starts to burn in laminar fashion in the fore-chamber  21 , with the laminar front spreading with a relatively low speed in a direction toward through-openings  24  formed in the separation plate  18 . The laminar flame front displaces the uncombusted air-fuel gas mixture in front of it, with the uncombusted air-fuel gas mixture flowing through the openings  24  into the main chamber  22  and generating, in the main chamber  22 , turbulence and pre-compression. When the flame front reaches the openings  24 , the flame, penetrates into the main chamber  22  in form of flame jets which generate a further turbulence in the main chamber  22 . The turbulent air-fuel gas mixture in the main chamber  22  is ignited over the entire surface of the flame jets. The air-fuel gas mixture burns now with high speed which results in sharp increase of the efficiency of combustion. 
   The generated pressure impacts the piston  8  which moves with high speed in a direction toward the bottom wall  7  of the guide cylinder  5  forcing the air out of the guide cylinder  5  through openings formed in the cylindrical wall  6  (not shown). After a setting process has been completed or after completion of the combustion of the air-fuel gas mixture, the piston  8  return to its initial position due to the thermal feedback which results from cooling of flue gases that remain in the combustion chamber  1  and in the guide cylinder  5  behind the piston  8 . The combustion chamber  1  remains sealed until the piston  8  returns in its initial position. After the piston  8  returns into its initial position, the return plate  18  and the movable combustion chamber wall  14  return to their initial position adjacent to the bottom wall  3  of the combustion chamber  1  by spring forces. The flue gases are evacuated through outlet valves (not shown) provided in the bottom wall  3 , and the setting cycle ends. 
   As it has already been discussed for driving a fastening element into an object, the setting tool is pressed with its tip against the object by being pushed toward the object. The linear displacement of the respective elements of the setting tool is used for displacing the movable combustion chamber  14  wall away from the bottom wall  3  in order to expand the fore-chamber  21  and the main chamber  22 . The direction of the displacement fore is shown in  FIG. 1  with an arrow  25 . The combustion chamber wall  14  is displaced with appropriately arranged push-rods (not shown in detail). 
   Shortly before the complete expansion of the fore-chamber  21  and the main chamber  22 , a predetermined amount of fuel gas is fed in each of the fore-chamber  21  and the main chamber  22 . In the discussed embodiment, a liquefied fuel gas is separately injected into the fore-chamber  21  and the main chamber  22 . Below, the injection of the liquified fuel gas into the fore-chamber  21  and the main chamber  22  will be described in detail. 
   As shown in  FIG. 1 , two radial, axially spaced from one another, through-openings  26 ,  27  are formed in the cylindrical wall  2  of the combustion chamber  1 . Two feeding channels  28 ,  29  of respective metering valves  30 ,  31  project into the through-openings  26 ,  27 , respectively. The metering valves  30 ,  31  are provided in a metering head  32 . Liquified fuel gas is fed to the metering valves  30 ,  31  from a flask  33 . The metering valves  30 ,  31  inject the predetermined amount of the liquified fuel gas through the feeding channels  28 ,  29  into the fore-and main chambers  21 ,  22  when the metering head  32  is pressed against the cylindrical wall  2  which results in a forward movement of the feeding channels  28 ,  29  which, in turn, leads to opening of the preferring valves  30 ,  31 . To provide for opening of the metering valves  30 ,  31  when the feeding changes  28 ,  29  project further into the openings  26 ,  27 , the openings  26 ,  27  taper inward, forming stops for the feeding channels  28 ,  29 . 
   The displacement of the metering head  32  toward the cylindrical wall  2  of the combustion chamber  1  is controlled by control device  34  which is shown in the drawings. 
   The control device  34  has two support members,  35 ,  36  which are secured to the outer circumference of the cylindrical wall  2  of the combustion chamber  1 . A drive axle  37  is rotatably supported between the two support members  35 ,  36 . Two, spaced from each other, cam plates  38 ,  39  are supported on the drive axle  37 . The cam plate  39 , which is shown in  FIG. 1 , has two cams  40 ,  41  which project from the circumference of the cam plate  39 . The cams  40 ,  41  are displaced against adjusting nose  42  provided on the rear side of the metering head  32 , i.e., on the side of the metering head  32  remote from the combustion chamber  1 . The other cam plate  38 , which is shown in  FIGS. 6-7 , has only one cam  43  projecting from its circumference and engageable with the adjusting nose  42 . The two cam plates  38 ,  39  are displaced in the longitudinal direction of the axle  37  by a switching device  44 , whereby, alternatively, cams  40 ,  41  and the cam  43  are aligned with the adjusting nose  42 . 
   When the setting tool is pressed against an object into which a fastening element is to be driven in, the movable combustion chamber wall  14  is displaced in the direction of arrow  25  by drive rods (not shown). Upon displacement of the combustion chamber wall  14 , the drive axle  37  begins to rotate in a counterclockwise direction, as shown in FIG.  1 . The rotation of the drive axle  37  provides for rotation of the cam plates  38 ,  39 . With the rotation of the cam plates  38 ,  39 . With the rotation of the cam plate  39  in the counterclockwise direction, first, the cam  40  impacts the adjusting nose  42 , and then the cam  41  impacts the adjusting nose  42 . In both cases, the metering head  32  is displaced in the direction toward the combustion chamber  1 , which leads to a two-times actuation of the metering valves  30 ,  31 . 
   In the condition shown in  FIG. 1 , both metering valves  30 ,  31  are filled with the liquified fuel gas but still remain closed. When the cam  40  impacts the adjusting nose  42 , the metering head  32 , together with the metering valves  30 ,  31  moves in the direction toward the combustion chamber  1 , and the metering valves  30 ,  31  open. Upon openings of the metering valves  30 ,  31 , a predetermined amount of the liquified fuel gas is injected in each of the fore chamber  21  and the main chamber  22 . The cam  40  has, in the circumferential direction of the cam plate  39 , a relatively small length. Therefore, the corresponding injection process is rather short. After the injection, the metering head  32 , together with the metering valves  30 ,  31 , moves away from the combustion chamber. The metering valves  30 ,  31  close and are again filled with the liquified gas. Meanwhile, the cam  41  impacts the adjusting nose  42 , and against the metering head  42 , together with the metering valves  30 ,  31 , moves in the direction toward the combustion chamber  1 . Because the length of the cam  41 , in the circumferential direction of the cam plate  39 , is greater than that of the cam  40 , the metering head  32  is pressed against the cylindrical wall  2  of the combustion chamber  1  for a longer time. As a result, the metering valves  30 ,  31  are likewise open for a longer time, resulting in additional injection of the liquified fuel gas into the fore-chamber  21  and the main chamber  22 . Only after the conclusion of the second injection of the liquified fuel gas, the fore-chamber  21  and the main chamber  22  completely expand, and the displacement of the movable combustion chamber wall  14  away from the bottom wall  3  stops. Now, an ignition can be initiated by the ignition device  23 . The above-described operational mode represents an intermittent metering operational mode according to which before each ignition process, the fuel gas is fed into the combustion chamber  1  several times. In principle, it is possible to effect feeding of the fuel gas into the combustion chamber  1  more than two times by increasing a number of cams carried by the cam plate. 
   However, with the switching device  44 , the cam plates  38 ,  39  can be so displaced on the axle  37  that the cam plate  38  is brought into contact with the adjusting nose  42  upon rotation of the axle  37 . Because the cam plate  38  has only one cam  43  projecting from its circumference (FIGS.  6  and  7 ), before each ignition process, the fuel gas is injected into the combustion chamber  1  only once. In this case, a so-called basic operational mode is effected. Which operational mode is selected, the intermittent or basic, is left to a setting tool operator who appropriately actuates the switching device. Naturally, the selection of the operational mode can be effected automatically, dependent on environmental conditions and operational parameters of the setting tool. 
   FIGS.  3 A/B- 5 A/B show a cycle of the intermittent metering operational mode for a case when the cam plate  39  is provided with two cams  40 ,  41 , i.e., with two injections for each ignition process. In  FIGS. 3A-3B , the cam  40  process the adjusting nose  42  and, thereby, the metering head  32  downward, so that the liquified fuel gas is injected from the metering valves  30 ,  31  into the fore-chamber  21  and the main chamber  22 , respectively, for the first time.  FIGS. 4A-4B  show a condition in which the outlets of the metering valves  30 ,  31  are closed again. The adjusting nose  42  is not actuated, and the metering head  32  occupies its initial position. At that time, the metering valves  30 ,  31  are again filled with the liquified fuel gas. 
     FIGS. 5A-5B  show a condition in which after a further rotation of the cam plate  39  is counterclockwise direction, the second cam  41  impacts or engages the adjusting nose  42 . The metering head  32  is again displaced in the direction toward the combustion chamber  1 . The metering valves  30 ,  31  open again, and the liquified fuel gas is injected into the fore-chamber  21  and the main chamber  22  for a second time. Then ignition takes place. 
   FIGS.  6 A- 7 A/B show an operational cycle of a basic operational mode. In  FIGS. 6A-6B , the adjusting nose  42  is not yet actuated, and the metering head  32  occupies its initial position. The metering valves  30 ,  31  are filled with the liquified fuel gas from the flask  33 . After rotation or the cam plate  38  counterclockwise, the condition shown in  FIGS. 7A-7B  is reached. A single cam  43  of the cam plate  38  presses the adjusting nose  42 , displacing the metering head  32  in the direction toward the combustion chamber  1 . Both metering valves  30 ,  31  open, and a predetermined amount of the liquified fuel gas is injected into the fore-chamber  21  and the main chamber  22 . Then, the ignition process takes place. 
   The construction of the metering valves  30 ,  31  which is shown in  FIGS. 1 through 7B , is basically known; however, the used metering valve will be briefly described below, for completeness sake, with reference to  FIG. 8. A  channel  45  connects the metering chambers  30   a ,  30   b  of the metering valves  30 ,  31  with a hollow pin  46  of the metering head  32 , and the metering chambers  30   a ,  30   b  are filled with the liquified fuel gas when the outlets  47 ,  48  are not located within the metering chambers  30   a ,  30   b , i.e., when the metering valves  30 ,  31  are closed. In this case, the liquified fuel gas is not delivered to the feeding channels  28 ,  29 . Rather, the liquified fuel gas flows from the flask  33  through the channel  45  and through the inlets  49 ,  50  into the metering chambers  30   a ,  30   b . Thereby, metering of a predetermined amount of the liquified fuel gas takes place. The compression springs  51 ,  52 , which are supported on bottoms of the valves  30 ,  31 , respectively, bias respective spools  53 ,  54  into a position in which the outlets  47 ,  48  are located outside of the respective chambers  30   a ,  30   b , i.e., into a position in which the metering valves  30 ,  31  are closed. 
   When the metering head  32  is displaced toward the combustion chamber  1 , the outlets  28 ,  29  are displaced into the interior of the respective chambers  30   a ,  30   b , i.e., against the biasing fore of the compression springs  51 ,  52 , respectively. With the outlets  28 ,  29  being located in the respective metering chambers  30   a ,  30   b , and the liquified fuel gas which fills the respective metering chambers  30   a ,  30   b , flows through the outlets  47 ,  48  into respective feeding channels  28 ,  29 . Simultaneously, the inlets  49 ,  50  become closed by corresponding enlargements  55 ,  56  of the respective valve spools  53 ,  54 . Upon release, of the feeding channels  28 ,  29 , the compression springs  51 ,  52  bias the respective spools  53 ,  54  forward, and the outlets  47 ,  48  become closed. The inlets  49 ,  50  open again. The metering head  32  is fixedly secured to the flask  33  with its collar  57 . 
   A second embodiment of the present invention is shown in  FIGS. 9-13 . Contrary to the first embodiment, in this embodiment, the metering valves  30 ,  31  are electromagnetically actuated. They are equipped, respectively, with coils  58 ,  59  for displacing the respective valve spools  60 ,  61  longitudinally, whereby the respective valve outlets  28 ,  29  become closed or open. When current flows through the coils  58 ,  59 , the valve spools  60 ,  61  are displaced into the coils  58 ,  59 . This condition corresponds to a position of the metering head  32  in which it is pressed against the cylindrical wall  2  of the combustion chamber  1 . In this position of the metering head  32 , a predetermined amount of the fuel gas can be injected into the fore-chamber  21  and the main chamber  22 . With no current flow through the spools  58 ,  59 , the springs  62 ,  63  bias the valve spools  60 ,  61 , respectively, into their initial position. The feeding channels  28 ,  29  become closed again. Now, the metering chambers  30   a ,  30   b  (See  FIG. 8 ) can again be filled with the liquified fuel gas over the channel  45  (FIG.  8 ). 
   How often an injection process is effected before an ignition process depends from the selected operational mode. With the intermittent metering operational mode, the coils  58 ,  59  are traversed by current several times, as shown in  FIG. 11 , which shows the change of the spool voltage U i  in time. With the basic operational mode, the current passes through the coils  58 ,  59  only once so that the metering valves  30 ,  31  open only once for injection of the liquified fuel gas before the ignition starts. The basic operational mode is shown in FIG.  12 . 
     FIG. 13  shows an embodiment in which an operational mode is selected automatically dependent on an operational temperature of the setting tool. In this embodiment again, electromagnetically actuated valves according to  FIGS. 9-10  are used. The flow of current through the coils  58 ,  59  is controlled by a microprocessor  64 . The operational temperature of the setting tool is measured with a temperature sensor  65  in the region of the combustion chamber  1  adjacent to the metering valves  30 ,  31 . The temperature sensor  65  communicates its signal through a conductor  66  to a switching device  67  in the microprocessor  64 . When the temperature, which is measured by the temperature sensor  65 , corresponds to a normal temperature of above 20° C., the switching device is so actuated that microprocessor selects the basic operational mode, so that the liquified fuel gas is fed only once before the start of the ignition process. When the temperature measured by the temperature sensor  65  is much smaller than 20° C., e.g., is close to the freezing point, the switching device  67  actuates the microprocessor  64  so that the intermittent metering operational mode is selected, with feeding of the liquified fuel gas into the fore-chamber  21  and the main chamber  22  several times. Instead of the operational temperature, other or additional parameters can control the selection of the operational mode, e.g., air pressure and the like. 
   Though the present invention was shown and described with references to the preferred embodiments, such are merely illustrative of the present invention and are not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiments or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.