Abstract:
A method of controlling operation a portable, internal combustion engined power tool including a combustion chamber ( 1 ), at least one suction/discharge valve ( 34 )associated with the combustion chamber ( 1 ), and an operational piston ( 8 ) the method including inquiring a displacement position of the operational piston ( 8 ) of the power tool when it is being displaced to its initial position after having performed an operational stroke upon ignition of a fuel gas mixture filling the combustion chamber, and releasing the at least one suction/discharge valve ( 34 ) upon the piston ( 8 ) reaching its initial position; and a power tool which is controlled by the method.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a portable, internal combustion engined power tool, in particular a setting tool for driving in fastening elements, and including a combustion chamber, at least one suction/discharge valve connected wit the combustion chamber, and an operational piston which performs an operational stroke upon ignition of a fuel gas mixture filling the combustion chamber. The present invention also relates to a method of controlling operation of such a power tool. 
     2. Description of the Prior Act 
     With a power tool of a type described above, the drive energy is generated by burning a fuel gas mixture in a combustion chamber and is transmitted by the operational piston to a fastening element. Upon pressing a power tool against a constructional component into which a fastening element is to be driven, an ignitable fuel gas mixture is injected into the combustion chamber. Upon actuation of a trigger, a spark is produced which ignites the fuel gas mixture, initiating a combustion process. The operational piston, which adjoins the combustion chamber, is driven by the combustion gases. At the end of its operational stroke, the piston passes past discharge openings through which the waste gases can be at least partially discharged. The piston, after having performed the operational strokes, returns in its initial position as a result of underpressure which was created in the combustion chamber by cooling of the waste or residual gases. During the period of thermal return of the piston to its initial position, the combustion chamber should remain sealed from the surrounding it environment. Therefore, the suction/discharge valves, which provide for delivery of fresh air into the combustion chamber, should be open only after the piston has returned into its initial position. Generally, the time necessary for the return of the piston into its initial position increases with the increase of the tool temperature which heats during operation. In addition, high-energy power tools require a large expansion volume which results in that a greater time becomes necessary for the return of the piston into its initial position. 
     In some conventional power tools, closing of a suction/discharge valve can be effected with a pawl connected by, e.g., a toggle lever with the trigger. In this way, the suction/discharge valve becomes open as soon as the trigger returns into its initial position. This means that by the time the trigger returns to its initial position, the piston also must return into its initial position. 
     The locking of the suction/discharge valve by the trigger means that the switching point of the trigger cannot any more be arbitrary selected. The ignition switch can only then be actuated when the locking of the suction/discharge valve has been completed, i.e., long after the start of the displacement of the trigger. However, a long trigger displacement adversely affects acceptance by the customers. Moreover, as it has already been discussed above, with a heated tool, the return of the piston into its initial position lasts longer. The tool user must, in this case, hold the trigger in its pulled condition longer to prevent the piston from occupying a erroneous position. 
     In order to increase the time available for return of the piston into its initial position, the trigger displacement can be damped. However, damping negatively influences operational characteristics of the trigger as a larger force is needed for actuating the trigger, and the trigger does not return sufficiently rapidly into its initial position. A user does not look at dampening favorably as it reduces the maximum setting rate and requires a greater force for actuating the trigger, which the user has to apply. 
     German Publication DE 19962 598.0 suggests detecting of the gas pressure in the combustion chamber after the fuel gas mixture has been ignited and locking the suction discharge valve(s) dependent on the detected gas pressure. 
     Accordingly, an object of the present invention is to provide a method which would permit to precisely determine when the piston returns into its initial position and thereby would provide for a more precise control of release of the suction/discharge valve. 
     Another object of the present invention is to provide a power tool which would contain means that would permit more precisely determine the return of the piston into its initial position. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention, which would become apparent hereafter, are achieved by providing a method according to which the displacement position off the piston is inquired, and the suction/discharge valve is released after the return of the piston in its initial position has been ascertained. 
     The tool according to the present invention includes inquiry means for inquiring the piston position and which actuates a locking/release device associated with the suction/discharge valve for opening same. 
     According to the present invention, the method for operating a portable internal combustion engined power tool including a combustion chamber, at least one suction/discharge valve for supplying fresh air into the combustion chamber and for discharging waste gases therefrom, and an operational piston displaceable in an operational direction upon ignition of a fuel gas mixture filling the combustion chamber, includes inquiring a displacement position of the piston when it is being displaced to its initial position after having performed an operational stroke; and releasing the at least one valve upon the piston reaching its initial position. 
     The foregoing method permits to precisely determined the geometrical position of the piston, in particular, its initial position. The release or opening of the suction/discharge valve is effected only when the piston has reached its initial position. The pressure variations of the residual gases in the combustion chamber do not lead any more to a faulty control during the valve release. The danger of the valve being released before th piston reaches its initial position is completely eliminated. The initial position of the piston is a position in which the opening between the combustion chamber and the guide cylinder is completely closed. 
     In principle, for inquiring the displacement position of the piston, a position of any portion of the piston can be monitored. However, because the piston is guided in a guide cylinder and has a piston rod at least a portion of which projects from the guide cylinder, according to embodiments of the invention, it is the position of the piston rod which is being ascertained. In this case, the access to the inside of the cylinder or the combustion chamber is not any more necessary. Therefore, the inquiring element or device for determining the displacement position of the piston can be formed much simpler, and it can be mounted much easier. The displacement position of the piston can be easily ascertained from the displacement position of the piston rod. Based on the principle that it is the initial position of the piston that need be ascertained, a corresponding clearly defined point on the piston rod can be monitored, e.g., by using a stationary sensor. Passing of the defined point of the piston rod past the sensor, upon returning of the piston to its initial position, is a clear indication that the piston has reached its initial position. 
     According to one embodiment of the present invention, as a defined point which has to be monitored for determining the displacement position of the piston rod, the end surface of the piston rod is used. The position of the end surface of the piston rod can be monitored very easily and very precisely. This permits to precisely determine the return of the piston to its initial position. 
     In case, the piston rod has not returned to its initial position after the performance of the operational stroke, according to a further development of the present invention, a forceful release of the suction/discharge valve is provided for. The forceful release is provided in order to discharge the waste or residual gases from the combustion chamber. In this case, the piston must be brought into its initial position by other means, namely, manually or by a subsequent ignition of a new fuel gas mixture injectable into the combustion chamber. 
     According to the present invention, the portable, internal combustion engined power tool includes a combustion chamber, at least one suction/discharge valve for supplying fresh air into the combustion chamber, and for discharging waste gases therefrom, an operational piston displaceable in an operational direction upon ignition of a fuel gas mixture filling the combustion chamber, a locking/release device for release the at least one suction/discharge valve after the piston has reached its initial position after having performed operational stroke, and inquiry means for inquiring a displacement position of the piston when it is being displaced to its initial position after having performed operational stroke and for actuating the locking/release device when the piston has reached its initial position. 
     According to the invention, it is, thus, provided for a piston location-dependent control of the locking/release device, with the control being independent from pressure variation of the residual gases in the region of the combustion chamber. 
     As it has already been discussed previously, the piston, which is displaceable in a guide cylinder, has a piston rod a portion of which projects from the guide cylinder. Therefore, according to the invention, the inquiry element for determining the position of the piston is located outside of the guide cylinder in vicinity of the guide cylinder. This simplifies the design of the tool and insures an easy mounting and monitoring of the inquiry element. This position of the inquiry element insures a precise determination of the displacement position of the piston rod. 
     The piston rod and the inquiry element are located immediately opposite each other, whereby the inquiry of the displacement position of the piston rod is effected directly. 
     As it has already been discussed above, for inquiring the displacement position of the piston, a displacement position of the piston rod is monitored. As it has further been discussed above, a clearly defined point on the piston rod, which corresponds to the initial position of the piston, can be monitored. As it has still further been discussed above, as a defined point on the piston rod, the piston rod end surface is used, which is monitored with suitable sensor means. As sensor means mechanical, electrical, optical, and magnetic sensors can be used. Besides the end surface of the piston rod, as a defined point, other geometrical elements of the piston or the piston rod can be used. Also, an external elements mounted on the piston or the piston rod, such as magnets, soft iron cores, optical bar codes, can be used. However, the most advantageous element for ascertaining the return of the piston into its initial position, proved to be the end surface of the piston rod as its position can be most easily detected. 
     According to one embodiment of the present invention, an inquiring element can be formed as a sensor located adjacent to the displacement path of the piston rod and generating an electrical release signal as soon as the piston rod leaves the region of the sensor. The sensor generates the electrical release signal when the free end of the piston rod passes the sensor, which position of the piston rod corresponds to the initial position of the piston. To this end, the distance of the location of the sensor from the initial position of the piston is so selected that the sensor is located immediately in front of the end surface of the piston rod when the piston occupies its initial position. 
     The electrical release signal actuates the locking/release device which releases the suction/discharge valve(s). When the locking/release device has a locking lever, the electrical release signal can be used for a actuating an setting device that lifts the locking lever off its locking position. The locking lever is brought into its locking position by the tool trigger. The setting device retains the locking lever in its locking position until it is actuated by the electrical release signal. 
     According to another embodiment of the present invention, the inquiry element for inquiring or ascertaining the displacement position of the piston rod is formed as a resilient feeler biased against the circumference of the piston rod and extending into the displacement path of the piston rod when the piston has returned to its initial position. In the initial position of the piston, the feeler end is located immediately in front of the free end surface of the piston rod. Forming the inquiry element as a feeler simplifies its structure and, at the same time, provides for an inquiry element which is robust and require little maintenance. It is also not sensitive to contamination. The end of the feeler adjacent to the piston rod can be formed as a spring element, in form, e.g., of a leaf spring with a convex end, with the piston rod extending tangentially to the convex end in the displacement direction of the piston. 
     The locking/release device locks or releases the suction/discharge valve(s) dependent on whether the feeler engages the piston rod or extends into the piston rod displacement path, respectively. The locking or release of the valve(s) can be effected by the locking lever of the locking/release device engageable by the feeler. 
     The feeler can be connected with the free end of the locking lever which is displaceable into its locking position by the tool trigger. The locking lever remains in its locking position after the ignition of the fuel gas mixture in the combustion chamber as long as the feeler engages the circumference of the piston rod. 
     According to an advantageous embodiment of the present invention, there is provided a setting device that displaces the locking lever in its release position as soon as the piston has returned into its initial position. As the feeler element is formed as a mechanical element engageable with the circumference of the position rod, it is formed as a resilient element as it need be compressed upon the pivotal movement of the locking lever in its release position. 
     As a setting device, e.g., an actuation button, which is actuated manually and is connected with the locking lever, can be used. 
     However, the setting device can be also formed as an angular lever pivotally supported at its apex, with one leg of the angular displacing the locking lever of the locking/release device into its locking position when the other leg of the angular lever is displaced by a press-on element, which is supported in the tool housing, upon the displacement of the press-on element to the rear of the tool. The displacement of the other leg provides for pivoting of the angular lever in a respective direction. If, for some reasons, the piston has not returned in its initial position after performing its operational stroke, the tool can again be pressed against the constructional component to displace the locking/release device or its locking lever into the release position, which insures release of the suction/discharge valves(s) and deaeration of the combustion chamber. With a subsequent press-against process, the fuel gas again is injected into the combustion chamber, and the working process is conducted with the piston spaced from its initial position. However, in this condition of the piston, no fastening element is located in the outlet channel of the tool, so that there is no any danger of injury. Upon return of the piston in its initial position, the normal operation of the tool is resumed. 
     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 is construction 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 DRAWINGS: 
     The drawings show: 
     FIG. 1 an axial cross-sectional view of a first embodiment of a power tool according to the present invention in the region of its combustion chamber in a completely collapsed condition of the combustion chamber sections; 
     FIG. 2 an axial cross-sectional view of the power tool shown in FIG. 1 in its pressed-on condition with expanded combustion chamber sections; 
     FIG. 3 a view similar to that of FIG. 2 in an ignited condition of the combustion chamber; 
     FIG. 4 a view similar to those of FIGS. 1-3 illustrating a condition during return of the piston into its initial position; 
     FIG. 5 a view similar to that of FIG. 1 after the piston has returned into its initial position; 
     FIG. 6 a view similar to that of FIG. 1 in a release condition of the power tool; 
     FIG. 7 an axial cross-sectional view of a second embodiment of a power tool according to the present invention in the region of its combustion chamber; 
     FIG. 8 an axial cross-section view of a third embodiment of a power tool accordingly to the present invention in the region of its combustion chamber; 
     FIG. 9 an axial cross-sectional view of a fourth embodiment of a power tool according to the present invention in the region of its combustion chamber; and 
     FIG. 10 a cross-sectional view of a portion of a front region of the power tool shown in FIG. 9 at an increased scale. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A combustion engined power tool according to the present invention, which is shown in FIGS. 1-6, includes a cylindrical combustion chamber  1  with a cylindrical wall  2  and an adjoining the cylindrical wall  2 , circular bottom  3   a,    3   b.  In the center of the bottom  3   a,    3   b,  there is provided an opening  4   a,    4   b.  A guide cylinder  5 , which has a cylindrical wall  6  and a bottom  7 , adjoins the opening  4   a,    4   b.  A piston  8  is slidably displaceable in the guide cylinder  5  in its longitudinal direction. The piston  8  is formed of a piston plate  9  adjoining the combustion chamber  1  and a piston rod  10  located centrally with respect of the piston plate  9 . The piston rod projects through an opening  11  formed in the bottom  7  of the guide cylinder  5 . 
     In FIG. 1, the piston  8  is shown in its initial position corresponding to a non-operational condition of the power tool which is formed as a setting tool for driving fastening elements into constructional components. A surface of the piston plate  9 , which is adjacent to the combustion chamber, adjoins, to a lesser or greater degree, the bottom  3   a,    3   b,  and the free end of the piston rod  10  only slightly projects past the bottom  7  of the guide cylinder  5 . The diameter of the piston plate  9  is stepwise reduced in the direction of the combustion chamber  1 , with a smaller diameter portion lying in the opening portion  4   b  and a larger diameter portion lying in the opening portion  4   a.  In this way, the larger diameter portion of the piston plate  9  abuts the bottom plate  3   b  which forms a stop for the piston  8  in the initial position of the piston. For sealing the space on opposite sides of the piston plate  9 , sealing rings (not shown) can be provided in the outer circumference of the piston plate  9 . 
     Within the combustion chamber  1 , there is located a cylindrical plate which can be called a combustion chamber wall  14 . The combustion chamber wall  14  is displaceable in the longitudinal direction of the combustion chamber  1  and is provided in its outer circumference with an annular sealing to seal the space in front of and behind the combustion chamber wall  14 . The combustion chamber wall  14  has a central opening  16  in the wall of which there is arranged an annular sealing. 
     An annular separation plate  18  is arranged between the bottom plate  3   b  and the combustion chamber wall  14 . The separation plate  18  has a diameter that corresponds to the inner diameter of the combustion chamber  1 . At its surface adjacent to the combustion chamber wall  14 , the separation plate  18  is provided with a cylindrical lug  19  which extend through the central opening  16  of the combustion chamber wall  14 . The length of the lug  19  exceeds the thickness of the combustion chamber wall  14  in several times. The sealing, which is provided in the wall of the opening  16 , snuggly surrounds the lug  19 . At its upper end, the lug  19  is provided with a shoulder  20  the outer diameter of which is greater than the inner diameter of the opening  16 . A hollow cylindrical lug  17  adjoins the combustion chamber wall  14  at the edge of the opening  16 . The hollow lug  17  surrounds the lug  19 . The free end of the hollow lug  17  is located beneath the shoulder  20  and, in the position shown in FIG. 1, is spaced from the shoulder  20 . A web  2   a,  which is connected with the cylindrical wall  2  of the combustion chamber  1 , serves as a stop for the lug  19  and thereby insures a proper positioning of the separation plate  18  in an expanded condition of the combustion chamber  1 . 
     In a non-operative position of the power tool, the separation plate  18  lies on the bottom plate  3   b,  and the combustion chamber wall  14  lies on the separation plate  18 . This position of the separation plate  18  and the combustion chamber wall  14  corresponds to a completely collapsed condition of the combustion chamber  1 . When the power tool is pressed against a constructional component (not shown) into which a fastening element is to be driven in, the combustion chamber wall  14  is, as it will be explain later, lifted and becomes spaced from the separation plate  18  or the bottom plate  3   b,  as the case may be. After a certain time period, the combustion chamber wall  14  engages the shoulder  20  of the lug  19  of the separation plate  18 . In this position of the combustion chamber wall  14 , it is separated from the separation plate  18  a predetermined distance, forming a so-called fore-chamber section of the combustion chamber  1 . The fore-chamber section is designated with a reference numeral  21  (FIG.  2 ). Upon lifting of the combustion chamber wall  14  further, the combustion chamber wall  14  and the separation plate  18  are displaced together parallel to each other, and a further chamber section is formed between the separation plate  18  and the bottom plate  3   b  or the piston plate  9 . This chamber section is called a main chamber section and is designated with a reference numeral  22  (FIG.  2 ). FIG. 2 shows a condition of the combustion chamber  1  in which both combustion chamber sections, the fore-chamber section  21  and the main chamber section  22 , are completely expanded. In this position of the combustion chamber  1 , the shoulder  20  of the lug  19  of the separation plate  18  engages the stop-forming web  2   a.    
     For displacing the combustion chamber wall  14 , there are provided several, e.g., three actuation or drive rods  23  uniformly distributed along the circumference of the combustion chamber wall  14  and fixedly connected therewith. Only one of the drive rods  23  is shown in the figures. The drive rods  23  extend parallel to the axis of the combustion chamber  1  and outside of the cylindrical wall  6  of the guide cylinder  5 . The drive rods  23  extend through openings  24 , respectively, formed in the separation plate  18  and through corresponding openings  25   a,    25   b  formed in the in the bottom  3   a,    3   b.  The openings  25   a,    25   b  simultaneously serve as ventilation openings, with the openings  25   b  having a conical shape. The drive rods  23  and the combustion chamber wall  14  are connected with each other, e.g., by screws in per se known manner. The free ends of the drive rods  23  are connected with the drive ring  28  which, thus, connects the drive rods  23  with each other. The drive ring  28  is arranged concentrically with the combustion chamber axis and surrounds the guide cylinder  5 . The drive ring  28  is connected with the drive rods  23  by screws, but other suitable connecting means can also be used. A shoulder  26 , through which the drive rods  23  extend, is formed on the guide cylinder wall  6  in a spaced relationship to the drive ring  28 . Compression springs  27  extend between the drive ring  28  and the shoulder  26 . The compression springs  27  are so arranged that they always pull the combustion chamber wall  14  in a direction toward the bottom plate  3   b.    
     As it has already been discussed above, the openings  25   a,    25   b  serve also as ventilation openings, and valve tappets  32  are provide for displacement into the openings  25   a.  In the open condition of the openings  25   a,    25   b,  the valve tappets  32  are located outside of the combustion chamber  1 , i.e., beneath the bottom plate  3   a.  The valve tappets  32  are supported on a shoulder  33   a  formed on the cylindrical wall  6  of the guide cylinder  5 . Compression springs  33   b  are arranged between the valve tappets  32  and the edges of respective openings  25   a,  applying pressure to the valve tappets  32 , biasing them toward the shoulder  33   a.  When the drive ring  28  is displaced in a direction toward the bottom plate  3   a,  shoulders  33  provided on the drive rods  23  engage the valve tappets  32  and carry them, against the biasing force of the compression springs  33   b,  into the openings  25   a,    25   b,  closing the valves  34 . The valves  34  are formed as suction/discharge valves. The shoulders  33 , which are provided on the drive rods  23  are displaced through the openings  33   c  provided in the shoulders  33   a.    
     A plurality of openings  38  are distributed over the circumference of the separation plate  18  at the same distance from the combustion chamber axis. In the lower end of the guide cylinder  5 , there are formed a plurality of outlet openings  39  for evacuating air from the guide cylinder  5  when the piston  8  is displaced toward the bottom  7  of the guide cylinder  5 . At the lower end of the guide cylinder  5 , there is provided damping means  40  for damping the movement of the piston  8 . When the piston  8  passes past the openings  39 , an exhaust gas can escape through the openings  39 . 
     Two radial, axially spaced openings  41  and  42  are formed in the cylindrical wall  2  of the combustion chamber  1 . A liquefied fuel gas is delivered into the combustion chamber  1  though the radial openings  41 ,  42 . 
     FIG. 2, as it has been discussed above, shows the inventive power tool, which is formed as a setting tool, in the expanded condition of the combustion chamber sections, i.e., in the expanded condition of the fore-chamber section  21  and the main chamber section  22 . The displacement positions of the combustion chamber wall  14  and the separation plate  18  is fixed upon the valve tappets  32  entering the ventilation openings  25   a,    25   b,  which prevents further displacement of the drive rods  23  and thereby the displacement of the combustion chamber wall  14  and with the shoulder  20  of the lug  19  of the separation plate  18  abutting the stop-forming web  2   a.    
     The lug  19  forms, in its region adjacent to the separation plate  18 , an ignition cage  51  for receiving an ignition element  52 . The ignition element  52  serves for generating an electrical spark for the ignition of the air-fuel gas mixture in the fore-chamber  21 . As it will be described in more detail below, the ignition element  52  is located in the central region of the cage  51  having openings  53  formed in the cage circumference. Through these openings  53 , a laminar flame front exits from the ignition cage  51  into the fore-chamber  21 . 
     As it is further shown in FIG. 1, adjacent to the guide cylinder  5 , there is provided a locking/release device  54  which is connected with an inquiry element  55  for inquiring the displacement position of the piston  8  or the piston rod  10 . 
     The locking/release device  54  serves for locking the drive ring  28  and thereby the suction/discharge valves  34  in their closed positions. To this end, the locking/release device  54  includes a locking lever  56  that extends parallel to the longitudinal extend of the guide cylinder  5  at a small distance from the cylindrical wall  6  of the guide cylinder  5 . The locking lever  56  is pivotally supported at its rear end on the bottom plate  3   a.  For supporting the locking lever  56 , a pivot support  57  is provided on the outer side of the bottom plate  3   a.  The locking lever  56  has its end remote from the support  57  extending through an opening  58  formed in the drive ring  28 . The locking lever  56  is formed integrally as one-piece with a feeler that forms the inquiry element  55 . The end of the inquiry element  55  is located immediately below an end surface of the piston rod  10  when the piston  8  is located in its initial position. The part, which forms the locking lever  56  and the feeler-shaped inquiry element  55  can be stamped out of a strong metal sheet. On its side remote from the guide cylinder  5 , the locking lever  56  has a locking edge  59  with which the locking lever  56  can engage the drive ring  28  from behind when the drive ring  28  has been pushed sufficiently far toward the bottom  3   a,    3   b  of the combustion chamber  1 . A portion of the locking lever  56  that extends from the support  57  to the locking edge  59  has a relatively small width which increase to form the locking edge  59 . The locking lever  56  is pivoted, at the support  57 , toward the guide cylinder  5  by a spring  60  supported against an element of power tool housing. Upon pivoting of the locking lever  56 , the locking edge  59  becomes disengaged from the edge of the opening  58  of the drive ring  28 , when the locking edge  59  is located behind the drive ring  28 , and the inquiry element  55  has its free end located in the displacement path of the piston rod  10 . 
     Sidewise of the guide cylinder  5 , there is provided a trigger  61  which is pivotally supported on the cylindrical wall  6  of the guide cylinder  5 . For pivotally supporting the trigger  61 , a support  62  is provided on the cylindrical wall  6 . The trigger  61  pivots in a direction toward the bottom  3   a,    3   b  against a biasing force of a compression spring  63 . An actuation section of the trigger  61  lies outside of the locking lever  56 . The trigger  61  is formed integrally with a lug  64  extending from the support  62  toward the bottom  7  of the guide cylinder  5 . Upon a pivotal movement of the trigger  61  in a counter clockwise direction, the lug  64  engages an edge of the locking lever  56 , pivoting the locking lever about the support  57  against a biasing force of the spring  60 . 
     Now, the operation of the first embodiment of a power tool according to the present invention will be described with reference to FIG. 2 in which the same elements are designated with the same reference numerals as in FIG.  1 . 
     FIG. 2 shows a condition in which the tool, which is formed as a setting tool, is pressed with its tip against a constructional component into which a fastening element has to be driven in. Upon the tip being pressed against the constructional component, the drive ring  28  is displaced by a press-on cage (not shown) in a direction toward the combustion chamber  1 , causing an expansion of the combustion chamber sections  21  and  22  (with drive rods  23 ), simultaneously closing the discharge/suction valves  34 . Shortly before the combustion chamber sections are completely expanded, the liquefied fuel gas is injected through the openings  41 ,  42 . In its displaced condition, the drive ring  28  is located in front of the locking edge  59 . However, the locking edge  59  cannot yet engage the drive ring  28  from behind, as the trigger  61  has not yet been actuated. The free end of the inquire element  55  remains in the path of the piston rod  10  immediately in front of the end surface  10   a  of the piston rod  10 . 
     FIG. 3 shows a position in which the trigger  61  is actuated, i.e., is pivoted counter clockwise about its support  62  against a biasing force of the spring  63 . Upon the pivotal movement of the trigger  61 , the lug  64  pivots the locking lever  56  also in the counter clockwise direction about the locking lever support  57 , and the locking edge  59  engages from behind the driving ring  28 . Simultaneously with the pivotal movement of the locking lever  56 , the inquiry element  55  moves out of the path of the piston rod  10 . During the last portion of the pivotal movement of the trigger  61  and after the inquiry element  55  has moved out of the path of the piston rod  10 , ignition of the gas mixture filling the combustion chamber sections  21 ,  22  is effected with the ignition element  52 . The ignition is effected with a spark produced by the ignition element  52  within the cage  51 . First, the mixture starts to burn luminary in the fore-chamber section  21 , and the flame front spreads rather slowly in a direction of the openings  38 . The unconsumaable air-fuel gas mixture is displaced ahead and enters, through the openings  38 , the main section chamber  22 , creating there turbulence and precompression. When the flame front reaches the openings  38 , it enters the main chamber section  22 , due to the reduced cross-section of the openings  38 , in the form of flame jets, creating there a further turbulence. The thoroughly mixed, turbulent air-fuel gas mixture in the main chamber section  22  is ignited over the entire surface of the flame jets. It buns with a high speed which significally increases the combustion efficiency. 
     The combustible mixture impacts the piston  8 , which moves with a high speed toward the bottom  7  of the guide cylinder  5 , forcing the air from the guide cylinder  5  out through the openings  39 . Upon the piston plate  9  passing the openings  39 , the exhaust gas is discharged therethrough. The piston rod  10 , moving in the direction indicated with an arrow, drives the fastening element in the constructional component against which the power tool is pressed. 
     Shortly after the ignition of the fuel gas mixture, the trigger  61  can be released. This results in the locking lever  56  moving, together with the inquiry element  55 , toward the guide cylinder  5  under the action of a biasing force applied by the spring  60 . However, this movement of the locking lever  56  does not lead to the disengagement of the locking edge  59  from the drive ring  28  because the free end of the inquiry element  55  engages the piston rod  10 , preventing further pivotal movement of the locking lever  56  about the support  57  in the clockwise direction. Thus, the drive ring  28  remains in its displaced position, the suction/discharge valves  34  remain, therefore, closed and the combustion chamber sections  21 ,  22  remain in their expanded condition. 
     FIG. 4 shows a condition of the inventive power tool after setting of the fastening element or following the combustion of the air-fuel gas mixture. The piston  8  is being brought to its initial position as a result of thermal feedback produced by cooling of the flue gases which remain in the combustion chamber  1  and the guide cylinder  5 . As a result of cooling of the flue gases, an underpressure is created behind the piston  8  or behind the piston plate  9  which provides for return of the piston  8  to its initial position. The inquiry element  55  slides along the piston rod  10  as the piston  8  is being brought to its initial position so that the drive ring  28  remains engaged by the locking edge  59 , and the suction/discharge valves  34  remain closed as the piston  8  has not yet reached its initial position. 
     FIG. 5 shows a condition of the inventive power tool in which the piston  8  has completely returned to its initial position, with the piston plate  9  completely closing the central bottom opening  4   a,    4   b.  The piston  8  has been retracted into the guide cylinder  5  to such an extent that the free en  10   a  becomes located outside of the region of the inquiry element  55 . 
     FIG. 6 shows a position in which both the locking lever  56  and the inquiry element  55  have been pivoted to their original position, with the locking edge  59  being disengaged from the drive ring  28  which can now move to its initial position. 
     In a next step, not shown in the drawings, the drive ring  28  is pushed away from the bottom  3   a,    3   b  of the combustion chamber  1  by springs  27 , entraining with it the drive rods  23 . The shoulders  33 , which are provided on the drive rods  23 , likewise move away from the bottom  3   a,    3   b,  and the springs  33   b  push respective valve tappets  32  out of the ventilation openings  25   a,    25   b.  With the displacement of the drive rods  23 , toward the front of the power tool, the combustion chamber wall  14  and the separation plate  18  move toward the bottom  3   a,    3   b  of the combustion chamber  1 , leading to the collapse of the combustion chamber sections  21 ,  22 . The residual gases, which remain in the combustion chamber  1  are discharged through the openings  25   a,    25   b,  with the suction/discharge valves  34  being open. The valves  34  also serve for admitting fresh air into the combustion chamber  1  upon the movement of the combustion chamber wall  14  and the separation plate  18  away from the bottom  3   a,    3   b.    
     FIG. 7 shows, as discussed above, a second embodiment of the power tool according to the present invention which differs from the first embodiment in that the inquiry element  55  is formed as an electric sensor unit. The sensor unit can be formed, e.g., as an electric photo barrier consisting of a light source, sender  65 , and a light receiver  67  arranged at the outer side of the bottom  7  of the guide cylinder  5  on opposite sides of the opening  11  in the bottom  7  in such a way that the light path extends beneath the end surface  10   a  of the piston rod  10  of the piston  8  in the initial position of the piston  8 , i.e., in the position of the piston  8  in which the piston plate  9  completely closes the opening  4   a,    4   b  in the bottom  3   a,    3   b  of the combustion chamber  1 . 
     After the ignition of the fuel gas mixture in the combustion chamber sections  21 ,  22  and displacement of the piston  8  into its operational position, the piston rod  10  is located between the light sender  65  and a the light receiver  66  so that no light reaches the light receiver  66 . An electrically actuated setting device  67  continues to retain the locking lever  56  in its locking position even after release of the trigger  61 , with the locking edge  59  engaging the drive ring  28 . Only after the return of the piston  8  in its initial position, the piston rod  10  unblocks the light path between the sender  65  and the receiver  66 . After receiving a light signal, the receiver  66  sends a release signal to the setting device  67  which provides for pivotal movement of the locking lever  56  about its support  57  in the clockwise direction. As a result of this pivotal movement of the locking lever  56 , the locking edge  59  becomes disengaged from the drive ring  28 , providing for displacement of the drive ring  28  away from the combustion chamber  1  and for opening of the suction/discharge valves  34 . 
     The embodiment of the power tool shown in FIG. 8 differs from the previously shown and discussed embodiments in that the inquiry element  55  is formed as a flexible element in form of a leaf spring. The rear end of this leaf spring is fixedly attached to a projecting heel  56   a  provided at the free end of the locking lever  56 . The opposite, front end of the leaf spring is convexly bent and is pressed against the circumference of the piston rod  10 . The front, convexly bent end of the leaf spring-shaped, inquiry element  55  is biased into engagement with the piston rod  10  by the spring  60 . However, the biasing force applied by the spring  60  is not sufficiently large to dislodge the locking edge  59  from engagement with the drive ring  28 . If for some reason, the piston  8  is not returned into its initial position, the condition shown in FIG. 8 does not changed. Still, the release of the drive ring  28  can be effected with a push-button  68  which is displaceably arranged in the housing of the power tool. The push-button  68  is located in a support element  69  and is displaced against a biasing force of a return spring  70 . Upon displacement of the push-button  68  the actuation rod  71  displaces the locking lever  56  in a direction toward the piston rod  10 . The leaf spring-shaped inquiry element  55  bents resiliently further, and the locking edge  59  becomes disengaged from the drive ring  28 . With the drive ring  28  moving away from the combustion chamber  1 , the combustion chamber becomes deaerated. Upon release of the push-button  68 , the return spring  70  pushes the push-button  68  in its initial position in which the shoulder  72  provided on the rod  71  lies on the surface edge of the support  69 . The locking lever  56  remains in its release position because the drive ring  28  is located leftward of the locking edge  59 . 
     Upon subsequent displacement and locking of the drive ring  28  and ignition of the fuel gas mixture in the combustion chamber  1 , and a subsequent return of the piston  8  into its initial position, the front end of the leaf spring-shaped inquiry element  55  can be displaced, under the action of the spring  60 , into a position beneath the end surface  10   a  of the piston rod  10 , with the locking edge  59  being disengaged from the drive ring  28 . 
     FIGS. 9-10, which show a fourth embodiment of the inventive power tool, show a condition of the power tool in which the piston  8  has not yet reached its initial position after the completion of the setting process and is immovable. In this case, likewise, a forced released is provided, which is needed because the setting tool is again pressed with its tip against the constructional component. 
     To this end, an angular lever  73 , which is supported for a pivotal movement about an axle  74  on the outer side of the bottom  7  of the guide cylinder  5 , is provided. The axle  74  is located in the apex region of the angular lever  73  which has two legs, a first leg  75  and a second leg  76  connected with each other at the apex. Both legs  75  and  76  lie in a plane extending perpendicular to the axial extent of the axle  74  which, e.g., can extend tangentially to the guide cylinder  5 . The first leg  75  extends toward the front of the setting tool whereas the second leg  76  extends toward the piston rod  10 . 
     At the free end of the first leg  75 , there is provided an axle stub  78  which is located in a slot  77  provided in the free end  56   a  of the locking lever  56 . The free end  56   a  is formed by tangent-bending an end section of the locking lever  56  remote from the locking lever support  57 . The slot  77  extends in the longitudinal direction of the setting tool or the piston rod  10 . A leaf spring-shaped inquiry member  55  is secured at its rear end to the free end of the free end section  56   a  of the locking lever  56 . The front end of the leaf-spring-shaped inquiry element  55  is convexly bent and is pressed against the piston rod  10 . The inquiry element  55  serves for sensing the position of the piston rod  10 . 
     The second leg  76  of the angular lever  73  is provided at its free end with an axle stub  79  on which set lever  80  is pivotally supported. The set lever  80  is formed as a unidirectionally extending section. The lever  80  has a slot  81  having a stop edge  82  against which a leg  83  of a leg spring  84  is pressed. The leg spring  84  is wound about the axle  74  and has its other leg  85  supported on the cylindrical wall  6  of the guide cylinder  5 . The leg  83  of the leg spring  84  is constantly pressed against the stop edge  82  for applying a biasing force, via the set lever  80 , to the angular lever  73  for rotating same about the axle  74  in the clockwise direction. The position of the stop edge  82  in the set lever  80  is so selected that the leg spring  84  so positions the set lever  80  that it extend toward the front end of the setting tool and at a predetermined angle to the piston rod  10 . This angle is so selected that, in case the piston  8  does not reach its initial position and the front end of the leaf-spring-shaped inquiry element  55  is pressed against the piston rod  10 , the free end  80   a  of the set lever  80  is surrounded by an extension  86  of a press-on cage (not shown) that upon the setting tool being pressed against a constructional component, is displaced toward the rear end of the setting tool. 
     The operation of the setting tool shown in FIGS. 9-10 will be now described for a case when the piston  8  does not reach its initial position and is immovable, i.e., is in the condition shown in FIGS. 9-10. 
     For releasing the drive ring  28  and for dearating the combustion chamber  1 , the setting tool is again pressed with its tip against the constructional component. Upon the setting tool tip being pressed against the constructional component, the press-on cage, which was mentioned above, is displaced inward, together with its extension  86 . Because the leaf-spring inquiry element  55  is pressed against the piston rod  10 , the leg spring  84  can pivot the angular lever  73  only a small predetermined amount. The spring leg  83 , which is pressed against the stop edge  82  of the set lever  80 , so aligns the set lever  80  that the free end  80   a  of the set lever  80  is grasped by the extension  86 . The extension  86  presses the set lever  80  backward. As a result, the angular lever  73  is pivoted by the set lever  80  in a clockwise direction about the axle  74 . At that, the first leg  75  of the angular lever  73  moves toward the piston rod  10 , compressing the inquiry element-forming leaf spring. Upon displacement of the first leg  75  toward the piston rod  10 , the locking lever  56  is pivoted by the stub  78  about the support  57 , which results in disengagement between the locking edge  59  and the drive ring  28 . The drive ring  28  moves toward the front of the setting tool, causing deaeration of the combustion chamber  1 . When the press-on cage, together with the extension  86 , is displaced back to the front end of the setting tool, the angular position of the angular lever  73  does not change because locking lever  56  cannot move back because its thick section lies in the opening  58  of the drive ring  28  because of the movement of the drive ring  28  to the front of the setting tool. The locking lever  56  retains, with its slot  77 , the stud  78  and thus, the angular lever  73 , in its new position. 
     During the displacement of the set lever  80  by the extension  86  backward, the set lever  80  becomes engaged by a convex section of the extension  86  to prevent the set lever  80  from turning back. The displacement of the set lever  80  causes the rotation of the angular lever  73  in FIG. 10 in the clockwise direction and, as a result, the leg  83  of the leg spring  84  forms an acute angle with the stop edge  82 . This acute angle opens in a direction toward the leg  85 . When, after the release or lifting off of the locking lever  56 , the extension  86  moves forward, the set lever  80  is further pivoted in the clockwise direction by the leg  83  about the axle stub  79 . The set lever  80 , at a subsequent application of the tool against the constructional component and rearward movement of the extension  86 , would not be engaged by the extension  86 , and the set lever  80  would be located above the extension  86 . 
     Upon the second application of the tool against the constructional component, the combustion chamber will again be filled with the fuel gas mixture that will be ignited. The piston  8  would be again actuated and finally would return into its initial position. As soon as the piston rod  10  passes the inquiry element-forming leaf spring, it can move back into the region of the piston rod  10 , with its front end lying immediately beneath the end surface  10   a  of the piston rod  10 . 
     It follows from the foregoing discussion that in the release condition of the tool, i.e., in the release condition of the drive ring  28 , the extension  86  is uncapable of engaging the set lever  80  during initial application of the tool against the constructional component. When after a subsequent application of the tool against the constructional component, the drive ring  28  becomes locked as a result of actuation of the trigger  61 , and the locking lever  56  is pivoted away from the guide cylinder  5 , the angular lever  73  in FIG. 10 would pivot in the counterclockwise direction about the axle  74 . As a result the inquiry element-forming leaf spring would be displaced out of the path of the piston rod  10 , the free end  80   a  of the set lever  80  would engage the upper portion of the extension  86 , and the leg spring  84  would become preloaded, as a result of the actuation of the trigger  61  and the pivotal movement of the locking lever  56 . The engagement of the free end  80   a  of the set lever  80  with the extension  10  leads to an additional excursion of the leg  83  of the leg spring  84  which, however, ascends again as soon as the extension  86 , as a result of a recoil, moves relative to the guide cylinder  5 . When after the ignition, the piston rod  10  passes, on its displacement back, the inquiry element-forming leaf spring, it again is displaced by the leg spring  84  into the path of displacement of the piston rod  10 . As a result, a conventional release of the drive ring  28  takes place. The drive ring  28  becomes released as a result of pivoting of the locking lever  56  which leads to disengagement of the locking edge  59  from the drive ring  28 . In this case, a compression spring  60 , which is used in the embodiments of FIGS. 1 and 3, becomes unnecessary. 
     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.