BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a combustion engined setting tool for driving fastening elements, such as nails, bolts, and the like in an object and including a combustion chamber in which fuel is combusted, a piston guide adjoining the combustion chamber, and a drive piston arranged in the piston guide and displaceable in a setting direction under action of expanding gases produced in the combustion chamber upon combustion of the fuel.  
           [0003]    2. Description of the Prior Art  
           [0004]    Setting tools of the type described above can be operated with gaseous or vaporized liquid fuel which is combusted in the combustion chamber, providing for driving of the drive piston. It is generally desirable to achieve a good thermal efficiency. German Publication DE-42 43 617 A1 discloses a setting tool in which a cylinder with a pre-combustion chamber is arranged beneath the piston in the setting direction. In the initial position of the setting tool, the piston is located directly above the pre-combustion chamber in its maximum remote, from the main combustion chamber, position. The pre-compression of the fuel-air mixture, which fills the main combustion chamber, in this setting tool, is effected by ignition of a fuel-air mixture in the pre-combustion chamber, whereby the piston is accelerated in a direction toward the main combustion chamber. This results in an isentropical compression of the air-fuel mixture in the main combustion chamber.  
           [0005]    The drawback of this solution consists in that the setting tool of DE-42 43 617 A is mechanically very complicated and is susceptible to wear.  
           [0006]    Accordingly, an object of the present invention is a setting tool in which the drawback of the known prior art tool is eliminated and the thermal efficiency is increased.  
         SUMMARY OF THE INVENTION  
         [0007]    This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing, in the setting tool, an electrically driven device for pre-compressing oxidation medium necessary for effecting a combustion process in the combustion chamber and/or the fuel. The provision of an electrically driven pre-compressing device permits to feed, into the combustion chamber, a fuel-oxidation medium mixture compressed during operation of the setting tool. The combustion of the pre-compressed fuel-air mixture leads to a very high thermal efficiency of combustion.  
           [0008]    The pre-compression of the oxidation medium and/or fuel or of a fuel-oxidation medium mixture can be effected, e.g., with an electrically driven compression device. The energy for the pre-compression can be the result of efforts of a setting tool user or be obtained from the impact energy of the setting tool. Ideally, the electrically driven pre-compression device is controlled automatically by electronic control means, without intervention of the use.  
           [0009]    The compression device can be formed, e.g., as an electrically driven compressor.  
           [0010]    According to an advantageous embodiment of the setting tool, there is provided a storage reservoir which can be formed, e.g., as a pressure container. The storage reservoir can be used for storing of the compressed oxidation medium, or the compressed fuel, or the oxidation medium-fuel mixture. The use of the compressed fuel medium permits, advantageously, to conduct quickly following one another, setting processes. Alternatively, a process with multiple impacts becomes possible. The pre-compressing device can be formed with relatively small dimensions, as it can operate continuously during operation of the setting tool.  
           [0011]    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 its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Single FIGURE of the drawings shows a side, partially cross-sectional view of a combustion-engine setting tool according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    A combustion-engined setting tool  1  according to the present invention, which is shown in the FIGURE, can be driven with a fuel gas or with a vaporized liquid fuel. The setting tool  1  includes a housing  10  and a setting mechanism located in the housing  10 . Upon the setting tool being pressed, with its bolt guide  16 , against an object (not shown), the setting mechanism drives a fastening element such as, e.g., a nail, a bolt, or the like in the constructional component. The setting mechanism includes, among others, a combustion chamber  11 , a piston guide  17 , a drive piston  15  displaceably supported in the piston guide  17 , and bolt guide  16  for a fastening element. The fastening element is driven into the object by an end of the drive piston  15  facing in the setting direction  40 . The bolt guide  16  is adjoined, at its end facing in the direction opposite to the setting direction, by the piston guide  17 . Fastening elements are stored, e.g., in a magazine  21  secured to the setting tool.  
         [0014]    An ignition device  13 , which includes ignition means, is arranged in the combustion chamber  11 . As the ignition means, e.g., a spark plug  14  can be used, which ignites an oxidation medium-fuel mixture fed into the combustion chamber  11 . The fuel is fed in the combustion chamber  11  from a fuel tank  12  or another fuel source through a conduit  19 . A valve  18 , e.g., a mechanical or electronic metering valve, is arranged in the fuel conduit  19 . With the valve  18 , which is controlled by mechanical or electronic control means (not shown), an amount of fuel fed into the combustion chamber  11  can be controlled. In addition, pressure sensing means (not shown) such as, e.g., a pressure sensor, can be arranged in the combustion chamber  11 . With a pressure sensor, the oxidation medium-fuel mixture can be automatically ignited when the pressure in the combustion chamber  11  reaches a predetermined level. The automatic ignition of the oxidation means-fuel mixture can be effected with evaluating electronics which is connected with the pressure sensor and the ignition device  13  and which evaluates the pressure pulse generated by the pressure sensor and compares it with a set valve. In the transition region between the combustion chamber  11  and the piston guide  17 , there can be provided magnet means (not shown). The magnet means can be used for retaining the drive piston  15  with a predetermined holding force in its initial position at an end of the piston guide  17  adjacent to the combustion chamber  11 .  
         [0015]    The setting tool  1  further includes an electrically driven device  5  for pre-compression which includes a compression device  20 . The compression device  20  includes, e.g., a compressor provided with a piston  30  which is displaceably arranged in a cylinder  29  and is driven by an electrical drive  21 . The piston  30  is connected with the electrical drive  21  by a connecting rod  31 . As an electrical drive, a contact device  32  can be used. The electrical contact device  32 , e.g., a network cable with a plug, can be connected to an electrical supply circuit or a generator, which supplies the electrical energy necessary for the operation. However, an accumulator, a battery or the like, which can be provided in or on the setting tool, can also be used as a source of electrical energy. The cylinder  29  has an inlet  36  through which an oxidation air is aspirated thereinto, and an outlet  35  through which a compressed air can be expelled. The air inlet  36  is provided with valve means  26  that provides for entering of the air into the cylinder. The valve means  26  blocks flow of the air in opposite direction, preventing the air from flowing out through the inlet  36 .  
         [0016]    Valve means  25 , which is provided at the outlet  35 , enables air flow out of the cylinder  29  in its open condition and block flow of air into the cylinder  29  in its closed condition. The outlet  35  opens into a chamber  24  that communicates with a cylinder chamber  33  in the open condition of the outlet  35 . A conduit  23  connects the chamber  24  with a storage reservoir  22 , e.g., a pressure container. The storage reservoir  22  is connected with the combustion chamber  11  via a pressure conduit  27  in which a control valve  28  is arranged. The opening and closing of the control valve  28  is controlled by appropriate control means (not shown) to fed a compressed air from the storage reservoir  21  into the combustion chamber  11 . As has already been discussed above, the storage reservoir  22  is filled, upon operation of the setting tool  1 , with air, oxidation medium, by the compression device  20  via the conduit  23 . In the storage reservoir  22 , the air is retained under pressure.  
         [0017]    The setting tool  1  is actuated, upon having been pressed against a constructional component or another object, with a trigger switch  42  provided on the setting tool  1 . It should be noted that instead of the oxidation medium, the entire oxidation medium-fuel mixture can be pre-compressed by the compression device and be fed, in its compressed state, into the combustion chamber  11 .  
         [0018]    It should also be pointed out that the oxidation means or the oxidation means-fuel mixture can be pre-compressed directly in the combustion chamber  11 . In this case, the storage reservoir can be eliminated.  
         [0019]    Though the present invention was shown and described with references to the preferred embodiment, such is merely illustrative of the present invention and is 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 embodiment 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.
A combustion-engined setting tool for driving fastening elements, such as nails, bolts, and the like in an object includes a combustion chamber ( 11), a piston guide ( 17) adjoining the combustion chamber ( 11), a drive piston ( 15) arranged in the piston guide ( 17) and displaceable in a setting direction under action of expanding gases produced in the combustion chamber ( 11) upon combustion of fuel, and an electrically driven device ( 5) for pre-compressing oxidation medium necessary for effecting a combustion process in the combustion chamber ( 11) and/or the fuel.