Patent Publication Number: US-7210431-B2

Title: Combustion-type power tool with exhaust gas flow regulating rib

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a combustion-type power tool, such as combustion-powered fastener-driving tool for driving fasteners, such as nails, into a workpiece. 
   2. Description of the Related Art 
   Compressors and hoses as required in conventional compressed-air type power tool are not needed in combustion-type power tools and the combustion-type power tools are good in operability. Various proposals have been made with respect to the combustion-type power tools as disclosed in U.S. Pat. Nos. 5,197,646 and 4,483,474. 
   Unlike a compressed-air type power tool that uses compressed air as a driving source, the combustion-type power tool requires no compressor and is, therefore, much easier to transport to a construction site or the like. Further, the combustion-type power tool includes an internal power source, such as a battery, so that the tool can be used in any environment without requiring a commercial power supply. 
     FIG. 6  is a cross-sectional view showing a conventional combustion-type nail gun  101 . 
   In the combustion-type nail gun  101 , a cylinder  105  is fixedly disposed within an outer frame  102 . A piston (not shown) is slidably movably disposed within the cylinder  105 . A combustion chamber frame  119  is disposed to surround the cylinder  105  and movable in the direction in which the piston moves. The outer frame  102  is partitioned by a partition wall  103  into spaces S 1  and S 2 . The cylinder  105  and the combustion chamber frame  119  are accommodated in space S 1 , and a gas cartridge cylinder  122  in space S 2 . A through-hole  103   a  is formed in the partition wall  103 , allowing spaces S 1  and S 2  to be in fluid communication with each other. A combustion chamber frame holding rod  125  is generally disposed within space S 2 . One end of the rod  125  is inserted into the through-hole  103   a  and projected into space S 1  so that the end of the rod  125  engages the lower potion of the combustion chamber frame  119 . The combustion chamber frame holding rod  125  moves in cooperation with a trigger switch  124  and serves to hold the combustion chamber frame  119  when the trigger switch  124  is turned off. 
   In use, the nail gun  101  is moved downward toward a workpiece W from the state shown in  FIG. 6 . When a push lever  121  is brought in abutment with the workpiece W and pushed thereagainst, the push lever  121  moves upward against the biasing force of a spring  120  biasing the push lever  121  downward. The combustion chamber frame  119 , which is coupled with the push lever  121  via an arm  131 , is also moved upward. In this manner, the combustion chamber frame  119  moves upward along the cylinder  105 . Raising the combustion chamber frame  119  to the uppermost position forms a hermetically sealed combustion chamber S by a cylinder head  104 , the combustion chamber frame  119 , the cylinder  105 , and the piston. Specifically, the combustion chamber S is formed by the engagement of the upper inner periphery of the combustion chamber frame  119  with the cylinder head  104 , and the engagement of the middle inner periphery of the combustion chamber frame  119  with the cylinder  105 . Flammable gas stored in a gas cartridge cylinder  122  is injected into the combustion chamber S. The flammable gas is agitated and mixed with air in the combustion chamber S by a fan  115 . A spark plug exposed in the combustion chamber S produces a spark for igniting and burning the gaseous mixture. The combusted gas expands to move the piston downward. A driver blade (not shown) secured to the piston strikes the nail into the workpiece W. A push switch  132  is provided for detecting that the combustion chamber frame  119  has elevated to a predetermined position. 
   When the piston has downwardly moved to a position near the lower dead center, an exhaust hole formed in the cylinder  105  is open to the atmosphere. High temperature, high pressure combusted gas in the cylinder  105  is discharged out to atmosphere through the exhaust hole and a stop valve disposed in the exhaust hole. Then, the pressure in the combustion chamber S is gradually lowered. When the pressure in the combustion chamber S has reached atmospheric pressure, a check valve is closed to thereby hermetically seal the combustion chamber S. Thermal vacuum caused by rapid cooling of the combustion chamber S draws the piston back to its initial upper dead center shown in  FIG. 6 . 
   The user subsequently lifts the nail gun  101  so as to be separated from the workpiece W. When the user releases the trigger switch  124  (turns the trigger switch  124  off), the combustion chamber frame holding rod  125  is disengaged from the lower portion of the combustion chamber frame  119 . Due to the biasing force of the spring  120 , the combustion chamber frame  119  returns to the initial position shown in  FIG. 6 . Therefore, the combustion chamber S is not hermetically sealed but is open to atmosphere. In this state, a motor  113  has been driven by a control circuit (not shown) and thus a fan  115  continues rotating. The rotating fan  115  draws fresh air through an inlet  112   a  formed in a cylinder head  112 . The fresh air is introduced into the combustion chamber S through a flow channel  128 , thereby performing a scavenging operation in which the fresh air introduced into the combustion chamber S expels the exhaust gas remaining in the combustion chamber S. 
   Continuous nail driving operations with the conventional nail gun  101  accumulate heat generated when the flammable gaseous mixture is combusted, resulting in heating up the nail gun  101 , particularly the combustion chamber frame  119  and the cylinder  105 . In the scavenging operation performed after the nail driving operation, these heated-up members are cooled. It should be noted that the exhaust gas primarily flows toward the lower portion of the combustion chamber frame  119  and is discharged out to the combustion chamber frame  119  through a discharge port  119   a  and then out to the outer frame  102  through an opening  130 . 
   The through-hole  103   a  formed in the partition wall  103  allows a part of high temperature exhaust gas to pass therethrough. That is, the high temperature gas existing in space S 1  in which the cylinder  105  and the combustion chamber frame  119  are accommodated flows into space S 2  in which the gas cartridge cylinder  122  is accommodated. As a result, the gas cartridge cylinder  122  is heated up, causing the temperature of the gas cartridge cylinder  122  to increase. 
   The pressure of the fuel confined in the gas cartridge cylinder  122  changes greatly depending upon the change in temperature. Accordingly, the temperature rise of the gas cartridge cylinder  122  causes a fuel ejection amount to vary and so a constant amount fuel ejection is not ensured. With the combustion-type nail gun  101 , the gaseous mixture in the combustion chamber S can be ignited only when the density of the flammable gas is within a predetermined range. If the density of the flammable gas is too low or too high to be outside the predetermined range, the gaseous mixture may not be ignited. Even if the gaseous mixture could successfully be ignited, the output power would be dramatically reduced, prohibiting stable performance of the nail driving operation. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to provide a combustion-type power tool with a structure capable of maintaining a substantially constant temperature of a gas cartridge cylinder. 
   It is another object of the present invention to provide a combustion-type power tool with a structure capable of stabilizing an amount of fuel ejected from the gas cartridge cylinder. 
   It is still another object of the present invention to provide a combustion-type power tool with a structure that ensures ignition of the gaseous mixture in the combustion chamber. 
   It is yet another object of the present invention to provide a combustion-type power tool with a structure that stably and constantly outputs required power in performing the fastener driving operation. 
   In order to attain the above and other objects, a combustion-type power tool according to one aspect of the invention includes an outer frame, a gas cylinder cartridge receiving portion, a cylinder head, a combustion chamber frame, a partition wall, and an exhaust gas flow regulating member. In use, a gas cartridge cylinder is placed in the gas cartridge cylinder receiving portion. The cylinder is fixedly disposed within the outer frame. The cylinder head is fixed to the outer frame. The piston is slidably movable along the inner surface of the cylinder. The combustion chamber is disposed within the outer frame to surround the cylinder to be movable along the cylinder. The combustion chamber is formed by the cylinder head, the cylinder, the piston, and the combustion chamber frame when the combustion chamber frame is in abutment with the cylinder head. The combustion chamber is capable of accommodating a gaseous mixture of existing air in the combustion chamber and fuel injected therein from the gas cartridge cylinder. The partition wall is disposed to divide the inner space of the outer frame into a first space in which the cylinder and the combustion chamber frame are disposed and a second space in which the gas cartridge cylinder is disposed. The partition wall is formed with a through-hole through which the first space and the second space are in fluid communication with each other. The exhaust gas flow regulating member is provided for regulating an exhaust gas flow generated by combustion of the gaseous mixture in the combustion chamber so that exhaust gas discharged from the combustion chamber is not directed toward the through-hole. 
   According to another aspect of the invention, there is provided a combustion-type power tool that includes an outer frame, a gas cartridge cylinder receiving portion, a cylinder head, a combustion chamber frame, a partition wall, and a plurality of ribs. In use, a gas cartridge cylinder is placed in the gas cartridge cylinder receiving portion. The cylinder is fixedly disposed within the outer frame. The cylinder head is fixed to the outer frame. The piston is slidably movable along the inner surface of the cylinder. The combustion chamber is disposed within the outer frame to surround the cylinder to be movable along the cylinder. The combustion chamber is formed by the cylinder head, the cylinder, the piston, and the combustion chamber frame when the combustion chamber frame is in abutment with the cylinder head. The combustion chamber is capable of accommodating a gaseous mixture of existing air in the combustion chamber and fuel injected therein from the gas cartridge cylinder. The partition wall is disposed to divide the inner space of the outer frame into a first space in which the cylinder and the combustion chamber frame are disposed and a second space in which the gas cartridge cylinder is disposed. The partition wall is formed with a through-hole through which the first space and the second space are in fluid communication with each other. The outer surface of the cylinder has a confronting portion confronting the partition wall. The confronting portion is divided into a cylinder-head-side portion and an anti-cylinder-head-side portion with respect to a position of the through-hole. The pair of ribs is formed on the cylinder-head-side portion and extends obliquely with respect to the imaginary longitudinal axis to be apart farther from each other. 
   According to still another aspect of the invention, there is provided a combustion-type power tool that includes an outer frame, a gas cartridge cylinder receiving portion, a cylinder head, a combustion chamber frame, a partition wall, and a plurality of ribs. In use, a gas cartridge cylinder is placed in the gas cartridge cylinder receiving portion. The cylinder is fixedly disposed within the outer frame. The cylinder head is fixed to the outer frame. The piston is slidably movable along the inner surface of the cylinder. The combustion chamber frame is formed with a discharge port. The combustion chamber is disposed within the outer frame to surround the cylinder to be movable along the cylinder. The combustion chamber is formed by the cylinder head, the cylinder, the piston, and the combustion chamber frame when the combustion chamber frame is in abutment with the cylinder head. The combustion chamber is capable of accommodating a gaseous mixture of existing air in the combustion chamber and fuel injected therein from the gas cartridge cylinder. The partition wall is disposed to divide the inner space of the outer frame into a first space in which the cylinder and the combustion chamber frame are disposed and a second space in which the gas cartridge cylinder is disposed. The partition wall is formed with a through-hole through which the first space and the second space are in fluid communication with each other. The pair of ribs is formed on the outer surface of the combustion chamber frame to extend in the imaginary longitudinal axis along the discharge port. 
   According to yet another aspect of the invention, there is provided a combustion-type power tool that includes an outer frame, a gas cartridge cylinder receiving portion, a cylinder head, a combustion chamber frame, a partition wall, and a plurality of ribs. In use, a gas cartridge cylinder is placed in the gas cartridge cylinder receiving portion. The cylinder is fixedly disposed within the outer frame. The cylinder head is fixed to the outer frame. The piston is slidably movable along the inner surface of the cylinder. The combustion chamber is disposed within the outer frame to surround the cylinder to be movable along the cylinder. The combustion chamber is formed by the cylinder head, the cylinder, the piston, and the combustion chamber frame when the combustion chamber frame is in abutment with the cylinder head. The combustion chamber is capable of accommodating a gaseous mixture of existing air in the combustion chamber and fuel injected therein from the gas cartridge cylinder. The partition wall is disposed to divide the inner space of the outer frame into a first space in which the cylinder and the combustion chamber frame are disposed and a second space in which the gas cartridge cylinder is disposed. The partition wall is formed with a through-hole through which the first space and the second space are in fluid communication with each other. The plurality of ribs is formed on the outer surface of and in one end portion of the combustion chamber frame and has a portion extending in a direction perpendicular to the imaginary longitudinal axis. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
       FIG. 1  is a vertical cross-sectional view showing a combustion-powered nail gun according to an embodiment of the present invention; 
       FIG. 2  is another vertical cross-sectional view showing the combustion-powered nail gun shown in  FIG. 1 ; 
       FIG. 3  is a partial vertical cross-sectional view showing a combustion chamber frame accommodated in an outer frame, as viewed from direction A indicated by an arrow in  FIG. 1 ; 
       FIG. 4  is a partial horizontal cross-sectional view showing a cylinder and ribs cut along a line B-B indicated in  FIG. 1 ; 
       FIG. 5  is a side view showing the surface of a cylinder as viewed from direction C indicated by an arrow in  FIG. 1 ; and 
       FIG. 6  is a vertical cross-sectional view showing a conventional combustion-powered nail gun. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A combustion-powered nail gun according to one embodiment of a combustion-type power tool will be described with reference to the accompanying drawings. Hereinafter, the terms “upward”, “downward”, “upper”, “lower”, “above”, “below”, “beneath” and the like will be used throughout the description assuming that the combustion-powered nail gun  1  is disposed in an orientation as shown in  FIGS. 1 and 2 . Also, the terms “front-side” and “rear-side” used hereinafter correspond to “left-side” and “right-side” of  FIGS. 1 and 2 , respectively. 
     FIGS. 1 and 2  are vertical cross-sectional views showing a combustion-powered nail gun  1 , and particularly a nail-driving tool. The components and operations of the nail-driving tool are described below with reference to  FIGS. 1 and 2 . 
   As shown in  FIG. 1 , the nail gun  1  includes an outer frame  2  that is vertically divided by a partition wall  3  into two spaces S 1  and S 2  where space S 1  is larger than space S 2 . In space S 2 , there is provided a gas cartridge cylinder receiving portion where a gas cartridge cylinder is placed at the time of using the nail gun  1 . 
   A cylinder head  4  is fixedly attached to the upper portion of space S 1 . A cylinder  5  is fixedly disposed below the cylinder head  4 . As shown in  FIG. 2 , a piston  6  is slidably movably disposed within the cylinder  5 . A driver blade  7  extends downward from the center of the piston  6 . A bumper  8  made from rubber is disposed at the bottom portion of the cylinder  5  against which the piston  6  collides, thereby limiting the downward movement of the piston  6 . A plurality of exhaust holes  9  of a rectangular shape is formed in the cylinder  5  in portions near the bumper  8 . A check valve  10  is provided in each exhaust hole  9  for selectively opening and closing the exhaust hole  9 . When the check valve  10  opens the exhaust hole  9 , the exhaust gas is allowed to flow from interior to exterior of the cylinder  5 . A seal ring  11  is fitted into the groove formed in the upper outer periphery of the cylinder  5  to hermetically seal the gap between the cylinder  5  and a combustion chamber frame  19  to be described later. 
   The cylinder head  4  is covered by a head cover  12  attached to the upper portion of the outer frame  2 . An intake port  12   a  open to atmosphere is formed in the head cover  12 . A motor  13  is disposed in the center portion of the cylinder head  4 . The motor  13  has an output shaft (motor shaft)  14  extending downward to which a fan  15  is fixedly attached. The cylinder head  4  is formed with a fuel injection passage  16  which allows the flammable gas to pass therethrough. One end of the fuel injection passage  16  serves as an injection port  16   a  that opens at the lower surface of the cylinder head  4 . Another end of the fuel injection passage  16  is in communication with a gas cartridge cylinder  22  (see  FIG. 1 ). The cylinder head  4  has an outer periphery formed with a groove into which a seal ring  17  is fitted. A spark plug  18  is secured to the bottom portion of the cylinder head  4  for generating a spark when a trigger switch  24  is operated. 
   The combustion chamber frame  19  is disposed in space S 1  to surround the cylinder  5  and is vertically movable along the longitudinal direction of the cylinder  5 . The combustion chamber frame  19  is coupled with a push lever  21  via an arm  31  and is constantly biased downward by means of a spring  20 . The front-side of the combustion chamber frame  19  (left-side in  FIG. 1 ) is formed with two discharge ports  19   a  of a rectangular shape as shown in  FIG. 3 . The discharge ports  19   a  are aligned in the circumferential direction of the combustion chamber frame  19 . The combustion chamber frame  19  is coupled with a push lever  21  projecting downward from the outer frame  2  and is vertically movable together with the push lever  21 . As shown in  FIGS. 1 and 2 , the upper inner periphery of the combustion chamber frame  19  is formed with a plurality of ribs  19   b.    
   The gas cartridge cylinder  22 , which stores flammable gas (liquid gas), is detachably accommodated in space S 2 . A handle  2   a  extending rearwardly of space  2  is also a part of the outer frame  2  of the nail gun  1 . A battery  23  used as a power source of the motor  13  is detachably accommodated in the handle  2   a . The handle  2   a  is provided with the trigger switch  24  with which a combustion chamber frame holding rod  25  is coupled. 
   As shown in  FIGS. 1 and 2 , a through-hole  3   a  is formed in the partition wall  3  which divides the outer frame  2  into two spaces S 1  and S 2 , allowing spaces S 1  and S 2  to be in fluid communication with each other. One end of the combustion chamber frame holding rod  25  extends through the through-hole  3   a  into space S 1  so as to selectively engage the lower portion of the combustion chamber frame  119  accommodated in space S 1 . Another end of the holding rod  25  is coupled with the trigger switch  24 . 
   A magazine  26  filled with a plurality of the nails is detachably provided along the handle  2   a . A tail cover  27  is disposed between the magazine  26  and the push lever  21  for guiding the nails supplied from the magazine  26  and sequentially setting the nails in a predetermined position in confrontation with the driver blade  7 . 
   As will be described hereinbelow, a plurality of ribs is formed on the outer surface of the cylinder  5  for regulating the exhaust gas flow generated as a result of combustion of a gaseous mixture in a combustion chamber S so that the gas does not flow in the direction toward the through-hole  3   a  formed in the partition wall  3 . 
     FIG. 5  shows the outer surface of the cylinder  5  facing the partition wall  3 , in which the rectangular portion  3   a ′ indicated by two-dotted chain line designates the corresponding position of the through-hole  3   a  formed in the partition wall  3 . That is, if light is horizontally irradiated toward space  1  from space  2 , then the through-hole  3   a  will be projected as the rectangular portion  3   a ′ on the outer surface of the cylinder  5 . As shown in  FIG. 5 , a plurality of ribs  5   a ,  5   b  is formed in both the upper and lower parts of the cylinder  5 . Here, the upper part of the cylinder  5  refers to the part above the projected through-hole  3   a ′, and the lower part thereof refers to the part below the projected through-hole  3   a ′. The exhaust gas generally flows downwardly along the surface of the cylinder  5 . However, due to a pair of obliquely formed ribs  5   b , the exhaust gas flow is directed away from the through-hole  3   a . The ribs  5   b  are formed in the upper part of the cylinder  5  to obliquely downwardly extend and to be apart farther from each other. The ribs  5   a  extending vertically downward are formed in both the upper and lower parts of the cylinder  5 . 
   As shown in  FIGS. 1 and 3 , the combustion chamber frame  19  is formed with vertically elongated side shielding ribs  19   c  in opposing sides of the discharge ports  19   a  in order to prevent the exhaust gas from circulating into the space formed between the cylinder  5  and the combustion chamber frame  19  at the rear-side and entering into the through-hole  3   a . As shown in  FIGS. 1 and 4 , the combustion chamber frame  19  is further formed with horizontally extending bottom shielding ribs  19   d  in the lower corners at the rear-side of the combustion chamber frame  19 . The bottom shielding ribs  19   d  are formed between the outer frame  2  and the combustion chamber frame  19  to shield the gap formed therebetween. 
   To drive nails into the workpiece W with the nail gun  1 , the user grips the handle  2   a  and moves the nail gun  1  downward toward the workpiece W from the initial state shown in  FIGS. 1 and 2  in which the push lever  21  is separated from the workpiece W. After the push lever  21  is brought into contact with the workpiece W, the user further pushes the nail gun  1  against the workpiece W. Then, the push lever  21  opposes the biasing force of the spring  20  and the push lever  21  and the combustion chamber frame  19  coupled therewith are upwardly moved so that the combustion chamber frame  19  moves above the cylinder  5 . In this state, the inner peripheries of upper and middle portions of the combustion chamber frame  19  are in hermetical contact with the cylinder head  4  and the cylinder  5  with the aid of seal rings  17 ;  11 , respectively. At the same time, flow channels  28 ,  29  are closed which are formed between the combustion chamber frame  19  and the cylinder head  4  and between the combustion chamber frame  19  and the cylinder  5 , respectively. Consequently, a combustion chamber S is formed in which a mixture of a flammable gas and air is burned. The combustion chamber S is a space enclosed by the combustion chamber frame  19 , the cylinder head  4 , the cylinder  5 , and the piston  6 . 
   In accordance with the upward movement of the combustion chamber frame  19 , fuel (flammable gas) stored in the gas cartridge cylinder  22  is injected into the combustion chamber S from the fuel injection port  16   a  through the fuel injection passage  16 . Here, the flammable gas stored in the gas cartridge cylinder  22  is a pressurized, liquid gas that becomes gasified when injected into the combustion chamber S. When the combustion chamber frame  19  is at the uppermost position and either one of the push switch  32  or the trigger switch  24  is turned on, the motor  13  is driven, causing the fan  15  to rotate. The flammable gas injected into the combustion chamber S is agitated and mixed with air in the combustion chamber S by the fan  15  rotating within the hermetically sealed combustion chamber S in cooperation with the ribs  19   b  protruding inside the combustion chamber S. 
   When the user pulls the trigger switch  24  provided on the handle  2   a , the spark plug  18  produces a spark for igniting and burning the gaseous mixture. The combusted gas expands to move the piston  6  downward and the driver blade  7  secured to the piston  6  strikes the nail into the workpiece W. When the trigger switch  24  is pulled by the user, the combustion chamber frame holding rod  25  is brought into engagement with the lower outer surface of the combustion chamber frame  19  to hold the latter and prevent its downward movement. 
   After striking the nail, the piston  6  collides with the bumper  23 , and the discharge ports  9  formed in the cylinder  5  are open to the combustion chamber S. High temperature and high pressure exhaust gas produced in the combustion chamber S is discharged out to atmosphere through the discharge ports  9 . As described above, the check valve  10  is disposed in each discharge port  9 . This check valve  10  is closed after the combusted gas has been discharged from the cylinder  5  at the point that the interior of the cylinder  5  and the combustion chamber S have reached atmospheric pressure. Again, the combustion chamber S is hermetically sealed by the check valves  10 . Cooling down the combustion chamber S creates thermal vacuum, causing the piston  6  to move upward along the cylinder  5  and return to the initial state shown in  FIG. 2 . 
   When the user subsequently lifts the nail gun  1  so as to be separated from the workpiece W and then releases the trigger switch  24  (turns the trigger switch  24  off). In accordance with the releasing operation of the trigger switch  24 , the combustion chamber frame holding rod  25  is disengaged from the combustion chamber frame  19 , allowing push lever  21  and the combustion chamber frame  19  to move downward by the biasing force of the spring  20  and return to the initial state shown in  FIGS. 1 and 2 . The downward movement of the combustion chamber frame  19  opens the combustion chamber S to atmosphere. At this time, the motor  13  is continuously energized by a control circuit (not shown) so that the fan  15  continues rotating. In this state, the rotating fan  15  draws fresh air through the intake port  12   a  and supplies the fresh air into the combustion chamber S through the flow channel  28 . As a result, residual gas is expelled outside the combustion chamber S, thereby scavenging the air in the combustion chamber S. 
   With the scavenging operation as described above, high temperature exhaust gas (residual gas) is expelled out from the combustion chamber S while flowing through the channel  29 . The exhaust gas further flows downwardly to pass through a gap between the combustion chamber frame  19  and the cylinder  5 . A part of the exhaust gas flows outside the combustion chamber frame through the discharge ports  19   a  formed in the combustion chamber frame  19 , passes through the gap between the outer frame  2  and the cylinder, and discharged to atmosphere through the opening  30  formed at the front-side lower portion of the outer frame  2 . At this time, the side shielding ribs  19   c  formed on the outer surface of the combustion chamber frame  19  serve to prevent the exhaust gas from circulating to the rear-side part of the combustion chamber frame  19 , i.e., the side opposing the through-hole  3   a  formed in the partition wall  3 . Therefore, a major part of the exhaust gas flowing outside the combustion chamber frame  19  through the discharge ports  19   c  is discharged to atmosphere through the opening  30 . 
   The remaining exhaust gas flows downwardly into a space formed between the cylinder  5  and the combustion chamber frame  19 . The bottom shielding ribs  19   d  formed at the rear-side lower portion of the combustion chamber frame  19  narrows the gap formed between the combustion chamber frame  19  and the outer frame  2 . Thus, the bottom shielding ribs  19   d  serve to prevent the exhaust gas flowing out through the lower opening of the combustion chamber frame  19  from circulating to the gap between the partition wall  2  and the combustion chamber frame  19 . The oblique ribs  5   b  formed at the upper part of the rear-side outer surface of the cylinder  5  serve to regulate the exhaust gas flow so that the gas is not directed toward the through-hole  3   a  but directed to the passages that are apart from the through-hole  3   a . A major part of the exhaust gas flowing in the rear-side gap between the combustion chamber frame  19  and the cylinder  5  is discharged to atmosphere through the opening  30  at the front-side lower end of the outer frame  2 . 
   As described above, with the scavenging operation, the high temperature exhaust gas remaining in the combustion chamber S is prevented from circulating to the rear-side of the combustion chamber frame  19  by the side shielding ribs  19   c . Also, the exhaust gas is prevented from circulating to the gap between the partition wall  3  and the combustion chamber frame  19  by the bottom shielding ribs  19   d . The exhaust gas flowing in the rear-side gap between the combustion chamber frame  19  and the cylinder  5  is changed its flow direction by the oblique ribs  5   b  to flow in the front-side. A major part of the exhaust gas flowing in the front-side gap between the combustion chamber frame  19  and the cylinder  5  is discharged from the opening  30  formed at the front-side lower end of the outer frame  2 . As such, the exhaust gas flow which may advance toward the through-hole  3   a  formed in the partition wall  3  is blocked. Consequently, temperature rise of the nail gun  1  resulting from successive nail driving operations does not allow high temperature gas to flow into space S 2  through the through-hole  3   a . Therefore, the gas cartridge cylinder  22  is not heated up by the high temperature gas so that the temperature of the gas cartridge cylinder  22  is maintained at substantially constant, ignitions to the gaseous mixture can stably achieved, and the required power can constantly be output by the combustion of the gaseous mixture containing a predetermined density of flammable gas so as to enable stable nail driving operations. 
   When the push lever  21  is separated from the workpiece W and a predetermined period of time has been expired after the turn-off operation of the push switch  32 , the motor  13  is deenergized to stop rotating the fan  15 . Then, the nail gun  1  returns to the initial state and is placed to a condition for the subsequent nail driving operation. 
   While the invention has been described in detail with reference to a specific embodiment thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein. 
   For example, while the embodiment describes the nail gun in which the trigger switch  24  is turned on and off each time the nail driving operation is performed, the present invention is applicable to a nail gun of a continuous type in which the nails are driven continuously by holding the trigger switch  24  in on-state and repeatedly carrying out the push-and-release operations with respect to the workpiece