TECHNICAL FIELD The invention concerns improvements for a gas-powered fixing tool and a gas-powered fixing tool including at least one of those improvements.

PRIOR ART

The prior art includes in particular the documents EP- Bl-123 717, EP-Bl-1 243 383 and EP-B1-2 087 220.

So-called gas-powered fastening or fixing tools comprise an internal combustion engine operating by igniting an air-fuel mixture in a combustion chamber, fuel from a fuel cartridge being injected into the chamber by an injection device. Such tools are intended to drive fixing elements into support materials (such as wood, concrete or steel) to fix components thereto. Gas-powered tools are in very widespread use nowadays and make it possible to install fixing elements of staple, nail, spike, pin, etc. type. By way of internal combustion engine fuel there may be cited petrol, alcohol, in liquid and/or gas form, for example.

Such a tool is generally portable and includes a casing in which the internal combustion engine propelling a piston driving a fixing element is mounted. Such a tool may also include a battery for supplying electrical power and a handle for holding, manipulating and firing it on which a trigger for actuating the tool is mounted . The present invention aims to improve this technology. SUMMARY OF THE INVENTION

In accordance with a first aspect, the invention concerns a combustion or precombustion chamber for a gas-powered fixing tool, comprising a casing defining a combustion cavity having a generally elongate form of longitudinal axis X, characterized in that said cavity has a variable cross section along said axis X. The invention can therefore make it possible to reduce the overall size of the chamber, for example to reduce its length. This length reduction can reduce the travel time necessary for the flame to cross the chamber longitudinally, which commensurately reduces the duration of a firing cycle by the tool. The invention can further make it possible to optimize the spatial distribution of the mass of the chamber within the tool, for example in order to shift the center of gravity of the tool into a predetermined area.

The chamber in accordance with the invention may have one or more of the following features, considered separately from one another or in combination with one another :

- said cavity has a generally staged form and comprises at least one first portion of cross section SI and one second portion of cross section S2, with SI different from S2,

the ratio S2/S1 is between 1.1. and 3.0 inclusive, for example, or even greater; in one particular case, it may be between 1.1 and 1.5 inclusive, and preferably between 1.2 and 1.5 inclusive,

ignition means, such as a spark plug, are situated at a longitudinal end of said cavity,

- said ignition means are situated in a portion of smaller cross section of said cavity,

said cavity comprises a longitudinal end opposite said ignition means, which is fluidically connected with a second combustion cavity, said cavity has, in longitudinal section, a generally L or T-shaped form.

In accordance with a second aspect, the invention concerns a combustion or precombustion chamber for a gas-powered fixing tool, comprising a casing defining a combustion cavity, characterized in that said cavity has at least partly a spherical or ovoid form. The invention is advantageous because it makes it possible to reduce sharp edges and intersections inside the cavity, the inventors having realized that these elements create combustion and flow dead areas that reduce the efficiency of combustion and filling (and purging) and therefore the performance of the tool.

The chamber in accordance with the invention may have one or more of the following features, considered separately from one another or in combination with one another:

said casing defines three openings, two of which are aligned on a same axis U and a third of which is aligned on an axis Y substantially at right angles to the axis U,

- said casing comprises a first half-shell comprising a first wall in the form of a portion of sphere,

said first wall is a median wall which is situated between two end walls each in the form of a portion of cylinder,

said end walls partly define said openings of axis

U,

said casing comprises a second half-shell comprising two end walls each in the form of a portion of cylinder and partly defining said openings of axis U, and a cylindrical wall defining said opening of axis Y. In accordance with a third aspect, the invention concerns a working chamber for a gas-powered fixing tool, comprising a casing defining a housing in which a piston is mounted and can slide to drive a fixing element, said piston being configured to be translationally displaced in said housing from a rest position to a working position, the chamber further comprising dynamic sealing means between said piston and said casing to ensure a seal during said displacement, characterized in that it further comprises static sealing means between said piston and said casing to ensure a seal when said piston is in its rest position, said static sealing means being independent of said dynamic sealing means.

The sealing means therefore have distinct functions. In addition to the known dynamic sealing means, the chamber is equipped with static sealing means, that is to say means designed to establish a seal between the piston and the casing of the chamber outside of any relative movement between them. This seal is established when the piston is in its rest position, which makes it possible to close the combustion chamber in a sealed manner, which chamber communicates with the internal cavity in which the piston moves, and to optimize the combustion of the air-fuel mixture in the combustion chamber.

The chamber in accordance with the invention may have one or more of the following features, considered separately from one another or in combination with one another :

the dynamic sealing means are configured to be operational/ functional (to cooperate with a sealing surface for example) when the piston is in its rest and working positions, and the static sealing means are configured to be operational/ functional when the piston is in its rest position and not to be so when it is in its working position, said static sealing means are borne by said casing,

said static sealing means are borne by said piston,

- said dynamic sealing means are borne by said piston,

said piston comprises a first outer cylindrical surface comprising an annular groove housing a dynamic seal ,

- said piston comprises a second inner or outer cylindrical surface comprising an annular groove housing a static seal,

said piston has an elongate form and comprises a head and a rod that are coaxial, and in which said second surface is situated at a longitudinal end of said head, which is opposite said rod.

In accordance with a fourth aspect, the invention concerns a device for injecting a fuel gas for a gas- powered fixing tool, characterized in that it comprises an evaporator block comprising:

a fuel evaporation cavity,

a fuel evaporation duct outgoing from said cavity, and

a housing, preferably upstream of said cavity, in which is mounted a substantially planar (for example slightly curved) filter configured to retain impurities of said fuel. The complex evaporation means of the prior art are replaced by a plane filter and evaporation spaces, which makes it possible to simplify the evaporator block and to reduce the cost thereof. The device in accordance with the invention may have one or more of the following features, considered separately from one another or in combination with one another : said evaporator block comprises a housing for receiving a member for actuating a fuel cartridge, said member having an elongate form of axis Z and being configured to be translationally displaced along said axis between a rest position and a position for releasing fuel from said cartridge, said member comprising an internal bore for the passage of fuel which comprises a generally L or T-shaped form of which a first axial part emerges at a longitudinal end of said member and of which a second radial part emerges on an outer peripheral surface of said member and is intended to be situated facing said filter at least when said member is in said release position,

said duct has a generally L or S-shaped form, - said duct is formed of a single piece with at least a part of said evaporator block.

The present invention further concerns a gas-powered fixing tool comprising a chamber or a plurality of chambers as described above and/or a device as defined above .

BRIEF DESCRIPTION OF THE FIGURES The invention will be better understood and other details, features and advantages of the present invention will become more clearly apparent on reading the following description given by way of nonlimiting example and with reference to the appended drawings, in which:

figure 1 is a diagrammatic view of a gas-powered fixing tool in accordance with the invention,

figure 2 is a diagrammatic view of a fuel gas injection device in accordance with the invention, - figure 3 is a diagrammatic perspective view of the device from figure 2,

figures 4a and 4b are diagrammatic views corresponding to figure 2 and showing two respective positions of an actuating member of the device, figure 5 is a diagrammatic view in axial section of chambers of a prior art gas-powered fixing tool,

figures 6 to 8 are diagrammatic views in axial section of chambers of a gas-powered fixing tool in accordance with the invention,

figures 9a to 9c are diagrammatic views in perspective and/or in axial section of a combustion chamber in accordance with the invention, and

figures 10a to lOe are diagrammatic views in axial section of a working chamber in accordance with the invention .

DETAILED DESCRIPTION The tool 10 shown in figure 1 includes a casing 12 in which is located an internal combustion engine 14 with a combustion chamber intended to contain a mixture of air and fuel the ignition of which causes the propulsion of a piston provided to drive a fixing element extracted from a feed magazine 16, the fixing element being intended to be anchored into a support material on leaving a spike guide 18 at the front of the casing 12. All these components of gas-powered fixing tools are perfectly familiar to the person skilled in the art and therefore do not all need to be shown in the drawings .

The casing of the tool has an axis 20 along which the drive piston and the fixing elements move, the latter inside the spike guide 18.

The tool includes a handle 22 for holding and manipulating the tool. It extends, from the casing and externally thereof, substantially perpendicularly to the axis 20, being slightly inclined to it depending on the application of the tool and the ergonomics of its use. The handle 22 is also used to fire it by means of an actuating trigger 24 mounted on it in the area 26 in which it is connected to the casing 12. The combustion chamber of the engine 14 is fed with fuel from a fuel gas cartridge 30 via an injection device 28.

The injection device 28 and the cartridge 30 are advantageously housed in an arm 32 connected to the casing 12 that is substantially perpendicular to the axis 20 in front of the handle 22 and in which the magazine 16 is also located.

Another arm 34 substantially parallel to the axis 20 extends between the handle 22 and the arm 32 so as to form a bridge between them on the (lower) side opposite the casing 12.

There will now be described the various aspects of the invention that may be incorporated in the tool 10 from figure 1 independently of one another or in combination with one another.

Injection device

One aspect of the invention illustrated by figure 2 concerns the device 28 for injecting fuel into the engine from a fuel cartridge 30.

The fuel is in the liquid state in the cartridge and must be evaporated, the fuel gas being intended to be mixed with air before being burned in the combustion chamber of the internal combustion engine.

An injection device of a gas-powered fixing tool must therefore make it possible to evaporate the fuel.

The document EP-B1-2 087 220 describes a system for feeding and evaporating liquid fuel to convert a liquid fuel into a gaseous fuel. That system includes an evaporator element associated with a casing that is heated in order to heat the evaporator element. The evaporator element is made from sintered metal and has a conical or frustoconical general shape. This technology is complex and relatively bulky, notably because of the particular shape of the evaporator element. This technology is also relatively costly .

Moreover, this evaporator element is relatively fragile and has a low resistance to the vibrations and shocks generated during the operation of a fixing tool. Additionally, as the fuel used to operate these tools may contain lubricants, additives and even impurities, the evaporator element may become clogged, therefore blocking the passage of the fuel through it. The result of this situation is malfunctioning of the tool, which necessitates demounting and cleaning of the evaporator element and possibly its replacement, because the cleaning operation may damage this element.

The invention is able to solve all the problems mentioned above. As well as attempting to manage the clogging of the evaporator element, the inventors have proposed a filter element notably having the aim of trapping the various materials contained in the fuel leaving the cartridge.

Various filters have been tested. The filters essentially consist of a screen, a mesh, a grid, a fabric, a woven material, a foam or fibers. These filters are made of metal or plastic or from mineral or natural fibers. The aim of these filters is to trap particles contained in the fuel whilst enabling the fuel to pass through the filter.

With the aim of simplifying the prior art injection device, the evaporator element is dispensed with. Surprisingly, the use of a filter disposed in the simplified injection device combined with an evaporation cavity makes possible optimum vaporization of the fuel in order to feed the combustion chamber of the tool.

Figure 2 represents one embodiment of the injection device 28.

A valve 40 intended to measure out a quantity of liquid fuel is disposed between the liquid fuel cartridge 30 and the simplified evaporator block 42. A filter 44 is disposed in a housing or bore 46 in the block 42. A predetermined quantity of liquid fuel is discharged from the cartridge 30 via the valve 40 into the block 42, passing through the filter 44, and arrives in the evaporation cavity 47. The block 42 is made from a thermally conductive material, such as metal. The liquid fuel passing through the filter 44 is at least partly converted into gaseous fuel thanks to the input of heat from the surrounding environment, which transmits thermal energy to the evaporator block 42.

Downstream of the filter 44 and the cavity 47, the at least partially vaporized fuel continues to circulate in the block 42 and absorbs additional heat from the environment. The downstream part of the block 42 includes an evaporation duct 48 acting as a distribution manifold leading to the combustion chamber 50 of the fixing tool.

The dimensional parameters of the device 28, and in particular of the cavity 47 and of the duct 48, such as the width, the diameter, the thickness, etc., are chosen so that the fuel is entirely converted into gas when it exits a downstream discharge port 51 of the duct 48. The block 42 and/or the duct 48 may comprise one or more fins 52 disposed on at least one of their surfaces to assist the transfer of heat from the surrounding environment. On leaving the discharge port 51, the gaseous fuel can be injected directly into the combustion chamber 50. An option is for the gaseous fuel leaving the discharge port 51 to feed one or more fuel outlet nozzles 54 feeding the combustion chamber 50. The fuel gas may alternatively feed a Venturi-type jet pump 56 in which surrounding air is drawn into the jet pump 56 and mixed with the gaseous fuel injected via the nozzle or nozzles 54 so as to form an air-fuel mixture for feeding the combustion chamber 50.

This evaporator block 42 is therefore easier to manufacture at lower cost. The filter is plane and therefore relatively simple. It lies substantially in a plane parallel to the axis Z of the cartridge 30. It has the shape of a pastille, disk or block, for example. It is much simpler and less fragile than the complex parts used in the prior art. Consequently, the simplified evaporator block is also easier to maintain when necessary, although the necessity to maintain such a block is also significantly reduced.

Figure 3 is a diagrammatic perspective view of the device 28 from figure 2 and notably shows that the duct 48 is formed in one piece with a portion of the evaporator block 42.

As seen in figure 2, the duct 48 has the general shape of an S or an L. The cavity 47 has a section in the shape of a T of which the upstream portion with the greatest transverse dimension forms the housing 46 receiving the filter. The cavity 47 communicates with a rectilinear end portion of the duct 48. The duct includes another rectilinear end portion that defines the discharge port 51. These two portions are parallel and connected to each other by a rectilinear median portion of the duct substantially parallel to the longitudinal axis Z of the cartridge 30. This rectilinear portion may be shut off in a sealed manner by a screw at the level of its connection to the rectilinear end portion that defines the discharge port 51.

The evaporator block 42 includes a bore in which an actuator member 58 is mounted and able to slide along the longitudinal axis Z of the cartridge 30. This actuator member has an elongate rectilinear shape and includes an internal bore 60 in the shape of a T or an L. This bore includes a first axial section that extends along the member 58 and discharges at the lower end thereof and a radial portion that extends between the upper end of the axial portion and the periphery of the member. The outlet of this radial portion is situated facing the filter 44.

The member 58 is mobile between two positions: a high or rest position shown in figure 4a and a low or working position shown in figure 4b. In both cases, the aforementioned radial outlet of the bore is situated facing the filter 44. Seals are provided between the member 58 and the bore in which it is mounted. The lower end of the member 58 is configured to cooperate through mutual nesting with a connection end- piece of the cartridge 30.

The movement of the member 58 from its rest position to its working position causes the release of a calibrated quantity of fuel from the cartridge 30. This fuel, in liquid form, flows in the bore 60 of the member 58 and passes through the filter 44, which retains any impurities, before penetrating into the cavity 47 in which the transformation of the liquid fuel into gaseous fuel is initiated. The fuel flows in the duct 48 to complete its evaporation and reaches the gaseous state at the level of the nozzle 54. It is then atomized in the jet pump 56 and mixed with air that penetrates into the pump by virtue of the Venturi effect, the air-fuel mixture then being injected into the chamber 50 of the internal combustion engine. As shown in figure 2, the block 42 is advantageously situated above the cartridge 30, the duct 48 advantageously extends in part on one side of the cartridge, and the jet pump 56 advantageously has a substantially perpendicular orientation relative to the longitudinal axis Z of the cartridge or to the duct 48. The cartridge 30, the block 42 and the duct 48 are advantageously housed in the arm 32 and the jet pump ideally lies in the arm 34, the combustion chamber 50 then being housed in the handle 22 of the tool from figure 1.

The filter 44 has a permeability less than 50 darcy and preferably between 10 and 33 darcy inclusive, for example, which makes it possible to filter particles with a diameter between approximately 7 μπι and 14 μπι inclusive with an efficiency of 98 to 99.9%.

Precombustion chamber An internal combustion engine of a gas-fired fixing tool includes a combustion chamber and a working chamber in which a piston driving a fixing element is able to move because of the effect of the explosion of the air-fuel mixture in the combustion chamber.

As shown in figure 5, which represents the prior art described in the document EP-Bl-1 243 383, the engine advantageously includes a precombustion chamber 60 and a combustion chamber 50. The first combustion chamber or precombustion chamber 60 makes it possible to initiate the combustion of the air-fuel mixture. This chamber 60 includes a casing 62 that defines a combustion cavity 64 in which are mounted ignition means such as a sparkplug 65. The chambers 60, 50 are separated from each other by a valve 66. The precombustion of the mixture in the chamber 60 causes an increase in pressure in the cavity 64. When this pressure exceeds a certain threshold, the valve opens and allows the combustible mixture to pass into the chamber 50.

The chamber 50 includes a casing 68 defining a combustion cavity 70. The mixture arrives in the chamber 50 at a relatively high pressure. The flame coming from the chamber 60 reaches the chamber 50, the high-pressure combustion in the chamber 50 making it possible to improve the performance of the tool. The combustion in the chamber 50 causes an increase of pressure in the cavity 70 that forces the piston 78 to move in the working chamber 80.

As can be seen in figure 5, it is known to provide a precombustion chamber 60 of elongate shape, one longitudinal end of which is connected to the combustion chamber 50 and the opposite longitudinal end of which includes the sparkplug 64. The output power of the combustion chamber 50 can be increased by up to fifty per cent (50%) merely by lengthening the precombustion chamber 60.

In the document EP-Bl-1 243 383, the precombustion chamber 60 has a predetermined length B and a predetermined width A, where the length B is significantly greater than the width A. To be more specific, the ratio of the length B to the width A, known as the aspect ratio of the precombustion chamber 60, is at least 2:1, and can be much higher with an optimum around 10:1 according to the same document.

It is also indicated in the document EP-Bl-1 243 383 that discontinuities or irregularities present in or on the internal surfaces of the precombustion chamber must be avoided because such structures tend to reduce the power of the engine. Moreover, a precombustion chamber can have a round, oval, rectangular or other shape in 3 cross section provided that its length is greater than its width.

Thus the prior art precombustion chamber 60 has a length B that is detrimental to the overall size of the ) tool.

Another disadvantage of this precombustion chamber 60 is that the longer the precombustion chamber, the greater the delay between igniting the spark and 3 igniting the combustion chamber 50. This can increase the duration of the firing cycle of the tool, which is a problem in some fixing applications.

Finally, the design of the precombustion chamber 60 is ) not the optimum in terms of ergonomics.

The following improvements make it possible to optimize the overall size of the tool, to optimize its operation and/or to shorten the duration of a firing cycle and in 3 particular the duration between ignition in the precombustion chamber 60 and combustion in the chamber 50, at the same time as maintaining good combustion chamber performance.

) To be in a position to compare the effect of the new precombustion chamber design against the prior art, the inventors have maintained the total volume of the chambers 50, 60 constant. The total quantities of air- fuel mixture are therefore comparable and consequently

3 the same total quantities of raw energy are available.

VI denotes the volume of the precombustion chamber 60 and V2 denotes the principal volume of the combustion chamber 50. VI + V2 is constant for all the tests. Moreover, as the object of the invention is to improve the performance of the precombustion chamber 60, the inventors have kept VI the same for all the embodiments .

The inventors have noted that, by keeping VI constant, a beneficial effect is achieved by changing the configuration of the precombustion chamber 60 from an elongate shape of constant cross section to an elongate shape in which the cross section varies along the longitudinal axis of the chamber. It can have a cross section that is staggered or has a frustoconical shape.

This means that the precombustion chamber preferably has, starting from the sparkplug 65, in the direction of the combustion chamber 50, an increasing section. The precombustion chamber 60 preferably includes two portions, the first portion including the sparkplug 65 and having a maximum first inside diameter that is smaller than the minimum inside diameter of the second portion .

At least one diameter, and preferably both diameters of the first and second portions, is or are preferably constant. For example, as shown in figure 6, the elongate chamber of constant cross section is replaced by two portions of which an upper one has a cross section S2 greater than that SI of the other, lower portion. The chamber 60 therefore has in longitudinal section the general shape of a T. Consequently, whilst maintaining the volume VI constant, this embodiment has a length less than the prior art length B. Consequently, the overall size of the tool may be reduced .

The reduction of the length of the precombustion chamber 60 makes it possible to reduce the distance between the sparkplug 65 and the combustion chamber 50, which has the advantage of reducing the time to ignite the chamber 50 and the overall duration of a firing cycle .

The invention therefore provides an efficient precombustion chamber for a tool that is less bulky and can operate faster than those of the prior art.

Figure 7 shows a variant embodiment of the precombustion chamber 60. This figure shows a precombustion chamber 60 that includes a portion having a component of forward horizontal extension such that the shortest fluid flow line between the spark plug 65 and the connection to the combustion chamber 50 has (at least in part) , from the sparkplug, a horizontal component inclined toward the rear of the tool.

This design leads to improved ergonomics because it is more beneficial in terms of the balance of the tool. With this design, the precombustion chamber is no longer situated entirely on one side of the tool and so the combustion chamber and the working chamber 80 do not necessarily form a conventional L-shaped architecture, i.e. a tool resembling a "pistol". This new design is more practical in terms of ergonomics given that the masses of the working chamber and of the magazine containing the fixing elements are no longer all situated on the same side of the tool and on the same side of the handle of the tool.

The precombustion chamber 60 preferably includes at least two portions; the first of these portions is that connected to the combustion chamber 50 and the second portion is that farthest from the combustion chamber 50. The lateral wall 82 of the precombustion chamber 60 in the first portion is nearer the rear end of the tool than the lateral wall of the precombustion chamber in the second portion. The second portion preferably includes the sparkplug 65. The tool is configured so that the tool fits closely around the precombustion chamber .

At least one diameter, and preferably both diameters of the first and the second portion, is or are preferably constant. For example, as shown in figure 7, the elongate chamber of constant cross section is replaced by two portions of which an upper one has a cross section S2 larger than that SI of the other, lower one. The chamber 60 therefore has in longitudinal section the general shape of an L. Consequently, whilst maintaining the volume VI constant, this embodiment has a length less than the length B of the prior art. Consequently, the overall size of the tool can be reduced.

As seen in figure 7, in one embodiment of the invention the precombustion chamber 60 is no longer rectilinear, but comprises a curvature in order to shift the handle of the tool (which contains the precombustion chamber) closer to the center of gravity of the tool. In the example shown, a horizontal portion is present. The (left-hand) lateral wall 83 of the precombustion chamber in the portion with the sparkplug is positioned nearer the (right-hand) lateral wall 84 of the portion connected to the combustion chamber.

Whilst maintaining VI constant relative to the prior art, the invention makes it possible to maintain a comparable or even identical level of performance, in terms of production of energy, in a tool that is much better balanced.

Combustion chamber

As shown in figure 5, the combustion chamber 50 of a tool is generally adjacent the working chamber 80 in which the piston 78 is moved by the effect of the combustion of the air-fuel mixture. Consequently, as the casing of the working chamber 80 still has a cylindrical shape and the piston 78 also has a cylindrical shape, the combustion chamber 50 has a cylindrical general shape at the end adjoining the working chamber 80.

As seen in figure 5, this combustion chamber 50 has the shape of a flat cylinder having a diameter D and a height H and its cavity 70 has a volume V2.

The inventors have found that this chamber 50 does not yield an optimum output of energy. They have found an improved shape for the combustion chamber that makes it possible to improve the production of energy.

A preferred embodiment is shown in figure 8 in which the combustion chamber defines a spherical or oval combustion cavity.

This spherical/oval shape leads to improved mixing and to correct distribution of fuel and purging of the combustion gases. In actual fact, the inventors have discovered that this shape features no dead areas caused by the presence of edges in the cavity. These edges affect both the flow and the combustion flame. The flow tends to stop on approaching the edges, resulting in dead areas. The flame is also affected by these edges because it tends to be extinguished on approaching the edges. The new shape eliminates most if not all of the harmful dead points that exist in the prior art. Even if the combustion volume is not a perfect sphere, any edge that can be removed from the volume of the combustion chamber makes it possible to optimize the entry and exit flows into and out of the chamber for optimum feeding with the air-fuel mixture and optimum scavenging of the combustion gases. Moreover, the mixture can burn much more efficiently in any area of the combustion chamber, minimizing the dead areas. As the main reason for this improvement is the elimination of edges and dead corners, a partially spherical shape may also be replaced by a partially oval shape or any other shape that has no or a minimum number of edges, for example a shape in which the radius of curvature of the upper portion of the bottom wall (here on the left) of the combustion chamber 50 is 25%, preferably 50%, greater than the smallest diameter of the prior art combustion chamber (for example, H) .

Figures 9a to 9c show a more concrete embodiment of this aspect of the invention.

The combustion chamber 50 includes a casing 68 defining three openings, of which two openings 50a, 50b are aligned on the same axis U, which corresponds to the longitudinal axis of the precombustion chamber or a portion thereof, and a third opening 50c is aligned on an axis Y substantially perpendicular to the axis U.

The casing 68 includes a first half-shell 68a including a part-spherical first wall 68aa. This first wall 68aa is a median wall that is situated between two end walls 68ab each of which is a part-cylinder. The end walls 68ab define in part the openings 50a, 50b with axis U. The casing 68 includes a second half-shell 68b including two end walls 68bb each of which is a part- cylinder and defining the rest of the openings with axis U and a cylindrical wall 68ba defining the opening on the axis Y.

The opening 50a provides fluid communication with the cavity of the precombustion chamber. The opening 50c provides fluid communication with the internal cavity of the working chamber, and the opening 50b provides fluid communication with the atmosphere. The opening 50a can be shut off by the aforementioned valve 66 and the opening 50b can be shut off by a valve 84 the mobile body of which is carried by a rod also carrying the valve 66. Working chamber

The performance of a combustion-actuated fixing tool is notably based on the capacity of the piston to convert efficiently the pressure energy generated by the combustion of the explosive mixture into kinetic energy transferred to the fixing element. This efficient conversion is affected by the leaks that occur between the piston and the casing of the working chamber. These pistons and the casings are very well known because these are used in all the tools. The design of the combustion chamber and the combustion technology may vary from one tool to another, but the piston reciprocating in the casing remains essentially the same for the various fixing tools.

This is well known to the person skilled in the art, as explained in the document EP-B1-123 717. Combustion occurs and the pressure generated moves the piston to drive the fixing element into a support material. Slightly before the piston reaches the bottom or the end of its driving travel, where it comes to abut against an elastic shock absorber, the piston passes ports in the wall of the casing that serve to evacuate the combustion gases. These ports make it possible to facilitate the elimination of the combustion gases to facilitate the establishing of a partial vacuum so that air at atmospheric pressure can penetrate under the piston and facilitate the return of the latter into its rest or upper position.

The piston used in such a tool conventionally includes dynamic sealing means, that is to say means used to provide a seal between the piston and the casing of the working chamber during the movement of the piston over its travel. This travel results from a pressure difference between the two sides of the piston (combustion for driving and vacuum for return) . The seals in accordance with the prior art are configured to provide a dynamic seal.

The presence of a precombustion chamber makes it possible to increase the efficiency of combustion and to increase the pressure inside the tool.

In its initial retracted position, the piston must be sealed firstly to contain the pressure generated by the combustion of the air-fuel mixture. As mentioned above, the seal must be maintained and the combustion chamber must not leak each time that the mixture is boosted or in the presence of the pre-pressure generated by the precombustion chamber before ignition in the combustion chamber when the combustion technology employs a precombustion chamber. During this preliminary phase, the piston must therefore be sealed as perfectly as possible. Ideally, the piston must also remain stable to maintain the small volume of the combustion chamber in order to maximize the pressure until combustion is almost complete. Ideally, in this preliminary phase, the piston must also be retained until a pressure peak occurs and combustion finishes. This requirement to retain the piston during a preliminary phase has been addressed in the prior art by employing magnets or mechanisms, notably balls, springs and/or cams. All these piston retaining mechanisms are generally bulky, complex and costly.

Consequently, in this preliminary phase, the requirement is to provide a maximum seal between the piston and the casing of the working chamber and therefore to have a maximum static seal when the piston is in the rest position. Ideally, the piston must be retained in this position, in a sealed manner, until the pressure peak is reached, in order to maximize the transformation of energy in the form of combustion pressure to kinetic energy driving the piston.

Releasing the piston is the second stage of the operation, in which the piston accelerates along its travel until it reaches its opposite working position and drives the fixing element into the support material. During this second stage, the requirement for a seal between the piston and the casing is less problematic. The dynamic sealing means are severely stressed by the acceleration of the piston and rubbing against the casing but provide a satisfactory response to this requirement.

There is therefore a compromise in respect of the sealing means between the first phase demanding static sealing performance and the second phase demanding dynamic sealing performance.

The person skilled in the art generally considers that static seals are generally flexible seals (O-rings, etc.) made of flexible materials such as rubber, silicone, etc. These are effective if there is no relative movement between the parts or if the movements are limited and slow. The same person skilled in the art knows that dynamic seals are more capable of providing a seal between two parts in motion, even if the seal as such is not as good with a static seal.

For internal combustion engines, the dynamic seals for pistons may be piston rings made of metals such as steel, which function efficiently at high speed and at high temperature. Other dynamic seals also exist, such as lip seals or composite seals, for example, although they are not generally as effective as steel seals because of the high temperatures encountered in internal combustion engines.

This confirms the compromise mentioned above between the static seal required in the first phase of operation of the tool and the dynamic seal required in the second phase. This compromise is further justified by the particular structure of the fixing tools, which have one or more exhaust ports situated inside the casing of the working chamber, between the two extreme positions of the travel of the piston. These exhaust ports are responsible for evacuating the burned gases. Unfortunately, when the piston passes these exhaust ports, the dynamic sealing means are strongly compressed and tend to expand into the open exhaust port. This situation is relatively well tolerated by steel seals but not by flexible seals. Flexible seals therefore tend to wear rapidly if they are exposed to repeated passages at the level of the exhaust ports because they tend to be extruded into the exhaust ports .

The inventors have sought to provide an improved seal between the piston and its casing when the piston is in its rest position, this seal not being degraded by the passage of the piston at the level of the exhaust ports. Ideally, these improved sealing means should retain the piston in its rest position until the pressure of the combustion gases in the chamber reaches a certain threshold.

In accordance with the invention, the working chamber includes a casing, for example a cylindrical casing, a piston and a first seal to seal the piston in the retracted or rest position of the piston (static seal) and a second seal - that is different from the first seal - to seal the piston during its movement (dynamic seal ) . Using two different seals, each seal may be optimally adapted to the necessary sealing function and no compromise has to be found between a dynamic seal and a static seal.

The second seal is preferably fixed to the piston (for example housed in a groove in the piston) . The first seal and the second seal are preferably both fixed to the piston and the casing preferably has a sealing surface for the first seal that is radially inside the sealing surface for the second seal. For example, the casing therefore includes a radial projection towards the interior of the interior cylindrical surface opposite the first seal before/in the rest position. More preferably, the first seal is fixed to the casing (for example housed in a groove in the casing) . In this case, there is preferably no radially inward projection present that holds the seal or serves as a radial sealing surface (for example in the form of a cylindrical lateral surface) .

In attempting to solve the problems and address the compromises listed above, the inventors have produced a number of embodiments that are shown in figures 10a to lOe.

All the embodiments show a working chamber 80 including a casing 90 in which a piston 78 is slidably mounted, the internal cavity 92 of the working chamber communicating with the internal cavity of a combustion chamber as described above.

The piston 78 is represented in its retracted or rest position, as known in the prior art and already explained above, and moves (downward in the orientation of the figures) in the casing 90 to drive in a fixing element. During its travel, the piston may eventually pass an exhaust port 94. Figure 10a refers to the first embodiment of the invention. The piston 78 includes a static seal 96 used to seal the piston in the preliminary phase of actuation of the tool. In this embodiment, the static seal 96 is carried by the piston and housed in a groove in the piston. The piston also includes a dynamic seal 98 housed in a groove in the piston.

Each seal provides the performance described above. The piston is designed so that the sealing surfaces for the seals are different. In this example, the diameter of the sealing surface for the static seal 96 is smaller than the diameter of the sealing surface for the dynamic seal 98. When the piston moves downwards, the dynamic seal remains in contact with its sealing surface throughout its travel. As the dynamic seal is able to resist repeated passages at the level of the exhaust port 94, there is no problem in respect of the durability of this seal. At the same time, while the piston is moving (downwards) along its travel, the static seal 96 provides the seal at the start of its travel, until it disengages from its smaller diameter sealing surface in the casing 90. Consequently, when the piston continues its travel, the static seal is no longer in contact with its surface or with any other surface of the casing.

In particular, thanks to this design, the static seal 96 is never in contact with the exhaust port 94 and therefore little loaded by friction. This static seal consequently provides a seal only during the first phase of the operation. This situation makes it possible to use the static seal as efficiently as possible without requiring any compromise because it is no longer exposed to dynamic loads.

The static seal may be made of flexible material, such as rubber, because it will never be in contact with the exhaust port 94 and will therefore never be damaged by friction. Moreover, the static seal may be a tight fit so that the seal is optimized. The other advantage of this tight fit is that the static seal participates in retaining the piston in its rest position. The static seal therefore acts also as a mechanism retaining the piston in accordance with the optimum combustion performance requirements.

Referring now to figure 10b, the general advantages described above remain applicable except that the groove for retaining the static seal 96 is situated on the surface of the casing that must be sealed. Figures 10a and 10b represent two solutions to achieve the same effects of sealing and retaining the piston.

Figure 10c is another embodiment of the invention. It represents a simplification of the structure. The static seal 96 is held in place in a groove in the casing of the tool and not in the piston. There is no necessity for the sealing surfaces of the seals to be different. As the static seal does not follow the piston along its travel, there is no risk of the static seal encountering the exhaust port, even if the surfaces of the seals are the same. In other words, the diameter of the surface of the static and dynamic seals may be identical and the piston 78 may be designed with only one diameter. Consequently, this simplified embodiment also procures all the advantages of the invention in respect of the static seal, the dynamic seal and the retention of the piston in its rest position .

Figures lOd and lOe are other embodiments of the invention. They are in fact another design of the embodiments from figures 10a and 10b. The piston utilizes two different sealing surfaces for the static seal and the dynamic seal. The difference being that in figures 10a and 10b the piston is the male part of the sealing surface of the static seal whereas in figures lOd and lOe the piston is the female part of the sealing surface of the static seal. Once again, the advantages of the invention are the static seal, the dynamic seal and the retention of the piston in its rest position.

In the various embodiments, the piston 78 has an elongate shape and comprises a head and a rod that are coaxial. The static seal 96 is situated in an area of the piston head near a longitudinal end thereof that is opposite the rod.