BACKGROUND OF THE INVENTION 
     Patent application No. 97/11 294 of Sep. 11, 1997 described an internal combustion engine comprising at least one exhaust manifold branch connecting at least two cylinders, this exhaust manifold branch comprising a controlled valve and being connected to the exhaust gas outlet tract via a rotary spool. 
     This arrangement can work with any kind of combustion engine whether it be a two-stroke or a four-stroke engine. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a simpler and therefore less expensive device intended for a two-stroke engine of the uniflow type. 
     It relates to a two-stroke engine of the type comprising a cylinder, a piston, exhaust valves, inlet ports and a blower for sending a stream of scavenging air into the cylinder, characterized in that it comprises an additional volume communicating with the cylinder via closure and opening means, these means being controlled so that they are synchronous with or can be phase-shifted with respect to the movements of the piston in the cylinder so that, during the combustion/expansion phase, burnt gases compress the air located in said additional volume by at least partially entering it; that this air/burnt gases mixture is trapped under pressure therein; then that this mixture is let into the cylinder during the inlet phase; and that finally, fresh air from the blower is introduced into said additional volume. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     By way of non-limiting examples, and to make the invention easier to understand, the appended drawings depict: 
     FIG. 1 ( 1   a - 1   g ) : a diagrammatic view of a first embodiment of the invention, the parts being depicted in seven successive positions; 
     FIG. 2 ( 2   a - 2   h ): a diagrammatic view of a second embodiment of the invention, the parts being depicted in eight successive positions; 
     FIG. 3 ( 3   a - 3   h ) : a diagrammatic view of a third embodiment of the invention, the parts being depicted in eight successive positions; 
     FIG. 4 ( 4   a - 4   h ) : a diagrammatic view of a fourth embodiment of the invention, the parts being depicted in eight successive positions; 
     FIG.  5 : a perspective view on a larger scale of one embodiment of the closure means of the embodiment of FIG. 3; 
     FIGS. 6 to  9 : four diagrams illustrating the working of the engine in FIGS. 1 to  4 . 
    
    
     In all these figures, the same elements bear the same references. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The internal combustion engine depicted is a two-stroke engine which has a cylinder  1  in which there moves a piston  2  coupled by a connecting rod  3  to the wrist pin  4  of a crankshaft  5 . The cylinder  1  is equipped at its upper part with one or more exhaust valves  6 . Inlet is via a number of ports  7  made in the base of the cylinder  1 , supplied by a ring  8  itself connected to a blower  9 . 
     An additional volume  10 , which may have any appropriate shape, communicates with the top and bottom of the cylinder  1 , via two rotary shutters  11 ,  12  and/or  21 . 
     The rotary shutter  11  is, for example, a two-way rotary spool connected to the pipes  13 ,  14 ,  15  and  16 . The pipe  13  comes from the cylinder  1  downstream of the exhaust valve(s)  6 ; the pipe  14  leads to the additional volume  10 ; the pipes  15  and  16  lead to the exhaust manifold, not depicted. 
     The rotary shutter  12  is, for example, a multiway rotary spool connected to the pipes  17 ,  18  and  19 . The pipe  17  leads to the volume  10 ; the pipe  18  leads to the inlet ring  8 ; the pipe  19  leads into the cylinder  1  near its bottom. 
     The rotary shutter  21  is depicted in detail in FIG.  5 : it consists of a hollow spool  22  with four ports  23 ,  24 ,  26  and  27 , two of these openings  23  and  24  being connected to one another by a duct  25  arranged inside the spool  22  so that this shutter  21  allows the communication either of the openings  23  and  24  via the duct  25  or the openings  26  and  27  via the spool  22 . When a rotary shutter  21  such as this is used (FIGS.  3  and  4 ), the volume of the spool  22  adds to the volume of the pipe  19  to define the amount of fresh air stored. 
     The rotary movements of the rotary shutters  11 ,  12  and  21  are linked in any appropriate way, known to those skilled in the art and therefore not described, to the rotary movement of the crankshaft  5 , in a 1:1 ratio or some ratio other than 1:1 which may be in phase with or phase-shifted with respect to the movement of the crankshaft  5 . 
     The way in which the device works is described hereinafter in relation to FIGS. 1 a  to  1   g.    
     In FIG. 1 a , the cylinder  1  is at its top dead center (TDC) considered as being the starting position at 0°; in FIG. 1 b , the cylinder  1  has begun its downstroke corresponding to the expansion phase, the wrist pin  4  having described an angle of 94° for example; in FIG. 1 c , the cylinder continues its downward movement, the wrist pin  4  having described an angle of 127°; in FIG. 1 d , the cylinder  1  is at its bottom dead center (BDC), at the end of the inlet phase, the ports  7  having been uncovered and the cylinder  1  filled with air by the blower  9 , the wrist pin  4  having described an angle of 180°; in FIG. 1 e , the cylinder  1  has begun its compression stroke, the ports  7  being blocked off, the wrist pin  4  having described an angle of 233°; in FIG. 1 f , the cylinder  1  continues its upward compression stroke, the wrist pin  4  having described an angle of 266°; in FIG. 1 g , the cylinder  1  approaches its TDC, the wrist pin  4  having described an angle of 338°. 
     In FIG. 1 a , the valve or valves  6  are closed as are the shutters  11  and  12 . 
     In FIG. 1 b , the valve or valves  6  open and the shutter  11 , which has pivoted, for example in the same direction as the crankshaft  5 , causes the pipes  13  and  14  to communicate. The shutter  12  has also rotated by the same amount and in the same direction but this had not led to any communication between the pipes: the pipe  17  leading from the volume  10  is closed. What this means is that a blast of burnt gases under pressure is discharged into said pipes  13  and  14 , which compresses the air in the volume  10  while at the same time introducing a portion of burnt gases thereinto, this portion corresponding to the angular transfer period. 
     In FIG. 1 c , the valve or valves  6  are still open; the shutter  11  having rotated has placed the pipes  13  and  15  in communication while closing the pipe  14 ; the shutter  12  has also rotated but without causing any communication. What this means is that the air/burnt gases mixture previously introduced under pressure (about 3.5 bar at full load) into the volume  10  is trapped therein and that the burnt gases escape via the pipe  15  to the exhaust manifold. 
     In FIG. 1 d , the exhaust valve or valves  6  are still open; the shutter  11 , although it has continued to rotate, maintains the communication between the pipes  13  and  15 ; the shutter  12  has also rotated but without causing any communication; the ports  7  are uncovered. What this means is that air from the blower  9  performs scavenging which discharges the burnt gases through the pipe  15 , the cylinder  1  fills with air at the fairly low pressure of the blower (about 1.2 bar) and the air/burnt gases mixture is still trapped under pressure (about 3.5 bar) in the volume  10 . 
     In FIG. 1 e , the valve or valves  6  are closed; the shutter  11  has continued to rotate, the pipes  13  and  15  are still in communication but this has no effect because the valve is closed; the cylinder  1  has covered over the openings  7 ; but the shutter  12  has placed the pipes  17  and  19  in communication. What this means is that the air and/or burnt gases mixture, which was trapped under pressure in the volume  10 , escapes and, under pressure, fills the cylinder  1 . This achieves both the supercharging of the cylinder and/or partial recirculation of burnt gases, an operation known by the name of EGR (for Exhaust Gas Recirculation), and which has the effect of reducing nitrous oxides emissions at low speed. 
     In FIG. 1 f , the valve or valves  6  are closed as are the shutters  11  and  12  which means that the compression phase continues. 
     In FIG. 1 g , the valve or valves  6  are closed; the shutter  11  has placed the pipes  14  and  16  in communication; the shutter  12  has placed the pipes  17  and  18  in communication. What this means is that the fresh air, driven by the blower  9  into the inlet ring  8 , takes the pipes  18  then  17  to enter the volume  10  and that the residual air and/or burnt gases mixture in said volume  10  is discharged through the pipes  14  then  16 . 
     When the piston reaches TDC, the cycle is ready to begin again. 
     FIGS. 2 to  4  depict three alternative forms of embodiment in which elements the same as those of FIG. 1 carry the same references. 
     The only difference between the devices of FIGS. 2 and 3 stems from the fact that the rotary shutters  11 ,  12  and  21  produce different communications which means that the circulations are not the same. 
     What changes is that in the case of FIGS. 2 and 3, the volume  10  is filled and emptied via its bottom through the shutter  12  or  21  whereas in the previous example it was filled from the top, through the shutter  11  and emptied via its bottom through the shutter  12 . 
     In FIG. 2 a , with the piston  2  at its TDC, the rotary shutter  12  causes the pipes  17  and  18  to communicate, while the rotary shutter  11  causes the pipes  14  and  16  to communicate. What this means is that the volume  10  is scavenged with fresh air from the blower  9  which passes through the ring  8 , the pipe  18 , then the pipe  17 , any residual air and/or burnt gases mixture in the volume  10  being discharged through the pipes  14  and  16 . 
     It can be seen in FIG. 2 b , where the wrist pin  4  has described an angle of 94° (which corresponds to FIG. 1 b ), that the valve or valves  6  are not yet open but that the piston  2  is uncovering the opening of the pipe  19 . The combusting gases under pressure in the cylinder  1  therefore escape down the pipe  19 , through the shutter  12  to the pipe  17  as far as the volume  10 . As the pipe  14  is closed, the burnt gases compress the air in the volume  10  and partially enter it. 
     In FIG. 2 c , in which the wrist pin  4  has described an angle of 109°, the shutter  12  has rotated so that it closes the pipe  17  (and the pipe  19 ). The pressurized air and/or burnt gases mixture in the volume  10  is thus trapped therein. The valve or valves  6  can then open to allow exhaust, the shutter  11  having placed the pipes  13  and  15  in communication. 
     In FIG. 2 d , in which the wrist pin  4  has described an angle of 127° (which corresponds to FIG. 1 c ), the shutter  11  is still providing communication between the pipes  13  and  15 , while the shutter  12  establishes communication between the pipes  18  and  19 . What this means is that fresh air, driven by the blower  9 , passes through the pipe  18 , passes through the shutter  12 , arrives in the cylinder  1  via the pipe  19  and begins to scavenge said cylinder, the burnt gases being driven through the valve  6 . 
     In FIG. 2 e , which corresponds to FIG. 1 d , the wrist pin  4  has described an angle of 180° and the cylinder  1  is at its BDC. Scavenging continues and the cylinder is filled with fresh air at the low pressure delivered by the blower (about 1.2 bar). 
     In FIG. 2 f , which corresponds to FIG. 1 c , the wrist pin  4  has described an angle of 233°, and the compression phase begins; however, the air and/or burnt gases mixture trapped in the volume  10  has not yet been introduced into the cylinder, as it had been previously. 
     In FIG. 2 g , the wrist pin  4  has described an angle of 251°. In this position, the shutter  12  establishes the communication between the pipes  17  and  19 . What this means is that the air and/or burnt gases mixture trapped in the volume  10  can escape down the pipe  17 , pass through the shutter  12 , and enter the cylinder  1  via the pipe  19 . This simultaneously produces supercharging in the cylinder  1  and partial recirculation of the burnt gases, which operation was performed in FIG. 1 e  in the previous example. 
     An operation similar to the operation described in relation to FIG. 1 is thus obtained. 
     In FIGS. 1 and 2, the spool  12  is equipped only with ports. It is found that, in the case of FIG. 2, with such a spool, it is not possible to exceed about 15° crank angle to use the exhaust blast, then to use the same time to recover the air stored in the duct or volume  10 : this time may not be long enough for the system to be effective. 
     It is therefore preferable to use a shutter like the one described in FIG. 5, that is to say a shutter  21  equipped with a hollow rotary spool  22 . 
     This arrangement is described in relation to FIG.  3 . In that figure, the rotary shutter  11  is a two-way shutter, whereas the ported shutter  12  of FIG. 2 is replaced by a rotary shutter  21 , which is the one depicted in FIG.  5 . 
     Like FIGS. 1 and 2, FIG. 3 is broken down into eight FIGS. 3 a  to  3   h  illustrating eight positions occupied by the moving parts in the course of one cycle. 
     Position  3   a  corresponds to the combustion phase, all the openings of the cylinder  1  being closed and the piston  2  being at its top dead center. In this position, the upper rotary shutter  11  causes the pipes  14  and  16  to communicate while the lower rotary shutter  21  causes the pipes  18  and  17  to communicate via the hollow spool  22  of said shutter  21 . What this means is that air from the blower  9  enters the ring  8 , then the pipe  18 , passes through the shutter  21 , takes the pipe  17 , passes through the volume  10  and exits via the pipe  14 , the shutter  11  and the pipe  16 . The volume  10  is thus scavenged and filled with fresh air. 
     In position  3   b , the crankshaft  5  has rotated through 94°. The expansion phase is in progress, the piston  2  has sunk in the cylinder  1  and begun to uncover the orifice via which the pipe  19  opens into the cylinder  1 . The upper shutter  11  has closed the pipe  14  while the lower shutter  21  has placed the pipes  17  and  19  in communication: it then follows that a blast of pressurized burnt gases begins to enter the volume  10 . 
     In position  3   c , the expansion movement continues and the blast of combustion gases continues under pressure to enter the volume  10 . 
     In position  3   d , the lower shutter  21  closes any communication between the pipe  17  and the pipes  18  or  19  so that the air in the volume  10  and the burnt gases which have entered this volume are trapped under pressure in said volume  10 . 
     However, the internal architecture of said spool  21  means that it places the pipes  18  and  19  in communication. At the same time, the upper shutter  11  has placed the pipes  13  and  15  in connection. Because, in this position  3   d , the valve or valves  6  are open, the cylinder  1  is scavenged with fresh air from the blower  9 , the burnt gases being discharged through the pipe  15  to the exhaust manifold. 
     In position  3   e , this scavenging continues. 
     In position  3   f , the piston  1  has moved past its bottom dead center and has begun the compression phase, the two shutters  11  and  21  being in a closed position, as is or are the valve or valves  6 . 
     In position  3   g , the valve or valves  6  are closed, the shutter  11  is closed, but the shutter  21  opens the communication between the pipes  17  and  19 , that is to say between the volume  10  and the cylinder  1 . The air and/or gas mixture which was trapped in the volume  10  can then escape and enter the cylinder  1 . 
     In position  3   h , the two shutters  11  and  21  are closed and the compression phase continues to TDC. 
     FIG. 4 illustrates another embodiment of the invention. In this figure, elements identical to those of FIGS. 1,  2  and  3  carry the same references. FIGS. 4 a  to  4   h  correspond to the same positions of the wrist pin  4  as FIGS. 2 a  to  2   h  and  3   a  to  3   h.    
     In this exemplary embodiment, there is now just one rotary shutter  21  communicating with an additional volume  20 , which is a closed volume, this rotary shutter  21  being a three-way shutter. 
     In FIG. 4 a , the combustion/expansion stroke is beginning. 
     In FIG. 4 b , the wrist pin  4  has described an angle of 94°, the piston  2  is therefore uncovering the orifice via which the pipe  19  opens into the cylinder  1 . In this position, the rotary shutter  21  establishes communication between the pipes  19  and  17  so that a blast of pressurized burnt gases discharges into the volume  20  the air from the pipe  19  and/or the gases. 
     The same is true of FIG. 4 c.    
     In FIG. 4 d , the wrist pin  4  having rotated through 127°, the rotary shutter  21  has interrupted any communication between the pipe  17  and the other pipes  18  and  19 , which means that the pressurized air and/or burnt gases mixture is trapped in the volume  20 . The valve or valves  6  open. 
     In FIG. 4 e , the valve or valves  6  are open, the openings  7  are no longer closed off by the piston  2 . Air which will later be used for supercharging is in the pipe  19  and in the hollow spool  22  of the rotary shutter: the respective dimensions of said spool  22  and of the pipe  19  can therefore be determined at will to store the desired volume of air. The air blown by the blower  9  scavenges the cylinder, which discharges the burnt gases through the valve or valves  6  and fills the cylinder with fresh air. 
     In FIG. 4 f , the position of the rotary shutter  21  is such that the fresh air from the blower  9 , which was previously introduced into the hollow spool of said shutter, is trapped therein. 
     In FIG. 4 g , the position of the rotary shutter  21  is such that the pipe  17  communicates with the pipe  19 . What this means is that the pressurized mixture, trapped in the volume  20 , post-fills the cylinder  1  by discharging the fresh air which was previously trapped in the spool of the rotary shutter  21  together with the fresh air contained in the pipe  19 . 
     In FIG. 4 h , the opening via which the pipe  19  communicates with the cylinder  1  is closed off. 
     Thus, fresh-air supercharging and/or introduction of burnt gases are obtained simultaneously. 
     FIG. 5 depicts the rotary spool  22  of the rotary shutter  21 . It has four openings  23 ,  24 ,  26  and  27 , two of these openings  23  and  24  being connected by a duct  25  so that the two pipes  23  and  24  can communicate only with one another and never with the other two. 
     Note that, as depicted, it is possible to use any appropriate means  30  to cool the volume  10  or  20 , which makes it possible to increase the density of the air and/or gases mixture trapped in said additional volume and improves the output of the engine. 
     FIGS. 6 to  9  are diagrams which correspond to FIGS. 1 to  4  respectfully. 
     FIG. 6 illustrates the operation of the exemplary embodiment of FIG.  1 . Curve A is the cylinder pressure curve; curve B is the exhaust pressure curve; EV means exhaust valve; IP means inlet port; the symbol V⇄E denotes transfers, in one direction or the other, between the volume  10  and the exhaust; the symbol V⇄C denotes transfers between the volume  10  and the cylinder; the symbol I→V denotes transfer from the inlet to the volume  10 . 
     It can be seen in particular that the exhaust valve is open between 94° and 288° of rotation of the crankshaft  5 ; that the inlet ports are uncovered by the piston  2  between 127° and 233° that between 94° and 127° and between 315° and 360° there is communication between the volume  10  and the exhaust (in FIG. 1 c ,it can be seen that this communication is from the exhaust to the volume  10 ; and in FIG. 1 h  it can be seen that this communication is from the volume to the exhaust) that between 233° and 266° there is communication between the volume  10  and the cylinder  1  (which corresponds to FIG. 1 g ); and that between 315° and 360° there is communication between the inlet (ports  7 , manifold  8 ) and the volume  10  (via the pipe  18 , through the shutter  11  and the pipe  17 ; which corresponds to FIG. 1 g ) 
     Similarly, FIG. 7 corresponds to the operation of the embodiment of FIG. 2; FIG. 8 to FIG.  3  and FIG. 9 to FIG.  4 .
The invention concerns a two-stroke engine comprising a cylinder ( 1) wherein moves a piston ( 2), exhaust ( 6) and intake ( 7) devices and a blower ( 9) for blowing in the cylinder ( 1) a scavenging air stream, wherein an additional volume ( 10) communicates with the cylinder ( 1) through closing and opening means ( 11, 12) whereof the movements are controlled synchdnously or out shift with those of the piston ( 2) in the cylinder ( 1) such that, during the combustion/expansion phase the burnt gases compress the air located in the additional volume ( 10) by penetrating therein at least partially, the air/burnt gas mixture is trapped therein under pressure; then the mixture is induced into the cylinder ( 1) during the intake phase.