EP0459848B1 - Improvements in two-stroke internal combustion engine of the reciprocating piston type - Google Patents

Description

The invention relates to reciprocating internal combustion engines, of the two-stroke type, with distribution by intake and exhaust valves, and with combustion and sweep prechamber, whether they are controlled ignition or spark ignition. compression.

• a) en augmentant la perméabilité du cylindre, c'est-à-dire en laissant passer le débit d'air requis par le moteur, sous une différence de pression entre admission et échappement et/ou avec une durée d'ouverture simultanée des soupapes d'admission et d'échappement qui soient aussi petites que possible ;
• b) en diminuant le court-circuit d'air entre l'admission et l'échappement, grâce à une orientation du courant des particules d'air frais entrant dans le cylindre qui les empêche de passer directement de l'admission à l'échappement, ce qui améliore le rendement de balayage ;
• c) en empêchant autant que possible l'air frais admis au cours d'un cycle dans le cylindre de se mélanger, pendant le balayage, aux gaz brûlés provenant du cycle précédent et quittant le cylindre à travers le ou les orifices d'échappement ; et
• d) pour un moteur à combustion non homogène et à allumage par compression, en créant dans la chambre de combustion d'intenses mouvements d'air bien synchronisés avec l'injection de carburant pour améliorer le mélange entre air et carburant.

In modern engines of this type, efforts are generally made to improve operation.

• a) by increasing the permeability of the cylinder, that is to say by letting pass the air flow required by the engine, under a pressure difference between intake and exhaust and / or with a duration of simultaneous opening of the valves intake and exhaust that are as small as possible;
• b) by reducing the short circuit of air between the intake and the exhaust, thanks to an orientation of the current of the particles of fresh air entering the cylinder which prevents them from passing directly from the intake to the exhaust, which improves the sweeping efficiency;
• c) by preventing as much as possible the fresh air admitted during a cycle into the cylinder from mixing, during the sweeping, with the burnt gases originating from the preceding cycle and leaving the cylinder through the exhaust port (s); and
• d) for a non-homogeneous combustion engine with compression ignition, creating in the combustion chamber intense air movements well synchronized with the injection of fuel to improve the mixture between air and fuel.

In order to ensure centrifugal combustion stratification, it has already been proposed, in document US-A-4,224.9O5, to organize the circulation of the air admitted into the cylinder in such a way that this air performs at each cycle, inside the cylinder, a helical movement around an axis practically coincident with that of the cylinder but the construction proposed in this document is complicated because it requires to open the prechamber tangentially in the cylinder, so that the air is introduced tangentially in the plane of the cylinder head, and to provide a central exhaust valve and an intake valve directly supplying the prechamber, as well as in general two additional intake valves opening in the cylinder and placed on either side of the central exhaust valve.

The invention aims to organize a circulation helical air intake into the cylinder, which simultaneously ensures the reduction of the air short circuit and the formation of a vortex in the combustion chamber. The invention also aims to arrange a two-stroke reciprocating engine with valve distribution in such a way that criterion b) defined above is particularly well respected, and this by moving the exhaust port (s) away particles of fresh air introduced into the cylinder, taking into account the orientation of the paths imposed on these particles. The invention also aims to improve the two-stroke engines described in French patent FR-A-2,592,430 and which will be chosen below to define the state of the art.

• au moins un cylindre dépourvu de lumières latérales ;
• une chambre de combustion limitée dans ce cylindre par une culasse fixe par rapport à ce cylindre et par un piston animé d'un mouvement alternatif à l'intérieur de ce cylindre ;
• une préchambre de combustion et de balayage ménagée dans la culasse , munie d'un siège de soupape d'admission et communiquant avec la chambre de combustion par un canal de transfert dont la section droite perpendiculaire à l'axe (X-X) du cylindre , au débouché dans la chambre de combustion , est une surface oblongue sensiblement tangente intérieurement à la paroi intérieure du cylindre ;
• une soupape d'admission d'axe (Y-Y) au moins approximativement orthogonal à l'axe (X-X) du cylindre et de préférence sécant avec ce dernier axe (X-X), disposée de façon telle que la tige de cette soupape soit plus éloignée de ce dernier axe (X-X) que la tête de celle-ci et que cette tête puisse se déplacer à l'intérieur de ladite préchambre en vue de s'écarter et de se rapprocher du susdit siège de soupape d'admission ;
• un conduit d'admission qui aboutit directement en amont du susdit siège ; et
• au moins une soupape d'échappement disposée de façon telle que son axe (Z-Z) soit au moins approximativement parallèle à l'axe (X-X) du cylindre et qu'elle puisse coopérer, en laissant passer les gaz d'échappement sur la majeure partie au moins de sa périphérie, avec un siège de soupape d'échappement ménagé dans ciel de la culasse à l'opposé du canal de transfert , ce dernier siège étant de préférence agencé de telle façon que la surface transversale de la tête du piston ne laisse subsister au point mort haut que le jeu de fonctionnement nécessaire avec la soupape d'échappement et le ciel de la culasse ,
     la préchambre , en dehors de son raccord avec le canal de transfert , ayant sensiblement une forme de révolution autour d'un axe parallèle à l'axe (Y-Y) de la soupape d'admission et de préférence pratiquement confondu avec ce dernier axe (Y-Y) ou légèrement décalé par rapport à celui-ci ;
     caractérisé en ce que des moyens déflecteurs disposés à l'intérieur de la partie terminale du conduit d'admission , c'est-à-dire aussi directement que possible en amont du siège de la soupape d'admission , sont agencés de façon à produire une déflexion dans un seul sens, autour de l'axe (Y-Y) de ladite soupape , de la masse d'air qui arrive par ce conduit lorsque cette soupape est ouverte ; et
     en ce que le canal de transfert a une forme convergente vers son débouché dans le cylindre et s'épanouit à son raccordement avec le cylindre en formant ainsi avec la préchambre un profil sensiblement en Ω de sorte que, d'une part, la déflexion ainsi produite dans la préchambre par les moyens déflecteurs pendant la phase de balayage engendre un tourbillon sensiblement hélicoïdal dans le cylindre et que, d'autre part, lors de la remontée du piston , ledit tourbillon sensiblement hélicoïdal engendre à son tour dans la préchambre un tourbillon dans le même sens.

The subject of the invention is a two-stroke internal combustion engine, which comprises:

• at least one cylinder without side lights;
• a combustion chamber limited in this cylinder by a cylinder head fixed relative to this cylinder and by a piston driven by a reciprocating movement inside this cylinder;
• a combustion and sweeping prechamber made in the cylinder head, fitted with an intake valve seat and communicating with the combustion chamber by a transfer channel, the cross section of which is perpendicular to the axis (XX) of the cylinder, outlet in the combustion chamber, is an oblong surface substantially tangent internally to the inner wall of the cylinder;
• an axis inlet valve (YY) at least approximately orthogonal to the axis (XX) of the cylinder and preferably intersecting with the latter axis (XX), arranged in such a way that the stem of this valve is further from this last axis (XX) that the head thereof and that this head can move inside said prechamber in order to move away and to approach the aforesaid intake valve seat;
• an intake duct which terminates directly upstream of the above-mentioned seat; and
• at least one exhaust valve arranged so that its axis (ZZ) is at least approximately parallel to the axis (XX) of the cylinder and that it can cooperate, allowing the exhaust gases to pass over the major part at least on its periphery, with an exhaust valve seat arranged in the breech of the cylinder head opposite the transfer channel, this latter seat preferably being arranged in such a way that the transverse surface of the piston head does not leave remain in top dead center as the necessary operating clearance with the exhaust valve and the head of the cylinder head,
  the prechamber, apart from its connection with the transfer channel, having substantially a shape of revolution around an axis parallel to the axis (YY) of the intake valve and preferably practically coincident with this latter axis (YY ) or slightly offset from it;
  characterized in that deflector means arranged inside the end part of the intake duct, that is to say as directly as possible upstream of the seat of the intake valve, are arranged so as to produce a deflection in only one direction, around the axis (YY) of said valve, of the mass of air which arrives via this conduit when this valve is open; and
  in that the transfer channel has a shape that converges towards its outlet in the cylinder and expands at its connection with the cylinder, thus forming with the prechamber a profile substantially in Ω so that, on the one hand, the deflection thus produced in the prechamber by the deflector means during the scanning phase generates a substantially helical vortex in the cylinder and that, on the other hand, during the raising of the piston, said substantially helical vortex in turn generates in the prechamber a vortex in the same way.

In other words, it can be said that the vortex induced in the cylinder during the scanning generates a vortex in the prechamber during the compression phase. We therefore find ourselves in the presence of a vortex generated in the prechamber for the combustion phase and which does not reverse during expansion, unlike known structures such as the "RICARDO" prechamber.

The aforementioned deflector means, which can be constituted by the actual drawing of the intake duct upstream of the seat but which are preferably constituted by blades, can be linked either to the intake valve being disposed with clearance inside the end part of the intake duct, this intake valve then comprising guide means preventing it from turning on itself, either at the seat of the valve intake by loosely surrounding the stem of this valve.

The prechamber preferably has a height (that is to say a dimension parallel to said axis) which is substantially equal to the stroke of the intake valve. It is advantageous, although not necessary, to give the assembly of the prechamber and the transfer channel a shape which is substantially symmetrical with respect to the plane passing through the axis of the cylinder and through the axis of the intake and give the transfer channel a shape which facilitates the generation of the vortex in the cylinder during scanning as well as the induction of a vortex in the prechamber during compression.

It should be noted that, unlike the aforementioned document US-A-4,224.9O5, the introduction of air into the cylinder is not tangential with a central intake valve but the air particles thus introduced are directed downwards obliquely so as to deflect their trajectory and to move them away from the vicinity of the exhaust.

As will be explained in more detail below with the aid of the drawings, the engine according to the invention does indeed achieve the aims which had been assigned to it.

• les figures 1 à 3 représentent schématiquement un moteur conforme à l'invention dans les positions occupées par le piston et les soupapes lors du balayage, respectivement en perspective et, à plus petite échelle, en coupe par le plan P (figure 3) passant par les axes du cylindre et des soupapes d'admission et d'échappement et par un plan perpendiculaire à ce dernier-plan ;
• les figures 4 à 6 sont des vues semblables à celle de la figure 1 mais correspondant respectivement à la compression, à la combustion et à la détente ;
• la figure 7 représente schématiquement la culasse du moteur vue par le dessous de la figure 2 ;
• les figures 8 et 9 représentent schématiquement un système d'alimentation en combustible établi selon une première variante, respectivement par une vue analogue à une partie de la figure 2 et en coupe selon la ligne IX-IX de la figure 8; et
• les figures 10 et 11 représentent schématiquement un système d'alimentation en combustible établi selon une deuxième variante, respectivement par des vues analogues à celles des figures 8 et 9, la figure 11 étant une vue en coupe selon la ligne XI-XI de la figure 10.

The invention will now be described in more detail. using these drawings including:

• Figures 1 to 3 schematically represent an engine according to the invention in the positions occupied by the piston and the valves during scanning, respectively in perspective and, on a smaller scale, in section through the plane P (Figure 3) passing through the axes of the cylinder and of the intake and exhaust valves and by a plane perpendicular to this latter plane;
• Figures 4 to 6 are views similar to that of Figure 1 but corresponding respectively to compression, combustion and expansion;
• FIG. 7 schematically represents the cylinder head of the engine seen from below of FIG. 2;
• Figures 8 and 9 schematically represent a fuel supply system established according to a first variant, respectively by a view similar to a part of Figure 2 and in section along the line IX-IX of Figure 8; and
• Figures 10 and 11 schematically represent a fuel supply system established according to a second variant, respectively by views similar to those of Figures 8 and 9, Figure 11 being a sectional view along line XI-XI of the figure 10.

• au moins un cylindre 1 dépourvu de lumières latérales ;
• une chambre de combustion 2 limitée dans le cylindre 1 par une culasse 3 fixe par rapport au cylindre 1 et par la surface transversale 5 d'un piston 4 animé d'un mouvement alternatif à l'intérieur de ce cylindre 1 ;
• une préchambre de combustion et de balayage 6 ménagée dans la culasse 3, munie d'un siège de soupape d'admission 7 (figure 2) et communiquant avec la chambre de combustion 2 par un canal de transfert 8 dont la section perpendiculaire à l'axe X-X du cylindre 1, au débouché dans la chambre de combustion 2, est une surface oblongue 9 sensiblement tangente intérieurement à la paroi intérieure du cylindre 1 à peu près parallèlement à la grande dimension de cette surface oblongue, comme le montre la figure 7;
• une soupape d'admission 1O, d'axe Y-Y au moins approximativement orthogonal à l'axe X-X du cylindre 1 et de préférence sécant avec ce dernier axe X-X, cette soupape d'admission 10 étant disposée de façon telle que sa tige 11 soit plus éloignée de l'axe X-X que la tête 12 de cette soupape 10 et que cette tête 12 puisse se déplacer à l'intérieur de la préchambre 6 en vue de s'écarter et de se rapprocher du susdit siège 7 ;
• un conduit d'admission 13 qui aboutit directement en amont du siège 7,
• au moins une soupape d'échappement 15 disposée de façon telle que son axe Z-Z sont au moins approximativement parallèle à l'axe X-X du cylindre 1 et qu'elle puisse coopérer, en laissant passer les gaz d'échappement sur la majeure partie au moins de sa périphérie, avec un siège 16 ménagé dans le ciel de la culasse 3 à l'opposé du canal de transfert 8, ce dernier siège 16 étant de préférence agencé de telle façon que la surface transversale 5 de la tête du piston 4 ne laisse subsister au point mort haut (figure 5) que le jeu de fonctionnement nécessaire avec la soupape d'échappement 15 et le ciel de la culasse 3, la soupape d'échappement 15 ou l'ensemble des soupapes d'échappement (quand il en existe plus d'une) étant de préférence au moins approximativement symétrique par rapport au plan P (figure 3) passant par l'axe X-X du cylindre 1 et par l'axe Y-Y de la soupape d'admission 1O. Il est à noter que ce plan P est confondu avec celui de la figure 2.

The engine shown in Figures 1 to 3 includes:

• at least one cylinder 1 devoid of lateral lights;
• a combustion chamber 2 limited in the cylinder 1 by a cylinder head 3 fixed relative to cylinder 1 and by the transverse surface 5 of a piston 4 driven by a reciprocating movement inside this cylinder 1;
• a combustion and sweeping prechamber 6 formed in the cylinder head 3, provided with an intake valve seat 7 (FIG. 2) and communicating with the combustion chamber 2 by a transfer channel 8 whose section perpendicular to the axis XX of the cylinder 1, at the outlet in the combustion chamber 2, is an oblong surface 9 substantially tangent internally to the internal wall of the cylinder 1 roughly parallel to the large dimension of this oblong surface, as shown in FIG. 7;
• an inlet valve 1O, of axis YY at least approximately orthogonal to the axis XX of the cylinder 1 and preferably intersecting with the latter axis XX, this inlet valve 10 being arranged so that its stem 11 is more distant from the axis XX that the head 12 of this valve 10 and that this head 12 can move inside the prechamber 6 in order to move away from and approach the above-mentioned seat 7;
• an intake duct 13 which terminates directly upstream of the seat 7,
• at least one exhaust valve 15 arranged in such a way that its axis ZZ are at least approximately parallel to the axis XX of the cylinder 1 and that it can cooperate, allowing the exhaust gases to pass over the at least most of its periphery, with a seat 16 formed in the sky of the cylinder head 3 opposite the transfer channel 8, the latter seat 16 preferably being arranged in such a way that the transverse surface 5 of the head of the piston 4 leaves at top dead center (FIG. 5) only the operating clearance required with the exhaust valve 15 and the head of the cylinder head 3, the exhaust valve 15 or all of the exhaust valves ( when there is more than one) preferably being at least approximately symmetrical with respect to the plane P (FIG. 3) passing through the axis XX of the cylinder 1 and through the axis YY of the intake valve 1O. It should be noted that this plane P is confused with that of FIG. 2.

The prechamber 6 is given, apart from its connection with the transfer channel 8, substantially a form of revolution around an axis parallel to the axis YY of the inlet valve 1O and preferably practically coincident with the latter. YY axis or slightly offset from it.

A motor as described hitherto with reference to the figures is known from the above FR-A-2 592 430) However, in accordance with the invention, there is disposed inside the terminal part of the conduit inlet 13, that is to say as directly as possible upstream of the seat 7, deflector means 17 arranged so as to produce a deflection in one direction, around the axis YY, of the mass of air which arrives via line 13 when the inlet valve 1O is open.

In addition, the transfer channel 8 has a shape such, on the one hand, that the deflection thus produced in the prechamber 6 by the deflector means 17 during the scanning phase generates a substantially helical vortex in the cylinder 1 and, on the other hand, that, during the raising of the piston 4, said substantially helical vortex in turn generates in the pre-chamber 6 a vortex in the same direction.

Although they may be constituted by a particular geometry, in particular a helical or "corkscrew" shape, of the intake duct 13, the deflector means 17 are generally constituted by blades and can in this case be linked either to the valve 10 (solution not shown) while being disposed with play inside the terminal part of the intake duct 13, in which case the valve 10 includes guide means preventing it from turning on itself, either to seat 7, with clearance around rod 11 of valve 1O as shown diagrammatically in FIG. 2.

The assembly of the prechamber 6 and the transfer channel 8 preferably has a shape that is substantially symmetrical with respect to a plane parallel to the axis XX of the cylinder 1 and passing through the axis YY of the intake valve 1O, preference over plane P.

Preferably, there is associated with cylinder 1 only one exhaust valve 15. Finally, the surface transverse 5 of the piston 4 and the head of the cylinder head 3 are preferably planes (subject to the groove or grooves 20 which will be discussed below) and perpendicular to the axis XX of the cylinder 1.

The lips of the transfer channel 8 are advantageously organized in such a way that the air jet emerging from the prechamber 6 towards the cylinder 1 makes an angle A of the order of 30 ° with the plane parallel to the axis XX of the cylinder 1 and passing through the axis YY of the intake valve 10 (see FIG. 3). This angle A obviously depends on the rate of deflection of the mass of air passing through the seat 7 of the inlet valve 1O after having crossed the deflector means 17 as well as the shape of the transfer channel 8. This angle will preferably evolve in a opposite direction to that of the stroke / bore ratio. If this angle A is too high, the paths of the air particles do not penetrate far enough into the combustion chamber 2 towards the piston 4 and remain too close to the exhaust port (s), which tends to cause passage direct from the exhaust intake, to the detriment of the quality of the sweep. Conversely, if this angle A is too small, the air particles tend to be returned by the piston 4 to the exhaust port (s), which produces the same harmful effect. The optimal situation is in the vicinity of a 30 ° angle for a stroke / bore ratio of the order of 1.25.

As shown in Figure 3, the transfer channel 8 has a shape converging towards its outlet in the cylinder 1, it opens out at its connection with the cylinder 1, thus forming with the prechamber a profile substantially in Ω and preferably, it is substantially symmetrical with respect to the above-mentioned plane parallel to the axis XX of cylinder 1 and passing through axis YY of inlet valve 1O.

The geometry of the intake valve 10 prevents it from being put in place by the sky of the cylinder head 3. It is therefore advisable, in known manner, to associate with this intake valve 10 a chapel 21 (FIG. 2) allowing it to be put in place with its seat 7 by crossing the cylinder head 3 (that is to say from right to left according to FIG. 2).

As shown in FIG. 7, it is advantageous to provide in the piston 4 or, preferably, in the part of the cylinder head 3 which is external to the outlet (or oblong surface) 9 of the transfer channel 8 and to the seat 16 of the exhaust valve 15, at least one groove 20 which leads to the outlet 9 and whose cross section and / or depth decrease as one moves away from this outlet 9. This groove (as shown in solid lines) or each of these grooves (as shown in solid lines and broken lines) advantageously has the shape of a half-crescent.

In the case of a compression-ignition engine, the injection orifice (s) 18 can be placed in the prechamber 6, either on the side wall of the latter, opposite the inlet valve 1O and preferably distributed on at least one circle centered on the axis of the prechamber 6, or on the peripheral wall thereof.

According to the first solution, there may be only one annular injection orifice 18 centered on the axis of the prechamber, as shown diagrammatically in FIG. 2, or else a plurality of orifices distributed over a centered circle on the axis of the prechamber and oriented radially towards the periphery thereof, as shown diagrammatically in FIG. 5. But it seems preferable, in order to increase the distance offered to the fuel between the orifice (s) 18 and the wall, as shown in FIGS. 8 and 9, to provide several injection orifices 18, for example eight in number, advantageously distributed along one (as shown) or several circles C preferably centered on the axis of the prechamber 6. These injection orifices are advantageously oriented towards the periphery of the prechamber 6, as appears from FIG. 8 in the manner of one of the families of the generators of a hyperboloid of revolution. In the case (shown) where these orifices 18 are distributed along a single circle C, this circle advantageously has a diameter d of the order of 50% of the diameter D of the prechamber 6 ..

According to the second solution which is illustrated diagrammatically in FIGS. 10 and 11, there is preferably only one injection orifice 18 in the form of a sheet (nipple injector) which is located on the dome of the prechamber 6, it that is to say on the part of the wall peripheral of the latter which is opposite to the transfer channel 8. This injection orifice 18, which flows towards the transfer channel 8, is then oriented so that the jet or layer of fuel 19 which it discharges is placed in a plane substantially perpendicular to the axis of the prechamber 6.

According to either of these solutions (FIGS. 8 to 11), better homogeneity of the air / fuel mixture is obtained and, by promoting the initiation of combustion on the periphery of the prechamber 6, formation is avoided. troublesome to a hot area near the injection port.

An engine is thus obtained, the operation of which is as follows, the arrows plotted in FIGS. 1 to 6 diagramming the movements of the fluids inside the cylinder 1 as well as at the inlet and at the outlet thereof. During scanning (Figures 1 to 3), the piston 4 being first in the vicinity of the bottom dead center, then during the first part of its ascent, the air passing through the seat 7 is deflected by the deflector means 17, the valve 1O intake and the walls of the prechamber 6 so as to penetrate into the cylinder 1 in the form of a jet inclined relative to the axis XX of the cylinder 1 (in the manner of a sling). Due to its cylindrical shape, the trajectories of the air particles emerging obliquely from the prechamber 6 wind in a helix inside the cylinder 1, which has the effect of preventing these particles from passing near of the seat 16 of the exhaust valve 15 which is then open. Thanks to the absence of a short circuit, the operating efficiency is high and good thermal uniformity is obtained for the piston 4 and the cylinder 1 (or the jacket thereof).

During compression (Figure 4), the helical rotation movement, induced in the cylinder 1 during the scanning phase, is maintained during the ascent of the piston 4 which tends to reduce the pitch of the propeller.

The air pushed by the ascent of the piston 4 invades the prechamber 6 while keeping the tangential speed induced by the rotation in the cylinder 1, which causes the rotation of the air in the prechamber 6, coaxial with said prechamber and, this which is important, without reversing the direction of rotation induced in the prechamber 6 during scanning.

During combustion (Figure 5), almost all of the air trapped in the cylinder 1 is pushed, by the pinching effect of the piston 4 cooperating with the head of the cylinder head 3 and with the exhaust valve 15 resting on its seat 16, inside the prechamber 6 while keeping its rotational movement coaxial with said prechamber. In the preferred case of a diesel engine, the injection orifice (s) 18 atomize fuel jets 19 which interfere with the rotational movement of the air and thus facilitate mixing between air and fuel. This is to limit the speed of transfer of gases from the chamber main 2 to the prechamber 6 and vice versa, which advantageously the above-mentioned groove (s) 20 will be arranged, preferably in the cylinder head 3 and not in the piston 4, in particular when the piston 4 has the possibility of turning around its axis in operation (in particular in the case of a spherical articulation of the piston 4 with the connecting rod associated therewith), as shown in FIG. 7.

During expansion (Figure 6), there is a phenomenon opposite to that of compression, the gases leaving the prechamber 6 with an inclination similar to that which had occurred during the sweep.

By this mechanism, the rotation of the air and the gases is maintained without reversing the direction both in the cylinder 1 and in the prechamber 6. At each scanning cycle, the rotational movement is reactivated by the deflector means 17 acting on fresh air admitted. We will thus obtain an aerodynamic process generating the minimum losses since there will be re-acceleration at each cycle, without reversing the direction of flow.

In the foregoing, it has been assumed that the engine comprises only one cylinder, but it goes without saying that the explanations which have been given above remain valid in the case where the engine has two or more cylinders. Likewise. although the engine according to the invention has so far been described as having a single exhaust valve 15, it could include two or more. Although Figure 3 shows a prechamber 6 having a shape of revolution around an axis coincident with that YY of the inlet valve 10, the axis of the prechamber 6 could be slightly offset with respect to this axis YY.

The invention is particularly advantageously applicable to engines supercharged by turbocharger driven by the exhaust gases.

Claims (18)

1. A two-stroke cycle internal combustion engine which comprises :
      at least one cylinder (1) devoid of lateral ports;
      a combustion chamber (2) limited in said cylinder (1) by a cylinder head (3) fixed relative to the cylinder (1) and by a piston (4) undergoing a reciprocating motion inside the cylinder (1) ;
      a combustion and scavenging prechamber (6) provided in the cylinder head (3), including an intake valve seat (7) and communicating with the combustion chamber (2) through a transfer passage (8) having an outlet which opens into the combustion chamber and which has a cross-section perpendicular to the axis (X-X) of the cylinder (1) which is an oblong surface (9) and is internally substantially tangent to the inner wall of the cylinder (1) ;
      an intake valve (10) having an axis (Y-Y) which is at least approximately orthogonal to the axis (X-X) of the cylinder (1) and preferably secant with respect to the axis (X-X) and being so disposed that the stem (11) of the valve (10) is spaced further away from the axis (X-X) than the head (12) of the valve (10) and said head (12) is movable inside said prechamber (6) for the purpose of moving away from and toward the seat (7) of the intake valve (10) ;
      an intake pipe (13) which directly leads to the upstream side of said seat (7) ; and
      at least one exhaust valve (15) so disposed that its axis (Z-Z) is at least approximately parallel to the axis (X-X) of the cylinder (1) and it is capable of cooperating, while allowing passage of exhaust gases on at least the major part of its periphery, with an exhaust valve seat (16) provided in the ceiling of the cylinder head (3) opposite the transfer passage (8), this seat (16) being preferably so arranged that the transverse surface (5) of the head of the piston (4) only leaves when at top dead centre the required operating clearance with the exhaust valve (15) and the ceiling of the cylinder head (3),
      the prechamber (6), apart from its connection with the transfer passage (8), has substantially a shape of revolution about an axis parallel to the axis (Y-Y) of the intake valve (10) and preferably substantially coincident with or slightly offset from the axis (Y-Y) ;
      characterized in that deflecting means (17), disposed inside the end part of the intake pipe (13), i.e. as directly as possible on the upstream side of the seat (7) of the intake valve (10), are so arranged as to produce a deflection in a single direction about the axis (Y-Y) of said intake valve (10), of the mass of air which arrives through the intake pipe (13) when the intake valve (10) is open ; and
      in that the transfer passage (8) has a shape which is convergent toward the outlet at which it opens into the cylinder (1) and is flared at its connection with the cylinder (1), thereby forming with the prechamber substantially a Ω section, so that, on one hand, the deflection thus produced in the prechamber (6) by the deflecting means (17) during the scavenging stage creates a substantially helical swirling in the cylinder (1) and , on the other hand, when the piston (4) rises, said substantially helical swirling in turn creates a swirling in the same direction in the prechamber (6).
2. Engine according to claim 1, characterized in that the deflecting means (17) are constituted by vanes around the stem (11) of the intake valve (10).
3. Engine according to claim 2, characterized in that the vanes (17) are connected to the intake valve (10) and are disposed with clearance inside the end part of the intake pipe (13), the intake valve (10) comprising guide means preventing it from rotating about itself.
4. Engine according to claim 2, characterized in that the vanes (17) are connected to the seat (7) of the intake valve (10) and surround with clearance the stem (11) of the intake valve (10).
5. Engine according to any one of the claims 1 to 4, characterized in that the prechamber (6) has a height which is substantially equal to the travel of the intake valve (10).
6. Engine according to any one of the claims 1 to 5, characterized in that the transfer passage (8) has a shape which is substantially symmetrical relative to a plane parallel to the axis (X-X) of the cylinder (1) and containing the axis (Y-Y) of the intake valve (10).
7. Engine according to any one of the claims 1 to 6, characterized in that the whole of the prechamber (6) and transfer passage (8) has a shape which is substantially symmetrical relative to a plane parallel to the axis (X-X) of the cylinder (1) and containing the axis (X-X) of the intake valve (10).
8. Engine according to any one of the claims 1 to 7, characterized in that the cylinder has only one exhaust valve (15).
9. Engine according to any one of the claims 1 to 8, which is supplied with fuel by at least one injection orifice (18) opening into the prechamber (6), characterized in that the injection orifice or orifices (18) are located in the lateral wall of the prechamber (6) opposite the intake valve (10), preferably coaxially of the latter.
10. Engine according to claim 9, characterized in that the injection orifice or orifices (18) are oriented toward the periphery of the prechamber (6).
11. Engine according to claim 10, characterized in that it has a plurality of injection orifices (18) spaced apart on at least one circle (C) centred on the axis of the prechamber (6).
12. Engine according to claim 10, characterized in that the injection orifices (18) are spaced apart on a single circle (C) whose diameter (d) is of the order of 50% of the diameter (D) of the prechamber (6).
13. Engine according to any one of the claims 1 to 8, supplied with fuel by an injection orifice (18) opening into the prechamber (6), characterized in that the injection orifice (18) is located on the peripheral wall of the prechamber (6) opposite the transfer passage (8) and is preferably oriented toward the transfer passage (8).
14. Engine according to any one of the claims 1 to 12, characterized in that the transverse surface (5) of the piston (4) and the ceiling of the cylinder head (3) are substantially planar and perpendicular to the axis (X-X) of the cylinder (1).
15. Engine acording to any one of the claims 1 to 14, characterized in that the lips of the transfer passage (8) are so arranged that the jet of air emerging from the prechamber (6) toward the cylinder (1) makes an angle (A) of the order of 30° with the plane parallel to the axis (X-X) of the cylinder (1) and containing the axis (Y-Y) of the intake valve (10), when the ratio between the stroke and the bore of the engine is of the order of 1.25.
16. Engine according to any one of the claims 1 to 15, characterized in that it comprises in the piston (4) or preferably in the part of the cylinder head (3) which is outside the outlet (9) at which the transfer passage (8) opens into the cylinder and outside the seat (16) of the exhaust valve (15), at least one groove (20) which leads to said outlet (9) and whose cross-section and/or depth decrease as one moves away from said outlet (9).
17. Engine according to any one of the claims 1 to 16, characterized in that a valve case (21) is associated with the intake valve (10) and its seat (7).
18. Engine according to any one of the claims 1 to 17, characterized in that it is supercharged.

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