EP1706612A1 - Direct injection two-stroke engine - Google Patents

Direct injection two-stroke engine

Info

Publication number
EP1706612A1
EP1706612A1 EP04817611A EP04817611A EP1706612A1 EP 1706612 A1 EP1706612 A1 EP 1706612A1 EP 04817611 A EP04817611 A EP 04817611A EP 04817611 A EP04817611 A EP 04817611A EP 1706612 A1 EP1706612 A1 EP 1706612A1
Authority
EP
European Patent Office
Prior art keywords
fuel
engine
combustion chamber
cylinder
axis
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04817611A
Other languages
German (de)
French (fr)
Inventor
Michael Pontoppidan
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Marelli Argentan France SAS
Original Assignee
Magneti Marelli Motopropulsion France SAS
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Magneti Marelli Motopropulsion France SAS filed Critical Magneti Marelli Motopropulsion France SAS
Publication of EP1706612A1 publication Critical patent/EP1706612A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B27/00—Use of kinetic or wave energy of charge in induction systems, or of combustion residues in exhaust systems, for improving quantity of charge or for increasing removal of combustion residues
    • F02B27/04—Use of kinetic or wave energy of charge in induction systems, or of combustion residues in exhaust systems, for improving quantity of charge or for increasing removal of combustion residues in exhaust systems only, e.g. for sucking-off combustion gases
    • F02B27/06—Use of kinetic or wave energy of charge in induction systems, or of combustion residues in exhaust systems, for improving quantity of charge or for increasing removal of combustion residues in exhaust systems only, e.g. for sucking-off combustion gases the systems having variable, i.e. adjustable, cross-sectional areas, chambers of variable volume, or like variable means
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B23/00—Other engines characterised by special shape or construction of combustion chambers to improve operation
    • F02B23/08—Other engines characterised by special shape or construction of combustion chambers to improve operation with positive ignition
    • F02B23/10—Other engines characterised by special shape or construction of combustion chambers to improve operation with positive ignition with separate admission of air and fuel into cylinder
    • F02B23/104—Other engines characterised by special shape or construction of combustion chambers to improve operation with positive ignition with separate admission of air and fuel into cylinder the injector being placed on a side position of the cylinder
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B25/00—Engines characterised by using fresh charge for scavenging cylinders
    • F02B25/14—Engines characterised by using fresh charge for scavenging cylinders using reverse-flow scavenging, e.g. with both outlet and inlet ports arranged near bottom of piston stroke
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B25/00—Engines characterised by using fresh charge for scavenging cylinders
    • F02B25/20—Means for reducing the mixing of charge and combustion residues or for preventing escape of fresh charge through outlet ports not provided for in, or of interest apart from, subgroups F02B25/02 - F02B25/18
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B23/00—Other engines characterised by special shape or construction of combustion chambers to improve operation
    • F02B23/08—Other engines characterised by special shape or construction of combustion chambers to improve operation with positive ignition
    • F02B23/10—Other engines characterised by special shape or construction of combustion chambers to improve operation with positive ignition with separate admission of air and fuel into cylinder
    • F02B23/101—Other engines characterised by special shape or construction of combustion chambers to improve operation with positive ignition with separate admission of air and fuel into cylinder the injector being placed on or close to the cylinder centre axis, e.g. with mixture formation using spray guided concepts
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • the present invention relates to a two-stroke engine with direct injection of liquid fuel. More particularly, the invention relates to a two-stroke engine with direct injection comprising a combustion chamber delimited by: a cylinder having a longitudinal axis, which is provided with at least one intake lumen and at least one lumen exhaust; a piston having a substantially flat bottom and displaced along the longitudinal axis by a connecting rod connected to a crankshaft; a cylinder head fitted with a spark plug and an injector adapted to spray a jet of pressurized fuel into the combustion chamber along a spraying axis, the combustion chamber having a first diametrical plane comprising the longitudinal axis of the cylinder and centered on the exhaust port and a second diametral plane perpendicular to said first diametrical plane, the spark plug being arranged in a first portion of the cylinder head extending from the second diametrical plane towards the intake port, the injector being arranged on a second portion of the cylinder head complementary to the first portion, and the
  • the operating cycle of two-stroke engines includes, for each crankshaft revolution, a first intake / compression time and a second combustion / exhaust time.
  • the piston performs a translational movement from a low neutral position to a high neutral position by successively closing the intake and exhaust ports of the cylinder.
  • Fresh gases compressed in the crankcase are then admitted, via a transfer channel, into the combustion chamber through the intake ports until they are closed by the piston.
  • the fresh gases admitted into the combustion chamber are then compressed until the piston reaches top dead center, while fresh gases are drawn into the crankcase.
  • the piston translates from top dead center to bottom dead center by successively unmasking the exhaust and intake ports.
  • the ignition is caused by the spark plug when the piston is approximately in its top dead center position.
  • This type of engine has the advantage of offering a relatively high power compared to a four-stroke engine of similar weight, due to the existence of an engine time for each revolution of the crankshaft. In addition, its manufacturing cost is particularly low, because the number of parts is less than that of a four-stroke engine.
  • this type of engine generally has the disadvantage of a high fuel consumption and a significant emission of pollutants compared to a four-stroke engine.
  • the object of the present invention is to improve two-stroke direct injection engines, in particular in order to satisfy the anti-pollution standards in force and to come, and this, by minimally modifying the geometry of the combustion chamber so that the present invention can be applicable to existing engines.
  • the subject of the present invention is a engine of the aforementioned type, characterized in that the opening angle ⁇ of the fuel jet is between 15 ° and 75 °, in that the injection of fuel begins when the crankshaft is located in an angular position between 45 ° and 20 ° before the angular closing position of the exhaust port, and in that the fuel injection pressure and the orientation of the spray axis are determined as a function of the gas circulation in the chamber combustion to obtain a substantially stoichiometric air / fuel mixture in the region of the spark plug at the time of ignition.
  • the opening angle ⁇ of the jet limited to 75 ° makes it possible to form the fuel droplets in a limited area of the combustion chamber where the gases have a particular speed profile, and above all avoids the spraying of droplets against the walls of the combustion chamber, which would increase pollutant emissions.
  • the fact of starting the injection of the fuel at least 20 ° before the closing of the exhaust lights, that is to say in advance compared to the prior direct injection devices in which the injection generally begins when the exhaust light is closed to prevent the passage of fuel droplets towards the exhaust, increases the time the fuel droplets mix with the fresh gases and the vaporization of the fuel, so as to obtain a more homogeneous air / fuel mixture time of ignition.
  • the orientation of the spray axis included in the values of the first angle ⁇ and the second angle ⁇ mentioned reduces the passage of fuel through the exhaust port during the compression phase, despite the early injection of the fuel.
  • This adaptation must be made as a function of the gas circulation in the combustion chamber which can be determined by numerical simulation. From the profile of the gas stream lines in the combustion chamber, which is substantially constant during the intake / compression phase, it is possible to adapt the orientation of the spray axis so that that the fuel droplets sprayed by the injector meet gases flowing against them.
  • the fuel injection pressure is variable depending on the engine speed and / or engine load, in order to obtain an optimal reduction in pollutant emissions over the entire engine operating range;
  • the fuel injection pressure is between 50 and 150 bars;
  • the fuel injection pressure is adjusted to different values according to an engine speed / load map;
  • the fuel injection pressure is constant over the entire operating range of the engine, the engine preferably having a displacement of at most equal to 125 cm 3 , to reduce pollutant emissions with a relatively simple injection system;
  • the injector is disposed in a bore of the cylinder head oriented along a given axis and in which the spray axis forms a non-zero angle ⁇ with said axis of the bore;
  • the injector is arranged through the cylinder head at the level of the first diametrical plane, which allows it to be mounted in a small displacement engine; fuel injection begins when the crankshaft is located in an angular position included between 40 ° and 30 ° before the ang
  • FIG. 1 is a simplified view in section on a diametrical plane the cylinder, a two-stroke direct injection engine produced according to the invention
  • - Figure 2 is a simpli sectional view along the line II-II of Figure 1
  • FIG. 3 is a view obtained by numerical simulation representing the gas flow lines in a two-stroke engine
  • - Figures 4 to 6 show the propagation of the fuel jet and the evolution of the region where a substantially stoichiometric mixture is obtained in an engine produced according to the invention between the start of injection and the time of ignition.
  • identical references have been kept to designate identical or similar elements.
  • FIG. 1 a section of a single-cylinder two-stroke engine provided with a direct injection system.
  • the structure of this engine, excluding the injection device, is known and in all respects similar to the structure of a two-stroke carburetor engine produced in large series today.
  • This structure includes a pump casing 2, inside which a crankshaft 3 is rotatably mounted.
  • the crankshaft 3 is connected to a piston 4 by means of a connecting rod 5.
  • the piston 4 has a bottom 4a, a head 4b provided with sealing segments and a skirt 4c.
  • the bottom 4a of the piston may be flat as in the mode of shown or slightly curved realization.
  • the piston 4 is movable in a cylinder 6 along the longitudinal axis X of the cylinder.
  • the wall 6a of the cylinder is provided with intake lights (7, 8) and an exhaust light 9. More particularly, the intake lights comprise a main light 7 disposed opposite the exhaust light 9 and four additional intake lights 8, called scanning lights, which are arranged on either side of the main intake light.
  • the intake and exhaust lights could have other known configurations, such as for example a single intake light, scanning lights 8 arranged in a non-symmetrical manner with respect to the main light 7 or even multiple exhaust lights 9.
  • the end of the cylinder 6 opposite the piston 4 is closed by a cylinder head 10, substantially hemispherical in the embodiment shown, and provided in known manner with a spark plug 11.
  • the bottom 4a of the piston, the internal wall 6a of the cylinder and the internal face of the cylinder head 10 delimit the combustion chamber 12 of the engine.
  • Fresh gases are admitted into the pump casing 2 through an intake duct 15, in particular under the effect of the vacuum created therein when the piston 4 rises towards the cylinder head 10, that is to say during the admission / compression time.
  • the fresh gases contained in the pump casing 2 are transferred by a transfer channel 16 to the intake ports (7, 8).
  • the intake duct 15 can, in known manner, be fitted with non-return valves and / or be masked by the flanges of the crankshaft to prevent a backflow of fresh gases through the intake duct during the combustion time / exhaust.
  • the intake lights (7, 8) are located at. a longitudinal distance from the cylinder head 10 greater than the exhaust port 9, so that they are closed by the piston 4 before the exhaust port 9 during the intake / compression phase.
  • the exhaust port 9 is closed by the piston 4 from a certain angular position of the crankshaft, which is called the angular position of closing the exhaust port or even angle closing the exhaust. This angular position is precisely defined by the structure of the motor.
  • Two-stroke engines with such a structure are well known and can be produced in very large series at a particularly competitive price. Their displacement varies quite significantly depending on their use. For example, to motorize portable tools such as a chainsaw or a hand-held brushcutter, the displacement is generally between fifteen and forty cubic centimeters while to motorize a two-wheeled vehicle of the moped, motorcycle or recreational vehicle type, the displacement generally varies between 50 cm 3 and 400 cm 3 . However, the total displacement of the engine can be even greater in the case of a multi-cylinder engine.
  • a first diametrical plane of the combustion chamber is defined, which comprises the longitudinal axis X of the cylinder and which is centered on the exhaust port 9.
  • the first diametrical plane must be centered on a fictitious light having a geometric surface equivalent to all of the surfaces of the exhaust lights.
  • This first diametral plane corresponds to the section plane of the embodiment shown in FIG. 1 and its trace (Pl-Pl) is visible in FIG. 2.
  • a second diametral plane is also defined which is perpendicular to the first diametral plane (Pl- Pl) and the trace of which (P2-P2) is visible in FIGS. 1 and 2.
  • the second diametral plane (P2-P2) delimits a first portion of the internal face of the cylinder head 10, including the second diametral plane, which s extends towards the main intake lumen 7.
  • the spark plug 11 is arranged in this first portion of the cylinder head, that is to say that the spark plug well must open into this region, either at an angle with the longitudinal axis X as in the embodiment shown, either by being collinear or coincident with the longitudinal axis X.
  • the engine 1 is equipped with an injection device comprising an injector 20 adapted to spray liquid and pressurized fuel in the room re combustion 12 along a spray axis P.
  • the injector 20 is arranged in a second portion of the cylinder head complementary to the first portion of the cylinder head, that is to say that the spray end of one injector 20 opens into the second portion of the internal face of the cylinder head. More particularly, as is apparent from FIGS.
  • the injector 20 is arranged in the cylinder head at the level of the first diametrical plane (Pl-Pl) centered on the exhaust, to allow its mounting in a small displacement engine.
  • the spray axis P defined by the axis of symmetry of the fuel jet created by the injector, forms a first angle ⁇ which is measured from a transverse plane (TT) of the cylinder, that is to say perpendicular to the longitudinal axis X.
  • TT transverse plane
  • the spray axis P also forms a second angle ⁇ , visible in FIG.
  • the spray axis P having the first and second angles ⁇ , ⁇ , included in these values is generally directed towards the half-portion of the cylinder opposite the exhaust port.
  • the fuel jet has in the embodiment shown a conical shape with a symmetry of revolution about the axis P, but it is possible to use a fuel jet of more complex shape, such as for example a jet having a cross section transverse oval.
  • the opening angle ⁇ of the fuel jet which is defined by two opposite edges of the droplet jet, must be between 15 and 75 ° in order to be able to be directed towards a relatively localized area of the combustion chamber, and above all, so that the droplets do not directly reach the walls of the combustion chamber, which would have a very unfavorable effect on pollutant emissions.
  • the injection device of course comprises a control device, not shown, which makes it possible to control the time when the injection must start as well as the duration of the latter.
  • the control device is connected to means for determining the angular position of the crankshaft to send a signal to open the injector 20 at an appropriate time, and to means for determining the operating parameters of the engine, such as for example an engine speed sensor and / or a load sensor to determine the duration of the injection and, consequently, the quantity of fuel injected.
  • the control device acts on the injector 20 so that, at least for certain operating ranges, the injection of fuel begins when the crankshaft is in an angular position between 45 and 20 ° before the closed position of the exhaust port, and preferably in an angular position between 40 and 30 ° before the closing angle of the exhaust. This injection is relatively early, since it begins when the exhaust gases are still discharged to the exhaust port.
  • the injection pressure and the orientation of the spray axis P are preferably determined using a digital simulation of the circulation of gases in the combustion chamber during the intake / compression phase.
  • the opening angle ⁇ due to the compromise between the opening angle ⁇ and the injection pressure (momentum of the jet), the fuel particles near the cylinder head are not yet finely atomized and their kinetic energy is high, so that the gas flows flowing near the injector 20 have little effect on the propagation of the jet in the combustion chamber 12.
  • the second angle ⁇ of the spray axis P must be substantially zero.
  • the angle ⁇ should be more or less pronounced depending on the importance of this vortex movement, so that the propagation of the jet is as much as possible against the flow fresh gas.
  • the positive or negative value of the second angle ⁇ is of course determined as a function of the direction of rotation of the vortex movement of the gases.
  • the injection pressure must also be adapted as a function of the circulation of gases in the combustion chamber. This injection pressure must not be too high because it must be avoided the projection of fuel droplets directly on the wall 6a of the cylinder or on the bottom 4a of the piston. However, this pressure must be high enough for the fuel droplets to reach a region where they will meet flows of gas circulating against the current and not to be entrained towards the exhaust port 9 by flows of gas circulating along of the cylinder head wall 10.
  • the suitable injection pressure can be determined using a gas velocity diagram, which is also obtained by numerical simulation.
  • This velocity diagram consists of vectors oriented along the current lines and of more or less important length as a function of the speed of the gases at the point considered.
  • the jet has a frustoconical shape which has a symmetry of revolution around the axis P.
  • the opening angle ⁇ of the jet is approximately 50 °.
  • the curves 23 indicate the outline of the zones of the combustion chamber where there are different values of ⁇ (Lambda), ⁇ being defined by the ratio between the proportion of air and fuel actually present, and the proportion of air and theoretical fuel which is necessary to have a stoichiometric mixture.
  • ⁇ Limbda
  • a mixture of air and fuel is said to be stoichiometric when the oxidation of the C-H chains is ideally consumed at one hundred percent.
  • a region of the combustion chamber where there is a value ⁇ equal to 1 therefore means that the air / fuel mixture is stoichiometric therein.
  • the region defined by the contours 23 is slightly offset towards the exhaust port relative to the spray axis P, but however, this portion of mixture is not entrained up to the exhaust port 9
  • the large amount of movement of the injected fuel drives it towards the half-portion of the cylinder located on the side of the intake ports (7,8).
  • the situation shown in Figure 5 corresponds to around the time when the exhaust light is closed, about forty degrees after Figure 4. In this situation, the amount of movement of the fuel is canceled out by the amount of movement of the fresh gases, the movement continues although the intake lights are closed. Note that the fuel injection can be extended after closing the exhaust port 9.
  • the injection pressure varies as a function of the speed or of the engine load, or as a function of these two parameters. This variation of the injection pressure can be controlled in a known manner by the injection device.
  • the injection device can be connected to an engine speed sensor and an intake gas throttle opening sensor, and include pressure regulation means prevailing in a pressurized fuel accumulator.
  • pressure regulation means prevailing in a pressurized fuel accumulator.
  • the injection control device is also suitable for controlling the injection duration so as to inject only the quantity of fuel required.
  • the injection pressure can vary continuously over the entire operating range of the engine, or according to different discrete values according to a speed / load map of the engine.
  • small-displacement engines that is to say with a displacement of approximately 125 cm 3 or less, it is possible to adopt a constant injection pressure over the entire operating range. engine while having a significant reduction in pollutant emissions and consumption.
  • the embodiment shown in the different figures corresponds to a two-stroke engine comprising a main intake light, four scanning lights and an exhaust light arranged symmetrically with respect to the first diametrical plane (Pl-Pl), but it will be clear to those skilled in the art that the injection device according to the invention can be adapted to a two-stroke engine comprising a different number of lights, or arranged in a non-symmetrical manner, as well as to a multi-cylinder two-stroke engine.

Abstract

The invention concerns a two-stroke engine comprising a combustion chamber (12), a cylinder (6) provided with an exhaust port (9) whereon is centered a first diametrical plane of the cylinder, a piston (4), a cylinder head (10) equipped with a spark plug (11) located on the side of the exhaust port relative to a second diametrical plane (P2-P2) perpendicular to the first, and an injector (20) adapted to inject a fuel jet into the combustion chamber, which is located on the other side of the second diametrical plane, the jet injection axis (P) forming an angle alpha between 30 DEG and 70 DEG from a transverse plane (T-T) of the cylinder, and an angle beta between +45 DEG and -45 DEG from the first diametrical plane. The aperture angle gamma of the jet is between 10 DEG and 75 DEG , the fuel injection starts when the crankshaft (3) lies between 45 DEG and 20 DEG prior to the closure of the exhaust port (9), the pressure of the injection and orientation of the jet injection axis are determined based on the circulation of gases to obtain a stoichiometric air/fuel mixture in the region of the spark plug at the time of ignition.

Description

MOTEUR DEUX TEMPS A INJECTION DIRECTE DIRECT INJECTION TWO-STROKE ENGINE
La présente invention se rapporte à un moteur deux temps à injection directe de carburant liquide. Plus particulièrement, l'invention se rapporte à un moteur deux temps à injection directe comprenant une chambre de combustion délimitée par : un cylindre présentant un axe longitudinal, qui est muni d'au moins une lumière d'admission et d'au moins une lumière d'échappement ; un piston présentant un fond sensiblement plat et déplacé selon l'axe longitudinal par une bielle reliée à un vilebrequin; une culasse munie d'une bougie et d'un injecteur adapté pour pulvériser un jet de carburant sous pression dans la chambre de combustion selon un axe de pulvérisation, la chambre de combustion présentant un premier plan diamétral comprenant l'axe longitudinal du cylindre et centré sur la lumière d'échappement et un deuxième plan diamétral perpendiculaire au dit premier plan diamétral, la bougie étant agencée dans une première portion de la culasse s ' étendant depuis le deuxième plan diamétral vers la lumière d'admission, l' injecteur étant agencé sur une deuxième portion de la culasse complémentaire à la première portion, et l'axe de pulvérisation formant un premier angle α mesuré à partir d'un plan transversal du cylindre qui est compris entre 30° et 70°, et un second angle β mesuré à partir du premier plan diamétral qui est compris entre + 45° et - 45° . Le cycle de fonctionnement des moteurs deux temps comprend pour chaque tour de vilebrequin un premier temps d'admission/compression et un deuxième temps de combustion/échappement . Au cours du premier temps, le piston effectue un mouvement de translation depuis une position de point mort bas jusqu'à à une position de point mort haut en fermant successivement les lumières d'admission et d'échappement du cylindre. Des gaz frais comprimés dans le carter du vilebrequin sont alors admis, via un canal de transfert, dans la chambre de combustion par les lumières d'admission jusqu'à ce que celles-ci soient fermées par le piston. Les gaz frais admis dans la chambre de combustion sont ensuite comprimés jusqu'à ce que le piston atteigne le point mort haut, tandis que des gaz frais sont aspirés dans le carter du vilebrequin. Au cours du temps de combustion/échappement, le piston effectue un mouvement de translation du point mort haut au point mort bas en démasquant successivement les lumières d'échappement et d'admission. L'allumage est provoqué par la bougie lorsque le piston est approximativement en sa position de point mort haut. Le mélange gazeux est alors enflammé et le piston est repoussé vers le point mort bas sous l'effet de la pression régnant dans la chambre de combustion. Lors du mouvement du piston vers le point mort bas, les gaz frais contenus dans le carter du vilebrequin sont comprimés et les gaz brûlés contenus dans la chambre de combustion s ' échappent par la lumière d'échappement à partir du moment où celle-ci est démasquée par le piston. Ce type de moteur a l'avantage d'offrir une puissance relativement élevée par rapport à un moteur quatre temps de poids similaire, du fait de l'existence d'un temps moteur pour chaque tour de vilebrequin. De plus, son coût de fabrication est particulièrement bas, car le nombre de pièces est inférieur à celui d ' un moteur quatre temps . Toutefois, ce type de moteur présente généralement l'inconvénient d'une consommation de carburant élevée et d'une émission importante de polluants par rapport à un moteur quatre temps . Ceci est dû à la concomitance des phases d'admission et de compression, ainsi que de la phase d'échappement. Au cours du balayage de la chambre de combustion par les gaz frais, des gaz chargés de carburant peuvent passer par l'échappement. Pour pallier ces inconvénients, il est possible d'effectuer une injection directe du carburant dans la chambre de combustion, comme par exemple décrit dans la demande de brevet WO-A-02/086310. Dans certains moteurs deux: temps à injection directe, on a cherché à améliorer l'injection du carburant en assistant celle-ci avec l'injection d'air comprimé. Toutefois, cela diminue la quantité de mouvement du jet injecté et augmente la complexité du système d'injection, ainsi que son coût de fabrication. Dans le but de réduire la consommation de carburant, certains moteurs deux temps à injection directe ont également été conçus de manière à fonctionner en mélange pauvre grâce à une stratification de la richesse du mélange gazeux. Mais ce résultat a été obtenu en modifiant de manière importante la forme du piston et de la culasse et, par conséquent, ce type de solution est plus difficilement applicable aux moteurs deux temps actuellement produits en grande série. De plus, un fonctionnement en mélange pauvre favorise la création de certains polluants, comme par exemple les NOx, ce qui ne permet pas de respecter les normes antipollution en vigueur sans utiliser un système de dépollution complexe. La présente invention a pour- but d'améliorer les moteurs deux temps à injection directe, afin notamment de satisfaire les normes antipollution en vigueur et à venir, et ce, en modifiant de manière minimale la géométrie de la chambre de combustion pour que la présente invention puisse être applicable aux moteurs existants. A cet effet, la présente invention a pour objet un moteur du type précité, caractérisé en ce que l'angle d'ouverture γ du jet de carburant est compris entre 15° et 75°, en ce que l'injection du carburant débute lorsque le vilebrequin est situé dans une position angulaire comprise entre 45° et 20° avant la position angulaire de fermeture de la lumière d'échappement, et en ce que la pression d'injection du carburant et l'orientation de l'axe de pulvérisation sont déterminées en fonction de la circulation des gaz dans la chambre de combustion pour obtenir un mélange airr/carburant sensiblement stoechiométrique dans la région de la bougie au moment de l'allumage. L'angle d'ouverture γ du jet limité à 75° permet de former les gouttelettes de carburant dans une zone limitée de la chambre de combustion où les gaz ont un profil de vitesse particulier, et évite surtout la pulvérisation de gouttelettes contre les parois de la chambre de combustion, ce qui augmenterait les émissions de polluants. Le fait de commencer l'injection du carburant au moins 20° avant la fermeture des lumières d'échappement, c'est-à-dire de manière anticipée par rapport aux dispositifs d'injection directe antérieurs dans lesquels l'injection débute généralement lorsque la lumière d'échappement est fermée pour éviter le passage de gouttelettes de carburant vers l'échappement, aigmente le temps de brassage des gouttelettes de carburant avec les gaz frais et la vaporisation du carburant, de manière à obtenir un mélange air/combustible plus homogène au moment de l'allumage. L'orientation de l'axe de pulvérisation compris dans les valeurs du premier angle α et du second angle β mentionnées, réduit le passage de carburant par la lumière d'échappement lors de la phase de compression, et ce malgré l'injection précoce du carburant. Enfin, en adaptant la pression d'injection du carburant et l'orientation de l'axe de pulvérisation dans les plages angulaires mentionnées, il est possible d'obtenir un mélange air/carburant stoechiométrique dans la région de la bougie au moment de l'allumage. Cette adaptation doit être effectuée en fonction de la circulation des gaz dans la chambre de combustion qui peut être déterminée par simulation numérique. A partir du profil des lignes de courant des gaz dans la chambre de combustion, qui est sensiblement constant au cours de la phase d'admission/compression, il est possible d'adapter l'orientation de l'axe de pulvérisation de manière à ce que les gouttelettes de carburant pulvérisées par l' injecteur rencontrent des gaz circulant à contre-courant de celles- ci. De plus, en adaptant la pression d'injection du carburant, on peut modifier la quantité de mouvement du carburant injecté de manière à ce que la quantité de mouvement des gaz frais, circulant en sens sensiblement inverse, provoque un arrêt, et même un recul des gouttelettes et de la vapeur de carburant, pour obtenir un mélange stoechiométrique à proximité de la bougie lors de 1 'allumage. Les essais effectués en appliquant l'ensemble de ces dispositions ont permis de constater une diminution très significative des émissions polluantes, de sorte que les moteurs deux temps ainsi réalisés satisfont les normes d' antipollution actuellement en vigueur, mais aussi les futures normes connues à ce jour, sans nécessiter de système de dépollution complexe et onéreux. De plus, on a également constaté une réduction extrêmement importante de la consommation de carburant, de l'ordre de 30 % par rapport à un moteur identique alimenté par un carburateur, ce qui est bien supérieur à la réduction de consommation attendue, étant donné qu'il ne s'agit pas d'un moteur à charge stratifiée fonctionnant en mélange pauvre. On notera que ces dispositions peuvent s'appliquer à la plupart des moteurs deux temps existants alimentés par carburateur, puisque pour les mettre en œuvre, il suffit de réaliser un alésage pour l' injecteur dans la culasse, mais que les géométries du piston, du cylindre et de la culasse, n'ont pas à être modifiées. Dans des formes de réalisations préfé_τées de l'invention, on a recours, en outre, à l'une et/ou à l'autre des dispositions suivantes : la pression d'injection du carburant est variable en fonction du régime moteur et/ou de la charge du moteur, afin d'obtenir une réduction optimale des émissions de polluants sur l'ensemble de la plage de fonctionnement du moteur ; la pression d'injection du carburant est comprise entre 50 et 150 bars ; la pression d'injection du carburant est ajustée à différentes valeurs selon une cartographie régime/charge du moteur ; la pression d'injection du carburant est constante sur l'ensemble de la plage de fonctionnement du moteur, le moteur présentant de préférence une cylindrée au plus égale à 125 cm3, pour réduire les émissions de polluants avec un système d'injection relativement simple ; l' injecteur est disposé dans un alésage de la culasse orienté selon un axe donné et dans lequel l'axe de pulvérisation forme un angle δ non nul avec ledit axe de l'alésage ; l' injecteur est agencé à travers la culasse au niveau du premier plan diamétral, ce qui permet son montage dans un moteur de petite cylindrée ; l'injection du carburant commence lorsque le vilebrequin est situé dans une position angulaire comprise entre 40° et 30° avant la position angulaire de fermeture de la lumière d'échappement. D'autres caractéristiques et avantages de 1 ' invention apparaîtront au cours de la description qui va suivre, donnée à titre d'exemple non limitatif, en référence aux dessins annexés dans lesquels : la figure 1 est une vue simplifiée en coupe selon un plan diamétral du cylindre, d'un moteur deux temps à injection directe réalisé selon l'invention ; - la figure 2 est une vue en coupe simpli iée selon la ligne II-II de la figure 1 ; la figure 3 est une vue obtenue par simulation numérique représentant les lignes de flux de gaz dans un moteur deux temps ; - les figures 4 à 6 représentent la propagation du jet de carburant et l'évolution de la région où l'on obtient un mélange sensiblement stoechiométrique dans un moteur réalisé selon 1 ' invention entre le début de l'injection et le moment de l'allumage. Sur les différentes figures, on a conservé des références identiques pour désigner des éléments identiques ou similaires. A la figure 1 est représentée une coupe d'un moteur deux temps monocylindre muni d'un système d'injection directe. La structure de ce moteur, à l'exclusion du dispositif d'injection, est connue et en tous points semblables à la structure de moteur deux temps à carburateur produit en grande série actuellement. Cette structure comprend un carter pompe 2 , à l'intérieur duquel un vilebrequin 3 est monté rotatif. Le vilebrequin 3 est relié à un piston 4 par l'intermédiaire d'une bielle 5. Le piston 4 présente un fond 4a, une tête 4b munie de segments d'étanchéité et une jupe 4c. Le fond 4a du piston peut être plat comme dans le mode de réalisation représenté ou légèrement bombé. On notera qu'il s'agit d'un piston de forme tout à fait standard, et non pas d'un piston présentant des reliefs ou des cavités prononcés sur son fond, comme dans certains moteurs deux temps expérimentaux qui sont destinés à fonctionner en mélange pauvre. Le piston 4 est mobile dans un cylindre 6 selon l'axe longitudinal X du cylindre. La paroi 6a du cylindre est munie de lumières d'admission (7, 8) et d'une lumière d'échappement 9. Plus particulièrement, les lumières d'admission comprennent une lumière principale 7 disposée en regard de la lumière d'échappement 9 et quatre lumières d'admission supplémentaires 8, appelées lumières de balayage qui sont disposées de part et d'autre de la lumière d'admission principale. Toutefois, les lumières d'admission et d'échappement pourraient présenter d'autres configurations connues, comme par exemple une lumière d'admission unique, des lumières de balayage 8 disposées de manière non symétrique par rapport à la lumière principale 7 ou encore de multiples lumières d'échappement 9. L'extrémité du cylindre 6 opposée au piston 4 est fermée par une culasse 10, sensiblement hémisphérique dans le mode de réalisation représenté, et munie de manière connue d'une bougie d'allumage 11. Le fond 4a du piston, la paroi interne 6a du cylindre et la face interne de la culasse 10 délimitent la chambre de combustion 12 du moteur. Les gaz frais sont admis dans le carter pompe 2 par un conduit d'admission 15, notamment sous l'effet de la dépression créée dans celui-ci lorsque le piston 4 remonte vers la culasse 10, c'est-à-dire lors du temps d'admission/compression. Lorsque le piston 4 descend vers le vilebrequin 3 au cours de la phase de combustion/échappement, les gaz frais contenus dans le carter pompe 2 sont transférés par un canal de transfert 16 vers les lumières d'admission (7, 8) . Le conduit d'admission 15 peut, de manière connue, être équipé de clapets anti-retour et/ou être masqué par les flasques du vilebrequin pour éviter un reflux des gaz frais à travers le conduit d'admission au cours du temps de combustion/échappement . Les lumières d'admission (7, 8) sont situées à. une distance longitudinale de la culasse 10 plus importante que la lumière d'échappement 9, de sorte qu'elles sont ferrnées par le piston 4 avant la lumière d'échappement 9 au cours de la phase d'admission/compression. Au cours de la phase d'admission/compression, la lumière d'échappement 9 est fermée par le piston 4 à partir d'une certaine position angulaire du vilebrequin, qui est appelée position angulaire de fermeture de la lumière d'échappement ou encore angle de fermeture de l'échappement. Cette position angulaire est définie précisément par la structure du moteur. Les moteurs deux temps présentant une telle structure sont bien connus et peuvent être produits en très grande série à un prix particulièrement compétitif. Leur cylindrée varie de manière assez importante en fonction de leur utilisation. Par exemple, pour motoriser un outillage portatif comme une tronçonneuse ou une débroussailleuse à main, la cylindrée est généralement comprise entre une quinzaine et une quarantaine de centimètres cubes tandis que pour motoriser un véhicule à deux roues de type mobylette, moto ou engin de loisir, la cylindrée varie généralement entre 50 cm3 et 400 cm3. Toutefois, la cylindrée totale du moteur peut être encore plus importante dans le cas d'un moteur multi-cylindres . On définit un premier plan diamétral de la chambre de combustion, qui comprend l'axe longitudinal X du cylindre et qui est centré sur la lumière d'échappement 9. Si le cylindre 6 comprend plusieurs lumières d'échappement, le premier plan diamétral doit être centré sur une lumière fictive présentant une surface géométrique équivalente à l'ensemble des surfaces des lumières d'échappement. Ce premier plan diamétral correspond au plan de coupe du mode de réalisation représenté à la figure 1 et sa trace (Pl-Pl) est visible sur la figure 2. On définit également un deuxième plan diamétral qui est perpendiculaire au premier plan diamétral (Pl-Pl) et dont la trace (P2-P2) est visible sur les figures 1 et 2. Le deuxième plan diamétral (P2-P2) délimite une première portion de la face interne de la culasse 10, incluant le deuxième plan diamétral, qui s'étend vers la lumière d'admission principale 7. La bougie 11 est agencée dans cette première portion de la culasse, c'est-à-dire que le puits de la bougie doit déboucher dans cette région, soit en formant un angle avec l'axe longitudinal X comme dans le mode de réalisation représenté, soit en étant colinéaire ou confondu avec l'axe longitudinal X. Le moteur 1 est équipé d'un dispositif ci ' injection comprenant un injecteur 20 adapté pour pulvériser un carburant liquide et sous pression dans la chambre de combustion 12 selon un axe de pulvérisation P. L' injecteur 20 est agencé dans une deuxième portion de la culasse complémentaire à la première portion de la culasse, c'est-à-dire que l'extrémité de pulvérisation de 1' injecteur 20 débouche dans la deuxième portion de la face interne de la culasse. Plus particulièrement, comme cela ressort des figures 1 et 2 , l' injecteur 20 est agencé dans la culasse au niveau du premier plan diamétral (Pl-Pl) centré sur l'échappement, pour permettre son montage dans un moteur de petite cylindrée. L'axe de pulvérisation P défini par l'axe de symétrie du jet de carburant créé par l' injecteur, forme un premier angle α qui est mesuré à partir d'un plan transversal (T-T) du cylindre, c'est-à-dire perpendicul ire à l'axe longitudinal X. La manière de déterminer précisément ce premier angle α en fonction de la géométrie de la chambre de combustion sera explicitée ci-après, mais il doit être compris entre 30° et 70° pour les moteurs deux temps les plus courants. L'axe de pulvérisation P forme également un second angle β, visible sur la figure 2, qui est mesuré à pa tir du premier plan diamétral (Pl-Pl) centré sur la lumière d'échappement 9. Cet angle doit être compris entre + 45° et - 45°, la manière de déterminer sa valeur précise étant également explicitée ci-après. L'axe de pulvérisation P présentant les premier et second angles α, β, compris dans ces valeurs est globalement dirigé vers la demi-portion du cylindre opposée à la lumière d'échappement. Le jet de carburant présente dans le mode de réalisation représenté une forme conique avec une symétrie de révolution autour de l'axe P, mais il est possible d'utiliser un jet de carburant de forme plus complexe, comme par exemple un jet présentant une section transversale ovale. Toutefois, l'angle d'ouverture γ du jet de carburant qui est défini par deux bords opposés du jet de gouttelettes, doit être compris entre 15 et 75° afin de pouvoir être dirigé vers une zone relativement localisée de la chambre de combustion, et surtout, afin que les gouttelettes n'atteignent pas directement les parois de la chambre de combustion, ce qui aurait un effet très défavorable sur les émissions de polluants. Le dispositif d'injection comprend bien entendu, un dispositif de commande, non représenté, qui permet de commander l'instant où doit débuter l'injection ainsi que la durée de celle-ci. Le dispositif de commande est relié à des moyens de détermination de la position angulaire du vilebrequin pour envoyer un signal d'ouverture de l' injecteur 20 à un instant approprié, et à des moyens de détermination des paramètres de fonctionnement du moteur- , comme par exemple un capteur de régime moteur et/ou un capteur de charge pour déterminer la durée de l'injection et, par conséquent, la quantité de carburant injecté. Le dispositif de commande agit sur l' injecteur 20 de manière à ce que, au moins pour certaines plages de fonctionnement, l'injection du carburant débute lorsque le vilebrequin est dans une position angulaire comprise entre 45 et 20° avant la position de fermeture de la lumière d'échappement, et de préférence dans une position angulaire comprise entre 40 et 30° avant l'angle de fermeture de l'échappement. Cette injection est relativement précoce, puisqu'elle commence lorsque les gaz d'échappement sont encore évacués vers la lumière d'échappement. Elle est notamment plus précoce que dans la plupart des systèmes à injection directe connus où l'on cherche à retarder le début de l'injection pour éviter qu'une partie du mélange de gaz et de carburant non brûlé ne passe par la lumière d' échappement . Toutefois, en adaptant la pression d'injection du carburant et l'orientation de l'axe de pulvérisation P dans les plages angulaires précédemment indiquées, et de la manière explicitée ci-après, on évite un passage de gaz non brûlés par la lumière d'échappement. L'injection anticipée permet d'injecter une quantité de carburant plus importante au cours du cycle , par conséquent elle est particulièrement avantageuse lorsque le moteur fonctionne à pleine charge et à haut régime. Mais l'invention n'exclue pas la possibilité de débuter l'injection plus tardivement, après la fermeture de l'échappement, pour certaines plages de fonctionnement du moteur. Pour adapter la pression d'injection de carburant et l'orientation de l'axe de pulvérisation P de manière correcte, il faut que celles-ci soient déterminées en fonction de la circulation des gaz dans la chambre de combustion, de manière à obtenir un mélange stoechiométrique de gaz dans la région de la bougie 11 au moment de l'allumage. L'allumage provoqué par une étincelle entre les électrodes lia de la bougie est effectué de manière habituelle, lorsque le vilebrequin est dans une position angulaire inférieure de quelques degrés à la position du point mort haut du piston, cette avance à l'allumage pouvant être plus ou moins importante en fonction du régime de rotation du moteur ou de sa charge. La pression d'injection et l'orientation de l'axe de pulvérisation P sont préférentiellement déterminées à l'aide d'une simulation numérique de la circulation des gaz dans la chambre de combustion lors de la phase d'admission/compression. La simulation numérique permet de connaître de manière précise le trajet des flux de gaz dans la chambre de combustion, comme le montre la figure 3 qui représente les lignes de courant des gaz lorsque le piston est dans une position encore relativement basse. Toutefois, il faut noter que du fait de l'énergie cinétique importante des gaz entrant dans la chambre combustion, la forme de ces lignes de courant ne change pas de manière sensible au cours de toute la phase d'admission/compression. Comme on peut le voir sur la figure 3, les gaz effectuent essentiellement un mouvement de rouleau (appelé tumble) , c'est-à-dire un mouvement de rotation autour d'un axe parallèle à l'axe du vilebrequin. Ceci est dû à la position diamétralement opposée de la lumière principale d'admission 7 et de la lumière d'échappement 9, ainsi qu'à la disposition symétrique des lumières de balayage 8 de part et d'autre de la lumière d'admission principale. Mais dans le cas de lumières disposées non symétriquement par rapport au premier plan diamétral (Pl-Pl) , on introduit une composante de mouvement de tourbillon (appelé swirl) , c'est-à-dire un mouvement partiel de rotation autour de l'axe longitudinal X du cylindre. L'angle α de l'axe de pulvérisation P est ajusté dans la plage de 30 à 70°, de manière à ce que le jet de carburant soit projeté à contre-courant de flux de gaz frais provenant des lumières d'admission (7, 8) . On notera que la base du jet de carburant traverse des flux de gaz circulant à proximité immédiate du débouché de l' injecteur 20 sur la face interne de la culasse 10. Toutefois, du fait du compromis entre l'angle d'ouverture γ et la pression d'injection (quantité de mouvement du jet), les particules de carburant à proximité de la culasse ne sont pas encore finement pulvérisées et leur énergie cinétique est importante, de sorte que les flux de gaz s ' écoulant à proximité de l' injecteur 20 affectent peu la propagation du jet dans la chambre de combustion 12. Dans le mode de réalisation représenté, il n'y a pas de phénomène de tourbillon autour de l'axe X de la chambre de combustion, et par conséquent, le second angle β de l'axe de pulvérisation P doit être sensiblement nul. Par contre, en présence d'un mouvement tourbillonnaire, l'angle β devrait être plus ou moins prononcé en fonction de l'importance ce mouvement tourbillonnaire, de manière à ce que la propagation du jet soit le plus possible à contre- courant des flux de gaz frais. La valeur positive ou négative du second angle β est bien entendu déterminée en fonction du sens de rotation du mouvement tourbillonnaire des gaz. Par ailleurs, la pression d'injection doit également être adaptée en fonction de la circulation des gaz dans la chambre de combustion. Cette pression d'injection ne doit pas être trop élevée car il faut éviter la projection de gouttelettes de carburant directement sur la paroi 6a du cylindre ou sur le fond 4a du piston. Mais, cette pression doit être suffisamment élevée pour que les gouttelettes de carburant atteignent une région où elles rencontreront des flux de gaz circulant à contre-courant et pour ne pas être entraînées vers la lumière d'échappement 9 par des flux de gaz circulant le long de la paroi de la culasse 10. La pression d'injection adaptée peut être déterminée à l'aide d'un diagramme des vitesses des gaz, qui est également obtenu par simulation numérique. Ce diagramme des vitesses, non représenté, est constitué de vecteurs orientés selon les lignes de courant et de longueur plus ou moins importants en fonction de la vitesse des gaz au point considéré. Une fois l'orientation de l'axe de pulvérisation P déterminé, il est possible de déterminer la pression d'injection de manière à ce que la quantité de mouvement des gouttelettes de carburant dans la région de diffusion du jet soit sensiblement égale, inférieure ou légèrement supérieure, à la quantité de mouvement des gaz circulant à contre-courant selon le profil de mélange désiré. Bien que l'inventeur ait adapté l'axe de pulvérisation P et la pression d'injection à l'aide de la simulation numérique, il est envisageable de déterminer ces paramètres dans les plages de valeurs indiquées à l'aide d'essais et des connaissances empiriques que l'homme du métier peut avoir sur la circulation des gaz en fonction de la géométrie de la chambre de combustion. Toutefois, il est important que l'injection débute de manière précoce, c'est- à-dire entre 45 et 20° avant l'angle de fermeture de l'échappement, et de préférence entre 40 et 30° avant cet angle de fermeture . En adaptant ainsi l'axe de pulvérisation P et la pression d'injection, on obtient la propagation du jet de carburant et l'évolution de la région présentant un mélange air/carburant sensiblement stoechiométrique, qui est représenté aux figures 4 à 6, au cours de la phase de compression. Le terme air désigne les gaz frais aspirés au cours de la phase d'admission, mais aussi un éventuel reliquat de gaz brûlés lors du précédent cycle ou des gaz d'échappement réintroduit dans la chambre par un système EGR. A la figure 4, on peut voir la propagation du jet de carburant peu après le début de l'injection qui a commencé 40° avant l'angle de fermeture de l'échappement pour le mode de réalisation représenté. Le jet a une forme tronconique qui présente une symétrie de révolution autour de l'axe P. L'angle d'ouverture γ du jet est d'environ 50°. Les courbes 23 indiquent le contour des zones de la chambre de combustion où l'on a différentes valeurs de λ (Lambda) , Λ étant défini par le rapport entre la proportion d'air et de carburant effectivement présente, et la proportion d'air et de carburant théorique qui est nécessaire pour avoir un mélange stoechiométrique. Un mélange d'air et de carburant est dit stoechiométrique lorsque l'oxydation des chaînes C-H est idéalement consommée à cent pour cent . Une région de la chambre de combustion où l'on trouve une valeur λ égale à 1 signifie donc que le mélange air/carburant est stoechiométrique dans celle-ci . On notera que la région définie par les contours 23 est légèrement décalée vers la lumière d'échappement par rapport à l'axe de pulvérisation P, mais toutefois, cette portion de mélange n'est pas entraînée jusqu'à la lumière d'échappement 9. En effet, comme on peut le voir à la figure 5, la quantité de mouvement importante du carburant injecté entraîne celui-ci vers la demi-portion du cylindre située du côté des lumières d'admission (7,8). La situation représentée à la figure 5 correspond à peu près au moment où l'on obtient la fermeture de la lumière d'échappement, soit environ une quarantaine de degrés après la figure 4. Dans cette situation, la quantité de mouvement du carburant est annulée par la quantité de mouvement des gaz frais dont le mouvement se poursuit bien que les lumières d'admission soient fermées. On notera que l'injection du carburant peut être prolongée après la fermeture de la lumière d'échappement 9. Lorsque le piston arrive à proximité du point mort haut, la portion de la chambre de combustion où règne un mélange stoechiométrique a évolué de manière à occuper la région de la bougie, comme on peut le voir sur la figure 6 où les électrodes lia de la bougie sont représentées symboliquement. Cette situation est obtenue grâce au mouvement de retour vers l' injecteur 20 du carburant sous l'effet de l'énergie cinétique des gaz admis. Dans cette situation, le carburant est vaporisé et forme un mélange stoechiométrique avec les gaz frais. L'allumage est alors commandé en créant une étincelle entre les électrodes lia de la bougie qui enflamme le mélange stoechiométrique . On notera que le mélange stoechiométrique occupe la majeure partie de la chambre de combustion. Seule, une faible portion de la chambre de combustion, située du côté de la lumière de échappement, contient un mélange pauvre, ce qui garantit un déroulement régulier de la combustion. L'injection directe ainsi réalisée permet de réduire considérablement les émissions de polluants d'un moteur deux temps de type existant, et notamment de satisfaire les futures normes antipollution. Par ailleurs, les essais effectués avec des moteurs deux temps mettant en œuvre l'injection directe selonThe present invention relates to a two-stroke engine with direct injection of liquid fuel. More particularly, the invention relates to a two-stroke engine with direct injection comprising a combustion chamber delimited by: a cylinder having a longitudinal axis, which is provided with at least one intake lumen and at least one lumen exhaust; a piston having a substantially flat bottom and displaced along the longitudinal axis by a connecting rod connected to a crankshaft; a cylinder head fitted with a spark plug and an injector adapted to spray a jet of pressurized fuel into the combustion chamber along a spraying axis, the combustion chamber having a first diametrical plane comprising the longitudinal axis of the cylinder and centered on the exhaust port and a second diametral plane perpendicular to said first diametrical plane, the spark plug being arranged in a first portion of the cylinder head extending from the second diametrical plane towards the intake port, the injector being arranged on a second portion of the cylinder head complementary to the first portion, and the spray axis forming a first angle α measured from a transverse plane of the cylinder which is between 30 ° and 70 °, and a second angle β measured at from the first diametrical plane which is between + 45 ° and - 45 °. The operating cycle of two-stroke engines includes, for each crankshaft revolution, a first intake / compression time and a second combustion / exhaust time. During the first time, the piston performs a translational movement from a low neutral position to a high neutral position by successively closing the intake and exhaust ports of the cylinder. Fresh gases compressed in the crankcase are then admitted, via a transfer channel, into the combustion chamber through the intake ports until they are closed by the piston. The fresh gases admitted into the combustion chamber are then compressed until the piston reaches top dead center, while fresh gases are drawn into the crankcase. During the combustion / exhaust time, the piston translates from top dead center to bottom dead center by successively unmasking the exhaust and intake ports. The ignition is caused by the spark plug when the piston is approximately in its top dead center position. The gas mixture is then ignited and the piston is pushed back to bottom dead center under the effect of the pressure prevailing in the combustion chamber. During the movement of the piston towards bottom dead center, the fresh gases contained in the crankshaft casing are compressed and the burnt gases contained in the combustion chamber escape through the exhaust port from the moment when this is unmasked by the piston. This type of engine has the advantage of offering a relatively high power compared to a four-stroke engine of similar weight, due to the existence of an engine time for each revolution of the crankshaft. In addition, its manufacturing cost is particularly low, because the number of parts is less than that of a four-stroke engine. However, this type of engine generally has the disadvantage of a high fuel consumption and a significant emission of pollutants compared to a four-stroke engine. This is due to the concomitance of the intake and compression phases, as well as the exhaust phase. During the sweeping of the combustion chamber by the fresh gases, gases loaded with fuel can pass through the exhaust. To overcome these drawbacks, it is possible to inject fuel directly into the combustion chamber, as for example described in patent application WO-A-02/086310. In some two-stroke direct injection engines, attempts have been made to improve fuel injection by assisting it with the injection of compressed air. However, this reduces the amount of movement of the injected jet and increases the complexity of the injection system, as well as its manufacturing cost. In order to reduce fuel consumption, some two-stroke direct injection engines have also been designed to operate in a lean mixture by stratifying the richness of the gas mixture. However, this result has been obtained by significantly modifying the shape of the piston and of the cylinder head and, consequently, this type of solution is more difficult to apply to two-stroke engines currently produced in large series. In addition, a lean mixture operation promotes the creation of certain pollutants, such as NOx for example, which does not make it possible to comply with the anti-pollution standards in force without using a complex pollution control system. The object of the present invention is to improve two-stroke direct injection engines, in particular in order to satisfy the anti-pollution standards in force and to come, and this, by minimally modifying the geometry of the combustion chamber so that the present invention can be applicable to existing engines. To this end, the subject of the present invention is a  engine of the aforementioned type, characterized in that the opening angle γ of the fuel jet is between 15 ° and 75 °, in that the injection of fuel begins when the crankshaft is located in an angular position between 45 ° and 20 ° before the angular closing position of the exhaust port, and in that the fuel injection pressure and the orientation of the spray axis are determined as a function of the gas circulation in the chamber combustion to obtain a substantially stoichiometric air / fuel mixture in the region of the spark plug at the time of ignition. The opening angle γ of the jet limited to 75 ° makes it possible to form the fuel droplets in a limited area of the combustion chamber where the gases have a particular speed profile, and above all avoids the spraying of droplets against the walls of the combustion chamber, which would increase pollutant emissions. The fact of starting the injection of the fuel at least 20 ° before the closing of the exhaust lights, that is to say in advance compared to the prior direct injection devices in which the injection generally begins when the exhaust light is closed to prevent the passage of fuel droplets towards the exhaust, increases the time the fuel droplets mix with the fresh gases and the vaporization of the fuel, so as to obtain a more homogeneous air / fuel mixture time of ignition. The orientation of the spray axis included in the values of the first angle α and the second angle β mentioned, reduces the passage of fuel through the exhaust port during the compression phase, despite the early injection of the fuel.  Finally, by adapting the fuel injection pressure and the orientation of the spray axis within the angular ranges mentioned, it is possible to obtain a stoichiometric air / fuel mixture in the region of the spark plug at the time of ignition. This adaptation must be made as a function of the gas circulation in the combustion chamber which can be determined by numerical simulation. From the profile of the gas stream lines in the combustion chamber, which is substantially constant during the intake / compression phase, it is possible to adapt the orientation of the spray axis so that that the fuel droplets sprayed by the injector meet gases flowing against them. In addition, by adapting the fuel injection pressure, it is possible to modify the momentum of the fuel injected so that the momentum of the fresh gases, circulating in substantially opposite directions, causes a stop, and even a decline. droplets and fuel vapor, to obtain a stoichiometric mixture near the spark plug during ignition. Tests carried out by applying all of these provisions have shown a very significant reduction in polluting emissions, so that the two-stroke engines thus produced meet the emission standards currently in force, but also the future standards known for this purpose. day, without the need for a complex and expensive depollution system. In addition, there has also been an extremely significant reduction in fuel consumption, of the order of 30% compared to an identical engine powered by a carburetor, which is much greater than the reduction in consumption expected, given that 'he ... not  it is not a stratified charge engine operating in a lean mixture. It will be noted that these provisions can apply to most of the existing two-stroke engines powered by carburetor, since to implement them, it suffices to make a bore for the injector in the cylinder head, but that the geometries of the piston, of the cylinder and cylinder head, do not have to be modified. In preferred embodiments of the invention, use is made of one and / or the other of the following arrangements: the fuel injection pressure is variable depending on the engine speed and / or engine load, in order to obtain an optimal reduction in pollutant emissions over the entire engine operating range; the fuel injection pressure is between 50 and 150 bars; the fuel injection pressure is adjusted to different values according to an engine speed / load map; the fuel injection pressure is constant over the entire operating range of the engine, the engine preferably having a displacement of at most equal to 125 cm3, to reduce pollutant emissions with a relatively simple injection system; the injector is disposed in a bore of the cylinder head oriented along a given axis and in which the spray axis forms a non-zero angle δ with said axis of the bore; the injector is arranged through the cylinder head at the level of the first diametrical plane, which allows it to be mounted in a small displacement engine; fuel injection begins when the crankshaft is located in an angular position included between 40 ° and 30 ° before the angular position of closing the exhaust port. Other characteristics and advantages of the invention will become apparent during the description which follows, given by way of nonlimiting example, with reference to the appended drawings in which: FIG. 1 is a simplified view in section on a diametrical plane the cylinder, a two-stroke direct injection engine produced according to the invention; - Figure 2 is a simpli sectional view along the line II-II of Figure 1; FIG. 3 is a view obtained by numerical simulation representing the gas flow lines in a two-stroke engine; - Figures 4 to 6 show the propagation of the fuel jet and the evolution of the region where a substantially stoichiometric mixture is obtained in an engine produced according to the invention between the start of injection and the time of ignition. In the various figures, identical references have been kept to designate identical or similar elements. In Figure 1 is shown a section of a single-cylinder two-stroke engine provided with a direct injection system. The structure of this engine, excluding the injection device, is known and in all respects similar to the structure of a two-stroke carburetor engine produced in large series today. This structure includes a pump casing 2, inside which a crankshaft 3 is rotatably mounted. The crankshaft 3 is connected to a piston 4 by means of a connecting rod 5. The piston 4 has a bottom 4a, a head 4b provided with sealing segments and a skirt 4c. The bottom 4a of the piston may be flat as in the mode of shown or slightly curved realization. It will be noted that this is a piston of quite standard shape, and not of a piston having reliefs or pronounced cavities on its bottom, as in certain two-stroke experimental engines which are intended to operate in poor mixture. The piston 4 is movable in a cylinder 6 along the longitudinal axis X of the cylinder. The wall 6a of the cylinder is provided with intake lights (7, 8) and an exhaust light 9. More particularly, the intake lights comprise a main light 7 disposed opposite the exhaust light 9 and four additional intake lights 8, called scanning lights, which are arranged on either side of the main intake light. However, the intake and exhaust lights could have other known configurations, such as for example a single intake light, scanning lights 8 arranged in a non-symmetrical manner with respect to the main light 7 or even multiple exhaust lights 9. The end of the cylinder 6 opposite the piston 4 is closed by a cylinder head 10, substantially hemispherical in the embodiment shown, and provided in known manner with a spark plug 11. The bottom 4a of the piston, the internal wall 6a of the cylinder and the internal face of the cylinder head 10 delimit the combustion chamber 12 of the engine. Fresh gases are admitted into the pump casing 2 through an intake duct 15, in particular under the effect of the vacuum created therein when the piston 4 rises towards the cylinder head 10, that is to say during the admission / compression time. When the piston 4 descends towards the crankshaft 3 during the combustion / exhaust phase, the fresh gases contained in the pump casing 2 are transferred by a transfer channel 16 to the intake ports (7, 8). The intake duct 15 can, in known manner, be fitted with non-return valves and / or be masked by the flanges of the crankshaft to prevent a backflow of fresh gases through the intake duct during the combustion time / exhaust. The intake lights (7, 8) are located at. a longitudinal distance from the cylinder head 10 greater than the exhaust port 9, so that they are closed by the piston 4 before the exhaust port 9 during the intake / compression phase. During the intake / compression phase, the exhaust port 9 is closed by the piston 4 from a certain angular position of the crankshaft, which is called the angular position of closing the exhaust port or even angle closing the exhaust. This angular position is precisely defined by the structure of the motor. Two-stroke engines with such a structure are well known and can be produced in very large series at a particularly competitive price. Their displacement varies quite significantly depending on their use. For example, to motorize portable tools such as a chainsaw or a hand-held brushcutter, the displacement is generally between fifteen and forty cubic centimeters while to motorize a two-wheeled vehicle of the moped, motorcycle or recreational vehicle type, the displacement generally varies between 50 cm3 and 400 cm3. However, the total displacement of the engine can be even greater in the case of a multi-cylinder engine. A first diametrical plane of the combustion chamber is defined, which comprises the longitudinal axis X of the cylinder and which is centered on the exhaust port 9. If the cylinder 6 comprises several exhaust lights, the first diametrical plane must be centered on a fictitious light having a geometric surface equivalent to all of the surfaces of the exhaust lights. This first diametral plane corresponds to the section plane of the embodiment shown in FIG. 1 and its trace (Pl-Pl) is visible in FIG. 2. A second diametral plane is also defined which is perpendicular to the first diametral plane (Pl- Pl) and the trace of which (P2-P2) is visible in FIGS. 1 and 2. The second diametral plane (P2-P2) delimits a first portion of the internal face of the cylinder head 10, including the second diametral plane, which s extends towards the main intake lumen 7. The spark plug 11 is arranged in this first portion of the cylinder head, that is to say that the spark plug well must open into this region, either at an angle with the longitudinal axis X as in the embodiment shown, either by being collinear or coincident with the longitudinal axis X. The engine 1 is equipped with an injection device comprising an injector 20 adapted to spray liquid and pressurized fuel in the room re combustion 12 along a spray axis P. The injector 20 is arranged in a second portion of the cylinder head complementary to the first portion of the cylinder head, that is to say that the spray end of one injector 20 opens into the second portion of the internal face of the cylinder head. More particularly, as is apparent from FIGS. 1 and 2, the injector 20 is arranged in the cylinder head at the level of the first diametrical plane (Pl-Pl) centered on the exhaust, to allow its mounting in a small displacement engine. The spray axis P defined by the axis of symmetry of the fuel jet created by the injector, forms a first angle α which is measured from a transverse plane (TT) of the cylinder, that is to say perpendicular to the longitudinal axis X. The way to precisely determine this first angle α as a function of the geometry of the combustion chamber will be explained below, but it must be between 30 ° and 70 ° for the most common two-stroke engines. The spray axis P also forms a second angle β, visible in FIG. 2, which is measured from the first diametrical plane (Pl-Pl) centered on the exhaust port 9. This angle must be between + 45 ° and - 45 °, the manner of determining its precise value being also explained below. The spray axis P having the first and second angles α, β, included in these values is generally directed towards the half-portion of the cylinder opposite the exhaust port. The fuel jet has in the embodiment shown a conical shape with a symmetry of revolution about the axis P, but it is possible to use a fuel jet of more complex shape, such as for example a jet having a cross section transverse oval. However, the opening angle γ of the fuel jet which is defined by two opposite edges of the droplet jet, must be between 15 and 75 ° in order to be able to be directed towards a relatively localized area of the combustion chamber, and above all, so that the droplets do not directly reach the walls of the combustion chamber, which would have a very unfavorable effect on pollutant emissions. The injection device of course comprises a control device, not shown, which makes it possible to control the time when the injection must start as well as the duration of the latter. The control device is connected to means for determining the angular position of the crankshaft to send a signal to open the injector 20 at an appropriate time, and to means for determining the operating parameters of the engine, such as for example an engine speed sensor and / or a load sensor to determine the duration of the injection and, consequently, the quantity of fuel injected. The control device acts on the injector 20 so that, at least for certain operating ranges, the injection of fuel begins when the crankshaft is in an angular position between 45 and 20 ° before the closed position of the exhaust port, and preferably in an angular position between 40 and 30 ° before the closing angle of the exhaust. This injection is relatively early, since it begins when the exhaust gases are still discharged to the exhaust port. It is notably earlier than in most of the known direct injection systems where it is sought to delay the start of the injection in order to prevent part of the mixture of gas and unburned fuel from passing through the light of exhaust. However, by adapting the fuel injection pressure and the orientation of the spray axis P within the angular ranges previously indicated, and as explained below, a passage of gases not burnt by the light d is avoided. 'exhaust. The early injection allows to inject a larger amount of fuel during the cycle, therefore it is particularly advantageous when the engine is running at full load and at high speed. However, the invention does not exclude the possibility of starting the injection later, after closing the exhaust, for certain operating ranges of the engine.  To adapt the fuel injection pressure and the orientation of the spray axis P correctly, these must be determined as a function of the gas flow in the combustion chamber, so as to obtain a stoichiometric mixture of gases in the region of the spark plug 11 at the time of ignition. The ignition caused by a spark between the electrodes 11a of the spark plug is carried out in the usual way, when the crankshaft is in an angular position a few degrees lower than the position of the top dead center of the piston, this advance in ignition being able to be more or less important depending on the engine rotation speed or its load. The injection pressure and the orientation of the spray axis P are preferably determined using a digital simulation of the circulation of gases in the combustion chamber during the intake / compression phase. Numerical simulation makes it possible to know precisely the path of the gas flows in the combustion chamber, as shown in Figure 3 which shows the gas flow lines when the piston is in a still relatively low position. However, it should be noted that due to the high kinetic energy of the gases entering the combustion chamber, the shape of these current lines does not change significantly during the entire intake / compression phase. As can be seen in Figure 3, the gases essentially perform a roller movement (called a tumble), that is to say a rotational movement around an axis parallel to the axis of the crankshaft. This is due to the diametrically opposite position of the main intake light 7 and the exhaust light 9, as well as to the symmetrical arrangement of the scanning lights 8 on either side of the main intake light. . But in the case of lights arranged not symmetrically by compared to the first diametral plane (Pl-Pl), we introduce a component of vortex movement (called swirl), that is to say a partial rotational movement around the longitudinal axis X of the cylinder. The angle α of the spray axis P is adjusted in the range from 30 to 70 °, so that the fuel jet is projected against the flow of fresh gas flow from the intake ports (7 , 8). It will be noted that the base of the fuel jet crosses gas flows circulating in the immediate vicinity of the outlet of the injector 20 on the internal face of the cylinder head 10. However, due to the compromise between the opening angle γ and the injection pressure (momentum of the jet), the fuel particles near the cylinder head are not yet finely atomized and their kinetic energy is high, so that the gas flows flowing near the injector 20 have little effect on the propagation of the jet in the combustion chamber 12. In the embodiment shown, there is no vortex phenomenon around the axis X of the combustion chamber, and therefore the second angle β of the spray axis P must be substantially zero. On the other hand, in the presence of a vortex movement, the angle β should be more or less pronounced depending on the importance of this vortex movement, so that the propagation of the jet is as much as possible against the flow fresh gas. The positive or negative value of the second angle β is of course determined as a function of the direction of rotation of the vortex movement of the gases. Furthermore, the injection pressure must also be adapted as a function of the circulation of gases in the combustion chamber. This injection pressure must not be too high because it must be avoided the projection of fuel droplets directly on the wall 6a of the cylinder or on the bottom 4a of the piston. However, this pressure must be high enough for the fuel droplets to reach a region where they will meet flows of gas circulating against the current and not to be entrained towards the exhaust port 9 by flows of gas circulating along of the cylinder head wall 10. The suitable injection pressure can be determined using a gas velocity diagram, which is also obtained by numerical simulation. This velocity diagram, not shown, consists of vectors oriented along the current lines and of more or less important length as a function of the speed of the gases at the point considered. Once the orientation of the spray axis P has been determined, it is possible to determine the injection pressure so that the momentum of the fuel droplets in the region of diffusion of the jet is substantially equal, less or slightly greater than the momentum of the gases flowing against the current according to the desired mixing profile. Although the inventor adapted the spray axis P and the injection pressure using digital simulation, it is conceivable to determine these parameters within the ranges of values indicated using tests and empirical knowledge that a person skilled in the art can have on the circulation of gases as a function of the geometry of the combustion chamber. However, it is important that the injection starts early, i.e. between 45 and 20 ° before the closing angle of the exhaust, and preferably between 40 and 30 ° before this closing angle . By thus adapting the spray axis P and the injection pressure, the propagation of the jet of spray is obtained. fuel and the evolution of the region having a substantially stoichiometric air / fuel mixture, which is represented in FIGS. 4 to 6, during the compression phase. The term air designates the fresh gases sucked in during the intake phase, but also a possible residue of gases burnt during the previous cycle or the exhaust gases reintroduced into the chamber by an EGR system. In FIG. 4, the propagation of the fuel jet can be seen shortly after the start of the injection which started 40 ° before the closing angle of the exhaust for the embodiment shown. The jet has a frustoconical shape which has a symmetry of revolution around the axis P. The opening angle γ of the jet is approximately 50 °. The curves 23 indicate the outline of the zones of the combustion chamber where there are different values of λ (Lambda), Λ being defined by the ratio between the proportion of air and fuel actually present, and the proportion of air and theoretical fuel which is necessary to have a stoichiometric mixture. A mixture of air and fuel is said to be stoichiometric when the oxidation of the C-H chains is ideally consumed at one hundred percent. A region of the combustion chamber where there is a value λ equal to 1 therefore means that the air / fuel mixture is stoichiometric therein. It will be noted that the region defined by the contours 23 is slightly offset towards the exhaust port relative to the spray axis P, but however, this portion of mixture is not entrained up to the exhaust port 9 In fact, as can be seen in FIG. 5, the large amount of movement of the injected fuel drives it towards the half-portion of the cylinder located on the side of the intake ports (7,8). The situation shown in Figure 5 corresponds to around the time when the exhaust light is closed, about forty degrees after Figure 4. In this situation, the amount of movement of the fuel is canceled out by the amount of movement of the fresh gases, the movement continues although the intake lights are closed. Note that the fuel injection can be extended after closing the exhaust port 9. When the piston comes close to top dead center, the portion of the combustion chamber where a stoichiometric mixture prevails has evolved so as to occupy the region of the candle, as can be seen in FIG. 6 where the electrodes 11a of the candle are symbolically represented. This situation is obtained thanks to the return movement towards the injector 20 of the fuel under the effect of the kinetic energy of the admitted gases. In this situation, the fuel is vaporized and forms a stoichiometric mixture with the fresh gases. The ignition is then controlled by creating a spark between the electrodes 11a of the spark plug which ignites the stoichiometric mixture. It will be noted that the stoichiometric mixture occupies most of the combustion chamber. Only a small portion of the combustion chamber, located on the side of the exhaust port, contains a lean mixture, which guarantees a smooth course of combustion. The direct injection thus produced makes it possible to considerably reduce pollutant emissions from an existing two-stroke engine, and in particular to meet future anti-pollution standards. In addition, the tests carried out with two-stroke engines implementing direct injection according to
1 ' invention ont permis de constater des réductions tout à fait spectaculaires de la consommation de carburant . En effet, sur certains moteurs, on a constaté une réduction sur un cycle réglementaire d' antipollution, de 30 % de la consommation en passant d'une alimentation par carburateur à une alimentation par injection directe selon l'invention. Cette réduction, plus importante que celle généralement obtenue avec les systèmes d'injection directe pour moteurs deux temps connus, peut en partie s'expliquer par la précocité du début de l'injection qui augmente le temps pendant lequel le carburant se vaporise et permet d'obtenir un mélange stoechiométrique dans une large partie de la chambre de combustion. La circulation des gaz dans la chambre de combustion, et principalement le diagramme de vitesse de ces gaz, peut varier assez sensiblement en fonction du régime et de la charge du moteur. Dans certains modes de réalisations, et notamment pour les moteurs de cylindrée relativement importante, il peut être préférable que la pression d'injection varie en fonction du régime ou de la charge du moteur, ou en fonction de ces deux paramètres. Cette variation de la pression d'injection peut être commandée de manière connue par le dispositif d'injection.1 invention have shown quite dramatic reductions in fuel consumption. Indeed, on some engines, there has been a reduction on a regulatory antipollution cycle, 30% of consumption by switching from a carburetor supply to a direct injection supply according to the invention. This reduction, greater than that generally obtained with direct injection systems for known two-stroke engines, can partly be explained by the precocity of the start of injection which increases the time during which the fuel vaporizes and allows '' obtain a stoichiometric mixture in a large part of the combustion chamber. The circulation of gases in the combustion chamber, and mainly the speed diagram of these gases, can vary quite significantly depending on the engine speed and load. In certain embodiments, and in particular for engines of relatively large displacement, it may be preferable that the injection pressure varies as a function of the speed or of the engine load, or as a function of these two parameters. This variation of the injection pressure can be controlled in a known manner by the injection device.
Par exemple, le dispositif d'injection peut être relié à un capteur de régime moteur et un capteur d ' ouverture du papillon de gaz d'admission, et comprendre des moyens de régulation de la pression régnant dans un accumulateur de carburant sous pression. En ajustant la pression d'injection du carburant entre 50 et 150 bars, on peut obtenir une combustion optimale sur l'ensemble de la plage de fonctionnement du moteur. Bien entendu, le dispositif de commande de l'injection est également adapté pour contrôler la durée d'injection afin de n'injecter que la quantité de carburant nécessaire. La pression d'injection peut varier de manière continue sur 1 ' ensemble de la plage de fonctionnement du moteur, ou selon différentes valeurs discrètes selon une cartographie régime/charge du moteur. Toutefois, dans le cas de moteurs de petite cylindrée, c'est-à-dire de cylindrée environ égale ou inférieure à 125 cm3, il est possible d'adopter une pression d'injection constante sur l'ensemble de la plage de fonctionnement du moteur tout en ayant une réduction significative des émissions de polluants et de la consommation. Le fait d'injecter le carburant à une pression constante, par exemple de 80 bars pour un moteur 50 cm3, permet d'utiliser un dispositif d'injection relativement simple qui n'augmente pas de manière excessive le coût du moteur. Par ailleurs, comme on peut le voir sur la figure 1, il est possible de disposer l' injecteur 20 dans un alésage de la culasse 10 orienté selon un axe I qui n'est pas colinéaire avec l'axe de pulvérisation P, c'est- à-dire que l'axe I de l' alésage forme un angle δ non nul avec l'axe de pulvérisation P. Cette disposition offre de plus—grandes-possibilités d'implantation de l' injecteur 20 dans la culasse 10 pour pulvériser le carburant selon un axe P déterminé. Ceci peut être particulièrement avantageux lorsqu'on cherche à implanter le dispositif d'injection dans un moteur deux temps existant dont la géométrie de la culasse limite les possibilités de réalisation de l'alésage de l' injecteur 20. Le mode de réalisation représenté aux différentes figures correspond à un moteur deux temps comportant une lumière principale d'admission, quatre lumières de balayage et une lumière d'échappement disposées symétriquement par rapport au premier plan diamétral (Pl-Pl) , mais il apparaîtra clairement à l'homme du métier que le dispositif d'injection selon l'invention peut être adapté à un moteur deux temps comportant un nombre de lumières différent, ou disposées de manières non symétrique, ainsi qu' à un moteur deux temps multicylindres. For example, the injection device can be connected to an engine speed sensor and an intake gas throttle opening sensor, and include pressure regulation means prevailing in a pressurized fuel accumulator. By adjusting the fuel injection pressure between 50 and 150 bars, optimal combustion can be obtained over the entire operating range of the engine. Of course, the injection control device is also suitable for controlling the injection duration so as to inject only the quantity of fuel required. The injection pressure can vary continuously over the entire operating range of the engine, or according to different discrete values according to a speed / load map of the engine. However, in the case of small-displacement engines, that is to say with a displacement of approximately 125 cm 3 or less, it is possible to adopt a constant injection pressure over the entire operating range. engine while having a significant reduction in pollutant emissions and consumption. The fact of injecting the fuel at a constant pressure, for example of 80 bars for a 50 cm 3 engine, makes it possible to use a relatively simple injection device which does not excessively increase the cost of the engine. Furthermore, as can be seen in FIG. 1, it is possible to arrange the injector 20 in a bore of the cylinder head 10 oriented along an axis I which is not collinear with the spray axis P, c ' that is to say that the axis I of the bore forms a non-zero angle δ with the spray axis P. This arrangement offers more — great possibilities of implantation of the injector 20 in the cylinder head 10 for spray the fuel along a determined axis P. This can be particularly advantageous when seeking to install the injection device in an existing two-stroke engine whose geometry of the cylinder head limits the possibilities of producing the bore of the injector 20. The embodiment shown in the different figures corresponds to a two-stroke engine comprising a main intake light, four scanning lights and an exhaust light arranged symmetrically with respect to the first diametrical plane (Pl-Pl), but it will be clear to those skilled in the art that the injection device according to the invention can be adapted to a two-stroke engine comprising a different number of lights, or arranged in a non-symmetrical manner, as well as to a multi-cylinder two-stroke engine.

Claims

REVENDICATIONS
1. Moteur deux temps à injection directe comprenant une chambre de combustion (12) délimitée par : un cylindre (6) présentant un axe longitudinal (X), qui est muni d'au moins une lumière d'admission (7,8) et d'au moins une lumière d'échappement (9) ; un piston (4) présentant un fond (4a) sensiblement plat et déplacé selon l'axe longitudinal par une bielle (5) reliée à un vilebrequin (3) ; une culasse (10) munie d'une bougie (11) et d'un injecteur (20) adapté pour pulvériser un jet de carburant liquide sous pression dans la chambre de combustion selon un axe de pulvérisation (P) , la chambre de combustion (12) présentant un premier plan diamétral (Pl-Pl) comprenant l'axe longitudinal (X) du cylindre et centré sur la lumière d'échappement et un deuxième plan diamétral (P2-P2) perpendiculaire a —d-±-fe- premier plan diamétral (Pl-Pl) , la bougie (11) étant agencée dans une première portion de la culasse s ' étendant depuis le deuxième plan diamétral (P2-P2) vers la lumière d'admission (7), l'injecteur (20) étant agencé sur une deuxième portion de la culasse complémentaire à la première portion, et l'axe de pulvérisation (P) formant un premier angle α mesuré à partir d'un plan transversal (T-T) du cylindre, qui est compris entre 30° et 70°, et un second angle β mesuré à partir du premier plan diamétral (Pl-Pl) qui est compris entre + 45° et - 45°, caractérisé en ce que l'angle d'ouverture γ du jet de carburant est compris entre 15° et 75°, en ce que l'injection du carburant débute lorsque le vilebrequin (3) est situé dans une position angulaire comprise entre 45° et 20° avant la position angulaire de fermeture de la lumière d'échappement (9), et en ce que la pression d'injection du carburant et l'orientation de l'axe de pulvérisation (P) sont déterminées en fonction de la circulation des gaz dans la chambre de combustion (12) pour obtenir un mélange air/carburant sensiblement stoechiométrique dans la région de la bougie (11) au moment de l'allumage. 1. Two-stroke direct injection engine comprising a combustion chamber (12) delimited by: a cylinder (6) having a longitudinal axis (X), which is provided with at least one intake lumen (7,8) and at least one exhaust port (9); a piston (4) having a substantially flat bottom (4a) and displaced along the longitudinal axis by a connecting rod (5) connected to a crankshaft (3); a cylinder head (10) fitted with a spark plug (11) and an injector (20) adapted to spray a jet of liquid fuel under pressure into the combustion chamber along a spraying axis (P), the combustion chamber ( 12) having a first diametral plane (Pl-Pl) comprising the longitudinal axis (X) of the cylinder and centered on the exhaust port and a second diametral plane (P2-P2) perpendicular to —d- ± -fe- first diametral plane (Pl-Pl), the spark plug (11) being arranged in a first portion of the cylinder head extending from the second diametrical plane (P2-P2) towards the intake lumen (7), the injector (20 ) being arranged on a second portion of the cylinder head complementary to the first portion, and the spray axis (P) forming a first angle α measured from a transverse plane (TT) of the cylinder, which is between 30 ° and 70 °, and a second angle β measured from the first diametrical plane (Pl-Pl) which is between + 45 ° and - 45 °, characterized in that the opening angle γ of the fuel jet is between 15 ° and 75 °, in that the injection of fuel begins when the crankshaft (3) is located in an angular position between 45 ° and 20 ° before the angular position for closing the exhaust port (9), and in that the fuel injection pressure and the orientation of the spray axis (P) are determined as a function of the gas flow in the combustion chamber (12) to obtain a substantially stoichiometric air / fuel mixture in the region of the spark plug (11) at the time of ignition.
2. Moteur selon la revendication 1, dans lequel la pression d'injection du carburant est variable en fonction du régime moteur et/ou de la charge du moteur. 2. Engine according to claim 1, wherein the fuel injection pressure is variable depending on the engine speed and / or the engine load.
3. Moteur selon la revendication 2, dans lequel la pression d'injection du carburant est comprise entre 50 et 150 bars. 3. Engine according to claim 2, wherein the fuel injection pressure is between 50 and 150 bars.
4. Moteur selon la revendication 2 ou 3 , dans lequel la pression d'injection du carburant est ajustée à différentes valeurs selon une cartographie régime/charge du moteur. 4. Engine according to claim 2 or 3, wherein the fuel injection pressure is adjusted to different values according to a speed / load map of the engine.
5. Moteur selon la revendication 1, dans lequel la pression d'injection du carburant est constante sur l'ensemble de la plage de fonctionnement du moteur, le moteur présentant de préférence une cylindrée au plus égale à 125 cm3. 5. Engine according to claim 1, wherein the fuel injection pressure is constant over the entire operating range of the engine, the engine preferably having a displacement at most equal to 125 cm 3 .
6. Moteur selon l'une quelconque des revendications précédentes, dans lequel l' injecteur (20) est disposé dans un alésage de la culasse (10) orienté selon un axe (I) et dans lequel l'axe de pulvérisation (P) forme un angle δ non nul avec ledit axe (I) de l'alésage. 6. Engine according to any one of the preceding claims, in which the injector (20) is arranged in a bore of the cylinder head (10) oriented along an axis (I) and in which the spray axis (P) forms a non-zero angle δ with said axis (I) of the bore.
7. Moteur selon l'une quelconque des revendications précédentes, dans lequel l' injecteur (20) est agencé à travers la culasse (10) au niveau du premier plan diamétral (Pl-Pl) . 7. Engine according to any one of the preceding claims, wherein the injector (20) is arranged through the cylinder head (10) at the first diametrical plane (Pl-Pl).
8. Moteur selon l'une quelconque des revendications précédentes, dans lequel l'injection du carburant commence lorsque le vilebrequin (3) est situé dans une position angulaire comprise entre 40° et 30° avant la position angulaire de fermeture de la lumière d'échappement (9) . 8. Engine according to any one of the preceding claims, in which the injection of fuel begins when the crankshaft (3) is located in an angular position between 40 ° and 30 ° before the angular position of closing of the light. exhaust (9).
EP04817611A 2003-12-31 2004-12-28 Direct injection two-stroke engine Withdrawn EP1706612A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR0315612A FR2864578B1 (en) 2003-12-31 2003-12-31 TWO-STROKE MOTOR WITH DIRECT INJECTION
PCT/FR2004/003400 WO2005073533A1 (en) 2003-12-31 2004-12-28 Direct injection two-stroke engine

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EP1706612A1 true EP1706612A1 (en) 2006-10-04

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WO (1) WO2005073533A1 (en)

Families Citing this family (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101482053B (en) * 2008-01-10 2012-06-27 光阳工业股份有限公司 Collocation mechanism for injection cylinder end in cylinder
AT507635B1 (en) * 2008-12-02 2013-08-15 Kirchberger Roland Dipl Ing Dr Techn TWO STROKE COMBUSTION ENGINE
NL2003099C2 (en) * 2009-06-29 2010-12-30 Univ Delft Tech A direct injection two-stroke engine.
US9512779B2 (en) 2010-04-27 2016-12-06 Achates Power, Inc. Swirl-conserving combustion chamber construction for opposed-piston engines
US8800528B2 (en) 2010-04-27 2014-08-12 Achates Power, Inc. Combustion chamber constructions for opposed-piston engines
US10180115B2 (en) 2010-04-27 2019-01-15 Achates Power, Inc. Piston crown bowls defining combustion chamber constructions in opposed-piston engines
WO2012158756A1 (en) 2011-05-18 2012-11-22 Achates Power, Inc. Combustion chamber construction for opposed-piston engines
EP2606202B1 (en) 2010-08-16 2016-03-09 Achates Power, Inc. Fuel injection spray patterns for opposed-piston engines
CN102116212B (en) * 2011-03-18 2012-09-26 重庆长安汽车股份有限公司 Air-entraining direct injection engine cylinder head
WO2012162717A1 (en) * 2011-06-01 2012-12-06 Ocampo Peter Selwyn Air cooled ported piston for internal combustion engines
US20130104848A1 (en) 2011-10-27 2013-05-02 Achates Power, Inc. Fuel Injection Strategies in Opposed-Piston Engines with Multiple Fuel Injectors
US9211797B2 (en) 2013-11-07 2015-12-15 Achates Power, Inc. Combustion chamber construction with dual mixing regions for opposed-piston engines
CN103982314A (en) * 2014-05-26 2014-08-13 浙江派尼尔机电有限公司 Two-stroke direct injection diesel outboard engine
JP6255318B2 (en) * 2014-08-01 2017-12-27 本田技研工業株式会社 Uniflow 2-stroke engine
FR3064300A1 (en) * 2017-03-23 2018-09-28 New Times TWO-TIME EXPLOSION ENGINE
CN112253307A (en) * 2019-07-06 2021-01-22 罗天珍 Intercooling method for instantaneous conduction, throttling and intercooling of combustion chamber and heat-insulating internal combustion engine
CN110761889A (en) * 2019-09-30 2020-02-07 广西擎芯动力科技有限公司 Ignition type two-stroke heavy oil direct injection engine combustion chamber in cylinder
DE102020000989A1 (en) * 2020-02-15 2021-08-19 Andreas Stihl Ag & Co. Kg Two-stroke engine and method of operating a two-stroke engine

Family Cites Families (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3154059A (en) * 1962-10-25 1964-10-27 Southwest Res Inst Stratified spark ignition internal combustion engine
US5042442A (en) * 1990-04-10 1991-08-27 Hale Fire Pump Company Internal combustion engine
US6390071B1 (en) * 1997-09-03 2002-05-21 Sanshin Kogyo Kabushiki Kaisha Engine component layout for outboard motor
US6112726A (en) * 1997-09-03 2000-09-05 Sanshin Kogyo Kabushiki Kaisha Fuel supply system for injected engine
US6302753B1 (en) * 1997-09-03 2001-10-16 Sanshen Kogyo Kabushiki Kaisha Accessory drive for direct injected outboard motor
JP4170427B2 (en) * 1997-12-08 2008-10-22 ヤマハマリン株式会社 Direct cylinder injection type 2-cycle engine
US6092494A (en) * 1998-01-27 2000-07-25 Brunswick Corporation Controlled pressure rise in two-cycle internal combustion engine having cylinder wall fuel injection
JP2000213431A (en) * 1999-01-22 2000-08-02 Yamaha Motor Co Ltd Jet propulsion boat
US6481394B1 (en) * 1999-09-27 2002-11-19 Sanshin Kogyo Kabushiki Kaisha Control system for two-cycle engine
JP2001115845A (en) * 1999-10-18 2001-04-24 Yamaha Motor Co Ltd Cylinder fuel injection engine
FR2824113B1 (en) 2001-04-25 2003-08-15 Magneti Marelli France DIRECT FUEL INJECTION ARRANGEMENT FOR AN INTERNAL COMBUSTION ENGINE
JP3788275B2 (en) * 2001-06-26 2006-06-21 日産自動車株式会社 In-cylinder direct injection internal combustion engine

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO2005073533A1 *

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US20090139485A1 (en) 2009-06-04
WO2005073533A1 (en) 2005-08-11
FR2864578B1 (en) 2006-03-24
CN100489282C (en) 2009-05-20
BRPI0417886A (en) 2007-04-27
CN1902387A (en) 2007-01-24
FR2864578A1 (en) 2005-07-01

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