WO2005049988A1

WO2005049988A1 - Combustion method for reciprocating engines

Info

Publication number: WO2005049988A1
Application number: PCT/FR2004/002907
Authority: WO
Inventors: Jean Frédéric MELCHIOR
Original assignee: Melchior Jean F
Priority date: 2003-11-17
Filing date: 2004-11-12
Publication date: 2005-06-02
Also published as: FR2862343B1; FR2862343A1

Abstract

The invention relates to a combustion method for a reciprocating engine which is adapted to operate with a high level of recirculated burnt gases. According to the invention, during the suction stroke of the piston, a gaseous mass is created from a first zone containing essentially recirculated burnt gases and a second zone containing essentially all of a fresh air combustion charge, said zones being kept separate during the compression of the piston. Subsequently, a fuel is introduced into the aforementioned first zone in order to create a fuel-air mixture which comprises recirculated burnt gases and fuel vapour and which is as homogenous as possible at the end of the compression. Finally, the first and second zones are compressed in order to heat separately the fresh air combustion charge and the fuel-air mixture and said combustion charge and fuel-air mixture are mixed close to the top dead centre of the piston in order to maintain a diffusion flame with the mixtures brought into contact therewith.

Description

The present invention relates to a combustion method for a reciprocating engine. The method involves maintaining or introducing a charge of recycled burnt gases into the cylinder and introducing a charge of fresh air and keeping them separated during compression. Preferably the rate of burnt gases recycled is sufficient to suppress the formation of NOX, for example greater than 30% of the total mass of the gases present in the cylinder. The fuel is distributed in the charge of burnt gases before or during the compression in order to obtain at the end of the compression a fuel mixture where the vapor concentration is substantially homogeneous and where the concentration of burnt gases is everywhere sufficient to limit the flame temperature below NOX formation threshold, regardless of oxygen supply. Combustion develops at the rate of mixing by diffusion of the carburetted charge into the charge of fresh air during transfers between the cylinder and the combustion chamber in the vicinity of the top dead center. The absence of local fuel accumulation in the reactive zone guarantees particle-free combustion.

Problem not resolved by the prior art. In compression ignition engines, efforts are made to homogenize the oxidizing charge composed of fresh air and recycled burnt gases to limit the local flame temperature to the threshold of NOX formation. The fuel is generally introduced in the form of a fog at top dead center when the gas charge has reached a temperature which allows rapid self-ignition. Despite improvements in common rail injection devices, this method creates local accumulations of particle-generating fuel. In addition, the small premixed areas light up with an unpleasant knock in urban traffic. In recent years other methods of introducing fuel into the homogeneous gaseous charge have demonstrated the possibility of eliminating NOX and particles at the source. They consist in creating during filling and / or during compression a more or less homogeneous premix of oxidizing gas and fuel vapor which ignites massively by self-ignition at the end of compression. These processes are known by the acronym HCCI (Homogeanous Charge Compression Ignition). In fact, in a perfectly homogeneous mixture of air, more or less cooled burnt gases and fuel vapor, the richness of which is less than stoichiometry, we can say in all points:

That every fuel molecule is surrounded by enough oxygen for total combustion without the formation of soot. That the oxidizer and the fuel are heated simultaneously during compression to the self-ignition threshold without overheating of the oxidizing medium. That the presence at all points of gas not participating in the reaction makes it possible to maintain the temperature everywhere below the threshold for the formation of NOX. Unfortunately, the implementation of these methods is limited by the explosive nature of self-ignition when it occurs simultaneously at all points of a volume. Self-ignition is progressive only at very diluted low charges where it starts at certain discrete points distributed in the mass, the preoxidation of which was initiated early by active radicals present in the recirculated gases, by their order of arrival in oxidizer or by small temperature gradients. After ignition, the kinetics of combustion due to heating by compression are no longer controllable for effective average pressures above 8 bars where the noise level becomes unacceptable. The other difficulty is to place the ignition at TDC (top dead center). This requires acting on the slow oxidation time, that is to say on the history of the fuel charge. Exercise is possible in a stabilized but perilous regime in the rapid transients of automobile engines. In addition, the development of cold flames promotes the formation of CO and HC which will be difficult to oxidize in the cylinder given the low temperatures at the end of the cycle, which are, moreover, insufficient for turbocharging. In conclusion it is very difficult to simultaneously control the initiation and the development of the combustion of a perfectly homogeneous charge. Turbocharging makes it possible to recycle a large part of the burnt gases throughout the field of use of the engine, as shown by the patent application filed in France on March 26, 2003 under number 03 03728, the content of which is incorporated here by reference. The object of the present invention is to obtain at all engine loads combustion without knocking, free of NOX and of particles by a ^' new spaciotemporal sequence of bringing together the species which will participate in the combustion.

Description of the invention. The subject of the invention is a combustion method for an alternative engine adapted to operate with a high rate of recycled burnt gases, said engine comprising at least one cylinder delimited by a piston and a cylinder head determining a working chamber of a gaseous mass. and a combustion chamber, characterized in that: - during the suction phase of the piston, said gaseous mass is created from a first zone containing essentially burnt recycled gases having an insufficient concentration of residual oxygen for allow early self-ignition and a second zone containing substantially all of an oxidizing charge of fresh air, said zones being kept separate during the compression of the piston, - a fuel is introduced into the first zone to create a mixture fuel from recycled burnt gases and fuel vapor as homogeneous as possible at the end of compression, and - by compression of the first and second zones, the oxidizing charge of fresh air and the fuel mixture are heated separately and mixed with near the top dead center of the piston, said oxidizing charge and said fuel mixture for maintaining a diffusion flame with the mixtures brought into contact. The invention also relates to motors suitable for this method. Unlike the prior art where the mixture of fresh air and recycled burnt gases present in the cylinder is homogeneous at the end of compression, the present invention provides for limiting the mixing zone as much as possible to preserve until the end of the compression an area filled with burnt gases containing only residual oxygen from the previous cycle and an area filled with fresh air. In the mixed intermediate zone, the oxygen concentration therefore varies between the residual value of the previous combustion and that of the fresh air. Unlike the prior art where the fuel is introduced into an oxidizing mixture where the oxygen concentration is sufficient to ensure its oxidation by local diffusion, the present invention provides for introducing the fuel into the heart of the zone filled with gas. burned with little oxygen. A more or less stratified mixture of burnt gases and fuel vapor is homogenized by heating up to the end of compression in a zone of the cylinder while the fresh air and the mixing zone heat up in a other part of the cylinder. The fuel mixture must be organized so that, at the end of compression, the dilution of the fuel vapor by the burnt gases is sufficient everywhere to prevent the formation of NOX in a locally stoichiometric combustion. Schematically the prior art dilutes by burnt gases all the oxygen charge before introducing the liquid fuel. The local temperature is limited by the mass concentration of oxygen whatever the local concentration of fuel created by the injector which accumulates the fuel around the droplets by generating particles. On the contrary, the present invention dilutes by burnt gases all the fuel vapor before introducing fresh oxygen into the reaction zone. The local temperature is limited by the low local concentration of fuel which simultaneously eliminates the formation of NOX and the rich points which generate particles. At the end of compression, the temperatures of the carburetted and oxidizing zones are sufficient for an auto-ignition to be triggered as soon as the hydrocarbon molecules and oxygen molecules are brought into contact with one another without significant premix formation. Combustion can start by self-ignition of the fuel premix, which is added to the compression to activate the products to be burned. The concentration of residual oxygen in the fuel zone is low enough for the possible self-ignition of the fuel premix to develop in a silent HCCI mode. The possible point of ignition of the premix can be controlled by the injection point, by the richness, by the compression ratio, by the temperature of the recycled gases or any other known means for controlling the HCCI process. The main combustion develops in the vicinity of the top dead center at the rate of the turbulent diffusion of the fuel charge in the oxidizing charge following the flushing of gas from the cylinder into the combustion chamber caused by the movement of the piston. This mixing process must be rapid and complete to guarantee total oxidation of the fuel. It is advantageous to generate the mixing energy during the filling phase by communicating to the gaseous charge a kinetic energy of rotation which will degrade in turbulence during the centripetal transfer in a combustion chamber of diameter much smaller than the bore. If the combustion gas and fuel gas meet at the periphery of the chamber in this rotational movement, the gases burned at high temperature evacuate towards the axis of the chamber to leave the periphery for fresh reaction products. The mixing phenomenon is thus accelerated by combustion. An additional advantage of this mechanism is to concentrate the hot gases at the heart of the load and to limit the heat losses at the walls of the working chamber. According to other characteristics of the invention: - the fuel is introduced in the form of a mist propelled by a high pressure injector towards the first zone when said first zone reaches a temperature sufficient to vaporize the liquid droplets of the fuel , - the fuel is injected into the burnt gases recycled from the first zone during their introduction into the cylinder, - the burnt gases recycled from the first zone contain the majority of the unburnt products from the previous cycle, - the burnt gases recycled from the first zone contain the majority of gases having undergone cooling in contact with the walls of the working chamber during the previous cycle, - the working chamber has a symmetry of revolution around the axis of the corresponding cylinder, - the combustion chamber is formed by a cavity coaxial with the corresponding cylinder formed in the piston and / or the cylinder head and whose diameter is less than that of the cylinder by 50 to 60%, - the fuel mixture of the first zone is animated by '' a rapid rotational movement around the axis of the corresponding cylinder and, at the end of compression, this fuel mixture is fully transferred into the corresponding combustion chamber when the oxidizing charge is still in the annular space between the piston and the cylinder head , - the fuel mixture of the first zone is located against the corresponding piston, - the cavity forming the combustion chamber is formed by a combustion bowl arranged in the piston, - the cylinder head has a protuberance intended to penetrate into the combustion bowl at top dead center of the corresponding piston to provide a variable annular clearance depending on the position of this piston and to maintain a speed of trans fert gas substantially independent of the position of this piston, - the oxidizing charge of fresh air from the second zone is concentrated against the corresponding cylinder and the recycled burnt gases are concentrated in a central zone of this cylinder and the combustion chamber is half provided in the piston and half in the corresponding cylinder head, - the gas mass is ignited at the periphery of the combustion chamber where the oxidizing charge of fresh air from the second zone begins to oxygenate the fuel mixture of the first zone, - the gas mass is ignited within the fuel mixture of the first zone when this mixture reaches the self-ignition temperature and / or the oxidizing charge of fresh air in the second area begins to enter the combustion chamber, - the gas mass is ignited immediately after the injection of the fuel into the hot recycled burnt gases of the first zone a little before the top dead center of the corresponding piston, - the fuel mixture of the first zone is stratified and the oxidizing charge of fresh air from the second zone by successively introducing this fuel mixture and this oxidizing charge into the corresponding cylinder and preferably by introducing the fuel mixture from the first zone before the oxidizing charge, - the stratification of the fuel mixture of the first zone and the oxidizing charge of fresh air of the second zone is prepared in an oblong stratification chamber external to the corresponding cylinder and opening on one side into a fresh air supply manifold and on the other side in an intake duct of the corresponding cylinder head. For example, in an alternative engine suitable for the four-stroke cycle, the stratification chamber is supplied with recycled burnt gas by an exhaust duct of the corresponding cylinder distinct from the supply duct of a turbine, by circulating them in a refrigerant. and in an intake duct of the corresponding cylinder head. For example in an alternative engine suitable for the two-stroke cycle, a refrigerant for recycled burnt gases located in the part of the stratification chamber opening into the intake duct of the corresponding cylinder head and a non-return valve located between the stratification and the fresh air supply manifold prevents any flow from said chamber to said manifold and the hot gases to be cooled and recycled are transferred from the cylinder to the corresponding stratification chamber by opening an inlet valve at the end of the expansion stroke of the corresponding piston, the non-return valve being thereby in the closed position, and the gaseous mass thus prepared is introduced into the stratification chamber, into the corresponding cylinder by the intake duct by opening a valve exhaust to exhaust gases hot contained in said cylinder, the pressure drop thus created in the laminating chamber opening the non-return valve to introduce the charge of fresh air for the next cycle. Operation of the invention The self-ignition of the very rich premix, when it exists, develops without knocking, lack of oxygen to accelerate the reaction in the ballast of burnt gases and excess fuel. It develops without NOX formation due to the very low oxygen concentration. It develops very below the soot-forming temperature in an environment free of local fuel accumulation. The main combustion by diffusion does not form NOX because the dilution by burnt gases of fuel vapor limits the local temperature even in excess of oxygen. The main combustion by diffusion does not form particles because the fuel vapor had time to disperse in the mass of burnt gases by eliminating the accumulation points. The main combustion is not noisy because it develops at the rate of the gaseous diffusion of the oxidizing charge in the carburetted charge. The self-ignition of the carburetted charge must occur before its mixing with the oxidizing charge is sufficiently advanced to be the site of a detonating combustion in premix. The possible combustion in HCCI mode of the premixed oxygen in the carburetted charge is associated with the movement of the piston to heat by compression the oxidizing charge above the self-ignition threshold.

^• The process implies that the gaseous charge remains stratified during compression to avoid the encounter of oxygen and fuel before the piston approaches the top dead center. The gaseous charge trapped in the cylinder is constructed from the following components: Fresh air taken from the atmosphere for each cycle, for example, at around 350 ° K, The burnt gases from previous cycles retained in the cylinder at 900 ° K approximately, for example, if necessary, the burnt gases from the preceding cycles recirculated in the cylinder after external cooling down to, for example, approximately 450 ° K. In a rotational movement around the cylinder axis, these three components are advantageously laminated radially with the fresh air against the cylinder, the burnt gases cooled inside the layer of fresh air and the column of hot gases. retained in the center of the cylinder. The mixtures between layers constitute at the end of compression a radial gradient of maximum oxygen concentration against the cylinder and low in the center as well as a radial gradient of low temperature in the cylinder and maximum in the center. The combustion according to the invention is mainly organized during transfers between the cylinder and the combustion bowl created by the piston on, for example, 20 to 30 degrees of crankshaft on either side of the top dead center. The fuel introduction process depends on its volatility and flammability. Diesel is advantageously atomized at high pressure early during the compression stroke in the center of the cylinder filled with burnt gases too poor in oxygen to trigger an explosive combustion at top dead center. Under these conditions, the possible self-ignition of the premix is regulated by the angle of start of injection which fixes the origin of the pre-oxidation phenomena. The mass of recycled burnt gases must be sufficient to keep the flame temperature everywhere below the threshold for the formation of thermal NOX. It is advantageous to give the recycled gases a strong turbulence in order to homogenize the carburetted charge after the injection of the fuel and to accelerate the subsequent diffusion of oxygen during the gas transfers between the cylinder and the combustion chamber in the vicinity of neutral high. When the recycled gases are too hot to avoid early self-ignition, the fuel must be injected later into the burnt gases, for example at the start of the transfer period of the oxidizing charge in the combustion chamber. At high temperature the vaporization is almost instantaneous and the mixing between the vapor and the residual gases can take place immediately before the diffusing penetration of the oxygen into the reaction zone. The organization of the mixture between the fuel and the burnt gases is, in this case, essentially ensured by the injector. The charge of fresh air admitted to oxidize the injected fuel is kept separate from the fuel charge up to the vicinity of top dead center by a buffer zone comprising fresh air outside and burnt gases inside.

^• During compression, the fuel charge, the oxygen charge and the buffer zone heat up separately to the self-ignition temperature. To limit the temperature of the nitrogen in the oxidizing charge, it is advantageous to bring the fuel mixture to a higher temperature to reach the self-ignition threshold associated with the presence of the preoxidized active radicals. This strategy takes into account the difficulty of cooling the flue gases to the same level as the fresh air. A little before top dead center the carburetted area may ignite in HCCI mode

^• Immediately afterwards, the two charges are mixed by the movement of the piston at the rate of the gas diffusion generated by the internal aerodynamics in the reaction zone. The main combustion develops at the rate of this diffusion. After the possible self-ignition of the very oxygen-poor premix, the release of energy is punctuated by the development of the diffusion between the two reactive charges which must be organized in the vicinity of the top dead center. Turbulent diffusion requires a speed gradient between carbide gases and oxidizing gases. This speed up can be generated by the movement of the piston, by the filling process, by the injection of fuel and by the local expansion of the gases due to combustion. The solutions are numerous and depend on the structure of the stratification generated by the transfers in open and closed phases. This process assumes that the combustion is close to stoichiometry at least in the zone of the chamber which will be recycled and irrigated and that the carburetted charge mixes little with the oxygenated zone until the end of the compression stroke. Any unburnt materials present in this area can thus be recycled. On the contrary, the area of the chamber which will be evacuated, for example, towards a turbine can be more oxygenated to remove unburnt in the exhaust gases. It is therefore preferable for the gases to be recycled to be directed to a circuit independent of the turbine supply circuit. At the end of the rebound stroke, there are two areas in the cylinder: An area, poor in unburnt hydrocarbons which may contain residual oxygen for possible post-treatments. It will be relaxed in the turbine. Preferably, this zone does not include burnt gases having undergone cooling at the walls of the working chamber. An area which optionally includes unburnt hydrocarbons and which preferably includes the burnt gases which have undergone cooling in contact with the walls. It will be retained in the cylinder or cooled externally for subsequent recycling in order to oxidize the unburnt. This area will be irrigated by the new fuel charge. It is advantageous to organize the combustion chamber to maintain the rich zone which will be recycled against the walls of the working chamber in order to include the heat losses of the closed cycle in the losses by cooling of the recycled burnt gases. The hottest part of the gas charge is first evacuated to the turbine. The delicate phases of the process are successively: The generation of the stratification and the maintenance during compression of the separation between the charge of burnt gases and the oxidizing charge. The creation of an almost homogeneous mixture between the burnt gases and the fuel vapor. The rapid and total diffusion of the oxidizing charge in the carburetted charge at high combustion dead center. Stratification generation. There are many methods for generating the stratification and will be described below only some examples which are easy to implement in 4 and 2-stroke cycles. The general principles are as follows: Unlike the homogeneous approach where the recycled gases are dosed simultaneously for all the cylinders in a common mixer with fresh air, the stratified approach imposes a cylinder by cylinder dosage of the recycled gases. Unlike the fresh air which is delivered by a compressor common to all the cylinders, the gases are emitted by each cylinder individually during the precise and repetitive emptying process. The present invention provides for using this process to dose the quantity of gas addressed to the receiving cylinder. The geometry of the cylinder supply and discharge circuits must be identical to ensure a balanced metering of recycled gases and fresh air. The parasitic mixing zone between the fresh air and the recycled gases develops substantially in proportion to the time of presence in the cylinder. The 2-stroke cycle is more favorable for stratification than the 4-stroke cycle because the duration of coexistence of the two charges in the cylinder is about three times shorter there. The simplest solution for recycling hot gases is to keep them in the cylinder from one cycle to the next. A precise metering of the retained mass can be ensured by the timing of the valves which can be adjusted in operation by a camshaft phase shifter. In a 4-stroke cycle, the early closing of the exhaust is accompanied by an unnecessary phase of expansion compression of the recycled gases, generating friction losses. In a 2-stroke cycle, it suffices to sub-sweep the cylinder to retain the burnt gases from the previous cycle. To stratify the load, the fresh air must be directed towards a part of the cylinder, avoiding as much as possible the phenomena of mixing with the gases already present. The disadvantage of this method is the high temperature of the recycled gases which may prove to be too high to remove the NOX. This solution can be viable in a very supercharged asymmetrical 2-stroke cycle with a high expansion rate to decrease the temperature at the end of expansion and at a low compression rate to reduce heating during compression. The fuel will advantageously be injected at the start of compression in order to cool the gases before compressing them. In general, the gases outside the cylinder must be cooled. In order to be able to recycle the polluted zone of the combustion chamber, only the stratification processes will be described where the traceability of the recycled gases is saved between the emitting cylinders and the receiving cylinders. This implies that the emptying of the cylinder takes place in two distinct phases towards the turbine and towards the recycling circuit by preferably different orifices. It is very difficult to install a stratification by simultaneously introducing fresh air and burnt gases into the cylinder. We therefore prefer the processes where the gases and air are introduced successively and particularly where the gases are introduced first. Stratification could then be generated during filling by successively opening separate intake orifices for the recirculated burnt gases and for the fresh air. The passage from one orifice to the other nevertheless involves a mixing phase where one valve closes while the other opens. In order not to interrupt the filling and to take advantage of the entire intake section, it is more advantageous to prepare the stratified charge outside the cylinder and to introduce it without discontinuity through the same orifice which will pass successively gases recycled then fresh air for example. To achieve this aim, the invention advantageously provides for an individual stratification chamber in the intake duct of each cylinder. Although a 2-stroke engine of axisymmetric architecture would allow optimal application of the method, the invention will be described on a 4-stroke engine of conventional architecture to facilitate the presentation. Reference is now made to the following description, given by way of nonlimiting example and referring to the appended drawing, in which: FIGS. 1 and 2 represent a 4-stroke engine implemented in the invention, FIGS. 3 to 9 schematically represent the aerodynamics in meridian section of a cylinder during operation, FIGS. 10 and 11 schematically represent the diffusions for two forms of combustion chambers, FIG. 12 represents an engine in a simplified mode of implementation. Referring now to Figure 1 which shows a 4-stroke engine, 4 cylinders and 4 valves per cylinder actuated by two camshafts. The diametrically opposite intake valves A1, A2 are associated with a directional channel to create air jets tangent to the cylinder and very flat on the cylinder head plane in order to generate a helical flow of revolution which invades the cylinder from the cylinder head, for example, in accordance with French patent application No. 03 03728. A first camshaft AàC1 actuates an exhaust valve And which supplies the turbine and an intake valve A1. The other camshaft A to C2, preferably phase-shiftable with respect to the first, actuates an exhaust valve Er which feeds the recycling circuit and the other intake valve A2. The stratification chamber 1 of volume close to the unit displacement 2 preferably has an oblong shape to limit the contact surface 3 between the gases and the fresh air. The chamber 1 is placed between the intake orifices 4 of the cylinder and the manifold 5 for the arrival of cooled air from the compressor 6. During each cycle, the chamber 1 undergoes a filling phase and an emptying phase in the cylinder with which it is associated. It is filled with fresh air by its junction 7 with the intake manifold 5 and cooled burnt gases from the same cylinder by its 5 junction 8 with the intake ports 4. Its supply of burnt gases must be repetitive of cycle with cycle and identical for all the cylinders. We have previously seen the advantage of emptying the cylinder in two stages fixed by the offset opening of two exhaust orifices

L0 respectively supplying the turbine and the recycling circuit. The phase shift between the valves can be adjusted in operation by a camshaft phase shifter to dose the mass of gas to be recycled. To keep this dosage up to the receiving cylinder, it is advantageous to provide an individual recycling circuit comprising a refrigerant L5 9 per cylinder. An adjustment valve 10 located at the outlet of the recycling orifice can replace or supplement the variable setting with the advantage of making it possible to completely eliminate recycling during accelerations. An adjustable bypass 11 of the refrigerant 9 makes it possible to adjust the temperature 20 of the recycled gases independently of their flow rate. A single body can perform these two functions. For example, a cylindrical bushel pierced with lights communicating with the inlet and the outlet of the refrigerant can be controlled in translation to adjust the flow rate and in rotation to adjust the temperature. 25 The valves and bypass of all cylinders are regulated simultaneously by a common control device. We now refer to FIG. 2 which describes the movements of recycled gas in the conduits. Cylinder C1 is in top crossing neutral position at the start of its 0 filling stroke. The charge of cooled gases is on standby in the stratification chamber 1. The hot gases to be cooled emitted during the preceding exhaust stroke occupy the entire volume comprised between the valve Er and the intake valve A2. The cylinder C2 is at the bottom intake neutral point where all the valves are closed. During the intake stroke, the exhaust valves Et and Er being closed, the cylinder sucked in via the intake valves the charge of cooled burnt gases waiting in the stratification chamber 1 followed by the charge of contiguous fresh air . The gases present in the refrigerant are being cooled. Cylinder C3 is in neutral high compression. The gases present in the refrigerant 9 continue to cool while contracting. The border with fresh air moves to the refrigerant. The cylinder C4 is in neutral, low trigger. The gases present in the refrigerant 9 finish their cooling by bringing the fresh air to the limit of the latter. The exhaust stroke which will follow will push the border 3 back to its position at the cylinder 1. FIGS. 3 to 9 describe schematically the internal aerodynamics of the cylinder during the compression combustion and expansion phases in a meridian section. If the intake ports are oriented tangentially to the cylinder with a slight inclination in the plane of the cylinder head, the rotating gas charge approaches a stratification of revolution around the axis of the cylinder as shown in fig. 3 The charge of burnt gases G1 occupies the lower central part of the cylinder. The fresh air G2 occupies the upper peripheral part of the cylinder. Figure 4 shows the fuel irrigation of recycled gases during compression with its fuel-rich zone Z1 and its lean zone Z2. FIG. 5 shows the situation at the start of combustion where the carbide gases Z occupy the entire combustion bowl 12 located in the piston. It is at this point that the self-ignition of the fuel mixture may possibly occur. Figure 6 shows combustion by diffusion in the vicinity of top dead center following the transfer of the fuel charge composed essentially of fresh air G2, located in the annular space between the piston and the cylinder head in the combustion chamber full of burnt gases carbides Z. FIG. 7 shows the situation at the end of expansion where the zone Z1 transformed after combustion and expansion may contain unburnt materials and the gases cooled by the walls whereas the transformed zone Z2 which contains an excess of oxygen is the hottest . The transformed zone Z2 will be evacuated first during the expansion of the gases towards the turbine via the valve E. And FIG. 8 shows the exhaust stroke where the transformed zone Z1 is discharged by the piston via the valve Er towards the recycling circuit in moving the cooled gases present in the refrigerant to the stratification chamber 1 to take their place until the next cycle. FIG. 9 shows a section perpendicular to the axis of the cylinder of the reaction zone of FIG. 6 where the products of combustion are evacuated towards the center by the centrifugation of the heavier carbide gases. A simpler method consists in supplying gas to the stratification chamber by open intake openings during the emptying of the cylinder. This involves placing the refrigerant in the stratification chamber. The gas charge goes back and forth in the refrigerant during transfers. The disadvantage of the method is the short residence time of the gases in the refrigerant (during the closed phase of the cycle the refrigerant is full air) as well as the heterogeneous cooling of the load (the gases close to the air intake are much more cooled than the gases close to the valves. The advantage of the method is a precise dosage of the recycled mass. a 4-stroke cycle the transfer sequence is as follows: During the emptying of the cylinder, the burnt gases are partially evacuated to the turbine via the exhaust ports and partially to the refrigerant filled with fresh air via the intake ports After the gases have been evacuated, the cylinder successively sucks back, via the intake ports, the burnt gases from the previous cycle stored and cooled in the intake coolant and the fresh charge which replaces the gases directed to the turbine. The intake and exhaust ports are open during the discharge stroke. During the suction stroke, the intake ports are open and the exhaust ports are closed. admission weights do not interfere with the piston, it is not necessary to close them in top crossing neutral position. The percentage of recycled gas can be adjusted by the phase difference between the opening of the intake ports and that of the exhaust ports at the end of expansion by means of an angular camshaft phase shifter. In a 2-stroke cycle, the transfer sequence is as follows: The recycled cooled burnt gases must precede the fresh air to sweep the burnt gases intended for the turbine. The transfer sequence then comprises the following successive phases: • Transfer of the fraction to recycle burnt gases to the gas cooler via the intake ports. • Return to the cylinder, via the intake ports, of the previous gases cooled by the refrigerant to sweep away some of the gases hot to the turbine via the exhaust ports. • Stratified introduction of the fresh charge via the refrigerant and the intake ports to complete the sweeping of hot gases to the turbine via the exhaust ports. To accomplish this complex sequence in the short period allocated to the sweep, (120 degrees crankshaft approximately for a three-cylinder engine) the invention provides for using the potential energy available in the burnt gases at the end of expansion which is generally lost in the exhaust puff. To do this, the stratification chamber contains the refrigerant and communicates with the air supply manifold via a non-return valve. To describe the transfer sequences we assume that: the purge air pressure delivered by the compressor is 3 bars the gas pressure at the turbine inlet is 2.5 bars the pressure of the burnt gases at the end of expansion is 10 bars At the end of expansion, the intake openings are open to communicate the cylinder filled with hot gases at 10 bars and the stratification chamber filled with fresh air at 3 bars isolated from the air intake duct by its check valve. return. The gases at 10 bars which expand and cool compress this air up to approximately 5 bars to occupy the closed cavity of volume equal to the sum of the cylinder and the stratification chamber. The volume of the refrigerant is fixed by these pressure conditions and the temperature of the gases cooled when the exhaust orifices are opened. The exhaust ports are then opened to communicate the overall volume at 5 bars with the turbine at 2.5 bars. The gases cooled to 5 bars sweep the hot gases present in the cylinder up to 3 bars where the non-return valve opens to let a new charge pass cool which propels the fresh charge stored in the cavity into the cylinder to complete the scanning of hot gases up to 2.5 bar. If the refrigerant is of the tubular type, it is possible to take advantage of the vacuum resulting from the speeding up of the gases to suck in the fresh charge via the non-return valve when the turbocharging is not initiated. The flows are very rapid taking into account the strong pressure drops between cavities. The refrigerant is driven alternately by the fresh air and gases ^Hentai recycled by turbulence associated with rapid flows that accelerate heat transfer. The tangential speed induced in the cylinder during sweeping will be used in the closed phase to homogenize the carburetted charge after injection and accelerate the diffusion during combustion. During the closed phase of the cycle, an acoustic back and forth can be maintained in the refrigerant to accelerate the cooling of the fresh charge pending. Generation of the mixture between the recycled gases and the fuel vapor. To allow maximum time for the formation of the mixture, the fuel will be distributed in the charge of burnt gases as soon as possible, preferably after closing the exhaust ports. For liquid fuels, the boiling temperature determines the optimal time and method to obtain a mixture as homogeneous as possible at the end of compression. The sprayer will ensure that the mist is distributed as evenly as possible in the recycled load. The turbulence of the recycled gases will complete the distribution of the fuel vapor during the compression stroke and especially during their centripetal transfer in the combustion bowl whose diameter is, for example, of the order of 50 to 60% of that of the cylinder and where the internal friction is very intense. Avoid the impact of liquid fuel on the walls of the working chamber. Petrol, very volatile, or gaseous fuels can be injected into the intake ports simultaneously with the introduction of recycled gases whose temperature always exceeds 450 ° K. Diesel, which is more difficult to vaporize, will be sprayed more or less later during the compression stroke depending on the temperature of the recycled gases. Beyond the temperature of the recycled gases generating an early self-ignition of the fuel charge, the injection will take place immediately before the transfer of fresh air into the combustion bowl and the quality of the fuel mixture will largely depend on the injector. Diffusion of the oxidizing charge in the carburetted charge near the top dead center. The combustion rate is a function of the intensity of the diffusion between the products which enter the reaction zone, itself a function of the level of turbulence. The most efficient mechanism is to communicate to the recycled gases and to the fresh air a kinetic moment of rotation around the cylinder axis which generates a strong turbulence in their centripetal transfer to the combustion bowl. It is also very important to quickly evacuate the products during combustion outside the reaction zone. This is done naturally when the latter is located at the periphery of the combustion bowl. In fact, the hot burnt gases are expelled towards the axis of the bowl while the denser fresh products are centrifuged towards the periphery as shown in FIGS. 6 and 9. An optimal configuration is possible in radial stratification with a half combustion bowl in the piston and half a combustion bowl in the cylinder head. The fresh air disk entering the equatorial plane of the reaction zone develops two zones of mixing with the carburetted charge located in the upper and lower hemispheres of the combustion chamber. Figures 10 and 11 show two possible aerodynamic regimes depending on the respective tangential speeds of fresh air and burnt gases and the geometries of the chambers. The preferred geometry for preserving the stratification in the cylinder and the kinetic energy of the load comprises a symmetry of revolution around the axis of the cylinder to receive rotary flows of revolution around this axis. In a 4-stroke cycle, the classic architecture of a flat cylinder head with 4 valves associated with a combustion bowl located in the center of the piston makes it possible to approach an internal aerodynamics with symmetry of revolution between the end of filling and the end of the evacuation of the cylinder. The intake ducts must generate jets tangential to the cylinder very flat on the plane of the cylinder head in order to form an axial stratification of the rotating gaseous mass. At the end of filling, the recycled gases are concentrated against the piston and fresh air against the cylinder head. In a 2-stroke cycle, an architecture with coaxial valves to the cylinder is best suited to the present process. It makes it possible to locate the combustion chamber for half in the piston and for half in the cylinder head. The working chamber is supplied and emptied by an intake valve and an exhaust valve concentric with the cylinder and located in the cylinder head; The openings and closings can advantageously be out of phase with respect to the motor shaft during operation in order to modulate the angular duration of scanning as a function of the operating parameters. Once closed, the working chamber has a symmetry of revolution around the cylinder axis. The exhaust port being located in the center. The mass evacuated to the turbine occupies the central and hot part of the cylinder, while the recycled mass occupies the cooled parietal zone of the cylinder. In some cases, it may be advantageous to provide a circular prominence in the center of the cylinder head or of the piston which penetrates into the combustion chamber located in the opposite to increase and control the hunting speed generated by the movement of the piston during the transfer of the last charge. To limit the formation of the mixture, fresh air is introduced into the cylinder after the burnt gases recycled in the form of a sheet tangential to the cylinder to form an oxygenated torus in rotation against the wall of the cylinder. The central part of the cylinder closed by the piston, a central cavity of which constitutes a half combustion chamber, is occupied by burnt gases which are substantially not very oxygenated. During compression the inert load occupies the central part of the cylinder while the fresh load remains against its wall. As soon as the compression has sufficiently heated the burnt gases, the fuel is sprayed into the latter to form a mixture of fuel vapor and burnt gases which is homogenized by entering the combustion half bowls. Towards the end of the compression, the burnt carbide gases occupy the entire combustion bowl and the fresh air occupies the annular space limited by the cylinder, by the face of the piston, by the cylinder head and by the external surface of the prominence of the breech. At the end of its compression stroke, the piston transfers the oxygenated charge into the bowl full of carburetted gas to trigger self-ignition by total diffusion of the two charges. Combustion continues during the reverse transfer of the burnt gases from the bowl to the cylinder at the start of the expansion stroke using at passage of the oxygen remaining in the clearance between the annular face of the piston and the headspace. For an implementation of the invention on an engine with a recycling circuit common to all the cylinders, reference is made to FIG. 12, where the metering of the recycled gases is ensured by identical nozzles 2 for all the cylinders. This method does not however allow a total stratification because, at the end of filling, the fresh charge is introduced with the gas flow rate from the nozzle of the cylinder concerned. This corresponds to a loss of around 25% of the mass of burnt gas which will be fueled.

Claims

Claims 1. Combustion method for an alternative engine adapted to operate with a high rate of recycled burnt gases, said engine comprising at least one cylinder delimited by a piston and a cylinder head determining a working chamber of a gaseous mass and a combustion, characterized in that: - during the suction phase of the piston, said gaseous mass is created from a first zone G1 essentially containing recycled burnt gases having an insufficient concentration of residual oxygen to allow self-ignition early and a second zone G2 containing substantially all of an oxidizing charge of fresh air, said zones G1 and G2 being kept separate during the compression of the piston, - a fuel is introduced into the first zone G1 to create a mixture fuel Z of recycled burnt gases and fuel vapor as homogeneous as possible at the end of compression, and - by compression d e the first and second zones, the oxidizing charge of fresh air and the fuel mixture are heated separately, and said oxidizing charge and said fuel mixture are mixed near the top dead center of the piston to maintain a diffusion flame with the mixtures put in contact.

2. Method according to claim 1, characterized in that the fuel is introduced in the form of a mist propelled by a high pressure injector towards the first zone G1 at the moment when said first zone reaches a temperature sufficient to vaporize the liquid fuel droplets.

3. Method according to claim 1, characterized in that the fuel is injected into the burnt gases recycled from the first zone G1 during their introduction into the cylinder.

4. Method according to claim 1, characterized in that the burnt gases recycled from the first zone G1 contain the majority of the unburnt from the previous cycle.

5. Method according to claim 1, characterized in that the burnt gases recycled from the first zone G1 contain the majority of the gases having undergone cooling in contact with the walls of the working chamber during the previous cycle.

6. Method according to claim 1, characterized in that the working chamber has a symmetry of revolution around the axis of the corresponding cylinder.

7. Method according to claim 1, characterized in that the combustion chamber is formed by a cavity coaxial with the corresponding cylinder formed in the piston and / or the cylinder head and whose diameter is less than that of the cylinder from 50 to 60%.

8. Method according to any one of the preceding claims, characterized in that the fuel mixture of the first zone G1 is animated by a rapid rotational movement around the axis of the corresponding cylinder and in that, at the end of compression , this fuel mixture is fully transferred to the corresponding combustion chamber when the oxidizing charge is still in the annular space between the piston and the cylinder head.

9. Method according to claim 8, characterized in that the fuel mixture of the first zone G1 is located against the corresponding piston.

10. Method according to any one of claims 6 to 9, characterized in that the cavity forming the combustion chamber is formed by a combustion bowl arranged in the piston.

11. Method according to any one of claims 7 to 10, characterized in that the cylinder head comprises a protuberance intended to penetrate into the combustion bowl at the top dead center of the corresponding piston to provide a variable annular clearance depending on the position of this piston and to maintain a gas transfer speed substantially independent of the position of this piston.

12. Method according to claim 8, characterized in that the oxidizing charge of fresh air from the second zone G2 is concentrated against the corresponding cylinder and the recycled burnt gases are concentrated in a central zone of this cylinder and in that the chamber combustion is provided for half in the piston and half in the corresponding cylinder head.

13. Method according to any one of claims 1 to 12, characterized in that one causes the ignition of the gas mass at the periphery of the combustion chamber where the oxidizing charge of fresh air from the second zone G2 begins to oxygenate the fuel mixture of the first zone G1.

14. Method according to any one of claims 1 to 12, characterized in that one causes the ignition of the gaseous mass within the fuel mixture of the first zone G1 at the moment when this mixture reaches the auto temperature ignition and / or the oxidizing charge of fresh air from the second zone G2 begins to penetrate the combustion chamber.

15. Method according to any one of claims 1 to 12, characterized in that one causes the ignition of the gaseous mass immediately after the injection of the fuel into the hot recycled burnt gases of the first zone G1 a little before the top dead center of the corresponding piston.

16. Method according to claim 1, characterized in that one carries out a stratification of the fuel mixture of the first zone G1 and of the oxidizing charge of fresh air of the second zone G2 by successively introducing this fuel mixture and this oxidizing charge in the corresponding cylinder and preferably by introducing the fuel mixture from the first zone G1 before the oxidizing charge.

17. Method according to claim 16, characterized in that the stratification of the fuel mixture of the first zone G1 and of the oxidizing charge of fresh air of the second zone G2 is prepared in an oblong stratification chamber external to the corresponding cylinder and 5 opening on one side into a fresh air supply manifold and on the other side into an intake duct of the corresponding cylinder head.

18. Method according to any one of claims 1 to 17, adapted to the cycle of a four-stroke reciprocating engine, characterized in that the stratification chamber is supplied with burnt gases recycled by a

10 exhaust duct of the corresponding cylinder, distinct from the supply duct of a turbine, by circulating them in a coolant and in the intake duct of the corresponding cylinder head.

19. Method according to any one of claims 1 to 17, adapted to the cycle of a two-stroke reciprocating engine, characterized in that a

L5 refrigerant for recycled burnt gases is located in the part of the stratification chamber opening into the intake duct of the corresponding cylinder head and a non-return valve located between the stratification chamber and the air supply manifold fees prohibits any flow from said chamber to said collector and in that:

! 0 - the hot gases to be cooled and recycled are transferred from the cylinder to the corresponding stratification chamber by opening an inlet valve at the end of the expansion stroke of the corresponding piston, the non-return valve being thereby in the closed position, and - the gaseous mass thus prepared is introduced into the stratification chamber, into the corresponding cylinder by the intake duct by opening an exhaust valve to evacuate the hot gases contained in said cylinder, the fall of pressure thus created in the stratification chamber opening the non-return valve to introduce the charge of fresh air for the next cycle.