Ammonia-powered reciprocating internal combustion engine with auto-ignition combustion

Abstract

The present invention concerns the production of carbon-free thermal energy by reciprocating engines. It relates to a compression auto-ignition combustion method. This method is best implemented in a highly turbocharged 2-stroke cycle designed to burn methane or ammonia without using a combustion promoter. It relies on the thermal stratification of the reaction medium for gradual auto-ignition without the risk of detonation.

Description

[0001] DESCRIPTION

[0002] Title: Constant-volume reciprocating internal combustion engine

[0003] Disclosure area

[0004] The invention relates to the field of carbon-free thermal energy production.

[0005] State of the art

[0006] Global warming will doom hydrocarbons in favor of decarbonized fuels, such as hydrogen and ammonia, to power internal combustion engines. The difficulties of storing hydrogen will favor ammonia for transportation and decentralized electricity generation.

[0007] Reciprocating internal combustion engines burn hydrocarbons in the gas phase via the flame front propagation mechanism where the already burned products diffuse into the fresh products to bring the latter to their auto-ignition temperature which triggers the chain reactions of live combustion. The slowness of gaseous diffusion handicaps the thermal efficiency of reciprocating engines by the angular duration of combustion which produces negative work before the top dead center of the pistons and insufficiently expanded gases after said top dead center. The very low flame speed of ammonia aggravates this problem to the point where it is necessary to mix ammonia with hydrogen to initiate stable combustion.

[0008] To achieve optimal efficiency of constant volume combustion in the Otto cycle, efforts were made to concentrate combustion around top dead center using the HCCI (Homogeanous Charge Compression Ignition) concept. This flameless combustion process involves heating a homogeneous fuel mixture by compression to its auto-ignition temperature. Unlike the progressive surface heating of a flame front, this volumetric heating process triggers auto-ignition simultaneously throughout the fuel charge, with the risk of destructive detonation for the engine. Attempts were made, unsuccessfully, to break the homogeneity of the reactive medium by stratifying the local fuel concentration or by adding cooled recycled burnt gases. This failure was predictable given the predominant role of the temperature of the reactive medium in the chemical kinetics, a temperature that is little modified by the spatial stratification of the local fuel concentration.Besides the risk of detonation, to reach the high auto-ignition temperature of pure ammonia, this process would require an unrealistic volumetric compression ratio greater than 35 / 1.

[0009] Disclosure Statement

[0010] To overcome the above-mentioned problems, the object of the present invention is to stratify the temperature of the reactive medium to confer progressiveness to the self-ignition by compression.

[0011] The invention can be applied to all fuels in a 4-stroke cycle and in a 2-stroke cycle via different gas exchange strategies. The 2-stroke cycle is best suited to low-carbon, low-flammability fuels such as methane and ammonia.

[0012] In order to eliminate detonation, the invention relates to a method for operating a reciprocating internal combustion engine in which each cylinder receives, at each cycle, a fresh mass MA of an isothermal mixture of combustion air and a combustible gas which replaces the mass of gases burned in the previous cycle, characterized in that a hot mass MB of burnt gases present in the cylinder diffuses throughout the fresh mass MA, during the gas exchange phase and the compression phase, to form a reactive mixture which is totally stratified in temperature, the hottest isothermal stratum of which reaches its self-ignition temperature before top dead center under the effect of its compression by the piston and the less hot isothermal strata of which successively reach their self-ignition temperature under the effect of their compression following the thermal expansion of the isothermal strata which have already been burned.

[0013] The temperature and mass concentration of combustible gas can be identical at all points of the fresh mass MA in order to standardize the temperature of the gases resulting from combustion.

[0014] The coldest isothermal layer of the reacting mixture can reach its autoignition temperature after the piston's top dead center in order to limit the maximum pressure and temperature of the cycle.

[0015] The surface area of ​​the isothermal strata of the reactive mixture can decrease when their temperature increases in order to confer a progressiveness to the rate of heat release of combustion.

[0016] The hottest area of ​​the reactive medium may be located near the center of the combustion chamber. This area may also be located against a heat-insulated piston. This can help to provide a progressive heat release rate from combustion and limit heat leakage to the piston body.

[0017] A sensor may be located in a cylinder to continuously transmit the angular position of the combustion relative to the top dead center of the piston to an engine control computer which controls actuators acting on the temperatures of the isothermal strata of the reactive mixture to place said angular position on a value conforming to a mapping of the torque / operating speed field of the engine stored in the computer's memory.

[0018] The temperatures of the isothermal strata of the stratified reactive mixture can be controlled via the MB / MA ratio of the masses retained in the cylinder.

[0019] The temperatures of the isothermal strata of the stratified reactive mixture can be controlled via the effective compression ratio of the piston that compresses the MA+MB mixture.

[0020] A pressure sensitive sensor located in a cylinder can detect the angular position of the maximum pressure of the cycle and the engine control computer can position said maximum pressure after the top dead center of the piston so that it coincides with the end of combustion according to the invention.

[0021] The engine control computer can act on the temperatures of the isothermal strata of the stratified reactive mixture by individually or collectively controlling at least some of the following actuators: camshaft phasers, variable area turbine distributors, variable area turbine bypasses, variable area air cooler bypasses.

[0022] The method according to the invention is particularly effective in the case of a reciprocating internal combustion engine operating on the under-swept 2-stroke cycle, each cylinder of which receives at each cycle a fresh mass MA of an isothermal mixture of combustion air and a combustible gas which replaces the mass of gases burned in the previous cycle, characterized in that a hot mass MB of burnt gases retained in the cylinder diffuses throughout the fresh mass MA, during the gas exchange phase and the compression phase, to form a reactive mixture which is totally stratified in temperature, the hottest isothermal stratum of which reaches its self-ignition temperature before top dead center under the effect of its compression by the piston and the less hot isothermal strata of which successively reach their self-ignition temperature under the effect of their compression following the thermal expansion of the isothermal strata which have already been burned.Said engine may comprise several identical cylinders provided with exhaust ports and intake nozzles closed according to a chronology which successively comprises: in the vicinity of the bottom dead center of the piston, the simultaneous opening of the exhaust ports followed by the simultaneous opening of the intake nozzles; between the bottom dead center and the top dead center of the piston, the simultaneous closing of the intake nozzles followed by the simultaneous closing of the exhaust ports, the engine control computer controlling the closing angle of the exhaust ports to position the combustion by simultaneously modulating the MB / MA ratio and the effective compression ratio of the stratified reactive mixture MA+MB.Said engine may comprise several cylinders provided with intake nozzles connected to an intake plenum and exhaust ports connected to an exhaust plenum; the intake plenum being pressurized to a pressure PA by the compressor of a high-pressure turbocharger HP via an air cooler HP, the exhaust plenum pressurized to a pressure PE slightly lower than the pressure PA feeding the HP turbine of the HP turbocharger via a power turbine TC.

[0023] The said engine may include a LP turbocharger whose compressor supplies air to the HP compressor via an HP refrigerant and whose turbine is supplied with burnt gases by the turbine of the HP turbocharger.

[0024] The camshafts of the said engine can be mechanically connected to the crankshaft via a common phase shifter controlled by the computer to angularly position the combustion via simultaneously the compression ratio and the burnt gas ratio of the MA+MB mixture.

[0025] A variable flow area of ​​the HP turbocharger turbine distributor can be controlled by the engine control computer to adjust the fuel richness of the MA mass.

[0026] A variable flow section of the TC power turbine distributor can be controlled by the engine control computer to position the combustion and a variable flow section of the HP turbocharger turbine can be controlled by the engine control computer to set the richness of the MA mass in accordance with a map stored in the computer's memory.

[0027] Each cylinder of said engine can be closed, on the one hand, by the flat face of an axisymmetric cylinder head comprising an axial intake nozzle formed around an intake valve and four identical peripheral exhaust ports arranged in a square around the intake nozzle and closed by synchronous valves, and on the other hand, by the face of an axisymmetric piston formed to return the jet of carbureted air from the intake nozzle to the cylinder head in order to structure the thermal stratification of the reactive medium.

[0028] The piston face may be covered by a heat shield that exchanges its heat with the carbureted air jet to cancel the heat flow to the piston body.

[0029] All the valves of the said engine can be actuated by cams mechanically connected to the crankshaft via a common phase shifter device.

[0030] The exhaust valves of said engine may be actuated by camshafts connected to the engine shaft via a common phase shifter device and the intake valve may be actuated by a camshaft mechanically connected to the engine shaft via a second phase shifter device.

[0031] The stroke of the pistons of the said engine may be close to the diameter of the cylinders, the combustible gas of the mass MA being ammonia NH3, the ratio MB / MA being greater than 0.70, the volumetric expansion ratio being between 7 and 9 and the volumetric compression ratio of the pistons being between 3 and 5.

[0032] The cold start of the said engine comprising a camshaft phase shifter, a carburetor and a starter can be controlled by the engine control computer according to the following chronology:

[0033] Cutting off the fuel supply, closing the carburetor throttle valve and dephasing the camshafts to close the cylinders at bottom dead center of the pistons and open them prematurely during the gas expansion stroke.

[0034] Simultaneous opening of the fuel supply and the carburetor throttle when the air temperature at the end of compression reaches the fuel autoignition temperature

[0035] Simultaneous control of the phase shifter and the throttle to position combustion at top dead center and stabilize idle speed.

[0036] Brief description of the figures

[0037] Other features and advantages of the present disclosure will become apparent from the following detailed description, with reference to the accompanying drawings in which:

[0038] [Fig. 1] is a schematic view of the air loop of a heavily supercharged turbo-compound engine

[0039] [Fig. 2] is a view of the thermodynamic cycle described by the turbo-compound engine of Figure 1

[0040] [Fig. 3] is a schematic view of the internal aerodynamics of a cylinder of the turbo-compound engine of Figure 1

[0041] [Fig. 4] is a cross-section of a cylinder of the turbo-compound engine of Figure 1

[0042] Detailed Description of Disclosure

[0043] Of course, the disclosure is in no way limited to the embodiment(s) described for illustrative, non-limiting purposes.

[0044] Thus, the key to the invention is the elimination of detonation of a homogeneous carbureted charge, which ruined the hopes raised by HCCI combustion at the end of the last century. It opens the way to rapid self-ignition by compression for fuels that are difficult to ignite by raising the thermal level of the reactive medium by means other than pure compression. It also makes it possible to reduce the production of nitrogen oxides by diluting the reactive medium without reducing the combustion start temperature.

[0045] The invention achieves its three objectives by constructing, in a reciprocating engine cylinder, a fully temperature-stratified reactive medium by mixing a fresh isothermal carbureted mass MA with a hot mass MB of burnt gases retained in the cylinder. The invention is based on the laws of chemical kinetics which govern the oxidation of a combustible mixture in the gas phase, stipulating that the rate of reactions increases exponentially with the local temperature. The exponential effect of temperature means that volumetric heating by compression of a temperature-stratified reactive medium triggers a self-ignition front which circulates against the temperature gradient between its hottest isothermal stratum and its coldest isothermal stratum until the fuel is exhausted.

[0046] The speed of movement of the self-ignition front depends on the temperature gradient and the richness of the active layer. The speed of the front is always subsonic given the high temperature of the reactive medium and therefore does not produce a shock wave.

[0047] The subject of the invention is therefore a reciprocating internal combustion engine in which each cylinder receives, at each cycle, a fresh isothermal mass MA of a mixture of combustion air and a combustible gas which replaces the mass of gases burned in the previous cycle, characterized in that a hot mass MB of burnt gases retained in the cylinder diffuses throughout the fresh mass MA, during the gas exchange phase and the compression phase, to form a reactive mixture which is totally stratified in temperature, the hottest isothermal stratum of which reaches its self-ignition temperature under the effect of its compression by the piston before its top dead center TDC and the less hot isothermal strata of which successively reach their self-ignition temperature under the effect of their compression following the thermal expansion of the isothermal strata which have already been burned.

[0048] The method of the invention is no longer operative after TDC when the volumetric expansion of the piston exceeds the thermal expansion due to combustion. Nevertheless, at maximum engine power, it is possible to terminate combustion after TDC given that, when heating by compression becomes insufficient to bring the coldest layers to the self-ignition threshold, the diffusion of the self-ignition front in the fresh products can supplement heating resulting from compression.

[0049] Combustion is optimal when the fuel concentration in the fresh mass MA is constant in order to standardize the end-of-combustion temperature in the cylinder. In order to control the fuel mass concentration at the molecular scale of chemistry, the invention recommends that the carburetion of the mass MA is carried out in an intake plenum of sufficient volume so that the diffusion of the combustible gas in the combustion air has time to reach thermodynamic equilibrium. The thermal stratification according to the invention thus generates in the reactive medium an inverse gradient of fuel mass concentration which homogenizes the final combustion temperature. Indeed, the hottest strata rich in burnt gases and poor in fuel undergo low chemical heating, while the cooler strata poor in burnt gases and rich in fuel undergo high chemical heating.This homogeneity of combustion temperatures optimizes the power of the cycle and eliminates hot spots that generate nitrogen oxides.

[0050] Since the end of compression temperatures are proportional to the start of compression temperatures, the invention makes it possible to achieve self-ignition of all fuels, regardless of the effective compression ratio of the reactive medium and the self-ignition temperature of the fuel, by controlling the MB / MA ratio of the masses retained in the cylinder before their compression.

[0051] The chemically inert ballast of recycled burnt gases MB which is parked in the cylinder retains its internal energy with the exception of thermal leaks at the walls. During compression-expansion cycles it releases internal energy to the reactive medium to initiate combustion and simultaneously recovers it by diluting said combustion to limit the final temperature. The thermal efficiency of the cycle is thus optimized by a complete, rapid and non-detonating combustion which takes place at a substantially constant volume near TDC.

[0052] The geography of the thermal stratification of the reactive medium defines the heat release law as a function of time. The instantaneous energy release is proportional to the surface area and the richness of the isothermal stratum undergoing self-ignition. The stratification according to the invention, where the hottest strata that ignite first are also the least rich in fuel, gives a structural progressiveness to the heat release independently of the evolution of the surface areas of the isothermal strata as a function of their temperature, which constitutes the main parameter governing the energy release law. To optimize the progressiveness of the heat release, the surface area of ​​the isothermal strata must therefore decrease when their temperature increases. The positioning of the hottest zone of the reactive medium in the center of the combustion chamber satisfies this requirement.Additionally, positioning the hottest zone against a hot piston covered with a heat shield reduces wall heat losses by delaying flame interaction with the cooled cylinder head wall and reducing heat flow to the piston body.

[0053] The geography of stratification depends on the position and orientation of the intake nozzles, the position of the exhaust ports and the geometry of the cylinder and the combustion chamber limited by the piston and the cylinder head. The geography of stratification also depends on the initial temperatures of the fresh mass MA and the hot mass MB, the ratio of the two masses MB / MA, the turbulence of the reactive medium and the duration of cohabitation of the two masses in the cylinder which determines the rate of advancement of the mixture.

[0054] Thermal stratification naturally begins in a 2-stroke engine cylinder when the fresh mass MA sweeps away the burnt gases from the previous cycle. During this rapid transfer, the gaseous diffusion that develops at the boundary between the two masses in contact does not have time to reach thermodynamic equilibrium and generates a mixed zone stratified in temperature and fuel concentration.

[0055] Unlike prior art 2-stroke engines where the gas exchange is structured to minimize the volume of the mixed zone and where the cylinder is closed when the mixed zone is partially expelled to provide combustion with as pure a combustion air as possible, the invention structures the gas exchange to diffuse burnt gases throughout the entire fresh mass MA without discharging carbureted air to the exhaust.

[0056] The conflicting objectives in a 2-stroke cycle of intensifying gas mixing and avoiding an intake-exhaust bypass imply the design of a specific gas exchange architecture of the invention with the help of a 3D numerical simulation of reactive fluid mechanics.

[0057] The invention comprises means for positioning the combustion optimally in the vicinity of the high point. For this purpose, a sensor located in the combustion chamber detects the angular position of the combustion in real time. This sensor continuously informs an engine control computer which controls actuators acting on the thermal level of the reactive medium, to position the self-ignition at the end of the compression stroke in accordance with a mapping of the engine torque / operating speed field stored in the computer's memory.

[0058] For example, a gas pressure sensitive sensor detects the maximum pressure of the cycle which coincides with the end of combustion. Among the actuators which act instantly on the thermal level of the reactive medium we will cite, without limitation, camshaft phase shifters, turbine distributors with variable flow area, turbine bypasses with variable flow area, air refrigerant bypasses with variable flow area.

[0059] To have a maximum number of parameters for controlling the engine, the inventor has chosen to continue the description of the invention in the architecture of 4-stroke engines where the intake and exhaust ports are located in a cylinder head and are closed by valves actuated by dephasing camshafts making it possible to disconnect the gas distribution diagram from the movement of the pistons, unlike single-current scavenging engines which have ports located in the thickness of the cylinders closed by the pistons. In the following, the invention is presented in its preferred application in a light generator set, for land and sea transport and for decentralized electricity production, driven by a two-stroke turbocharged turbo-compound engine with low compression ratio, fueled with pure ammonia and rotating at constant speed.This particular application exploits all the advantages that characterize the invention.

[0060] We now refer to Figure 1.

[0061] A generator engine (1) operating at 1500 rpm on the 2-stroke turbo-compound cycle has 6 cylinders (2) in line which fire successively every 60 dv (crankshaft degrees) to separate the gas exchanges which take place through cylinder heads.

[0062] In order to ensure identical internal aerodynamics in all cylinders, the intake nozzles are connected to an intake plenum (3) pressurized to a pressure PA, and the exhaust ports are connected to an exhaust plenum (4) pressurized to a pressure PE lower than PA. The plenums are sufficiently large compared to the volume of gas transferred in each cycle so that PA and PE are substantially constant in steady state at constant load.

[0063] We now refer to Figures 3 and 4.

[0064] In order to ensure axisymmetric internal aerodynamics, each cylinder is closed, on the one hand, by the flat face (5) of an axisymmetric cylinder head comprising an axial intake nozzle (6) formed around an intake valve (7) and four identical peripheral exhaust ports (8) arranged in a square around the intake nozzle and closed by synchronous valves (9), and on the other hand, by the face (10) of an axisymmetric piston formed to return the jet of carbureted air (11) from the intake nozzle to the cylinder head by diffusing in the burnt gases to structure the stratification according to the decreasing isotherms (22), (23), (24). The face of the piston is covered by a hot heat shield which exchanges its heat with the jet of carbureted air to cancel the heat flow towards the piston body.

[0065] The piston stroke is equal to the cylinder diameter at 180 mm to provide a flow section of transfer ports in the cylinder head compatible with the cylinder capacity.

[0066] To minimize the angular duration of port opening / closing, the valves are biased to their seats by air springs (not shown) and their seats are recessed into the cylinder head to delay the effective opening of the ports during valve acceleration and advance their effective closing during valve deceleration.

[0067] We now refer to Figure 2 which represents the pressure / volume diagram of the closed cycle. The intake valve and the exhaust valves are closed once per cycle by two identical overhead camshafts (12) synchronized on the engine shaft via a common controllable device (not shown) to phase shift the entire angular timing of the transfers set by the camshafts. The exchange of gases begins at the opening OA of the intake nozzle which follows the simultaneous opening OE of the exhaust ports at the end of expansion of the burnt gases when the piston slows down near its bottom dead center PMB, and ends at the simultaneous closing FE of the exhaust ports which follows the closing FA of the intake nozzle when the piston has accelerated towards its top dead center TDC and occupies a position P controlled by the common phase shifter of the camshafts.

[0068] When the intake nozzle and the exhaust ports are open simultaneously, the pressure drop PA-PE generates a flow in the cylinder which is interrupted when the intake nozzle closes to fix the mass of carbureted air MA which will participate in the cycle. When the flow section of the 4 exhaust ports is greater than 2.5 times that of the intake nozzle, the mass MA introduced into the cylinder is proportional to the time duration of opening of the intake nozzle which does not vary with the phase shift of the camshafts.

[0069] The burnt gases of a cycle are evacuated according to three successive mechanisms: between OE and OA the cylinder overpressure is suddenly discharged until the PE pressure of the exhaust plenum; between OA and FA the fresh charge expels burnt gases towards the exhaust ports which are added to those discharged by the piston; between FA and FE, the piston continues alone to discharge the burnt gases until the cylinder closes. Contrary to the mass MA which does not depend on the phase shift of the camshafts, the mass MA+MB retained in the cylinder and its compression ratio depend on the position P of the piston at the closing of the cylinder which varies with the phase shift of the exhaust cams. It follows that the camshaft phase shifter, which controls the mass MA+MB whose MA remains constant, simultaneously controls the MB / MA ratio which fixes the proportion of burnt gases in the stratified mixture and its compression ratio.

[0070] To conclude, the camshaft phase shifter simultaneously and in the same direction controls the effective compression ratio of the mass MA+MB, and its fraction of burnt gases MB / MA which combine their effects on the thermal level of the reactive medium at the end of compression. By controlling the camshaft phase shifter, the computer therefore acts doubly on the thermal level of the reactive medium to adjust the position of combustion in the thermodynamic cycle.

[0071] We now refer to Figure 1.

[0072] The intake plenum (3) is supplied with combustion air via a carburetor (13) which diffuses the combustible gas therein to standardize the local temperature and richness. The engine torque is controlled by the flow of fuel injected into the carburetor. The volumetric ratio is close to 7 and the effective compression ratio is close to 4 to accept the intake pressure of close to 20 bars delivered by two turbochargers working in cascade without exceeding the maximum pressure of 240 bars. The intake plenum is supplied with combustion air, via an air cooler (14), by a high-pressure HP turbocharger (15) rotated by an HP turbine (16) supplied with burnt gases by the exhaust plenum (4) via a turbo-compound working turbine (17) which completes the useful expansion of the burnt gases aborted by the piston. The turbo-compound turbine 17 can be mechanically connected via a reducer 21 to the engine shaft or to an auxiliary alternator.The flow section of the HP turbine distributor can be controlled to adjust the fuel richness in the intake plenum. The HP turbocharger (15) is supplied with combustion air via an air cooler (18) by a low-pressure LP turbocharger (19) rotated by a LP turbine (20) supplied with burnt gases by the HP turbine (16).

[0073] The enthalpy available in the exhaust plenum is shared between the useful work of the turbo-compound turbine and the work of the turbochargers which sets the pressure PA and the richness in the intake plenum. A variation in the flow area of ​​the HP turbine distributor, which modifies the expansion ratio of the working turbine, simultaneously modifies the work of the turbochargers which sets the richness in the intake plenum. The camshaft phase shifter and the variable area distributor of the HP turbine are the two actuators which allow the computer to simultaneously control the angular position of the combustion and the richness of the mass MA measured by an oxygen sensor located in the exhaust gases.

[0074] Operating the engine at constant speed has the advantage of operating the turbochargers at their optimum isentropic efficiency and simplifying the engine control strategy.

[0075] The position of the working turbine upstream of the HP and LP turbines optimizes the engine's efficiency at partial loads. Indeed, the reductions in engine torque controlled by the flow of fuel injected into the carburetor successively impact the rotation speed of the LP turbocharger then that of the HP turbocharger before impacting the expansion ratio of the working turbine which determines the thermal efficiency of the cycle.

[0076] When the engine is started from cold, the cylinders do not contain any hot gases from previous cycles. To avoid additional devices to ensure this function, the invention proposes a process that can be controlled by the engine control computer that controls the carburetor and the camshaft phase shifter.

[0077] At each engine stop, the fuel supply is cut off, the carburetor throttle is closed and the phase shifter positions the cams to advance the cylinder closing to bottom dead center of the piston to maximize the air compression ratio and simultaneously advance the cylinder opening before bottom dead center to reduce the gas expansion rate.

[0078] With this timing of the distribution diagram, the rotation of the crankshaft by a starter, electric for example, triggers at each rotation a strong heating by compression of the air and a cooling reduced by an aborted expansion which discharges into the exhaust plenum, via the exhaust ports, a fraction of the hot air partially re-drawn into the cylinder by the residual stroke of the piston up to its bottom dead center with a mass of fresh air from the intake plenum, via the intake ports opened later, regulated by the carburetor butterfly.

[0079] The first rotation will begin its cycle at room temperature and each subsequent rotation will add heat to the cylinder to reach the fuel auto-ignition temperature at the piston's top dead center.

[0080] Fuel is then injected into the carburetor to decouple the starter motor, initiating a naturally aspirated, slightly supercharged idle engine operation regulated by the engine control computer via the fuel injector and throttle valve, which respectively control the rotational speed and angular position of combustion via the quantity and temperature of the recycled burnt gases.

[0081] Engine acceleration and loading are handled by the fuel injector and the camshaft phaser which takes over from the wide open throttle to position combustion in the cycle

Claims

DEMANDS 1. Method of operating a reciprocating internal combustion engine in which each cylinder receives at each cycle a fresh mass MA of an isothermal mixture of combustion air and a fuel gas which replaces the mass of the burnt gases in the previous cycle characterized in that a hot mass MB of burnt gases present in the cylinder diffuses throughout the fresh mass MA, during the gas exchange phase and the compression phase, to form a reactive mixture totally stratified in temperatures, the hottest isothermal stratum of which reaches its auto-ignition temperature before top dead center under the effect of its compression by the piston and the less hot isothermal strata successively reach their auto-ignition temperature under the effect of their compression following the thermal expansion of the isothermal strata already burned.

2. A method of operation according to claim 1 of a reciprocating internal combustion engine operating on the 2-stroke underswept cycle, in which each cylinder receives at each cycle a fresh mass MA of an isothermal mixture of combustion air and a fuel gas which replaces the mass of the burnt gases in the previous cycle, characterized in that a hot mass MB of burnt gases retained in the cylinder diffuses throughout the fresh mass MA, during the gas exchange phase and the compression phase, to form a reactive mixture totally stratified in temperatures, the hottest isothermal layer of which reaches its auto-ignition temperature before top dead center under the effect of its compression by the piston, and the less hot isothermal layers successively reach their auto-ignition temperature under the effect of their compression following the thermal expansion of the already burned isothermal layers.

3. A method for operating an engine according to claim 1, characterized in that the coldest isothermal layer of the reactive mixture reaches its auto-ignition temperature after top dead center of the piston 4. A method for operating an engine according to claim 1, characterized in that the temperature and mass concentration of combustible gas are identical at all points of the fresh mass MA 5. Method of operating an engine according to claim 1, characterized in that the surface area of ​​the isothermal layers of the reactive mixture decreases when their temperature increases.

6. Method of operating an engine according to claim 1, characterized in that the hottest area of ​​the reactive medium is located near the center of the combustion chamber.

7. A method for operating an engine according to claim 1, characterized in that a sensor located in a cylinder continuously transmits the angular position of the combustion relative to the top dead center of the piston to an engine control computer which controls actuators acting on the temperatures of the isothermal layers of the reactive mixture to place said angular position on a value conforming to a map of the engine's operating torque / speed field stored in the computer's memory.

8. Method of operating an engine according to claim 7, characterized in that a pressure-sensitive sensor located in a cylinder detects the angular position of the maximum pressure of the cycle and the engine control computer positions said maximum pressure after the top dead center of the piston so that it coincides with the end of combustion according to the invention.

9. A method for operating an engine according to claim 7, characterized in that the engine control computer acts on the temperatures of the isothermal layers of the stratified reactive mixture by individually or collectively controlling at least some of the following actuators: camshaft phasers, variable area turbine distributors, variable area turbine bypasses, variable area air cooler bypasses 10. Method of operating an engine according to claim 7, characterized in that the temperatures of the isothermal layers of the stratified reactive mixture are controlled via the MB / MA ratio of the masses present in the cylinder.

11. Method of operating an engine according to claim 7, characterized in that the temperatures of the isothermal layers of the stratified reactive mixture are controlled via the effective compression ratio of the piston which compresses the MA+MB mixture.

12. A method for operating a multi-cylinder engine according to claim 2, wherein each cylinder is provided with intake nozzles connected to an intake plenum and exhaust ports connected to an exhaust plenum, closed by valves actuated by cams in a sequence comprising successively: in the vicinity of bottom dead center of the piston, the simultaneous opening of the exhaust ports followed by the simultaneous opening of the intake nozzles; between bottom dead center and top dead center of the piston, the simultaneous closure of the intake nozzles followed by the simultaneous closure of the exhaust ports; the intake plenum being pressurized to a pressure PA by the compressor of a high-pressure turbocharger HP via an air cooler HP, the exhaust plenum pressurized to a pressure PE slightly lower than the pressure PA supplying the turbine HP of the turbocharger HP via a power turbine TC.

13. A method of operating an engine according to claim 12 comprising a low-pressure turbocharger whose compressor supplies air to the high-pressure compressor via a high-pressure refrigerant and whose turbine is supplied with exhaust gases by the turbine of the high-pressure turbocharger 14. Method of operating an engine according to claims 9, 10, 11, 12 wherein the camshafts are mechanically connected to the crankshaft via a common phaser controlled by the computer to angularly position the combustion via simultaneously the compression ratio and the burnt gas ratio of the MA+MB mixture.

15. Method of operating an engine according to claim 12 wherein a variable flow section of the turbocharger turbine distributor is controlled by the engine control computer to control the fuel mixture of mass MA.

16. Method of operating an engine according to claims 9 and 12 wherein a variable flow section of the power turbine distributor TC is controlled by the engine control computer to position the combustion and wherein a variable flow section of the turbocharger turbine HP is controlled by the engine control computer to fix the mass mixture MA in accordance with a map stored in the computer memory.

17. Method of operating an engine according to claim 12, characterized in that each cylinder is closed, on the one hand, by the flat face of an axisymmetric cylinder head comprising an axial intake nozzle formed around an intake valve and four identical peripheral exhaust ports arranged in a square around the intake nozzle and closed by synchronous valves, and on the other hand, by the face of an axisymmetric piston formed to return the jet of carbureted air from the intake nozzle to the cylinder head, mixing with the recycled burnt gases to structure the thermal stratification of the reactive medium.

18. Method of operating an engine according to claim 17, characterized in that the face of the piston is covered by a heat shield which exchanges its heat with the jet of carbureted air to cancel the heat flow towards the body of the piston.

19. Method of operating an engine according to claim 12, characterized in that all the valves are actuated by cams mechanically connected to the crankshaft via a common phase-shifting device.

20. Method of operating an engine according to claim 12, characterized in that the exhaust valves are actuated by cams connected to the crankshaft via a common phase-shifting device and the intake valve is actuated by a cam mechanically connected to the engine shaft via a second phase-shifting device.

21. Method of operating an engine according to claim 12, in which the stroke of the pistons is close to the diameter of the cylinders, the fuel gas of mass MA being ammonia NH3, the ratio MB / MA being greater than 0.70, the volumetric expansion ratio being between 7 and 9 and the volumetric compression ratio of the pistons being between 3 and 5.

22. A method for cold starting an engine according to claims 12 and 19 comprising a camshaft phaser, a carburetor and a starter controlled by the engine control computer in the following sequence: Fuel supply cut-off, carburetor butterfly valve closing and camshaft phasing to close the cylinders at piston bottom dead center and open them prematurely during the gas expansion stroke. Simultaneous opening of the fuel supply and the carburetor throttle when the air temperature at the end of compression reaches the fuel auto-ignition temperature Simultaneous control of the phase shifter and the throttle to position the combustion at top dead center and stabilize an idle speed.

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