FR3136514A1 - Reciprocating internal combustion engine of pure ammonia NH3 - Google Patents

Abstract

The present invention is part of the production of carbon-free thermal energy. Its purpose is a highly turbocharged, low compression ratio 2-stroke reciprocating engine designed to burn pure ammonia without using a combustion promoter gas. It relies on the under-scanning of the cylinder to stratify the temperature of a reactive medium resulting from the mixture of a fresh carburized mass and a recycled burnt mass which achieves self-ignition by compression without risk of detonation with a reduced compression ratio. Figure for abstract: Figure 2

Description

Reciprocating internal combustion engine of pure ammonia NH3 Area of disclosure

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

State of the art

Global warming will condemn hydrocarbons in favor of carbon-free fuels, such as hydrogen and ammonia, to power internal combustion engines. The difficulties of storing hydrogen will favor ammonia for transport and decentralized electricity production.

Reciprocating internal combustion engines burn hydrocarbons in the gas phase via the mechanism of the propagation of a flame front where the products already burned diffuse into the fresh products to bring the latter to their self-ignition temperature which triggers the reactions in chains of live combustion. The slowness of gas 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 compounds this problem to the point where ammonia must be mixed with hydrogen to initiate stable combustion.

To achieve the optimal efficiency of constant volume combustion of the Otto cycle, we sought to concentrate combustion around the top dead center via the HCCI (Homogeanous Charge Compression Ignition) concept. This flameless combustion process consists of heating a homogeneous fuel mixture by compression to its auto-ignition temperature. Unlike the progressive surface heating of a flame front, this volume heating process triggers auto-ignition simultaneously throughout the entire fuel charge with the risk of a destructive detonation for the engine. We tried, without success, to break the homogeneity of the reactive medium by stratifying the local concentration of fuel or by adding cooled recycled burnt gases. This failure was predictable given the predominant role of the temperature of the reactive medium in chemical kinetics, a temperature little modified by the spatial stratification of the local fuel concentration. In addition to the risk of detonation, to achieve the high auto-ignition temperature of pure ammonia, this process would require an unrealistic volumetric compression ratio greater than 35/1.

Disclosure Statement

To remedy the aforementioned problems, the object of the present invention is an underbrushed 2-stroke reciprocating engine for recycling a mass of burnt gases (N2+H2O) whose internal energy initiates the combustion of pure ammonia and whose the chemically inert mass limits the maximum temperature of the cycle.

In order to eliminate detonation, the invention relates to a reciprocating internal combustion engine operating on the 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 burned 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 of which the hottest isothermal stratum reaches its self-ignition temperature under the effect of its compression by the piston before its top dead center and of which the less hot isothermal strata successively reach their temperature self-ignition under the effect of their compression following the thermal expansion of the isothermal strata already burned.

The invention also relates to a method of operating such an engine.

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.

The coldest isothermal stratum of the reactive mixture can reach its auto-ignition temperature after the top dead center of the piston in order to limit the maximum pressure and temperature of the cycle.

The surface extent of the isothermal strata of the reactive mixture can decrease when their temperature increases in order to provide progressiveness to the rate of heat release from combustion.

The hottest stratum of the reactive medium can be located near the center of the combustion chamber and against a heat-insulated piston in order to provide progressiveness to the rate of heat release from combustion and to limit thermal leaks towards the piston body.

A sensor can be located in a cylinder to permanently 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 position angular on a value consistent with a map of the torque/motor operating speed field stored in the computer memory.

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

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

A sensor sensitive to gas pressure can detect the angular position of the maximum pressure of the cycle which coincides with the end of combustion according to the invention.

The engine control computer can act on the temperatures of the isothermal layers of the stratified reactive mixture by individually or collectively controlling some of the following actuators: camshaft phase shifters, variable section turbine distributors, by-passes turbine with variable area, air refrigerant bypasses with variable area.

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 bottom dead center and 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 pressurized at a pressure PA, and provided with exhaust ports connected to an exhaust plenum pressurized at a pressure PE less than PA, the plenums being sufficiently voluminous in relation to the volume of gas transferred at each cycle so that PA and PE are substantially constant in stabilized regime when the torque and the speed of rotation of the engine are constant so that the flows are identical in all the cylinders.

Each cylinder 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 orifices 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 coming from the intake nozzle towards the cylinder head in order to structure thermal stratification axisymmetric.

The face of the piston can be covered by a heat shield which exchanges its heat with the jet of carbureted air to cancel the heat flow towards the piston body.

The exhaust valves and the intake valve can be actuated by camshafts connected to the engine shaft via a common controllable phase shifter to position the combustion in the cycle.

The exhaust valves can be actuated by camshafts connected to the engine shaft via a common phase shifter device and the intake valve can be operated by a camshaft mechanically connected to the engine shaft via a second phase shifter device controllable depending on the engine rotation speed.

The intake plenum can be supplied with combustion air by a high pressure HP turbocharger via an HP air refrigerant and the exhaust plenum can supply burnt gas to an HP turbine of the HP turbocharger whose flow section of the distributor is controlled by the engine control computer to adjust the richness of the fuel mixture in the intake plenum measured by an oxygen probe immersed in the exhaust gas, in accordance with a map of the torque/engine operating speed field stored in the memory from the computer.

The engine may include a working turbine located between the exhaust plenum and the HP turbine to supplement the low gas expansion rate by the piston.

The engine may include a low pressure LP turbocharger whose compressor supplies the HP compressor with combustion air via a LP air refrigerant and whose LP turbine is powered by the burnt gases coming from the HP turbine in order to increase the supercharging rate. of the motor.

The engine control computer can simultaneously control an exhaust camshaft phase shifter and the flow section of the HP turbine distributor to position the combustion and fix the richness in the intake plenum in accordance with a map stored in its memory.

The stroke of the pistons can be close to the diameter of the cylinders to have orifice sections compatible with the cylinder capacity, the combustible gas of mass MA being ammonia NH3 so as not to emit CO2, the MB/MA ratio being greater than 0.70 to reach the auto-ignition temperature with an effective piston volumetric compression ratio between 3 and 5 and a volumetric expansion ratio between 7 and 9.

Brief description of the figures

Other characteristics and advantages of the present disclosure will appear in the following detailed description, referring to the appended drawings in which:

is a schematic view of the air loop of a heavily supercharged turbo-compound engine

is a view of the thermodynamic cycle described by the turbo-compound engine of the

is a schematic view of the internal aerodynamics of a cylinder of the turbo-compound engine of the

is a cross-section of a cylinder of the turbo-compound engine of the

Detailed description of the disclosure

Of course, the disclosure is in no way limited to the embodiment(s) described by way of illustration, not limitation.

Thus, the key to the invention is the elimination of the detonation of a homogeneous fuel 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 of difficult-to-ignite fuels by raising the thermal level of the reactive medium by means other than pure compression. As a bonus, it makes it possible to reduce the production of nitrogen oxides by diluting the reactive medium without reducing the combustion start temperature.

The invention achieves its three objectives by constructing, in an under-flushed 2-stroke engine cylinder, a reactive medium fully stratified in temperature by mixing a fresh isothermal carbureted mass MA with a hot mass MB of burnt gas 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 speed of the reactions increases exponentially with the local temperature. The exponential effect of temperature means that the volume heating by compression of a reactive medium stratified in temperature triggers a self-ignition front which circulates against the temperature gradient between its hottest isothermal stratum and its hottest isothermal stratum. colder until the fuel runs out.

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

The object of the invention is therefore an alternative internal combustion engine operating on the 2-stroke cycle, each cylinder of which 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 burned 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 of which the hottest isothermal stratum reaches its self-ignition temperature under the effect of its compression by the piston before its top dead center TDC and of which the less hot isothermal strata successively reach their temperature d self-ignition under the effect of their compression following the thermal expansion of the isothermal strata already burned.

The process of the invention is no longer effective after TDC when the volumetric expansion of the piston exceeds the thermal expansion due to combustion. However, at maximum engine power, it is possible to terminate combustion after TDC taking into account that, when the heating by compression becomes insufficient to bring the coldest strata to the self-ignition threshold, the diffusion of the front self-ignition in fresh products can supplement the heating resulting from compression.

Combustion is optimal when the concentration of fuel in the fresh mass MA is constant in order to standardize the end-of-combustion temperature in the cylinder. In order to control the mass concentration of fuel at the molecular scale of chemistry, the invention recommends that the carburization of the mass MA takes place 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 weak chemical heating, while the cooler strata poor in burnt gases and rich in fuel undergo strong chemical heating. This homogeneity of combustion temperatures optimizes the power of the cycle and eliminates hot spots that generate nitrogen oxides.

The end of compression temperatures being proportional to the start of compression temperatures, the invention makes it possible to auto-ignite all fuels, regardless of the effective compression ratio of the reactive medium and the auto-ignition temperature. -ignition of the fuel, by controlling the MB/MA ratio of the masses retained in the cylinder before their compression.

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

The geography of thermal stratification of the reactive environment defines the law of heat release as a function of time. The instantaneous release of energy is proportional to the surface area and the richness of the isothermal stratum being self-ignited. The stratification according to the invention where the hottest strata which ignite first are also the least rich in fuel gives a structural progressiveness to the release of heat independently of the evolution of the surface extents of the isothermal strata as a function of their temperature which constitutes the main parameter governing the law of energy release. To optimize the progressiveness of heat release, the surface extent 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. In addition, positioning the hottest zone against a hot piston covered with a heat shield reduces wall heat losses by delaying the interaction of the flame with the cooled wall of the cylinder head and reducing the heat flow towards the piston body.

The geography of the 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 the 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 sets the mixing progress rate.

Thermal stratification is initiated naturally 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 gas diffusion which develops at the boundary between the two masses brought into contact does not have time to reach thermodynamic equilibrium and generates a mixed zone stratified in temperature and fuel concentration.

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

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

The invention includes means for positioning combustion optimally near the high point. To this end, a sensor located in the combustion chamber detects the angular position of the combustion in real time. This sensor permanently informs an engine control computer which controls actuators acting on the thermal level of the reactive medium, to position the auto-ignition at the end of the compression stroke in accordance with a map of the torque/operating speed field of the engine stored in computer memory.

For example, a sensor sensitive to gas pressure 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 section, turbine by-passes with variable flow section, bypasses of air refrigerant with variable flow section.

To have a maximum number of parameters to control the engine, the inventor chose 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 phase-shifting camshafts making it possible to disconnect the gas distribution diagram from the movement of the pistons, unlike single-current scavenging engines which have orifices 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, for land and maritime transport and for the decentralized production of electricity, driven by a turbo-compound turbocharged engine with a low compression ratio. pure ammonia and rotating at constant speed. This particular application exploits all the advantages which characterize the invention.

We now refer to the .

A generator engine (1) operating at 1500 revolutions per minute on the 2-stroke turbo-compound cycle has 6 cylinders (2) in line which successively fire every 60 dv (crankshaft degrees) to separate the exchange of gases which are carried out through cylinder heads.

In order to guarantee 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 at a pressure PE lower than PA. The plenums are sufficiently large in relation to the volume of gas transferred at each cycle so that PA and PE are substantially constant in stabilized conditions at constant load.

We now refer to Figures 3 and 4.

In order to guarantee 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 orifices (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) coming from the intake nozzle towards the cylinder head by diffusing into the burned gases to structure the stratification according to the decreasing isotherms (22), (23), (24). The piston face is covered by a hot heat shield which exchanges heat with the jet of carbureted air to cancel the heat flow to the piston body.

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

To minimize the angular duration of opening/closing of the ports, the valves are returned to their seats by pneumatic springs (not shown) and their seats are recessed in the cylinder head to delay the effective opening of the ports during acceleration of the ports. valves and advance their effective closing during valve deceleration.

We now refer to the 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 shift the entire angular chronology of transfers fixed 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 BDC, and ends with 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 shafts cams.

When the intake nozzle and the exhaust ports are opened simultaneously the PA-PE pressure drop generates a flow in the cylinder which is interrupted when the intake nozzle is closed to fix the mass of carbureted air MA who 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 temporal duration of opening of the intake nozzle which does not vary not with the phase shift of the camshafts.

The burnt gases of a cycle are evacuated according to three successive mechanisms: between OE and OA the overpressure of the cylinder is suddenly discharged up to the pressure PE 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 alone continues the delivery of the burned gases until the cylinder closes. Unlike 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 rate depend on the position P of the piston when the cylinder closes which varies with the phase shift of the cams exhaust. It follows that the camshaft phase shifter, which controls the mass MA+MB of which MA remains constant, simultaneously controls the MB/MA ratio which sets the proportion of gases burned in the stratified mixture and its compression ratio.

To conclude, the camshaft phase shifter controls simultaneously and in the same direction the effective compression ratio of the mass MA+MB, and its fraction of burned gases MB/MA which combine their effects on the thermal level of the reactive medium at the end 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.

We now refer to the .

The intake plenum (3) is supplied with combustion air via a carburetor (13) which diffuses the combustible gas there to standardize the local temperature and richness. 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 inlet pressure 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) driven in rotation 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 reduction gear 21 to the motor 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 refrigerant (18) by a low pressure LP turbocharger (19) driven in rotation by a LP turbine (20) supplied with burnt gases by the HP turbine (16) .

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 of the flow section of the HP turbine distributor, which modifies the expansion rate of the working turbine, simultaneously modifies the work of the turbochargers which fixes the richness in the intake plenum. The phase shifter of the camshafts and the variable section 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 probe located in the exhaust gas.

Operating the engine at constant speed has the advantage of operating the turbochargers at their optimal isentropic efficiency and simplifying the engine control strategy.

The position of the working turbine upstream of the HP and LP turbines optimizes engine 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 rate of the working turbine which sets the thermal efficiency of the cycle.

Claims (21)

Reciprocating internal combustion engine operating on the 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 the gases burned in the previous cycle characterized in what 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 including the stratum the hottest isothermal reaches its self-ignition temperature under the effect of its compression by the piston before its top dead center and whose less hot isothermal strata successively reach their self-ignition temperature under the effect of their compression following the thermal expansion of the already burned isothermal strata. Engine according to claim 1, characterized in that a sensor located in a cylinder permanently 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 strata isotherms of the reactive mixture to place said angular position on a value consistent with a map of the torque/motor operating speed field stored in the computer memory. Engine according to claim 2, characterized in that the temperatures of the isothermal layers of the stratified reactive mixture are controlled via the MB/MA ratio of the masses retained in the cylinder. Engine according to claim 2 or 3, 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. Engine according to one of claims 2 to 4, characterized in that a sensor sensitive to gas pressure detects the angular position of the maximum pressure of the cycle which coincides with the end of combustion according to the invention. Motor according to one of Claims 2 to 5, characterized in that the motor control computer acts on the temperatures of the isothermal layers of the stratified reactive mixture by individually or collectively controlling some of the actuators below: shaft phase shifters cams, variable area turbine distributors, variable area turbine by-passes, variable area air coolant by-passes. Engine according to claim 6, comprising several cylinders provided with intake nozzles and exhaust ports 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 bottom dead center and 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. Engine according to any one of the preceding claims, said engine comprising several cylinders provided with intake nozzles connected to an intake plenum pressurized at a pressure PA, and provided with exhaust ports connected to a pressurized exhaust plenum at a PE pressure lower than PA, the plenums being sufficiently voluminous in relation to the volume of gas transferred at each cycle so that PA and PE are substantially constant in stabilized mode when the torque and the rotation speed of the motor are constant. Engine according to claims 7 and 8, 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 orifices identical peripheral exhausts 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 coming from the nozzle admission to the cylinder head by mixing with the burnt gases to structure the thermal stratification of the reactive medium. Engine according to claim 9, 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. Engine according to claim 9 or 10, characterized in that the exhaust valves and the intake valve are actuated by camshafts connected to the engine shaft via a common controllable phase shifter device. Engine according to one of claims 9 to 11, characterized in that the exhaust valves are actuated by camshafts connected to the engine shaft via a common phase shifter device and that the intake valve is actuated by a shaft cam connected mechanically to the motor shaft via a second controllable phase shifter device. Engine according to one of claims 8 to 12, characterized in that the inlet plenum is supplied with combustion air by a high pressure HP turbocharger via an HP air cooler and that the exhaust plenum supplies a burnt gas to a HP turbine of the HP turbocharger whose flow section of the distributor is controlled by the engine control computer to adjust the richness of the fuel mixture in the intake plenum measured by an oxygen probe immersed in the exhaust gases, in accordance with a mapping of the torque/motor operating speed field stored in the computer memory. Engine according to claim 13, characterized in that it comprises a turbo-compound working turbine located between the exhaust plenum and the HP turbine. Engine according to claim 13 or 14, characterized in that it comprises a low pressure LP turbocharger whose compressor supplies the HP compressor with combustion air via a BP air refrigerant and whose LP turbine is supplied by the burnt gases coming from the HP turbine. Engine according to claims 8 and 13, characterized in that the engine control computer simultaneously controls an exhaust camshaft phase shifter and the flow section of the distributor of the HP turbine to position the combustion and fix the richness in the intake plenum in accordance with a map stored in its memory. Engine according to claim 14, in which the stroke of the pistons is close to the diameter of the cylinders, the combustible gas of mass MA being ammonia NH3, the ratio MB/MA being greater than 0.70, the volumetric expansion rate being between 7 and 9 and the volumetric compression ratio of the pistons being between 3 and 5. Method of operating an engine according to one of the preceding claims, in which the temperature and the mass concentration of combustible gas are identical at all points of the fresh mass MA. Method according to claim 18, in which the coldest isothermal stratum of the reactive mixture reaches its auto-ignition temperature after the top dead center of the piston. Method according to claim 18 or 19, in which the surface extent of the isothermal strata of the reactive mixture decreases as their temperature increases. Method according to one of claims 18 to 20, in which the hottest stratum of the reactive medium is located near the center of the combustion chamber.