Diesel and ammonia dual-fuel engine combustion system and combustion method based on a composite EGR circuit

Abstract

The object of the present invention is to provide a combustion system and a combustion method for a diesel and ammonia dual-fuel engine based on a composite EGR circuit, including a combustion system, an electronic control system, a composite EGR circuit system, a variable valve system, a high-pressure common rail fuel system, and an ammonia supply system. The composite EGR circuit system includes three circuits: 1. an in-cylinder high-temperature EGR circuit based on variable valve timing, 2. an out-of-cylinder low-flow low-temperature EGR circuit, and 3. an out-of-cylinder high-flow low-temperature EGR circuit. The electronic control system controls the two concentric double-needle valve fuel injectors and the in-cylinder direct injection ammonia injector with a flexible injection strategy in combination with the composite circuit EGR system according to different working conditions and the combustion characteristics of the fuel, which can effectively control the mixing characteristics and ignition characteristics in the cylinder, thereby optimizing the in-cylinder combustion and significantly reducing emissions at the same time. The present invention can improve the performance of the engine under all working conditions, achieve high combustion efficiency and low emissions, and has good adaptability to low-temperature environments.

Description

Technical Field

[0001] The present invention relates to an ammonia fuel engine, specifically to an engine combustion system and method. Background Art

[0002] The demand for global fossil energy is increasing continuously, and a large amount of greenhouse gases and pollutants are generated when fossil fuels are burned. In order to further prevent global warming and reduce people's dependence on non-renewable resources such as fossil fuels, a series of regulations have been issued internationally for industries such as industry, shipping, and automobiles to decarbonize. Using alternative fuels with renewable low-carbon or carbon-free characteristics in diesel engines is the most effective solution to achieve zero carbon emissions. Nowadays, many renewable low-carbon and zero-carbon energy sources have been widely studied and applied to engines.

[0003] Ammonia fuel is a zero-carbon fuel that does not produce carbon emissions, and its octane number is as high as 130, enabling the engine to achieve a higher compression ratio. However, the flame propagation speed of ammonia is small, which makes the time required for combustion and ignition in the engine longer, the temperature higher, and it is not easy to ignite. How to reasonably organize the combustion method, reduce the loss of compression work, and improve the system stability has become a challenge in the development of ammonia fuel engines.

[0004] Most existing patents focus on improving combustion by adding hydrogen to improve the activity of ammonia fuel. The patent with publication number CN114483299A, a system and method for reducing unburned ammonia emissions in an ammonia engine, designs various injection methods of ammonia fuel to be adjusted according to different working conditions, and at the same time uses waste heat of exhaust gas to displace hydrogen to achieve the purpose of assisting in burning ammonia fuel. However, it is difficult to actually achieve using waste heat of exhaust gas and cracking of unburned ammonia to produce hydrogen to enhance the reaction activity, and the system stability is poor. The patent with publication number CN114738140A, a zero-carbon engine and control method for ammonia-hydrogen hybrid combustion ignited by hydrogen, realizes the practical application of hydrogen igniting ammonia fuel in a compression ignition engine by increasing the intake air temperature. However, the cost of hydrogen storage and transportation is very high and a reasonable combustion organization method is not designed, lacking reliability. Summary of the Invention

[0005] The purpose of the present invention is to provide a diesel and ammonia dual-fuel engine combustion system and combustion method based on a composite EGR circuit that can eliminate knocking while improving the power performance of the dual-fuel engine, and ensure the engine works efficiently, cleanly and stably under all working conditions.

[0006] The purpose of the present invention is achieved as follows:

[0007] The combustion system of a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to the present invention is characterized in that it includes an intake pipe, an exhaust pipe, a cylinder, a cylinder head, a piston, a large-flow low-temperature EGR circuit, and a small-flow low-temperature EGR circuit. The cylinder, the cylinder head, and the piston form a combustion chamber. A concentric dual-needle valve injector, an in-cylinder direct injection ammonia injector, an intake passage, and an exhaust passage are provided on the cylinder head. The concentric dual-needle valve injector includes an external large-flow injector and an internal small-flow injector. The external large-flow injector includes an external large-flow injector needle valve body and an internal small-flow injector needle valve body. The external large-flow injector needle valve body is located outside the internal small-flow injector needle valve body, and an external large-flow injector oil passage is formed between the two. The internal small-flow injector includes an internal small-flow injector needle valve body and an internal small-flow injector needle valve. The internal small-flow injector needle valve body is located outside the internal small-flow injector needle valve, and an internal small-flow injector oil passage is formed between the two.

[0008] The intake passage is connected to the intake pipe, the exhaust passage is connected to the exhaust pipe. A venturi tube is provided on the intake pipe, and a compressor is provided behind the venturi tube. A turbine is provided on the exhaust pipe. The compressor and the turbine are coaxial. The large-flow low-temperature EGR circuit and the small-flow low-temperature EGR circuit are parallel circuits. The inlet end of the parallel circuit is connected to the exhaust pipe downstream of the turbine, and the outlet end of the parallel circuit is connected to the throat of the venturi tube. A large-flow EGR valve and a large-flow EGR cooler are provided on the large-flow low-temperature EGR circuit, and a small-flow EGR valve and a small-flow EGR cooler are provided on the small-flow low-temperature EGR circuit.

[0009] The combustion system of a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to the present invention may further include:

[0010] 1. An exhaust back-pressure valve is provided on the exhaust pipe.

[0011] 2. The external large-flow injector is a pintle injector, and the internal small-flow injector is a multi-hole injector. The number of spray holes is 6 - 8. Both the external large-flow injector and the internal small-flow injector include independent needle valves, needle valve bodies, springs, solenoid valves, and oil passages.

[0012] 3. The flow rate of the internal small-flow injector needle valve body reaches the linearity range earlier than that of the external large-flow injector needle valve body.

[0013] 4. The in-cylinder direct injection ammonia injector has a multi-hole structure, and the spray holes are axially symmetrically distributed about the central axis of the in-cylinder direct injection ammonia injector.

[0014] 5. The central axis of the concentric dual-needle valve injector coincides with the central axis of the cylinder.

[0015] 6. The central axis of the in-cylinder direct injection ammonia injector and the central axis of the concentric dual-needle valve injector point to the center of the combustion chamber in the same spatial plane.

[0016] The combustion method of a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to the present invention is characterized in that: it includes a pure diesel mode:

[0017] Under starting, idle and low-load conditions, a small-flow injector of a concentric double-needle valve injector injects a small flow of diesel into the cylinder before top dead center; as the flow requirement increases and exceeds the fuel supply capacity of the small-flow injector, it switches to the large-flow injector of the concentric double-needle valve injector to inject fuel; under medium and high-load conditions, control the external large-flow injector, internal small-flow injector of the concentric double-needle valve injector, and the large-flow low-temperature EGR and small-flow low-temperature EGR circuits to work simultaneously, so that part of the exhaust gas flows back into the cylinder after cooling, reducing the in-cylinder combustion temperature, reducing high-temperature NOx emissions and effectively eliminating the knocking phenomenon.

[0018] The combustion method of a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to the present invention may further include:

[0019] 1. It includes a dual-fuel mode:

[0020] (1) When the engine is at low load, diesel diffusion combustion is dominant, and the ammonia fuel substitution rate is less than 30%; in the exhaust stage, the exhaust valve is closed in advance to achieve in-cylinder high-temperature EGR; in the second half of the compression stroke, high-pressure liquid ammonia is injected, and the injection duration is less than half of the compression stroke. The ammonia fuel atomizes and evaporates and mixes non-uniformly with air. Before top dead center, a small-flow injector injects diesel, and the diesel injection duration partially overlaps with the ammonia fuel injection duration, forming a large concentration gradient stratification to achieve the ignition and stable combustion of the ammonia fuel;

[0021] (2) When the engine is at medium load, diesel diffusion combustion is dominant, and the ammonia fuel substitution rate is greater than 30% and less than 50%; control the small-flow low-temperature EGR circuit to work, so that part of the exhaust gas forms a pre-mixture with air and then enters the cylinder together; adjust the ammonia fuel supply pressure and timing, inject half of the high-pressure liquid ammonia in the first half of the compression stroke, and the injection duration is less than half of the compression stroke. Before top dead center, a small-flow injector injects diesel, and the other half of the high-pressure liquid ammonia is supplemented after injecting the ignition diesel, and the injection is completed before top dead center, forming a local rich equivalence ratio in the diesel injection path to form an active stratification of diesel and ammonia fuel, achieving the coupled stratified combustion of high-reactivity diesel and ammonia fuel;

[0022] (3) When the engine is at high load, ammonia is used as the main fuel and diesel is used as the pilot fuel; the intake valve closing moment is postponed to make the actual compression ratio of the engine less than the expansion ratio. At the same time, the large-flow low-temperature EGR circuit is controlled to work, so that part of the exhaust gas is cooled and then flows back into the cylinder together with air; the ammonia fuel supply pressure and timing are adjusted, and low-pressure liquid ammonia is injected in the first half of the compression stroke. The injection duration is greater than half of the compression stroke. Before top dead center, a small-flow injector is used to inject pilot diesel as the ignition source to achieve high-efficiency and clean combustion with ammonia fuel as the main fuel.

[0023] The advantages of the present invention are as follows: The present invention gives the specific combustion mode of ammonia fuel in the application of internal combustion engines. The concentric double-needle valve injector is adopted to avoid arranging too many injectors on the cylinder head. By designing a composite EGR circuit and synergistically controlling the work of the concentric double-needle valve injector and the in-cylinder direct injection ammonia injector, multi-mode combustion is achieved, the problems existing in the ammonia / diesel dual-fuel engine under different loads are solved, and high-efficiency, clean and stable combustion of ammonia fuel in the cylinder is realized. While meeting the high-power requirements, the full operating conditions are covered, meeting the requirements of fuel economy and emission regulations. High-temperature EGR is adopted at low load to increase the cylinder temperature, accelerate the evaporation of liquid ammonia and increase the molecular activation energy at the same time; small-flow low-temperature EGR is adopted at medium load to reduce the maximum combustion temperature and reduce NOx emissions; at high load, the intake valve closing moment is postponed and large-flow low-temperature EGR is adopted to reduce NOx emissions and suppress the knocking phenomenon. Brief Description of the Drawings

[0024] Figure 1 is a schematic structural diagram of the present invention;

[0025] Figure 2 is a schematic structural diagram of the engine combustion system;

[0026] Figure 3 is a schematic specific structural diagram of the concentric double-needle valve injector;

[0027] Figure 4 is a schematic diagram of the injection characteristic curve of the concentric double-needle valve injector;

[0028] Figure 5 is a schematic diagram of the in-cylinder fuel distribution during diesel injection at low load in the dual-fuel mode;

[0029] Figure 6 is a schematic diagram of the in-cylinder fuel distribution during diesel injection at medium load in the dual-fuel mode;

[0030] Figure 7 is a schematic diagram of the in-cylinder fuel distribution during diesel injection at high load in the dual-fuel mode. Detailed Embodiment

[0031] The present invention will be described in more detail with reference to the accompanying drawings as follows:

[0032] Combined with Figures 1-7 , Figure 1 is a schematic diagram of the system structure of the present invention. The present invention includes a combustion system, an electronic control system, a composite EGR loop system, a variable valve timing system, a high-pressure common rail fuel system, and an ammonia supply system. An air filter 8, a venturi tube 9, a compressor 10, an intercooler 11, and a throttle valve 12 are successively installed on the intake pipe. A turbine 2 and an exhaust back pressure valve 3 are installed on the exhaust pipe. The composite EGR loop system includes three loops: 1. An in-cylinder high-temperature EGR loop based on variable valve timing; 2. An out-of-cylinder low-flow low-temperature EGR loop; 3. An out-of-cylinder high-flow low-temperature EGR loop. The out-of-cylinder high- and low-flow low-temperature EGR loops are parallel loops. The inlet end is connected to the exhaust pipe downstream of the turbine 2, and the outlet end is connected to the throat of the venturi tube 9 installed on the intake main pipe upstream of the compressor 10. The out-of-cylinder high- and low-flow low-temperature EGR loops are provided with independent EGR coolers and EGR valves, and have different flow capacities. Precise control of small-flow exhaust gas is achieved through the low-flow low-temperature EGR loop, and timely response to large-flow exhaust gas is achieved through the high-flow low-temperature EGR loop. The EGR cooler is used to cool the exhaust gas flowing through this loop, and the EGR valve is used to control the on / off of the loop. The venturi tube 9 includes an inlet section, a converging section, a throat, and a diverging section. The air flow velocity is high and the pressure is low at the throat, so that the exhaust gas flows into the diverging section together with the air, is pressurized by the compressor 10 and cooled by the intercooler 11, and then flows into the manifold through the throttle valve 12, ensuring the turbine efficiency and better combustion consistency of each cylinder. The specific heat ratio of the exhaust gas containing polyatomic gases is greater than that of air, and the high specific heat capacity effect causes the maximum in-cylinder combustion temperature to decrease, so as to reduce NOx emissions under high load and suppress knocking phenomena.

[0033] The specific working mode of the in-cylinder high-temperature EGR loop based on variable valve timing is as follows: The electronic control system realizes the capture of internal high-temperature exhaust gas by controlling the closing time and lift curve of the intake valve 19 and the exhaust valve 17, and solves the problems of difficult ignition and slow combustion speed of low-reactivity ammonia fuel under low load. The working mode of the out-of-cylinder low-flow low-temperature EGR loop is as follows: The electronic control system controls the opening degree of the exhaust back pressure valve and opens the low-flow EGR valve 7, so that part of the exhaust gas behind the turbine flows to the out-of-cylinder low-flow low-temperature EGR loop, forms a pre-mixture with air and then enters the cylinder together. The high specific heat capacity effect of the exhaust gas molecules causes the maximum in-cylinder combustion temperature to decrease, so as to reduce NOx emissions of the engine under medium load. The working mode of the out-of-cylinder high-flow low-temperature EGR loop is as follows: The electronic control system controls the opening degree of the exhaust back pressure valve and opens the high-flow EGR valve 6, so that part of the exhaust gas behind the turbine flows to the out-of-cylinder high-flow low-temperature EGR loop, increases the EGR rate, forms a pre-mixture with air and then flows back into the cylinder together, reduces the in-cylinder combustion temperature, and reduces NOx emissions under high load and suppresses knocking phenomena.

[0034] Figure 4It is a schematic structural diagram of the engine combustion system of the present invention. The combustion system includes a piston 13, a cylinder 14, a cylinder head 16, an intake pipe, an exhaust pipe, an intake valve 19 and an exhaust valve 17; a vertically installed concentric double-needle valve injector 18 and an inclined in-cylinder direct injection ammonia injector 20 are arranged on the cylinder head 16. The concentric double-needle valve injector 18 is located at the center of the cylinder head 16, and its axis coincides with the center line of the cylinder 14. The single high-pressure common rail fuel supply system is used to supply fuel to the concentric double-needle valve injector 18. Both injectors of the concentric double-needle valve adopt a single injection strategy to respectively achieve precise control of the injection pulse width and injection timing of the two needle valves; the in-cylinder direct injection ammonia injector 20 is located on one side of the cylinder head 16 close to the intake port 21, and its axis forms a certain angle with the bottom plane of the cylinder head 16. The angle of the angle is determined according to the actual structure of the cylinder head 16. The variable rail pressure ammonia supply system is used to supply ammonia fuel to the in-cylinder direct injection ammonia injector 20, and the in-cylinder high and low pressure multi-injection technology is used to achieve a reasonable stratification of the concentration of the ammonia-air mixture;

[0035] Figure 2 It is a specific structural schematic diagram of the concentric double-needle valve injector of the present invention. The concentric double-needle valve injector consists of an external large-flow injector oil passage 22, an internal small-flow injector oil passage 23, an internal small-flow injector needle valve 24, an internal small-flow injector needle valve body 25, and an external large injector needle valve body 26. The concentric double-needle valve injector 18 is composed of an external large-flow injector and an internal small-flow injector. The external large-flow injector includes an external large-flow injector oil passage 22, an internal small-flow injector needle valve body 25 and an external large-flow injector needle valve body 26; the internal small-flow injector includes an internal small-flow injector oil passage 23, an internal small-flow injector needle valve 24 and an internal small-flow injector needle valve body 25. The internal small-flow injector needle valve body 25 is also used as the needle valve of the external large-flow injector. The external large-flow injector and the internal small-flow injector are coaxially arranged inside the concentric double-needle valve injector 18; the external large-flow injector is a pintle injector, and the internal small-flow injector is a multi-hole injector with 6-8 injection holes. Both the external large-flow injector and the internal small-flow injector have independent needle valves, needle valve bodies, springs, solenoid valves and oil passages.

[0036] Figure 3 It is a schematic diagram of the injection characteristic curve of the concentric double-needle valve injector of the present invention. In a preferred embodiment, as Figure 3 shown, under the condition of the same pulse width, the flow rate of the internal small-flow injector needle valve body 25 is much smaller than that of the external large-flow injector needle valve body 26, but the linearity intervals are different. The flow rate of the internal small-flow injector needle valve body 25 reaches the linearity interval earlier to achieve more precise control in the case of less fuel required during starting, idling or medium and low loads.

[0037] It includes a pure diesel mode and a dual-fuel mode. In the pure diesel mode, the power can cover the entire operating conditions of the engine. During starting, idling, and low-load conditions, a small-flow injector of the concentric double-needle valve injector 18 injects a small amount of diesel into the cylinder before top dead center. As the fuel flow requirement increases and exceeds the fuel supply capacity of the small-flow injector, it switches to the large-flow injector of the concentric double-needle valve injector 18 to inject fuel. Under medium and high load conditions, the electronic control system controls the large-flow injector, small-flow injector of the concentric double-needle valve injector, and the large and small-flow low-temperature EGR circuits to work simultaneously, so that part of the exhaust gas flows back into the cylinder after cooling, reducing the in-cylinder combustion temperature, reducing high-temperature NOx emissions, and effectively eliminating the knocking phenomenon.

[0038] As Figures 4-6 shown in the in-cylinder fuel distribution schematic diagram, in the dual-fuel mode, (1) when the engine is at low load, the engine mainly uses diesel diffusion combustion, and the ammonia fuel substitution rate is less than 30%. During the exhaust stage, the electronic control system controls the variable valve timing system to close the exhaust valve 17 in advance according to the operating conditions to achieve in-cylinder high-temperature EGR, increasing the in-cylinder temperature and molecular activity, and solving problems such as difficult ignition and incomplete combustion of direct-injected ammonia fuel in the cylinder at low load. In the second half of the compression stroke, high-pressure liquid ammonia is injected, and the injection duration is less than half of the compression stroke. The ammonia fuel quickly atomizes and evaporates, non-uniformly mixes with air. Before top dead center, the needle valve 24 of the internal small-flow injector of the concentric double-needle valve injector 18 lifts, and the needle valve 25 of the external large-flow injector moves down, injecting a small amount of diesel, precisely controlling the ignition timing and flow rate of the ignition diesel. The diesel injection duration partially overlaps with the ammonia fuel injection duration, forming a large concentration gradient stratification, as Figure 4 shown, to achieve the ignition and stable combustion of ammonia fuel;

[0039] (2) When the engine is at medium load, the engine mainly uses diesel diffusion combustion, and the ammonia fuel substitution rate is greater than 30% and less than 50%. Control the small-flow low-temperature EGR circuit to work, so that part of the exhaust gas forms a pre-mixture with air and enters the cylinder together. The high specific heat capacity effect of the exhaust gas molecules causes the maximum in-cylinder combustion temperature to decrease, reducing NOx emissions. And by controlling the ammonia fuel supply system, adjusting the ammonia fuel supply pressure and timing, inject half of the high-pressure liquid ammonia in the first half of the compression stroke, and the injection duration is less than half of the compression stroke, mixing more uniformly with air. Before top dead center, the needle valve 24 of the internal small-flow injector of the concentric double-needle valve injector 18 lifts, and the needle valve 25 of the external large-flow injector moves down, injecting a small amount of diesel, and then injecting the other half of the high-pressure liquid ammonia after injecting the ignition diesel, completing the injection before top dead center, forming a local rich equivalence ratio in the diesel injection path, forming an active stratification of diesel and ammonia fuel, as Figure 5 shown, to achieve the efficient coupled stratified combustion of high-reactivity diesel and ammonia fuel;

[0040] (3) When the engine is under high load, ammonia serves as the main fuel and diesel as the pilot fuel; the electronic control system controls the variable valve train to delay the closing moment of the intake valve 19, so that the actual compression ratio of the engine is less than the expansion ratio. At the same time, it controls the operation of the large-flow low-temperature EGR circuit to increase the EGR rate, so that part of the exhaust gas is cooled and then flows back into the cylinder together with air, reducing the in-cylinder combustion temperature, reducing high-load NOx emissions and suppressing knocking; and by controlling the ammonia fuel supply system, adjusting the ammonia fuel supply pressure and timing, injecting low-pressure liquid ammonia in the first half of the compression stroke, with the injection duration being greater than half of the compression stroke, to obtain a lean ammonia mixture with a low concentration gradient. Before top dead center, the needle valve 24 of the small-flow injector inside the concentric double-needle valve injector 18 is lifted, and the needle valve 25 of the large-flow injector outside is lowered to inject the pilot diesel, as Figure 6 shown, to achieve high-efficiency and clean combustion with ammonia fuel as the main fuel.

[0041] Among them, the fuel spray does not limit the form of the fuel, and the fuel can be gaseous or in a supercritical fluid state in addition to being liquid.

Claims

1. A combustion system for a diesel and ammonia dual-fuel engine based on a composite EGR circuit, Characterized in that: It includes an intake pipe, an exhaust pipe, a cylinder, a cylinder head, a piston, a large-flow low-temperature EGR circuit, and a small-flow low-temperature EGR circuit. The cylinder, cylinder head, and piston form a combustion chamber. A concentric double-needle valve injector, an in-cylinder direct injection ammonia injector, an intake passage, and an exhaust passage are provided on the cylinder head. The concentric double-needle valve injector includes an external large-flow injector and an internal small-flow injector. The external large-flow injector includes an external large-flow injector needle valve body and an internal small-flow injector needle valve body. The external large-flow injector needle valve body is located outside the internal small-flow injector needle valve body, and an external large-flow injector oil passage is formed between the two. The internal small-flow injector includes an internal small-flow injector needle valve body and an internal small-flow injector needle valve. The internal small-flow injector needle valve body is located outside the internal small-flow injector needle valve, and an internal small-flow injector oil passage is formed between the two; The intake passage is connected to the intake pipe, the exhaust passage is connected to the exhaust pipe. A venturi tube is provided on the intake pipe, and a compressor is provided behind the venturi tube. A turbine is provided on the exhaust pipe. The compressor and the turbine are coaxial. The large-flow low-temperature EGR circuit and the small-flow low-temperature EGR circuit are parallel circuits. The inlet end of the parallel circuit is connected to the exhaust pipe downstream of the turbine, and the outlet end of the parallel circuit is connected to the throat of the venturi tube. A large-flow EGR valve and a large-flow EGR cooler are provided on the large-flow low-temperature EGR circuit, and a small-flow EGR valve and a small-flow EGR cooler are provided on the small-flow low-temperature EGR circuit.

2. The combustion system for a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to claim 1, Characterized in that: An exhaust backpressure valve is provided on the exhaust pipe.

3. The combustion system for a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to claim 1, Characterized in that: The external large-flow injector is a pintle injector, and the internal small-flow injector is a multi-hole injector. The number of spray holes is 6 - 8. Both the external large-flow injector and the internal small-flow injector include independent needle valves, needle valve bodies, springs, solenoid valves, and oil passages.

4. The combustion system for a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to claim 1, Characterized in that: The flow rate of the internal small-flow injector needle valve body reaches the linearity range earlier than that of the external large-flow injector needle valve body.

5. The combustion system for a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to claim 1, Characterized in that: The in-cylinder direct injection ammonia injector has a porous structure, and the spray holes are axially symmetrically distributed about the central axis of the in-cylinder direct injection ammonia injector.

6. The combustion system for a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to claim 1, Characterized in that: The central axis of the concentric double-needle valve injector coincides with the central axis of the cylinder.

7. The combustion system for a diesel and ammonia dual-fuel engine based on a composite EGR circuit according to claim 1, Characterized in that: The central axis of the in-cylinder direct injection ammonia injector and the central axis of the concentric double-needle valve injector point to the center of the combustion chamber in the same spatial plane.

8. The engine combustion method of a diesel and ammonia dual-fuel engine combustion system based on a composite EGR circuit according to any one of claims 1 to 7, characterized in that: It includes a pure diesel mode: Under starting, idle and low-load conditions, a small-flow injector of a concentric double-needle valve injector is used to inject a small amount of diesel into the cylinder before top dead center; As the fuel flow requirement increases and exceeds the fuel supply capacity of the small-flow injector, it is switched to the large-flow injector of the concentric double-needle valve injector to inject fuel; under medium and high load conditions, the external large-flow injector, internal small-flow injector of the concentric double-needle valve injector, and large-flow low-temperature EGR and small-flow low-temperature EGR circuits are controlled to work simultaneously, so that part of the exhaust gas flows back into the cylinder after cooling, reducing the in-cylinder combustion temperature, reducing high-temperature NOx emissions and effectively eliminating the knocking phenomenon.

9. The engine combustion method of a diesel and ammonia dual-fuel engine combustion system based on a composite EGR circuit according to claim 8, characterized in that: It includes a dual-fuel mode: (1) When the engine is at low load, diesel diffusion combustion is dominant, and the ammonia fuel substitution rate is less than 30%; during the exhaust stage, the exhaust valve is closed in advance to achieve in-cylinder high-temperature EGR; in the second half of the compression stroke, high-pressure liquid ammonia is injected, and the injection duration is less than half of the compression stroke. The ammonia fuel atomizes and evaporates and mixes non-uniformly with air. Before top dead center, a small-flow injector is used to inject diesel, and the diesel injection duration partially overlaps with the ammonia fuel injection duration, forming a large concentration gradient stratification to achieve the ignition and stable combustion of ammonia fuel; (2) When the engine is at medium load, diesel diffusion combustion is dominant, and the ammonia fuel substitution rate is greater than 30% and less than 50%; the small-flow low-temperature EGR circuit is controlled to work, so that part of the exhaust gas forms a pre-mixture with air and enters the cylinder together; the ammonia fuel supply pressure and timing are adjusted. In the first half of the compression stroke, half of the high-pressure liquid ammonia is injected, and the injection duration is less than half of the compression stroke. Before top dead center, a small-flow injector is used to inject diesel, and the other half of the high-pressure liquid ammonia is supplemented after injecting the ignition diesel. The injection is completed before top dead center, forming a local rich equivalence ratio in the diesel injection path, forming an active stratification of diesel and ammonia fuel, and realizing the coupled stratified combustion of high-reactivity diesel and ammonia fuel; (3) When the engine is at high load, ammonia is the main fuel and diesel is the ignition fuel; the intake valve closing time is postponed to make the actual compression ratio of the engine less than the expansion ratio. At the same time, the large-flow low-temperature EGR circuit is controlled to work, so that part of the exhaust gas flows back into the cylinder together with air after cooling; the ammonia fuel supply pressure and timing are adjusted. In the first half of the compression stroke, low-pressure liquid ammonia is injected, and the injection duration is greater than half of the compression stroke. Before top dead center, a small-flow injector is used to inject ignition diesel as an ignition source to achieve high-efficiency and clean combustion with ammonia fuel as the main fuel.

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