Combustion chamber structure and injection strategy collaborative optimization method for ultra-lean hydrogen combustion

Abstract

The invention discloses a combustion chamber structure and injection strategy collaborative optimization method for ultra-lean-burn hydrogen combustion, which comprises the following steps of: establishing a matching relationship between combustion chamber structure characteristic parameters and hydrogen injection characteristic parameters through collaborative optimization of combustion chamber structure characteristic parameters and hydrogen injection characteristic parameters of an engine, and forming multiple control strategies; under the ultra-lean combustion condition that the air-fuel ratio lambda is larger than or equal to 2.5, cooperative control over the turbulence intensity, mixed gas concentration distribution and ignition time in the ultra-lean hydrogen mixed gas combustion process is achieved. Wherein the structural characteristic parameters of the combustion chamber comprise the compression ratio, the shape of a pit on the top surface of a piston and the mounting position of a hydrogen direct injection nozzle; the hydrogen injection characteristic parameters comprise hydrogen injection time sequence, injection proportion, injection duration and injection flow. According to the combustion chamber structure and injection strategy collaborative optimization method for ultra-lean hydrogen combustion, starting from the two aspects of combustion chamber design and injection control, combustion chamber structure parameters such as the compression ratio, the nozzle arrangement and the turbulence scale and hydrogen injection strategy parameters such as the injection time, the injection duration, the injection position and the injection flow are regulated in a collaborative mode; optimized control over the ultra-lean-burn hydrogen combustion process is achieved, the combustion stability and heat efficiency of an engine are optimized under the hydrogen fuel ultra-lean-burn combustion condition, harmful emission is reduced, and the backfire explosion risk is restrained.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of hydrogen internal combustion engines, and particularly relates to a combustion chamber structure and injection strategy coordination optimization method for super-dilute hydrogen combustion. BACKGROUND

[0002] Hydrogen, as a zero-carbon renewable fuel, has a broad application prospect in internal combustion engines. Hydrogen has the advantages of fast combustion speed, good diffusivity, wide flammable equivalence ratio range, and near-zero carbon emission. The overall thermal efficiency of a hydrogen direct injection internal combustion engine under dilute conditions can exceed 40%, and further reducing heat loss can make the thermal efficiency exceed 50%. However, there are still many challenges to achieve high efficiency and high power density of hydrogen internal combustion engines: the super-wide flammable limit and extremely low ignition energy of hydrogen make it prone to abnormal combustion phenomena such as premature self-ignition (backfire) and knock under super-dilute conditions, and the combustion speed significantly decreases when the air excess coefficient (λ) is very high, resulting in a decrease in thermal efficiency. Therefore, under the super-dilute combustion conditions of hydrogen internal combustion engines, stable combustion and prevention of knock backfire become urgent technical problems to be solved.

[0003] In view of the above problems, some improvement strategies are proposed in the prior art. For example, by stratified dilution, slightly rich mixture is formed near the hydrogen direct injection nozzle, and leaner mixture is formed near the cylinder wall, which can accelerate the combustion speed and suppress the generation of knock. Delaying the hydrogen direct injection time (such as injecting in the late compression stroke) can shorten the residence time of hydrogen in the high-temperature environment, thereby effectively preventing the risk of premature ignition and backfire. Using the Miller cycle or exhaust gas recirculation (EGR) can also reduce the temperature at the end of compression to some extent to alleviate knock. Increasing the compression ratio of the engine helps to improve thermal efficiency, but it also exacerbates the tendency of knock and increases NOx emissions, which needs to be combined with late ignition and EGR and other means to alleviate. However, these methods often have to sacrifice some performance or efficiency, such as delaying ignition and using a large amount of EGR.

[0004] The present application provides a combustion chamber structure and injection strategy coordination optimization method for super-dilute hydrogen combustion, particularly regarding how to optimize the combustion stability and thermal efficiency of the engine under super-dilute hydrogen fuel combustion conditions, reduce harmful emissions, and suppress the risk of backfire and knock. SUMMARY

[0005] The present application aims to at least solve one of the technical problems existing in the prior art. To this end, the present application provides a combustion chamber structure and injection strategy coordination optimization method for super-dilute hydrogen combustion, which aims to optimize the combustion stability and thermal efficiency of the engine under super-dilute hydrogen fuel combustion conditions, reduce harmful emissions, and suppress the risk of backfire and knock.

[0006] In order to achieve the above object, the technical scheme adopted by the present application is: a method for synergistically optimizing the combustion chamber structure and injection strategy for ultra-lean hydrogen combustion, which establishes the matching relationship between the combustion chamber structure characteristic parameters and the hydrogen injection characteristic parameters by synergistically optimizing the engine combustion chamber structure characteristic parameters and the hydrogen injection characteristic parameters, and realizes the synergistic control of the turbulence intensity, the mixture concentration distribution and the ignition timing in the ultra-lean hydrogen mixture combustion process under the condition of ultra-lean air-fuel ratio λ≥2.5.

[0007] The combustion chamber structure characteristic parameters include the compression ratio, the piston top surface pit shape and the installation position of the hydrogen direct injection nozzle; the hydrogen injection characteristic parameters include the hydrogen injection timing, the injection ratio, the injection duration and the injection flow rate.

[0008] The optimization of the combustion chamber structure parameters includes:

[0009] The compression ratio is selected in the range of 9-12 (preferably 9-11), which is determined by experiments to be the balance interval for improving thermal efficiency while suppressing the risk of knocking;

[0010] The piston top surface adopts a structure with pits, and after the compression ratio is determined, the pit shape and size are adjusted to change the tumble intensity and flow field characteristics.

[0011] The arrangement direction of the piston top surface pit is coordinated with the hydrogen injection timing:

[0012] When the early injection strategy is adopted, the pit arranged on the piston top surface is biased to the side of the hydrogen direct injection nozzle, guiding the hydrogen jet to form counterclockwise tumble flow consistent with the natural tumble direction in the cylinder;

[0013] When the late injection strategy is adopted, the pit arranged on the piston top surface is biased to the side away from the hydrogen direct injection nozzle, guiding the jet to form clockwise tumble flow.

[0014] The hydrogen direct injection nozzle is installed on one side of the cylinder head, and the injection direction is towards the lower side of the opposite side and points to the pit area of the piston top surface.

[0015] The injection pressure of the hydrogen direct injection nozzle is set to 2-11 MPa, wherein 2-5 MPa is adopted for low load conditions, and 9-11 MPa is adopted for medium and high load conditions, preferably 10 MPa.

[0016] The hydrogen injection adopts a double-pulse segmented direct injection mode, including first injection and second injection; the first injection occurs at the end of the intake stroke or the beginning of the compression stroke; the second injection occurs at the late compression stroke and before ignition.

[0017] The distribution ratio and timing of the two injections are optimized, the first injection of hydrogen accounts for 50-80% of the total hydrogen injection amount, and the second injection of hydrogen accounts for 20-50% of the total hydrogen injection amount; the first injection is performed in the interval of 90°CA-60°CA before the top dead center after the intake valve is closed, and the second injection is performed in the interval of 40°CA-10°CA before the top dead center.

[0018] The second injection of hydrogen accounts for 20-40% of the total hydrogen injection amount, and the second injection is preferably started at about 30°CA before the top dead center (SOSI≈30°CA BTDC) and is completed before the ignition time (about 5°CA ATDC after the top dead center).

[0019] The first injection of hydrogen preferably accounts for about 70% of the total hydrogen injection amount, and is performed at the end of the intake stroke to ensure the formation of a basic homogeneous lean mixture.

[0020] The second injection of hydrogen preferably accounts for about 30% of the total hydrogen injection amount, at which time the engine can obtain the best indicated thermal efficiency and the lowest NOx emission level.

[0021] The combustion chamber structure and injection strategy optimization method for super-lean hydrogen combustion of the application optimizes the super-lean hydrogen combustion process by synergistically controlling the compression ratio, nozzle arrangement, turbulent flow scale and other combustion chamber structure parameters, and the injection time, injection duration, injection position, injection flow and other hydrogen injection strategy parameters, so as to optimize the combustion stability and thermal efficiency of the engine under the condition of super-lean hydrogen combustion, reduce harmful emissions and suppress backfire and knock risk. BRIEF DESCRIPTION OF DRAWINGS

[0022] The present specification includes the following drawings, the contents shown in each of which are as follows:

[0023] Figure 1 An engine test bench schematic diagram for researching and verifying the method of the application;

[0024] Figure 2 A geometric model schematic diagram of the engine and its combustion system studied;

[0025] Figure 3a A diagram showing the agreement between the calculated hydrogen laminar flame speed and the experimental data in the literature;

[0026] Figure 3b A convergence verification curve diagram of cylinder pressure and heat release rate under different grid refinement levels;

[0027] Figure 3c and Figure 3d A comparison diagram of cylinder pressure and heat release rate between experiments and simulations under two working conditions;

[0028] Figure 4a is a schematic diagram of a flat-top piston structure;

[0029] Figure 4b is a schematic diagram of a right-dish piston structure;

[0030] Figure 4c is a schematic diagram of a left-dish piston structure;

[0031] Figure 5 is a curve diagram of tumble ratio changing with crank angle under three piston design schemes;

[0032] Figure 6a is a velocity vector flow field of different schemes at a certain moment in compression stroke;

[0033] Figure 6b is an equivalent specific concentration distribution nephogram of a corresponding moment section;

[0034] Figure 6c is a mass fraction distribution histogram of mixture uniformity of three schemes at ignition moment and a comparison diagram of uniformity index (UI);

[0035] Figure 7a is a curve diagram of in-cylinder pressure and pressure rise rate changing with crank angle of three schemes;

[0036] Figure 7b is a heat release rate (HRR) curve diagram of three schemes;

[0037] Figure 8 is a temperature field distribution nephogram of three schemes at different crank angles in the combustion process;

[0038] The marks in the figure are:

[0039] 1, encoder; 2, exhaust pipe; 3, computer; 4, data collector; 5, turbocharger; 6, intercooler; 7, electromagnetic valve; 8, hydrogen guide rail; 9, intake pipe; 10, dynamometer; 11, hydrogen tank group; 12, flame arrester; 13, hydrogen flowmeter; 14, pressure reducing valve. DETAILED DESCRIPTION

[0040] The specific embodiments of the present application will be further described in detail below with reference to the accompanying drawings, and the purpose is to help the skilled in the art to have a more complete, accurate and in-depth understanding of the concept and technical solution of the present application, and to help its implementation.

[0041] The embodiment of the present application provides a combustion chamber structure and injection strategy collaborative optimization method for ultra-lean hydrogen combustion, which optimizes the combustion chamber structure characteristic parameters and hydrogen injection characteristic parameters of the engine through collaborative optimization, establishes the matching relationship between the combustion chamber structure characteristic parameters and the hydrogen injection characteristic parameters, forms multiple control strategies, and realizes the collaborative control of the turbulent intensity, the mixture concentration distribution and the ignition time in the ultra-lean hydrogen mixture combustion process under the condition of the air-fuel ratio λ≥2.5. The combustion chamber structure characteristic parameters include the compression ratio, the piston top surface pit shape and the installation position of the hydrogen direct injection nozzle; the hydrogen injection characteristic parameters include the hydrogen injection timing, the injection ratio, the injection duration and the injection flow rate. Through collaborative control, the technical problems of combustion instability, low thermal efficiency and high backfire risk are solved under the condition of the air-fuel ratio λ≥2.5.

[0042] Specifically, in the embodiment of the present application, a combustion chamber structure and injection strategy collaborative optimization method for ultra-lean hydrogen combustion is used to optimize the combustion stability and thermal efficiency of the engine under the condition of hydrogen fuel ultra-lean combustion, so that the lean fuel is more efficiently and completely combusted, and the harmful emissions are reduced and the backfire risk is inhibited.

[0043] The embodiment of the present application provides a control mechanism and method for collaborative optimization of the combustion chamber structure and hydrogen injection strategy, which starts from the combustion chamber design and injection control, and realizes the optimization control of the ultra-lean hydrogen combustion process by collaborative regulation of the compression ratio, the nozzle arrangement, the turbulent scale and other combustion chamber structure parameters, and the injection timing (SOI), the injection duration (DOI), the injection position, the injection flow rate and other hydrogen injection strategy parameters.

[0044] The combustion chamber structure and injection strategy collaborative optimization method for ultra-lean hydrogen combustion provided by the embodiment of the present application includes the following steps:

[0045] 1. Combustion chamber structure optimization: design or improve the engine combustion chamber structure, including selecting a suitable compression ratio and piston shape. Preferably, a piston top surface structure with a pit (bowl groove) is used to generate strong in-cylinder rolling flow movement.

[0046] The shape and orientation of the piston pit are designed according to the injection strategy: according to the coordination of the injection timing and the pit orientation, the direction, scale and intensity of the in-cylinder rolling flow are systematically different, which will be described in the following scheme 2 and scheme 3.

[0047] Scheme 2 (right pit piston + early injection):

[0048] With right offset dimple and early injection timing (SOFI ≈ 90°CA BTDC), the in-cylinder tumble flow is consistent with the natural tumble flow direction, which is counterclockwise. The tumble flow scale is larger and the overall strength is higher. The hydrogen jet slides along the dimple surface and produces significant entrainment, which promotes fast and more uniform mixing of hydrogen-air (UI ≈ 0.90-0.95), is conducive to stable ignition and flame propagation under ultra-lean conditions, and reduces the cycle-to-cycle variation (COV IMEP ) of NOx emissions.

[0049] Scheme 3 (left dimple piston + late injection):

[0050] With left offset dimple and late injection (SOSI ≈ 30°CA BTDC, which needs to be completed before ignition ≈ 5°CA ATDC), the in-cylinder tumble flow is clockwise, and the tumble flow direction may reverse during injection. The tumble flow scale is smaller but the strength is higher, forming a moderate hydrogen-rich stratification near the ignition zone / hydrogen direct injection nozzle, with lower overall mixing uniformity (UI ≈ 0.65-0.70) and relatively larger cycle-to-cycle variation, which is more prone to high-temperature zones and leads to an increase in NOx emissions.

[0051] 2. Injection system arrangement: The hydrogen direct injection nozzle is installed on one side of the cylinder head, with the injection direction pointing downward and towards the dimple area on the piston top surface on the opposite side, so that the injected hydrogen jet is coordinated with the direction of the tumble flow in the combustion chamber. The number of nozzle holes and the injection angle are designed to ensure that the hydrogen jet can fully penetrate the lean mixture and avoid direct impact on the cylinder wall, causing fuel retention. The injection pressure range is 2-11 MPa, with a typical working condition of about 10 MPa, and the injection flow rate is also set according to the requirements to ensure the appropriate distribution and mixing of hydrogen in the cylinder.

[0052] 3. Injection strategy optimization: Multi-pulse segmented injection strategy is adopted to coordinate the formation of mixture and combustion process. Specifically, the direct hydrogen injection of each working cycle (each working cycle refers to the period of time for the engine to complete a complete energy conversion process, which corresponds to one working cycle of the engine, including four consecutive strokes of intake, compression, work, and exhaust) is divided into two times: the first injection is carried out at the end of the intake stroke or the early stage of the compression stroke (for example, near 90° crank angle before top dead center), to provide a basic fuel amount to form a homogeneous lean mixture; the second injection is carried out at the late stage of the compression stroke before ignition (for example, around 30° crank angle before top dead center), to form a slightly rich local mixture cloud around the hydrogen direct injection nozzle. The time and duration of the two injections can be adjusted according to the engine operating conditions, and the second injection is immediately adjacent to the spark ignition time but ensures sufficient mixing time, so that the equivalence ratio around the hydrogen direct injection nozzle is slightly higher than the surrounding, to increase the combustion rate. The fuel distribution ratio of the first and second injections (i.e. the percentage of the second injection fuel in the total injection amount) is a key parameter for optimization, and the preferred range is 20% to 40% of the total fuel. For example, in a specific embodiment, when the second injection accounts for about 30% of the total injection amount, the engine exhibits the best indicated thermal efficiency and lower NOx emissions. By adjusting the ratio of the two injections, the ignition delay and combustion duration can be shortened while reducing the temperature peak of combustion and reducing NOx generation.

[0053] 4. Synergistic control: The above-mentioned combustion chamber structure parameters and injection parameters are matched to form an optimized control strategy. Specifically, according to the engine load, speed and air-fuel ratio and other operating conditions, select the appropriate combination scheme: when fast and stable combustion is needed, increase the tumble intensity and adopt double pulse injection to speed up the combustion rate; when knock or backfire needs to be suppressed, reduce the temperature and avoid premature self-ignition by increasing the mixture dilution and delaying the injection time. Through model simulation and experimental calibration, the best combination of parameters is determined to realize automatic adjustment of injection time and injection pulse in real-time control, so as to maintain stable and efficient ultra-lean combustion in typical operating conditions and most medium and low load operating conditions.

[0054] In the embodiment of the present application, the optimization of the combustion chamber structure parameters includes selecting appropriate compression ratio and piston top surface shape to ensure high thermal efficiency and suppress the tendency of knock under ultra-lean combustion conditions. The compression ratio is preferably in the range that increases the thermal efficiency of the engine while not excessively increasing the risk of knock, and other measures are taken to avoid self-ignition at high temperature; the piston top surface adopts a structure with a recess to enhance the in-cylinder tumble, and the tumble intensity and flow field characteristics are adjusted by adjusting the shape and size of the recess when the compression ratio is determined.

[0055] The compression ratio is selected in the range of 9 to 12 (preferably 9 to 11), which is determined by experiments to be the balance interval for increasing thermal efficiency while suppressing the risk of knock;

[0056] The piston top surface adopts a structure with a recess, and the tumble flow intensity and flow field characteristics are changed by adjusting the recess shape and size after the compression ratio is determined.

[0057] In the embodiment of the present application, the combustion chamber structure optimization adopts a piston recess with a specific shape to generate large-scale in-cylinder tumble flow movement. The piston top surface recess is arranged according to the different directions and timing of the selected injection strategy: the arrangement direction of the piston top surface recess is coordinated with the timing of the hydrogen injection:

[0058] When the early injection strategy is adopted, the recess arranged on the piston top surface is biased toward the side of the hydrogen direct injection nozzle, guiding the hydrogen jet to form a counterclockwise tumble flow consistent with the direction of the natural tumble flow in the cylinder;

[0059] When the late injection strategy is adopted, the recess arranged on the piston top surface is biased toward the side away from the hydrogen direct injection nozzle, guiding the jet to form a clockwise tumble flow.

[0060] By adjusting the shape and orientation of the recess, the hydrogen jet interacts favorably with the combustion chamber wall and the airflow, improving the diffusion uniformity and turbulent energy of hydrogen before ignition.

[0061] In the embodiment of the present application, the hydrogen direct injection nozzle is installed on one side of the cylinder head, with the injection direction pointing downward to the opposite side and pointing to the piston top surface recess area. The orientation and hole type design of the hydrogen direct injection nozzle make the hydrogen jet interact with the piston shape: the jet slides along the recess surface and entrains the surrounding gas, forming a dominant tumble flow structure, avoiding direct impact of the fuel on the cylinder wall. The arrangement position of the hydrogen direct injection nozzle ensures that the hydrogen is evenly distributed in space in the combustion chamber and concentrated near the hydrogen direct injection nozzle, providing an ideal mixed gas environment for ignition and combustion.

[0062] In the embodiment of the present application, the injection pressure range of the hydrogen direct injection nozzle is 2-11 MPa, with 2-5 MPa for low load, 9-11 MPa for medium-high load, and preferably 10 MPa.

[0063] In the embodiment of the present application, the hydrogen injection adopts a double-pulse segmented direct injection method, including a first injection and a second injection; the first injection occurs at the end of the intake stroke or the beginning of the compression stroke, for injecting the main fuel quantity to form a uniform lean mixture; the second injection occurs in the late compression stroke and before ignition, for injecting the remaining fuel quantity to form a locally rich fuel mixture near the hydrogen direct injection nozzle. Through at least two segmented injections, a uniform and stratified mixture layout is achieved in the cylinder, taking into account the requirements of combustion stability and combustion rate.

[0064] In the embodiment of the present application, the distribution ratio and timing of the two injections are optimized, the first injection accounts for 50% to 80% of the total hydrogen injection amount, and the second injection accounts for 20% to 50% of the total hydrogen injection amount. The specific ratio is shown in Example 2.

[0065] The engine comprises at least a cylinder, a cylinder block, a piston, a cylinder head, an intake system and an exhaust system, and a fuel supply assembly for supplying hydrogen, which comprises a hydrogen tank and the like. The engine adopts spark ignition, and further comprises an electronic control unit (ECU) for monitoring and adjusting combustion parameters. The cylinder block, the piston and the cylinder head form a combustion chamber, and the cylinder head is provided with an intake valve and an exhaust valve for controlling the opening and closing of the intake port and the exhaust port, respectively. The intake system is connected to the intake port, and the exhaust system is connected to the exhaust port.

[0066] Preferably, the second injection accounts for 20% to 40% of the total hydrogen injection amount, and the second injection starts about 30°CA before the top dead center (SOSI ≈ 30°CA BTDC) and is completed before the ignition time (about 5°CA ATDC after the top dead center).

[0067] Preferably, the distribution ratio of the two injections is about 70% for the first injection and about 30% for the second injection, and the specific parameters are consistent with those in Example 2.

[0068] In the embodiment of the present application, the optimization of the injection strategy also includes controlling the injection duration (DOI) and the injection flow rate. In the embodiment of the present application, the injection duration and the injection pressure are matched according to the engine speed and the load. Specifically, in the low-speed and low-load working condition (such as a speed of about 2500 rpm, a brake mean effective pressure BMEP ≈ 0.2 MPa, and an air-fuel ratio λ ≈ 3.0), a lower injection pressure (about 2-5 MPa) is adopted, and the injection duration is appropriately prolonged, so as to ensure that the hydrogen has sufficient diffusion time at the end of the intake stroke, thereby forming a homogeneous lean mixture and ensuring the ignition stability and the combustion integrity; in the medium-speed and medium-high load working condition (such as BMEP ≥ 0.5 MPa), the injection pressure is increased to 9-11 MPa, so as to enhance the kinetic energy of the jet flow, enable the hydrogen to quickly penetrate to the cylinder center region, avoid the hydrogen to stay near the cylinder wall, and shorten the injection duration, so as to prevent the abnormal combustion caused by excessive accumulation of the local hydrogen-rich area.

[0069] Preferably, the present application adopts a double-pulse segmented injection mode: the first injection is performed at about 90°CA before the top dead center, and the injection amount accounts for 60-80% of the total injection amount, for forming a basic homogeneous lean mixture; the second injection is completed at the late compression stroke (about 30°CA BTDC), and the injection amount accounts for 20-40% of the total injection amount, so that a moderate hydrogen-rich local mixture is formed near the hydrogen direct injection nozzle at the ignition moment, to accelerate the flame kernel generation and improve the combustion rate. The research results show that when the second injection ratio is about 30%, the engine exhibits the best indicated thermal efficiency (about 46.3%) and the lowest NOx emission level.

[0070] Therefore, the present application realizes the matching calibration of the hydrogen injection parameters and the combustion chamber structure by synergistically adjusting the injection pressure and duration under different working conditions, so that the engine can maintain stable and efficient combustion under most working conditions, especially under the condition of medium and low load and ultra-lean combustion, while effectively suppressing the risk of knocking and backfire, to ensure that the hydrogen jet has sufficient penetration depth into the core area of the combustion chamber and does not penetrate too long to cause excessive fuel lag and unburned.

[0071] In the embodiment of the present application, the control of the in-cylinder turbulent flow scale and intensity is realized through the synergistic design of the combustion chamber structure and the injection parameters. The piston concave structure strengthens the intake and injection-induced rolling flow, increases the turbulent pulsation speed at the flame propagation moment, and at the same time, the segmented injection strategy further excites the turbulence through the jet action. The combination of the two makes the combustion maintain a high turbulent combustion speed under the condition of ultra-lean mixture. The method makes the overall in-cylinder turbulent flow level higher than that of the conventional design, the flame front surface is more wrinkled, and the combustion reaction zone is expanded, thereby significantly improving the rate of hydrogen lean combustion.

[0072] In the embodiment of the present application, the synergistic optimization control can prevent abnormal combustion. On the one hand, the optimization of the injection moment makes the hydrogen avoid entering the high-temperature and high-pressure environment too early, thereby preventing the intake port backfire and the in-cylinder pre-ignition; on the other hand, the stratified lean combustion strategy makes the mixture in the edge area of the combustion chamber extremely lean, and the combustion temperature is suppressed, which eliminates the condition of knocking caused by the self-ignition of the end gas. Even when running at a higher compression ratio, the engine using the present method will not knock or pre-ignite, realizing a safe and stable combustion process.

[0073] The hydrogen fuel engine applying the method of the embodiment of the present application can obtain high efficiency and low nitrogen oxide emission performance in the working mode of ultra-dilute combustion (air-fuel ratio λ≥2.5-3.0). With the synergistic regulation of the combustion chamber structure and the injection strategy, the method makes the engine indicated thermal efficiency increase to the level of more than 40%-50%, while the emissions such as NOx are significantly reduced to several tenths or even one hundredth of the traditional dilute combustion mode, and the combustion stability (cycle variation coefficient COV) is maintained below 5%. Therefore, the method of the present application endows the hydrogen internal combustion engine with the ability to obtain near-ideal thermodynamic efficiency and ultra-low emission under extremely dilute combustion.

[0074] The method of the embodiment of the present application can be implemented by the control strategy pre-marked by an electronic control unit (ECU), and the engine operating parameters are closed-loop regulated. The ECU dynamically adjusts the hydrogen injection timing, duration and distribution ratio, and the ignition time according to the information such as in-cylinder pressure, temperature and knock signal fed back by the sensor, so that the combustion is always maintained in a stable and efficient state far away from the knock boundary. When the working condition changes, the control system modifies the injection and ignition parameters according to the pre-stored optimization mapping, so as to realize the optimization control of the ultra-dilute hydrogen combustion under typical working conditions (such as 2500 rpm, medium and low load, λ=2-3), and improve the practicability and reliability of the engine.

[0075] Embodiment 1: Comparison of combustion chamber structure optimization and single injection strategy

[0076] This embodiment is carried out on a modified single-cylinder direct injection hydrogen fuel engine, aiming to verify the influence of different combustion chamber structures and injection strategies on ultra-dilute combustion.

[0077] The main specifications of the engine include: compression ratio 9:1, rotation speed 2500 rpm, excess air coefficient λ≈3 (ultra-dilute combustion), direct injection pressure 10 MPa, and ignition advance angle optimized according to different working conditions (so that 50% combustion center CA50 is about 8° after top dead center). Three different piston crown shapes (as shown in FIG. 4) are selected for comparison test:

[0078] · Scheme 1 (comparison reference): flat piston + early injection strategy. The top of the piston is a traditional flat shape ( Figure 4a ), and a hydrogen injection is completed at the end of the intake stroke (90°CA before top dead center) to form an ultra-dilute homogeneous mixture. This scheme provides a reference level of turbulence and mixing uniformity.

[0079] · Scheme 2 (one of the optimization schemes of the present application): right concave piston + early injection strategy. The piston crown surface has a right-side semispherical concave pit ( Figure 4b), the design intention is to generate strong counterclockwise tumble flow in conjunction with early injection. The injection strategy is the same as that of scheme 1 (SOI = 90°CA BTDC, one injection) to form homogeneous mixture. But due to the piston shape optimization, the hydrogen jet forms a larger scale tumble structure in the cylinder, significantly enhancing the intake turbulence. Figure 5 It is shown that the average tumble intensity obtained in scheme 2 is higher than that in scheme 1 during the injection process. At the end of compression stroke, the diffusion range of hydrogen in scheme 2 is wider and the concentration distribution is more uniform, and at the ignition moment, the hydrogen direct injection nozzle has slightly higher equivalence ratio than the global (see Figure 6b ), the overall uniformity index UI reaches 0.93, which is significantly higher than 0.90 in scheme 1. The combustion pressure curve ( Figure 7a ) shows that scheme 2 is similar to scheme 1, but the maximum pressure and pressure rise rate are slightly improved; correspondingly, the heat release rate of scheme 2 is slightly higher ( Figure 7b ), indicating that the combustion is accelerated. The combustion duration is shortened from about 19°CA in scheme 1 to 15°CA in scheme 2. Due to the faster combustion and more uniform mixing, scheme 2 slightly improves the indicated thermal efficiency while maintaining low NOx emissions. The data of combustion duration, ITE and NOx emissions under different schemes in Table 1 further confirm that the indicated thermal efficiency of scheme 2 is about 45.84%, which is comparable to or even slightly higher than that of scheme 1 (45.73%), while the NOx emission is only 0.03g / kW·h, which is much lower than that of scheme 1 (0.12g / kW·h).

[0080] · Scheme 3 (comparative non-optimized scheme): left concave piston + late injection strategy. The piston top surface is a left concave shape ( Figure 4c ), aiming to generate clockwise tumble flow in conjunction with late injection in the compression stroke to achieve stratified mixing. The hydrogen injection timing is significantly delayed, and all the fuel is injected at once at the end of the compression stroke (35°CA before top dead center). Due to the late injection, the mixture formation time is short, and a rich hydrogen zone is accumulated near the cylinder center at the ignition time, while the cylinder wall is extremely lean ( Figure 6b ). This strong stratification causes the flame to rapidly spread from the hydrogen direct injection nozzle to the surrounding, reaching the cylinder wall within 14°CA ( Figure 8 ), and the combustion duration is only about 12.6°CA, which is much faster than schemes 1 and 2. The rapid combustion of high concentration hydrogen causes the in-cylinder temperature to be extremely high ( Figure 8), resulting in a dramatic increase in NOx emissions - Table 1 shows that the NOx of Scheme 3 is as high as 9.49 g / kW·h, which is more than 300 times that of Scheme 2. At the same time, the excessively high heat release peak brings a large pressure rise rate, which may cause shock and noise problems. Although Scheme 3 avoids the long residence of pre-mixed hydrogen due to the delayed injection, theoretically reducing the risk of backfire, its deflagration combustion and ultra-high NOx make it undesirable. This scheme exposes new problems if only the late injection strategy is adopted without other optimization methods. Its flow field characteristics can be summarized as follows: clockwise dominant, reverse direction of tumble flow during injection, and smaller but higher intensity of tumble flow; this is consistent with the change of tumble ratio phase and the uniformity index UI≈0.69 shown in Figure 5 , which explains the results of the increase in the volume fraction of the high-temperature zone and the significant increase in NOx from a mechanism. Figure 6c

[0081] Table 1 Combustion duration, ITE and NOx emissions under different operating conditions.

[0082]

[0083] Example 2: Optimization of injection strategy and coordination with segmented injection

[0084] In view of the problems of Scheme 3 in Example 1, this embodiment introduces the double-pulse injection strategy of the present application on the basis of the combustion chamber structure of Scheme 3 to improve the performance of ultra-lean combustion.

[0085] The engine still uses a left concave piston and a compression ratio of 9:1, but the original fuel injection, which is completed once at 35°CA BTDC (before top dead center), is changed to two injections:

[0086] In the present embodiment 2, the double-pulse segmented direct injection method is used for hydrogen injection.

[0087] The first injection is preferably performed at the end of the intake stroke (such as the interval of 90°CA~60°CA after the intake valve is closed to before top dead center), and the injection amount accounts for about 70% of the total hydrogen injection amount, which is used to form a basic homogeneous lean mixture to ensure the stability of the spark plug ignition and the integrity of the combustion.

[0088] The second injection is preferably performed at the late compression stroke (in the interval of 40°CA~10°CA before top dead center, and completed before the ignition time (≈5°CA ATDC)), and the injection amount accounts for about 30% of the total hydrogen injection amount, which is used to form a moderate hydrogen stratification in the cylinder center area to speed up the flame propagation speed and improve the combustion efficiency.

[0089] ​The bench test and simulation analysis show that when the second injection ratio is 30%, the engine shows the best combustion performance: the indicated thermal efficiency (ITE) is about 46.3%, and the NOx emission level is about 0.02 g / kW·h, which is significantly better than other distribution ratios such as 20% and 40%. In comparison, when the second injection ratio is 20%, the combustion rate is slower, and the NOx emission is still high; when the second injection ratio is 40%, although the combustion rate is accelerated, a local hydrogen-rich area appears, resulting in a significant increase in NOx emission.

[0090] Therefore, by adopting the distribution ratio of "first injection about 70%, second injection about 30%", the best balance between combustion efficiency and emission control can be achieved under the condition of ultra-lean combustion.

[0091] Table 2 Main parameters of three ratio MFSI operation conditions

[0092]

[0093] By this injection strategy, the combustion chamber structure has both homogeneous and stratified mixing characteristics: the first injection provides the basic lean mixture to ensure overall combustion stability; the second injection locally enriches hydrogen near the direct injection nozzle before ignition to improve the combustion rate. The implementation results are as follows:

[0094] · From the mixture distribution, after the second injection, a stratified structure is formed in the cylinder, which is slightly rich in the center and lean in the periphery. At the ignition time, compared with the case where hydrogen gas is mainly biased to the right side of the cylinder when late injection is used once, when the second injection ratio increases to 30%, the rich hydrogen area is more centralized and uniformly distributed around the hydrogen direct injection nozzle. This ensures that the flame core area has sufficient fuel concentration and avoids over-concentration near the cylinder wall.

[0095] · The combustion characteristics are significantly improved. Compared with single late injection, after adding the first advance injection, the ignition delay is significantly shortened, and the total combustion duration is also shortened. Especially when the second injection ratio is 30%, the ignition delay time is the shortest, and the cylinder pressure rises rapidly but not excessively. Compared with the original scheme 3, the double pulse injection reduces the maximum pressure rise rate and the peak heat release rate by more than 35%, making the combustion process more stable and reducing the tendency of knocking and noise. At the same time, due to the more concentrated flame propagation and the reduction of high-temperature residence time, the wall heat loss is reduced, which helps to improve the thermal efficiency.

[0096] • Engine performance and emissions are significantly improved. The measured results show that the indicated thermal efficiency of scheme 3 is increased from 45.6% to about 46.3% (maximum at 30% of the second injection) after using double injection. More importantly, the NOxemissions are greatly reduced relative to single late injection, close to the level of scheme 2. This is because the double-stage injection achieves a more reasonable mixture distribution, reduces the over-rich combustion area, and significantly reduces the volume of high-temperature area. Therefore, the present application achieves high-efficiency, clean and stable combustion by synergizing the segmented injection with the combustion chamber shape while maintaining the advantages of lean-burn suppression of backfiring and knocking, greatly reducing the NOxpenalty caused by excessive stratification.

[0097] As can be seen from the above examples, the synergistic optimization method proposed by the present application is effective on hydrogen fuel ultra-lean internal combustion engine.

[0098] Example 1 demonstrates that the optimized combustion chamber structure (such as right concave piston) combined with early injection strategy can achieve better combustion performance and lower emissions than traditional design;

[0099] Example 2 further shows that by adjusting the injection strategy (using double pulse and optimizing the proportion), the combustion speed can be improved while the NOx is controlled at a very low level in the case of avoiding knocking.

[0100] Thus it is verified that the synergistic optimization control mechanism of the combustion chamber structure and injection strategy described in the present application can flexibly adapt to different working condition requirements, and comprehensively improve the performance and reliability of hydrogen internal combustion engine under ultra-lean combustion conditions.

[0101] The present application has the following advantages by synergistic optimization of the combustion chamber structure and injection strategy:

[0102] (1) Improve combustion speed and stabilize ultra-lean combustion: large-scale turbulent tumble flow generated by the optimized piston shape effectively promotes the diffusion mixing of hydrogen in the cylinder. Combined with appropriate double-stage injection strategy, a slightly rich fuel mixture core is formed near the hydrogen direct injection nozzle, significantly accelerating the flame propagation speed and combustion rate, so that the ultra-lean mixture can also be fully burned in a short combustion duration. This improves the combustion stability, reduces the cycle fluctuation and misfire tendency.

[0103] (2) Improve thermal efficiency and reduce emissions: By synergistically controlling the compression ratio and injection timing, the present invention can improve thermal efficiency while suppressing knock. Experimental results show that the indicated thermal efficiency of the engine using the present invention can reach a high level of about 46%. At the same time, due to the optimization of the mixture distribution, the occurrence of over-rich combustion regions in the combustion chamber is avoided, thereby minimizing the peak combustion temperature and the generation of NOx. Under the method of the present invention, nitrogen oxide emissions are significantly reduced, more than an order of magnitude lower than the unoptimized working condition. In particular, for the case of local over-concentration combustion caused by late injection strategy (high NOx emissions), the present invention optimizes the second injection to reduce NOx emissions to a very low level, balancing the high efficiency of lean combustion and low pollutant emissions.

[0104] (3) Suppress backfire and knock risk: The present invention controls the injection timing and distribution, and uses delayed injection when needed, so that most of the hydrogen is injected in the late compression stroke, thereby avoiding the premature entry of hydrogen into a high-temperature environment and effectively preventing the occurrence of backfire (premature ignition). At the same time, the stratified lean strategy is used to make the mixture near the cylinder wall area extremely lean and the combustion temperature relatively low, while the hydrogen direct injection nozzle is slightly rich to ensure combustion, thereby avoiding the overheating of the end mixture and triggering knock. Experiments show that under the control strategy of the present invention, the engine can run smoothly under ultra-lean conditions without any signs of knock, solving the problem of unavoidable knock in traditional high-compression-ratio lean combustion.

[0105] Figure 1 The engine test bench for studying and verifying the method of the present invention is shown in the figure. The structure and measurement arrangement of the engine bench are shown, which is used to obtain combustion pressure, heat release rate and other data, and to verify the numerical simulation model.

[0106] Figure 2 The geometric model of the engine and its combustion system is shown in the figure. It includes the position arrangement of the combustion chamber, intake and exhaust ports, direct injection nozzle and hydrogen direct injection nozzle, which is used for three-dimensional simulation analysis.

[0107] Figure 3a 、 Figure 3b 、 Figure 3c and Figure 3d are the comparison graphs of numerical model verification results and experimental data. Among them: Figure 3a shows the agreement between the calculated hydrogen laminar burning velocity and the experimental data from the literature; Figure 3b is a convergence verification curve graph of cylinder pressure and heat release rate under different grid refinement levels; Figure 3c and Figure 3d are comparison graphs of cylinder pressure and heat release rate between experiments and simulations under two working conditions. These verification results show that the established simulation model has good accuracy.

[0108] Figure 4a 、 Figure 4b 、 Figure 4c are schematic diagrams of three piston crown shapes and the interaction of hydrogen jet with the piston. Among them: Figure 4a is a flat-top piston (reference); Figure 4b is a right-dimple piston; Figure 4c is a left-dimple piston. The right-dimple piston ( Figure 4b ) is designed to generate counterclockwise tumble flow in the early injection case, while the left-dimple piston ( Figure 4c ) is used to generate clockwise tumble flow in the late injection case. The arrows show the situation of the hydrogen jet entering the combustion chamber and being guided by the piston shape to form tumble flow.

[0109] Figure 5 are graphs of tumble ratio variation with crank angle under three piston design schemes. The positive tumble ratio, negative tumble ratio and average tumble ratio around the X-axis with time are given respectively. It can be seen that the tumble ratio increases significantly during injection due to the action of hydrogen jet. Scheme 2 (right-dimple piston) produces stronger positive tumble than scheme 1 (flat-top piston), while scheme 3 (left-dimple piston) reverses the direction of tumble flow (negative tumble ratio) under the action of late hydrogen injection.

[0110] Figure 6a 、 Figure 6b 、 Figure 6c are flow field and equivalence ratio distribution comparison diagrams of three combustion chamber designs. Figure 6a is the velocity vector flow field at a certain time during the compression stroke of different schemes; Figure 6b is the equivalence ratio concentration distribution cloud diagram of the corresponding section at the same time; Figure 6c is the mass fraction distribution column chart of mixture uniformity at ignition time and the uniformity index (UI) comparison diagram of three schemes; the results show that scheme 2 has larger tumble scale and more uniform mixture distribution (UI>0.9), while scheme 3 forms significantly stratified mixture (UI≈0.69) due to late injection before ignition.

[0111] Figure 7a 、 Figure 7b are comparison curve diagrams of combustion performance under three combustion chamber designs. Figure 7a is the curve diagram of in-cylinder pressure and pressure rise rate with crank angle for three schemes; Figure 7b is the heat release rate (HRR) curve diagram of three schemes. From Figure 7a 、 Figure 7b , it can be seen that the pressure curve of scheme 1 and flat-top piston is close to that of scheme 2 right-dimple piston, but the slightly higher combustion rate of scheme 2 makes its peak pressure and pressure rise rate slightly higher than that of scheme 1. Scheme 3 has the highest pressure rise rate and HRR peak value due to the concentration of most of the mixture near the hydrogen direct injection nozzle and the highest turbulent intensity, which burns the fastest.

[0112] Figure 8 The temperature field distribution cloud pictures of the three schemes at different crank angles in the combustion process can be seen from the flame propagation process in the figures. The flames of schemes 1 and 2 propagate uniformly from the hydrogen direct injection nozzle to the outside, and the flame propagation speed of scheme 2 is slightly faster than that of scheme 1; while the flame propagation of scheme 3 is the fastest, and it propagates to the cylinder wall only about 14°CA after ignition (compared with about 32°CA and 28°CA of schemes 1 and 2, respectively). The scheme 3 combustion releases heat concentratedly, resulting in that the in-cylinder temperature is significantly higher than that of the other two schemes, and causing extremely high NOx emission.

[0113] The above describes the present application in conjunction with the drawings. Obviously, the specific implementation of the present application is not limited by the above manner. As long as various non-essential improvements are made by adopting the method concept and technical scheme of the present application; or the above concept and technical scheme of the present application are directly applied to other occasions without improvement, they are all within the protection scope of the present application.

Claims

1. A method for the synergistic optimization of the combustion chamber structure and injection strategy for ultra-lean hydrogen combustion, characterized by, The matching relationship between the combustion chamber structure characteristic parameters and the hydrogen injection characteristic parameters is established by optimizing the combustion chamber structure characteristic parameters and the hydrogen injection characteristic parameters in coordination, and the coordinated control of the turbulent intensity, the mixture concentration distribution and the ignition timing in the combustion process of the ultra-lean hydrogen mixture is realized under the ultra-lean combustion condition of the air-fuel ratio λ≥2.

5. The combustion chamber structure characteristic parameters include the compression ratio, the piston top surface pit shape and the installation position of the hydrogen direct injection nozzle; and the hydrogen injection characteristic parameters include the hydrogen injection timing, the injection ratio, the injection duration and the injection flow rate.

2. The method according to claim 1, characterized in that, The optimization of the combustion chamber structure parameters includes: The compression ratio is selected in the range of 9-12, which is determined by experiments as a balanced interval for improving the thermal efficiency while suppressing the risk of knocking; The piston top surface adopts a structure with pits, and the pit shape and size are adjusted to change the tumble intensity and the flow field characteristics after the compression ratio is determined.

3. The method according to claim 2, characterized in that, The arrangement direction of the piston top surface pits is coordinated with the hydrogen injection timing: When the early injection strategy is adopted, the pits arranged on the piston top surface are deviated to one side of the hydrogen direct injection nozzle, and the hydrogen jet flow is guided to form counterclockwise tumble flow consistent with the natural tumble direction in the cylinder; When the late injection strategy is adopted, the pits arranged on the piston top surface are deviated to the side away from the hydrogen direct injection nozzle, and the jet flow is guided to form clockwise tumble flow.

4. The method according to claim 2, characterized in that, The hydrogen direct injection nozzle is installed on one side of the cylinder head, and the injection direction is towards the lower side of the opposite side and points to the pit area of the piston top surface.

5. The method according to claim 4, characterized in that, The injection pressure of the hydrogen direct injection nozzle is set to 2-11 MPa.

6. The method according to any one of claims 1 to 5, characterized in that, The hydrogen injection adopts a double-pulse segmented direct injection mode, including first injection and second injection; the first injection occurs at the end of the intake stroke or the beginning of the compression stroke; and the second injection occurs at the late compression stroke and is performed before ignition.

7. The method of claim 6, wherein the method is a method of synergistically optimizing a combustion chamber structure and injection strategy for a lean burn hydrogen combustion engine, characterized by, The fuel distribution ratio and timing of the two injections are optimized and set, the hydrogen amount of the first injection accounts for 50%-80% of the total hydrogen injection amount, and the hydrogen amount of the second injection accounts for 20%-50% of the total hydrogen injection amount; the first injection is performed in the interval of 90°CA-60°CA before the top dead center after the intake valve is closed, and the second injection is performed in the interval of 40°CA-10°CA before the top dead center.

8. The method of claim 7, wherein the method is a method of synergistically optimizing a combustion chamber structure and injection strategy for a lean burn hydrogen combustion engine, characterized by, The hydrogen amount of the second injection accounts for 20%-40% of the total hydrogen injection amount, and the second injection is preferably started at about 30°CA before the top dead center (SOSI≈30°CA BTDC) and is completed before the ignition timing (about 5°CA ATDC after the top dead center).

9. The method of claim 7, wherein the method is a method of synergistically optimizing a combustion chamber structure and injection strategy for a lean burn hydrogen combustion engine, characterized by, The hydrogen amount of the first injection preferably accounts for about 70% of the total hydrogen injection amount, and is implemented at the end of the intake stroke to ensure the formation of a basic homogeneous lean mixture.

10. The method of claim 8, wherein the method is a method of synergistically optimizing a combustion chamber structure and injection strategy for a lean burn hydrogen combustion engine, characterized by, The hydrogen amount of the second injection preferably accounts for about 30% of the total hydrogen injection amount, and the engine can obtain the best indicated thermal efficiency and the lowest NOx emission level.

Images