Dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation and control

Abstract

The invention discloses a dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation and control, which comprises the following steps: 1) fuel ratio control: acquiring current operation state parameters of an engine, determining a target AER according to preset mapping, regulating ammonia gas supply quantity and hydrogen supply quantity to enable the actual AER to reach the target AER, and controlling the fuel ratio; the AER is adaptively set according to engine load and combustion stability conditions; 2) valve timing control: adjusting the opening time of an intake valve or the closing time of an exhaust valve through a variable valve timing mechanism so as to change a valve timing overlap angle VOA, wherein the VOA is set to be a negative value, a small overlap angle or a positive overlap angle according to the working condition of an engine; (3) injection time sequence optimization is conducted, specifically, the injection time of ammonia gas and the in-cylinder direct injection time of hydrogen are controlled respectively, and coordinated optimization of a fuel injection strategy and valve timing is achieved; and 4) closed-loop steady-state control: monitoring a combustion state and an emission index in real time, comparing the combustion state and the emission index with a threshold value to serve as a feedback signal, and correcting the target AER and VOA settings in time. According to the dual-fuel engine combustion control method based on regulation and control of the hydrogen-ammonia energy ratio and the gas distribution overlapping angle, the energy ratio of hydrogen fuel and ammonia fuel and the overlapping opening angle of an intake valve and an exhaust valve are dynamically adjusted under different working conditions, so that the combustion reaction rate and temperature are coordinated, and the stable and efficient combustion process is achieved; and harmful emissions of NOx, unburned ammonia and the like are obviously reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of combustion control of internal combustion engines, and particularly relates to a dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and valve overlap angle regulation. BACKGROUND

[0002] With increasing attention to energy security and environmental protection, developing carbon-neutral fuel engines has become a research hotspot. Among them, hydrogen is considered to be a very promising alternative fuel for engines due to its fast combustion rate, high combustion heat value, low ignition energy, and carbon-free combustion. At present, the global annual production of hydrogen has reached tens of millions of tons, and hydrogen energy application has a certain foundation. However, the application of hydrogen fuel in internal combustion engines also faces many challenges: hydrogen is prone to knock and pre-ignition at high load, which limits the power increase and stable operation of the engine; at the same time, hydrogen has low volumetric energy density, is difficult to store, and hydrogen molecules are highly permeable and easy to leak, leading to problems in storage and safety. In addition, due to the high combustion temperature of hydrogen, the nitrogen oxide (NOx) emissions in the engine exhaust are often significantly increased when only using hydrogen fuel, and additional aftertreatment measures are needed to control NOx.

[0003] Ammonia (NH3) as an ideal carrier of hydrogen has gradually attracted attention. Ammonia is easy to liquefy at room temperature, and the storage and transportation technology is mature, and the combustion products are mainly water and nitrogen, which do not directly produce carbon dioxide, so it is considered as a potential zero-carbon emission fuel. However, pure ammonia fuel combustion in engines faces serious difficulties: ammonia has a high ignition temperature, slow flame propagation speed, and high ignition energy requirement, making it difficult to start combustion and low combustion rate. To achieve stable and rapid combustion of ammonia, very high compression ratios and intake heating are usually required. In actual research, to improve the combustion performance of ammonia, high-reactivity pilot fuels such as diesel, gasoline or hydrogen are often added to form a dual-fuel mode, and the pilot fuel is ignited first to drive the combustion of ammonia. For example, a small amount of diesel is added as a guide in a compression ignition engine to help reduce the ignition difficulty of ammonia; a certain proportion of hydrogen can be added to a spark-ignition engine to improve the overall reactivity of the combustible mixture.

[0004] It has been shown that hydrogen and ammonia have strong complementarity in combustion characteristics. Hydrogen has high calorific value and fast burning rate, but high combustion temperature can easily increase NOx. Ammonia has low combustion temperature and can inhibit NOx, but it is difficult to ignite and has slow combustion rate. By adjusting the ratio of hydrogen-ammonia fuel, the combustion reaction rate and temperature can be controlled within a certain range, and the compromise optimization of combustion performance and emissions can be achieved. For example, Pyrc et al. found that in hydrogen-ammonia premixed combustion, stable combustion can only be achieved when the hydrogen energy ratio exceeds about 12%. Xin et al. added ammonia to a multi-cylinder gasoline engine and found that a small amount of ammonia mixed with hydrogen can improve the thermodynamic efficiency and power output of hydrogen combustion, but the low combustion rate of ammonia leads to a slight decrease in thermal efficiency. Wang et al. measured the stable combustion of at least 15% hydrogen energy ratio under the condition of passive pre-chamber, while the active pre-chamber and optimized injection strategy can reduce the hydrogen ratio to 3% and still obtain an indicated thermal efficiency of 42.5%. In general, increasing the hydrogen ratio can help improve the ammonia combustion rate and stability, but the increase in combustion temperature will lead to an increase in NOx emissions. Conversely, increasing the ammonia ratio can reduce the combustion temperature and thus reduce NOx, but too high a proportion of ammonia will lead to incomplete combustion, decreased thermal efficiency, and increased unburned ammonia emissions. Therefore, it is necessary to dynamically optimize the hydrogen-ammonia ratio under different working conditions to balance combustion stability, efficiency, and emissions.

[0005] In addition to fuel ratio, valve timing and valve overlap angle (VOA) also have important influence on the combustion and emissions of dual-fuel engines. VOA refers to the overlapping angle of the opening degree of the engine exhaust valve and the intake valve. When VOA is positive (the intake and exhaust valves are open at the same time for a period of time), a part of the residual exhaust gas will flow back to the intake port at the end of the exhaust stroke and enter the cylinder with the next cycle, forming internal exhaust gas recirculation (internal EGR). Moderate internal EGR can reduce combustion temperature and reduce NOx generation, but too high EGR rate will dilute the mixture and reduce the combustion rate and stability. Conversely, when VOA is negative (i.e. no overlap between intake and exhaust valves), there is no waste gas remaining in the cylinder during the gas exchange process, and the combustion is more complete and stable, but the intake efficiency and combustion pressure may be affected due to the increase in fresh charge and the change in pump work loss. Many existing gasoline engines use variable valve timing (VVT) technology to adjust VOA to optimize torque and emissions under different working conditions. However, in the context of hydrogen-ammonia dual fuel, the change of VOA not only changes the internal EGR rate, but also has a dynamic coupling effect with the hydrogen-ammonia ratio: for example, VOA affects the temperature and residual ammonia amount during the intake process, thereby affecting the ammonia combustion reaction activity and unburned ammonia (HC-like) emissions. How to coordinate the cooperation of fuel ratio and valve strategy is a key technical problem in the optimization of dual-fuel engines.

[0006] In summary, hydrogen-ammonia dual-fuel engines have significant carbon emission reduction potential, but existing technologies lack systematic research on the synergistic optimization of hydrogen-ammonia energy ratio (AER) and valve overlap angle (VOA) in different engine load and speed ranges. Traditional methods often only adjust the hydrogen-ammonia ratio for a fixed operating condition, or use fixed valve timing, making it difficult to balance combustion stability and low emission requirements under all operating conditions.

[0007] Therefore, a dual-fuel engine combustion control method is provided, particularly regarding how to adjust the hydrogen-ammonia fuel ratio and valve overlap angle in real time according to the dynamic operating conditions of the engine, and optimize the coordination of injection and intake strategies, to achieve efficient and clean combustion at high ammonia replacement rate, realize efficient and stable control of the engine combustion process, and reduce pollutant emissions. SUMMARY

[0008] 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 dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and valve overlap angle regulation, which aims to realize efficient and stable control of the engine combustion process by regulating the energy ratio of hydrogen fuel and ammonia fuel and the valve overlap angle of the engine, and reduce pollutant emissions.

[0009] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows: a dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and valve overlap angle regulation, comprising the following steps:

[0010] 1) Fuel ratio control: obtain the current operating state parameters of the engine, determine the target AER according to the preset mapping, adjust the ammonia supply amount and the hydrogen supply amount, so that the actual AER reaches the target AER, and the AER is adaptively set according to the engine load and combustion stability conditions;

[0011] 2) Valve timing control: adjust the intake valve opening time or the exhaust valve closing time through a variable valve timing mechanism to change the valve overlap angle VOA, and set the VOA to a negative value, a small overlap or a positive overlap according to the engine operating conditions;

[0012] 3) Injection timing optimization: control the injection timing of ammonia and the in-cylinder direct injection timing of hydrogen respectively to realize the coordinated optimization of fuel injection strategy and valve timing; wherein the injection timing of ammonia is adaptively controlled based on the valve overlap angle VOA and the engine speed: when the engine operates at medium and low speed and the VOA is in a small angle range, the ammonia injection is controlled to be completed in the intake valve opening phase; when the engine operates at high speed or the VOA is negative, the ammonia injection process is allowed to continue after the intake valve is closed;

[0013] The in-cylinder direct injection timing of hydrogen is set in the late compression stroke;

[0014] 4) Closed-loop steady state control: real-time monitoring of combustion state and emission indicators, comparing them with threshold values as feedback signals, timely modifying the target AER and VOA settings.

[0015] The target AER is self-adaptively adjusted according to engine load, with the maximum target AER limited to a preset percentage at low load; the target AER is gradually increased when the load rises above a first threshold value, with the target AER set to 60% to 80% at medium load, and the target AER raised to above 90% when the load rises above a second threshold value.

[0016] In the step 2), the VOA is set to approximately 0 or a negative value at low engine load or idle; the VOA is set to a positive value at medium to high engine load.

[0017] The VOA setting is modified according to engine speed, with the VOA reduced at high speed and increased at low speed; the VOA adjustment range includes several levels from negative overlap to positive overlap.

[0018] When the VOA is positive and greater than a predetermined angle, the control system limits the maximum allowed AER to a safe range; when the VOA is negative or zero, the AER is allowed to be raised to a higher level.

[0019] The end time of the hydrogen direct injection is set to a range of 0° to 60° crank angle after the compression top dead center.

[0020] When the engine is running at high speed or the VOA is negative, the ammonia is allowed to partially remain in the intake port; the remaining ammonia in the intake port is heated by the residual heat of the intake port wall surface during the period from the intake valve closing to the start of the next intake stroke.

[0021] In the closed-loop steady state control, the electronic control unit monitors CA50 and maintains CA50 at the target position by controlling the ignition advance angle or fuel ratio, and when the target AER adjustment or VOA change causes the combustion rate to change, the ignition time is automatically corrected to maintain the combustion phase in the range of 8° to 15° CA after the top dead center.

[0022] In the closed-loop steady state control, the concentrations of NOx and unburned ammonia in the exhaust gas are monitored in real time, and the following emission coupling control strategy is implemented:

[0023] When the NOx concentration exceeds the set threshold value, the target AER is increased or the ignition is delayed, and the VOA is increased to introduce internal EGR when necessary.

[0024] In the closed-loop steady state control, the concentrations of NOx and unburned ammonia in the exhaust gas are monitored in real time, and the following emission coupling control strategy is implemented:

[0025] When the unburned ammonia concentration exceeds the threshold value, the target AER is reduced or the ignition is advanced, while the VOA is reduced when needed.

[0026] The present application is based on a hydrogen-ammonia energy ratio and gas distribution overlap angle control dual-fuel engine combustion control method, which dynamically adjusts the energy ratio of hydrogen fuel and ammonia fuel and the overlap opening angle of the intake and exhaust valves under different working conditions to coordinate the combustion reaction rate and temperature, realize stable and efficient combustion process, and significantly reduce harmful emissions such as NOx and unburned ammonia, while maximizing the use of zero-carbon fuel ammonia gas while ensuring engine performance. BRIEF DESCRIPTION OF DRAWINGS

[0027] The present specification includes the following drawings, the contents of which are shown as follows:

[0028] Figure 1 is a structural schematic diagram of a dual-fuel engine;

[0029] Figure 2a is a trend chart of 50% combustion point (CA50) and cycle variation coefficient (COV) with AER;

[0030] Figure 2b is a chart of brake thermal efficiency (BTE) with AER;

[0031] Figure 2c is a chart of ignition delay (ignition delay period) and combustion duration with AER;

[0032] Figure 3a is a chart of average cylinder pressure under different ammonia energy ratios at 1500 rpm, 5 bar;

[0033] Figure 3b is the corresponding instantaneous heat release rate curve.

[0034] Figure 4a is a trend chart of average cylinder temperature in the compression stroke and the work stroke when AER increases from 0 to 57%;

[0035] Figure 4b is a corresponding combustion chamber pressure-volume (P-V) curve comparison chart, which shows that the increase of AER reduces the end-of-compression pressure and the pumping loss;

[0036] Figure 5 is a chart of unburned ammonia (NH3) emissions and NOx emissions characteristics under different engine loads and ammonia energy ratios;

[0037] Figure 6a is a chart showing the change of the proportion of energy loss (including heat transfer loss, exhaust loss, unburned loss and pumping loss) of the ammonia-hydrogen engine when AER changes from 0 to 86%;

[0038] Figure 6b is a schematic diagram of the corresponding exhaust flow rate and exhaust temperature variation;

[0039] Figure 8a is a curve diagram of BTE with speed variation when AER is 0 and 85% respectively;

[0040] Figure 8b is a schematic diagram of the corresponding ammonia injection pulse width (expressed in EOI relative to the time when the intake is closed) with speed variation, reflecting the trend of extending ammonia injection duration and delaying EOI at high speed;

[0041] Figure 9 is a diagram comparing the variation of unburned NH3 and NOx concentrations in the exhaust during the process of engine speed increasing from 1500 to 2500 rpm when AER is 0%, 60% and 85% respectively;

[0042] Figure 10a is a diagram of the variation trend of pumping loss with VOA when VOA gradually increases from negative to positive (achieved by advancing the opening time of the intake valve and delaying the closing of the exhaust valve);

[0043] Figure 10b is a comparison diagram of the average cylinder pressure curve during the intake stroke of the engine under the corresponding working condition;

[0044] In the figure, the following are marked:

[0045] 1, exhaust pipe; 2, hydrogen tank; 3, ammonia tank; 4, solenoid valve; 5, turbocharger; 6, intercooler; 7, heating jacket; 8, hydrogen guide rail; 9, intake pipe; 10, ammonia guide rail; 11, pressure stabilizing chamber; 12, flame arrester; 13, hydrogen flowmeter; 14, pressure reducing valve; 15, ammonia flowmeter. DETAILED DESCRIPTION

[0046] 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 technical personnel in the field 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.

[0047] The embodiment of the present application provides a dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation, which comprises the following steps:

[0048] 1) Fuel ratio control: obtain the current operating state parameters of the engine, determine the target AER according to the preset mapping, adjust the ammonia supply amount and the hydrogen supply amount, so that the actual AER reaches the target AER, and the AER is adaptively set according to the engine load and combustion stability condition;

[0049] 2) Valve overlap control: Adjust the intake valve opening time or exhaust valve closing time by variable valve timing mechanism to change the valve overlap angle VOA, which is set to negative, small overlap or positive overlap according to engine operating conditions;

[0050] 3) Injection timing optimization: Control the injection timing of ammonia and the in-cylinder direct injection timing of hydrogen respectively, realize the coordinated optimization of fuel injection strategy and valve timing; wherein the injection timing of ammonia is adaptively controlled based on the valve overlap angle VOA and engine speed: when the engine operates at medium and low speed (such as speed n≤3000rpm) and VOA is in a small angle range (such as VOA≤5°CA), control the ammonia injection to be completed in the intake valve opening stage; when the engine operates at high speed (such as speed n>3000rpm) or VOA is negative, allow the ammonia injection process to continue after the intake valve is closed;

[0051] The in-cylinder direct injection timing of hydrogen is set in the late compression stroke;

[0052] 4) Closed-loop steady-state control: Real-time monitoring of combustion state and emission indicators, comparing them with threshold values as feedback signals, timely correcting the target AER and VOA settings.

[0053] Specifically, the embodiment of the present application provides an engine combustion control method based on hydrogen-ammonia energy ratio and valve overlap angle regulation. The method aims to dynamically adjust the energy ratio of hydrogen fuel and ammonia fuel, and the overlap opening angle of intake and exhaust valves under different operating conditions, to coordinate the combustion reaction rate and temperature, realize stable and efficient combustion process, and significantly reduce harmful emissions such as NOx and unburned ammonia, while ensuring engine performance and maximizing the use of zero-carbon fuel ammonia.

[0054] In the above step 1), the current operating state parameters of the engine are obtained, including load, speed, etc., the target ammonia energy replacement rate AER is determined according to the preset mapping, and the ammonia supply amount and hydrogen supply amount are adjusted to make the actual AER reach the target value; wherein a lower AER is set under low load or unstable combustion conditions, and AER is as high as possible under medium and high load to increase the ammonia replacement rate. The preset mapping refers to a data model or Map table that reflects the corresponding relationship between engine operating conditions and control parameters including target ammonia energy replacement rate (AER) established by pre-experiment calibration or simulation calculation. The electronic control unit can directly determine the target AER under the current operating condition by querying the preset mapping after obtaining the real-time state parameters of the current engine, without real-time calculation, thereby realizing efficient control of fuel ratio.

[0055] In the above step 2), the intake valve opening time or the exhaust valve closing time is adjusted by the variable valve timing mechanism to change the valve overlap angle VOA; the VOA is set to be negative, small overlap or positive overlap according to the operating condition. The VOA is set to be approximately 0 or negative to reduce internal EGR and improve combustion stability at low load or idle; a moderate positive VOA is adopted to reduce pumping loss and partially introduce internal EGR to reduce temperature and emissions at medium-high load.

[0056] In the above step 3), the ammonia intake port injection time and the hydrogen in-cylinder direct injection time are controlled respectively to realize coordinated optimization of the fuel injection strategy and the valve timing; the ammonia injection is selected to be completed during the intake valve opening or partially continued after the intake valve closing according to the VOA and the speed, so as to utilize the intake port fuel retention preheating effect or avoid backfire risk; the hydrogen direct injection is set at the late compression stroke close to the ignition time to form a local hydrogen-rich area to ignite the mixture.

[0057] In the above step 4), the combustion state (cylinder pressure, knock, cycle fluctuation COV) and the emission index (NOx, unburned ammonia, etc.) are monitored in real time, which are compared with the threshold value as a feedback signal to timely correct the AER and VOA settings; when combustion instability (COV out of limit) is detected, the AER is reduced or the VOA is decreased; when NOx is too high, the AER is increased or the VOA is increased; when unburned ammonia is too high, the AER is reduced or the injection timing is adjusted to ensure stable combustion.

[0058] As shown in Figure 1 The dual-fuel engine includes at least one cylinder, a cylinder body, a piston, a cylinder head, a variable valve timing mechanism, an intake system and an exhaust system, and fuel supply assemblies for supplying hydrogen and ammonia respectively, which include hydrogen tanks and ammonia tanks and the like. The engine adopts spark ignition or compression ignition mode, and the dual-fuel engine further includes an electronic control unit (ECU) for monitoring and adjusting combustion parameters. The cylinder body, 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.

[0059] As shown in Figure 1As shown, the ammonia tank provides ammonia, which is injected into the intake port as the main fuel via the intake pipe in gaseous form. The hydrogen tank provides hydrogen, which is supplied as the auxiliary fuel to the combustion chamber, preferably via direct injection (DI) into the cylinder. The intake system includes an intake manifold, which communicates with the intake port, and an ammonia injector is arranged on the intake manifold. The ammonia tank is connected to the ammonia injector via an ammonia intake pipe, which conveys the ammonia in the ammonia tank to the ammonia injector. A pressure reducing valve is arranged in the ammonia intake pipe. The ammonia tank and the pressure reducing valve are used to inject the ammonia into the intake manifold in gaseous form in an appropriate amount, so as to realize quantitative injection of ammonia into the intake port. A hydrogen high-pressure direct injection nozzle is arranged on the cylinder head of the engine. The hydrogen tank is connected to the hydrogen high-pressure direct injection nozzle via a hydrogen intake pipe, which conveys the ammonia in the hydrogen tank to the hydrogen high-pressure direct injection nozzle. A hydrogen rail is arranged in the hydrogen intake pipe and connected to all the hydrogen high-pressure direct injection nozzles. The hydrogen high-pressure direct injection nozzles are used to inject the hydrogen into the combustion chamber at high pressure (e.g., 16 bar). The injection time and injection duration of the two fuels are independently controlled by the ECU, and the injection timing can be adjusted to optimize the formation of the mixture. The hydrogen high-pressure direct injection nozzles and the ammonia injector are electrically connected to the ECU.

[0060] In the embodiment of the present application, the ammonia injection pressure is about 7 bar, and the hydrogen direct injection pressure is about 16 bar. The two fuels are derived from high-pressure steel cylinders and are supplied through two-stage pressure reduction and stabilization. A flame arrester is installed on the hydrogen intake pipe and the ammonia intake pipe to ensure safety.

[0061] In the embodiment of the present application, the target AER (the percentage of the fuel energy provided by ammonia in the total fuel energy, i.e., the energy proportion of ammonia) is set according to the engine operating conditions (load, speed, etc.). The ECU controls the ammonia injection amount and the hydrogen injection amount by calculating the current fuel demand and the target AER, so that the energy proportion of ammonia contained in the mixture reaches the set value. For example, the AER is increased to increase the ammonia replacement rate at medium and high loads, and the AER is appropriately reduced to ensure sufficient hydrogen fuel to maintain stable ignition at low loads or special operating conditions. In the preferred embodiment, the AER can be gradually increased from 0 to about 85% at 2000 rpm and BMEP 5 bar, corresponding to an increase in ammonia from zero to providing most of the fuel energy; the AER can even exceed 95% at high loads with BMEP > 5 bar, approaching pure ammonia combustion conditions.

[0062] In step 1) described above, the hydrogen-ammonia dual-fuel engine adopts an ammonia intake port injection and hydrogen in-cylinder direct injection fuel supply mode. Ammonia is injected into air in gaseous form through the intake manifold to form a combustible mixture, and hydrogen is directly injected into the combustion chamber at high pressure through the oil nozzle. An electronic control unit controls the fuel metering and timing of the ammonia injector and the hydrogen high-pressure direct injection nozzle, respectively.

[0063] In the embodiment of the present application, the target AER is self-adaptively adjusted according to the engine load. When the load is low (at this time, the indicated pressure or BMEP is lower than the first threshold value), the target AER is limited to not more than a preset percentage, so as to ensure that the hydrogen gas in the mixture accounts for a sufficient proportion for ignition. When the load is increased and exceeds the first threshold value, the target AER is gradually increased. When the engine is in a medium load (BMEP is between 3 and 5 bar), the target AER is set to 60% to 80%. When the load is increased and exceeds the second threshold value, the target AER is increased to more than 90%, so as to maximize the ammonia gas to replace the conventional fuel. The second threshold value is greater than the first threshold value. For example, when the first threshold value is 3 bar, the second threshold value is 5 bar.

[0064] When the engine is in a low load, the indicated pressure refers to the pressure generated by the fuel acting on the piston in the cylinder of the engine (reflecting the working intensity in the cylinder). The BMEP (brake mean effective pressure) is an index for measuring the output power per unit displacement of the engine, and is directly related to the load level of the engine. The higher the value is, the greater the load is. When the indicated pressure value or the BMEP value during the operation of the engine is lower than a preset first threshold value, it is determined that the engine is in a low load condition. At this time, the maximum allowable value of the AER needs to be limited according to the strategy.

[0065] In the embodiment of the present application, the valve overlap angle (VOA) needs to be controlled. The variable valve timing mechanism is used to dynamically adjust the overlap opening angle VOA of the intake and exhaust valves according to different working conditions. The ECU selects an appropriate VOA setting according to the engine speed and load and other parameters, so as to optimize the charging efficiency and combustion stability. When the load is low to medium, a small VOA or a negative overlap angle (that is, the intake valve lags behind the closing of the exhaust valve) is preferably used to reduce the internal EGR, ensure the fresh mixture charge, and improve the combustion rate and stability, so as to allow a higher proportion of ammonia gas to participate in the combustion. When the load is medium to high, a moderate positive VOA can be used to utilize the exhaust inertia to improve the intake charge and reduce the pumping loss, and at the same time, a certain EGR is used to reduce the high combustion temperature to reduce the NOx emission. The specific control strategy of the VOA is determined by a pre-labeled mapping. For example, when the engine speed is increased, the positive overlap angle is gradually reduced to control the combustion duration. When the engine load is reduced, the VOA is adjusted to a negative value in time to suppress the unstable combustion caused by excessive EGR.

[0066] In the embodiment of the present application, in the above step 2), the VOA is set to be approximately 0 or a negative value when the engine is in a low load or idling. When the engine is in a medium to high load (for example, the load is greater than or equal to 50%), the VOA is set to a positive value.

[0067] In the above step 2), the setting of the valve overlap angle VOA is corrected according to the engine speed, and the VOA is reduced in the high speed region (e.g. n≥3000 rpm) to shorten the combustion duration and avoid slow combustion caused by excessive internal EGR; the VOA is increased in the low speed region (e.g. n<1500 rpm) to appropriately introduce exhaust gas to reduce pumping loss and suppress knock; the adjustment range of the VOA includes several gears from negative overlap (no overlap between intake and exhaust) to positive overlap, and the specific value is obtained by experimental calibration.

[0068] In the above step 2), when the VOA is positive and greater than a predetermined angle (e.g. VOA exceeds 50° crank angle after the intake valve opens), the control system (ECU) limits the maximum allowed target AER to a safe range to compensate for the negative effect of high internal EGR rate on combustion stability caused by positive overlap, and prevent the cycle variation coefficient COV from exceeding the allowed value; when the VOA is negative or zero, the target AER is allowed to rise to a higher level (e.g. AER≥50%), because internal EGR is very low at this time and will not significantly affect the combustion stability of ammonia gas.

[0069] In the above step 3), the injection timing of ammonia gas meets one of the following optimization conditions:

[0070] In the case of low engine speed and small VOA, the control of ammonia gas injection duration is not more than the closing time of the intake valve, to ensure that all ammonia gas enters the cylinder in the current intake stroke, thereby avoiding the risk of backfire caused by fuel retention;

[0071] When the engine is at high speed or the VOA is negative, ammonia gas is allowed to be partially retained in the intake port (i.e. the injection continues for several crank degrees after the intake valve closes), and the retained ammonia gas is heated by the wall heat after the valve closes until the next intake, and its temperature rises to help the next working cycle (each working cycle refers to the period of a complete energy conversion process of the engine, which corresponds to one working cycle of the engine, including four consecutive strokes of intake, compression, work and exhaust) combustion, thereby improving the combustion rate and efficiency.

[0072] In the embodiment of the present application, the injection timing of hydrogen direct injection is set to the late compression stroke, so that the local concentration of hydrogen fuel in the mixture near the ignition time is high to facilitate ignition. The end time of hydrogen direct injection is set to 0°-60° crank angle after the compression top dead center (i.e. the end of compression stroke), so as to avoid late injection causing under-mixing or early pre-ignition.

[0073] In the embodiment of the present application, when the engine operates at high speed (n≥3000 rpm) or VOA is negative, ammonia gas is allowed to stay in the intake port of the engine; the ammonia gas staying in the intake port is heated by the residual heat of the intake port wall surface in the period from the closing of the intake valve to the start of the next intake stroke. After the opening of the intake valve, the heated ammonia gas in the intake port enters the combustion chamber, and the ammonia gas with increased temperature helps combustion, which can improve the combustion rate and efficiency.

[0074] In the embodiment of the present application, the coordinated control of the dual-fuel injection strategy and the intake valve strategy is emphasized. First, the injection timing of ammonia gas is optimized relative to the opening / closing time of the intake valve: in certain working conditions, the delayed injection after valve closing (post-injection) mode is adopted, that is, part of the ammonia gas is injected after the closing of the intake valve, so that the ammonia gas stays in the intake port until the next intake stroke. This can utilize the residual heat of the cylinder wall to preheat the ammonia gas, increase the temperature of the mixture and the activity of ammonia gas in the next cycle, and promote the combustion rate and complete combustion. On the other hand, in other working conditions, the simultaneous injection during valve opening (injection during valve opening) mode is adopted, that is, the ammonia gas injection is completed during the opening of the intake valve, so that the ammonia gas enters the cylinder in time to participate in the current cycle combustion.

[0075] The ECU determines the switching of the two injection modes according to VOA and engine speed:

[0076] For example, when VOA is large (e.g. ≈50° crank), the intake duration is long, and the ammonia gas can be injected entirely during the opening of the valve; when VOA is small, the ammonia gas injection duration may exceed the closing time of the intake valve, and part of the ammonia gas will stay until the next working cycle. By optimizing the injection time of ammonia gas, the intake process and fuel supply are matched, which avoids the risk of backfire caused by late ammonia gas injection, and utilizes the heating effect of the retained ammonia gas to improve the quality of the mixture, thereby improving the combustion efficiency. Hydrogen direct injection is preferably performed in the late compression stroke to form a local hydrogen-rich area to promote ignition: for example, the hydrogen direct injection is controlled to be completed between about 120° and 60° crank before top dead center, so that the hydrogen concentration is high near the ignition time, which improves the spark ignition reliability and guides the combustion of the surrounding lean ammonia mixture.

[0077] In the above step 4), in the closed-loop steady-state control, the electronic control unit monitors CA50 (combustion center position or combustion center offset) and maintains CA50 at the target position by controlling the ignition advance angle or fuel ratio, when the target AER adjustment or VOA change causes the combustion rate to change, the ignition time is automatically corrected to maintain the combustion phase (CA50) in the range of 8°-15° CA after top dead center, so as to ensure the combustion completeness and the stability of engine torque output.

[0078] In step 4) above, in the closed-loop steady-state control, the concentrations of NOx and unburned ammonia (NH3) in the exhaust gas are monitored in real time, and the following emission coupling control strategy is performed:

[0079] When the NOx concentration exceeds the specified threshold, the target AER is increased or the ignition is delayed to reduce the combustion temperature, and the VOA is increased to introduce internal EGR when necessary to further suppress the generation of NOx;

[0080] When the unburned ammonia concentration exceeds the threshold, the target AER is decreased or the ignition is advanced to increase the combustion temperature to promote the complete combustion of ammonia, and the VOA is reduced to decrease the internal EGR rate to increase the combustion reaction rate. Through the above-mentioned bidirectional adjustment, the dynamic balance optimization between NOx and unburned ammonia emissions is achieved.

[0081] In an embodiment of the present application, the ignition timing or other control parameters are fine-tuned according to combustion feedback information (such as knock sensor, cylinder pressure sensor, etc.) to ensure proper combustion phase and cycle stability. Preferably, the ECU monitors indicators such as combustion center (CA50) and cycle variation coefficient (COV) of each cycle, controls CA50 at a position close to a certain angle (e.g. 8-10° CA) after top dead center to obtain the best thermal efficiency, and controls COV within 3% by adjusting the hydrogen / ammonia injection ratio or ignition advance angle to avoid unstable phenomena such as misfire or knock. When the COV increases and tends to be unstable under high AER conditions, the control system can temporarily reduce the AER or adjust the VOA to restore stability; when the engine is running at high speed and the ammonia injection duration is close to the intake window, the control unit can increase the ammonia injection pressure or inject in multiple pulses to ensure that the ammonia injection is completed before the intake valve closes, avoiding backfiring and uneven combustion.

[0082] The present application is based on a dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation, which has the following advantages:

[0083] 1. High ammonia replacement rate and efficient combustion: Through fuel ratio regulation and injection optimization, this method can achieve the highest possible ammonia replacement rate under various operating conditions while maintaining high combustion efficiency and stability. The ammonia energy ratio can be increased to 90% or even more than 95% at medium and high loads, far exceeding the ammonia replacement rate limited by stability in traditional strategies, which is limited to 50-80%. Even if ammonia is the main fuel, the engine can still maintain a high thermal efficiency (for example, the brake thermal efficiency is only slightly lower than that of pure hydrogen when ammonia accounts for 86%), which proves that this application can fully utilize the heat value of ammonia without sacrificing combustion quality.

[0084] 2. Significant reduction of NOx emissions: Since increasing the ammonia proportion can significantly reduce the peak combustion temperature, the present invention can effectively reduce the generation of thermal NOx. Research results show that under the condition of 2000 rpm and 5 bar, the AER is increased from 0 to 86%, and the in-cylinder peak temperature is reduced from about 2700 K to 2000 K, which corresponds to a reduction of about 64% in NOx emissions. By coordinating VOA and AER, the present invention keeps the combustion temperature at a low level under most working conditions, and the engine emissions of NOx are significantly lower than the pure hydrogen combustion mode. In addition, for fuel-type NOx generated by ammonia combustion, the present method can also inhibit its generation to a certain extent by optimizing the completeness of ammonia combustion and segmented EGR control. For example, when a small amount of ammonia is initially added at low load, a slight increase in NOx may occur (the contribution of fuel-type NOx supplied by ammonia decomposition increases), but as the AER is further increased and the combustion temperature is overall reduced, NOx decreases instead. The present invention can balance this process and achieve a monotonic decrease in NOx under various loads.

[0085] 3. Unburned ammonia (HC) emissions are controlled: The present invention controls fuel ratio and injection strategy to alleviate the problem of increased unburned ammonia emissions at high ammonia fuel proportion. When only the ammonia replacement rate is increased without taking other measures, a large amount of ammonia gas may remain unburned at the corners of the combustion chamber due to slow burning speed. On the one hand, the present method actively reduces AER or optimizes injection in the case of high speed or high VOA leading to slow combustion, to avoid incomplete combustion caused by excessive ammonia participation. On the other hand, it uses post-valve retention injection to improve the preheating and uniformity of the mixture, and to improve the combustion rate and completeness of ammonia combustion. Experimental results show that at low and medium ammonia proportion (AER < 60%), unburned ammonia emissions are almost unaffected by engine speed or even slightly reduced, while at high ammonia proportion (AER ≈ 85%), although the combustion is slow at high speed, the absolute amount of unburned ammonia emissions is still lower than that without control strategy. At the same time, since the unburned ammonia emissions are reduced, the present invention also reduces the burden on the aftertreatment system to deal with ammonia slip.

[0086] 4. Dynamic coupling optimizes emission balance: The present invention reveals and utilizes the dynamic coupling relationship between valve overlap angle and NOx / unburned ammonia emissions. When VOA increases, the increased internal EGR leads to a decrease in combustion temperature, which reduces NOx, but at the same time, the combustion slows down, requiring a decrease in ammonia proportion to maintain stability, which in turn makes unburned ammonia emissions decrease with the increase of VOA (because less ammonia fuel is input). Conversely, when VOA decreases from positive to negative, the decrease in internal EGR increases the combustion rate, which can increase the input of ammonia gas but NOx may increase. The control method of the present invention can adjust VOA and AER in real time, so that they compensate for each other: increase VOA and decrease AER when it is necessary to reduce NOx, and decrease VOA and increase AER when it is necessary to increase the combustion rate, so as to achieve the synergistic optimization of NOx and unburned ammonia (HC) emissions. Experiments show that when VOA increases from 0 to 50°CA, to maintain COV < 3%, the maximum allowed AER needs to decrease from about 78% to 25%, which corresponds to a 30% increase in combustion duration, but NOx decreases while unburned ammonia emissions also decrease; while VOA is negative to 0, internal EGR changes little, AER and combustion rate remain almost unchanged, maintaining a low unburned ammonia emission. This dynamic balance relationship is fully utilized in the present invention to maintain optimal emissions.

[0087] 5. Improve thermal efficiency and reduce pumping loss: By optimizing VOA and injection strategy, the present invention also improves the charging and thermodynamic process of the engine. Proper negative VOA makes the intake valve close early and the exhaust valve open late, reducing the loss of fresh mixture being taken away and backflushing, increasing the charging coefficient and combustion pressure; at the same time, negative VOA eliminates overlap and reduces pumping power consumption, improving indicated efficiency. In the experiment, the engine brake thermal efficiency is slightly improved when using negative VOA. Even if VOA changes from negative to positive and pumping loss continues to decrease, due to the increase in internal EGR, the present invention suppresses the overall thermal efficiency decline by adjusting the fuel ratio. In addition, the increase in ammonia proportion also reduces the cylinder wall heat loss, from about 43% to 32%, which uses more fuel energy for work output. Under the combined action, the thermal efficiency and effective power of the engine remain at a high level while using high ammonia fuel.

[0088] 6. Adaptability to All Operating Conditions: The method of this invention achieves closed-loop control via the ECU, automatically adjusting parameters according to the engine's operating status, exhibiting excellent real-time response and universality. Whether at low-load idling (requiring stable ignition), medium-load cruising (pursuing high ammonia substitution rate and high efficiency), or high-load acceleration (focusing on avoiding knocking and maximizing power), this method can meet the requirements by adjusting the hydrogen-ammonia ratio, injection timing, and VOA. For example, in the high-speed range of the engine, backfire can be avoided by shortening the ammonia injection duration or increasing the injection pressure; at low speed and high load, misfire can be avoided by reducing VOA and lowering EGR; and at high speed and high load, the combustion duration can be prevented from being too long by appropriately reducing AER. Therefore, this invention provides a combustion control scheme applicable to a wide range of operating conditions, significantly superior to existing control strategies calibrated only for a single operating condition.

[0089] As can be clearly seen from the above description of specific embodiments, the control method provided by the embodiments of the present invention can dynamically coordinate the ratio and gas phase of hydrogen fuel and ammonia fuel, giving full play to the advantages of the two fuels and complementing their shortcomings, thus achieving high-efficiency, low-pollutant-emission combustion control of dual-fuel engines. This method can be applied to automotive engines to reduce carbon emissions and meet stringent pollutant regulations, and can also be used in high-power engines for power generation, marine engines, and other fields, showing broad application prospects.

[0090] In summary, this embodiment of the invention organically combines the advantages of rapid and efficient combustion of hydrogen fuel with the low-temperature NOx suppression characteristics of ammonia fuel, and through unique parameter coordination and control, achieves the optimal balance between power, economy and emissions in the dual-fuel engine.

[0091] Figure 2a The figure shows the trends of 50% ignition point (CA50) and coefficient of variation (COV) with AER. Figure 2b The middle part represents the variation of braking thermal efficiency (bTE) with AER. Figure 2c The values ​​in the middle represent the changes in ignition delay (ignition delay period) and combustion duration with AER.

[0092] Figure 3a and Figure 3b The in-cylinder pressure and heat release rate (HRR) curves are shown under different AER conditions. Figure 3a The diagram illustrates the average cylinder pressure variation at different ammonia energy percentages under conditions of 1500 rpm and 5 bar. Figure 3b The curve in the middle shows the corresponding instantaneous heat release rate.

[0093] Figure 4a and Figure 4b Comparison of in-cylinder temperature and PV diagrams under different AER conditions. Figure 4a The middle figure shows the trend of average cylinder temperature changes during the compression and power strokes as the AER increases from 0 to 57%.Figure 4b The middle section shows a comparison of the corresponding combustion chamber pressure-volume (PV) curves. It can be seen that increasing AER reduces the pressure at the end of compression and decreases pumping losses.

[0094] Figure 5 This diagram shows the emission characteristics of unburned ammonia (NH3) and NOx under different engine loads and ammonia energy ratios. The changes in NH3 and NOx emissions with increasing AER are compared under low load (2 bar BMEP) and medium-high load (5 bar, 8 bar BMEP) conditions.

[0095] Figure 6a and Figure 6b The energy loss distribution and exhaust parameters under different AER conditions are shown. Figure 6a The diagram illustrates the changes in the percentage of energy loss from various components (including heat transfer loss, exhaust loss, unburned loss, and pumping loss) in an ammonia-hydrogen engine as the AER (Average Energy Response) varies from 0 to 86%. Figure 6b The diagram illustrates the corresponding changes in exhaust flow rate and exhaust temperature.

[0096] Figure 7a and Figure 7b A comparison of combustion stability and duration under different engine speeds and AER conditions. Figure 7a The data shows the changes in the position of the combustion center (CA50) and the coefficient of variation of the cycle (COV) as the engine speed increases from 1200 rpm to 2400 rpm when the AER is 60% and 85% respectively. Figure 7b The data shows the changes in combustion duration and ignition delay with engine speed under the same conditions.

[0097] Figure 8a and Figure 8b The braking thermal efficiency (bTE) and ammonia injection termination time (EOI) are calculated under different engine speeds and AER conditions. Figure 8a The curves showing the change in BTE with rotational speed when AER is 0% and 85% are shown in the middle. Figure 8b The middle part shows the change of the corresponding ammonia injection pulse width (represented by the EOI relative to the moment of intake closure) with the speed, reflecting the trend of prolonged ammonia injection duration and delayed EOI at high speeds.

[0098] Figure 9 The emission characteristics of NH3 and NOx under different engine speeds and AER conditions. Figure 9 The changes in the concentration of unburned NH3 and NOx in the exhaust gas as the engine speed increased from 1500 to 2500 rpm were compared when AER=0%, 60%, and 85%. The results showed that NH3 emissions were more sensitive to engine speed at high AER, while NOx emissions decreased with increasing engine speed at high AER.

[0099] Figure 10a and Figure 10b The engine valve timing and pumping loss and in-cylinder pressure variation under different valve overlap angle (VOA) are shown. Figure 10a The variation trend of pumping loss with VOA when VOA gradually increases from negative to positive (by advancing the intake valve opening time and retarding the exhaust valve closing time) is shown in FIG. 2; Figure 10b The average cylinder pressure curves in the intake stroke of the engine under corresponding conditions are compared in FIG. 3, and increasing VOA leads to reduced intake stroke vacuum degree and increased cylinder pressure, thereby reducing pumping work loss.

[0100] The dual-fuel engine combustion control method of the embodiment is realized by ECU in the form of software algorithm. The ECU determines the target ammonia energy ratio AER and valve VOA setting under the current condition by table lookup or model calculation, and outputs control signals to adjust the fuel injection amount and valve timing. When testing the dual-fuel engine on a test bench, the control process is as follows:

[0101] 1. Obtain the working condition parameters: real-time acquisition of engine speed, throttle opening or intake flow, current cylinder pressure index (used to estimate the load BMEP), and data such as COV, knock sensor signal of the last several cycles.

[0102] 2. Determine the target AER: according to the engine speed and load, the target ammonia energy replacement rate is interpolated in the calibrated AER control map. For example, in the low load (such as BMEP<3bar) region, limit AER not higher than 40% to ensure stable combustion; in the medium load (3-5bar) region, linearly increase AER to about 80%; in the high load (>5bar) region, pursue the maximum ammonia replacement rate, and take AER>90%. If special conditions (such as high speed COV exceeding the standard) are monitored, the target AER can be corrected immediately.

[0103] 3. Determine the VOA setting: according to the current load and speed, look up the VOA control mapping to get the target value of intake and exhaust valve timing. Generally, low load corresponds to negative VOA (for example, -30°CA), medium load is close to 0°, and high load can take small positive VOA (such as +10°-+20°CA). If the combustion temperature is too high or the NOx is too high, the VOA can be appropriately increased to introduce internal EGR; on the contrary, if the COV is found to rise, indicating that the combustion is deteriorating, the VOA is reduced to reduce EGR. The adjustment rate of VOA can be set with a limit value according to the working condition change rate to avoid the impact of drastic changes on stability.

[0104] 4. Calculate fuel injection amount: Calculate total fuel energy required according to target AER and engine current torque demand, then distribute into ammonia and hydrogen injection amount respectively. Injection pulse width is calculated according to fuel heat value and nozzle flow characteristics. Preferably add 5-10% margin to ammonia injection amount to compensate for fuel remaining in intake port (especially when VOA is negative or zero). Hydrogen injection is precisely controlled to provide minimum excess energy for pilot ignition, avoiding excessive hydrogen leading to increased NOx.

[0105] 5. Set injection timing: According to the "dual injection strategy" of the invention, determine the start time and duration of ammonia injection, and the time of hydrogen injection. Generally set the start of ammonia injection at the early stage of intake stroke (e.g. around 30°CA after intake valve opening) to ensure sufficient mixing, and the injection duration spans the intake process; if calculated ammonia injection continues after intake valve closing, then accept a certain amount of post-valve injection fuel remaining to the next cycle (at this time the ECU will appropriately reduce the injection amount in the next cycle to avoid cumulative errors). Hydrogen direct injection time is set in the compression stroke, for example, in this embodiment, hydrogen injection is set to start at 226°CA before top dead center and stop at about 60°CA after top dead center. This ensures that hydrogen has a certain evaporation mixing time before spark ignition, and avoids early injection leading to early ignition.

[0106] 6. Ignition control: Adjust spark advance angle according to target CA50 position. When AER changes or VOA changes, the combustion rate may change, and the ECU advances or delays the ignition to keep CA50 close to the nominal value (e.g. 10°CA after top dead center). If knock tendency is detected, immediately delay the ignition angle and reduce the hydrogen injection amount to quell the knock; conversely, if there is a slow combustion (COV increases), then appropriately advance the ignition or temporarily increase the hydrogen proportion to stabilize the combustion.

[0107] 7. Cycle feedback correction: The combustion heat release parameters (ignition delay, combustion duration, peak pressure, etc.) calculated in real time through cylinder pressure analysis, as well as the NOx and NH3 concentrations obtained by the exhaust sensor, are used as closed-loop feedback signals. The ECU compares these actual values with the expected range, and if it finds that NOx is higher than the threshold, it further increases VOA or reduces the hydrogen proportion (increases the relative share of ammonia to lower the temperature); if it detects an increase in unburned ammonia concentration and the COV is within limits, then slightly reduce VOA or increase hydrogen ratio (increase combustion temperature to help ammonia burn fully). Through fine-tuning, the comprehensive optimization of NOx and unburned ammonia emissions is achieved. If the COV exceeds the predetermined upper limit (e.g. 3%), temporarily reduce AER or change injection timing to recover.

[0108] The engine was tested under various working conditions according to the above control scheme, and the results showed that the method could effectively improve the utilization rate of ammonia fuel and improve the emission. Under typical medium load (2000 rpm, BMEP 5 bar), when AER increased from 0 to 86%, the engine ran smoothly without misfire, the peak cylinder pressure decreased with the increase of AER, the combustion heat release peak decreased (Figure 3), and the combustion duration was significantly prolonged by about 2 times Figure 2c ) but still within an acceptable range; the brake thermal efficiency BTE first increased slightly and then decreased slightly, and finally was slightly lower than that of pure hydrogen combustion ( Figure 2b ). In this process, the maximum in-cylinder temperature decreased significantly ( Figure 4a ), from about 2700K when burning pure hydrogen to about 2000K when burning 86% AER, so that the NOx emission monotonously decreased with the increase of AER, with a total decrease of more than 60%. On the contrary, the unburned NH3 emission increased sharply ( Figure 5 a、 Figure 5 b) when AER increased from 0 to more than 60%, mainly because ammonia combustion was more difficult to completely oxidize due to the decrease in temperature. However, thanks to the control measures of the application, the increase rate of unburned ammonia emission slowed down and was controlled when AER>80%, and the NH3 emission at high AER was more affected by engine load than by the continuous increase of AER. Energy loss analysis ( Figure 6a 、 Figure 6b ) showed that the benefit brought by the increase of AER was that the heat transfer loss ratio decreased from 43% to 32%, while the exhaust loss slightly increased (from 25% to about 28%); the total of unburned and pumping losses was still less than 5%, so the total efficiency remained at a high level.

[0109] Under different speeds, the application also showed superior control effect. When the engine speed was increased while keeping BMEP 5 bar and AER fixed, it was found that when AER was not higher than 60%, the combustion COV at each speed was less than 2%, and the combustion center CA50 was basically not disturbed ( Figure 7a ); but when AER reached 85%, with the increase of speed from 1500 rpm to 2500 rpm, COV increased from about 2% to nearly 5%, indicating that high speed operation under high ammonia condition had a certain degree of instability. For this, the application re-controlled the COV at high speed and high AER to within 3% by ending the ammonia injection in advance, appropriately reducing AER, etc., avoiding misfire. Combustion duration and ignition delay generally increased with speed ( Figure 7b ), but the slow combustion caused by high speed was partially offset by the increase in combustion rate caused by hydrogen direct injection. Figure 8bIt is shown that without measures, the extension of ammonia injection pulse width at high speed leads to the EOI lagging behind the intake valve closing, which brings the risk of backfire. After applying the control strategy, ammonia injection ends early or pressurizes and speeds up, so that the EOI is always less than the intake valve closing time, and thus the backfire phenomenon does not occur. Figure 9 The trend of NH3 and NOx emissions under different AERs with the change of speed is shown: for low and medium AERs (≤60%), NH3 emissions do not increase or even decrease slightly with the increase of speed, which can be attributed to the increase of turbulent intensity at high speed, which promotes the completeness of combustion; while under high AER (85%), the unburned NH3 increases at high speed, but NOx further decreases, both of which change in opposite directions. The present application utilizes the synergistic regulation of VOA and AER to appropriately reduce the amount of ammonia at high speed and high AER to reduce NH3 emissions, while the EGR effect caused by the slight increase of VOA further inhibits NOx, so that the emissions are still controlled at a low level.

[0110] Gas distribution overlap angle effect verification: under low and medium load (BMEP 3bar) of the engine, the maximum stable AER and combustion characteristics that the engine can reach are tested by adjusting VOA, and the results are shown in Figure 10a and Figure 10b When VOA is gradually increased from-50°CA to +50°CA, the pumping loss shows a trend of first decreasing and then slightly rising ( Figure 10a ): when VOA is negative, the intake and exhaust are completely staggered without overlap, the intake vacuum is larger and the pumping loss is higher; when VOA increases to around 0, the late closing of intake reduces the compression negative work, and the pumping loss is the lowest; when it continues to increase to +50°, the excessive backflow of exhaust gas leads to the reduction of charge, and the pumping loss increases slightly. Comparison of cylinder pressure curves ( Figure 10b)also shows that the average cylinder pressure in the intake stroke gradually increases and then slightly falls with the increase of VOA from negative to positive, which fully illustrates that moderate valve overlap can use exhaust gas inertia to improve the intake charge, but excessive overlap will cause the mixture to dilute. More importantly, with the increase of VOA, the internal EGR rate increases significantly: when VOA=+50°, a large amount of exhaust gas recirculation makes the combustion rate decrease significantly, and the test measures that the combustion duration is about 30% longer than when VOA=0 (from 20°CA to 26°CA), and the maximum AER allowed to maintain stable combustion is reduced from about 78% to 25%. Thus it is verified that there is a coupling restriction relationship between VOA and ammonia replacement rate. According to this principle, the present application avoids using excessive positive VOA at low load in actual operation, so as to avoid that high EGR leads to limited ammonia replacement rate; and appropriately introduces positive VOA and reduces ammonia at high load to reduce NOx, so as to obtain lower NOx without excessively increasing unburned ammonia emissions. The experiment also found that when the VOA increases from negative to moderate positive, the engine brake thermal efficiency BTE first increases slightly and then decreases, which shows that negative overlap angle is beneficial to efficiency, but too much EGR will cause slow combustion, which will offset the benefits brought by the reduction of pumping work. Therefore, the present application optimizes fuel ratio and injection timing under different VOA conditions, so that the engine always works at the best balance point of efficiency and emission.

[0111] The present application is described above with reference to 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 using the method concept and technical scheme of the present application; or without improvement, the above-mentioned concept and technical scheme of the present application are directly applied to other occasions, which are within the protection scope of the present application.

Claims

1. A combustion control method for a dual-fuel engine based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation, characterized in that, The method comprises the following steps: 1) Fuel ratio control: Obtain engine current operating state parameters, determine target AER according to preset mapping, adjust ammonia supply amount and hydrogen supply amount to make actual AER reach target AER; 2) Valve timing control: Adjust intake valve opening time or exhaust valve closing time through variable valve timing mechanism to change valve overlap angle VOA, VOA is set to negative value, small overlap or positive overlap according to engine operating condition; 3) Injection timing optimization: Control ammonia injection time and hydrogen in-cylinder direct injection time respectively to realize coordinated optimization of fuel injection strategy and valve timing; wherein, ammonia injection time is adaptively controlled based on valve overlap angle VOA and engine speed: when engine operates at medium-low speed and VOA is in a small angle range, ammonia injection is controlled to be completed in the intake valve opening stage; when engine operates at high speed or VOA is negative, ammonia injection process is allowed to continue after intake valve closing; Hydrogen in-cylinder direct injection time is set in the late compression stroke; 4) Closed-loop steady-state control: Real-time monitor combustion state and emission indicators, compare them with threshold value as feedback signal, and timely correct the target AER and VOA setting.

2. The dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation according to claim 1, characterized in that, The target AER is adaptively adjusted according to engine load, and the target AER is limited to not more than a preset percentage at low load; the target AER is gradually increased when the load rises above a first threshold value, the target AER is set to 60% to 80% at medium load, and the target AER is increased to above 90% when the load rises above a second threshold value.

3. The dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation of claim 1, wherein, In the step 2), VOA is set to be approximately 0 or negative at low load or idle speed of the engine; VOA is set to be positive at medium-high load of the engine.

4. The dual-fuel engine combustion control method based on the hydrogen-ammonia energy ratio and gas distribution overlap angle regulation according to any one of claims 1 to 3, characterized in that, The setting of VOA is corrected according to engine speed, VOA is reduced at high speed area and increased at low speed area; the adjustment range of VOA includes several gears from negative overlap to positive overlap.

5. The dual-fuel engine combustion control method based on the hydrogen-ammonia energy ratio and gas distribution overlap angle regulation according to any one of claims 1 to 3, characterized in that, When VOA is positive and greater than a predetermined angle, the control system limits the maximum allowed AER to drop to a safety range; when VOA is negative or zero, the AER is allowed to rise to a higher level.

6. The dual-fuel engine combustion control method based on the hydrogen-ammonia energy ratio and gas distribution overlap angle regulation according to any one of claims 1 to 3, characterized in that, The end time of hydrogen direct injection is set in the range of 0° to 60° crank angle after top dead center of compression.

7. The dual-fuel engine combustion control method based on the hydrogen-ammonia energy ratio and gas distribution overlap angle regulation according to any one of claims 1 to 3, characterized in that, When the engine operates at high speed or VOA is negative, the control system allows part of ammonia to remain in the intake port; the remaining ammonia in the intake port is heated by the residual heat of the intake port wall surface in the period from the intake valve closing to the start of the next intake stroke.

8. The dual-fuel engine combustion control method based on the hydrogen-ammonia energy ratio and gas distribution overlap angle regulation according to any one of claims 1 to 3, characterized in that, In the closed-loop steady-state control, the electronic control unit monitors CA50 and maintains CA50 at a target position by controlling ignition advance angle or fuel ratio, and automatically corrects ignition time to maintain the combustion phase in the range of 8° to 15° CA after top dead center when the target AER adjustment or VOA change causes the combustion rate to change.

9. The dual-fuel engine combustion control method based on the hydrogen-ammonia energy ratio and gas distribution overlap angle regulation according to any one of claims 1 to 3, characterized in that, In the closed-loop steady-state control, the concentration of NOx and unburned ammonia in the exhaust gas is monitored in real time, and the following emission coupling control strategy is executed: When the concentration of NOx exceeds the threshold value, the target AER is increased or the ignition is delayed, and the VOA is increased to introduce internal EGR when needed.

10. The dual-fuel engine combustion control method based on hydrogen-ammonia energy ratio and gas distribution overlap angle regulation of claim 9, wherein, In the closed-loop steady-state control, the concentrations of NOx and unburned ammonia in exhaust gas are monitored in real time, and the following emission coupling control strategy is executed: When the concentration of unburned ammonia exceeds a threshold value, the target AER is reduced or the ignition is advanced, while the VOA is reduced as needed.

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