An optimization method for integrated optimization matching of EGR system and supercharging system

Abstract

The present invention belongs to the field of engine technology, and discloses an optimization method for integrated optimization matching of an EGR system and a supercharging system, wherein the optimization method for integrated optimization matching of an EGR system and a supercharging system includes: establishment and verification of a simulation model of a supercharged engine working process; simulation analysis of system characteristics of a Miller cycle supercharged engine; construction of an optimization algorithm for integrated optimization matching of an EGR system and a supercharging system; and parameter optimization of an EGR system and a supercharging system of a Miller cycle engine using a genetic algorithm. The present invention uses a domestic engine as a prototype engine for the research paper, establishes a mathematical model of the working process of each subsystem of a turbocharged gasoline engine, and compares a gasoline engine simulation calculation model established based on GT‑POWER with test data to verify the accuracy of the model, which is helpful to promote the design of a high-efficiency domestic hybrid gasoline engine and improve the overall engine performance, and has important academic value and engineering application value.

Description

Technical Field

[0001] The present invention belongs to the technical field of engines, and in particular relates to an optimization method for integrated optimization matching of an EGR system and a supercharging system. Background Art

[0002] Faced with the dual pressures of increasingly severe carbon emissions and rapidly growing energy consumption, China's energy structure faces severe challenges. Therefore, it is necessary to actively promote and apply advanced engine energy-saving technologies under the strict national energy conservation and emission reduction and fuel consumption restrictions. Due to the characteristics of energy saving and low emissions, hybrid vehicles have become a focus of automobile research and development. Compared with the traditional Otto cycle, the Miller cycle has the advantages of small pumping loss and high effective thermal efficiency, so it has become an important thermodynamic cycle for hybrid electric vehicles. As advanced engine technologies, exhaust gas recirculation (EGR) and turbocharging have a positive effect on engine energy conservation and emission reduction. Therefore, engine Miller cycle technology, exhaust gas recirculation technology (EGR) and turbocharging technology are becoming more and more eye-catching due to their energy-saving potential and development space.

[0003] The Miller cycle delays the closing time of the intake valve, thereby increasing the engine intake pressure and reducing pumping losses, pushing some of the gas back into the intake manifold. By closing the intake valve late or early, the Miller cycle achieves an intake reflux effect, thereby achieving a process in which the engine expansion ratio is greater than the compression ratio. Since the compression ratio of the Miller cycle intake process is less than the expansion ratio, the engine pumping losses are reduced, thereby significantly improving the engine thermal efficiency. Therefore, the use of Miller cycle technology has become one of the main measures to improve the effective thermal efficiency of engines at this stage.

[0004] There are two ways to implement the Miller cycle: one is to achieve a variable compression ratio of the engine through mechanical structures such as connecting rods and gears; the other is to make the effective compression ratio when the engine intake valve is closed less than the geometric compression ratio through early intake valve closing (Early Intake-Valve-Closing) or late intake valve closing (Late Intake-Valve-Closing).

[0005] For gasoline engines, the use of variable valve timing technology can realize the application of Miller cycle in gasoline engines. At the same time, the thermal efficiency of the engine is improved by taking advantage of the characteristic that the compression ratio is less than the expansion ratio.

[0006] Domestic and foreign scholars have conducted a large number of simulation studies on the Miller cycle. Ballarat University in Australia used a 2.0L GDI engine as the original engine and used GT-POWER software to improve the engine expansion ratio on the model to achieve the Miller cycle. The results showed that the engine using the Miller cycle had improved power and economy. Sheffield University in the UK applied the Miller cycle to a four-cylinder gasoline engine to study the full-load power and partial-load economy of the engine. The results showed that the fuel economy of a gasoline engine at partial load can be close to that of a diesel engine. Minho University in Portugal compared the economy of the Otto cycle, Miller cycle, diesel cycle and mixed cycle engines under partial load conditions, and found that the Miller cycle had the best fuel economy under low load.

[0007] Wolfram Gottachalk and others from the IAV laboratory of Volkswagen in Germany conducted research on variable Miller cycle technology. They equipped a high geometric compression ratio turbocharged DISI engine with variable valve technology to study the variable Miller cycle valve timing in the turbocharged area. The research shows that Miller cycle technology combined with VVT and ignition angle control can effectively suppress the knock phenomenon caused by high compression ratio.

[0008] In China, Bai Wentao and others used GT-POWER software to build a diesel engine simulation model and studied the impact of Miller degree on engine characteristics. The results showed that as Miller degree increases, the maximum explosion pressure, power, and charging efficiency in the cylinder decrease accordingly.

[0009] Zu Bingfeng studied the effects of two different Miller cycle forms on engine fuel consumption and knock. The results show that both Miller cycle forms can improve the fuel economy of the engine. Compared with LIVC, EIVC has relatively small pumping loss; in the high load area, EIVC will cause combustion to slow down. Therefore, LIVC is generally used to suppress the knock phenomenon caused by high compression ratio.

[0010] Professor Wang Changyuan and others from Beijing Institute of Technology studied the performance of the Miller cycle on a gasoline engine. The results showed that the thermal efficiency of the engine can be greatly improved by optimizing the engine valve timing.

[0011] Jiang Wei from Jilin University used GT-Power to explore the improvement effect of Miller cycle (LIVC) on the fuel economy of GDI engines. He compared the fuel economy of Miller cycle and Otto cycle under different loads. The fuel economy was improved by 6.15% at 25% load, 4.11% at 50% load, and 1.19% at 75% load.

[0012] Liu Ran et al. studied the effect of Miller cycle on engine fuel economy on a 3.0L supercharged engine. The results showed that when the effective pressure in the cylinder is less than 0.6Mpa, the late closing of the intake valve has a greater improvement on the Miller cycle pumping loss, while when the effective pressure in the cylinder is greater than 0.6Mpa, in order to ensure the power of the engine, the engine's boost pressure needs to be increased. When the effective pressure in the cylinder is greater than 0.8MPa, the Miller cycle improves the engine's fuel economy.

[0013] Guan Li and others studied the effect of Miller cycle on supercharged Miller cycle engine fuel consumption. The results show that after applying Miller cycle, under low load conditions, the engine pumping loss can be effectively reduced and the engine effective thermal efficiency can be improved; however, under low speed and high load conditions, the engine compression ratio is greater than the original engine compression ratio, which makes the engine fuel consumption deteriorate to a certain extent. Therefore, in the later development process, the engine needs to balance the relationship between different operating points and compression ratio to reduce engine fuel consumption.

[0014] In summary, experts, scholars and engineering technicians at home and abroad have never stopped their research on the Miller cycle. As hybrid vehicles are gaining more and more attention, research on the Miller cycle has gradually become a hot topic. Compared with the traditional Otto cycle, the Miller cycle reduces engine pumping losses, improves effective thermal efficiency, and thus achieves the effect of improving the engine's fuel economy. Although the Miller cycle engine has a higher thermal efficiency, the intake air reflux causes the engine power to be low, especially under low-load conditions. Hybrid vehicles use electric motors to drive under low-load conditions, which avoids this weakness and allows the engine to always operate in a high-efficiency economic zone, giving full play to the energy-saving advantages of the Miller cycle. In addition, Miller cycle technology can also improve fuel economy under partial load conditions and reduce NO. x Emissions, and suppress detonation caused by high compression ratio.

[0015] Since the Miller cycle reduces the intake volume, the engine torque is poor at low speed, so it needs to be compensated by supercharging. At present, most hybrid vehicles in my country still use the traditional Otto cycle technology. It can be found that there is still a big gap between our technology and that of foreign countries. Therefore, it is of great practical significance to conduct research on Miller cycle engines in my country.

[0016] The principle of EGR technology is to re-enter the combustion chamber through the control valve to participate in the next new cycle. EGR was first used in diesel engines to reduce combustion temperature and inhibit NO x Emissions, while reducing combustion temperature and inhibiting gasoline engine NO x On the other hand, since it increases the intake volume of the mixed gas, in order to ensure the same power output, the engine throttle opening needs to be increased.

[0017] The working principle of the exhaust gas turbocharger system is to use the exhaust gas discharged by the engine to drive the high-speed rotation of the turbine, and then drive the compressor coaxial with the turbine to rotate. The compressor compresses fresh air, thereby increasing the intake volume of the cylinder and improving the output power of the engine. At the same time, the exhaust gas turbocharger technology can also improve the thermal efficiency and mechanical efficiency of the engine.

[0018] Research status of exhaust gas recirculation technology and supercharging technology

[0019] (1) Research status of exhaust gas recirculation technology

[0020] EGR systems are mainly divided into two categories: internal EGR and external EGR. The internal EGR system mainly adjusts the valve overlap angle through the engine's valve timing mechanism, and uses the exhaust gas remaining in the cylinder to achieve exhaust gas recirculation. Since internal EGR does not require the installation of a special EGR device, it can be achieved by opening and closing the intake and exhaust valves, so its structure is simple, but it is difficult to accurately control the EGR rate. For external EGR, its structure is more complicated and requires special pipelines, but the EGR rate can be accurately controlled through the electronic control system, which is effective, so it is more commonly used.

[0021] If the amount of exhaust gas recirculation is to be determined, the EGR rate, which is expressed as the mass ratio of the recirculated exhaust gas to the intake air, is usually chosen to measure the exhaust gas recirculation amount, that is:

[0022]

[0023] In the formula, G r Indicates the mass of recirculated exhaust gas, G a Indicates the quality of fresh air.

[0024] Many foreign scholars have done a lot of research on EGR technology. Among them, Helmut et al. studied that in order to reduce NO x Emissions, intake port injection gasoline engines can achieve this goal by changing the EGR rate.

[0025] Research by Heywood et al. shows that EGR technology can effectively improve the fuel economy of gasoline engines for two main reasons: first, after EGR is added to the engine, the combustion temperature in the cylinder is greatly reduced due to the influence of exhaust gas, thereby reducing the heat transfer loss in the engine cylinder; second, under partial load, EGR reduces the intake and exhaust pressure difference, thereby reducing pumping losses.

[0026] Haozhong Huang's research shows that under full-load engine conditions, in order to ensure that the engine power does not decrease, the engine intake pressure must be increased through supercharging after adding EGR.

[0027] Galloni et al. studied the application of EGR technology in small-displacement gasoline engines. The results showed that EGR technology reduces the engine's knock index and intake temperature, and can effectively improve the engine's effective thermal efficiency while ensuring that the engine's output torque remains unchanged.

[0028] Potteau et al. conducted an experimental study on the EGR installed on a 2.0L turbocharged engine and found that under full load conditions, the use of EGR can improve fuel economy; in terms of emissions, NO x The content is reduced to a certain extent; in the study of the impact of EGR on turbocharged engines, when using high-pressure EGR, increasing the EGR rate only increases the pressure ratio; when using low-pressure EGR, increasing the EGR rate can increase the pressure ratio while increasing the intake flow.

[0029] Lurun Zhong et al. adopted a method of using high-pressure EGR in high-speed conditions and low-pressure EGR in low-speed conditions. The results showed that this method can effectively improve the fuel economy of the engine.

[0030] In China, Peng Yourong and others used EGR systems on gasoline engines and conducted some research on the impact of EGR technology on gasoline engines. The results showed that under high-speed engine conditions, the use of EGR in gasoline engines has a significant impact on NO x The generation of emissions has a relatively obvious inhibitory effect. At the same time, EGR can reduce both the pumping loss during engine operation and the exhaust heat loss of the engine, thereby achieving a significant fuel saving effect. From the exhaust point of view, EGR also has a certain effect on reducing the combustion temperature in the engine cylinder and suppressing knock.

[0031] Zeng Qi et al. conducted an experimental study on the effects of high-pressure EGR and low-pressure EGR on the engine on a gasoline engine. The effects of high-pressure EGR and low-pressure EGR technology on engine power, fuel economy and intake and exhaust under full-load and partial-load conditions were compared, and the reasons for these changes were analyzed. The results show that adding an EGR system to a gasoline engine can optimize combustion in the cylinder and reduce pumping losses, thereby improving fuel economy. The fuel consumption rate of the low-pressure EGR system is lower than that of the high-pressure EGR at medium and low speeds. The main reason is that in the engine of the low-pressure EGR system, the exhaust gas temperature entering the engine is lower than that of the high-pressure EGR, so the combustion in the cylinder can be optimized.

[0032] Corning et al. conducted an experimental study on EGR technology for small-displacement turbocharged gasoline engines in response to traditional port injection gasoline engines. The results showed that after adding the EGR system to a small-displacement turbocharged gasoline engine, the emission of CO was reduced by about 15.3%, the emission of HC was increased by about 5.8%, and the emission of NO x The emission of NO was reduced by 30.7%. x The emissions have been significantly reduced.

[0033] Zhu Tianyu et al. studied the effect of EGR system on gasoline engine performance. The study showed that under full load conditions, increasing the EGR rate significantly reduced the output power of the gasoline engine, but NO x Emissions will be significantly reduced.

[0034] Yu Guangyao et al. used low-pressure EGR technology on a 1.5L port injection (PFI) gasoline engine. The simulation results show that low-pressure EGR can significantly improve the fuel economy, anti-knock and CO emissions of the engine. x Emissions decrease significantly under medium and low load conditions, but deteriorate under high engine load conditions.

[0035] For direct injection gasoline engines, Wu Da and others from Shanghai Jiaotong University studied the mechanism of the influence of EGR technology on the fuel consumption rate of turbocharged direct injection gasoline engines. The results show that when the engine is under full load, adding an EGR system reduces the combustion temperature in the cylinder, reduces the heat transfer loss of the engine, and improves the fuel consumption rate of the engine.

[0036] Yin Xiaojun et al. conducted a simulation analysis on the effects of different EGR systems on engine performance. The results show that by reducing the turbine size, a single-path EGR system can produce a higher EGR rate, but the fuel consumption rate is higher.

[0037] From the above domestic and foreign research, it can be seen that after adding EGR, the temperature of the gas in the cylinder is reduced, the heat transfer loss of the engine is reduced, and the engine fuel consumption rate is reduced. After adding EGR, the power of the engine is affected to a certain extent. In order to solve the contradiction between EGR and engine power, the engine intake volume can be increased by increasing the throttle opening under partial load, and the engine can use turbocharging technology to increase the intake volume under full load. In terms of emissions, after adding EGR, NO x The emission effect is significantly reduced, CO emissions are reduced, but the impact is small, HC emissions will increase, but the increase is not significant.

[0038] (2) Research status of supercharging technology

[0039] Many scholars abroad have conducted extensive research on turbocharging systems. Dominique et al.'s research shows that turbocharged gasoline engines have 30% more power than non-turbocharged gasoline engines, and their fuel consumption is reduced by 8-10%, while also improving the engine's acceleration performance.

[0040] SMShahed et al. built a GT-POWER simulation model to study the impact of turbocharging technology on engine fuel-saving performance. The results show that while ensuring the same power, turbocharging technology can increase engine power by 37.2% and reduce fuel consumption by 21.6%.

[0041] Yang Shiyou et al. studied the VGT turbocharger system. The results showed that the VGT turbocharger system can significantly increase the engine output torque and reduce the engine's fuel consumption.

[0042] Bertrand et al. studied the supercharging technology of direct injection engines. The results of the comparative study of Renault 1.8IDE engine and 3.0L naturally aspirated engine showed that direct injection engines can improve the anti-knock performance of engines under high load conditions, and the fuel economy of direct injection engines is significantly improved while ensuring the same power.

[0043] Domestically, Wang Fei and others used AVL-BOOST software to study the impact of turbochargers on engine performance. The results showed that the engine's power and fuel economy were improved after being equipped with a turbocharger.

[0044] Professor Wang Shuqing and others from Hunan University matched a 1.0L naturally aspirated engine turbocharger and optimized the engine's intake and exhaust manifolds. The study showed that after the turbocharger was installed, the engine's output torque increased by 41.7% compared to the original model, but the fuel consumption rate was higher than the original model. The two engines were installed on the same vehicle and the vehicle fuel consumption test under cyclic conditions was carried out. The results showed that while keeping the vehicle's power unchanged, the engine equipped with a turbocharger can reduce the vehicle's fuel consumption by 5.18% compared to the original model.

[0045] Based on the current research status at home and abroad, after being equipped with a turbocharger, the engine can achieve the goals of improving power, reducing fuel consumption, and improving engine thermal efficiency. At the same time, the acceleration performance of the engine is improved. In order to meet the national emission reduction indicators, the exhaust gas turbocharger system and EGR have better application prospects.

[0046] Research status of integrated matching of EGR and supercharging system: In terms of integrated matching, domestic and foreign scholars have also done a lot of research. Guven et al. combined the Miller cycle with turbocharging technology on diesel engines. The results show that when the application of the Miller cycle reduces the engine's braking power and braking thermal efficiency to 6.5% and 10%, turbocharging increases the engine's braking power and braking thermal efficiency to 18% and 12%. The experimental and theoretical results were compared in terms of torque, specific fuel consumption, braking power and braking thermal efficiency. The results obtained from the theoretical model have been verified with empirical data with a maximum error of less than 7%. Therefore, the developed combination scheme can improve the performance of the engine, and the method is easy to apply on diesel engines.

[0047] Fontana et al. achieved integrated matching of EGR and Miller cycle by controlling VVT. The results showed that compared with the original engine, the matched engine had lower fuel consumption rate. Gentile et al. also conducted related research. By combining EGR, Miller cycle and hydraulic technology, they reduced the gas loss of the engine pump and reduced fuel consumption in the engine cycle test.

[0048] Bozza et al. built a simulation model for a Miller cycle, high-pressure EGR gasoline engine. By optimizing the intake valve closing angle, throttle opening and EGR rate, the effective thermal efficiency of the engine under low load was improved. Compared with the original engine before optimization, the effective thermal efficiency of the optimized engine was increased by 7%.

[0049] In China, Fan Zhenyang from Harbin Institute of Technology optimized the matching of VVT and EGR for a 2.0L naturally aspirated gasoline engine with port injection, and conducted simulation analysis through GT-POWER. The results show that under medium-high speed and full-load conditions, when the EGR rate is 5%, the integrated matching of EGR and VVT can increase the effective thermal efficiency of the engine by about 7% while ensuring that the engine power is not reduced.

[0050] Geng Qi used GT-POWER software to model and simulate gasoline engines and superchargers, and studied the application of EGR technology in turbocharged gasoline engines, analyzing the impact of different EGR rates on the economic characteristics, power performance and emission performance of turbocharged gasoline engines under different working conditions. The results showed that the use of EGR technology in turbocharged gasoline engines can effectively suppress NO x However, each operating condition has an optimal EGR rate. It is necessary to control HC emissions from increasing significantly and reduce NO emissions as much as possible while ensuring that the engine torque and fuel consumption and other power and economic performance do not change significantly. x emissions in order to achieve the purpose of emission standards.

[0051] For direct injection engines, Wang Lei et al. based on a 4-cylinder 2.0T Miller cycle GDI engine, used low-pressure EGR with cooling. Then they analyzed the impact of VVT technology and EGR technology on engine fuel economy and emissions under different loads. The conclusion shows that without a significant upgrade of the combustion system, EGR technology mainly improves fuel economy at medium and high loads, and Miller technology mainly improves fuel economy at high and low loads. In view of the respective characteristics of Miller cycle and EGR technology, combined with the throttle and turbocharger bleed valve, it is recommended to use different control strategies for different engine operating conditions to achieve the lowest specific fuel consumption and the best power performance.

[0052] Based on the GT-Power platform, Wang Xiaoyan established simulation models for a turbocharged GDI gasoline engine equipped with a low-pressure EGR and high-pressure EGR system. By changing the engine ignition advance angle and EGR rate, she simulated the power, effective thermal efficiency and emissions of the modified engine. The results showed that both high-pressure and low-pressure EGR systems can reduce the maximum combustion pressure in the cylinder, delay the peak combustion phase, save fuel consumption, and inhibit NO x At low speeds, the low-pressure EGR system has more stable combustion in the cylinder than the high-pressure EGR system, and the effective fuel consumption of the two systems is reduced by about 9.4% and 17% respectively. The low-pressure EGR system can more effectively reduce NO x Emissions dropped by up to 75%.

[0053] Regarding the control strategy, Shi Naichen proposed an EGR valve opening control model based on intake flow correction and a VNT blade opening control model based on intake pressure correction based on the common control strategy, and combined the two to form a coordinated control strategy for the supercharger and EGR system. A simulation model was established using GT-Power to verify the feasibility of the strategy. The results show that the coordinated control strategy for the supercharger and EGR system has achieved a relatively ideal control effect.

[0054] Shi Lei and his colleagues studied the Miller cycle and EGR technology based on a direct injection turbocharged gasoline engine. The results showed that for traditional passenger cars that do not have particularly high requirements for external characteristics, the Miller cycle angle can be appropriately reduced, and combined with low-pressure EGR technology, the fuel consumption of the vehicle can be further reduced; combined with the engine low-pressure EGR technology, at the economic operating point of the test, CO and NO x Emissions were significantly reduced, and HC changes showed certain fluctuations.

[0055] It can be seen that through the combination of Miller cycle and supercharging system, at high speed and medium and high load, Miller cycle can be used to reduce the effective compression ratio to suppress the knocking effect caused by supercharging, and also reduce the fuel consumption rate of the engine. However, at low speed and medium and high load, the fuel consumption rate will increase. Miller cycle combined with low-pressure EGR technology still has shortcomings in emissions, which need to be solved by optimizing the injection timing or controlling the EGR rate. Therefore, the integrated matching research of EGR and supercharging system for Miller cycle engine needs to be solved urgently.

[0056] In terms of integrated matching, domestic and foreign scholars have conducted research involving diesel engines, gasoline engines, and hybrid engines, but the research is still mainly focused on the integrated matching of the turbocharged EGR system of the traditional Otto cycle internal combustion engine, while there are fewer studies on hybrid engines, especially Miller cycle engines. The EGR system, turbocharging system, and Miller cycle technology can all achieve the goal of reducing engine emissions and improving fuel economy. Therefore, the three are combined and applied to hybrid gasoline engines. By evaluating the matching of the turbocharging and EGR systems of the Miller cycle gasoline engine, the problems in the matching are found, the corresponding solutions are proposed, and simulation analysis is performed to achieve the best power, fuel economy, and emissions.

[0057] Therefore, Miller cycle, EGR and supercharging technology each have their own advantages and disadvantages in terms of energy conservation and emission reduction. If these three technologies are used together on the engine, it can be used as a better solution for energy conservation and emission reduction in the near future.

[0058] Through the above analysis, the problems and defects of the prior art are as follows:

[0059] (1) Although the Miller cycle engine has a high thermal efficiency, the intake air recirculation causes the engine power to be low, especially under low load conditions.

[0060] (2) Since the internal EGR system does not require the installation of a special EGR device and can be achieved simply by opening and closing the intake and exhaust valves, its structure is simple, but it is difficult to accurately control the EGR rate.

[0061] (3) In terms of emissions, after adding EGR, NO x The emission effect is significantly reduced, CO emissions are reduced, but the impact is small, HC emissions will increase, but the increase is not significant.

[0062] (4) Through the Miller cycle combined with low-pressure EGR technology, the fuel consumption rate will increase at low speed and medium and high load, but there are still shortcomings in terms of emissions. Summary of the invention

[0063] In view of the problems existing in the prior art, the present invention provides an optimization method for integrated optimization matching of an EGR system and a supercharging system.

[0064] The present invention is implemented as follows: an optimization method for integrated optimization matching of an EGR system and a supercharging system, wherein the optimization method for integrated optimization matching of an EGR system and a supercharging system comprises the following steps:

[0065] Step 1: Establishment and verification of the supercharged engine working process simulation model;

[0066] Step 2: Simulation analysis of system characteristics of Miller cycle supercharged engine;

[0067] Step 3: constructing an optimization algorithm for integrated optimization matching of the EGR system and the supercharging system;

[0068] Step 4: Parameter optimization of Miller cycle engine EGR system and supercharging system using genetic algorithm.

[0069] Furthermore, in step one, the supercharged engine working process simulation model includes intake and exhaust system models, in-cylinder thermal process models, intercooler models and supercharging system models.

[0070] Further, in step 2, the system characteristic simulation analysis of the Miller cycle supercharged engine includes:

[0071] (1) Determination of EGR system characteristics of Miller cycle turbocharged engine

[0072] (2) Optimization of bypass valve opening of Miller cycle engine supercharging system

[0073] Furthermore, in step 3, the optimization algorithm for the integrated optimization matching of the EGR system and the supercharging system is constructed, including: multi-objective optimization is to use a suitable optimization method to optimize the relevant model parameters in the model based on the established simulation model so that the simulation model can achieve the optimization goal. The mathematical model of the multi-objective optimization problem generally includes three elements: optimization variables, objective functions and constraints.

[0074] Further, in step 4, the genetic algorithm parameter optimization of the Miller cycle engine EGR system and the supercharging system includes:

[0075] (1) Establishment of a joint simulation model based on the coupling of GT-Power and Simulink

[0076] (2) Genetic algorithm and its parameter design

[0077] (3) According to the established optimization simulation model, the optimization variable parameters, objective function parameters, constraint condition parameters and genetic algorithm parameters in the model are set respectively, including:

[0078] In combination with all the above technical solutions, the advantages and positive effects of the present invention are as follows: the optimization method for integrated optimization matching of the EGR system and the supercharging system provided by the present invention, under the support of the national key R&D plan project "Hybrid Engine Whole Machine Design and Integrated Development" (Project No.: 2017YFB0103404), the integrated optimization matching of the EGR system and the supercharging system of the Miller cycle engine is studied, and the effective thermal efficiency and torque of the engine are optimized while ensuring that the emissions meet the standards. The research work is helpful to promote the design of high-efficiency domestic hybrid gasoline engines and improve the overall performance of the engine. The research on the powertrain system of new energy electric vehicles has important academic value and engineering application value.

[0079] This paper takes a domestic engine as the prototype engine for the paper research, establishes a mathematical model of the working process of each subsystem of the turbocharged gasoline engine, and compares the gasoline engine simulation calculation model established based on GT-POWER with the test data to verify the accuracy of the model.

[0080] The present invention carries out a simulation study on the system characteristics of a Miller cycle engine, analyzes the influence of the Miller cycle in the form of late intake valve closing on the engine ventilation process under the driving conditions of a hybrid vehicle, and studies the influence of different Miller cycle intake valve late closing angles on the engine power and fuel economy; analyzes the influence of the EGR system on the emission, power and fuel economy of the Miller cycle engine, and studies the problem of backflow in the high-pressure EGR line; by studying the influence of the bypass valve opening on the engine performance, the bypass valve opening is re-optimized and calibrated, the optimal opening of the engine bypass valve under all working conditions is obtained, and the Miller cycle engine and the turbocharger are re-matched.

[0081] The genetic algorithm is determined to be the optimization algorithm of the present invention, and the multi-objective optimization problem of the Miller cycle engine is mathematically described. The GT-POWER and MATLAB / Simulink coupling program is developed, and the parameters of the genetic algorithm are designed. Finally, the optimization result of the genetic algorithm is obtained, and the bypass valve opening MAP diagram, EGR rate MAP diagram, boost pressure MAP diagram and intake flow rate MAP diagram are obtained according to the optimization results, completing the integrated optimization matching of the EGR system and the boost system of the Miller cycle engine.

[0082] The subject of this invention is derived from the national key R&D project "Hybrid Engine Development" (Project No.: 2017YFB0103404). The subject will focus on the design and integrated development of hybrid engine, with the goal of optimizing the effective thermal efficiency of hybrid engine. Taking a domestic gasoline engine as the prototype engine, the supercharging system, EGR system, and Miller cycle system are optimized and matched to optimize the economy and power of the engine, and achieve the goal of effective thermal efficiency higher than 40%.

[0083] With the support of the national key R&D project "Hybrid Engine Overall Design and Integrated Development" (Project No.: 2017YFB0103404), the present invention has carried out integrated optimization matching research on the EGR system and supercharging system of the Miller cycle engine, and optimized the effective thermal efficiency and torque of the engine under the premise of ensuring that the emission standards are met. The present invention established a simulation model of the Miller cycle supercharged engine based on the GT-Power software. For hybrid electric vehicles, the effects of the Miller cycle timing and EGR rate on the engine power, fuel economy and emissions were studied, and the supercharger operation MAP was re-optimized and calibrated. At the same time, the present invention also established a coupling program between GT-POWER and MATLAB / Simulink, and used a genetic optimization algorithm and a coupling program to optimize the performance matching between the EGR system and the supercharging system of the Miller cycle engine. The main work of the present invention and the conclusions obtained are as follows:

[0084] (1) The present invention mathematically describes the simulation model of the engine working process, including the calculation models of the working process in the cylinder, the intake and exhaust system, the turbocharging system, and the intercooler. Taking a domestic engine as the prototype engine, simulation modules such as the engine cylinder, the intake and exhaust system, and the turbocharging system are designed and built, and the overall simulation model of the Miller cycle supercharged engine is built. The present invention also verifies the performance of the engine simulation model. The analysis and verification results show that the errors of the fuel consumption rate, the maximum combustion pressure in the cylinder, the torque and the power of the engine simulation model and the experimental data are all less than 5%, indicating that the constructed Miller cycle supercharged engine GT-POWER simulation model has good accuracy.

[0085] (2) The system characteristics of the Miller cycle supercharged engine were simulated and analyzed for the operating conditions of 50%-100% engine load and 2000-4500r / min speed. The results show that:

[0086] ①After adopting Miller cycle technology, pumping loss has been greatly reduced. At the same time, during the combustion process in the cylinder, the temperature of the gas has dropped to a certain extent, which ultimately leads to a decrease in fuel consumption rate. As the Miller degree increases, the effective compression ratio continues to decrease, the effective thermal efficiency decreases, and the fuel consumption rate increases. Due to the decrease in combustion temperature in the cylinder, NO in engine emissions x The decrease of and the increase of CO led to the subsequent research object being determined as a gasoline engine with a Miller cycle late closing angle of 30°.

[0087] ②EGR system can effectively reduce engine NO x and CO emissions, NO x The maximum reduction is 45%, and the maximum reduction of CO is 32.8%. However, it has an inhibitory effect on engine power. The effect of the EGR system on the fuel economy of the Miller cycle supercharged engine was studied. When the EGR rate is small, the temperature of the combustion process in the cylinder can be reduced, thereby reducing the heat transfer loss of the high-temperature gas in the cylinder, and the exhaust loss will also be reduced, thereby improving fuel economy. When the EGR rate is high, the combustion process deteriorates and the fuel economy deteriorates. At the same time, when the load is large and the speed is high, EGR mainly inhibits the combustion process, thereby increasing the fuel consumption rate.

[0088] ③ The present invention analyzes the backflow problem of the EGR pipeline of the Miller cycle supercharged engine. The simulation analysis shows that the speed is one of the main factors affecting the EGR backflow phenomenon. The EGR will have a backflow phenomenon at low speed, but not at high speed. Load is also a factor that affects the EGR pipeline backflow phenomenon. At high load, the EGR pipeline backflow phenomenon is more serious than at low load. The maximum value of the outlet pressure difference occurs at the maximum engine speed of 6000r / min, and as the throttle opening increases, the maximum pressure difference also gradually increases.

[0089] (3) The present invention recalibrates the bypass valve opening of the engine supercharger. The bypass valve opening under the engine external characteristic working condition is optimized and calibrated to obtain the bypass valve under the external characteristic of the Miller cycle engine. The results show that the bypass valve opening is compared with that before optimization, and the joint curve passes through the high-efficiency area and is far away from the surge line. The expected optimization result is achieved.

[0090] (4) A coupling program between GT-Power and MATLAB / Simulink was established, and a genetic optimization algorithm was used to optimize the performance matching between the EGR system and the supercharging system of the Miller cycle engine.

[0091] ① Using genetic algorithm as the optimization algorithm of the present invention, a mathematical description of the multi-objective optimization model was carried out, including optimization variables, objective functions, constraints and Pareto optimal solution sets. A coupling program of GT-Power and MATLAB / Simulink was developed. The parameters of the genetic algorithm were designed.

[0092] ②Analyze the optimization results of the genetic algorithm. The engine performance was optimized under full load conditions, and the highest torque point and maximum effective thermal efficiency point conditions were analyzed. By optimizing the engine under full working conditions, the effective thermal efficiency was 41.4837%, the torque was 211.28N·m, and the corresponding working point was 3647r / min, the load was 75.6%, the EGR rate was 7.4%, and the bypass valve opening was 17%. According to the optimization results, the bypass valve opening MAP map, EGR rate MAP map, boost pressure MAP map, and intake flow MAP map were obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0093] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0094] Figure 1 It is a flow chart of an optimization method for integrated optimization matching of an EGR system and a supercharging system provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0095] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0096] In view of the problems existing in the prior art, the present invention provides an optimization method for integrated optimization matching of an EGR system and a supercharging system. The present invention is described in detail below with reference to the accompanying drawings.

[0097] like Figure 1 As shown, the optimization method for integrated optimization matching of the EGR system and the supercharging system provided by the embodiment of the present invention includes the following steps:

[0098] S101, establishment and verification of supercharged engine working process simulation model;

[0099] S102, simulation analysis of system characteristics of Miller cycle supercharged engine;

[0100] S103, constructing an optimization algorithm for integrated optimization matching of the EGR system and the supercharging system;

[0101] S104, parameter optimization of Miller cycle engine EGR system and supercharging system based on genetic algorithm.

[0102] The technical solution of the present invention is further described below in conjunction with embodiments.

[0103] 1. This paper uses a domestic engine as the prototype engine for the research, establishes a mathematical model of the working process of each subsystem of the turbocharged gasoline engine, and compares the gasoline engine simulation calculation model established based on GT-POWER with the test data to verify the accuracy of the model.

[0104] The present invention carries out a simulation study on the system characteristics of a Miller cycle engine, analyzes the influence of the Miller cycle in the form of late intake valve closing on the engine ventilation process under the driving conditions of a hybrid vehicle, and studies the influence of different Miller cycle intake valve late closing angles on the engine power and fuel economy; analyzes the influence of the EGR system on the emission, power and fuel economy of the Miller cycle engine, and studies the problem of backflow in the high-pressure EGR line; by studying the influence of the bypass valve opening on the engine performance, the bypass valve opening is re-optimized and calibrated, the optimal opening of the engine bypass valve under all working conditions is obtained, and the Miller cycle engine and the turbocharger are re-matched.

[0105] The genetic algorithm is determined to be the optimization algorithm of the present invention, and the multi-objective optimization problem of the Miller cycle engine is mathematically described. The GT-POWER and MATLAB / Simulink coupling program is developed, and the parameters of the genetic algorithm are designed. Finally, the optimization result of the genetic algorithm is obtained, and the bypass valve opening MAP diagram, EGR rate MAP diagram, boost pressure MAP diagram and intake flow rate MAP diagram are obtained according to the optimization results, completing the integrated optimization matching of the EGR system and the boost system of the Miller cycle engine.

[0106] This invention originates from the national key R&D project "Hybrid Engine Development" (Project No.: 2017YFB0103404). The project will focus on the design and integrated development of hybrid engine, with the goal of optimizing the effective thermal efficiency of hybrid engine. Taking a domestic gasoline engine as the prototype engine, the supercharging system, EGR system, and Miller cycle system are optimized and matched to optimize the economy and power of the engine, and achieve the goal of effective thermal efficiency higher than 40%.

[0107] Main contents of the present invention

[0108] (1) Establishment and verification of the simulation model of the working process of the Miller cycle supercharged engine

[0109] Analyze the characteristics of the working process of the Miller cycle hybrid vehicle gasoline engine, establish the physical and mathematical models of the engine cylinder; establish a simulation analysis platform for the working process of the Miller cycle hybrid vehicle gasoline engine, and establish a simulation model of the working process of the gasoline engine with a supercharging system; verify the accuracy of this model based on the engine bench test results.

[0110] (2) Simulation and analysis of Miller cycle turbocharged engine system characteristics

[0111] Based on the established simulation model of the working process of the supercharged gasoline engine, the influence of different Miller timings on the engine performance is studied; the influence of different EGR rates on the performance of the Miller cycle engine is studied; the backflow problem of the EGR pipeline of the Miller cycle engine is analyzed; and the opening of the exhaust bypass valve of the engine and turbocharger system is re-optimized and calibrated.

[0112] (3) Research on the integrated matching of Miller cycle gasoline engine supercharging system and EGR system

[0113] Determine the optimization objectives and optimization parameters of integrated optimization matching, and select a suitable optimization algorithm; establish a joint simulation model of GT-Power and Siumlink coupling, and perform parameter design of the genetic algorithm; obtain the EGR rate MAP and exhaust bypass valve MAP under different working conditions, and analyze the results of integrated optimization matching.

[0114] 2. Establishment and verification of the simulation model of the supercharged engine working process

[0115] Since the experimental research method requires a long R&D cycle, a large workload and a high R&D cost, simulation calculation has become one of the main modern design research methods due to its high work efficiency and low R&D cost. This paper studies the EGR system and supercharging system of the Miller cycle prototype supercharged gasoline engine by simulation calculation, so it is necessary to use GT-POWER to establish a simulation model of the supercharged engine working process.

[0116] 2.1 Establishment of supercharged engine simulation model

[0117] In order to facilitate the establishment of the simulation model of the engine working process, the mathematical model of the entire engine working process is divided into several independent, instantaneous thermodynamic equilibrium subsystems. The instantaneous gas pressure, temperature, and components at each point in the subsystem are uniform, that is, they are in an instantaneous thermodynamic equilibrium state. Each subsystem is connected through heat exchange and mass exchange, and serves as a boundary condition for each other. The engine system is mainly divided into five subsystems: intake system, exhaust system, cylinder, turbocharger system, and intercooler system. The present invention discusses the relevant calculation models under the GT-POWER software platform.

[0118] GT-POWER is a simulation tool developed by Gamma Technologies that meets the engine industry standards and is used by most engine and automobile manufacturers and suppliers in the world. GT-POWER is part of the GT-Suite series of software, covering six aspects: engine body, drive system, cooling system, fuel supply system, crankshaft mechanism, and valve mechanism. The software uses the finite volume method to calculate the fluid, and the calculation step size is automatically adjustable. It has powerful auxiliary modeling pre-processing tools, comes with a wealth of combustion models, has a wealth of control functions, can be coupled with SIMULINK for solution, can be coupled with three-dimensional CFD software for calculation, has its own optimization design function, can perform direct optimization, DOE design / optimization, can perform intake and exhaust system noise analysis, and can optimize the design of silencer components for intake and exhaust systems.

[0119] 2.1.1 Establishment of intake and exhaust system model

[0120] The calculation models of the intake and exhaust systems have the same essence, and both systems have pipes and throttles. The accuracy of the mathematical calculation models of pipes and throttles has a crucial impact on the overall simulation calculation of the engine. During the operation of the engine, the flow states of fresh air in the intake system and exhaust gas in the exhaust system are quite complex, with typical unsteady flow characteristics. In order to simplify the calculation model, one-dimensional calculations are usually performed on the intake and exhaust pipes. The solution methods for describing the one-dimensional unsteady flow control equations in the pipe include characteristic line method, wavelet perturbation method, finite volume method, etc. GT-POWER software uses the finite volume method to solve the one-dimensional unsteady flow control equations.

[0121] (1) Basic assumptions of simulation

[0122] It is relatively complicated to simulate the actual three-dimensional unsteady flow. In order to simplify the calculation model, the working medium flow in the intake and exhaust pipes is treated as a one-dimensional unsteady flow. The basic assumptions are as follows:

[0123] (a) Ignore the propagation, reflection and superposition of pressure waves in the intake and exhaust systems along the length of the pipe, and assume that the pressure is independent of the spatial coordinate position in the pipe;

[0124] (b) The radial flow effect in the pipeline is ignored, and the pipeline is considered to be a one-dimensional unsteady flow, and the state parameters of the working fluid on each section of the pipeline are average values;

[0125] (c) The change of state parameters in the pipeline is only related to the axial coordinate and time, and within a sufficiently small calculation step, the unstable flow in the pipeline is simplified to a quasi-stable flow;

[0126] (d) The cross section of the pipe changes slowly when the gas flows through it, and the pipe wall is considered to be rigid and elastic deformation is ignored;

[0127] (e) Considering the friction and heat transfer of the tube wall, the heat exchange between the tube wall and the gas inside the tube is simplified into a one-dimensional non-isentropic unsteady flow model;

[0128] (f) The gas in the tube is considered to be an ideal gas and gravity is ignored.

[0129] (2) Basic differential equations in the intake and exhaust pipes

[0130] In the simulation calculation of gasoline engines, the basic control equations of the one-dimensional unsteady flow in the intake and exhaust pipes are shown in the following formula:

[0131] (a) Continuity equation

[0132]

[0133] (b) Momentum equation

[0134]

[0135] (c) Energy equation

[0136]

[0137] In the formula: u represents gas flow rate; ρ represents gas density; A represents pipe cross-sectional area; D represents pipe diameter; D=4F / l, l represents the average circumference of pipe cross section; f represents friction coefficient inside pipe, which is generally taken as 0.005; q represents heat transfer rate per unit mass of fluid.

[0138] (3) Establishment of intake and exhaust pipe model

[0139] In the software, the finite volume method is used to specifically solve the one-dimensional unsteady flow of gas in the intake and exhaust pipes. This algorithm has good adaptability to the region and is most commonly used in numerical calculations. The finite volume method is used to solve the one-dimensional unsteady flow in the intake and exhaust pipes. The following formula is the general form of the conservation equation when the finite volume method is used for calculation:

[0140]

[0141] In the formula: φ represents the conservative flow rate of the fluid; ρ represents the fluid density; S represents the surface area of ​​the control body; V represents the volume of the control body; n represents the external normal of the control body surface; v represents the fluid velocity; T represents the diffusion coefficient; q represents the source term in the control body.

[0142] In the process of numerical integration calculation of conservation control equations, in order to obtain a stable solution, the CFL (Courant-Friedrichs-Lewy) criterion must be satisfied, as shown in the following formula:

[0143]

[0144] Where: Δt represents the time step; Δx represents the step length along the length of the pipeline; u represents the gas flow rate; a represents the gas sound speed.

[0145] GT-POWER software uses a one-dimensional staggered grid. Scalars are calculated at the center of the grid, such as pressure, temperature, etc. Vectors are calculated at the intersection of the grid, such as velocity, mass flow, etc. The discretization length of the grid is defined by the user. For common internal combustion engine performance calculations, it is recommended to:

[0146] The discretization length of the intake system is ≈0.4×D; the discretization length of the exhaust system is ≈0.55×D.

[0147] When gas flows in a pipeline, it will cause pressure loss due to friction with the wall. The friction coefficient is related to the Reynolds number and the roughness of the wall. When it is a smooth wall, the calculation formula is as follows:

[0148]

[0149]

[0150] Where: C f Re represents the friction coefficient; D Reynolds number.

[0151] When the wall surface is rough, the friction coefficient will increase. The friction coefficient is corrected by the formula Nikuradse as shown in the following formula:

[0152]

[0153] Where: D represents the equivalent pipe diameter; h represents the roughness height.

[0154] When the pipeline is bent or the cross-section is irregular, it will cause pressure loss to the airflow. The pressure loss coefficient is introduced here to represent this loss, as shown in the following formula:

[0155]

[0156] Where: p out Indicates the total inlet pressure; p in represents the total outlet pressure; ρ represents the inlet gas density; v1 represents the inlet gas velocity.

[0157] When the gas flows in the pipe, heat transfer occurs with the pipe wall. The heat transfer coefficient is calculated according to the following formula:

[0158]

[0159] Where: C f represents the friction coefficient; ρ represents the inlet gas density; u represents the gas velocity; C p represents the specific heat of gas; Pr represents the Prandtl number. In the temperature range of 20 to 1500°C, the Prandtl number is considered to be a constant, and Pr is taken as 0.71.

[0160] 2.1.2 Establishment of in-cylinder thermal process model

[0161] The modeling of the thermal process in the cylinder of a gasoline engine mainly includes the in-cylinder combustion model and the in-cylinder heat transfer model, and the Weber combustion model of the gasoline engine and the Woschni wall heat transfer model are selected.

[0162] (1) In-cylinder combustion model

[0163] The quasi-dimensional combustion model is also called the two-zone combustion model. This model assumes that the flame front divides the combustion chamber into two zones: the unburned zone in front of the flame front and the burned zone behind the flame front. The gasoline engine quasi-dimensional combustion model is based on the following assumptions:

[0164] (a) The combustion reaction takes place only within a very thin flame front, and the thickness of the flame front is negligible; the flame front is a spherical surface centered on the spark plug; the flame front divides the combustion chamber into a burned zone and an unburned zone.

[0165] (b) The burned zone and the unburned zone each constitute an independent subsystem and each satisfies the assumptions of the zero-dimensional combustion model. The working fluid in the burned zone is in an instantaneous chemical equilibrium state. The two zones are in an instantaneous pressure equilibrium state, that is, the pressure in the two zones is considered to be the same at the same instant.

[0166] (c) The working fluid in both the burned and unburned zones is considered to be an ideal gas.

[0167] (d) The flame propagation velocity is calculated using a semi-empirical formula.

[0168] (e) There is no heat transfer at the frontal interface where the burned zone and the unburned zone contact each other. The working fluid in the two zones only transfers heat to the combustion chamber wall surface they contact.

[0169] Based on the above assumptions, the entire working process is linked together through the gas state equation, mass conservation equation and energy conservation equation.

[0170] Gas state equation:

[0171] pV=mRT

[0172] The mass conservation equation:

[0173]

[0174] Energy conservation equation:

[0175]

[0176] Where: m represents the total mass of the working fluid in the cylinder; m in 、m out Indicates the mass of the working fluid entering and exiting the cylinder respectively; m B Indicates the mass of the working fluid injected into the cylinder; Q B Indicates the heat released by the combustion of fuel in the cylinder; Q w Indicates the heat transferred to the outside through the cylinder wall; h in Indicates the specific enthalpy of the medium entering the heat exchanger; h out It represents the specific enthalpy of the working fluid discharged from the cylinder; u represents the internal energy of the working fluid in the cylinder; p represents the pressure of the working fluid in the cylinder; V represents the working volume of the cylinder; T represents the temperature of the working fluid in the cylinder.

[0177] The instantaneous combustion heat release rate of fuel combustion in the cylinder is determined according to the following formula:

[0178]

[0179] Where: g f Indicates the cyclic injection amount of each cylinder; H u Indicates the lower calorific value of fuel combustion; X indicates the mass of fuel burned at a certain crankshaft angle and g f ratio; Indicates the heat release rate of combustion.

[0180] The combustion process in the cylinder is relatively complex. The heat release law of combustion is affected by physical and chemical changes in the combustion process and factors such as the structure and operating parameters of the engine. It is difficult to accurately describe this process using a mathematical method. There are usually two methods used to simulate and calculate this combustion process, namely the actual measurement indicator diagram method and the semi-empirical formula method. The present invention uses the second method to calculate the combustion heat release process using the Weber semi-empirical formula.

[0181] Weber's semi-empirical formula is as follows:

[0182]

[0183] Where: m1 represents the combustion quality index; Indicates the crankshaft angle; represents the combustion duration angle, Indicates the combustion starting angle; Indicates the combustion end angle.

[0184] (2) In-cylinder heat transfer model

[0185] The heat transfer in the cylinder is calculated using the classic Woschni heat transfer model. The heat transfer surface area of ​​the entire combustion chamber usually includes three parts: the piston gas side surface, the cylinder head gas side surface and the cylinder liner. However, the average wall temperatures of these three heat transfer surfaces are different. The heat transfer coefficient uses the average instantaneous surface heat transfer coefficient to calculate the wall heat transfer. The formula for calculating the cylinder wall heat transfer per unit crankshaft angle is as follows:

[0186]

[0187] Where: n represents the engine speed; α g Indicates the instantaneous average heat transfer coefficient; A i Indicates the heat exchange area; T g Indicates the instantaneous temperature of the working fluid in the cylinder; T wi Indicates the average temperature of the wall. In the formula, i=1 represents the cylinder head; i=2 represents the cylinder liner; i=3 represents the piston top.

[0188] In the GT-POWER software, the instantaneous average heat transfer coefficient is calculated using the Woschni criterion formula:

[0189]

[0190] Where: p represents the cylinder pressure; T represents the cylinder temperature; D represents the cylinder diameter; C m represents the average piston speed; p a , T a 、V a Indicates the cylinder pressure, temperature and cylinder volume when the crankshaft angle is at the bottom dead center; V s represents the cylinder working volume; p0 represents the cylinder pressure of the engine in reverse; C1 represents the air flow velocity coefficient; C2 represents the current coefficient of the combustion chamber.

[0191] The instantaneous average heat transfer coefficient is calculated using the Woschni formula, and then the heat transfer between the cylinder and the outside world can be calculated by substituting it into the cylinder wall heat transfer calculation formula.

[0192] 2.1.3 Intercooler model establishment

[0193] In a supercharged gasoline engine, the supercharged air is intercooled to increase the amount of fresh air in the cylinder at the same intake pressure. The air temperature drops after passing through the intercooler. The intercooler calculation is mainly to calculate the outlet temperature T of the intake air after passing through the intercooler. s , outlet pressure p s . T kis the air temperature at the supercharger outlet. The subscript "s" represents air, "w" represents cooling water, "i" represents inlet conditions, and "o" represents outlet conditions.

[0194] Intercooler outlet air temperature T s The calculation formula is:

[0195]

[0196] Intercooler outlet cooling water temperature T wo The calculation formula is:

[0197]

[0198] The pressure loss of the charge air when it flows through the intercooler is Δp s It can be calculated as follows:

[0199]

[0200] In the formula, Δp so Indicates the pressure loss of the intercooler under design conditions, which can generally be taken as Δp so =0.3-0.5kpa; Indicates actual air flow; Indicates the air flow rate under design conditions.

[0201] 2.1.4 Establishment of supercharging system model

[0202] The exhaust gas turbocharger system consists of a compressor and a turbine. The calculation of the exhaust gas turbocharger system should meet three requirements: energy conservation, that is, the turbine output power is balanced with the compressor power consumption; the turbine and the compressor are installed coaxially to ensure equal speed; mass conservation, that is, the air flow through the compressor is equal to the exhaust gas flow through the turbine. For an engine running under stable conditions, in order to match a perfect supercharging system, the following conditions should be met:

[0203]

[0204] Where: η s,T represents the isentropic efficiency of the turbine; c p,T represents the average constant-pressure specific heat in the turbine; T1 represents the turbine inlet temperature; p2 / p1 represents the turbine expansion ratio.

[0205] (1) Conservation of energy:

[0206] P T =P C

[0207] Where: P T Represents the average power provided by the turbine; P CIndicates the average power consumed by the compressor.

[0208] The average power provided by the turbine is determined by the mass flow through the turbine and the enthalpy difference before and after the turbine. It is customary to allocate the loss of the turbocharger to the power provided by the turbine. The calculation formula is as follows:

[0209]

[0210] Where: represents the turbine mass flow rate; η m,TC It represents the mechanical efficiency of the turbocharger; h1 represents the enthalpy value at the turbine inlet; h2 represents the enthalpy value at the turbine outlet.

[0211] The average power consumed by the compressor depends on the mass flow rate through the compressor and the enthalpy difference through the compressor. The calculation formula is as follows:

[0212]

[0213] Where: represents the compressor mass flow rate; h3 represents the enthalpy value at the compressor inlet; h4 represents the enthalpy value at the compressor outlet.

[0214]

[0215] Where: η s,C represents the isentropic efficiency of the compressor; c p,C It represents the average constant pressure specific heat in the compressor; T3 represents the compressor inlet temperature; p4 / p3 represents the compressor compression ratio.

[0216] The overall efficiency formula for a turbocharger is as follows:

[0217] η TC =η m,TC ·η s,T ·η s,C

[0218] (2) Equal speed:

[0219] n T =n C

[0220] Where: n T Indicates turbine speed; n C Indicates the compressor speed.

[0221] When the engine is running unsteadily, the momentum balance of the turbocharger should be considered in order to obtain the rate of change of the angular velocity of the turbocharger rotor. The formula is as follows:

[0222]

[0223] Where: TC Indicates the turbocharger rotor angular velocity; I TC Indicates the inertia of the turbocharger rotor.

[0224] (3) Conservation of mass:

[0225]

[0226] Where: Indicates the mass flow rate through the wastegate valve; Represents the fuel mass flow rate and can be ignored.

[0227] 2.2 Parameter description of supercharged engine working process simulation analysis

[0228] 2.2.1 Main technical parameters of the engine

[0229] A domestic supercharged gasoline engine is a 1.2L turbocharged direct injection engine with the advantages of small displacement and strong power performance. It is suitable as a prototype engine for hybrid engine development. The performance index requirements of this type of gasoline engine are shown in Table 1. The present invention establishes a GT-POWER simulation model of this type of supercharged gasoline engine for simulation calculation and analysis, matches the engine with the turbine, and provides a basis and guidance for improving the performance of the supercharged engine.

[0230] Table 1 Main parameters of a domestic supercharged gasoline engine

[0231]

[0232] (1) Main parameters of combustion system

[0233] The combustion system parameters mainly include in-cylinder geometric structure parameters, valve timing parameters, combustion model parameters, injection model parameters, wall heat transfer model parameters, etc.

[0234] The in-cylinder parameters include in-cylinder structural parameters and related operating parameters, as shown in Table 2.

[0235] Table 2 Main parameters of engine cylinder

[0236]

[0237] (2) Main parameters of valve train

[0238] According to the relevant parameters provided by the enterprise, the engine valve train parameters are determined, as shown in Table 3.

[0239] Table 3 Engine valve train parameters

[0240]

[0241] Define the compression top dead center as 0°, draw the corresponding valve lift curve, according to the above engine valve train parameters, in Table 3, due to the influence of valve clearance, the valve opening and closing angles are the crankshaft angles corresponding to the moment when the valve lift is 0.2mm. The cam timing of the intake valve corresponds to the crankshaft angle when the intake valve opens; the cam timing of the exhaust valve corresponds to the crankshaft angle when the exhaust valve closes.

[0242] (3) Main parameters of turbocharger system

[0243] The parameters of the compressor and turbine are mainly determined by the pressure after the supercharger and the pressure before the turbine. The main parameters of the turbocharger system are given in Table 4.

[0244] Table 4 Main parameters of turbocharger system

[0245]

[0246] 2.2.2 Main modules of the simulation model

[0247] The prototype supercharged engine working process simulation model includes a cylinder simulation module, an intake and exhaust simulation module, and an injector simulation module, etc. The present invention first establishes each simulation module separately and simplifies the supercharger module, and then connects each simulation module to form a supercharged engine working process simulation model.

[0248] (1) Cylinder simulation module

[0249] The cylinder simulation module is set up and established using the EngCylinder module in the GT-POWER software. The parameter settings of the cylinder simulation module are relatively complex, including: the initial state of the working medium in the cylinder, the wall heat transfer model of the cylinder, the combustion model and the fluid flow model, as well as the initial temperature of the cylinder wall. The working medium in the first and third cylinders is set to the fresh air state after supercharging, and the second cylinder is set to the exhaust gas state after combustion; the Woschni model is selected as the cylinder wall heat transfer model; the Weber combustion model is selected as the cylinder combustion model, through which the ignition timing, ignition energy and other related parameters can be set; the Flow model is selected as the fluid flow model; the cylinder top, piston and cylinder side wall temperatures are set to 575K, 575K and 400K respectively.

[0250] The prototype supercharged engine is an inline four-cylinder engine, so cylinders 1, 2, 3 and 4 are set up according to the above method to establish a supercharged engine cylinder simulation module.

[0251] (2) Intake and exhaust system simulation module

[0252] The intake and exhaust systems of the engine include: throttle, intake pipe, exhaust pipe and intake and exhaust valves, etc. In the GT-POWER software, each part is established separately, and then each part is connected to form a complete intake and exhaust system simulation module.

[0253] (3) Turbocharger system simulation module

[0254] The main function of turbocharging is to increase the amount of air intake into the engine, thereby increasing the power and torque of the engine, making the car more powerful. The turbocharger is driven by the exhaust gas discharged by the engine. The turbine consists of two parts, one is the fresh air boost end (compression pump wheel), and the other is the exhaust gas drive end (exhaust gas turbine). There is an impeller at each end. There is also a pressure relief trigger (wastegate) between the two turbines on the same axis. It is located on one side of the exhaust gas turbine. When the pressure of the compression turbine is too high, the pressure will push the trigger to open the valve of the exhaust gas turbine to reduce the air pressure to prevent excessive boost.

[0255] (4) Supercharged engine simulation model

[0256] The steps to establish a GT model for four-cylinder turbocharged direct injection engine performance simulation are as follows:

[0257] (a) Based on the mathematical model of the prototype engine simulation modeling module and the simplified physical model, the complex engine is divided into several major subsystems.

[0258] (b) On this basis, the modules provided by GT-POWER software are used to establish the corresponding subsystem physical model. Different subsystems correspond to different GT modules.

[0259] (c) Input the initial parameters into the corresponding modules and preliminarily establish the engine performance simulation GT model.

[0260] According to the above modeling steps, the four-cylinder turbocharged direct injection engine is simplified into a simulation calculation model consisting of the intake system, cylinder working system, exhaust system, turbocharging system and environmental boundary conditions, and a general module for engine performance simulation is established.

[0261] 2.3 Verification of the simulation model of the supercharged engine working process

[0262] The engine parameters are imported into the simulation model, and the simulation results of torque, power, BMEP (mean effective pressure), and BSFC (effective fuel consumption rate) changing with speed are obtained under full load conditions. By comparing with the test values ​​of the bench test data provided by the project partner, it can be seen that:

[0263] ① The deviation between the experimental and simulation values ​​of engine power and torque. The maximum deviation of power and torque occurs at 5000r / min, which are 3.7% and 3.5% respectively;

[0264] ② The deviation between the experimental value and the simulation value of the engine pump air pressure BMEP is described. The maximum deviation value occurs at a speed of 3000r / min, which is 4.1%;

[0265] ③Describes the deviation between the experimental value and the simulation value of the engine fuel consumption rate BSFC. The maximum deviation value of BSFC occurs at a speed of 4500r / min, which is 4.5%.

[0266] In summary: the errors are all within the allowable range of 5%, so the simulation model reflects the performance of the prototype engine more accurately.

[0267] 2.4 The present invention mainly builds a GT-POWER simulation model for the prototype supercharged engine and verifies it, laying the foundation for the subsequent parameter analysis and matching optimization. The main work content and achievements of the present invention are as follows:

[0268] (1) A detailed mathematical description of the simulation model of the engine working process is given, including the calculation models of the in-cylinder working process, intake and exhaust system, turbocharger system, and intercooler.

[0269] (2) A domestically produced supercharged gasoline engine was selected as the prototype engine. Simulation modules such as the engine cylinder, intake and exhaust system, and turbocharger system were designed and built. The modules were connected to build an overall simulation model of the prototype supercharged engine.

[0270] (3) The performance of the engine simulation model was verified. The verification results showed that the relative errors between the engine simulation model and the experimentally obtained fuel consumption rate, maximum combustion pressure in the cylinder, torque and power were all less than 5%, which was within the allowable range. This shows that the constructed supercharged engine GT-POWER simulation model has good accuracy.

[0271] 3. Simulation analysis of system characteristics of Miller cycle supercharged engine

[0272] 3.1 Effect of different Miller timing on engine performance

[0273] Miller cycle technology is a key technology to improve engine thermal efficiency and is widely used in hybrid engines. Miller cycle is achieved by delaying the closing time of the intake valve or closing the intake valve early. The former reduces the compression ratio by pushing part of the gas back into the intake duct during the upward process after the piston reaches the bottom dead center, while the latter reduces the compression ratio by closing the intake valve before the piston reaches the bottom dead center, while maintaining a large expansion ratio, making the expansion ratio greater than the compression ratio. Miller cycle technology has the advantages of suppressing knock and reducing pumping losses, thereby improving the fuel economy of the engine.

[0274] There are two ways to implement the Miller cycle: late intake valve closing (Late Intake-Valve-Closing) and early intake valve closing (Early Intake-Valve-Closing). Late intake valve closing (LIVC) is based on the intake valve lift curve of the prototype engine. The intake valve opening angle is kept unchanged. By changing the duration of the maximum lift of the intake valve, the intake valve closing angle is pushed back to obtain the late intake valve closing (LIVC) lift curve; Early intake valve closing (EIVC) makes the cam profile narrower. In order to ensure the mechanical properties of the valve cam, it is necessary to appropriately reduce the intake valve lift. By consulting relevant literature, it can be seen that the LIVC form has a higher ability to suppress knock than the EIVC form, and the EIVC will reduce the intake volume and thus seriously affect the engine power. Therefore, the present invention studies the LIVC form and studies the influence of different Miller timings on engine performance. The influence of different intake valve closing times on the engine's ventilation process, power, fuel economy and combustion emissions is studied, and the reasons for the influence are explored.

[0275] Generally speaking, the engine of a hybrid vehicle does not need to consider the idle condition, and the high thermal efficiency area appears in the medium and high load conditions between 2000r / min and 4500r / min. Therefore, the present invention mainly performs simulation analysis under the conditions of 50% and 100% load and 2000r / min, 3000r / min and 4500r / min speed.

[0276] The intake valve timing and lift parameters of the Miller cycle are shown in Table 5.

[0277] Table 5 Miller cycle intake timing and lift

[0278]

[0279] 3.1.1 Effect of different Miller timing on engine ventilation process

[0280] (1) The influence of Miller cycle valve timing on engine ventilation process under full throttle condition

[0281] The ventilation process of the engine plays a very important role in the working process of the engine and directly affects the performance of the engine. Therefore, the present invention studies the variation of pumping loss and volumetric efficiency with different Miller degrees, so as to analyze the influence on the ventilation process of the engine.

[0282] The variation of engine pumping effective pressure (PMEP) with Miller angle at different loads and speeds, where negative values ​​represent pumping losses.

[0283] In general, under hybrid engine conditions, the pump effective pressure (PMEP) shows an increasing trend with the increase of Miller degree, and the increasing trend gradually increases, so the pump loss decreases with the increase of Miller degree. Under different working conditions, the maximum PMEP appears when the Miller degree is 60°. Under 50% load conditions, PMEP increases by 0.21 bar at most, and under 100% load conditions, PMEP increases by 0.16 bar at most.

[0284] Analysis shows that after applying the Miller cycle, the improvement of pumping loss is large, and the reduction rate of pumping loss even exceeds 25%. This is because after applying the Miller cycle, part of the gas with higher pressure in the cylinder is pushed back to the intake duct, so that the engine expansion ratio is greater than the compression ratio, which increases the expansion work and leads to a decrease in pumping loss. In summary, the application of Miller cycle technology can reduce pumping loss, and a significant reduction in pumping loss will play a certain role in improving the thermal efficiency of the engine.

[0285] Under hybrid engine conditions, the charging efficiency decreases with the increase of Miller degree. Among them, at the speed of 2000r / min and 3000r / min, when the Miller degree is 0-30°, the charging efficiency decreases with the increase of Miller degree. The pumping loss is slightly increased when the Miller degree is 30°. Regardless of the operating conditions, after the Miller degree is greater than 30°, the charging efficiency shows a significant downward trend with the increase of Miller degree. At 50% load, the charging efficiency drops by 0.193 at the maximum, which occurs at 4500r / min. At 100% load, the charging efficiency drops by 0.179 at the maximum, which occurs at 4500r / min.

[0286] In summary, as the Miller degree increases, the engine charging efficiency shows a downward trend. When the Miller degree is 60°, the Miller cycle engine charging efficiency drops by more than 20% compared with the original engine. This is because, in the Miller cycle mode where the intake valve closes after the bottom dead center, the piston will push out part of the mixture in the cylinder before the intake valve closes. This process is not conducive to improving the engine charging efficiency.

[0287] 3.1.2 Effect of different Miller timing on engine fuel economy

[0288] From the previous section, it can be seen that the Miller cycle will have a great impact on the engine ventilation process, so the engine performance will also change accordingly. The present invention studies the impact of Miller timing on engine fuel economy by analyzing the change of fuel consumption rate with Miller degree.

[0289] At 50% load, overall, the fuel consumption rate of the Miller cycle engine shows a trend of first decreasing and then increasing with the increase of Miller degree LIVC. When the speed is 2000r / min, the minimum fuel consumption rate occurs at Miller degree of 20°, with a maximum decrease of 12.4g / (kW·h); when the speed is 3000r / min, the minimum fuel consumption rate occurs at 30°, with a maximum decrease of 13.1g / (kW·h); when the speed is 4500r / min, the minimum fuel consumption rate occurs at 30°, with a maximum decrease of 12.3g / (kW·h).

[0290] At 100% load, the fuel consumption rate change trend is basically similar to that at 50% load. When the speed is 2000r / min, the minimum fuel consumption rate occurs at 30° Miller degree, with a maximum decrease of 11.8g / (kW·h); when the speed is 3000r / min, the minimum fuel consumption rate occurs at 20°, with a maximum decrease of 7.1g / (kW·h); when the speed is 4500r / min, the minimum fuel consumption rate occurs at 30°, with a maximum decrease of 14.2g / (kW·h).

[0291] Analyzing the reasons, after adopting Miller cycle technology, pumping loss is reduced, the effective thermal efficiency of the engine is improved, and thus fuel economy is improved. However, as the Miller degree gradually increases, more and more air is pushed back into the intake duct, resulting in a decrease in intake flow. Under the condition that the geometric compression ratio remains unchanged, the effective compression ratio continues to decrease, the effective thermal efficiency decreases, and the fuel consumption rate increases. When the Miller degree is small, the impact of pumping loss on fuel economy is greater than the effective compression ratio. As the Miller degree increases, the influencing factor of the effective compression ratio gradually increases and is higher than the influencing factor of pumping loss.

[0292] 3.1.3 Effect of different Miller timing on engine emissions

[0293] Since the purpose of developing hybrid engines is to improve effective thermal efficiency and reduce emissions, it is necessary to study the effect of Miller timing on engine emissions. x The present invention studies the changing rules of different pollutants with the Miller cycle LIVC angle, so as to analyze the impact on engine emissions. xThe amount generated at high speed and high load is significantly lower than that at other loads, so the present invention selects the working conditions of 100% load and 4500 r / min to analyze and study the engine emissions.

[0294] As the LIVC angle of the Miller cycle increases, NO x The emission of NO gradually decreased, with the maximum drop to 33.6ppm. x The generation condition is high temperature and rich oxygen. As the Miller cycle closing angle increases, part of the gas is pushed back into the intake duct, which gradually reduces the temperature in the cylinder and prevents NO x The generation conditions.

[0295] As the LIVC angle of the Miller cycle increases, the CO emissions gradually increase. Since the intake valve of the Miller cycle LIVC has not yet closed during the compression stroke, some gas is pushed back into the intake duct, which may affect the mixing of the mixture in the cylinder, resulting in uneven mixing of the mixture in the cylinder and increased CO emissions.

[0296] Based on the comprehensive performance parameters of the gasoline engine, the Miller cycle crankshaft angle of 30° was finally selected as the best solution to improve the engine performance, and the Miller cycle late closing angle of 30° was determined as the engine for subsequent research and analysis.

[0297] In summary, the fuel consumption rate shows a trend of decreasing first and then increasing with the increase of Miller degree. Since the adoption of Miller cycle technology, the pumping loss has been greatly reduced. At the same time, during the combustion process in the cylinder, the temperature of the gas has dropped to a certain extent, which ultimately leads to a decrease in fuel consumption rate. However, as the Miller degree gradually increases, the effective compression ratio continues to decrease, and the effective thermal efficiency decreases. Therefore, when the Miller degree causes the fuel consumption rate to increase. The decrease in the combustion temperature in the cylinder leads to NO in the engine emissions. x The decrease of and the increase of CO, current research shows that the use of EGR technology can effectively reduce CO emissions.

[0298] 3.2 Study on the EGR system characteristics of Miller cycle turbocharged engine

[0299] Exhaust gas recirculation (EGR) is a technology that introduces part of the exhaust gas into the intake manifold through a special pipeline, mixes with fresh gas, and then re-enters the cylinder to participate in combustion. The EGR rate can be accurately controlled by controlling the opening of the EGR valve to optimize engine performance. Since the exhaust gas enters the cylinder to participate in combustion, the intake amount of fresh gas is reduced, which will have a certain impact on the entire combustion process and ultimately affect the performance of the engine.

[0300] The present invention first establishes a simulation model of EGR technology, then studies the engine emissions under different EGR rates, and analyzes the NO x, CO and maximum combustion temperature with the changes in EGR rate, and explore the impact of exhaust gas recirculation technology on the fuel economy and power of the Miller cycle engine.

[0301] 3.2.1 Construction of EGR system simulation model

[0302] The EGR system consists of an EGR pipeline, an EGR valve and an EGR cooler. The present invention adds an intercooled high-pressure EGR system on the basis of the prototype engine, and simulates and analyzes the intercooled high-pressure exhaust gas recirculation technology, that is, directly connecting the turbine inlet and the intake manifold to achieve EGR. An intercooled low-pressure EGR system is established in the prototype engine GT model. The exhaust gas is drawn out from the exhaust manifold before the turbine, passes through the EGR valve and the EGR cooler, and then introduced into the supercharger. To achieve high-pressure EGR, the EGR controller adjusts the EGR valve opening by monitoring the mass flow at the throttle and the mass flow at the EGR valve, thereby achieving precise control of the target EGR rate.

[0303] Although low-pressure EGR has the advantage of easy control of the EGR rate, it is not commonly used because the exhaust gas is introduced in front of the compressor. High-pressure EGR takes the exhaust gas from the front of the turbine and then introduces it into the intake pipe after supercharging. The intake pipe after supercharging has a certain pressure, so high-pressure EGR may have pipe backflow. The problem of high-pressure EGR pipe backflow will be discussed in the following section.

[0304] 3.2.2 Study on the influence of EGR rate on emission performance of Miller cycle engine

[0305] As can be seen from 3.1.3, the influence of Miller cycle leads to increased CO emissions in the cylinder. Therefore, the present invention studies the influence of different EGR rates on engine emissions. Under full load and a speed of 4500r / min, the CO emissions change with the EGR rate.

[0306] When the engine is fully loaded and the speed is 4500r / min, as the EGR rate increases, CO emissions gradually decrease, and compared with before the addition of EGR, the maximum decrease is 32.8%, and the emission volume decreases by 643.1ppm. The lowest CO emission occurs when the EGR rate is 20%, which is 1361.8ppm.

[0307] Analysis of the reasons shows that CO is produced due to incomplete engine combustion. After EGR is added to the engine, part of the engine's exhaust gas enters the cylinder, causing the CO in the exhaust gas to re-burn and generate CO2. Therefore, EGR can reduce the engine's CO emissions.

[0308] Under the condition of full engine load and speed of 4500r / min, as the EGR rate increases, NOx The emissions gradually decreased, and compared with before the addition of EGR, the maximum decrease was 45%, the emissions decreased by 34.2ppm, NO x The lowest emission occurs at 42.2ppm when the EGR rate is 20%.

[0309] Analysis of the reasons shows that after the engine is added with EGR, part of the engine's exhaust gas enters the cylinder, thereby reducing the cylinder combustion temperature, and NO x The formation condition of NO is high temperature and rich oxygen. With the addition of exhaust gas, the oxygen concentration in the cylinder is also reduced. Therefore, as the EGR rate increases, NO is gradually destroyed. x The formation conditions.

[0310] The maximum combustion temperature in the cylinder changes with the EGR rate. When the engine is working at full load and 4500r / min, as the EGR rate increases, the maximum combustion temperature in the engine cylinder gradually decreases, and the greater the EGR rate, the greater the temperature drop. When EGR reaches 20%, the maximum combustion temperature drops by 498.3K.

[0311] The reason for the analysis is that as the EGR rate increases, the exhaust gas in the cylinder gradually increases, and the oxygen content in the cylinder decreases due to the dilution effect. Affected by the increase in exhaust gas, the combustion process gradually deteriorates, the heat release of combustion decreases, and the specific heat ratio of the exhaust gas molecules is greater than that of the fuel molecules. The heat capacity effect causes the combustion temperature in the cylinder to decrease.

[0312] 3.2.3 Study on the influence of EGR rate on the power and economic performance of Miller cycle engine

[0313] (1) Effect of EGR rate on Miller cycle engine performance under wide-open throttle conditions

[0314] Since the high-pressure EGR pipeline will have a backflow phenomenon, resulting in the EGR rate cannot be accurately controlled, the present invention analyzes the two speeds of 3000r / min and 4500r / min where EGR will not flow back, and studies the influence of the EGR rate on the engine performance under the throttle fully open condition.

[0315] As the EGR rate increases, the engine output torque increases first and then decreases, reaching the maximum when the EGR rate is 4%. As the EGR rate increases, the torque gradually decreases. When the EGR rate reaches 20%, the output torque decreases by more than 20%. The greater the EGR rate, the greater the decrease.

[0316] Analyzing the main reasons, with the addition of EGR, part of the exhaust gas is sucked into the cylinder, which reduces the cylinder combustion temperature and thus reduces the pumping loss, so the engine torque increases first. With the increase of the EGR rate, more and more exhaust gas is sucked into the cylinder, resulting in an increase in the proportion of exhaust gas in the cylinder, a decrease in the amount of fresh charge, a decrease in combustion heat release, and a decrease in the ability to do external work. In addition, the dilution effect and heat capacity effect of the exhaust gas gradually increase with the increase of the EGR rate, and the inhibitory effect on combustion becomes greater and greater. The combustion process in the cylinder is hindered, which increases the combustion duration and slows down the combustion heat release, resulting in a decrease in the ability to do external work. Therefore, the engine torque decreases, and it shows a gradual downward trend with the increase of the EGR rate, and the engine's power becomes weaker.

[0317] As the EGR increases, the fuel consumption rate of the engine shows a trend of decreasing first and then increasing, reaching the minimum value when the EGR rate is 4%. When the speed is 3000r / min, the fuel consumption rate increases by 0.7% at most and decreases by 1.5% at most. When the speed is 4500r / min, the fuel consumption rate increases by 4.9% at most and decreases by 4.5% at most. It can be seen that as the speed increases, the influence of the EGR rate on the fuel consumption rate becomes greater.

[0318] Analyzing the reasons, on the one hand, since EGR is introduced into the intake manifold after the throttle, this part of the intake air is not affected by the throttle intake throttling, reducing the throttling loss of the intake process; on the other hand, due to the introduction of EGR, the temperature of the combustion process in the cylinder can be reduced, thereby reducing the heat transfer loss of the high-temperature gas in the cylinder, and at the same time, the exhaust loss will also be reduced, thereby improving fuel economy. When the EGR rate is high, the dilution effect of the exhaust gas and the inhibitory effect of the heat capacity effect on combustion become the dominant factors, the combustion process deteriorates, and the fuel economy deteriorates. At the same time, when the load is large and the speed is high, EGR mainly inhibits the combustion process, reduces efficiency, and thus reduces fuel economy.

[0319] (2) Effect of EGR rate on Miller cycle engine performance at 50% load

[0320] The present invention studies the influence of the EGR rate on the engine performance under the working conditions of 3000r / min and 4500r / min with the throttle valve opened by 50%, and the influence of the EGR rate on the engine output torque under different speeds.

[0321] When the throttle is opened to 50%, the engine output torque shows a downward trend as the EGR rate increases. At a speed of 3000r / min, the torque drops by up to 44%, and at a speed of 4500r / min, the torque drops by up to 43%.

[0322] When the throttle is opened to 50%, the engine fuel consumption rate shows an upward trend as the EGR rate increases. At a speed of 3000r / min, the torque increases by a maximum of 1.3%, and at a speed of 4500r / min, the torque increases by a maximum of 4.2%.

[0323] 3.2.4 Analysis of EGR Pipe Backflow in Miller Cycle Engine

[0324] Since EGR pipeline backflow may occur in the experiment, the present invention analyzes the problem of high-pressure EGR pipeline backflow in the Miller cycle engine and explores the circumstances and conditions under which it occurs.

[0325] High-pressure EGR engine simulation model. The EGR system at this time takes exhaust gas from the front of the turbine and then introduces it into the intake pipe after supercharging. The backflow situation of the EGR pipe is determined by studying the inlet and outlet pressures of the EGR pipe.

[0326] The speed variation range is selected as 1000-6000r / min, a point is selected every 1000r / min, and the throttle openings of 10%, 30%, 50%, 70% and 100% are selected for simulation calculation.

[0327] At 10% throttle opening, as the speed increases, the EGR pipeline inlet pressure decreases slightly, while the pipeline outlet pressure decreases sharply. At a speed of 1000r / min, the EGR pipeline inlet pressure is less than the outlet pressure, so the EGR pipeline has a backflow phenomenon at this time. When the speed is greater than 1000r / min, the EGR pipeline inlet pressure is greater than the outlet pressure, so the EGR pipeline will not have a backflow phenomenon. In summary, when the load is 10% throttle opening, the EGR pipeline will only have a backflow situation at a speed of 1000r / min.

[0328] As the speed increases, the change rules of the inlet and outlet pressures of the EGR pipeline under the four working conditions are basically the same. When the throttle opening is 30%, the EGR pipeline will flow back at a speed of 1000-2000r / min. When the throttle opening is greater than 30%, the EGR pipeline will flow back at a speed of 1000-2500r / min.

[0329] In summary: Speed ​​is one of the factors that affect EGR backflow. At low speed, EGR will have backflow, but not at high speed. Load is also a factor that affects EGR pipeline backflow. At high load, EGR pipeline backflow is more serious than at low load, and the range is wider. The maximum value of the outlet pressure difference occurs at the maximum engine speed of 6000r / min, and as the throttle opening increases, the maximum pressure difference also gradually increases.

[0330] 3.3 Optimization of bypass valve opening of Miller cycle engine supercharging system

[0331] The working principle of the automotive supercharging system is to pre-compress the air or combustible mixture entering the engine cylinder or cool it after compression to increase the density of the air or combustible mixture entering the cylinder, thereby increasing the mass of the charge, and with the proper cooperation of the fuel supply system, more fuel can be burned well, thereby achieving the purpose of improving engine power, increasing specific power, improving fuel economy, and reducing exhaust emissions and noise.

[0332] 3.3.1 Establishment of the wastegate valve simulation model

[0333] The present invention will establish a wastegate bypass valve model based on the established Miller cycle engine simulation model. First, it is necessary to establish a wastegate bypass valve GT-POWER simulation model in the software. Before the engine exhaust enters the turbine, a bypass valve is added. The amount of exhaust gas passing through the turbine can be controlled by changing the bypass valve opening. In order to more accurately and conveniently control the size of the bypass valve opening, the bypass valve opening angle can be expressed from 0 degrees to 90 degrees. 0 degrees means that the wastegate bypass valve is closed, and 90 degrees means that the wastegate bypass valve is fully open.

[0334] 3.3.2 Optimization calibration of bypass valve opening under engine external characteristics

[0335] Since the previous engine uses the Miller cycle and adds an EGR system, the working characteristics of the turbocharger system matched with the original engine can no longer meet the requirements. The Miller cycle engine needs to adjust the structural parameters of the supercharging system according to the optimization target. For the optimization of the bypass valve opening, it is necessary to study the relationship between the power and fuel economy of the Miller cycle engine and the bypass valve opening. The operating condition is set to 3000r / min full load.

[0336] Under full load conditions at 3000r / min, the engine output torque first increases and then decreases with the bypass valve opening, reaching the maximum value under this condition when the bypass valve opening is 32°, and then the engine output torque gradually decreases with the increase of the bypass valve opening, reaching the minimum value when the bypass valve opening is 90°. Compared with the moment when the bypass valve is closed, the engine output torque drops by 93.8N·m at most.

[0337] Under full load conditions at 3000r / min, the fuel consumption rate of the engine shows an opposite trend with the change of bypass valve opening and output torque. The fuel consumption rate decreases first and then increases with the change of bypass valve opening. The minimum value under this condition is reached when the bypass valve opening is 32°. The maximum value under this condition is reached when the bypass valve opening is 90°. Compared with the moment when the bypass valve is closed, the engine fuel consumption rate increases by 73.5g / (kW·h).

[0338] In summary, by analyzing the influence of the bypass valve opening on the engine power and fuel economy, the optimal exhaust bypass valve opening of the Miller cycle engine at full load of 3000r / min is preliminarily calibrated to be 32°.

[0339] According to the calibration method, the optimal bypass valve opening is calibrated at each speed under full load conditions. The optimal bypass valve opening at different speeds under full load conditions of the engine can be obtained, and the optimal exhaust bypass valve opening calibration curve at each speed under full load conditions of the engine can be obtained.

[0340] The optimal opening law of the exhaust bypass valve under full load conditions of the engine is: as the speed increases, the optimal bypass valve opening gradually increases.

[0341] After completing the bypass valve opening calibration at various speeds of the Miller cycle engine under full load conditions, the engine and turbocharger are simulated and analyzed, and the simulation results can obtain the joint operation curve of the two.

[0342] The overall efficiency of the full-load operating point of the Miller cycle engine before optimization is low, and some of the operating conditions are on the surge line. It can be seen that the engine and the supercharger are poorly matched before optimization. From the optimized joint operation curve, it can be seen that the efficiency of the full-load operating point of the Miller cycle engine on the compressor is greater than 65%, which is significantly improved compared to the efficiency before optimization. And the entire joint operation curve is far away from the surge line, leaving a certain surge margin, and passing through the high-efficiency area. The results show that after calibrating the exhaust bypass valve opening, the Miller cycle engine and the supercharger are well matched.

[0343] 3.3.3 Optimization calibration of bypass valve opening under partial load

[0344] After studying the optimal bypass valve opening at different speeds of the Miller cycle engine under full load conditions, in order to obtain the optimal bypass valve opening of the Miller cycle under partial load conditions, it is necessary to further optimize and calibrate the bypass valve opening. The calibration method is the same as the calibration process in the previous section.

[0345] The actuator of the electronic waste gas bypass valve in the present invention is a pneumatic actuator, which uses the boost pressure at the rear end of the supercharger compressor as the air pressure source. When the Miller cycle engine is in idle and low-load conditions, due to the low intake pressure, even if the control solenoid valve is fully opened and the boost pressure is fully passed to the bypass valve actuator air chamber, the pressure is not enough to drive the bypass valve to open. Therefore, the present invention does not calibrate the bypass valve opening under engine idle and low-load conditions.

[0346] The present invention performs optimal bypass valve opening optimization calibration for all loads above 50%. The working condition selection engine load is 50%-100%, the step length is set to 10%, the speed range is 3000r / min-5000r / min, and the step length is set to 500r / min.

[0347] As the load increases, the optimal bypass valve opening gradually increases, and as the speed increases, the optimal opening gradually increases. The optimal exhaust bypass valve opening is the largest under high load and high speed conditions. The maximum value of the optimal bypass valve opening occurs under full load 5000r / min conditions, which is 70.3°.

[0348] 3.4 The present invention mainly uses the built simulation model to complete the simulation analysis of the Miller cycle engine system characteristics, and studies the influence of Miller cycle valve timing, EGR system, and supercharging system on engine performance, providing a theoretical basis for the subsequent Miller cycle engine integration matching. The main work content and achievements of the present invention are as follows:

[0349] (1) First, the valve lift curve of the Miller cycle with late intake valve closing (LIVC) was determined, and the effect of LIVC on the engine ventilation process, power, fuel economy and emissions under the conditions of full throttle and 50% throttle was studied. The research results show that the use of Miller cycle technology can effectively improve engine fuel economy and reduce NO x It will reduce emissions, but will increase CO emissions and will also have a certain negative impact on the engine's power.

[0350] (2) A high-pressure EGR model was built to study the effect of different EGR rates on the performance of the Miller cycle engine, the causes of the effect were analyzed, and the backflow problem of the EGR pipeline of the Miller cycle engine was analyzed. The research results show that as the EGR rate increases, the CO and NO x Emissions will be reduced, including NO x It will be greatly reduced. Reasonable use of EGR rate can improve the fuel economy of the Miller cycle engine, but it will cause a certain degree of decline in power.

[0351] (3) An exhaust bypass valve model was established in the Miller cycle engine model. The optimal bypass valve opening was optimized and calibrated under the full load condition of the engine to obtain the optimal bypass valve opening law under the full load condition of the Miller cycle engine. At the same time, the matching between the Miller cycle engine and the turbocharger before and after the bypass valve opening was compared and analyzed. The results showed that the engine and the turbocharger were well matched. The optimal bypass valve opening of the exhaust turbine under partial load conditions was then obtained.

[0352] 4. Research on the integration and matching of EGR system and supercharging system of Miller cycle engine

[0353] The present invention studies the simulation analysis of the characteristics of the Miller cycle, EGR system and supercharging system for each engine. However, the integrated matching problem of the Miller cycle engine is a multi-constraint, multi-dimensional and multi-objective nonlinear optimization problem. There is mutual coupling between the parameters of each system. Therefore, in order to achieve the goal of an effective thermal efficiency higher than 40%, the present invention is based on the engine working process simulation model established in Chapter 2, combined with genetic algorithms, to conduct a multi-objective optimization matching study on the performance parameters of the Miller cycle gasoline engine EGR system and supercharging system.

[0354] 4.1 Optimization algorithm for integrated optimization matching of EGR system and supercharging system

[0355] 4.1.1 Optimization Algorithm

[0356] The optimization matching of gasoline engine performance parameters needs to consider the coupling relationship between various engine working conditions, and adjust the control parameters so that all performance requirements of the engine are met in a certain working area. Since there are always contradictions between power, emissions and economy in the development process of gasoline engines, the optimization process of gasoline engine performance is to obtain a compromise overall optimal solution, which requires the optimization algorithm to be able to handle various constraints and optimization requirements and have sufficient flexibility.

[0357] GT-Power software can solve single-objective optimization problems. In the optimizer function of GT-Power software, there are two optimization algorithms, namely Brent method and discrete-grid method. Among them, the Brent method is based on the bisection method, uses the quadratic interpolation method to accelerate, and applies the reverse interpolation method to simplify the calculation. Its performance is very dependent on the initial independent variable values ​​defined in the instance setting, and it is easy to fall into local convergence. The discrete grid method traverses the entire constraint space with a fixed step size, discretizes the space into a series of data points, and then brings the discretized data into the objective function to find the maximum or minimum value that satisfies the function. The disadvantage of this algorithm is that the number of iterations is very large, and using this algorithm on GT-Power requires setting many unknown parameters.

[0358] For general mathematical problems, the traditional commonly used optimization algorithms include Lagrange multiplier method, golden section method, gradient descent method, Monte-Carlo method, etc. The usage conditions and limitations of the above optimization methods are summarized in Table 6. The engine performance parameter optimization problem belongs to a nonlinear programming problem, which requires finding the optimal value of the optimization target while satisfying various constraints. The above traditional optimization algorithms are difficult to solve the optimization of such problems.

[0359] In view of the shortcomings of traditional optimization algorithms, an optimization algorithm that can adapt to multiple constraints and multiple objectives, solve nonlinear programming problems, and is not restricted by objective functions is needed. Genetic Algorithm (GA) meets such requirements. Genetic algorithm is a computational model formed by simulating Darwin's natural selection and survival of the fittest. It is evolved from the selection and evolution mechanism of the biological world. It has high parallelism and randomness and is an adaptive global optimization algorithm. In recent years, genetic algorithms have had many successful applications in the field of internal combustion engine engineering. Such as application in electronic control engine calibration, application in optimization of structural parameters of various engine components, application in optimization of various engine control parameters, application in engine fault diagnosis, and application in engine modeling, etc.

[0360] Table 6 Comparison of different optimization algorithms

[0361]

[0362]

[0363] After comparing and analyzing various optimization methods and GT-Power's own optimization algorithm, the present invention finally selects the genetic algorithm to optimize the engine performance parameters for the following reasons:

[0364] (1) The parameter optimization problem of the Miller cycle engine involves many parameters, and there is a coupling relationship between the parameters. The genetic algorithm does not use deterministic rules, but emphasizes guiding its search process through probability transformation. Its evolutionary calculation method searches through a population rather than from a single initial point, making it very suitable for global optimization and fast convergence.

[0365] (2) The optimization problem of gasoline engine performance parameters is a multi-constraint, multi-dimensional, and multi-objective nonlinear optimization problem. There is mutual coupling between the parameters, and there is no accurate mathematical relationship between the input parameters and the output parameters. Unlike traditional optimization algorithms, genetic algorithms do not seek optimization based on the mathematical relationship between input parameters and output parameters, but operate on chromosomes based on fitness functions. Therefore, genetic algorithms can be used to solve objective function optimization problems without analytical expressions.

[0366] (3) Genetic algorithms cannot solve all problems. There is a class of problems called "GA-hard" that cannot be solved by genetic algorithms. "GA-hard" problems generally contain isolated optimal points, that is, the optimal point is surrounded by poor points, making it difficult for genetic algorithms to solve this optimal point by splicing genes together. For the global optimization problem of gasoline engine performance parameters, from the perspective of the principles of internal combustion engines, first, the field during the operation of the internal combustion engine is continuous; second, the performance output of the internal combustion engine is also continuously changing. Therefore, there will be no isolated optimal points in the output of the internal combustion engine, and the optimization problem does not belong to the "GA-hard" problem and can be solved by genetic algorithms.

[0367] 4.1.2 Mathematical Description of Multi-Objective Optimization Model

[0368] Multi-objective optimization is to use a suitable optimization method to optimize the relevant model parameters in the model based on the established simulation model so that the simulation model can achieve the optimization goal. The mathematical model of multi-objective optimization problem generally includes three elements: optimization variables, objective function and constraints.

[0369] (1) Optimization variables

[0370] The optimization variables are usually a set of parameters. Depending on the calibration objectives and objects, the selected optimization variables are also different. When optimizing the parameters, the parameter values ​​of the optimization variables can usually be given in advance, and some parameters that do not need to be optimized can be set as constants. A set of optimization variables constitutes a vector in the optimization space. The entire optimization calibration process is to find a set of optimization variable values ​​in the entire optimization space so that it meets the calibration objectives and the constraints at the same time.

[0371] n optimization variables a1, a2, a3, ..., a n Construct an array, represented as (a1, a2, a3, ..., a n ). In the process of optimization and calibration, many arrays will be generated. When there are more optimization variables, that is, the larger n is, the more arrays will be generated, the more complicated the problem will be, the more difficult it will be to calculate and solve it, and the longer it will take.

[0372] (2) Objective function

[0373] Usually a standard is used to measure the quality of each set of optimization variables, and this standard can be expressed as an objective function. When performing parameter optimization, this function is a computable function expressed by the optimization variables, which is used to evaluate the quality of a set of optimization variables. This calibration objective function can be expressed as:

[0374] maxy=f(X)=[f1(x),f2(x),...,f m (x)] T

[0375] Where: f1(x),f2(x),...,f m (x) represents m objective functions; f i (x) represents m mapping functions from the optimization variable space to the objective function space.

[0376] (3) Constraints

[0377] In the multi-objective optimization process, the optimization variables are constantly changing to achieve the optimal value of the objective function, but the value range of the optimization variables, the objective function value and other related parameters must be limited and constrained to improve the optimization speed and the accuracy of the optimization results. The constraints are as follows:

[0378] g i (x)≤0,i=1,2,...,q

[0379] h j (x)=0,j=1,2,...,p

[0380] Where: g i (x)≤0 defines q inequality constraints; h j (x)=0 defines p equality constraints.

[0381] The multi-objective optimization model based on genetic algorithm needs to give the definition of Pareto optimal solution set: there is usually no optimal solution for multi-objective optimization problems, because there are interactions between the objectives, and it is difficult to optimize all objectives at the same time. However, there is a solution set in which each solution is equal, which is called non-dominated solution set or Pareto optimal solution set. The mathematical description of Pareto optimal solution set is:

[0382] For a given multi-objective optimization problem maxf(x), if there is a feasible solution x*∈S and no other x∈S satisfies f i (x)≤f i (x*)(i=1,2,...m), then x* is called the Pareto optimal solution of maxf (x). {x*} is the Pareto optimal solution set.

[0383] For a given multi-objective optimization problem maxf(x) and a Pareto optimal solution set {x*}, its Pareto optimal frontier is defined as:

[0384] PF={f(x)=[f1(x),f2(x),...,f m (x)]|x∈{x *}}

[0385] The solution set of the Pareto optimal boundary definition formula generates the Pareto optimal boundary. The solid line AB represents the Pareto optimal boundary of the two objectives, and the points on this boundary are all Pareto optimal solutions; the points on the area enclosed by the dotted line AB and the solid line AB are the dominant solutions.

[0386] 4.2 Parameter optimization of Miller cycle engine EGR system and supercharging system based on genetic algorithm

[0387] 4.2.1 Establishment of joint simulation model based on coupling of GT-Power and Simulink

[0388] GT-POWER software is a complete engine working process simulation platform, and Simulink in MATLAB is a standard automatic control design platform. Joint simulation can give full play to the advantages of GT in engine performance calculation and Simulink in data transmission and control, becoming an important link in system optimization and matching.

[0389] The present invention uses the "SimulinkHarness" module in GT-POWER to connect the GT model with the Simulink model for joint simulation. There are two coupling modes between GT-POWER and Simulink: running from the GT-POWER environment; running from the Simulink environment. The present invention needs to obtain data in GT and use MATLAB for data processing, so the coupling mode running from the Simulink environment is selected for joint simulation.

[0390] The specific steps to establish the coupling program between GT-Power and Matlab / Simulink are as follows:

[0391] (1) In the built GT-Power engine working process simulation model, add the SimulinkHarness module to receive or transmit data to Matlab / Simulink. Use wired or non-wired methods to establish data transmission relationships between other components and this module. The data transmitted by GT-Power to MATLAB / Simulink are the indicated thermal efficiency, torque and NOx Emissions, the data transmitted by MATLAB / Simulink to GT-Power are engine speed, load, EGR rate and exhaust bypass valve opening.

[0392] (2) In the "SimulinkHarness" module, select "Siumilation Type" as: "run fromSimulink", and set "Simulink Model to Import (.dll / .so)" to: "ign".

[0393] (3) In the simulink interface, add the GT-SUITE model module to receive or transfer data to GT-Power. Add the constant module and the to workspace module to read or transfer data to the MATLAB script. The genetic algorithm program is written in the MALTLAB file.

[0394] 4.2.2 Genetic algorithm and its parameter design

[0395] Combined with the established GT and Simulink coupling model, the specific genetic algorithm optimization process is as follows:

[0396] 1. Initialize the genetic algorithm program, encode the algorithm parameters, and obtain the initial population t;

[0397] 2. The corresponding values ​​of the four variables contained in an individual in the current population are set by the MATLAB program to the input module of the Simulink model; then these values ​​are passed to the GT-Power model through the Simulink / GT-Power interface for calculation to obtain the effective thermal efficiency and torque data;

[0398] 3. The Simulink / GT-Power interface returns the calculation results to the Simulink model in real time;

[0399] 4. The MATLAB program collects the indicated thermal efficiency and other required results returned by the current GT-Power through the Simulink model, and then starts the calculation of the next individual;

[0400] 5. Repeat steps 2 to 4 until all individuals in the current population are completely calculated;

[0401] 6. The MATLAB program uses the calculation results of GT-Power to calculate the fitness value of each individual in the current population; the genetic algorithm performs selection, crossover and mutation operations to generate a new population based on the calculated fitness value if the termination condition is not met;

[0402] 7. Repeat steps 2 to 6 until the termination condition of the genetic algorithm is met, and the algorithm finds the global optimal solution to the current optimization problem.

[0403] According to the established optimization simulation model, the present invention sets the optimization variable parameters, objective function parameters, constraint condition parameters and genetic algorithm parameters in the model respectively.

[0404] (1) Selection of optimization variables

[0405] According to the requirements of the simulation calculation of this paper, the simulation model of the working process of the EGR system and the supercharging system of the Miller cycle engine is optimized and matched. Since the Miller cycle does not involve variable cam technology, different working conditions correspond to the same late closing angle of the intake valve; the EGR rate and bypass valve opening are mainly selected as optimization variables.

[0406] (2) Determination of the objective function

[0407] The present invention takes ensuring power as the premise and improving effective thermal efficiency as the main goal, and performs multi-objective optimization on the EGR system and supercharging system of the Miller cycle engine. Therefore, the effective thermal efficiency and torque index are used as objective functions at the same time, with the purpose of analyzing the relationship between effective thermal efficiency and torque under the premise of meeting the emission index requirements.

[0408] (3) Constraints

[0409] Light-duty vehicle pollution emission limits and measurement methods (China Phase VI) GB18352.6-2016, NO x The emission does not exceed 60mg / km, assuming that the three-way catalytic converter of the gasoline engine exhaust aftertreatment device can reduce NO x The emission is effectively reduced by 80%. Then, through calculation, it can be concluded that the emission limit of the present invention is: NO x The original emission of NO does not exceed 300mg / km, so x The original emission is limited to 300mg / km.

[0410] Finally, the optimization matching problem of Miller cycle starting and EGR system and supercharging system can be expressed as follows:

[0411]

[0412]

[0413] n=n c

[0414] P=P c

[0415] 0<xv ≤1

[0416]

[0417]

[0418] Where: η i Indicates the effective thermal efficiency of the engine, %; T i Indicates engine torque, N·m; n indicates engine speed, rpm, 1000≤nc≤6000; P indicates engine load, 0%≤P c ≤100%,x v Indicates the bypass valve opening Indicates EGR rate, %; NO x Indicates NO x Emissions, mg / km.

[0419] (4) Select operator

[0420] Selection is also called replication. The role of the selection operator is to ensure that good individuals in the group are selected, enter the next generation, and are inherited from generation to generation. In the evolutionary process, selection is performed based on the fitness value of each individual. Individuals with larger fitness values ​​have a greater chance of being inherited to the next generation, and individuals with smaller fitness values ​​have a smaller chance of being inherited to the next generation. The quality of the selection operator will directly affect the calculation results. Common selection methods and their characteristics are shown in Table 7. The present invention selects the random traversal sampling operator in the GATBX genetic algorithm toolbox to perform the selection operation.

[0421] Table 7 Common selection methods and characteristics

[0422]

[0423]

[0424] (5) Crossover operator

[0425] Crossover is also called recombination or mating, which is to select two individuals from a group, exchange some or a certain part of the two individuals, and produce new individuals in the next generation. And explore the area where good individuals may be produced. The crossover operation is the main difference between genetic algorithms and other optimization algorithms. It plays a key role in the global search ability of genetic algorithms. It is the main method for generating new individuals and has a direct impact on the performance and final implementation of the algorithm. Commonly used crossover operators include one-point crossover, two-point crossover, multi-point crossover, uniform crossover, mixed crossover, heuristic crossover, arithmetic crossover, etc. The present invention selects the most commonly used two-point crossover.

[0426] (6) Mutation operator

[0427] The mutation operator maintains the diversity of the population by introducing a certain amount of randomness, ensures that different areas are searched, and inhibits the premature convergence of the algorithm. Mutation generates new individuals in the genetic process. Compared with the crossover method, the mutation method plays a low role in the genetic algorithm and is not an important operation in parameter optimization. The present invention selects the mutbga operator suitable for real number coding.

[0428] (7) Other algorithm parameters

[0429] Genetic algorithm parameters have a great impact on algorithm performance. The larger the population size, the higher the solution accuracy, but the calculation time is greatly increased. When the crossover probability is too large, the search space expands, but the convergence speed slows down, and when the crossover probability is too small, the algorithm is prone to fall into the local optimum and cannot jump out. The mutation operation is responsible for introducing diversity. A small mutation rate cannot produce useful genes, and a large mutation probability will cause the genetic algorithm to degenerate into a random search. The number of iterations is the termination condition of the algorithm. If it is too large, the calculation time will be too long, and if it is too small, the evolution may not be thorough.

[0430] According to the relevant literature, the selection range of population size is generally 20 to 200, the number of iterations of genetic algorithm optimization calculation is generally 100 to 500, the crossover probability is generally selected from 0.4 to 0.99, and the mutation probability is selected from 0.0001 to 0.01. After multiple experiments, the population size was selected as 120. The initial population for each optimization was generated using the DOE method, the crossover operator was selected as 0.5, and the mutation operator was 0.001. During the optimization process, the maximum number of iterations of genetic algorithm optimization calculation is 300. When the difference between the optimal solutions of two adjacent generations in 10 consecutive generations is less than 1%, it is considered to have converged and no further iterations are performed.

[0431] 4.3 Analysis of matching optimization results

[0432] 4.3.1 Genetic Algorithm Optimization under Full Load Condition

[0433] The joint simulation platform is used to perform genetic algorithm optimization on the full-load operating point of the Miller cycle gasoline engine. The initial parameters are optimized after 120 generations of evolution, and the average distance between individuals of each generation under full-load optimization is shown.

[0434] The average distance represents the average distance between all individuals in each generation of the population, and is used to characterize the differences between individuals in the population. The larger the average distance, the greater the difference between individuals in the population. As the evolutionary process increases, individuals with poor performance are gradually eliminated, and better individuals are continuously retained. Finally, individuals in the population approach the optimal value, and the average distance between individuals in the population eventually approaches 0.

[0435] After the optimization calculation process is completed, the distribution diagram of effective thermal efficiency and torque of the last generation of population individuals is obtained.

[0436] 4.3.2 Genetic algorithm optimization under all working conditions

[0437] After verifying the full load condition, in order to find the lowest fuel consumption point of the engine, it is necessary to perform genetic algorithm optimization on all engine conditions. Under all engine conditions, the optimization was stopped when the evolutionary generation reached 210 generations and the convergence standard was reached.

[0438] 4.3.3 Obtaining MAP

[0439] The engine is optimized for other speed and load conditions, and the engine speed is selected as 1000r / min, 1500r / min, 2000r / min, 2500r / min, 3000r / min, 3500r / min, 4000r / min, 4500r / min, 5000r / min, 5500r / min, 6000r / min, and the load is 20%, 40%, 60%, 80%, 100%, a total of 55 groups of conditions. Combined with the previously optimized full load condition and full condition, the EGR rate MAP map and bypass valve opening MAP map are made by interpolation method.

[0440] In general, the EGR rate is smaller at low speed and low load, and larger at high speed and high load. x The emission is not high, so a small EGR rate is used; when the engine is fully loaded, in order to ensure the power and economy of the engine, even if NO x The emission is very high and EGR is not used. When the speed increases, the EGR rate is also reduced to ensure the fresh air impulse of the engine.

[0441] The bypass valve opening is smaller at low speed and high load, and larger at high speed. The reason is that when the engine speed decreases, in order to ensure the boost pressure and intake flow at low speed, the bypass valve opening decreases, the turbine speed increases, and the boost pressure increases; when the engine speed increases, a larger bypass valve opening is used to prevent the supercharger from overspeeding and causing damage to the supercharger. When the engine load increases, a smaller bypass valve opening is used to ensure that the supercharger has a larger speed, increase the boost pressure and intake volume, and thus ensure sufficient engine power.

[0442] The optimized EGR valve opening and bypass valve opening results are input into the GT-Power model to calculate the boost pressure and intake air flow rate under various operating conditions and generate a MAP diagram.

[0443] 4.4 The present invention adopts a parameter matching optimization method combining simulation model and optimization algorithm, establishes a coupling program between GT-Power and MATLAB / Simulink, and uses the coupling program to optimize the performance matching between the EGR system and the supercharging system of the Miller cycle engine. The specific work content is as follows:

[0444] (1) Genetic algorithm was determined as the optimization algorithm of the present invention. By comparing the optimization algorithm provided in GT-Power and the commonly used optimization algorithms in mathematics, the applicability and limitations of each algorithm were analyzed, and finally the genetic algorithm was selected for the research work of the present invention, and the feasibility of the genetic algorithm for solving the engine performance optimization problem was analyzed.

[0445] (2) The multi-objective optimization model is mathematically described, including optimization variables, objective function, constraints and Pareto optimal solution set.

[0446] (3) Developed a coupling program between GT-Power and MATLAB / Simulink. Combining the advantages of GT-Power in engine performance calculation and the advantages of MATLAB / Simulink in data transmission and control calculation, a joint simulation program was developed that uses GT-Power for engine performance simulation and MATLAB / Simulink for genetic algorithm optimization. The development steps of the coupling program and the workflow of genetic algorithm optimization were explained in detail.

[0447] (4) The parameters of the genetic algorithm were designed. The objective function was designed. The genetic operators were the traversal sampling selection operator, the two-point crossover operator, and the mutbga mutation operator. The crossover probability was selected as 0.5 and the mutation probability was selected as 0.001. The number of populations was 120, and the initial population was generated by the DOE method. The termination condition of the genetic algorithm program was that the difference between the optimal solutions of two consecutive generations in 10 consecutive generations was less than 1%, and the maximum number of iterations was 300.

[0448] (5) The optimization results of the genetic algorithm were analyzed. First, the optimization was performed under the full load condition of the engine, and the highest torque point and the maximum effective thermal efficiency point were found. Finally, the maximum effective thermal efficiency point was determined as the optimal point. Then, the optimal point optimized under the full working condition of the engine was found. The corresponding effective thermal efficiency was 41.4837%, the torque was 211.28N·m, the corresponding working point of the individual was 3647r / min, the load was 75.6%, the EGR rate was 7.4%, and the bypass valve opening was 17%. According to the optimization results, the bypass valve opening MAP map, EGR rate MAP map, boost pressure MAP map and intake flow MAP map were obtained.

[0449] 5. Summary

[0450] With the support of the National Key R&D Project "Hybrid Engine Overall Design and Integrated Development" (Project No.: 2017YFB0103404), this invention has carried out integrated optimization matching research on the EGR system and supercharging system of the Miller cycle engine, and optimized the effective thermal efficiency and torque of the engine under the premise of ensuring that the emission standards are met. This invention establishes a simulation model of the Miller cycle supercharged engine based on the GT-Power software. For hybrid electric vehicles, the effects of Miller cycle timing and EGR rate on engine power, fuel economy and emissions are studied, and the supercharger operation MAP is re-optimized and calibrated. A coupling program between GT-POWER and MATLAB / Simulink was established, and a genetic optimization algorithm and a coupling program were used to optimize the performance matching between the EGR system and the supercharging system of the Miller cycle engine. The main work of the paper and the conclusions obtained are as follows:

[0451] (1) The present invention mathematically describes the simulation model of the engine working process, including the calculation models of the working process in the cylinder, the intake and exhaust system, the turbocharging system and the intercooler. Taking a domestic engine as the prototype engine, the simulation modules of the engine cylinder, the intake and exhaust system, the turbocharging system and the like were designed and built, and the overall simulation model of the Miller cycle supercharged engine was built. The performance of the engine simulation model was verified, and the analysis and verification results showed that the errors of the fuel consumption rate, the maximum combustion pressure in the cylinder, the torque and the power between the engine simulation model and the experimental data were less than 5%, indicating that the constructed Miller cycle supercharged engine GT-POWER simulation model has good accuracy.

[0452] (2) Aiming at the operating conditions of engine load of 50%-100% and speed of 2000-4500r / min, this paper conducts simulation analysis on the system characteristics of Miller cycle supercharged engine. The research results show that:

[0453] ①After adopting Miller cycle technology, pumping loss has been greatly reduced. At the same time, during the combustion process in the cylinder, the temperature of the gas has dropped to a certain extent, which ultimately leads to a decrease in fuel consumption rate. As the Miller degree increases, the effective compression ratio continues to decrease, the effective thermal efficiency decreases, and the fuel consumption rate increases. Due to the decrease in combustion temperature in the cylinder, NO in engine emissions x The decrease of CO and the increase of CO. Finally, the subsequent research object was determined to be a gasoline engine with a Miller cycle late closing angle of 30°.

[0454] ②EGR system can effectively reduce engine NO x and CO emissions, NO xThe maximum reduction is 45%, and the maximum reduction of CO is 32.8%. However, it has an inhibitory effect on the engine power. The effect of the EGR system on the fuel economy of the Miller cycle supercharged engine was studied. When the EGR rate is small, the temperature of the combustion process in the cylinder can be reduced, thereby reducing the heat transfer loss of the high-temperature gas in the cylinder, and the exhaust loss will also be reduced, thereby improving the fuel economy. When the EGR rate is high, the combustion process deteriorates and the fuel economy deteriorates. At the same time, when the load is large and the speed is high, EGR mainly inhibits the combustion process, thereby increasing the fuel consumption rate.

[0455] ③ The present invention analyzes the backflow problem of the EGR pipeline of the Miller cycle supercharged engine. The simulation analysis shows that the speed is one of the main factors affecting the EGR backflow phenomenon. The EGR will have a backflow phenomenon at low speed, but not at high speed. Load is also a factor that affects the EGR pipeline backflow phenomenon. At high load, the EGR pipeline backflow phenomenon is more serious than at low load. The maximum value of the outlet pressure difference occurs at the maximum engine speed of 6000r / min, and as the throttle opening increases, the maximum pressure difference also gradually increases.

[0456] (3) The present invention recalibrates the bypass valve opening of the engine supercharger. The bypass valve opening under the engine external characteristic working condition is optimized and calibrated to obtain the bypass valve under the external characteristic of the Miller cycle engine. The results show that the bypass valve opening is compared with that before optimization, and the joint curve passes through the high-efficiency area and is far away from the surge line. The expected optimization result is achieved.

[0457] (4) A coupling program between GT-Power and MATLAB / Simulink was established, and a genetic optimization algorithm was used to optimize the performance matching between the EGR system and the supercharging system of the Miller cycle engine.

[0458] ① Using genetic algorithm as the optimization algorithm of the present invention, a mathematical description of the multi-objective optimization model was carried out, including optimization variables, objective functions, constraints and Pareto optimal solution sets. A coupling program of GT-Power and MATLAB / Simulink was developed. The parameters of the genetic algorithm were designed.

[0459] ②Analyze the optimization results of the genetic algorithm. The engine performance was optimized under full load conditions, and the highest torque point and maximum effective thermal efficiency point conditions were analyzed. By optimizing the engine under full working conditions, the effective thermal efficiency was 41.4837%, the torque was 211.28N·m, and the corresponding working point was 3647r / min, the load was 75.6%, the EGR rate was 7.4%, and the bypass valve opening was 17%. According to the optimization results, the bypass valve opening MAP map, EGR rate MAP map, boost pressure MAP map, and intake flow MAP map were obtained.

[0460] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When the use is implemented in whole or in part in the form of a computer program product, the computer program product includes one or more computer instructions. When the computer program instructions are loaded or executed on a computer, the process or function described in the embodiment of the present invention is generated in whole or in part. The computer may be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions may be stored in a computer-readable storage medium, or transmitted from one computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions may be transmitted from one website site, computer, server or data center by wired (e.g., coaxial cable, optical fiber, digital subscriber line (DSL) or wireless (e.g., infrared, wireless, microwave, etc.) mode) to another website site, computer, server or data center. The computer-readable storage medium may be any available medium that a computer can access or a data storage device such as a server or data center that includes one or more available media integrated. The available medium may be a magnetic medium (e.g., a floppy disk, a hard disk, a tape), an optical medium (e.g., a DVD), or a semiconductor medium (e.g., a solid-state hard disk SolidState Disk (SSD)), etc.

[0461] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. An optimization method for integrated optimization matching of an EGR system and a supercharging system, characterized in that: The optimization method for integrated optimization matching of the EGR system and the supercharging system includes: Establishment and verification of the simulation model of supercharged engine working process; Simulation analysis of system characteristics of Miller cycle supercharged engine; Construction of optimization algorithm for integrated optimization matching of EGR system and supercharging system; Parameter optimization of Miller cycle engine EGR system and supercharging system based on genetic algorithm; The optimization algorithm for the integrated optimization matching of the EGR system and the supercharging system is constructed, including: multi-objective optimization is to use a suitable optimization method to optimize the relevant model parameters in the model based on the established simulation model so that the simulation model can achieve the optimization goal; the mathematical model of the multi-objective optimization problem generally includes three elements: optimization variables, objective functions and constraints; (1) Optimization variables The optimization variables are usually a set of parameters. Depending on the calibration objectives and objects, the selected optimization variables are also different. When optimizing the parameters, the parameter values ​​of the optimization variables can usually be given in advance, and some parameters that do not need to be optimized can be set as constants. A set of optimization variables constitutes a vector in the optimization space. The entire optimization calibration process is to find a set of optimization variable values ​​in the entire optimization space so that it meets the calibration objectives and the constraints at the same time. n optimization variables a1, a2, a3, ..., a n Construct an array, represented as (a1, a2, a3, ..., a n ); In the process of optimization and calibration, many arrays will be generated. When there are more optimization variables, that is, the larger n is, the more arrays will be generated, the more complicated the problem will be, the more difficult it will be to calculate and solve, and the longer the time required; (2) Objective function Usually a standard is used to measure the quality of each set of optimization variables, and this standard can be expressed as an objective function. When performing parameter optimization, this function is a computable function expressed by the optimization variables, which is used to evaluate the quality of a set of optimization variables. This calibration objective function can be expressed as: maxy=f(X)=[f1(x),f2(x),...,f m (x)] T ; Where: f1(x),f2(x),…,f m (x) represents m objective functions; f i (x) represents m mapping functions from the optimization variable space to the objective function space; (3) Constraints In the multi-objective optimization process, the optimization variables are constantly changing to achieve the optimal value of the objective function. However, the value range of the optimization variables, the objective function value, and other related parameters must be limited and constrained to improve the optimization speed and the accuracy of the optimization results. The constraints are as follows: g i (x)≤0,i=1,2,…,q; h j (x)=0,j=1,2,…,p; Where: g i (x)≤0 defines q inequality constraints; h j (x) = 0 defines p equality constraints; The multi-objective optimization model based on genetic algorithm needs to give the definition of Pareto optimal solution set: there is usually no optimal solution for multi-objective optimization problems, because there are interactions between the objectives, which makes it difficult to optimize all objectives at the same time; but there is a solution set, in which each solution has no superiority or inferiority, called non-dominated solution set or Pareto optimal solution set. The mathematical description of the Pareto optimal solution set is: For a given multi-objective optimization problem maxf(x), if there is a feasible solution x*∈S and no other x∈S satisfies f i (x)≤f i (x*), i = 1, 2, ... m, then x* is called the Pareto optimal solution of maxf(x); {x*} is the Pareto optimal solution set; For a given multi-objective optimization problem maxf(x) and a Pareto optimal solution set {x*}, its Pareto optimal frontier is defined as: PF={f(x)=[f1(x),f2(x),...,f m (x)]|x∈{x * }}; The Pareto optimal boundary is generated by the solution set of the Pareto optimal boundary definition formula; among them, the solid line AB represents the Pareto optimal boundary of the two objectives, and the points on this boundary are all Pareto optimal solutions; the points on the area enclosed by the dotted line AB and the solid line AB are the dominant solutions.

2. The optimization method for integrated optimization matching of the EGR system and the supercharging system according to claim 1, characterized in that: The supercharged engine working process simulation model includes an intake and exhaust system model, an in-cylinder thermal process model, an intercooler model and a supercharging system model.

3. The optimization method for integrated optimization matching of the EGR system and the supercharging system as claimed in claim 2, characterized in that: The intake and exhaust system models are established, including: (1) Basic assumptions of simulation ① Ignore the propagation, reflection and superposition of pressure waves in the intake and exhaust systems along the length of the pipe, and assume that the pressure has nothing to do with the spatial coordinate position in the pipe; ② Ignore the radial flow effect in the pipeline, assume that the pipeline is a one-dimensional unsteady flow, and the state parameters of the working fluid on each section of the pipeline are average values; ③The change of state parameters in the pipeline is only related to the axial coordinate and time, and within a sufficiently small calculation step, the unstable flow in the pipe is simplified to a quasi-stable flow; ④ The gas changes slowly when flowing through the pipe cross section, and the pipe wall is considered to be rigid, and elastic deformation is ignored; ⑤ Considering the friction and heat transfer of the tube wall, the heat exchange between the tube wall and the gas in the tube is simplified into a one-dimensional non-isentropic unsteady flow model; ⑥The gas in the tube is considered to be an ideal gas and gravity is ignored; (2) Basic differential equations in the intake and exhaust pipes In the simulation calculation of gasoline engines, the basic control equations of the one-dimensional unsteady flow in the intake and exhaust pipes are shown in the following formula: ①Continuity equation ② Momentum equation ③Energy equation Where: u represents gas flow rate; ρ represents gas density; A represents pipe cross-sectional area; D represents pipe diameter; D = 4F / l, l represents the average circumference of the pipe cross section; f represents the friction coefficient inside the pipe, which is 0.005; q represents the heat transfer rate per unit mass of fluid; (3) Establishment of intake and exhaust pipe model In the software, the finite volume method is used to specifically solve the one-dimensional unsteady flow of gas in the intake and exhaust pipes; the finite volume method is used to solve the one-dimensional unsteady flow in the intake and exhaust pipes. The following formula is the general form of the conservation equation when the finite volume method is used for calculation: Where: φ represents the conservative flow rate of the fluid; ρ represents the fluid density; S represents the surface area of ​​the control body; V represents the volume of the control body; n represents the outer normal of the control body surface; v represents the fluid velocity; T represents the diffusion coefficient; q represents the source term in the control body; During the numerical integration of the conservation control equations, the CFL criterion must be met: Where: Δt represents the time step; Δx represents the step length along the length of the pipeline; u represents the gas flow rate; a represents the gas sound speed; GT-POWER software uses a one-dimensional staggered grid; scalars are calculated at the center of the grid, such as pressure and temperature, and vectors are calculated at the intersection of the grid, such as velocity and mass flow; the discretization length of the grid is defined by the user; for common internal combustion engine performance calculations, it is recommended to: The discretization length of the intake system is ≈0.4×D; the discretization length of the exhaust system is ≈0.55×D; When gas flows in a pipeline, it will cause pressure loss due to friction with the wall. The friction coefficient is related to the Reynolds number and the roughness of the wall. When it is a smooth wall, the calculation formula is as follows: Where: C f Re represents the friction coefficient; D represents the Reynolds number; When the wall surface is rough, the friction coefficient will increase. The friction coefficient is corrected by the formula Nikuradse as shown in the following formula: Where: D represents the equivalent pipe diameter; h represents the roughness height; When the pipeline is bent or the cross-section is irregular, it will cause pressure loss to the airflow. The pressure loss coefficient is introduced to represent this loss, as shown in the following formula: Where: p out Indicates the total inlet pressure; p in represents the total outlet pressure; ρ represents the inlet gas density; v1 represents the inlet gas velocity; When the gas flows in the pipe, heat transfer occurs with the pipe wall. The heat transfer coefficient is calculated according to the following formula: Where: C f represents the friction coefficient; ρ represents the inlet gas density; u represents the gas velocity; C p represents the specific heat of gas; Pr represents the Prandtl number. In the temperature range of 20 to 1500°C, the Prandtl number is considered to be a constant, and Pr=0.

71.

4. The optimization method for integrated optimization matching of the EGR system and the supercharging system as claimed in claim 2, characterized in that: The in-cylinder thermal process model is established, including: The modeling of the thermal process in the cylinder of a gasoline engine mainly includes the in-cylinder combustion model and the in-cylinder heat transfer model, and the Weber combustion model and the Woschni wall heat transfer model of the gasoline engine are used; (1) In-cylinder combustion model The quasi-dimensional combustion model is also called the two-zone combustion model. This model assumes that the flame front divides the combustion chamber into two zones: the unburned zone in front of the flame front and the burned zone behind the flame front. The gasoline engine quasi-dimensional combustion model is based on the following assumptions: ①The combustion reaction takes place only within a very thin flame front, and the thickness of the flame front is negligible; the flame front is a spherical surface centered on the spark plug; the flame front divides the combustion chamber into a burned area and an unburned area; ② The burned area and the unburned area each constitute an independent subsystem and each satisfies the assumptions of the zero-dimensional combustion model; the working fluid in the burned area is in an instantaneous chemical equilibrium state; the two areas are in an instantaneous pressure equilibrium state, that is, the pressure in the two areas is considered to be the same at the same instant; ③ The working fluids in the burned and unburned zones are regarded as ideal gases; ④The flame propagation speed is calculated using a semi-empirical formula; ⑤ There is no heat transfer at the frontal interface where the burned area and the unburned area contact each other; the working fluid in the two areas only transfers heat to the combustion chamber wall surface they contact; Based on the above assumptions, the entire working process is linked together through the gas state equation, mass conservation equation and energy conservation equation; Gas state equation: pV=mRT; The mass conservation equation: Energy conservation equation: Where: m represents the total mass of the working fluid in the cylinder; m in 、m out Indicates the mass of the working fluid entering and exiting the cylinder respectively; m B Indicates the mass of the working fluid injected into the cylinder; Q B Indicates the heat released by the combustion of fuel in the cylinder; Q w Indicates the heat transferred to the outside through the cylinder wall; h in Indicates the specific enthalpy of the medium entering the heat exchanger; h out represents the specific enthalpy of the working fluid discharged from the cylinder; u represents the internal energy of the working fluid in the cylinder; p represents the working fluid pressure in the cylinder; V represents the cylinder working volume; T represents the working fluid temperature in the cylinder; The instantaneous combustion heat release rate of fuel combustion in the cylinder is determined according to the following formula: Where: g f Indicates the cyclic injection amount of each cylinder; H u Indicates the lower calorific value of fuel combustion; X indicates the mass of fuel burned at a certain crankshaft angle and g f ratio; Indicates the heat release rate of combustion; The combustion process in the cylinder is relatively complex. The heat release law of combustion is affected by the physical and chemical changes in the combustion process and the structure and operating parameters of the engine. It is difficult to accurately describe this process with a mathematical method. There are usually two methods used to simulate and calculate this combustion process, namely the actual measurement indicator diagram method and the semi-empirical formula method. The second method is selected to use Weber's semi-empirical formula to calculate the combustion heat release process. Weber's semi-empirical formula is as follows: Where: m1 represents the combustion quality index; Indicates the crankshaft angle; represents the combustion duration angle, Indicates the combustion starting angle; Indicates the combustion end angle; (2) In-cylinder heat transfer model The heat transfer in the cylinder is calculated using the classic Woschni heat transfer model. The heat transfer surface area of ​​the entire combustion chamber usually includes the piston gas side surface, the cylinder head gas side surface and the cylinder liner. However, the average wall temperatures of the three heat transfer surfaces are different. The heat transfer coefficient is calculated using the average instantaneous surface heat transfer coefficient. The calculation formula for the cylinder wall heat transfer per unit crankshaft angle is as follows: Where: n represents the engine speed; α g Indicates the instantaneous average heat transfer coefficient; A i Indicates the heat exchange area; T g Indicates the instantaneous temperature of the working fluid in the cylinder; T wi represents the average temperature of the wall; where i=1 represents the cylinder head; i=2 represents the cylinder liner; i=3 represents the piston top; In the GT-POWER software, the instantaneous average heat transfer coefficient is calculated using the Woschni criterion formula: Where: p represents the cylinder pressure; T represents the cylinder temperature; D represents the cylinder diameter; C m represents the average piston speed; p a , T a 、V a Indicates the cylinder pressure, temperature and cylinder volume when the crankshaft angle is at the bottom dead center; V s represents the cylinder working volume; p0 represents the cylinder pressure of the engine in reverse; C1 represents the air flow velocity coefficient; C2 represents the current coefficient of the combustion chamber; The instantaneous average heat transfer coefficient is calculated using the Woschni formula, and then the heat transfer between the cylinder and the outside world can be calculated by substituting it into the cylinder wall heat transfer calculation formula.

5. The optimization method for integrated optimization matching of the EGR system and the supercharging system as claimed in claim 2, characterized in that: The intercooler model is established, including: In a supercharged gasoline engine, the supercharged air is intercooled to increase the amount of fresh air in the cylinder at the same intake pressure. After the air passes through the intercooler, the temperature drops. The intercooler calculation mainly calculates the outlet temperature T of the intake air after passing through the intercooler. s , outlet pressure p s ; T k is the air temperature at the supercharger outlet; the subscript "s" represents air, "w" represents cooling water, "i" represents inlet state, and "o" represents outlet state; Intercooler outlet air temperature T s The calculation formula is: Intercooler outlet cooling water temperature T wo The calculation formula is: The pressure loss of the charge air when it flows through the intercooler is Δp s It can be calculated as follows: In the formula, Δp so Indicates the pressure loss of the intercooler under design conditions, taking Δp so =0.3-0.5kpa; Indicates actual air flow; Indicates the air flow rate under design conditions.

6. The optimization method for integrated optimization matching of the EGR system and the supercharging system as claimed in claim 2, characterized in that: The boost system model is established, including: The exhaust gas turbocharger system consists of a compressor and a turbine. The calculation of the exhaust gas turbocharger system should meet three requirements: energy conservation, that is, the turbine output power is balanced with the compressor power consumption; the turbine and the compressor are installed coaxially to ensure equal speeds; mass conservation, that is, the air flow through the compressor is equal to the exhaust gas flow through the turbine; for an engine running under stable conditions, in order to match a perfect supercharging system, the following conditions should be met: Where: η s,T represents the isentropic efficiency of the turbine; c p,T represents the average constant pressure specific heat in the turbine; T1 represents the turbine inlet temperature; p2 / p1 represents the turbine expansion ratio; (1) Conservation of energy: P T =P C ; Where: P T Represents the average power provided by the turbine; P C Indicates the average power consumed by the compressor; The average power provided by the turbine is determined by the mass flow through the turbine and the enthalpy difference before and after the turbine. It is customary to allocate the loss of the turbocharger to the power provided by the turbine. The calculation formula is as follows: Where: represents the turbine mass flow rate; η m,TC Indicates the mechanical efficiency of the turbocharger; h1 indicates the enthalpy value at the turbine inlet; h2 indicates the enthalpy value at the turbine outlet; The average power consumed by the compressor depends on the mass flow rate through the compressor and the enthalpy difference through the compressor. The calculation formula is as follows: Where: represents the mass flow rate of the compressor; h3 represents the enthalpy value at the compressor inlet; h4 represents the enthalpy value at the compressor outlet; Where: η s,C represents the isentropic efficiency of the compressor; c p,C represents the average constant pressure specific heat in the compressor; T3 represents the compressor inlet temperature; p4 / p3 represents the compressor compression ratio; The overall efficiency formula for a turbocharger is as follows: or TC =the m,TC ·or s,T ·or s,C ; (2) Equal speed: n T =n C ; Where: n T Indicates turbine speed; n C Indicates compressor speed; When the engine is running unsteadily, the momentum balance of the turbocharger should be considered in order to obtain the rate of change of the angular velocity of the turbocharger rotor. The formula is as follows: Where: TC Indicates the turbocharger rotor angular velocity; I TC Indicates the inertia of the turbocharger rotor; (3) Conservation of mass: Where: Indicates the mass flow rate through the wastegate valve; Represents the fuel mass flow rate and can be ignored.

7. The optimization method for integrated optimization matching of the EGR system and the supercharging system according to claim 1, characterized in that: The supercharged engine working process simulation model includes: The cylinder simulation module is set and established using the EngCylinder module in the GT-POWER software. The parameter settings include the initial state of the working medium in the cylinder, the wall heat transfer model, combustion model and fluid flow model of the cylinder, and the initial temperature of the cylinder wall. The working medium in the first and third cylinders is set to the fresh air state after supercharging, and the second cylinder is set to the exhaust gas state after combustion. The Woschni model is selected as the cylinder wall heat transfer model. The Weber combustion model is selected as the cylinder combustion model, through which the ignition timing and ignition energy related parameters can be set. The Flow model is selected as the fluid flow model. The cylinder top, piston and cylinder side wall temperatures are set to 575K, 575K and 400K respectively. Intake and exhaust system simulation module: the intake and exhaust systems of the engine include the throttle, intake pipe, exhaust pipe and intake and exhaust valves; each part is established separately in the GT-POWER software, and then each part is connected to form a complete intake and exhaust system simulation module; The turbocharger system simulation module is used to increase the engine intake volume, thereby increasing the engine power and torque, making the car more powerful; the turbocharger is driven by the exhaust gas discharged by the engine. The turbine consists of two parts, one is the fresh air boost end, namely the compression pump wheel, and the other is the exhaust gas drive end, namely the exhaust gas turbine. There is an impeller at each end. There is also a pressure relief trigger between the two turbines on the same axis, which is located on one side of the exhaust gas turbine. When the compression turbine pressure is too large, the pressure will push the trigger to open the valve of the exhaust gas turbine; Supercharged engine simulation model, the steps to establish a four-cylinder turbocharged direct injection engine performance simulation GT model are as follows: ① Based on the mathematical model of the prototype engine simulation modeling module and the simplified physical model, the complex engine is divided into several main subsystems; ② Use the modules provided by GT-POWER software to establish the corresponding subsystem physical model. Different subsystems correspond to different GT modules; ③ Input the initial parameters into the corresponding module to preliminarily establish the engine performance simulation GT model; simplify the four-cylinder turbocharged direct injection engine into a simulation calculation model consisting of the intake system, cylinder working system, exhaust system, turbocharger system and environmental boundary conditions.

8. The optimization method for integrated optimization matching of the EGR system and the supercharging system according to claim 1, characterized in that: The system characteristics simulation analysis of the Miller cycle supercharged engine includes: (1) Determination of EGR system characteristics of Miller cycle turbocharged engine ① Construction of EGR system simulation model The EGR system consists of an EGR pipeline, an EGR valve and an EGR cooler; an intercooled high-pressure EGR system is added to the prototype engine, and the intercooled high-pressure exhaust gas recirculation technology is simulated and analyzed, that is, the turbine inlet is directly connected to the intake manifold to achieve EGR; an intercooled low-pressure EGR system is established in the prototype engine GT model, and the exhaust gas is drawn out from the exhaust manifold before the turbine, passes through the EGR valve and the EGR cooler, and then introduced into the supercharger to achieve high-pressure EGR. The EGR controller adjusts the EGR valve opening by monitoring the mass flow at the throttle and the mass flow at the EGR valve; ② Determine the effect of EGR rate on the emission performance of Miller cycle engine; ③ Determine the impact of EGR rate on the power and economic performance of Miller cycle engine; ④ Analysis of EGR pipeline backflow in Miller cycle engine; (2) Optimization of bypass valve opening of Miller cycle engine supercharging system ①Establishment of the wastegate valve simulation model On the basis of the established Miller cycle engine simulation model, the wastegate valve model is established. First, the wastegate valve GT-POWER simulation model is established in the software. ②Bypass valve opening optimization calibration under engine external characteristics The Miller cycle engine is calibrated to have the best wastegate valve opening of 32° at full load of 3000r / min; ③ Optimization calibration of bypass valve opening under partial load The maximum value of the optimal bypass valve opening occurs at full load 5000r / min, which is 70.3°.

9. The optimization method for integrated optimization matching of the EGR system and the supercharging system according to claim 1, characterized in that: The genetic algorithm for optimizing the parameters of the Miller cycle engine EGR system and the supercharging system includes: (1) Establishment of a joint simulation model based on the coupling of GT-Power and Simulink Use the "SimulinkHarness" module in GT-POWER to connect the GT model with the Simulink model for joint simulation. There are two ways to couple GT-POWER and Simulink: run from the GT-POWER environment; run from the Simulink environment; get the data in GT, use MATLAB for data processing, and choose the coupling method of running from the Simulink environment for joint simulation; The steps of establishing the coupling program between GT-Power and Matlab / Simulink are as follows: ①In the built GT-Power engine working process simulation model, add the Simulink Harness module to receive or transmit data to Matlab / Simulink; use wired or non-wired methods to establish the data transmission relationship between other components and this module, where the data transmitted by GT-Power to MATLAB / Simulink is the indicated thermal efficiency, torque and NO x Emissions, the data transmitted from MATLAB / Simulink to GT-Power are engine speed, load, EGR rate, and wastegate valve opening; ②In the "SimulinkHarness" module, select "run from Simulink" for "Siumilation Type" and set "Simulink Model to Import (.dll / .so)" to "ign"; ③In the simulink interface, add the GT-SUITE model module to receive or transfer data to GT-Power; add the constant module and the to workspace module to read or transfer data to the MATLAB script; the genetic algorithm program is written in the MALTLAB file; (2) Genetic algorithm and its parameter design Combining GT and Simulink coupling model, the genetic algorithm optimization process is as follows: ① Initialize the genetic algorithm program, encode the algorithm parameters, and obtain the initial population t; ② The corresponding values ​​of the four variables contained in an individual in the current population are set by the MATLAB program to the input module of the Simulink model; then these values ​​are passed to the GT-Power model through the Simulink / GT-Power interface for calculation to obtain the effective thermal efficiency and torque data; ③The Simulink / GT-Power interface returns the calculation results to the Simulink model in real time; ④The MATLAB program collects the results of the indicated thermal efficiency requirements returned by the current GT-Power through the Simulink model, and then starts the calculation of the next individual; ⑤ Repeat steps ② to ④ until all individuals in the current population are completely calculated; ⑥The MATLAB program uses the calculation results of GT-Power to calculate the fitness value of each individual in the current population; the genetic algorithm performs selection, crossover and mutation operations to generate a new population based on the calculated fitness value if the termination condition is not met; ⑦ Repeat steps ② to ⑥ until the termination condition of the genetic algorithm is met, and the algorithm finds the global optimal solution to the current optimization problem; (3) According to the established optimization simulation model, the optimization variable parameters, objective function parameters, constraint condition parameters and genetic algorithm parameters in the model are set respectively, including: ①Optimization variable selection According to the requirements of simulation calculation in this paper, the simulation model of the working process of the EGR system and the supercharging system of the Miller cycle engine is optimized and matched. Since the Miller cycle does not involve variable cam technology, different working conditions correspond to the same intake valve late closing angle; the EGR rate and bypass valve opening are mainly selected as optimization variables; ②Determination of objective function Multi-objective optimization is performed on the EGR system and supercharging system of the Miller cycle engine, with effective thermal efficiency and torque indicators simultaneously used as objective functions. The purpose is to analyze the relationship between effective thermal efficiency and torque while meeting emission index requirements. ③ Constraints Light-duty vehicle pollution emission limits and measurement methods GB18352.6-2016, NO x The emission does not exceed 60mg / km, assuming that the three-way catalytic converter of the gasoline engine exhaust aftertreatment device can reduce NO x If the emissions are effectively reduced by 80%, the emission limit can be calculated as follows: NO x The original emission of NO does not exceed 300mg / km, so x The original emission is limited to 300mg / km; The optimization matching problem of Miller cycle starting and EGR system and supercharging system is expressed by the following formula: n=n c ; P=P c ; 0＜x v ≤1； Where: η i Indicates the effective thermal efficiency of the engine, %; T i Indicates engine torque, N·m; n indicates engine speed, rpm, 1000≤nc≤6000; P indicates engine load, 0%≤P c ≤100%,x v Indicates the bypass valve opening Indicates EGR rate, %; NO x Indicates NO x Emission, mg / km; ④Select operator Selection is also called replication. The function of the selection operator is to ensure that good individuals in the group are selected, enter the next generation, and are inherited from generation to generation. In the evolutionary process, selection is performed based on the fitness value of each individual. Individuals with larger fitness values ​​have a greater chance of being inherited to the next generation, and individuals with smaller fitness values ​​have a smaller chance of being inherited to the next generation. The random traversal sampling operator in the GATBX genetic algorithm toolbox is selected for selection operation. ⑤Crossover operator Crossover, also known as recombination or mating, is to select two individuals from a population, exchange some or a certain part of the two individuals, generate new individuals in the next generation, and explore areas that may produce good individuals; Crossover operation is the main difference between genetic algorithms and other optimization algorithms. It plays a key role in the global search ability of genetic algorithms. It is the main method for generating new individuals and has a direct impact on the performance and final implementation of the algorithm; Commonly used crossover operators include one-point crossover, two-point crossover, multi-point crossover, uniform crossover, mixed crossover, heuristic crossover and arithmetic crossover. The most commonly used two-point crossover is selected; ⑥ Mutation operator The mutation operator maintains the diversity of the population by introducing a certain amount of randomness, ensures the search of different areas, and inhibits the premature convergence of the algorithm; mutation generates new individuals in the genetic process; compared with the crossover method, the mutation method plays a low role in the genetic algorithm and is not an important operation in parameter optimization; the present invention selects the mutbga operator suitable for real number coding; ⑦Other algorithm parameters Genetic algorithm parameters have a great impact on algorithm performance; the larger the population size, the higher the solution accuracy, but the calculation time is greatly increased; when the crossover probability is too large, the search space is expanded, but the convergence speed is slowed down, and when the crossover probability is too small, the algorithm is easy to fall into the local optimum and cannot jump out; the mutation operation is responsible for introducing diversity, a small mutation rate cannot produce useful genes, and a large mutation probability will cause the genetic algorithm to degenerate into a random search; the number of iterations is the termination condition of the algorithm. If it is too large, the calculation time will be too long, and if it is too small, it may lead to incomplete evolution; According to the relevant literature, the population size is selected in the range of 20 to 200, the number of iterations of the genetic algorithm optimization calculation is 100 to 500, the crossover probability is selected from 0.4 to 0.99, and the mutation probability is selected from 0.0001 to 0.01; after multiple experiments, the population size is selected as 120, and the initial population for each optimization is generated using the DOE method, with a crossover operator of 0.5 and a mutation operator of 0.001; during the optimization process, the maximum number of iterations of the genetic algorithm optimization calculation is 300. When the difference between the optimal solutions of two adjacent generations in 10 consecutive generations is less than 1%, it is considered to have converged and no further iteration is performed.

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