Forward Performance Development Of Opposite Piston And Opposite Cylinder Two-stroke Engine Based On Statistical Method And Design For Hybrid Application

Under the dual pressure of energy crisis and environment pollution,more and more stringent fuel consumption and emission regulations for vehicles are formulated all over the world.At present,internal combustion engine is the main power source of vehicles.Its performance development and optimization have direct effects on the fu el economy and emission performance of vehicles.Opposite piston and opposite cylinder(OPOC)two-stroke engine has unique structure which simplifies the valve system and removes the cylinder head so that OPOC engine has simple and light structure as well as good balance.Also,OPOC engine has higher power per liter and power density than those of traditional internal combustion engines.Comparing with traditional combustion engines,OPOC engine has more potential for energy saving and emission reduction so that ithas been widely investigated by experts and scholars at home and abroad.A forward design method for the engine coupling the statistics with simulation was proposed in this paper and the one-dimensional performance of OPOC engine was optimized.The scavenging performance of OPOC engine was simulated and compared by CFD and the spray characteristics of OPOC engine were researched experimentally.Finally,the key parameters,scavenging system and injection nozzle diameter was determined.Based on the developed OPOC engine,the powertrain system matching for hybrid electric vehicle was conducted.The configuration and control strategy were optimized to improve the energy consumption under driving conditions.The followings are the main research work of this thesis:Aiming at high time cost problem of traditional forward design method for internal combustion engine,this paper proposed a forward development process for engine based on the statistical methods.The test program was designed by Doe(Design of Experiments)and then the key influencing factors of the object were extracted.On this basis,the prediction model of object was established to for parameter optimization so as to shorten optimization time.First,the proposed process was validated for the torque optimization of a four-stroke engine.The results showed that torque was mainly impacted by 50% combustion location,intake and exhaust valve timing and the optimized torque under full speed conditions was increased by 4.62% on average.Then the on-dimensional engine performance simulation model of OPOC engine was constructed based on the structure and piston motion law of OPOC engine for the optimization of torque under the maximum speed condition.The results show that OPOC engine torque was mainly affected by intake,exhaust port timing offset,intercooler oultlet volume,exhaust manifold length and diameter,and exhaust pipe before the turbine.Finally,the OPOC engine torque under the maximum speed condition was improved by 2%.For the improvement of scavenging performance for OPOC engine,this paper adopted the port-to-port scavenging and chose the non-uniform intake chamber volume as the basic design scheme.Six scavenging system models were constructed based on the one-dimensional simulation optimization results and the effects of intake port number,swirl diameter and combustion chamber design on OPOC engine scavenging performance were analyzed by Converge.The results showed that less intake port number and larger swirl diameter strengthed the in-cylinder swirl motion.However,it was easier to cause the formation of concentration area of exhaust gas near the cylinder central axis,which led to higher residual gas fraction.Meanwhile,the combustion chamber design affected the swirl intensi ty.The rules of residual gas fraction and exhaust gas ratio were analyzed and then residual gas fraction was chosen as evaluation index.Finally,the optimum scavenging system was determined.In addition,for better performance of air-fuel mixture and combustion,the experiment and simulation were carried out for spray characteristics research of OPOC engine.The spray test was conducted experimentally in the constant chamber and the changing rules of the spray macroscopic characteristics including nozzle diameter,common rail pressure and environmental density were analyzed.The results show that when the nozzle diameter is 110μm,the engine has the best spray performance.Subsequently,for further research on spray characteristics,three-dimensional simulation was conducted by Kiva software.Based on the spray test results and statistical methods,the mathematical models of SMR(Sauter Mean Radius)and spray cone angle on nozzle diameter,common rail pressure and environmental density were established,whi ch is proved to have high accuracy and can be used to predict spray characteristics under different conditions.In order to give full play to high efficiency of OPOC engine,a traditional petrol vehicle was modified for hybrid power system,which utilized the OPOC engine as the traditional power source.On the basis of comparision of different hybrid system configurations,P2 configuration was selected as the basic configuration of hybrid electric vehicle and its model was established.The operating charact eristics of hybrid system under NEDC(New European Driving Cycle)were analyzed and the difference and causes of vehicle fuel economy under different gear ratios were compared and analyzed.Also,the shift and control strategies were optimized by genetic a lgorithm for better operating characteristics of engine and motor so as to improve fuel economy.Finally,aiming at the limitations of P2 hybrid system,P1+P2 hybrid system configuration was proposed and fuel economy under optimized control strategies for different hybrid systems was compared and analyzed.The results show that compared with P2 hybrid system,fuel consumption under NEDC for P1+P2 system decreased by 35.9g and was improved by 7.3%.Through the study of this paper,the problems of long develo pment time and high cos in the forward development process of internal combustion engine are solved and the performance optimization of OPOC scavenging system is provided guidance.Moreover,the prediction model of spray characteristics solve the problem t hat the parameter setting of spray and combustion simulation is overly dependent on engineering experience,and reduces the simulation difficulty.Also,the optimization of hybrid system configuration and control strategy is significant for the improvement of fuel economy of hybrid electric vehicles under road conditions.