| In the face of increasingly stringent emission regulations and the crisis of petroleum energy shortage,the advantages of lean combustion technology such as high-efficiency fuel economy and emission reduction have led it to extensive research and attention.The key to lean combustion technology is to operate the engine at high air-fuel ratio.The improvement of the air-fuel ratio can make the combustion more complete,improve the engine thermal efficiency,save more fuel,and reduce exhaust emissions.The increase in nitrogen oxide emissions is one of the reasons why lean combustion technology is difficult to promote and apply.In order to solve the problem of nitrogen oxide emissions,this paper proposed a technical route combining exhaust gas recirculation technology(EGR)and lean combustion technology.Exhaust gas recirculation technology suppresses the formation of nitrogen oxides by introducing exhaust gas into the cylinder,lowering the combustion temperature,and destroying oxygen-rich conditions in the cylinder.Both exhaust gas recirculation technology and lean combustion technology have strict requirements on the scope of operation.Therefore,reasonable selection of EGR rate and lean air-fuel ratio according to working conditions can combine economic and emission,while ensure the power of the engine.In this paper,the synergistic control of lean combustion and exhaust gas recirculation was studied,and the air-fuel ratio was accurately tracked.The simulation experiments under different working conditions were given.First of all,a gas path model containing exhaust gas circulation was established.Considering the entry of exhaust gas which brought about changes in both intake pressure and intake temperature,a two-state model was established to describe the dynamics of the intake manifold.In order to improve the accuracy of the air flow into the cylinder,considering the deviation caused by factors such as engine volumetric efficiency,a high gain observer was designed.What can not be ignored is that the air flow into the cylinder contains exhaust gas.The amount of fresh air flow into the cylinder was calculated according to the law of conservation of energy,which could be used for the derivation of air-fuel ratio control system.The simulation of the established model was carried out by GT-POWER and MATLAB/Simulink.The results showed that the control accuracy of the model was good,which laid a foundation for the controller design.Secondly,through the analysis of the effects of lean combustion technology and exhaust gas recirculation technology on fuel consumption and emissions,the idea of synergistic control was proposed.A nonlinear prediction model based on BP Neural Network was designed.The throttle,engine speed,air flow into the cylinder,EGR rate and air-fuel ratio were selected as the input,and the effective fuel consumption rate,nitrogen oxides and hydrocarbon emissions were selected as the output.Based on the nonlinear model predictive control method,an optimal control scheme was proposed,in which the state and control input of the system was limited.The EGR rate and the ideal lean air-fuel ratio were optimized under different working conditions by minimizing the performance indexes.Through simulation experiments,compared with the equivalent air-fuel ratio,engine emissions and fuel consumption under lean combustion were reduced to meet emission requirements,which verified the optimization control scheme.At last,the oil film evaporation model was designed,which described the relationship between the fuel injection amount of the injector and the fuel into the cylinder.Based on the triple-step nonlinear control method,the air-fuel ratio controller was presented.For the equivalent air-fuel ratio,both steady state conditions and transient conditions could achieve good tracking.For the lean air-fuel ratio,the controller could also track the given reference.From the simulation results,the controller had good control effects,which verified the accuracy and reliability of the control structure. |
PDF Full Text Request