Study On Dynamic Characteristic And Control Strategy Of Three-Way Catalytic Converter For Gasoline Engine

In order to meet increasingly stringent emissions regulations, the combination of the three-way catalytic converter and electronic control system of the engine are effective way to reduce the gasoline engine emissions of harmful gas HC, CO, NO. Because of the"window"feature of the three-way catalytic converter, the engines are usually controlled in the lambda closed-loop which fluctuates around stoichiometry. When the engine is in transient condition, emissions do not react completely in three-way catalytic converter.Because of Ceria in active layer of the three-way catalytic converter, when the mixture is lean, the excess of oxygen was absorbed. And when the mixture is rich, the absorbed oxygen is released. Taking use of the three-way catalytic converter dynamic characteristic of oxygen storage and release on the ceria well to reduce emissions of HC, CO, NO in transient conditions, dynamic characteristic and control strategy of the three-way catalytic converter were studied.As the three-way catalytic converter reaction mechanisms are complex, substance concentrations of reactors on noble metal and ceria surface could not be measured in experiment. According to the mass conservation and energy conservation equation, combined with chemical reaction kinetics reaction mechanisms, the three-way catalytic converter model was established. Lambda is rich and lean step with some period, concentration of inlet HC, CO, NO is step changed with three-way catalytic converter inlet lambda. Concentration of gas-phase, solid-phase of HC, CO, NO and Carbide on the surface of noble metals and Ceria at different positions of converter were known. Mechanisms of oxygen storage capacity effect exhaust of HC, CO, NO were analyzed.The study of the three-way catalytic converter control strategy is based on the engine and catalytic converter model. There is a deviation between the air flow measured by sensor and mass flow of air into the cylinder in the transient conditions, the mean value engine model was used to estimate it. In order to reduce the lambda fluctuations in transient conditions caused by engine film effect, which would impact the study of the dynamic characteristics of the three-way catalytic converter, the model was established. Film parameters of film evaporation time constantτf and fuel deposition rate X were estimated with the least square method.Because of complexity of chemical reaction kinetics model, the cost computed time is too long to satisfy the requirement of real time of control unit of the vehicle, and the control-oriented model of the three-way catalytic converter was established. Above light-off temperature, oxygen storage capacity on ceria surface is determined the emissions of HC, CO, NO. Inlet and outlet lambda noise were removed the using digital filtering and then estimated parameters which are oxygen storage capacity and oxygen relative conversion function of the converter. Because the oxygen storage capacity could not measured by sensor, control-oriented model was used to computed the value of it which is as inference sensor. Oxygen storage capacity on Ceria solved by control-oriented model of the three-way catalytic converter was proved by chemical reaction kinetics model solved. Oxygen relative conversions in different exhaust mass flows were made a map.Oxygen storage capacity on ceria surface solved by the control-oriented model of the three-way catalytic converter is used as the feedback signal. Control reference value is set as 0.5 and the relative oxygen storage capacity on the ceria surface can be controlled, but lambda changed very violently which led to the bad emissions. The modern control theory was used to improve the emissions. The engine in different state operations, the optimal value of the oxygen storage capacity solved using quadratic function. Using quadratic function to optimize the fluctuation of oxygen storage capacity and the emissions to keep them least at the same time, the optimization result is the system control target. Using predictive control to solve the optimization manipulated variable in control horizon, the oxygen storage capacity is predicted in future horizon and keep optimizing online. Oxygen storage capacity is controlled.Hardware and software system were developed to test the control strategy on the engine bench. Hardware system is composed by acquisition signal processing circuit. Based on the integration and high-efficiency development environment of Freescale CodeWarrior Company, the three-way catalytic converter is controlled in closed loop. Tasks were designed as mutual independence modular and software is stratified, so it can be transplanted and easy to upgrade and debug. Calibration software was developed with visual C++ 6.0, based on high speed and stable serial communication bus CAN, by which a real-time communication was made between MPC566 and computer. Interface is easy to be used. Using Advantech's high performance data acquisition card real-time data such as engine speed and were collected in high speed with differential methods. Using the calibration software, the parameters were modified online and data were collected and saved.The converter controller is in open loop and inlet lambda is rich and lean step with some period by changing the engine fuel injection pulse width, outlet lambda, absorption of Ceria surface and conversion of HC, CO, NO of three-way catalytic converter were affected by the exhaust mass flow, the step periods and amplitude of lambda, which were analyzed by the engine bench test. By comparing different control strategies on the engine bench test, the relationship between the ROC and NO, HC emissions are analyzed Using converter controller, the emissions of HC and NO are better than the ones controlled byλ=1 controller. The relative oxygen storage capacity is controlled around the target value and absolute value is smaller than the value inλ=1 controller, so NO is reduced.