Research On Cold Start Control Strategy Of GDI Engine

Facing with increasingly stringent emission regulations and fuel economy requirements, a combination of gasoline and diesel engine with high thermal efficiency and low emission advantages, direct injection gasoline (GDI) engine, comes into being. However, when the GDI engine under the cold start condition, the mixture in the cylinder is not completely combusted and the oxygen sensors and catalytic converters outside the cylinder are not working, which causes high UBHC (Unburned HC) emission. Compared with traditional PFI engine, UBHC emission of GDI engine can be reduced by 30%, thus research on cold start control strategy has practical significance.Combined with "863" project-"The Development of Direct injection gasoline engine", this paper designs the control strategy based on characteristics of cold start conditions. First of all, design low-pressure fuel pump, high pressure fuel pump control algorithm and electronic throttle control strategy of the relative position. Among them, use Toolbox of MATLAB to identify the mathematical models of GDI engine fuel supply system, build adaptive fuzzy PID closed loop control system model in Simulink GUI, and the design of cold start strategy is based on the simulation result. Then, calculate broadband oxygen sensor heating time and study the three-way catalytic converter light-off characteristics. And design broadband strategy of oxygen sensor heating control, two-stage fuel injection control and air-fuel ratio closed loop control. The hardware designed by DXP2004 software of the oxygen sensor control system is included in this paper. Based on the preliminary work of electronic control system and combined with oxygen sensor hardware and cold start program, the experimental platform is built up and research on cold start is made.Main experimental conclusions for GDI engine are as follows:1. Low pressure fuel pump control and high pressure fuel pump closed loop control can guarantee that the rail pressure can be stable at 5MPa in five cycles. 2. According to the use of the closed loop control of broadband oxygen sensor, excess air ratio control accuracy is±1.2%. So it can meet the requirement of catalyst.3. According to the experimental study in start-up stage, it is found that fuel injection and ignition advance angle of GDI engine have great influence on start-up progress. Under the conditions of the injection timing 90°BTDC and the ignition advance angle 30°BTDC, compared with other cases, it has acquired the quick start.4. According to the experimental study in catalyst light-off stage, two-stage injection strategy can make the engine stable combustion under lean burn conditions. Under the conditions of first injection timing 300°BTDC and second injection end timing phase 70°BTDC and the ignition advance angle 15°ATDC, compared with other four cases, the case of the ratio 5:5 between the first and second injection width has acquired the quick catalytic converter light-off time and low emissions.5. According to the experimental study in worm-up stage, the ignition advance angle has great influence on engine coolant temperature. Under the condition of injection timing 260°BTDC, The best ignition advance angle is 25°BTDC in the experiment.6. Using the optimal control parameters for each stage to control cold start process, in the motor 294r/min for the first time fuel injection ignition, the engine starts by three cycles successfully, the catalyst light-off time is 38s and the coolant temperature rises faster. Compared to previous experiments, overall UBHC emissions can be decreased by 19%, and the speed fluctuation rate is only 6%. Therefore, the designed control strategy can meet the general requirements for GDI engine cold start.