| Turbocharging and gasoline direct injection(TGDI)technologies make gasoline engines more economical and dynamic,but they also bring about problems such as high temperature in the engine cylinder,increased NOx emissions,and increased tendency to knock at large loads.The use of measures such as delaying ignition,reducing boost pressure,and enriching the mixture to suppress knocking will worsen fuel economy and deteriorate HC emissions under high loads,limiting the potential of TGDI engines.The introduction of water spray technology can reduce the temperature in the cylinder,alleviate knocking,improve thermal efficiency,and reduce NOx emissions.However,improper water injection will affect in-cylinder combustion and reduce engine output.In order to make the water spray technology play a better role,it is necessary to design the best control strategy,control the water spray volume and water spray timing under different working conditions,and achieve coordinated and comprehensive control with intake air and fuel injection.In this thesis,on the basis of the preliminary test of the turbocharged engine oil / water cylinder-intake port dual injection technology,the model-based gas path control strategy and the water injection control strategy are designed.The test data is simulated in the loop to verify the control effect.The main research results are as follows:(1)Research on water spray test.On the basis of the original engine,an intake port injection device is added to construct a dual injection system,and the combustion status of the engine is studied according to the measured pressure changes in the cylinder under different operating conditions.By adjusting the water injection volume and injection timing at different oil / water ratios,the effect on engine performance is studied,which provides a reference for the subsequent formulation of water injection control strategies.The test results show that under the conditions of medium and low speed and high load,the appropriate water injection amount and the appropriate injection timing can increase the average effective pressure of the engine,the fuel economy can be increased by up to 10%,and the maximum nitrogen oxide emissions can be reduced 60%.(2)Engine gas path modeling and design of gas path control strategy.Using the principle of average value,a control-oriented engine gas path model and crankshaft dynamics model are established.The former includes a flow characteristic model,a turbine pressure estimation model,and a turbine temperature estimation model.The latter is used to more intuitively reflect changes in engine operating characteristics.The numerical value is one of the basis for judging the working conditions in the water spray strategy.The pneumatic control strategy model is composed of throttle control and boost pressure control.The throttle control module calculates the cylinder air intake and the desired intake manifold pressure according to the engine torque demand and speed,and according to the flow characteristics to calculate the target opening of the throttle and PI closed-loop control.The boost pressure adopts the joint control method with throttle priority.Before the basic boost pressure,the intake air quantity is adjusted by the throttle valve;After the basic boost pressure,the throttle valve is fully opened,and the intake air quantity is adjusted by the wastegate valve;The desired intake manifold pressure at this time is the desired boost pressure.Under the premise of engine hardware protection,calculate the maximum expected boost pressure as the target value,determine the preset pressure of the wastegate vacuum regulator,and then control the opening of the wastegate valve and the flow of exhaust gas flowing through the turbine.For non-linear and complex variable relations,the control target value adopts experimentally calibrated control MAP.Using the engine bench dynamic test data,the effectiveness of the control model and control strategy are verified.The results show that under different transient conditions,but the throttle control accuracy is within 5%,which meets the overall accuracy control requirements.In the boost control strategy,the opening angle of the wastegate valve will accurately reach the target value,and the error is controlled within 10% of the engineering requirements.(3)Design of engine water spray control strategy.The water spray strategy mainly includes two modules: fuel control and water spray control.In the fuel control module,the starting air-fuel ratio,the operating air-fuel ratio and the transitional airfuel ratio are calculated separately,the target fuel amount is calculated according to the target air-fuel ratio,and the fuel amount is subjected to PI feedback control.In the water spray control module,the water spray function is determined according to the water spray enable condition,the water spray target value is calculated according to the target fuel quantity,and the water spray ratio is controlled by the MAP determined by the calibration experiment.Using the engine bench test data,the effect of water spray control is tested.The results show that after the water injection strategy is adopted,the air-fuel ratio control of the engine under transient conditions has a faster response speed,and the steady-state air-fuel ratio error control is less than 1%. |
