Control strategy of vehicle automatic transmissions matching with start-stop system

Abstract: The engine idling start-stop technology can realize energy conservation and emission reduction, thus it has a very promising application. However, the start-stop technology does not match well with conventional automatic transmissions because of the vehicle starting delay, starting shock and safety problems. In order to overcome these problems, in this paper, we conducted a research on system level, based on a start-stop vehicle platform and the interfaces between engine, automatic transmission and other parts were analyzed. Based on comprehensive analyzing of the start-stop principle, the technical scheme of TCU (transmission control unit) control system was designed and a coordinated control strategy between TCU, EMS (engine management system) and the electrical oil pump was built. The coordinated control module contained three parts corresponding the engine auto-stop process, engine auto-start process, and the electrical oil pump control strategy. The control strategy for engine auto-stop and auto-stop process was to guarantee the starter operating only when the driveline was open to avoid starting failure and damage to the engine starter device, and the boundary conditions and CAN communication signals were expressed in these parts. An intelligent control strategy of the electrical oil pump was developed to satisfy the automatic transmission pressure requirement during engine restart and auto-stop, as well as decreasing the energy consumption. The rotating speed of the electrical oil pump was controlled based on the engine starting process and engine speed. This coordinated control strategy can avoid engine starting failure and guarantee the system normal operation and driving safety. In order to satisfy the special starting requirements of start-stop function, a mathematical model of starting process and clutch control system was built. The model contained the engine, torque converter, AT starting clutch, and vehicle body, the current to pressure characteristic, and friction characteristic of clutch. Based on model simulation, abnormal shifting operations caused by driver such as shift selector changing during start-stop process were considered in the starting control strategy. In different conditions, the control pressure for five clutches was different. By this control logic design, vehicle launching delay can be eliminated, and the shifting shock caused by abnormal operations can be avoided. The starting control strategy improved vehicle launching responsiveness and comfort significantly. The TCU application software was developed based on 'V process' from control module to target code, then the code was integrated with the target controller. A test environment on a prototype vehicle was established to verify the feasibility of the proposed control strategies. The test results indicated that the transmission functioned favorably, the speed of output shaft changed smoothly instead of changing sharply, and a quick and smooth starting performance of intuitive feeling was achieved when the engine was restarted. The findings in this study are valuable for forward designs of an AT for matching with start-stop system and also have great engineering application value.