Abstract:

The shift of DCT is composed of two parts， the pre-engagement control of the fork and the shift control of the clutch. In the process of power downshift， the engagement time of the fork is very long， and the corresponding clutch torque must be reduced to zero before the pre-engagement control can be executed. Under some working conditions， the delay of the whole gear shift process will be caused by waiting for the pre-engagement shift， affecting the acceleration response of the whole vehicle， which is especially obvious in the coaxial power downshift process. In order to optimize the four-stage downshift performance of a DCT model， the STB based shift is proposed to optimize the shift time and improve the power response of vehicles. STB is a semi power interruption shift method applied to DCT. The advantages and disadvantages of STB is elaborated in this paper by making a detailed comparison of the whole shift process with the normal shifting. In order to compensate for the temporary power interruption caused by STB shifting， a complete control strategy is designed. On the one hand， a targeted kick down logic is developed， and the corresponding shift scheduling is adopted to control the timing of multi-stage downshift， so that the fourth-stage downshift is only triggered when kick down occurs. On the other hand， an open-loop algorithm is used to convert the engine speed control into time control during the shifting process， so as to achieve the effect that the engine speed is just synchronized when the pre-engagement shift is done. After comparison and verification on actual vehicles， the STB shifting greatly shortens the shift time of the four-stage downshift， and the power response of the whole vehicle is significantly optimized.

Key words: Kick Down, STB, DCT four-stage downshift, DCT coaxial downshift