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Mathematical Analysis of Clutch Thermal Energy during Automatic Shifting Coupled with Input Torque Truncation
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
A step-ratio automatic transmission system alters torque paths through engagement and disengagement of multiple clutches. It adjusts gear ratios to efficiently run torque sources while meeting driver demand. Yet, thermal energy loss during clutch slippage is one of the contributors to the fuel efficiency of a vehicle. In order to optimize drivetrain control strategy, including frequency of shifts, it is important to understand the cost of shift itself. During a power-on upshift, clutch thermal energy is primarily dissipated during inertia phase. The interaction between multiple clutches, coupled with input torque truncation, makes the decomposition of overall energy loss less obvious. This paper systematically presents the mathematical analysis of clutch thermal energy during the inertia phase of a typical single-transition gearshift. In practice, a quicker shift is generally favored, partly because the amount of energy loss is considered smaller. However, the analysis reveals that there is a critical input torque truncation level, as a function of transmission output torque, where a shorter shift actually results in a larger energy penalty. Numerical simulations of gear shifting as well as vehicle testing are conducted to examine clutch thermal energy characteristics above or below the critical input torque truncation level.