Abstract
Automotive grille shutters are used to reduce the aerodynamic drag of the vehicle when cooling requirements are low by blocking a portion of the airflow through the condenser, radiator, and fan module (CRFM); the lower aerodynamic drag improves fuel economy. A passively actuated SMA grille shutter was designed as a less complex and lower overall cost solution to current electro-mechanical designs. This paper presents the design and calibration process for the automotive grille shutter SMA actuator using passive temperature control. The initial design target opening and closing temperatures of the grille shutter system were determined by climatic wind tunnel and vehicle road tests measuring motor-driven grille shutter position versus CRFM exhaust temperature. The initial SMA actuator design almost met the temperature requirements; subsequent road testing confirmed the need to shift the start to close and fully closed temperatures at least 6 °C higher to meet specifications. The transformation temperatures of the SMA actuator were calibrated by varying stress on the SMA wires by means of the total cross-sectional area and biasing force. Tests were performed in an environmental chamber to characterize the temperature-position behavior during heating and cooling cycles, the best candidate improved the hysteresis band by 3 °C from the initial design. This study presents an industry need for the development of lower hysteresis SMA wires/springs for passive applications, which can maximize the benefit of SMA actuation by utilizing the temperature-stress-strain coupling behavior of SMA.
