Development of Design Principles for SMA Wave Springs

Electric coolant pumps in vehicles usually operate with a fixed characteristic curve, which is designed for an optimized operating point in terms of efficiency. Practically, however, the operating conditions change constantly, e.g., due to temperature changes during the warm-up phase, which change the coolant viscosity and thus the pressure conditions in the pump. In order to adapt the operating point to the current temperature, one option is to change the blade geometry of the rotating impeller. Shape-memory-alloys (SMA) are perfectly suited to fulfil this task. However, the existing flat and compact cross-section of the impeller as well as the required stiffness and force prohibit the use of SMA wire or spiral spring actuators. SMA wave springs represent an alternative actuator geometry to solve both the space- and the force and stiffness problems. In this paper, the design principles for SMA wave springs are developed using the finite element method. In particular, a numerical study is carried out to determine the influences of different geometrical parameters of the wave spring on the actuator performance. This is realized using an SMA FEA model of a NiTiCu-alloy based on Aurichio's approach, which is described in detail. The calculations show that it is possible to determine an optimum geometry for wave springs in terms of the achievable system work. Based on these results, SMA wave springs suitable for the application are manufactured and experimentally investigated to verify the model. In addition, a functional model of the water pump impeller including the actuator system is manufactured using additive manufacturing techniques to validate the approach in experiments.