Kinetic energy storage systems for powering vehicles currently exist but are not prevalent. Often the coupling between the flywheel and the vehicle is done using a separate actuator/generator system. This separate actuator system necessarily results in efficiency losses. In this paper we present a design for a spring-coupled variable inertia flywheel which directly couples the flywheel and vehicle. Simulation results for the non-linear dynamic behavior of the system are given and show that it can be used to store more than 99% of the energy of the vehicle when braking, but that there is a tradeoff between device size, deceleration rate, and energy stored. We found that a parameter exploration using three cost functions related to braking time, energy stored, and flywheel radius, shows that one can optimize at most two of the three cost functions. Analytic results are also given for a driven mass-flywheel model, which mitigates some of the problems of the linear spring model. However, this model, if it uses equivalent non-linear springs, is able to store at most 75% of the system energy. The driven-mass/non-linear spring model allows for a lower deceleration and smaller physical size than the linear spring model.

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