Analysis of Dual Mode Continuously Variable Transmission for Flywheel Energy Storage Systems

There are different types of energy storage devices which are used in today’s hybrid and electric vehicles. Batteries, ultra capacitors and high speed flywheels are the most commonly used ones. While batteries and supercapacitors store energy in the form of electric energy, the flywheel (FW) is the only device that keeps the energy stored in the original form of mechanical energy the same as the moving vehicle. The flywheel needs to be coupled to the driveshaft of the vehicle in a manner which allows it to vary its speed independently of the moving vehicle in order to vary its energy content. In other words a continuously variable transmission (CVT) is needed. The common mechanical variators used in automotive applications, namely the rolling traction drives and the belt drives, have the disadvantage that their speed ratio range defined as the maximum to minimum speed ratio is generally not sufficient for flywheel energy storage system (FESS). One of the ways to improve the ratio range is by using a dual mode transmission, where the ratio coverage of the variator is exploited twice. This paper presents the fundamental kinematics of such a transmission including its variants. The equations of speed ratio, power flow and efficiency are derived for a variator only transmission and a power split CVT (PSCVT) used in dual mode and the results compared.