5128 Etcheverry Hall, Mailstop 1740, University of California, Berkeley, CA 94720


The Problem

Over the past century, the automobile has been one of the crowning achievements of mechanical engineering and a driving force behind global economic development and culture. As large-scale automotive production enters its second century, concerns over climate change and oil dependency have focused attention on fuel efficiency and the reduction of greenhouse gases. One of the most promising technologies to meet these needs is the hybrid electric vehicle. Utilizing the high energy density of fossil fuels for steady-state power (cruising) along with electrical energy for short term power increases (accelerating) has led to relatively modest gains in fuel efficiency. However, one potential feature of hybrids, regenerative braking, the ability to recoup energy from deceleration, has thus far been relatively unsuccessful. This is due to the modern battery’s lack of power density; batteries are not capable of being charged as quickly as kinetic energy is dissipated by rapid braking.


The widespread adoption of hybrid and fully-electric vehicles is key to our transition away from fossil fuel dependence. However, this transition has been slow – the relatively limited fuel efficiency gains have not justified the increased costs. While regenerative braking certainly improves overall vehicle efficiency, the charging rates of current chemical batteries limit the amount of energy recovered.

30% of available regenerative braking energy is captured by the NiMH batteries in a Toyota Prius
45% of available regenerative braking energy is captured in the Li-Ion batteries in a Tesla Roadster

The Solution

The concept of INSTAR is to increase the efficiency of regenerative braking by increasing the power density of the hybrid system through the use of a flywheel as a “mechanical battery”. Flywheels are capable of much faster charge/discharge rates, and thus would be capable of filling in the areas where batteries fall short by providing high power density and efficiency for regenerative breaking and acceleration. Although flywheel energy storage systems have been researched for many years, there is still a need for the development of cost-effective, reliable systems. INSTAR seeks to develop an innovative, low-cost triple-hybrid (gasoline engine, electric motor, and mechanical flywheel) system that will have improved regenerative energy storage and acceleration when compared to traditional hybrids which employ only batteries in energy storage.


INSTAR employs a flywheel kinetic energy recovery system (KERS) to temporarily store the excess regenerative braking energy normally lost as heat. This energy is then sent either directly to the drive motor for acceleration or to the batteries at a rate they can accept.

90% of available regenerative braking energy is captured for reuse
5-15% extended range depending on driving conditions