Edison2 took a close look at the interplay between weight, drag, regenerative braking and acceleration before deciding on a conventional power source for the Very Light Car. Our examination demonstrated the key importance of low weight and low aerodynamic drag in automotive efficiency, and led us away from a hybrid or electric drivetrain.
Here is a graph showing power requirements for a Prius on the EPA city cycle. The specially written Edison2 software behind it is part of what allowed us to explore in detail the benefits and consequences of different drivetrain philosophies. The key think shown in this graph is that the Prius - by conventional standards aerodynamically "slippery" - takes appreciable power to overcome its drag. This energy is not available to regenerate: it's gone.
Further, Edison2's analysis of the EPA city cycle for a spread of vehicle attributes demonstrated that the benefits of regenerative braking are, at best, mixed. A hybrid vehicle squanders much of the recovered energy accelerating the equipment required to do the recovery. In fact, only in a narrow set of circumstances can the help from a hybrid drive overcome the weight and cost disadvantages of batteries and electric motor. Pure electric vehicles fare worse because their batteries are necessarily even larger and heavier.
This is illustrated below in a graph showing the energy consumption of varying the weight of a Prius while holding aero drag constant. The graph is best read horizontally: a 3000 lb hybrid Prius with regeneration is not mare efficient than the same car weighing 2000 lbs without the hybrid drive. Other conclusions that can be drawn include 1) there is less energy available to regenerate in a light car because less has been spent accelerating it 2) As weight shrinks, the benefit of regenerative braking becomes vanishingly small and 3) A 5000 lb SUV is a good candidate for a hybrid drive but no matter how good the hybrid system, it will still drink fuel.
In the Very Light Car we have an unprecedented combination of low weight and low aerodynamic drag. Our theoretical work told us this approach is the most efficient, and so far our testing is validating the theory, expressed in the following chart.
This graph is based on published car weights and manufacturers claimed drag coefficients. Edison2's Very Light Car goes about 5 times as far on the same amount of energy as a Chevy Tahoe and more than twice as far as a Prius. Aptera comes close, but faces serious limitations on range per charge and has only 2 seats.
The X Prize events begin at the Michigan International Speedway on April 26 with the Shakedown stage, followed by the Knockout stage in June and the Finals in July. Some cars are electric, some are hybrid, and a few are internal combustion. Soon we will find out just how successful the various approaches to auto efficiency are.