ENHANCING ROAD VEHICLE EFFIENCY BY REGENERATIVE BRAKING A M Walker, M U Lampérth and S Wilkins Mechanical Engineering Department Imperial College of Science, Technology and Medicine, London, UK ABSTRACT Regenerative braking is an established feature of rail vehicles. The subject is under development in the road vehicle industry through recent developments in Hybrid Electric (HEV) and pure Electric Vehicles (EV). Up to 30% of the overall energy demand can be satisfied by energy saved through regenerative braking, significantly improving a vehicle’s overall efficiency. In addition, the brake force requirement on friction brake modules is reduced and, in-turn, their size can be reduced. The paper discusses an overall approach to brake system, powertrain and energy storage system components. Computer codes have been developed to simulate component performance and examine control strategies, which demonstrate significant reduction in friction brake use. A notable finding has been the improved energy retention by the incorporation of ultracapacitors. NOMENCLATURE A Vehicle front area (m 2 ) t Time (s) c w Coefficient of aerodynamic drag v Speed (m/s) E Energy (W.h) η Efficiency g Gravity constant (9.81 m/s 2 ) µ Coefficient of rolling resistance h Height (m) ρ Density / Density of air (kg/m 3 ) m Mass (kg)