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Hybrid vehicles combine a powerplant with an energy-storage device, the presence of which permits several fuel-reducing capabilities. Among these is regenerative braking. Its impact on vehicle fuel consumption can be determined by vehicle testing and/or computer simulation. In this paper, equations are developed that complement these results by offering a means for readily quantifying the potential impacts of regenerative braking on a vehicle's tractive-fuel consumption. Driving schedules can be decomposed into three generic modes - powered driving, braking, and idling. Without regenerative braking, the tractive-fuel consumed for powered driving is determined by the tractive energy required to propel a vehicle along a driving schedule, and the efficiency with which this energy can be delivered by the powertrain. The addition of regenerative braking reduces the portion of tractive energy that must be directly supplied by the powerplant.

Driving schedules prescribed for fuel-economy regulation are composed of two generic modes: (1) accelerations and constant-speed travel, requiring a positive tractive force at a vehicle's driving wheels; (2) decelerations, requiring a negative or braking force at those wheels. In the first mode, a total tractive energy, ETR, is required to overcome a vehicle's tire rolling resistance, aerodynamic drag, and the inertia of its mass. In the second mode, all the kinetic energy that a vehicle's mass acquired in the first mode has to be removed. The inherent rolling resistance and aerodynamic drag remove some of it. The remainder, EBR, has to be removed by a wheel-braking force. In vehicles with conventional braking the wheel-braking force is frictional, and so all of EBR is dissipated. However, if this force is not inherently frictional some of EBR can be captured, stored, and subsequently used to provide part of the ETR required for propulsion.