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. This reduction is determined by the vehicle kinetic energy that must be removed by wheel braking, and the braking-wheel to traction-wheel effectiveness with which it is recovered, stored, and subsequently recycled for propulsion.
Correlations of tractive and braking energies are developed for broad ranges of vehicle mass, tire rolling resistance, and aerodynamic drag, and for the driving schedules used for fuel-economy regulation in the U.S., Europe, and Japan. Using these correlations, equations are developed that quantify the potential reductions in tractive-fuel consumption enabled by regenerative braking. The sensitivity of tractive-fuel consumption to reductions in the vehicle parameters is changed significantly by regenerative braking, and this is quantified as well. The mathematical formulations of the study facilitate physical understanding of the results.