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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.

The fuel economy of an automobile is a highly complex function of the detailed characteristics of the vehicle and its subsystems (particularly the engine, transmission and drivetrain), as well as being dependent on the manner in which the vehicle is driven. For existing vehicles, automotive manufacturers utilize laboratory test procedures to evaluate fuel economy. However, during new-vehicle design, and to assess the fuel economy potential of new technologies, computer programs that simulate the operation of the vehicle system over prescribed driving schedules are used. Of particular interest are the integrated fuel consumptions on the EPA Urban and Highway driving schedules since these are subject to Federal regulation. Since neither detailed subsystem test data nor simulation programs are typically used by those outside the automotive industry, the physics of fuel economy is not always well understood.