Hybrid vehicles achieve reduced fuel consumption by incorporating in the drive train, in addition to an internal combustion (IC) engine, both an energy storage device and a means of converting the stored energy into mechanical motion. Some hybrids are also able to convert mechanical motion into stored energy. In its most general sense, the storage device can be a battery, flywheel, compressible fluid, elastomer, or ultra capacitor.

The means of converting energy between storage and mechanical motion is through the use of one or more motors/ generators (e.g., electric, pneumatic, hydraulic). In motor mode, these devices convert stored energy into mechanical
motion to propel the vehicle, and in generator mode, these devices convert vehicle motion into stored energy by providing part of the vehicle braking function (regeneration).
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Although each vehicle manufacturer has a proprietary way of defining very precisely how its vehicle must perform, it is assumed here that the following parameters will remain essentially constant as the technologies that reduce fuel consumption are considered:

• Interior passenger volume;
• Trunk space, except for hybrids, where trunk space may be compromised;
• Acceleration, which is measured in a variety of tests, such as time to accelerate from 0 to 60 mph, 0 to 30, 55 to 65 (passing), 30 to 45, entrance ramp to highway, etc.;
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