usually used to heat the internal cabin. In the Prius, running the heater requires the engine to continue running to generate cabin-usable heat. This effect is most noticeable when turning the climate control (heater) off when the car is stopped with the engine running. Normally the HSD control system will shut the engine off as it is not needed, and will not start it again until the generator reaches a maximum speed. moderate acceleration: Because hybrid cars can throttle back or completely shut off the engine during moderate, but not rapid, acceleration, they are more sensitive than conventional cars to driving style. Hard acceleration forces the engine into a high-power state while moderate acceleration keeps the engine in a lower power, high efficiency state (augmented by battery boost). gradual braking: Regenerative brakes re-use the energy of braking, but cannot absorb energy as fast as conventional brakes. Gradual braking recovers energy for re-use, boosting mileage; hard braking wastes the energy as heat, just as for a conventional car. Use of the "B" (braking) selector on the transmission control is useful on long downhill runs to reduce heat and wear on the conventional brakes, but it does not recover additional energy. Constant use of "B" is discouraged by Toyota as it "may cause decreased fuel economy" compared to driving in "D". Most HSD systems have batteries that are sized for maximal boost during a single acceleration from zero to the top speed of the vehicle; if there is more demand, the battery can be completely exhausted, so that this extra torque boost is not available. Then the system reverts to just the power available from the engine. This results in a large decline in performance under certain conditions: an early-model Prius can achieve over on a 6 degree upward slope, but after about of altitude climb the battery is exhausted and the car can achieve only 55–60 mph on the same slope. (until the battery is recharged by driving under less demanding circumstances) Prius Platform Generations The design of the Toyota Hybrid System / Hybrid Synergy Drive has now had four generations since the original 1997 Japanese-market Toyota Prius. The power train has the same basic features, but there have been a number of significant refinements. The schematic diagrams illustrate the paths of power flow between the two electric motor-generators MG1 & MG2, the Internal Combustion Engine (ICE), and the front wheels