increases when charge level of HVB is too much to accept regen electricity from MG2, or increasing effort from driver pushing the brake pedal) Regenerative braking wheels -> MG2 -> HVB Hard braking Front disk/rear drum (rear disk in UK) -> wheels All disk -> wheels (2010 and newer, except 2012-current Prius c, which uses front disk, rear drum). MG1 and MG2 MG1 (Primary motor-generator): A motor to start the ICE and a generator to generate electrical power for MG2 and to recharge the high voltage traction battery, and, through a DC-to-DC converter, to recharge the 12 volt auxiliary battery. By regulating the amount of electrical power generated (by varying MG1's mechanical torque and speed), MG1 effectively controls the transaxle's continuously variable transmission. MG2 (Secondary motor-generator): Drives the wheels and regenerates power for the HV battery energy storage while braking the vehicle. MG2 drives the wheels with electrical power generated by the engine-driven MG1 and/or the HVB. During regenerative braking, MG2 acts as a generator, converting kinetic energy into electrical energy, storing this electrical energy in the battery. Transmission The mechanical gearing design of the system allows the mechanical power from the ICE to be split three ways: extra torque at the wheels (under constant rotation speed), extra rotation speed at the wheels (under constant torque), and power for an electric generator. A computer running appropriate programs controls the systems and directs the power flow from the different engine + motor sources. This power split achieves the benefits of a continuously variable transmission (CVT), except that the torque/speed conversion uses an electric motor rather than a direct mechanical gear train connection. An HSD car cannot operate without the computer, power electronics, battery pack, and motor-generators, though in principle it could operate while missing the internal combustion engine. (See: Plug-in hybrid) In practice, HSD equipped cars can be driven a mile or two without gasoline, as an emergency measure to reach a gas station. An HSD transaxle contains a planetary gear set that adjusts and blends the amount of torque from the engine and motor(s) as it's needed by the front wheels. It is a sophisticated and complicated combination of gearing, electrical motor-generators, and computer-controlled electronic controls. One of the motor-generators, MG2, is connected to the output shaft, and thus couples torque into or out of the drive shafts; feeding electricity into MG2 adds torque at the wheels. The engine end