demand placed by the driver. During traffic stops the internal combustion engine can even be turned off for even more economy. The combination of efficient car design, regenerative braking, turning the engine off for traffic stops, significant electrical energy storage and efficient internal combustion engine design give the HSD powered car significant efficiency advantages—particularly in city driving. Phases of operation The HSD operates in distinct phases depending on speed and demanded torque. Here are a few of them: Battery charging: The HSD can charge its battery without moving the car, by running the engine and extracting electrical power from MG1. The power gets shunted into the battery, and no torque is supplied to the wheels. The onboard computer does this when required, for example when stopped in traffic or to warm up the engine and catalytic converter after a cold start. Engine start: To start the engine, power is applied to MG1 to act as a starter. Because of the size of the motor generators, starting the engine is relatively fast and requires relatively little power from MG1. Additionally, the conventional starter motor sound is not heard. Engine start can occur while stopped or moving. Reverse gear (equivalent): There is no reverse gear as in a conventional gearbox: the computer reverses the phase sequence to AC motor-generator MG2, applying negative torque to the wheels. Early models did not supply enough torque for some situations: there have been reports of early Prius owners not being able to back the car up steep hills in San Francisco. The problem has been fixed in recent models. If the battery is low, the system can simultaneously run the engine and draw power from MG1, although this will reduce available reverse torque at the wheels. Neutral gear (equivalent): Most jurisdictions require automotive transmissions to have a neutral gear that decouples the engine and transmission. The HSD "neutral gear" is achieved by turning the electric motors off. Under this condition, the planetary gear is stationary (if the vehicle wheels are not turning); if the vehicle wheels are turning, the ring gear will rotate, causing the sun gear to rotate as well (the engine inertia will keep the carrier gear stationary unless the speed is high), while MG1 is free to rotate while the batteries do not charge. The owners manual warns that Neutral gear will eventually drain the battery, resulting in "unnecessary" engine power to recharge