Wednesday, October 21, 2009

Hybrid Synergy Drive

Hybrid Synergy Drive, (HSD) is a set of hybrid car technologies developed by Toyota and used in the company's Prius, Highlander Hybrid, Camry Hybrid, It is also used in the Nissan Altima Hybrid. HSD technology produces a 'full hybrid' vehicle and allows the car to perform on the electric motor only as opposed to other brand hybrids which cannot and are considered 'mild hybrids'; it also combines an electric drive and a continuously variable transmission.

The Synergy Drive is a drive-by-wire system with no direct mechanical connection between the engine and the engine controls: both the gas pedal/accelerator and the gearshift lever in an HSD car merely send electrical signals to a control computer.

General Motors and DaimlerChrysler's Global Hybrid Cooperation is similar in that it combines the power from a single engine and two motors. In contrast, Honda's Integrated Motor Assist uses a more traditional ICE and transmission where the flywheel is replaced with an electric motor.

In the "standard" car design the alternator (AC generator) and starter (DC motor) are considered accessories that are attached to the internal combustion engine (ICE) which normally drives a transmission to power the wheels propelling the vehicle. A battery is used only to start the car's internal combustion engine and run accessories when the engine is not running.

The alternator is used to recharge the battery and run the accessories when the engine is running. HSD replaces the gear box (transmission), alternator and starter motor with a pair of powerful motor-generators[1] (designated MG1 and MG2, ~60 Hp total) with a computerized shunt system to control them, a mechanical power splitter that acts as a second differential, and a battery pack that serves as an energy reservoir. The motor-generator uses power from the battery pack to propel the vehicle at startup and at low speeds or under acceleration. The ICE may or may not be running at startup.

When higher speeds, faster acceleration or more power for charging the batteries is needed the ICE is started by the motor-generator (acting as a starter). These features allow the ICE to normally be turned off for traffic stops—accessory power (including air conditioning if needed) is normally provided by the battery pack.

When a moving vehicle operator wants the vehicle to slow down the initial travel of the brake pedal engages the motor-generator(s) into generator mode converting much of the forward motion into electrical current flow which is used to recharge the batteries while slowing down the vehicle. In this way the forward momentum regenerates (or converts) much of the energy used to accelerate the vehicle back into stored electrical energy.

(See regenerative braking) Harder braking action engages standard front disk and rear drum brakes which are also provided for faster stops and emergency use. This wastes energy which could have been recovered and is discouraged for normal use.