Hybrid Synergy Drive, (HSD) is a set of hybrid car technologies developed by Toyota and used in that company's Prius, Highlander Hybrid, Camry Hybrid, Lexus RX 400h, Lexus GS 450h, and Lexus LS 600h/LS 600hL automobiles. It 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.
HSD is a refinement of the original Toyota Hybrid System (THS) used in the 1997–2003 Toyota Prius. As such it is occasionally referred to as THS II. The name was changed in anticipation of its use in vehicles outside the Toyota brand (Lexus; the HSD-derived systems used in Lexus vehicles were termed Lexus Hybrid Drive since 2006). The Lexus Hybrid Drive system has since been touted for its increase in vehicle power as well as environmental and efficiency benefits.
When required to classify the transmission type of an HSD vehicle (such as in standard specification lists or for regulatory purposes), Toyota describes HSD-equipped vehicles as having E-CVT (Electronically-controlled Continuously Variable Transmission).
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.
The Toyota HSD replaces a normal geared transmission with an electromechanical system. Because an internal combustion engine (ICE) delivers power best only over a small range of torques and speeds, the crankshaft of the engine is usually attached to an automatic or manual transmission by a clutch or torque converter that allows the driver to adjust the speed and torque that can be delivered by the engine to the torque and speed needed to drive the wheels of the car.
In the "standard" car design the alternator (AC generator) and starter (DC motor) are considered accessories that are attached to the gasoline/diesel engine which normally drives a transmission to power the wheels propelling the vehicle. A battery is used only to start the car's gasoline/diesel 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* (designated MG1 and MG2, ~60Hp total) with a computerized shunt system to control them, a blank">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 gasoline engine may or may not be running at startup. When higher speeds, faster acceleration or more power for charging the batteries is needed the gasoline engine is started by the motor-generator (acting as a starter). These features allow the gasoline engine 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 disk 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. The mechanical gearing design of the system allows the mechanical power from the gas/diesel engine 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 code running appropriate actuators 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 gasoline 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 in Toyota manuals; sometimes called "MG-T" for "Torque") is mounted on the drive shaft, and thus couples torque into or out of the drive shafts: feeding electricity into MG2 adds torque at the wheels. The engine end of the drive shaft has a second differential; one leg of this differential is attached to the gasoline engine and the other leg is attached to a second motor-generator (MG1 in Toyota manuals; sometimes "MG-S" for "Speed"). The differential relates the rotation speed of the wheels to the rotation speeds of the engine and MG1, with MG1 used to absorb the difference between wheel and engine speed. The differential is an epicyclic gear set (also called a "power split device"); that and the two motor-generators are all contained in a single transaxle housing that is bolted to the engine. Special couplings and sensors monitor rotation speed of each shaft and the total torque on the drive shafts, for feedback to the control computer. http://autos.yahoo.com/green_center-article_24/
The HSD drive works by shunting electrical power between the two motor generators, running off the battery pack, to even out the load on the gasoline engine. Since a power boost from the electrical motors is available for periods of rapid acceleration, the gasoline/diesel engine can be down sized to match only the average load on the car, rather than sized by peak power "needs" for acceptable acceleration. The smaller gasoline/diesel engine can be designed to run more efficiently. Furthermore, during normal operation the engine can be operated at or near its ideal speed and torque level for power, economy, or emissions, with the battery pack absorbing or supplying power as appropriate to balance the demand placed by the driver. During traffic stops the ICE 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 gasoline engine design give the HSD powered car significant efficiency advantages--particularly in city driving.
