Cars - Hybrid Synergy Drive

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	Cars - Hybrid Synergy Drive

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, and Lexus RX 400h automobiles. It combines the characteristics of an electric drive and a continuously variable transmission, using electricity and transistors in place of toothed gears. 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 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.

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 Advanced Hybrid System 2 is similar in that it combines the power from a single engine and two motors. To contrast Honda's Integrated Motor Assist uses a more traditional ICE and transmission where the flywheel is replaced with an electric motor.

Some early non-production Plug-in hybrid electric vehicle conversions have been based on the version of HSD found in the 2004 and 2005 model year Prius. Early Pba conversions by have demonstrated 10 miles of ev-only and 20 miles of double mileage mixed-mode range. A company planning to offer conversions to consumers named will be using Li-ion batteries and have 35 miles of electric range. Both of these systems leave the existing HSD system mostly unchanged and could be similarly applied to other hybrid powertrain flavors by simply replacing the stock NiMH batteries with a higher capacity battery pack and of course a charger to refill them for about $0.03 per mile from standard household outlets. Another provider of a plug-in module for the Toyota Prius is .

Theory of operation

HSD replaces a normal geared transmission with an electronic system. All car powertrains drive a driveshaft that turns the drive wheels of the car. Because an internal combustion engine delivers energy best only over a small range of torque and speed the crankshaft of the engine is usually attached to a switchable gear train that matches the needed torque at the wheels to the torque that can be delivered by the engine.

HSD replaces the gear box, alternator and starter motor with a pair of electrical motor-generators, a computerized shunt system to control them, a that acts as a second differential, and a battery pack that serves as an energy reservoir. A motor-generator is a transducer that converts electricity to motion or vice-versa. The mechanical connections of the system allow the computer to convert mechanical power from the engine between three forms: extra torque at the wheels (under constant rotation speed), extra rotation speed at the wheels (under constant torque), and electricity. This achieves the benefits of a continuously variable transmission, except that the torque/speed conversion uses electricity rather than direct mechanical connection. A HSD car cannot operate without the computer and both motor-generators, though in principle it could operate while missing either the battery pack or the gasoline engine (but not both). In practice, HSD cars can be driven several miles while out of gas, as an emergency measure to get to a gas station.

One of the motor-generators (MG2 in Toyota manuals; sometimes called "MG-T" for "Torque") is mounted on the driveshaft, and thus couples torque into or out of the driveshaft: feeding electricity into MG2 adds torque at the wheels. The engine end of the driveshaft 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 ensures that the rotation speed of the wheels is the sum of the rotation speeds of the engine and MG1, so MG1 is used to change the wheel (or engine) speed. In Prius models, the differential is an epicyclic sun gear design, and the two motor generators and differential are all contained in a single housing that is bolted to the engine. Special couplings and sensors monitor rotation speed of each shaft and the total torque on the driveshaft, for feedback to the control computer.

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