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Hybrid Synergy Drive
(HSD) is a set of hybrid car technologies developed by Toyota and used in the Lexus RX 400h, Lexus GS 450h, and Lexus LS 600h/LS 600hL 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.
he Lexus Hybrid Drive system has 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), thye describe 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.
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 CalCars 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 EDrive systems will be using Valence 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.
Theory of operation
high voltage unit of the HSDHSD 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 mechanical power splitter 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. An HSD car cannot operate without the computer and both motor-generators, though in principle it could operate while missing the gasoline engine. In practice, HSD cars can be driven a mile or two without gasoline, as an emergency measure to reach 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 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 gearset (also called a “power split device”); that and the two motor-generators 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.
The drive works by shunting electrical power between the two motor generators and the battery pack to even out load on the gasoline engine. Because a power boost is available for periods of acceleration, the gasoline engine can be sized to match only the average load on the car, rather than the peak load on the car: this saves fuel because smaller engines are more power efficient. Furthermore, during normal operation the gasoline engine can be operated at 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.
More information on Toyota Hybrid Synergy Drive (HSD) can be found here.
Some Hybrid models are:
Lexus RX 400h 2006 model year, second generation Hybrid Synergy Drive
Lexus GS 450h 2007 model, second generation Hybrid Synergy Drive
Lexus LS600hL 2008 model Luxury car, expected to go on sale June 2007
Other models are
Compact: IS 200 / 300, IS 250 / 300 / 350 / 220d
Mid-size: ES 250 ,ES 300, ES 300, ES 300 / 330, ES 350, GS 300, GS 300 / 400 / 430, GS 300 / 430 / 450h / 350
Full-size: LS 400, LS 400, LS 430, LS 460 / 460L / 600h / 600hL
Crossover: RX 300, RX 330 / 350 / 400h, RX 350 / 400h