Hybrid Synergy Drive, (HSD) is a set of hybrid car technologies developed by Toyota. HSD technology is used in the Prius, Highlander Hybrid sport-utility vehicle (SUV), 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. Toyota describes HSD-equipped vehicles as having E-CVT (Electronically-controlled Continuously Variable Transmission). We can classify the HSD-equipped vehicles as ―combined HEV‖. 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 (e.g. 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.
1. Theory of operation
HSD replaces a normal geared transmission with an electromechanical 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 (MG), 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. Each Motor-Generator (MG) can convert electricity to motion (mechanical power) 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. The HSD works by shunting electrical power between the two motor generators and the battery pack to even out the load on the gasoline engine. Since 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 its peak load: 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. The following graph shows the difference between a shift-gear ICE vehicle and a vehicle with HSD.
Red line = acceleration of a vehicle with IC petrol engine Green line = acceleration of a combined HEV
An HSD car cannot operate without the computer, power electronics, and MG2, though in principle it could operate while missing the gasoline engine. In practice, HSD cars can be driven several kilometers without gasoline, as an emergency measure to reach a gas station.
2. The planetary gear unit of Toyota Prius II
The motor (motor-generator 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. Between the engine and the driveshaft, a "power split device" is mounted. This is...