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, Lexus RX 400h, Lexus GS 450h, and Lexus LS 600h/LS 600hL automobiles. It is also used in the Nissan Altima Hybrid. 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 HYBRID SYNERGY DRIVE power unit integrates the strong elements of an electric motors and a gas/petrol engine. It is not merely a package that has an electric motor and a gas/petrol engine on board. Toyota applied cutting edge technologies based on our latest research to integrate the dual power sources in the most ideal way.
There are other models called "hybrid vehicles" in existence, but Toyota is fully confident that our HYBRID SYNERGY DRIVE is the most advanced hybrid powertrain in the world. In fact, HYBRID SYNERGY DRIVE is one of the very few systems in the world in 2005 that fully complies with the UN's definition of a "hybrid vehicle".

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.

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 program 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.

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 internal combustion 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.

Phases of operation

The HSD operates in distinct phases depending on speed and demanded torque. Here are a few of them:

  • Engine start: To start the engine, power is applied to MG1 to act as a starter. Because of the size of the motor generators, starting the engine requires relatively little power from MG1 and the conventional starter motor sound is not heard. Engine start can occur while stopped or moving.
  • Low gear (equivalent): When accelerating at low speeds in normal operation, the engine turns more rapidly than the wheels but does not develop sufficient torque. The extra engine speed is fed to MG1 acting as a generator. The output of MG1 is fed to MG2, acting as a motor and adding torque at the driveshaft.
  • High gear (equivalent): When cruising at high speed, the engine turns more slowly than the wheels but develops more torque than needed. MG2 then runs as a generator to remove the excess engine torque, producing power that is fed to MG1 acting as a motor to increase the engine speed. In steady state, the engine provides all of the power to propel the car unless the engine is unable to supply it (as during heavy acceleration, or driving up a steep incline at high speed). In this case, the battery supplies the difference. Whenever the required propulsion power changes, the battery quickly balances the power budget, allowing the engine to change power relatively slowly.
  • Reverse gear: There is no reverse gear as in a conventional gearbox: the computer feeds negative voltage to MG2, applying negative torque to the wheels. Early models did not supply enough torque for some situations: there have been reports of early Prius owners not being able to back the car up steep hills in San Francisco. The problem has been fixed in recent models. If the battery is low, the system can simultaneously run the engine and draw power from MG1, although this will reduce available reverse torque at the wheels.
  • Silent operation: At slow speeds and moderate torques the HSD can drive without running the gasoline engine at all: electricity is supplied only to MG2, allowing MG1 to rotate freely (and thus decoupling the engine from the wheels). This is popularly known as "Stealth Mode." Provided that there is enough battery power, the car can be driven in this silent mode for some miles even without gasoline.
  • Neutral gear: Most jurisdictions require automotive transmissions to have a neutral gear that decouples the engine and transmission. The HSD "neutral gear" is achieved by turning the engine off. Under this condition, the planetary gear is stationary (if the vehicle wheels are not turning); if the vehicle wheels are turning, the ring gear will rotate, causing the sun gear to rotate as well (the engine inertia will keep the carrier gear stationary unless the speed is large), while MG1 freewheels so no power is dissipated.
  • Regenerative braking: By drawing power from MG2 and depositing it into the battery pack, the HSD can simulate the deceleration of normal compression braking while saving the power for future boost. The regenerative brakes in an HSD system absorb a significant amount of the normal braking load, so the conventional brakes on HSD vehicles are undersized compared to brakes on a conventional car of similar mass.
  • Compression braking: The HSD system has a special transmission setting labelled 'B' (for Brake), that takes the place of a conventional automatic transmission's 'L' setting, providing engine braking on hills. This can be manually selected in place of regenerative braking. During braking when the battery is approaching potentially damaging high charge levels, the electronic control system automatically switches to conventional compression braking, drawing power from MG2 and shunting it to MG1, speeding the engine with throttle closed and decelerating the vehicle.
  • Electric boost: The battery pack provides a reservoir of energy that allows the computer to match the demand on the engine to a predetermined optimal load curve, rather than operating at the torque and speed demanded by the driver and road. The computer manages the energy level stored in the battery, so as to have capacity to absorb extra energy where needed or supply extra energy to boost engine power.
  • Battery charging: The HSD can charge its battery without moving the car, by running the engine and extracting electrical power from MG1. The power gets shunted into the battery, and no torque is supplied to the wheels.