Hybrid Synergy Drive

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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 Automobile. It is also used in the Nissan Altima Hybrid. It combines an Electric vehicle 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 Accelerator (car) and the Gear stick in an HSD car merely send electrical signals to a control Computer.

Lexus hybrid logo.png
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 Corporation 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 Internal combustion engine and transmission where the flywheel is replaced with an electric motor.



The Toyota HSD replaces a normal geared Transmission (mechanics) with an electromechanical system. Because an Internal combustion engine (ICE) delivers power best only over a small range of Torque and Speed, 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.

Power sources

high voltage unit of the HSD

In the "standard" car design the Alternator (auto) (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-generator[1] (designated MG1 and MG2, ~60 Hp total) with a computerized shunt system to control them, a mechanical power splitter that acts as a second Differential (mechanics), 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 front disk and rear drum 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.

MG1 and MG2

  • MG1 (motor generator 1) generates electrical power. MG1 recharges the EV battery and supplies electrical power to drive MG2. In addition, by regulating the amount of electrical power generated (thus varying MG1's internal resistance and rpm), MG1 effectively controls the transaxle's continuously variable transmission. MG1 also serves as the Engine starter.[2]
  • MG2 (motor generator 2) drives the vehicle. MG2 and the engine work together to drive the wheels. The addition of MG2's strong torque characteristics help achieve excellent dynamic performance, including smooth start-off and acceleration. During regenerative braking, MG2 converts kinetic energy into electrical energy, which is then stored in the EV battery.[2]


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 Filling 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 (mechanics); 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 gearing (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.[3]


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 Driving. 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

Hybrid Synergy Drive

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 brake: 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.


The Toyota Prius has modest acceleration but has extremely high efficiency for a mid sized four-door sedan: Usually significantly better than 40 mpg (US) is typical of brief city jaunts; 55 mpg is not uncommon, especially for extended drives at modest speeds (a longer drive allows the engine to warm up fully). This is approximately twice the fuel efficiency of a similarly equipped four-door sedan with a conventional power train. Not all of the extra efficiency of the Prius is due to the HSD system: the Atkinson cycle engine itself was also designed specifically to minimize engine drag via an offset Crankshaft to minimize piston drag during the Power stroke, and a unique intake system to prevent drag caused by Manifold vacuum versus the normal Otto cycle in most engines. Furthermore, the Atkinson cycle recovers more energy per cycle than the Otto because of its longer power stroke. The downside of the Atkinson cycle is much reduced torque, particularly at low speed; but the HSD has enormous low-speed torque available from MG2.

The Highlander Hybrid (also sold as the Kluger in some countries) offers better acceleration performance compared to its non-hybrid version. The hybrid version goes from 0–60 mph in 7.2 seconds, trimming almost a second off the conventional version's time. Net hp is 268 hp (200 kW) compared to the conventional 215 hp (160 kW). Top speed for all Highlanders is limited to 112 mph (180 km/h). Typical fuel economy for the Highlander Hybrid rates between 27 and 31 mpg. A conventional Highlander is rated by the EPA with 19 city, 25 highway mpg.

Ford Motor Company independently developed a system with key technologies similar to Toyota's HSD technology in 2004. As a result, Ford licensed 21 patents from Toyota in exchange for patents relating to emissions technology.[4] It is currently offered in an Sport utility vehicle, the Ford Escape, though a hybrid Ford Fusion will be released with Ford's second-generation hybrid drivetrain in the future.[5] The four-cylinder hybrid Escape achieves an increase in mileage, and is rated by the EPA with a combined 34 mpg, a 36% improvement over other similar sized SUVs from Subaru and Honda (Forester and CR-V, 25 mpg combined).[6]

There have been reports in the press of hybrid power trains not living up to the EPA fuel efficiency claims. Fundamentally this is due to the artificial and unrealistic EPA testing procedure that manufacturers have learned to "game". The EPA testing procedure fails to recognize the sensitivity of hybrid mileage to driving style. The mileage boost depends on using the gasoline engine as efficiently as possible, which requires:

  • extended drives, especially in winter: Heating the internal cabin for the passengers runs counter to the design of the HSD. The HSD is designed to generate as little waste heat as possible. In a conventional car, this waste heat in winter is usually used to heat the internal cabin. In the Prius, running the heater will then require the engine to continue running to generate cabin-usable heat. This effect is most pronounced by turning the climate control (heater) off when at a stop when the engine is running. Normally the HSD control system will shut the engine off as it is not needed, and will not start it again until the generator reaches a maximum speed.
  • moderate acceleration: Because hybrid cars can throttle back or completely shut off the engine during moderate, but not rapid, acceleration, they are more sensitive than conventional cars to driving style. Hard acceleration forces the engine into a high-power state while moderate acceleration keeps the engine in a lower power, high efficiency state (augmented by battery boost).
  • gradual braking: Regenerative brakes re-use the energy of braking, but cannot absorb energy as fast as conventional brakes. Gradual braking recovers energy for re-use, boosting mileage; hard braking wastes the energy as heat, just as for a conventional car. Use of the "B" (braking) selector on the transmission control is useful on long downhill runs to reduce heat and wear on the conventional brakes, but it does not recover additional energy[7]. Use of "B" constantly is discouraged by Toyota as it may promote excessive wear on certain gears[8].

Most HSD systems have batteries that are sized for maximal boost during a single acceleration from zero to the top speed of the vehicle; if there is more demand, the battery can be completely exhausted, so that this extra torque boost is not available. Then the system reverts to just the power available from the engine. This results in a large decline in performance under certain conditions: an early-model Prius can achieve over 90 mph (140 km/h) on a 6 degree upward slope, but after about 2,000 feet (610 m) of altitude climb the battery is exhausted and the car can only achieve 55–60 mph on the same slope (until the battery is recharged by driving under less demanding circumstances).


The basic design of the Toyota Hybrid System / Hybrid Synergy Drive has not changed since its introduction in the 1997 Japanese-market Toyota Prius, but there have been a number of refinements.

Toyota Hybrid System (THS)

The original Prius used shrink-wrapped 1.2 volt D cells; all subsequent THS/HSD vehicles have used custom 7.2 V battery modules mounted in a carrier. There has been a continuous, gradual improvement in specific capacity.

The Toyota Hybrid System relied on the voltage of the battery pack — between 276 and 288 V. The Hybrid Synergy Drive adds a DC to DC converter boosting the potential of the battery to 500 V or more. This allows smaller battery packs to be used, and more powerful motors.

Hybrid Synergy Drive (HSD)

Although not part of the HSD as such, all HSD vehicles from the 2004 Prius onwards have been fitted with an electric air-conditioning compressor, instead of the conventional engine-driven type. This removes the need to continuously run the engine when cabin cooling is required. Two Positive temperature coefficient heaters are fitted in the heater core to supplement the heat provided by the engine[9].

Vehicles such as the Lexus RX 400h and Toyota Highlander Hybrid added four-wheel drive operation by the addition of a third electric motor ("MGR") on the rear axle. In this system, the rear axle is purely electrically powered, and there is no mechanical link between the engine and the rear wheels. This also permits regenerative braking on the rear wheels. In addition, the Motor (MG2) is linked to the front wheel transaxle by means of a second Planetary gearset, thereby making it possible to increase the power density of the motor.

Ford Motor Company has developed a similar hybrid system, introduced in the Ford Escape Hybrid.

The latest addition to the family of Hybrid Synergy Drivetrains is used in the Lexus GS 450h / LS 600h. This system uses two clutches (or brakes) to switch the second motors gear ratio to the wheels between a ratio of 3.9 and 1.9, for low and high speed driving regimes respectively. This decreases the power flowing from MG1 to MG2 (or vice versa) during higher speeds. The electrical path is only about 70% efficient, thus decreasing the power flow there increases the overall performance of the transmission. The second planetary gearset is extended with a second carrier and sun gear to a ravigneaux-type gear with four shafts, two of which can be held still alternatively by a brake/clutch.

Third generation

The third generation hybrid system from Toyota is expected to debut in the 2010 Toyota Prius, due out in late 2009 or early 2010. Toyota CEO Katsuaki Watanabe said in a February 16, 2007 interview that Toyota was "aiming at reducing, by half, both the size and cost of the third-generation hybrid system."[10] However, plans to replace Nickel metal hydride battery batteries with Lithium-ion batteries have since been canceled or delayed.[11] Lithium-ion batteries have a higher energy capacity-to-weight ratio, but cost more, don't last as long as NiMH, and operate at higher temperatures, and are subject to thermal instability if not properly manufactured and controlled, raising safety concerns.[11][12]

Hybrid Synergy Drive
Hybrid Synergy Drive
Hybrid Synergy Drive

List of vehicles with HSD

¹Lexus Hybrid Drive


Some early non-production Plug-in hybrid electric vehicle Electric vehicle conversion have been based on the version of HSD found in the 2004 and 2005 model year Prius. Early Lead-acid battery conversions by CalCars have demonstrated 10 miles (16 km) of ev-only and 20 miles (32 km) 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 (56 km) 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 Nickel metal hydride 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 Hymotion.


As of autumn 2005, the Antonov Automotive Technology BV Plc company has sued Toyota, the Lexus brand mother company, over alleged patent infringement relating to key components in the RX 400h's drivetrain and the Toyota Prius hybrid compact car. The case has been pending in secret since April 2005, but settlement negotiations did not bring a mutually acceptable result. Antonov eventually took legal recourse in the German court system, where decisions are usually made relatively swiftly. The patent holder seeks to impose a levy on each vehicle sold, which could make the hybrid SUV less competitive. Toyota fought back by seeking to officially invalidate Antonov's relevant patents. The court motion in Microsoft Word document format can be read here.

On 1st September 2006 Antonov announced: The Board of Antonov plc announces that the Federal Patent Court in Munich has not upheld the validity of the German part of Antonov's patent (EP0414782) against Toyota. A few days earlier, a court in Düsseldorf had ruled that the Toyota Prius driveline breached the Antonov hybrid CVT patent. Equivalent patents are still in force in various countries worldwide. The position is therefore at an impasse, with the Toyota Prius probably breaching the Antonov hybrid CVT patents outside Europe, Antonov PLC lacking the financial muscle to enforce their patents, and Toyota prepared to try to get the Antonov patents cancelled elsewhere if Antonov tries to enforce them. [13]

See also

  • Comparison of Toyota hybrids
  • Hybrid car
  • Inverter (electrical)
  • IGBT transistor
  • Variable-frequency drive
  • Global Hybrid Cooperation
  • Integrated Motor Assist
  • List of hybrid vehicles


  1. All electric motors can be used as generators (and vice-versa) so the term motor-generator is normally used only when the same device is being used for both purposes.
  2. 2.0 2.1 Document:
  3. Bill Siuru. "Synergy Drive: Why Toyota’s Hybrids Rock". Green Car Journal. Yahoo. http://autos.yahoo.com/green_center-article_24/. Retrieved on 2008-03-12. 
  4. NussbaumOnDesign Is Ford Innovative? Part Two. - BusinessWeek
  5. Media.Ford.Com: Ford Hybrid Vehicle Leadership Continues With New Technology And Taxi Fleet In Nyc
  6. Gas Mileage of 2007 Sport Utility Vehicle
  7. http://www.techno-fandom.org/~hobbit/cars/b-mode.html
  8. 2007 Prius Owners Manual
  9. "6 - Body Electrical" (PDF). Toyota Hybrid System - Course 071. Toyota Technical Training. p. 1. http://www.autoshop101.com/forms/Hybrid06.pdf. Retrieved on 2008-10-15. 
  10. "Toyota's Bid for a Better Battery". http://www.businessweek.com/magazine/content/07_10/b4024075.htm. 
  11. 11.0 11.1 Halvorson, Bengt. Li-ion Not Ready for Prius. BusinessWeek, June 18, 2007. Retrieved on 2007-08-07.
  12. DailyTech - Toyota Shuns Lithium-ion Batteries for Next Gen Prius
  13. Toyota gets Antonov hybrid CVT patent cancelled in Europe.

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