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Hybrid Synergy Drive

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).

Phases of operation

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

• Engine start:
  To start the engine, MG1 is fed negative voltage, so that it acts as a starter motor. The engine is forced into forward motion. Because both motor generators are sized to drive the entire car, turning the engine does not stress the motors and the conventional starter motor sound is not heard: engine start is silent. Engine start can occur when the car is stopped or moving.
• Low gear (equivalent):
  When accelerating at low speeds in normal operation, the engine turns much more rapidly than the wheels, but does not develop as much torque as is needed. MG1 is forced rapidly backwards, and the computer pulls electricity from MG1. The electricity is shunted to MG2, adding torque at the driveshaft, so that the drive train develops power at low speed and high torque.
  High gear (equivalent): When cruising at high speed, the engine turns more slowly than the wheels, but develops more torque than is needed. The computer pulls electricity from MG2, reducing the torque available at the wheels. The electricity is shunted to MG1, which boosts the speed of the driveshaft. Because the engine supplies mechanical energy to the whole system, conservation of energy is not violated: the power that is shunted from MG2 to MG1 is less than the total power developed by the engine, and so power is delivered to the wheels.
• 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 breaking the electrical connection to both MG1 and MG2. Under this condition, MG1 is free running and no torque can be delivered to the wheels (MG1 rotates backwards when the engine rotates forward).
  Regenerative braking: by drawing power from MG2 and depositing it into the battery pack, the HSD can simulate normal compression braking while saving the power for future boost. The HSD system has a special transmission setting labelled 'B' (for Brake), that takes the place of a conventional automatic transmission's 'L' setting for engine braking on hills. If the battery is full, the system switches to conventional compression braking, drawing power from MG2 and shunting it to MG1, speeding the engine with throttle closed and so slowing the vehicle. 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.
• 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.

Performance

The Toyota Prius has decent, but not sport-car-like, acceleration but has extremely high mileage for a mid sized four-door sedan: 45 mpg (US) is typical of brief city jaunts; 55 mpg is not uncommon, especially for extended drives (which allows the engine to warm up fully). This is noticeably less than 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 with 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.

The Highlander Hybrid (also sold as the Kluger in some countries) offers better 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 compared with to the conventional 215 hp. Top speed for all Highlanders are limited to 112 mph. 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. It is currently offered in an SUV, the Ford Escape, though a hybrid Ford Fusion will be released with Ford's second-generation hybrid drivetrain in the future. 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).
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.

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 is a big difference in performance: an early-model Prius can achieve over 90 mph on a 6 degree upward slope, but after about 2,000 feet 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).

Hybrids available by Toyota
2000 Toyota Estima hybrid (Japanese market only)
2000 Toyota Prius US market
Toyota Kluger / Highlander Hybrid 2006 model
Toyota Camry Hybrid 2007 model, second generation Hybrid Synergy Drive
Toyota Estima / Previa hybrid minivan, second generation Hybrid Synergy Drive (Japanese market only)
Toyota Sienna (minivan) 2009 model
Toyota Prius (5 seat midsize) 2009 model year, third generation Hybrid Synergy Drive (weight and cost reduced by 50%), Lithium-ion batteries
Toyota Camry Hybrid 2012 model, third generation Hybrid Synergy Drive

Some other of Toyota's Current Vehicles
Aurion, Auris, Avalon, Avanza, Avensis, Corolla, Crown, Dyna, Estima, Innova, Matrix, Noah, Passo, Premio, Prius, Reiz, Sequoia, Sienta, Tarago, ToyoAce, Tundra, Vios, Vitz, Voxy, WISH, Yaris