Hybrid Synergy Drive

Hybrid Synergy Drive (HSD) is a set of hybrid car technologies developed by Toyota and used in that company's Prius and Lexus RX 400h automobiles. It combines the characteristics of an electric drive and a continuously variable transmission, using electricity and transistors to 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.

Theory of operation

HSD replaces a normal geared transmission with an electronic system. All car power trains 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 with a pair of electrical motor-generators, a computerized shunt system to control them, 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. A HSD car cannot operate without the computer and both motor-generators, though in principle it could operate while missing either the battery pack or the gasoline engine (but not both). In practice, HSD cars can be driven several miles while out of gas, as an emergency measure to get to a gas station.

One of the motor-generators (MG2 in Toyota manuals; here "MG-T" for "Torque") is mounted on the driveshaft, and thus couples torque into or out of the driveshaft: feeding electricity into MGT adds torque at the wheels. The engine end of the driveshaft has a second differential; one leg of the differential goes to the gasoline engine and the other leg goes to a second motor generator (MG1 in Toyota manuals; here "MG-S" for "Speed"). The HSD differential ensures that the rotation speed of the wheels is the sum of the rotation speeds of the engine and MG-S, so MG-S is used to change the wheel (or engine) speed. In Prius models, the differential is a sun gear design, and the two motor generators and differential 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.

Phases of operation

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

• Engine start: MGS is fed negative voltage, so that it attempts to drive the wheels backwards. The wheel torque is canceled by a forward voltage fed to MG-T. Because the differential forces the speed of the wheels (zero) to be the sum of the speeds of MG-S and the engine, 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.

• Low gear: When accelerating from a stop in normal operation, the engine turns much more rapidly than the wheels, but does not develop as much torque as is needed. MG-S is forced rapidly backwards, and the computer pulls electricity from MG-S. The electricity is shunted to MG-T, adding torque at the driveshaft, so that the drive train develops power at low speed and high torque.

• High gear: 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 MG-T, reducing the torque available at the wheels. The electricity is shunted to MG-S, 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 MG-T to MG-S 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 MG-T, 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.

• Silent operation: At slow speeds and moderate torques the HSD can drive without running the gasoline engine at all: electricity is supplied only to MG-T, allowing MG-S to rotate freely (and thus decoupling the engine from the wheels). 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 places 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 MG-S and MG-T. Under this condition, MG-S is free running and no torque can be delivered to the wheels (MG-S rotates backwards when the engine rotates forward).

• Regenerative braking: by drawing power from MG-T and depositing it into the battery pack, the HSD can simulate normal compression braking while saving the power for future boost. The Prius 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 Prius switches to conventional compression braking, drawing power from MG-T and shunting it to MG-S to drive the engine rapidly forward. The regenerative brakes in a HSD system absorb a significant amount of the normal braking load, so the conventional brakes on a Prius 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 MG-S. 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 is typical of brief city jaunts; 55 MPG is not uncommon, especially for extended drives (which allow the engine to warm up fully). This is about 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 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.

Ford motor company has licensed the HSD technology from Toyota, and manufactures a hybrid SUV, the Ford Escape. The six-cylinder hybrid Escape achieves good but not spectacular increases in mileage: 28–30 MPG, which is comparable to a conventional four-cylinder SUV such as the Honda CRV.

There have been reports in the press of hybrid power trains not living up to fuel efficiency claims. This is due in part to 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: HSD cars such as the Prius do not shut off the gasoline engine until the oil temperature reaches a set point. Brief drives do not allow the engine to warm up, burning extra fuel per mile traveled

• 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).

See also

• Hybrid car
• Advanced Hybrid System 2