[Success Story]
Setting The Standard For Hybrid Cars
Synergy between gasoline-engine, electroic-motor, and battery technologies vaults the Prius hybrid gasoline-electric vehicle to high fuel savings and low emissions.
With its sports-car look, avionics-like dashboard display, and an eerily silent and smooth start, it's no wonder that the Toyota Prius hybrid gasolineelectric vehicle is one of the fastest-selling cars of its type (Fig. 1) (see "Earning Market Share And Accolades," p. 44).
Aside from these assets, though, it features one discriminating characteristic—it's a gas miser. The U.S. Environmental Protection Agency (EPS) gives it a gas rating of 61 miles/gallon (mpg) on the streets and 50 mpg on the highway (under ideal test conditions). Realistically, a driver can expect a combined street/highway figure of 45 to 50 mpg.
Typically, hybrid cars start by using gasoline-engine power and then revert to battery power to boost acceleration. The Prius takes a different tack, though. It starts out by using battery power, then switches over to the gasoline engine at speeds of greater than 20 mph. Although the Prius has great pickup, drivers should not "floor" the gas pedal from a complete stop, for maximum gasoline savings (see "My Test Drive Of The Toyota Prius" at www.elecdesign.com, Drill Deeper 11346).
Coming on the heels of the highest gasoline prices ever experienced by the U.S. and the rest of the world, the Prius' gasoline-consumption figures are a welcome sign in an otherwise gloomy gasoline market.
HYBRID SYSTEM TECHNOLOGY The Prius design is based on what Toyota calls its Hybrid Synergy Drive concept, which aims for synergy between the electric motor's and gasoline engine's power. This concept makes it possible to maximize the car's power and environmental performance.
The latest Prius models operate under Toyota's Total Hybrid System-II (THS-II) technology. Each successive generation of the THS-II system improves on gasoline fuel efficiency and reduced emissions. This includes a four-cylinder in-line 1.5-l engine, a high-voltage nickel-metal-hydride (NiMH) battery, a clever hybrid transaxle with a planetary gear system, and a sophisticated engine control unit (Fig. 2).
A BLENDING OF TWO APPROACHES Basically, two types of hybrid car systems exist: series and parallel. Each has its own set of pros and cons. Unlike other hybrid gasoline-electric cars, the Prius uses a combination of both, maximizing the strength of each and complementing their weaknesses.
In a series hybrid system, the gasoline engine runs a generator, and the generated electricity enables an electric motor to drive a car's wheels (Fig. 3). The low-output-power engine used here operates at a practically constant speed in its most effective range, enabling it to efficiently recharge the battery while the car is in motion.
In a parallel hybrid system, both the gasoline engine and the electric motor directly drive the car's wheels. In addition to supplementing the motive force of the engine, the electric motor can serve as a generator to recharge the battery while the car is in motion.
The Prius uses two permanent-magnet 500-V ac motor generators—MG1 and MG2. Both motors are powered by an inverter assembly that changes the battery's dc voltage to a high ac voltage. Joining the engine and both generators are a hybrid transaxle and a planetary gear system (Fig. 4).
MG1 and MG2, which produce 67 hp of output from 1040 to 5600 rpm, have more than enough torque to get the car moving without the aid of the gasoline engine. The Prius' engine runs at a maximum speed of only 5000 rpm, providing it with a 75-hp output.
First, electric power is supplied from the high-voltage battery to MG2 to drive the wheels. While the wheels of the engine are being driven by the engine via the planetary gears, MG1 is rotated by the engine via the gears to supply the generated electricity to MG2. Next, the engine rotates MG1 through the planetary gears to charge the high-voltage battery. When the car decelerates, kinetic energy from the wheels is recovered and converted to electrical energy, which is used to recharge the high-voltage battery via MG2 (Fig. 5). This is known as "regenerative braking."
At the heart of the Prius lies a power-split device—a planetary gearbox that hooks the electrical engine, generator, and electric motors together (Fig. 6). It lets the car operate like a parallel hybrid system where an electric motor can power the car by itself, the gasoline engine can power the car by itself, or both can power the car together.
This power-split device enables the car to operate like a series hybrid. Thus, the gasoline engine can run independently from the car's speed, charging the high-voltage battery or providing power to the wheels as necessary. It also acts as a continuously variable transmission, eliminating the need for a manual or automatic transmission. Because the power-split device allows the generator to start the gasoline engine, the car doesn't need a starter.
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