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.
The electric motor connects to the ring gear of the gear set. It also links directly to the differential that drives the wheels. Therefore, the rate of spin of the electric motor and the ring gear determines the car's speed.
The generator is connected to the sun gear of the gear set, and the engine connects to the planet carrier. The ring gear's speed depends on all three components, which must work together at all times to control the output speed.
A proprietary 32-bit microprocessor unit is used for the engine-control module (ECM), the high-voltage electronic-control unit (ECU), the battery's ECU, and the skid-control ECU. This represents an upgrade from earlier 16-bit designs. Intercommunication takes place over a controller-area-network bus.
AN ADVANCED BATTERY DESIGN Consisting of 168 1.2-V dc cells, the NiMH battery is interconnected using 28-cell modules, for a nominal voltage of 201.6 V dc. It's housed in a crash-resistant case located under the backseat of the car. The constantly improving battery is now in its third generation. (The battery used for Toyota's Lexus RH 400h and Highlander sports utility vehicles is a fourthgeneration design.) Located under the hood are the engine, the inverter assembly, and the hybrid transaxle (Fig. 7).
Toyota says it has always been asked about the battery's lifetime and replacement cost. For one thing, the battery pack and all hybrid components are considered "life-of-the-car" components. They're warrantied for eight years or 100,000 miles of driving, whichever comes first. That warranty is even better (10 years and 150,000 miles) in U.S. states with California-type emission standards (California, Connecticut, Massachusetts, Maine, New Jersey, New York, Rhode Island, Vermont, Washington, and soon Oregon). And since the Prius went on sale in the U.S. five years ago, there has never been a failure of the battery requiring replacement.
"Our batteries have been tested to a simulated 180,000 miles without failure," claims a Toyota spokesman. "A first-generation Prius in taxi use in the Pacific Northwest has gone more than 200,000 miles on its original battery."
Still, much speculation persists about the replacement should it fail beyond the warranty period. While no specific cost figure exists due to no history of battery failure, a check with an energy storage expert estimated a price of $3000 to $4000.
According to the expert, "A key issue in a battery's life is good energy management. Using the battery properly (i.e. no deep discharges or charges, keeping it operating at the right temperature, etc.) adds to battery life."
He regards the Prius' synergistic design with its high-voltage battery as a milestone, though not necessarily the final and most efficient answer for energy storage. He foresees even newer technologies on the horizon that will make driving more efficient and more environmentally friendly.
One such technology is supercapacitors. They aren't batteries. Rather, they're ionicenergy storage devices that have greater lifetimes than batteries of eight to 10 years. Additionally, they possess 20% or more energy storage capability. But he credits the Prius design for starting this revolution in automotive energy savings and pollutionlowering technologies.
Indeed, the hybrid gasoline-electric car movement is just beginning, and it's certain to expand considerably. By 2008, other auto manufacturers worldwide are all expected to penetrate the market.
Last month, Toyota issued a voluntary recall for 75,000 Priuses sold in the U.S. (160,000 worldwide) for a potential software glitch that could cause the car to stall or shut down. According to Toyota, no accidents or injuries have occurred as a result. It says the problem is a minor one that the company already has dealt with successfully.
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Electric Drive Transportation Association
R.L. Polk & Co.
Toyota Motor Co.