The Prius Just Got Even Better
By Bill Moore
The Prius Just Got Even Better by Bill Moore
The readers of Automotive Engineering International just voted the Toyota Prius the best-engineered automobile for 2001. Their praise is well deserved because Toyota's engineers have made significant strides in improving the performance of the world's first mass-production hybrid electric vehicle.
Not only is the 2001 Prius more powerful with better fuel economy than its predecessor is but it also generates far fewer emissions. This is due, in part, because it operates in ZEV mode a dramatic 43% of the time by Toyota's calculations. This is a 43% increase over the 1998 version of the car.
This remarkable accomplishment is the result of numerous improvements including the car's upgraded battery technology, changes to its 1.5 liter gasoline engine and the installation of a special in-tank bladder to reduce evaporative emissions. The 2001 Prius also uses a combined hydrocarbon absorber and three-way catalyst. This allows the car to be rated in California as a "super-ultra low emission vehicle" or SULEV, which is according to Automotive Engineering magazine "about 75% more stringent than ULEV and nearly 90% cleaner than LEV for smog-forming exhaust gases."
First introduced in Japan in 1997, Toyota sold some 37,000 Priuses before it introduced the car in North America and Europe in 2000 just months behind the introduction of the Honda Insight gasoline-electric hybrid, its chief rival in the nascent HEV marketplace.
Shortly after its introduction in Japan, Toyota began loaning American journalists -- including EV World -- Japanese versions of the car. It quickly became apparent that any future North American or European models would have to be re-engineered to meet the more challenging driving conditions of these regions.
The original Prius was optimized for Japan's slower, more congested driving cycle where the average speed in downtown Tokyo is a sedate 19.4 kph (12mph). This explains, in part, why the car actually gets better fuel economy in town driving than on the highway, typically 45 mpg on the road and 52 mpg in town.
To compensate for US driving conditions Toyota has increased the horsepower rating of the Prius' 1.5 L gasoline engine from 43kW(58 hp). to 58kW (70 hp) by increasing the engine's maximum rpm from 4000 to 4500 rpm. This is still relatively slow enough to enable Toyota to take advantage of reduced engine stresses by using a smaller diameter crank shaft, lower tension in the piston rings and lower value spring loads. This translates directly into less friction and higher engine efficiency.
The ICE wasn't the only part of the drive train to be up-graded. While the basic design of the electric motors used in the Toyota Hybrid System or THS remains unchanged, the company introduced new motor windings and magnets, as well as a new motor control strategy. According to Automotive Engineering, several of the internal motor components were also changed, including elimination of certain oil seals and use of a new resin sealing material. “By reducing losses from oil pumping and windage,” the magazine writes, mechanical loss of the overall transmission (including the torque-split device) is reduced by about 40%.”
Instead of relying on pulse width modulation (PWM) alone to control the motors through their various speed regimens, Toyota also introduced one-pulse switching for high-speed range and PWM for low speed operation. “The use of one-pulse switching allowed a redesign of the electromagnetic circuit to enhance efficiency further,” AEI states. As a result, the basic wave voltage that can now be applied to the motor was increased by 27%. This enabled engineers to expand the ZEV-only driving mode of the vehicle from 30% to 43% of the time, a feat assisted by the use of a brand-new 7.2V mono-block battery module that is some 30% lighter than the old cylindrical modules and take up 60% less volume.
(Editor: ECD recently filed a lawsuit against Matsushita, Panasonic and Toyota for patent infringement on this battery technology. Click To Learn More.
In the original Prius, the NiMH battery pack was located between the back seat and the trunk. This prevented the rear seat being folded down and took space away from the trunk (boot) area. The new battery pack consisting of 38 modules instead of the original 40 and is rated at a nominal 274 volts with peak power of 25kW. It now fits under the under the trunk and rear seat area, enabling the rear seat to fold down. And the added trunk space now means the vehicle is classified as a “compact” car by the US EPA, moving it up a class.
Exterior styling and interior design remain similar to that of the original model. The 4-dooar car comfortably seats 4, but can easily accommodate another passenger in the back seat. For EV World’s impressions of the Japanese version of the car, see the following two stories.
The new 2001 Prius clearly demonstrates that we are only just beginning to see what can be accomplished in the way of cleaner, more efficient vehicles. While hybrid-drive costs still remain a critical issue, Toyota engineers have again shown the way to a better automotive future and set a demanding pace that other carmakers will be hard-pressed to follow. EV World congratulates Toyota on a job well done and recognition well earned!
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