Advanced Automotive Technology: Visions of a Super-Efficient Family Car

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increase in the number of gear steps between the available limits (that is, moving to five or more speeds

in an automatic transmission), with continuous variable transmissions (CVTs) being the extreme limit;

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electronic control of transmission shift points and torque converter lockup.

All of these improvements have been adopted, in some form, by automakers, but their

penetration of the fleet is incomplete and, in some cases, further technical improvements are

possible. For example, Mercedes-Benz and Nissan have recently (1993) introduced a five-speed

automatic transmission, while GM introduced a six-speed manual transmission. Product plans

reveal that such transmissions are likely to be more widely adopted by 2005. CVTs have been

introduced in Europe and Japan, and in the United States in one car model that has been since

discontinued.

Torque converter improvements.

Redesign of the torque converter to reduce flow losses will yield improved fuel economy.

Toyota has introduced a new “Super Flow” converter in its Lexus LS400 vehicle.^157 The new

converter was computer designed to optimize impeller blade angle and blade shape to reduce loss

of oil flow. In addition, new manufacturing techniques were developed for the impeller to increase

rigidity. As a result, Toyota claims the converter efficiency is the world’s best, and is 3 percent to

5 percent higher than other torque converters.^158 Such an improvement is expected to provide a

0.5 percent benefit in composite fuel economy.

Greater number of gears
Increasing the number of transmission gears can be used to provide a wider ratio spread
between first and top gears, or else to increase the number of steps with a constant ratio spread
for improved drivability and reduced shift shock. In addition, the wider ratio spread can be
utilized to provide higher performance in the first few gears while keeping the ratio of engine
speed to car speed in top gear constant, or else to maintain the same performance in the first few
gears and to reduce engine speed in top gear. Because the manufacturer is able to select among
these tradeoffs, different manufacturers have chosen different strategies in selecting gear ratios;
therefore, any fuel economy gain from increasing the number of gears is dependent upon these
strategies.

Five-speed automatic transmissions have only recently been commercialized in Japan and

Europe. Nissan has provided a comprehensive analysis of the effect of numbers of gears and

choice of first gear and top gear ratios on fuel economy.^159 They found declining benefits with

increasing numbers of gears, with little or no benefit above six gears. With a first gear ratio of 3.0

(similar to that of current automatics) they found no benefits in fuel economy in using overdrive

(^157) T. Kondo et al., "Toyota “ECT-i", A New Automatic Transmission with Intelligent Electronic Coolant System,” SAE paper 900550, 1990.
(^158) Ibid.
(^159) N. Hattori et al., “A New 5-speed Automatic Transmission forPassenger Cars,” SAE paper 900551, 1990.