The transmission in a vehicle matches the power requirements of the automobile to the power output available from an engine or motor; the automatic transmission’s selection of different gears keeps the engine operating in speed ranges that allow high levels of efficiency to be achieved. Most modem transmissions operate at efficiencies of over 85 percent on the city cycle and 92 to 94 percent on the highway cycle.
The efficiency losses that do occur are caused primarily by:
Hydraulic losses in the torque converter (current automatic transmissions use a hydraulic system to transmit the engine power to the drivetrain). Designs that avoid the operating point that would maximize fuel economy. If fuel economy were the only concern, the optimum point would maximize torque and minimize engine speed (rpm), which reduces throttling and fiction losses.
Designing the transmission for maximum efficiency leaves little or no reserve power, however, so that even modest changes in road load horsepower may require a downshift-and frequent downshifts are considered undesirable for customer satisfaction. In addition, operating at too low an rpm causes excessive driveline harshness and poor accelerator response.
Improvements to current transmissions can occur in the following areas: reduction in flow losses in the torque converter for automatic transmissions; increase in the ratio spread between top and first gear; 155 Mitsubishi, presentation to OTA, June 1993. 156 J.C. Bass, “Engine Test of Thermoelectric Generator,” paper presented at the Automotive Technology Development Contractors Coordination Meeting U.S. Department of Energy, November 1994.
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; and 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.
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