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

at ambient temperatures of 45°C (113°F). Improved systems could provide a benefit of 5 to 6

percent under urban driving conditions at 25oC, and as much as 10 percent at winter conditions.

Waste heat recovery from the exhaust alone could be a possibility for engines operating at near
constant output, as is theorized for some hybrid vehicle types. Mitsubishi^155 has experimented
with a turbocompound system where the turbine drives an auxiliary generator and obtained a 7
percent increase in output. Another possibility is a thermoelectric generator, which converts heat
directly into electricity. DOE is supporting the development of a thermoelectric generator with
Hi-Z Technology,^156 Inc. for heavy-duty truck applications. Data presented by Hi-Z indicates that
the power output of the current design of the thermoelectric generator is very low, (about 1kW)
in conjunction with a 250HP engine at fill power, which is only a 0.5 percent increase in output.
Mitsubishi confirms that these generators provide only about 100W of power in a light-duty
vehicle application, so that currently they do not appear to be cost effective.

IMPROVEMENTS TO AUTOMATIC TRANSMISSIONS

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:

l

l

l

l

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 formaximum 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 ThermoelectricGenerator," paper presented at the Automotive Technology Development ContractorsCoordination

Meeting U.S.Department of Energy, November 1994.