advanced ceramic recuperators, and ceramic turbine blades capable of operating at very high
temperature.A simple, all metal single shaft gas turbine engine of 150 HP attains relatively low efficiency
because of the low compression ratios employed (3:1), low turbine inlet temperature of 1,300oC,
and the heat loss in the exhaust. Typically, these efficiencies are in the range of 30 to 32
percent.^146 To improve efficiencies to over 40 percent, regenerators have been widely used. The
regenerator is usually a ceramic matrix that rotates through both the hot turbine exhaust and
cooler intake air from the compressor, transferring heat from exhaust to intake air. A major
problem area with regenerators is the dynamic seal between the turbine exhaust and compressor
discharge air, which tends to leak, leading to a substantial reduction in performance.^147 An
alternative is the fixed boundary heat exchanger, or recuperator. This eliminates leakage, but size,
weight, and cost are problems with regenerators that have persisted even after a decade of
research.
An additional way to increase efficiency, by another 5 to 7 percent, is through use of ceramic
parts in the “hot” section of the turbine, allowing higher temperatures. The development of
durable and reliable ceramic components is the focus of much research and such components
could be available commercially by 2005.
To date, the best automotive gas turbine cannot yet match the efficiency of a gasoline engine
over the entire drive cycle, and many now believe that it is never likely to exceed this moving
target of gasoline engine efficiency in an automotive environment.^148 Even ceramic gas turbines of
about 80HP now under development have project goals of reaching a 40 percent efficiency
(peak),^149 a level already attained by current production diesels.
More recent research has focused on the use of the ceramic gas turbine as a hybrid vehicle
powerplant, where it operates at constant rpm to drive an electric generator. If the generator
speed is increased to that of the turbine shaft, the size and weight of the generator can be reduced
by nearly a factor of 10 for equal output, and the gearbox between the turbine and output shaft is
eliminated. Such an approach has been used by Volvo in its High Speed Generation concept
included in the Volvo ECC prototype hybrid vehicle.^150 The HSG unit features a single-stage
radial compressor and turbine, which operates at speeds up to 90,000 rpm with an output of 56
HP. The gas turbine engine uses a recuperator to maximize energy efficiency. Anecdotal
information suggests that the Volvo gas turbine engine operates with an efficiency of about 35
percent, but there are no data on the durability of the recuperator seals or the efficiency and
durability of the high rpm electric generator.
It is unlikely that small gas turbines (20 to 40 kW) can have an efficiency of much more than 35
percent, because the 1aws of fluid dynamics affect the scaling laws for gas turbines. As the engine(^146) Ford Motor Co., “Conceptual Design Study of Automotive Gas Turbine,”report prepared for the National Aeronautics and Space
Administration, 1979. 147
148 R. Mackay, “Gas Turbine Generator Sets for Hybrid Vehicles,” SAE paper 920441, 1992.
C.A.Amman, “The Automotive Engine - A Future Perspective,” GM Research Publication, GMR-6653, n.d.
(^149) M. Bauer, "The European Ceramic Gas Turbine Programme -AGATA,” Automotive Technology Development Contractors Coordination
Meeting, U.S. 150 Department of Energy, October 1994.
Volvo ECC publicity brochure, n.d.