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Since the earliest days of the automobile, improved fuel economy has been an objective of passenger-car manufacturers. The original market paced fuel-economy phase gave way to the CAFE-regulated phase in the late 1970s. September 1993 marked the start of a third fuel-economy phase, the Partnership for a New Generation of Vehicles (PNGV). PNGV has as its objective the development of a mid-size “Supercar” achieving an EPA combined-schedule fuel economy of 80 mpge (80 mi/gal gasoline equivalent = 34 km/L = 2.94 L/100 km) without sacrificing other attributes of the current U.S. mid-size car. The PNGV program is differentiated from the two previous phases by its cooperative research effort between industry and government. A review of past automotive phases sets the stage for future PNGV projections.

Improving the fuel economy of a passenger car by installing a small-displacement low-power engine is a methodology long recognized, but the accompanying loss in vehicle performance is a tradeoff unacceptable to the customer. Recovering the power deficiency by boosting the engine with a turbocharger or an engine-driven supercharger has often been suggested as a remedy. Turning back the pages of history, about thirty years ago an unusual supercharging scheme was evaluated that involved injection of high-pressure air from a storage reservoir directly into the cylinders of a downsized engine. Makeup air was provided by a pair of 21-MPa (3000 psi) engine-driven compressors. Large gains in fuel economy were measured when the compressors were not required to recharge the storage reservoir, as might be expected, but in simulated city and highway driving, those gains were greatly diminished by the need to replace stored supercharging air.

The single-shaft gas turbine engine has been proposed as a reduced-cost alternate to the previously used two-shaft turbine engine for application to passenger cars. The power output characteristics of the fixed-geometry single-shaft engine have been found to create performance difficulties, particularly with respect to standing-start acceleration of the vehicle. A review of the fundamentals responsible for these difficulties leads to the observation that variable compressor geometry can provide relief from this situation. Use of variable inlet guide vanes is identified as the simplest means of gaining this relief. Design factors influencing the susceptibility of the compressor to control by inlet guide vanes are considered. A method by which inlet guide vanes can be used to improve vehicle acceleration, without penalizing fuel consumption, is illustrated.

The incentive for use of an interstage diffuser in a free-shaft gas turbine engine is briefly examined and some pertinent published background data reviewed. Tests of two annular diffusers behind an upstream turbine show the deleterious effects of turbine exit flow nonuniformity on diffuser behavior. The flow acceleration provided by the area contraction of a power turbine nozzle located at the diffuser exit substantially improves the nature of the flow previously found to exist at the diffuser exit in the absence of the nozzle.

Utilization of numerically controlled milling has been found particularly attractive in producing, in limited quantities, the three-dimensional curved surfaces characteristic of turbomachinery. In experimental and developmental programs its use can result in decreased fabrication cost, reduced lead time, and improved dimensional accuracy. Following a review of the general classifications of numerically controlled milling machines available for manufacture of such parts, illustrations are given of some of the procedures and techniques employed in their use. A variety of parts made using numerical control serve as examples.

The conflicts in the design of turbines for an automotive gas turbine engine are examined. Considerations of stress, efficiency, engine and vehicle acceleration requirements, and compatibility of the flow path are shown to impose a number of opposing requirements. The philosophies used to compromise the conflicts in two successive engine designs are presented. Following a discussion of turbine test facilities, test results are presented for a typical turbine.