Some Design Considerations of Automotive Gas Turbines

An examination of some design considerations of gas turbine automobile engines is made including an overview of the unsolved problems. Emphasis is placed on the design and manufacturing aspects.

Performance penalties due to size effects are discussed. The gains in cycle thermal efficiency with increasing peak temperatures are examined in view of these penalties. A cursory investigation of thermal efficiency is performed on a 75 horsepower and a 150 horsepower gas turbine. This was done considering both current and advanced design and manufacturing technology with regard to rotor clearances and blade thicknesses. Design and manufacturing limits used in defining the penalties in engines with advanced technology are qualified.

This investigation indicates an improvement of about 15 percent in thermal efficiency due to increasing the peak cycle temperature from 1850°F to 2500°F for a gas turbine engine suitable for an automobile but requiring advanced technology. It was also found that the size effects between 150 horsepower and 75 horsepower only result in about a 2 percent penalty. With current design and manufacturing technology, the increase in thermal efficiency reduces from 15 to 10 percent assuming that ceramics can be utilized to achieve a 2500°F turbine inlet temperature. If a ceramic turbine rotor does not become a reality and the cycle temperature is thereby limited by the capability of some type of metallic rotor, then the gain in thermal efficiency further reduces to about 5 percent.