Fuel Burn Reduction of Hybrid Aircraft Employing an Exhaust Heat Harvesting System

This study investigates the application of an exhaust gas energy harvesting hybrid powertrain to three general aviation aircraft with reciprocating spark-ignition piston engines. The hybridization process involves downsizing the internal combustion engine to save weight and installation of an exhaust heat harvesting system composed of a harvester, a generator, lithium-ion traction battery, an electrical conversion system, and an electric motor. Novel calculations for a hybrid powertrain performance envelope are presented that estimate the upper limits to the degree of downsizing and mass of the harvester. An organic Rankine cycle (ORC) has been proposed as a potential compact, high-efficiency harvester. Five hydrofluroolefins (which have no ozone depletion potential and very low global warming potential) are considered as organic fluids for the ORC. It is found that R-1336mzz(Z) performs best in this application. Each ORC design parameter is investigated, and an optimized design with 14.3% thermal efficiency is found. At this efficiency, the internal combustion engine may be downsized up to 34.5%. The fuel burn reduction at this degree of downsizing may be as high as 12.7-14.5%.