Automotive Application of Turbo-Expansion
Brutus, Jean Christian
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This research project focuses on a charge-air cooling concept, Turbo-Expansion, which offers promise to bring the application of Variable Geometry Turbine (VGT) technology toward its original aim of improving gasoline internal combustion (IC) engine performance and efficiency. In a conventional turbocharger, as the pressure increases, the exhaust gas bypasses the turbine through a wastegate. This represents a loss of energy since the exhaust gas goes directly to the ambient surroundings without performing any useful work within the system. The Turbo-Expansion concept can eliminate the need for the exhaust bypassing system by using a VGT as the exhaust turbine. It can also eliminate the turbo lag that exists in regular turbochargers by functioning as a"small turbo" at low engine speed and a"big turbo" at high engine speed. In addition, due to reduced operating temperatures the application of the Turbo- Expansion cooling system to the gasoline engine will not lead to knocking, an abnormal and engine-damaging combustion phenomenon that plagues naturally aspirated and conventionally turbocharged engines. Normal operation of gasoline engines produces higher exhaust gas compared to Diesel engines where turbocharger application is common. These higher temperatures lead to turbocharger component failures, which can be eliminated by the application of Turbo-Expansion technology. Engine performance modeling software, Gamma Technologies' GT-Suite, was previously used to prove the theoretical benefits of the new concept. The results of the investigation show that Turbo-Expansion will increase the power output of current gasoline engines. It will also save fuel costs and lower emissions by lowering intake air- fuel mixture temperature below ambient. These changes will not require the use of advanced combustion technology, high-grade fuel, or advanced materials. Fuel economy will result from engine downsizing and higher compression ratios. Reliability and durability will increase as well, due to lower engine speeds. Using expansion-cooling to attain these benefits has been questioned since it opposes the main idea of turbocharging, which is based on the compression of inlet air. In the present effort, an analysis of the components involved is presented along with theoretical calculations, which prove that the concept presents a certain potential. Suggestions are made on critical requirements concerning the effectiveness of the intercooler and the control system needed to realize that potential.