ANALYSIS OF A HYBRID PROPULSION SYSTEM WITH EXHAUST GAS ENERGY RECOVERY

As widely known internal combustion engines are not able to complete the expansion process of the gas inside the cylinder, causing theoretical energy losses of the order of 20%. Several systems and methods have been proposed and implemented to recover the unexpanded gas energy, such as turbocharging, which partially exploits this energy to compress the intake fresh charge, or turbo-mechanical and turbo-electrical compounding, where the amount of unexpanded gas energy not used by the compressor is dedicated to propulsion or transformed into electric energy. In all these cases, however, maximum efficiency improvements between 4% and 9% have been achieved. In this thesis, the author deals with an innovative propulsion system composed of a spark ignition engine equipped with a turbo-generator group specifically dedicated to the unexpanded exhaust gas energy recovery and with a separated electrical-driven turbocompressor. The system has been conceived specifically for hybrid propulsion architectures, being the electric energy produced by the turbine-generator easily storable in the on-board energy storage system and reusable for vehicle traction. The turbo-generator unit should be composed of two fundamental elements: an exhaust gas turbine expressly designed and optimized for the application, and a suitable electric generator necessary to convert the recovered energy into electric energy, which could hence be stored in the on-board energy storage system of the vehicle. Being optimized for quasi-steady power production, the exhaust gas turbine here dealt with is quite different from those commonly employed for turbocharging applications; for this reason, considering the power size of the machine, such a turbine is not available on the market, nor its development has been carried out in the scientific literature. On account of this, the author of the present thesis implemented a mean-line design algorithm to estimate the main turbine geometric dimensions. Once determined the turbine geometry, it was developed a turbine mean-line performance prediction model to evaluate the realistic performance of the electric turbo-compound system: as an overall result, it was estimated that, compared to a reference traditional turbocharged engine, the electric turbo-compound system could gain vehicle efficiency improvement between 3.1% and 17.9%, depending on the output power level, whit an average efficiency increment of 10.9% evaluated on the whole operating range.