Experimental investigation of an inverted Brayton cycle for exhaust gas energy recovery

Exhaust gases from an internal combustion engine (ICE) contain approximately 30% of the total energy released from combustion of the fuel. In order to improve fuel economy and reduce emissions, there are a number of technologies available to recover some of the otherwise wasted energy. The inverted Brayton cycle (IBC) is one such technology. The purpose of this study is to conduct a parametric experimental investigation of the IBC. The hot air from a turbocharger test facility is used. The system is sized to operate using the exhaust gases produced by a 2 l turbocharged engine at motorway cruise conditions. A number of parameters are investigated that impact the performance of the system such as turbine inlet temperature, system pressure drop, and compressor inlet temperature. The results confirm that the output power is strongly affected by the turbine inlet temperature and system pressure drop. The study also highlights the packaging and performance advantages of using an additively manufactured heat exchanger to reject the excess heat. Due to rotordynamic issues, the speed of the system was limited to 80,000 rpm rather than the target 120,000 rpm. However, the results show that the system can generate a specific work of up to 17 kJ/kg at 80,000 rpm. At full speed, it is estimated that the system can develop approximately 47 kJ/kg, which represents a thermal efficiency of approximately 5%.