In this work, two different options of waste heat recovery from internal combustion engines have been proposed and compared to each other. Both consider two thermodynamic cycles in order to maximize the energy recoverable. The first considers a Brayton-Joule cycle as top cycle, which is immediately interfaced with the exhaust gases and has a supercritical CO2 as working fluid; while an ORC is bottomed to this, using R1233zdE as the organic working fluid. In this way, the supercritical CO2 can have better efficiency and the bottomed ORC can further recover the low temperature energy discharged by the Brayton-Joule cycle.
The second option considers an Inverted Brayton Cycle as top thermodynamic cycle, which uses the same exhaust gases as working fluid and, so, has a lower complexity. The thermal power discharged at low temperature by the inverted Brayton Cycle is further recovered by the ORC bottom unit.
The two options will be studied using a mathematical model developed and tailored to a 3L turbocharged diesel engine, whose experimental data are available measuring the most relevant quantities (exhaust temperature and pressure, fuel consumption, brake efficiency, etc.) in typical operating points of a heavy duty vehicle. A comparison of the two combined systems assesses the most effective recovery option.