A Novel Option for Direct Waste Heat Recovery From Exhaust Gases of Internal Combustion Engines

Among the different opportunities to save fuel and reduce Co2 emissions from internal combustion engines, great attention has been done on the waste heat recovery: the energy wasted is, in fact, almost two thirds of the energy input and even a partial recovery into mechanical energy is really promising . Usually, thermal energy recovery has been referred to a direct heat recovery (furtherly expanding the gases expelled by the engine thanks to their high pressure and temperature) or an indirect one (using the thermal energy of the exhaust gases – or of any other thermal streams discharged into the atmosphere – as upper source of a conversion power unit which favour a thermodynamic cycle of a working fluid ). Limiting the attention to the exhaust gases, a novel opportunity can be represented by directly exploiting the residual pressure and temperature of the flue gases through an Inverted Brayton cycle (IBC), in which the gases are expanded at a pressure below the environmental one, cooled down and then recompressed to the environmental pressure. Considering the thermodynamic conditions of the exhaust gases, expansion and compression must be done into dynamic machines, so making profit of the technology of the turbocharging group. The useful power of an IBC-based recovery unit is strictly related to the behaviour of the machines chosen as IBC turbine and compressor (pressure ratio vs. mass flow rate and efficiencies), considering that they run at the same speed. Therefore, an experimentally based mathematical model has been developed to evaluate the coupling of an IBC-based recovery unit with a real turbocharged diesel engine operating on a dynamic test bed. In particular, experimental data of the engine have been used as boundary conditions of the direct recovery group, considered as a bottomed thermal plant. In this way, the room of recovery of the unit has been assessed evaluating also the possibility to accept a higher engine backpressure in order to have a higher recoverable power, optimizing the whole system. An overall net efficiency increase close to 4 % has been demonstrated with respect to the original efficiency of the engine with similar value of CO2 saved.