Integrated evaluation of Inverted Brayton cycle recovery unit bottomed to a turbocharged diesel engine

Internal combustion engines evolution has recently gained additional push, in relation to their environmental impact on global warming and air quality deterioration mainly in high congested areas, like urban environments. Several innovative technologies have been introduced in the market and many others are under development in order to reduce CO2 emissions during homologation cycles, including real driving emission tests. Among them, great attention has been paid to the waste heat recovery, since it can assure a significant improvement on overall engine efficiency and, so, fuel saving and CO2 emission reduction. 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 value. The real useful power of an IBC-based recovery unit is strictly related to the behavior 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, resulted by the equilibrium of a common shaft. Therefore, an experimentally based mathematical model has been developed to evaluate the coupling of an IBC-based recovery unit with a turbocharged diesel engine, operating on a dynamic test bed. In particular, experimental data of the engine have been used as boundary conditions of the IBC group and real operating maps of radial turbine and compressor have been considered. In this way, the actual room of recovery of the unit has been assessed, evaluating also the trade-off produced by backpressure induced on the engine by the IBC-based recovery unit, which hadn't been investigated yet. An overall net efficiency increase close to 3.4% has been demonstrated with respect to the original efficiency of the engine, when operates close to the maximum power. The analysis is concluded evaluating the correct functioning of the after-treatment devices of the engine: sufficiently higher temperature has to be assured to guarantee right pollutants abatement.