Full energy recovery from exhaust gases in a turbocharged diesel engine

Energy recovery has become an important solution that drives the transition to more sustainable propulsion, since the amount of thermal energy available from exhaust gases is huge. This energy can be transformed into electrical energy (without significantly affecting the powertrain) and directly used, for instance, in hybrid propulsion or driving auxiliaries.A turbocharged diesel engine equipped with a variable geometry turbine (VGT) was tested to assess the maximum energy recoverable from exhaust gases through two different recovery stages. The first was achieved using the pressure difference between the value at the exhaust valves and the atmospheric datum (turbo -com-pounding). The second recovery stage was achieved thanks to the temperature of the exhaust gases after the first recovery, and performed using an organic Rankine cycle (ORC)-based power unit.Therefore, these two combined stages of energy recovery were experimentally investigated in the medium-low load region of the ESC-13 homologation test, which is representative for real driving of heavy-duty engines. The first stage was performed on the turbocharging system by recovering the energy lost inside the VGT, through an additional turbine that operates in parallel with the main turbine that drives the compressor. It facilitates the recovery of a mechanical power of up to 3 kW, which was approximately equal to 5 % of the engine brake power at a specific medium-low load. The second stage was performed with an ORC-based unit bottomed to the first recovery section, exploiting the fact that after the first recovery, exhaust gases still have a high temperature, which can be used to feed the additional ORC based recovery unit. This second stage adds up to 3.5 kW of recovered mechanical energy, which represents 5 % of the engine brake power at the same medium-low engine load operating points. Therefore, a total of 10 % of the engine power was recovered in the two stages, which are characterized by proven technologies. Considering the engine working point at maximum power, the value was also higher, the combined recovery achieved a mechanical power of approximately 14 % of the engine brake power.In this study, the detrimental effects related to the engine backpressure produced by the two recovery units and the additional weight of the vehicle were assessed, demonstrating a net overall specific fuel consumption reduction of approximately 5-7% in the medium-low operating region of the engine considered, and higher than 8% at the maximum engine power. The increase in complexity related to the two recovery stages invites to consider this technology for heavy-duty engines for long-hauling vehicles, in which the detrimental effects does not significantly affects fuel consumption.