Inverted Brayton Cycle for waste heat recovery in reciprocating internal combustion engines

Energy recovery in reciprocating internal combustion engines is one of the most investigated topics for reducingfuel consumption and carbon dioxide emissions in the on-the-road transportation sector. An exhaust gas recoveryopportunity is represented by a power unit with a so-called inverted Brayton cycle (IBC). The gas is used as theworkingfluid, which expands inside a turbine when it falls below atmospheric pressure; after being cooled by anexternal source, it is re-compressed to the atmospheric value. The useful work is the difference between the oneproduced by the turbine and that absorbed by the compressor. In this study, a thermodynamic assessment of theopportunity to apply an IBC-based power unit to a turbocharged diesel engine was conducted, and the mostimportant parameters affecting the range of possible recovery (turbine and compressor efficiencies, pressuredrops) were evaluated, and the pressure ratio was optimized. A conventional bottomed layout shows a recoveryof approximately 1.5% of the engine’s mechanical power when a homologation heavy duty procedure is per-formed. An improved integration, in which the IBC turbine is placed upstream of the turbocharger one, makes itpossible to partially recover the energy losses related to the turbocharger control device, which leads to anaverage recoverable power of approximately 2% of the engine brake power. Concerns about possible watercondensation in the exhaust have also been thoroughly investigated, and they can be managed in temperateweather.