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

Abstract

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