Modeling and Simulation of an Inverted Brayton Cycle as an Exhaust-Gas Heat-Recovery System

Abstract

The exhaust gas from an internal combustion engine contains approximately 30% of the thermal energy of combustion. The exhaust-gas heat-recovery systems aim to reclaim a proportion of this energy in a bottoming thermodynamic cycle to raise the overall system thermal efficiency. The inverted Brayton cycle (IBC) considered as a potential exhaust-gas heat-recovery system is a little-studied approach, especially when applied to small automotive power-plants. Hence, a model of the inverted Brayton cycle using finite-time thermodynamics (FTT) is presented to study heat recovery applied to a highly downsizing automotive internal combustion engine. IBC system consists of a turbine, a heat exchanger (HE), and compressors in sequence. The use of IBC turbine is to fully expand the exhaust gas available from the upper cycle. The remaining heat in the exhaust after expansion is rejected by the downstream heat exchanger. Then, the cooled exhaust gases are compressed back up to the ambient pressure by one or more compressors. In this paper, the exhaust conditions available from the engine test bench data were introduced as the inlet conditions of the IBC thermodynamic model to quantify the power recovered by IBC, thereby revealing the benefits of IBC to this particular engine. It should be noted that the test bench data of the baseline engine were collected by the worldwide harmonized light vehicles test procedures (WLTP). WLTP define a global harmonized standard for determining the levels of pollutants and CO2 emissions, fuel consumption. The IBC thermodynamic model was simulated with the following variables: IBC inlet pressure, turbine pressure ratio, heat exchanger effectiveness, turbomachinery efficiencies, and the IBC compression stage. The aim of this paper is to analysis the performance of IBC system when it is applied to a light-duty automotive engine operating in a real-world driving cycle.

Experimental Investigation of an Inverted Brayton Cycle for Exhaust Gas Energy Recovery

“…The findings were that the IBC gave a fuel consumption improvement of about 6%, which was more than the pressurized Brayton (~ 4% improvement) but less than the organic Rankine cycle (~ 12% improvement). Chen et al [27] applied the IBC to a gasoline engine to determine the benefits over the Worldwide Harmonized Light Vehicle Test Procedure. The analysis was conducted using pre-catalyst temperatures, at a number of fixed turbomachinery isentropic efficiencies.…”

Study of an Electric Energy Generation System from Exhaust Waste Recovery from an Internal Combustion Engine

“…The remaining heat in the exhaust after expansion is rejected by the downstream heat exchanger. Then, the cooled exhaust gases are compressed back up to the ambient pressure by one or more compressors [8]. There are various technologies for exhaust gases waste heat recovery.…”

Maximizing specific work output extracted from engine exhaust with novel inverted Brayton cycles over a large range of operating conditions

“…Whether for economic reasons or to mitigate global warming, reducing engine fuel consumption is imperative. In internal combustion (IC) engines, approximately 30% of the energy of combustion is lost in exhaust gases [1]. A way to improve their overall energy conversion efficiency is to add a system capable of recovering the waste heat exiting the engine.…”

“…For example, Chen et al [1] simulated the performance of IBCs when it is coupled with a lightduty automotive engine operating in a real-world driving cycle where the exhaust flow rate varies in time. A reduction of fuel consumption of 3.15% was calculated when the turbine pressure ratio is constantly optimized.…”