Thermodynamic Cycle and Working Fluid Selection for Waste Heat Recovery in a Heavy Duty Diesel Engine

Abstract

Thermodynamic power cycles have been shown to provide an excellent method for waste heat recovery (WHR) in internal combustion engines. By capturing and reusing heat that would otherwise be lost to the environment, the efficiency of engines can be increased. This study evaluates the maximum power output of different cycles used for WHR in a heavy duty Diesel engine with a focus on working fluid selection. Typically, only high temperature heat sources are evaluated for WHR in engines, whereas this study also considers the potential of WHR from the coolant. To recover the heat, four types of power cycles were evaluated: the organic Rankine cycle (ORC), transcritical Rankine cycle, trilateral flash cycle, and organic flash cycle. This paper allows for a direct comparison of these cycles by simulating all cycles using the same boundary conditions and working fluids. To identify the best performing cycle, a large number of working fluids were evaluated with regards to the maximum power output of the power cycle for each heat source. Taking into account the constraints and boundary conditions, this study shows that the ORC gives the best performance with a power output of around 1.5 kW for the coolant, 2.5 kW for the exhaust gas recirculation cooler, and 5 kW for the exhaust with acetone, cyclopentane and methanol as the best performing working fluids.