Thermodynamic cycle and working fluid selection for waste heat recovery in a heavy duty Diesel engine

Paper #:
  • 2018-01-1371

  • 2018-04-03
Thermodynamic power cycles have been shown to provide an excellent basis for waste heat recovery (WHR) in internal combustion engines. By capturing and reusing the heat that is otherwise lost to the environment, the efficiency of the engine can be increased. This paper evaluates the maximum power output of different power cycles used for waste heat recovery in a heavy duty Diesel engine with the focus on the working fluid selection. Inside a heavy duty Diesel engine four distinct heat sources can be identified, two high-temperature heat sources: the exhaust gas recirculation cooler and the exhaust gas flow out, and two low-temperature heat sources: the charge air cooler and the engine coolant. Typically, only the high-temperature heat sources are evaluated for WHR in internal combustion engines, whereas this paper also includes the potential for WHR from the low-temperature heat sources. To recover the heat, four types of power cycles were selected: the Rankine cycle, the transcritical Rankine cycle, the trilateral flash cycle, and the single 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 establish the best performing cycle, a large number of working fluids was evaluated with regards to the maximum power output of the thermodynamic power cycle for each heat source. Models were built using Modelica with Python code for pre-/post-processing which was coupled to the CoolProp database for the fluid properties. The simulations were performed using a set of practical constraints (minimum temperature/pressure, maximum pressure, superheating, and pinch point) and boundary conditions determined by the corresponding heat source and the operating point of the engine. The constraints and boundary conditions were chosen so that they reflect typical engine operating values and realistic cycle conditions.
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