Combining Low- and High-Temperature Heat Sources in a Heavy Duty Diesel Engine for Maximum Waste Heat Recovery Using Rankine and Flash Cycles

Abstract

Waste heat recovery (WHR) systems enable the heat losses of an engine to be captured and converted to power, thereby increasing engine efficiency. This paper aims to identify the combination of working fluid and thermodynamic cycle that yields the best WHR performance for the most important engine operating points of a heavy duty Diesel engine. WHR cycles were simulated using two distinct configurations of the heat sources available in a typical heavy duty Diesel engine: Conf-1: CAC-Coolant-Exhaust-EGRC and Conf-2: CAC-Exhaust-EGRC. Simulations were performed for fifty working fluids and four thermodynamic cycles, with and without a recuperator: the organic Rankine cycle (ORC), the transcritical Rankine cycle (TRC), the trilateral flash cycle (TFC), and the organic flash cycle (OFC). An analysis of a 100kW operating point revealed important performance differences between the two heat exchanger configurations, with maximum net power outputs of 5–7 kW for the ORC and TRC, 3–5 kW for the TFC, and 0.5–4 kW for the OFC. The use of a recuperator increased the net power output by 15 to 25% for Conf-1 and helped reduce the condenser load for Conf-2. For the dominant engine operating points of long haul cycle, the best performance was achieved for Conf-2. With this configuration, the ORC and TRC showed maximum power outputs with acetone, methanol, cyclopentane, ethanol or isohexane as the optimum working fluid.