Modelling and Evaluation of Waste Heat Recovery Systems in the Case of a Heavy-Duty Diesel Engine

In the present study, the effects of Organic Rankine Cycle (ORC) and turbo-compound (T/C) system integration on a heavy-duty diesel engine (HDDE) is investigated. An inline six-cylinder turbocharged 11.5 liter compression ignition (CI) engine employing two waste heat recovery (WHR) strategies is modelled, simulated, and analyzed through a 1-D engine code called GT-Power. The WHR systems are evaluated by their ability to utilize the exhaust excess energy at the downstream of the primary turbocharger turbine, resulting in brake specific fuel consumption (BSFC) reduction. This excess energy is dependent on the mass flow rate and the temperature of engine exhaust gas. However, this energy varies with engine operational conditions, such as speed, load, etc. Therefore, the investigation is carried out at six engine major operating conditions consisting engine idling, minimum BFSC, part load, maximum torque, maximum power, and maximum exhaust flow rate. The results for the ORC and T/C systems indicated a 4.8% and 2.3% total average reduction in BSFC and also maximum thermal efficiencies of 8% and 10%, respectively. Unlike the ORC system, the T/C system was modelled as a secondary turbine arrangement, instead of an independent unit. This in turn deteriorated BSFC by 5.5%, mostly during low speed operation, due to the increased exhaust backpressure. It was further concluded that the T/C system performed superiorly to the ORC counterpart during top end engine speeds, however. The ORC presented a balanced and consistent operation across the engines speed and load range.