Thermodynamic analysis of a Rankine cycle applied on a diesel truck engine using steam and organic medium

A theoretical study is conducted to investigate the potential improvement of the overall efficiency of a heavy-duty truck diesel engine equipped with a Rankine bottoming cycle for recovering heat from the exhaust gas. To this scope, a newly developed thermodynamic simulation model has been used, considering two different working media: water and the refrigerant R245ca. As revealed from the analysis, due to the variation of exhaust gas temperature with engine load it is necessary to modify the Rankine cycle parameters i.e. high pressure and superheated vapor temperature. For this reason, a new calculation procedure is applied for the estimation of the optimum Rankine cycle parameters at each operating condition. The calculation algorithm is conducted by taking certain design criteria into account, such as the exhaust gas heat exchanger size and its pinch point requirement. From the comparative evaluation between the two working media examined, using the optimum configuration of the cycle for each operating condition, it has been revealed that the brake specific fuel consumption improvement ranges from 10.2% (at 25% engine load) to 8.5% (at 100% engine load) for R245ca and 6.1% (at 25% engine load) to 7.5% (at 100% engine load) for water.

► ORC improves bsfc from 10.7% to 8.4% as engine load increases from 25% to 100%.
► Increasing ORC high pressure increases thermodynamic efficiency and power output.
► Operating at high pressure the ORC is favorable for the engine cooling system.
► The low temperature values of the ORC favors heat extraction from the EGR gas.
► The impact of the exhaust gas heat exchanger on engine backpressure is limited.