Abstract
•Detailed model of high pressure ratio automotive ORC radial turbines is developed.•Backswept blade is incorporated for higher enthalpy drop.•Effects of turbine design and ORC system on engine performance are explored.•Efficient optimized radial turbine of 74.4% efficiency is designed.•Engine BSFC, power and NOx emission improved by 3.79%, 3.95% and 3.7%, respectively.
Due to increasing environmental restrictions and rising fuel prices, automobile manufacturers must extend their efforts to produce more efficient powertrains. Given that only up to 35% of fuel energy is converted into mechanical power, the wasted energy can be reused through waste heat recovery technologies. Compared to other waste heat recovery technologies, organic Rankine cycle system is regarded as the most candidate technology due to its simplicity, low cost and small back pressure impact. The expansion machine is the key component in organic Rankine cycle systems, and its performance has a direct and significant impact on overall cycle efficiency. This paper presents a detailed design method of high pressure ratio radial turbo-expanders integrated in organic Rankine cycles. The method is coupled with an optimization algorithm to optimize the input parameters specified by the designer. In addition, real fluids properties library is implemented in the design method in order to account for the thermodynamic properties at the inlet and exit of each turbine stage. Finally, the effects of the turbo-expander design and organic Rankine cycle system as a waste heat recovery technology on engine power, fuel consumption and NOx emissions are presented at three different engine operating points. Based on the steady-state cycle simulation, a radial turbo-expander with a pressure ratio of seven (7) was designed for an automotive application and demonstrated a total-to-static efficiency and power output of 74.4% and 13.6 kW, respectively, for a heavy duty diesel engine. Compared to the base diesel powertrain systems, the waste heat recovery system improved the brake specific fuel consumption, power and NOx emissions of the engine by 3.79%, 3.95% and 3.7%, respectively.