Waste heat recovery from engine coolant on mild hybrid vehicle using organic Rankine cycle

Considerable efforts have been invested in the automotive industry on electrified powertrains in order to reduce passenger cars’ dependence on fossil fuels. Powertrains electrification resulted in a wide range of mass-production hybrid vehicle models, ranging from micro-hybrid, to mild, full, and battery-extended hybrids such as plug-in and range-extender electric vehicles. Fuel savings of these powertrains strongly rely on the energy management strategy deployed on-board, as well as on the technology used to recover the waste heat energy. This paper investigates the fuel savings potential of a mild hybrid vehicle using an organic Rankine cycle for generating electricity from the engine-coolant circuit. The net mechanical power and electrical power generated from the organic Rankine cycle are determined based on experimental data recorded on a 1.2-L turbocharged engine. The coolant temperature is regulated at 85°C and 105°C depending on the engine load. The R-1234yf organic fluid is used and the Rankine operating pressure has been controlled to maximize the overall system efficiency under technological constraints. The dynamic programming control is used as a global optimal energy management strategy in order to define the best strategy for the engine operation and power-split between the electric and thermal paths of the powertrain. A sensitivity analysis is also performed to find the optimal size of the electric motor while taking into account the additional weight of the organic Rankine cycle system. Results show 2.4% of fuel economy improvement on The Worldwide Harmonized Light Vehicles Test Cycles.