While striving for more fuel-efficient vehicles, all possible measures are considered to increase the efficiency of the combustion engine powertrain. 48V mild hybrid technology is one such measure, SIDI (Spark Ignited Direct Injection) engines with Miller technology are another, while recovering energy from the engine’s waste heat (WHR) is yet another option.
In this paper, results will be published from an advanced engineering project at Volvo Cars including all of these components. An ethanol based Organic Rankine Cycle (ORC) WHR-system was successfully built around a 4-cylinder, 2.0 litre SIDI-engine, including 48V mild hybrid technology, with vehicle packaging considered. A dedicated control system was also developed for the ORC system including communication between it and the engine. The ORC system uses the engine exhaust as the heat source, for which a purpose-built evaporator was designed and built to fit in the vehicle tunnel. The expansion of the ethanol vapour occurs in an axial piston expander coupled both electrically to the hybrid system and mechanically to the engine crankshaft via a belt-drive. This dual power output from the expander was found to be of particular importance for a passenger car with a transient driving style, giving the opportunity to recover more waste heat energy by overcoming the slow response from the steam system with the fast response of the electrical hybrid system.
Initial results on the engine bench indicate, at high engine loads, a decrease in brake specific fuel consumption of 5-7% with stoichiometric engine operation. These results are in-line with our simulations also show that the combined powertrain system gives advantages in both city (mild hybrid), rural (optimized engine, WHR) and highway conditions (WHR).