Waste heat recovery of an ORC-based power unit in a turbocharged diesel engine propelling a light duty vehicle

Abstract

The present technologies in internal combustion engines for transportation purposes clearly demonstrate the room for improvement still achievable. In a recent past, harmful emission reduction was the main goal: wonderful technologies were developed which strongly reoriented the interest and the use of such engines. Actually, CO2 is the most important driver: it calls for fuel consumption reduction (energy saving) and energy recovery from that usually wasted. Considering that about one third of the fuel energy is in the flue gases, the possibility to recover this energy and re-use it for engine and vehicle needs is one of the smartest ways to participate to reduce fuel consumption and, therefore, CO2 emissions. In particular, Organic Rankine Cycle (ORC)-based power units fed by the exhaust gases are promising and technologically ready, but they have a significant impact on the exhaust line and engine behavior. A trade-off between energy recovered in mechanical form and energy lost due to the engine back pressure, vehicle weight increase, discharge energy at the condenser, and the management of the strong off-design operating conditions is a key point which could definitively open the way to this technology or limit it to particular applications. The paper discusses the effects of the pressure losses produced by an ORC-based power unit mounted on the exhaust line of a turbocharged IVECO F1C engine, operated on a test bench. The interactions produced on the turbocharged engine have been experimentally investigated: the presence of an Inlet Guide Vane (IGV) system to manage the turbocharger makes the effect of the back pressure not straightforward to be predicted. The IGV opening and closure degree, in fact, can compensate the effect of the back pressure which intrinsically tends to increase specific fuel consumption. A wide experimental testing of the turbocharging group in order to understand its reaction and the net effect in terms of engine specific fuel consumption is presented. Finally, once the engine performances were verified, the contribution due to the heat recovery inside the ORC-based power unit fed by the exhaust gases in terms of mechanical power was evaluated and experimentally verified in some points, considering the strong off design conditions produced by the engine operating point variations. In fact, exhaust gas flow rate and temperature variations lead the evaporator, even though properly designed, to severe off-design conditions which modify the inlet working fluid conditions till to make the mechanical recovery impossible. Under the hypothesis that the engine propels a light duty vehicle, the effect of the extra weight is discussed re-evaluating the propulsion power increase in terms of fuel consumption.