Experimental Analysis of an Organic Rankine Cycle Plant Bottoming a Heavy-Duty Engine Using Axial Turbine as Prime Mover

The use of reciprocating internal combustion engines (ICE) dominates the sector of the on-road transportation, both for passengers and freight. CO2 reduction is the present technological driver, considering the major worldwide greenhouse reduction targets committed by most governments in the western world. In the near future (2020) these targets will require a significant reduction with respect to today's goals, reinforcing the importance of reducing fuel consumption. In ICEs more than one third of the fuel energy used is rejected into the environment as thermal waste through exhaust gases. Therefore, a greater fuel economy could be achieved if this energy is recovered and converted into useful mechanical or electrical power on board. For long haul vehicles, which run for hundreds of thousands of miles per year at relatively steady conditions, this recovery appears especially worthy of attention. In this paper, an Organic Rankine Cycle power unit was tested on a heavy duty diesel engine with an axial single stage turbine as the prime mover. Key differences appear between theoretical predictions and measured performances, and this invites toward an experimental real verification of the assumptions made by predictions. The most critical aspects are certainly represented by the behavior of the expander machine in mechanical power range of 2-7 kW. In the unit tested, the single stage impulse axial turbine was operated driving an electric variable speed generator and an AC/DC converter. The experimental activity gave also the possibility to go deep inside the need of reducing the thermal power to be exchanged at low temperature (by means of a radiator) and inside the management of transients conditions of the recovery unit. An overall net efficiency of the power unit was around 2-3 %, with mechanical power production equal to 2.5 kW, when the thermal power transferred from the ICE exhaust gases is 55 kW.