Recovery of exhaust and coolant heat with R245fa organic Rankine cycles in a hybrid passenger car with a naturally aspirated gasoline engine

In internal combustion engines, only a part of the fuel energy flow is transformed into power available at the crankshaft, while the most part of the fuel energy flow is lost as coolant, exhaust gases and other waste heat flows. Recovery of waste heat from the exhaust gases, and the coolant with organic Rankine cycles (ORC) is considered here for a hybrid vehicle powered by a 1.8 L naturally aspirated gasoline engine. The ORC systems fitted on the exhaust and the coolant permit an increase in fuel conversion efficiency by up to 6.4% and 2.8% individually, and by up to 8.2% combined. The average improvements all over the map are 3.4%, 1.7% and 5.1% respectively. These gross improvements do not account for the less than uniform efficiency of the mechanical-to-electric-to-chemical-to-electric-to-mechanical loop when the ORC expanders are used to charge the battery of the hybrid vehicle. Nor do they account for the reduced efficiency of the thermal engine due to the back pressure effects on the indicated mean effective pressure (exhaust ORC) and friction mean effective pressure (coolant ORC). Nevertheless, these values serve as a reference point for the assessment of the current potential of a technology that is still being developed having major downfalls in the increase of weight, costs, packaging complexity and finally in difficulty in transient operation. (C) 2011 Elsevier Ltd. All rights reserved.