**4. Organic Rankine Cycle (ORC)**

The ORC system uses a series of devices in a closed loop in order to recover energy, which can then be fed back to the engine or vehicle as required. It consists of a heat exchanger (the evaporator) that the hot gases pass through and increase the temperature of the working fluid to the extent that it changes states from liquid to superheated vapor. After the evaporator, the working fluid expands isentropically providing work in an expansion device, which is connected to an electric generator. Then the expanded vapor passes through a condenser to change its state back to liquid and reaches the pump which increases its pressure and pushes the working fluid back to the evaporator to repeat the process. Figure 1a represents a temperature-enthalpy (T-S) diagram of the Rankine cycle, while Figure 1b illustrates a schematic view of a typical Rankine cycle with all major components. Theoretically, the ORC systems can offer several benefits to the whole powertrain configuration; at best, power output of the powertrain can be increased by a maximum of approximately 15% depending on the application [26–28]. As a consequence of the increased efficiency of the engine, the fuel consumption can be decreased, making the ORC systems desirable for various applications [1,9,29].

**Figure 1.** Working principle of typical Rankine cycle: (**a**) temperature-enthalpy diagram; and (**b**) schematic of Rankine cycle components.

Furthermore, a significant advantage of the ORC systems is that they do not depend on the pressure of the exhaust gases, as the only thing that affects the efficiency of the system are the exhaust gas temperature and mass flow rate. Comparing a conventional steam Rankine cycle to an ORC, the main difference arises from the fact that ORCs use an organic fluid (instead of water) and do not need a superheater in order for the fluid to reach the desired temperature for evaporating in order to spin the turbine [15,16,18].
