**1. Introduction**

The current trend of the automotive industry focuses not only on maximizing the vehicle's performance, but also in minimizing the emissions and fuel consumption of the vehicle [1,2]. Several technologies have been developed since the 1990s, when worldwide emissions standards started to impose boundaries on the levels of acceptable emissions of vehicles, which resulted in ever-reducing levels of emissions, as well as fuel consumption [3,4]. These technologies include the adoption of forced induction with the use of a supercharger and/or a turbocharger, usage of electric energy through high-capacity batteries, and combinations of both [5–7]. This study aims to examine vehicle technology that uses the combination of the above technologies, which are known as electric hybrid vehicles, and they use an internal combustion engine (ICE) combined with an electric powertrain as part of the propulsion architecture of the vehicle. As a significant amount of fuel energy used to move any vehicle equipped with an ICE is lost as heat, several systems are developed to exploit the energy of this wasted heat in order to offer it back to the engine or vehicle in the form of increased power or as an additional source of electrical energy [8–10]. The organic Rankine cycle (ORC), which is examined in this work, uses the exhaust gases of the engine to heat an organic fluid and pass it through a series of devices in order to produce mechanical or electrical power, which can be employed to enhance various aspects of engine or vehicle performance. The final stage of the current study includes the evaluation of the developed hybrid model through three driving cycles: the New European Driving Cycle (NEDC) and two EPA federal tests (FTP-75, US06) used in USA.

The motivation for this particular project arises from the fact that limited research has been conducted for the use of ORC waste heat recovery in combination with gasoline engine-equipped light hybrid electric vehicles. The ORC system is a new technology that is not ye<sup>t</sup> implemented in lightweight vehicles, not only because the cost is fairly high, but also because there is lack of evidence proving that this subsystem could operate efficiently with smaller capacity engines [11–14]. This study intends to assess the merits of implementing an ORC waste heat recovery system as part of a hybrid vehicle's powertrain, in particular its fuel consumption reduction potential. For this purpose the implementation of the ORC system is tested through two widely-used current drive cycles to further the utility of this assessment, thus increasing its value.

### **2. Operational Modes of Hybrid Vehicles**

When a vehicle is moving on the road surface three driving modes can be defined: traction mode, braking mode, and coasting mode. In traction mode the vehicle is accelerating and its force overcomes inertia, while in braking mode the vehicle is decelerating and the brakes dissipate the kinetic energy. The coasting mode refers to the phase where the vehicle is free-rolling without propulsive power from the engine or braking force from the brakes being supplied. Similarly, for hybrid electric vehicles the same operational modes apply, but are more complicated, as the electric motor extends the range of the available driving modes. Moreover, there is a distinction between the operational modes of a series hybrid vehicle and a parallel hybrid vehicle, as their power delivery operating principles differ. These operational modes include the engine drive, electric drive, hybrid, power split and braking mode, which engage different powertrain parts in each type of hybrid electric vehicle. Parallel hybrid electric vehicles use the engine drive mode as a propulsive mode when the electric motor is switched off. The electric drive mode, on the other hand, uses only the electric motor to provide vehicle motion while the engine is disengaged. In hybrid mode, the vehicle is driven by both the ICE and the electric motor, while in the power split mode the engine is not only providing motion, but also charging the battery pack [15–19]. Finally, the braking mode includes the regenerative brake, the energy of which is stored in the battery for further use. In a series hybrid electric vehicle these modes retain the same operating principles, while the differences are based on the components that are used in each mode. The engine is disengaged from the drivetrain and is only connected to a generator in a series hybrid electric vehicle and the battery alone, transfers power to the traction motors [20,21].
