**3. Waste Heat Recovery**

A large proportion of fuel energy in an internal combustion engine is lost as exhaust gas heat, a fact that reduces the overall engine efficiency of conventional vehicles [22]. Typical state-of-the-art waste heat recovery (WHR) technologies include mechanical or electric turbocompounding, bottoming cycles, and thermoelectric generators [1,5]. A mechanical turbocompound system uses the exhaust gas energy to spin a turbine, which can then be linked to the crankshaft, offering more output power and as much as 5% better fuel economy. On the other hand, an electric turbocompound setup can exploit the exhaust gas energy to gain electrical power that can be saved in a battery or used to support several electric components of the vehicle, increasing the fuel economy up to 10%. Currently, approximately a 2% reduction in fuel consumption can be offered by thermoelectric generators, which use the exhaust gas heat to produce electricity directly via thermoelectric conversion means [2,17,20]. This study focuses on the bottoming cycle type of waste heat recovery, which employs thermodynamic cycles to gain energy out of the exhaust gas heat. These cycles operate with a working fluid and heat exchangers, which absorb the heat of exhaust gases to change the fluid's state, which is then channeled towards a turbine for power generation. The most widely used thermodynamic cycles for this application are Rankine and Brayton, but in this particular work only the Rankine cycle is taken into consideration as the most widely-accepted and employed means of waste heat recovery [23,24]. A typical parameter that can predict the efficiency of a waste heat recovery system is the temperature of the exhaust gases, and as this temperature is increased, the amount and quality of energy that can be exploited will increase. Specifically, in the automotive industry the main types of vehicles that can benefit from a waste heat recovery system are the heavy-duty vehicles. These vehicles not only consume fuel in an excessive manner, but also produce grea<sup>t</sup> amounts of hazardous emissions, including NOx and CO2 gases. The implementation of a waste heat recovery system in such vehicles can offer energy recovery benefits in various forms that can further increase the efficiency of the powertrain by approximately 10–15% and also reduce harmful emissions [4,9,14,25].
