**5. Engine Model Calibration**

In order to validate the engine model, a MATLAB (2015b, MathWorks, Natick, MA, USA, 2015) code was employed to produce a Brake Specific Fuel Consumption (BSFC) map which was identical to a chosen theoretical one. This theoretical BSFC map corresponded to the selected engine profile of a Ford 2000cc Zetec-SE DOHC engine (Ford Motors Company, London, UK), which is a four-cylinder engine with a 16-valve design and has a maximum Brake Mean Effective Pressure (BMEP) value of approximately 10 bar and a maximum engine speed at 5000 rpm. The BSFC map is shown in the figures below.

Following the selection of the theoretical engine configuration and BSFC map, the MATLAB code needed to be tested to determine the level of its accuracy. To accomplish this, the map was discretized at 29 different points that represent respective engine speed values and BMEP values. Particularly, the BSFC map was divided into five columns representing engine speeds from 1000 to 5000 rpm, and six rows representing BMEP values from 2 bar until reaching the maximum curve. This discretization is shown in Figure 2a with red dots.

**Figure 2.** BSFC map of Ford 2.0 L Zetec-SE DOHC engine: (**a**) predicted and discretized BSFC map of 29 points; and (**b**) the BSFC map given from optimized MATLAB code.

All of these 29 points correspond to different BSFC values, which were estimated visually and gathered into a table that was later used as an input in the MATLAB code. The MATLAB code can produce different performance maps, depending on the input values given from the user and for this current simulation the BSFC values were used, along with the engine speed (*x*-axis) and BMEP values (*y*-axis). After the optimization a new BSFC map was obtained as shown in Figure 2b, pointing out to the satisfactory accuracy of the code. For validation of the code results the Ford engine was modelled in GT-Power and the results correlated against the MATLAB code data. The GT-Power models of both the engine and the dynamic HEV sub-models are shown in Figure 3a,b, respectively.

The complete model included a throttle controller that was adjusted accordingly in the simulations, while the values of the inlet and exhaust ports were estimated after benchmarking and suggestions of the software. The table that included these BSFC values was given as an input to the MATLAB code and the BSFC that was generated matched the BSFC values of the theoretical model accurately. The next step after validating that the engine model can produce accurate values was to modify the model so that the new engine displacement was implemented. When the changes were implemented, the simulations were run for thirty discrete engine/load cases. It is important to point out that this work did not aim to evaluate the effect of a dynamic ORC bottoming cycle on the overall driving cycle of the vehicle, but to provide a discrete-point assessment of a conventional ORC layout for implementation in a hybrid vehicle.

**Figure 3.** Ford 2.0 L Zetec-SE DOHC Engine modeled in GT-Power software (7.3b2, Gamma Technologies LLC, Westmont, IL, USA, 2015): (**a**) integrated conventional engine's components; (**b**) dynamic Hybrid Electric Vehicle (HEV) subsystem integrated with the base engine components.

### **6. Organic Rankine Cycle Fluid Selection and System Optimization**

The selection of the appropriate organic fluid for each respective application is a demanding challenge for engineers since the performance and efficiency of the organic Rankine cycle system is seriously affected by the working fluid. However, the properties of the fluid are not the only criteria for selection, as the cost and the environmental impact of each fluid may limit the list of available fluids. As far as the cost is concerned, the engineer should decide which fluid would decrease the payback period and offer the maximum output and thermal efficiency at the same time. The properties of the organic fluids can be divided into four categories, each one of them being equally important for the efficient and safe operation of the ORC system. Thermodynamic properties of the organic fluids vary in several aspects, as do the density, viscosity, boiling point temperature, pressure, and the latent heat of vaporization. Each one of these parameters affect not only the thermal efficiency of the system, but also the design and construction of the respective internal combustion engine configuration. Critical and maximum operating conditions constitute the process-related properties and are linked with the efficiency of the organic Rankine cycle system. As far as the safety and environmental aspects are concerned, the toxicity and flammability of the fluid concerns engineers, while the global warming potential and ozone depletion danger are the major dangers for the environment. The working fluids should compromise among several criteria specified below [10,11,20,21,25,30–34]:


Considering all of aforementioned requirements, R245fa was selected as the working fluid in this study entirely considering prior experience and potential for widespread use. This organic fluid has no ozone impact (ODP), low global warming impact (GWP), it is non-flammable, and its thermodynamic properties fulfill the above criteria. After the selection of the working fluid, the final model was created. In GT-Power software some assumptions were considered and are shown below [2,12,13,18,22]:


The input values for the ORC model, such as exhaust temperature and exhaust mass flow rate, were obtained from a table regarding thirty different cases from 2000 cc engines at several engine speeds and BMEP ranges. Figure 4a illustrates the schematic view of the ORC system modeled in GT-Power. Data obtained included parameters such as the evaporator energy, turbine power, pump efficiency, turbine efficiency, and the pressure rise in the pump. The design parameters of the ORC system used in the simulation are presented in Table 1.

**Figure 4.** Schematic view of the engine system model in GT-Power: (**a**) organic Rankine cycle system sub-model; and (**b**) complete model used for driving cycle testing.

**Table 1.** Organic Rankine cycle component design parameters using R245fa refrigerant.

