## 2 Materials and Methods
Figure 7: Block diagram of the prototype. As shown in Table 3, significant differences exist between the designed prototype and the systems reviewed in the introduction: active rectifying, a digitally controlled DC/DC converter, air-forced adjustable ventilation, a much higher output power, the replacement of backup batteries by a SCs capacitors array and four control loops, all of which are integrated in the same FPGA. Since SCs are quite new technology and quickly evolving, before explaining the operating principle of the prototype, it is necessary to briefly describe their properties. Supercapacitors, unlike batteries, do not store and release energy through chemical reactions but rather directly accumulate electrical charges, retaining energy in the form of an electrical field. Their main disadvantage, compared to batteries, is that the amount of energy they store is lower. In the case of Li-Ion batteries, the energy density ranges between 100 and 250 Wh/kg, while in current SCs, this value is around 4-6 Wh/kg, although this situation is changing with the recent emergence of graphene SCs, whose energy density is around 110-115 Wh/kg. The main advantage of SCs is their low equivalent series resistance (ESR), which allows them to handle high power with an excellent performance and very low heating. In addition to this, the number of use cycles of SCs can reach millions under standard conditions--well above any battery--and their recycling or waste disposal does not present any drawback. The use of SCs is especially suitable in applications that involve numerous charge and discharge cycles in which high power is handled. For this reason, they are usually combined in hybrid industrial applications with other energy storage systems with higher energy density but slower dynamics. Although a more detailed description of every piece of hardware, as well as preliminary results, is presented in the next subsections, the general operating principle of the prototype is as follows: the ICU and BLDC generator are linked with a low-weight high-precision flexible coupling. The ICU, because of its extremely high specific power, is started by means of an external small DC motor. A clutch system is also used for this purpose, releasing the ICU's shaft as soon as it is started. The variable-frequency AC output voltage generated by the BLDC generator is actively rectified. Very low-internal-resistance MOSFETs (3 m\(\Omega\)), in a three-branch parallel configuration (1 m\(\Omega\)), replace diodes and transistors. They are triggered from the FPGA, with an accurate delay, to obtain the optimal commutation point. Thus, losses are reduced a great deal, as well as the weight of the rectifier. A DC/DC boost converter is connected downstream to the active rectifier. Active rectifying is also used in the converter, reaching efficiencies that vary from 91.2% to 95% depending on the ICU RPM. The converter's point of operation is also controlled by the FPGA. The charging of the SC array is under its control as well. When the flying envelope implies a low power profile, the control system decreases the ICU RPM, reducing fuel consumption, motor wear and heating. The time between inspection and service stops is therefore increased. Forced \begin{table}
\begin{tabular}{c c} \hline
**Engine Type** & **Two-Cylinder Two-Stroke** \\ \hline Engine displacement & 111.2 cc \\ \hline Global dry weight & 9.2 kg \\ \hline Rated output power (limited) & 7.75 kW \\ \hline Operating voltage & 50 V \\ \hline Fuel type & Gasoline, 95 octanes \\ \hline Ignition & Capacitive discharge \\ \hline Fuel delivery & Carburettor (optionally, injection) \\ \hline Starting & Removable starter \\ \hline Cooling (electronically controlled) & Forced air \\ \hline \end{tabular}
\end{table} 
Table 3: Prototype general specifications. ventilation is also adjusted to keep cylinders at optimal temperature. Another control loop implemented in the FPGA adjusts the airflow to maintain a constant temperature.