# Properties of Car-Embedded Vibrating Type Piezoelectric Harvesting System
## 3 Results and Discussion
Figure 4b shows the output voltage and generated current of piezoelectric generator I (PG I) and piezoelectric generator II (PG II), operated at 37 and 52 Hz, respectively. Both generators have a maximum peak-to-peak voltage of 6 V under a resistive load of 100 k\(\Omega\) at vibrations of 37 and 52 Hz, along with the creation of 0.04 mA current. From the voltage and current results, both generators could be calculated to have a maximum power of 0.12 mW. The optimum condition and performance of the PGI and PG II are listed in Table 3. Consequently, in order to acquire electric power in a relatively broad vibration condition ranging from 2000 to 4000 rpm, two generators were installed onto the metal block, along with an electric charging circuit and LCD display. The LCD display was attached for the direct observation of power generation without any additional connection. Figure 3: Output voltage and power of piezoelectric generator as a function of payload. (**a**) Output voltage and power versus load at 37 and 52 Hz; (**b**) Output power–voltage–load resistance of PG I at 37 Hz and PG II at 52 Hz. Figure 4: Fabrication and measurement of a single PG. (**a**) Piezo-generator, electric charging circuit, block capsulation, and measurement system; (**b**) Output voltage and power of a single PG (I and II) with payloads of 0.5 and 1.0 g. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Piezoelectric** & **Piezoelectric** \\  & **Generator I** & **Generator II** \\ \hline Resonance Frequency (Hz) & 37 & 52 \\ Payload (g) & 1 & 0.5 \\ Load Resistance (k\(\Omega\)) & 100 & 100 \\ Voltage (Vrms) & 5.5 & 5 \\ Power (mW) & 0.152 & 0.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Optimum conditions and performance of two piezoelectric cantilever generators. Figure 5 shows each component, consisting of a piezoelectric generator, charging circuit, and display. The whole assembly process is shown in Figure 5. Then, measurement of the assembled harvesting block was observed, as shown in Figure 5. The power of 0.181 and 0.248 mW was observed in accordance with a vibration acceleration of 1 g (2200 rpm) and 1.2 g (3200 rpm), respectively. Figure 5 shows the installation of a piezoelectric harvesting system on the engine of a small car. Two images show the output power of 0.038 and 0.357 mW when the car runs with an engine velocity of 2200 and 4000 rpm, respectively. Figure 5 shows the relationship between the output power and engine velocity in the car. In a narrow range between 2200 and 3200 rpm, power generation was detected. However, no energy was stored in the harvesting block when the car ran in the other rpm condition. The generated power corresponds with 52% of that in the single piezoelectric generator operated in the car engine. When the system with a double piezoelectric generator was compared to the ones with a single piezoelectric generator, the rpm condition corresponding to the maximum output was somewhat different. The maximum rpm of a system with double piezoelectric generators was slightly shifted to the lower side. This may be the effective payload change in the harvesting system. The rpm or resonance frequency of PGs is affected by the effective stiffness and effective mass or payload. Piezoelectric-type power generation devices are dependent upon the vibration amplitude and frequency. The cantilever-type piezoelectric device exhibited a significant power drop due to a small deviation in frequency. A method to store considerable power is to design a geometry structure to possess several resonant frequencies. As an example, we designed and fabricated the two cantilever generators, which have different masses, showing several resonant frequencies. The system investigated in this study can be considered  
Figure 5: Fabrication and measurement of a harvesting system with double PGs (I + II). (**a**) Fabrication; (**b**) Measurement; (**c**) Installment in engine block; (**d**) Measurement of harvesting system with a single PG (I, II) and double PGs (I + II).