# Properties of Car-Embedded Vibrating Type Piezoelectric Harvesting System
## 3 Results and Discussion  
Figure 2 shows the general specifications of a piezoelectric generator, its employment on the engine block of a small passenger car, and the acceleration frequency of the engine block investigated in this study. A passenger car with 1000 cc capacity was chosen and the vibration of the engine block, with respect to frequency, was measured. The piezoelectric generators and their embedded energy harvesting system were designed according to the specifications of the car. Vibration amplitude versus frequency in the engine block was measured by an acceleration sensor (ReVibe Energy, Model D). The sensor was attached to the bottom of the car's engine block. As shown in Figure 1b, the 37 to 52 Hz range was defined as the resonance frequency in the block. The acceleration peaks of 23-28, 34-40, and 48-54 Hz, observed in the figure, correspond with 1400-1800, 2000-2400, and 2900-3300 rpm (revolutions per minute) of the engine, respectively. To match the resonance piezoelectric harvesting frequency to that of the block, the two piezoelectric generators were used. According to the frequency-engine rpm relation and the acceleration-frequency relation, the largest vibration of >1 g (g = 9.8 m/s) was generated from the engine in the rpm range of 2000-2400 and 2900-3300. Therefore, the piezoelectric-type generator was designed to operate in resonance mode in 34-40 Hz (37 Hz in peak) and 48-54 Hz (52 Hz in peak), because cars have an engine operation with an rpm of 2000-4000 during normal performance. The basic concept of energy harvesting, consisting of double piezoelectric generators with different payloads, is shown in Figure 3. The wide frequency range was selected as the resonance frequency range using a technique of serially connecting different masses. The masses were connected by a spring strip. Therefore, the resonance frequency characteristics of each mass could be observed, and a broad range of high vibration is presented. The position of the piezoelectric cantilever generator is the same, except for differences in mass. Figure 3 shows the output voltage and power versus payload in a piezoelectric generator operated at 37 and 52 Hz. At 37 and 52 Hz, the optimum piezoelectric generator was determined by a change in the payload of 0.25-2.0 g. In the case of 37 Hz, the piezoelectric generator with a payload of 1.0 g had maximums of 5.5 V and 0.152 mW. In the vibration at 52 Hz, a maximum voltage of 5 V and power of 0.190 mW were observed in the generator with a payload of 0.5 g. With a single piezoelectric device, a block consisting of a single piezoelectric generator and energy conversion circuit was designed, as shown in Figure 4. Each generator was separately connected to an electric storage circuit, due to the interference between generators. The circuit consists of a full diode bridge and charging capacitor. In addition, in the case of installation in the engine of a small car, the power was measured by connecting the LCD display with the power charging circuit. The LCD display shows the power (mW in unit) produced by the piezoelectric generators. Figure 2: Schematic illustrations of the piezoelectric generator, its employment in the engine block of a small passenger car, and acceleration–frequency relationship of the engine block. **(a)** Specifications of piezoelectric generator, energy harvesting system, and engine blocks of the passenger car investigated in this study; **(b)** Schematic illustration of the passenger car and acceleration–frequency relation of the engine block (23–28 Hz: 1400–1800 rpm, 34–40 Hz: 2000–2400 rpm, 48–54 Hz: 2900–3300 rpm). Figure 4 shows the system block, consisting of a single piezoelectric generator and charging circuit, and its measurement. Figure 4a contains photos showing the attachment of a piezoelectric generator onto the metal block, the positioning of an electric charging circuit inside the block, encapsulation of the block with an upper cover, and the measurement of a piezoelectric generator installed on the electromagnetic shaker.