# Properties of Car-Embedded Vibrating Type Piezoelectric Harvesting System
## 1 Introduction  
In recent years, automobiles have used multiple-sensor systems to monitor service conditions and optimize their operation [1, 2]. The multiple-sensor systems require many batteries and more than 100 km of electrical cables to connect them to the main electrical board installed in front of the driver's seat. The use of conventional batteries has environmental downsides, and electric cables produce a large amount of waste; therefore, there is a need to find self-powering methods for sensor systems. A self-powered system in vehicles and cars can be provided by energy harvesting, with consideration of the fact that vehicles generate large amounts of vibration and heat when in operation [3, 4, 5].  
Piezoelectric energy harvesting is a typical method of converting external mechanical vibration into electrical energy. Several considerations have been proposed for piezoelectric generators [6, 7, 8] to enhance the mechanical-to-electric energy conversion. Most cases are classified into either the mechanical impedance matching of a piezoelectric device with an external vibration source, or the electrical impedance matching between a piezoelectric device and a rectifier-storage circuit. In the electrical impedance matching between the variable electrical signal coming from piezoelectric devices and the electrical circuit designed by rectifying storage, actively controlled switching converters have been used to improve the conversion efficiency. Resonant rectifiers consisting of switching resistor-inductor-capacitor (RLC) circuits have been used to effectively transfer power from a piezoelectric device to a diode rectifier [9, 10, 11]. Furthermore, a suitable DC-DC converter was employed to regulate the DC output voltage and to draw the maximum possible power from the piezoelectric device for a specific electrical load [12, 13, 14, 15]. In mechanicalimpedance matching between an external mechanical vibration signal and the mechanical characteristics of a piezoelectric device, the usable approach is to match the resonant frequency of the piezoelectric generator with the ambient frequency of environmental vibration [16; 17]. Mechanically matching the resonance frequency of a piezoelectric generator with that of an external vibration source is the most realistic scavenging system method based on piezoelectricity [18; 19; 20]. However, matching two frequencies is difficult because the ambient vibration in a vehicle is in a variable state [21; 22; 23]. To overcome this difficulty, piezoelectric system supplements can be considered. One method is the adoption of multiple piezoelectric generators whose resonant frequencies are somewhat different. Therefore, the resonance mode covers a frequency range that matches the variable vibration frequency source. Another method is to make sure that the frequency of the modulated generator is considered in response to the variations in the external vibration source. It is also important to find the places where large vibrations in the car can be observed: the car engine and nearby. Therefore, with these two considerations, in this study, two piezoelectric generators were connected to the engine block of a small passenger car, and their effectiveness and the factors needed to control the harvesting system were evaluated. Unlike a single piezoelectric generator, multiple piezoelectric generators may interfere with each other in terms of resonant mode and power amplitude. For an example of multiple piezoelectric generators, in this study two piezoelectric generators were employed and compared with a single piezoelectric generator in terms of resonant vibration frequency and power amplitude.  
In this study, two cantilever-type piezoelectric generators were employed and encapsulated into a harvesting system box, which was installed in the engine of a small car. The performance of the piezo-generator was measured in the range of resonance frequency and evaluated at ambient vibrations.