Experimental Investigation and Theoretical Modeling of Piezoelectric Actuators Used in Fuel Injectors

Abstract

Piezoelectric stack actuators are increasingly used in micropositioning applications due to their precision and responsiveness. Advanced automotive fuel injectors have recently been developed that utilize multilayer piezoelectric actuators. Since these injectors must operate under high dynamical excitations at high temperatures, understanding their thermo-electro-mechanical performance under such operating conditions is crucial to their proper design.In this thesis, the effect on soft Lead Zirconate Titanate (PZT) piezoelectric actuators of different controlling parameters relevant to fuel injection is studied experimentally.These parameters include electric-field magnitude and frequency, driving-field rise time, DC offset, duty-cycle percentage, and ambient temperature. Soft PZT actuators generate a significant amount of heat when driven under high electric-field magnitudes and/or high frequency, both of which occur in fuel injectors. They also exhibit hysteretic nonlinear behavior when driven under high electric-field magnitudes. Self-heating and hysteretic nonlinearity are interconnected, and both are undesirable in applications that require precise positioning, such as fuel injection. Self-heating in PZT stacks is considered to be caused by ferroelectric hysteretic nonlinearity, originating from domainswitching. Theoretical studies of self-heating and domain-switching in PZT materials are developed in this thesis.An analytical self-heating model based on the first law of thermodynamics is presented that accounts for different parameters such as geometry, magnitude and frequency of applied electric field, duty-cycle percentage, and surrounding properties. It also directly relates self-heating in PZT actuators to displacement-electric field loss (displacement hysteresis), which is found to increase linearly with increased temperature. The model shows reasonable agreement with experimental results at low and high electric-field magnitudes. Abstract iiiA novel domain-switching model for PZT materials is developed. The model is based on changes in potential energy, and accounts for the temperature effect on domain switching. It also accounts for full thermo-electro-mechanical coupling. Additionally, different energy levels are assumed for different domain-switching types. It is assumed that 180 o switching is a two-step process caused by two 90 o switching events. A finite element implementation of a thermo-piezoelectric continuum, based on the proposed switching model, is presented. The model shows good agreement with experimental results at different temperatures and loading conditions. iv

Design and finite element analysis of a new stack ultrasonic motor based on in-plane mode

“…In a fuel injector, PZT stack actuators control the injection process via an applied electric field. The stroke of the actuator is used to activate a valve needle, allowing the pressurized fuel to be injected [9]. PZT stack actuators are replacing solenoid valves in such applications.…”