Date of Graduation
Statler College of Engineering and Mineral Resources
Mechanical and Aerospace Engineering
The free piston linear engine (FPLE) generator has the potential to displace existing crankshaft driven engine technology because of its relative simplicity, higher efficiency, and increased power density. Continued interest in hybrid-electric vehicles for transportation and tightening emissions regulations has created a challenging market for conventional piston engines. Combined with rising market interest in localized power generation means there are exciting opportunities for innovative technologies that can satisfy both regulatory and commercial demands. Many groups around the world are currently working to advance the state of the FPLE, and recent success at West Virginia University will lead to a working prototype device within the next three years.;This dissertation presents the analysis and optimization of a dual free piston, spring assisted, linear engine generator (SALEG). The primary moving part is a dual piston translator driven by 2-stroke homogeneous charge compression ignition combustion cycles such that the compression stroke for one cylinder corresponds to the expansion stroke of the other. The dynamics of the translator are augmented by the addition of springs that support higher frequency operation, provide energy storage to support cyclic stability, and can be tailored to achieve a desired translator dynamic profile. Current challenges for the device involve optimization for high efficiency performance at steady state and control of the translator position and combustion events.;Using numeric simulation tools in MATLABRTM and Simulink, the dynamic behavior of the translator is modeled in conjunction with the in-cylinder thermodynamics for each engine cylinder and the linear electric alternator load. Sweeps of the primary design parameters explore the design space while demonstrating the interdependency that is characteristic of the FPLE. Then, a genetic algorithm is employed to optimize the SALEG for efficiency based on target power and practical operating constraints. It is demonstrated that low maximum stroke to bore ratio and low intake temperature are favored. Also, the design space becomes more restrictive as target power is raised, but for a range of devices as high as 25 kW, efficiency greater than 40% can be achieved.;Control mechanisms for the simulated SALEG are demonstrated and compared. These entail the control of alternator force, engine fueling, and intake conditions through the use of proportional and integral control methods. The control methods are applied to achieve resonant start-up of the device and to respond to changes in load demand and misfire. Motored, resonant hot-start is simulated for a device with natural frequency of 40 Hz, and the linear motor and controller parameters are tested. Misfire is shown to lead to rapid loss of compression, so the motored resonant control mechanism is employed to recover after misfire. A map-based controller is used to control intake temperature in response to rapid change in load. For a 50% reduction in load, intake temperature is raised by 15% (40 °C) and results in an efficiency drop from 38% to 22% at steady state. Ultimately, the simulation tool represents a platform for future investigations where experimental data and more sophisticated modeling techniques might be included to enhance the research and advancement of the free piston linear engine.
Robinson, Matthew C., "Analysis and Optimization of a Dual Free Piston, Spring Assisted, Linear Engine Generator" (2015). Graduate Theses, Dissertations, and Problem Reports. 6516.