A Framework for Energy Optimization of Small, Two-Stroke, Natural Gas Engines for Combined Heat and Power Applications

Abstract

A Framework for Energy Optimization of Small, Two-Stroke, Natural Gas Engines for Combined Heat and Power Applications Mahdi Darzi This research was focused on the development of a framework for the energy optimization a natural gas engine for small combined heat and power (CHP) applications operating with lowpressure natural gas (NG) available at homes. The required targets of the ARPA-E GENSETS program for 1 kW electric power generation and previous GENSETS simulation at West Virginia University, suggested a design space for the engine which is currently dominated by simple twostroke designs. Hence this research focused on screening, modeling, and experimentally evaluation of cost-effective technologies to achieve a system design that maximized thermal efficiency and utilization factor, while reducing emissions. A baseline engine was selected which started with 8% brake thermal efficiency (BTE) on natural gas operation. By implementing intake and exhaust optimization BTE increased up to around 12% and with head design optimization and exhaust backpressure, the engine reached 15% BTE. Though the initial optimization methods almost doubled efficiency, it was realized that further increases in BTE must be achieved for a commercially competitive design. The engine was redesigned to utilize modified porting for enhanced breathing and scavenging and a method to deploy low-pressure direct injection (LPDI) was developed. With these enhancements, BTE increased to around 24%. A second round of optimization was performed then, by modeling the whole system in a 1D platform and using a genetic algorithm and experimental data to further optimize scavenging which increased the efficiency up to around 26%. Finally, a 3D computational fluid dynamics (CFD) was developed to investigate the stratification effects of LPDI and determine the optimal spark plug location. This effort increased the BTE to around 27.5%. To assess issues with commercial deployment, three different NG compositions and propane operation were investigated. With an energy balance and exergy distribution analysis for each fuel, fuel quality effects on the CHP system performance were determined. At the end, with combination literature review, experiments, 1D and 3D simulations, a framework for optimization of micro-CHP systems operating on gaseous fuels was developed that can serve industry to highlight methods that can more than triple brake thermal efficiency of small two-stroke engines while improving the energy distribution for CHP systems. iii Acknowledgment Special thanks to Dr. Johnson for his support and mentorship throughout my PhD program. I thank my dedicated PhD committee members for their guidance and valuable feedback to improve this dissertation. I would like to thank Mr.