Experimental Design, Testing, and Evaluation of Methods to Improve the Efficiency and Reduce Emissions from a Small Two-stroke Natural Gas Engine

Abstract

Decentralized power generation is a research area of interest due to possible improvements in electrical generation efficiency and grid resilience. Two-stroke engines are simple, inexpensive, power dense systems that could serve as the prime-movers for combined heat and power (CHP) systems. In addition, such systems could be fueled on natural gas (NG) that is readily available to serve as a reliable fuel source in most households. However, most two-stroke engines are inefficient and produce excessive emissions. This research focused on methods to simultaneously improve engine efficiency and decrease emissions from a 34 cc air-cooled, two-stroke engine retrofitted to operate on NG. The engine type and size were selected for decentralized household power generation at the 1-kilowatt (kW) level. The engine utilized resonant intake and exhaust systems designed for operation at a fixed frequency of 90 Hz (5400 RPM) using Helmholtz resonance theory. Testing was conducted at wide open throttle (WOT) with electronic injection timing and ignition timing set for maximum brake torque (MBT). Two different fueling strategies and two variations in resonant exhaust systems were examined. The engine was first tested with port injection (PI) of NG with a near optimum baseline exhaust, then using low pressure direct injection (LPDI) with the near optimum exhaust, and finally with LPDI and an optimized resonant exhaust. Maximum indicated efficiency was 31.2% for the optimized LPDI system. Using PI as a baseline, LPDI increased efficiency by a relative 81% and 70% for the optimized and near optimum exhausts, respectively. Brake specific fuel consumption (BSFC) decreased by 33% and 32%, while indicated power increased by 46% and 25%, respectively. Emission levels met EPA standards for non-methane hydrocarbon plus nitrogen oxides (NMHC+NOx) and carbonmonoxide (CO) under all test configurations. At the most efficient points LPDI operation decreased NMHC+NOx by 46% for both exhausts as compared to PI, while CO increased under LPDI. This research validates that a resonance tuned, LPDI, NG, two-stroke engine could be incorporated into an emissions compliant decentralized power generation system with high efficiency at the 1 kW level. The findings will support future efforts in designing efficient, economically competitive, and clean CHP systems. I would like to thank Dr. Thompson for having selected me as an undergraduate applied thermodynamics grader, since this position led me on a direct path for graduate school. I would also like to thank Dr. Thompson for his recommending me to the new thermodynamics instructor, Dr. Derek Johnson. My utmost thanks go to Dr. Johnson for all his efforts in taking me under his wing as a new graduate student and aiming me toward success. Dr. Johnson orchestrated the engine team research, aiding in selection of components, designs, and technologies used for engine efficiency improvement. I would like to thank all the members of the WVU ARPA-E GENSETS project. Mehar Bade performed engine simulation and assi...

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

“…Two-stroke engines were deployed in small vehicle transportation, marine engines, handheld equipment, and numerous other applications over the past 13 decades. There was a brief surge of interest in twostroke engines for automotive applications during the 1980's-90's by many manufacturers with the focus of direct injection and electronic control, however deep market penetration was not realized except for marine applications [13,14].…”