Baseline Evaluation of Ignition Timing and Compression Ratio Configurations on Efficiency and Combustion Stability of a Small-Bore, Two-Stroke, Natural Gas Engine

Two-stroke engines continue to dominate the small engine market based on cost and simplicity, though companies have incorporated small four-stroke engines into handheld equipment. On the other end of the displacement spectrum, two-stroke natural gas engines are common in large-bore applications within the natural gas compression industry. Nearly 60% of homes utilize natural gas and could therefore benefit from its use as fuel for decentralized power generation. Such use for home applications does not require significant investment in infrastructure, which has limited its penetration into the transportation sector. Companies already offer back-up power generation systems for home use fueled by either natural gas or propane. These systems are often cost prohibitive and rely on four-stroke engines. The ultimate goal is to apply advanced technologies, such as direct fuel injection, to improve efficiency of small two-stroke engines. To establish a baseline, researchers developed a micro-engine test facility to examine effects of ignition timing, compression ratio, and intake and exhaust systems on efficiency and combustion stability. This research focuses on an air-cooled, spark-ignited, two-stroke engine converted to operate on natural gas. In addition to fuel conversion, an electronic ignition system replaced the stock magneto driven coil. The added trigger wheel provided a signal for control of ignition and injection timing, and for in-cylinder pressure time alignment. Engine displacement was 29-cc with a bore and stroke of 35 mm and 30 mm. Tests were performed on gasoline and a natural gas blend at an engine speed of 5400 RPM. Fuel flow was adjusted for each case to produce maximum brake torque. Two different fuel delivery methods were tested for natural gas — a mass flow controller and an electronic port fuel injector. Tests examined the effects of two compression ratios for spark timings of 15, 20, 25, and 30 CAD BTDC. Fumigation and port injection decreased efficiency compared to gasoline by 24 and 32%, respectively. Brake power also decreased by 64 and 65% on average. A similar trend occurred for delivery ratio due to the volume of fresh air displaced by natural gas. Delivery ratio of fumigation and port injection decreased compared to gasoline by 12 and 27%, respectively. The coefficient of variation in indicated mean effective pressure varied from six to 27% over compression ratio and ignition timing sweeps.