Two-stroke engines are often used for their low cost, simplicity, and power density. However, these engines suffer efficiency penalties due to fuel short-circuiting. Increasing power density has previously been an area of focus for performance two-stroke engines — such as in dirt bikes. Smaller-displacement engines have also been used to power remote controlled cars, boats, and aircraft. These engines typically rely on gasoline or higher-octane liquid fuels. However, natural gas is an inherently knock-resistant fuel and small natural gas engines and generators could see increased market penetration. Power generators typically operate at a fixed frequency with varied load, which can take advantage of intake and exhaust system tuning. In addition, stationary engines may not be subject to size restrictions of optimal intake and exhaust systems. This paper examines methods to improve combustion stability, efficiency, and power density of a 29cc air-cooled two-stroke engine converted to operate on natural gas. Initial conversion showed significant penalties on delivery ratio, which lowered power density and efficiency. To overcome these issues a tuned intake pipe, two exhaust resonators, and a combustion dome were designed and tested. The engine was operated at 5400 RPM and fueling was adjusted to yield maximum brake-torque (MBT). All tests were conducted under wide-open throttle conditions. The intake and exhaust systems were designed based on Helmholtz resonance theory and empirical data. The engine utilized a two-piece cylinder head with removable combustion dome. The combustion dome was modified for optimal compression ratio while decreasing squish area and volume. With all designs incorporated, power increased from 0.22 kW to 1.07 kW — a factor of 4.86. Efficiency also increased from 7% to 12%. In addition to these performance gains, the coefficient of variation (COV) of indicated mean effective pressure (IMEP) decreased from just above 11% to less than 4%.

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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.

The Wiebe function is a simple and cost-effective analytical approach to approximate the burn rates in internal combustion (IC) engines. Previous studies indicated that a double-Wiebe function model can better describe the two-stage combustion process inside diesel engines retrofitted to natural gas (NG) spark ignition (SI) compared with a single-Wiebe function. Specifically, the two Wiebe functions are associated with the bowl burn and the squish burn. However, the long tail in the energy release at the end of combustion produces some differences between experiment and model, which can be attributed to the complexity of the late oxidation process inside the post-flame zone and the incomplete combustion of the unburned mixture flowing out from engine crevices. To improve the matching between the model and experimental data, this paper investigated the effect of adding a third Wiebe function just to describe the long tail in the energy release at the end of combustion. The results indicated that such a methodology greatly improved the fitting accuracy in terms of phasing and magnitude of the heat release rate in each combustion stage.