Methods to Improve Combustion Stability, Efficiency, and Power Density of a Small, Port-Injected, Spark-Ignited, Two-Stroke Natural Gas Engine

Abstract

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%.

Investigation of the Effect of Electrode Surface Roughness on Spark Ignition

“…To address these concerns, combustion systems have evolved over time, implementing a wide variety of approaches. Fuels are broadly utilized in premixed [1][2][3][4] and non-premixed combustion [5][6][7][8][9] using gas turbines and different forms of internal combustion (IC) engines such as spark ignition (SI) [10][11][12], homogeneous charge compression ignition (HCCI) [13][14][15], and reactivity controlled compression ignition (RCCI) [16,17]. Natural gas, as one of the cleanest fossil fuels, is one of the main sources of energy production and electricity generation in the United States and abroad.…”

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

“…In previous investigations using a port-injection (PI) configuration, I observed that two-stroke engines, even those with improved breathing and CR, suffered from fuel slip losses [57][58][59][60]. That is why direct injection was found to be necessary for energy optimization.…”

“…Alternatively, for two-stroke engines with fixed port geometries the exhaust design and exhaust throttling can serve as a method of inducing internal EGR. The lengths and angles of the convergent and divergent resonator sections directly impact the scavenging efficiency and thus level of internal EGR [10,18,42,44,46,59].…”

“…The multi cones have the ability to tune the engine over a wider range of engine speeds when compared to single cone [37]. Two exhaust resonators were designed and manufactured using Helmholtz theory to target operation at 5400 RPM [59]. The baseline engine came with an off the shelf (OTS) exhaust design.…”

“…Applying intake and exhaust optimization each added around 2% (absolute, each), making the engine about 12% efficient. On the original engine, we performed a study on combustion chamber shape and there were improvements in both power density and efficiency [59]. However, the engine brake power was still below 1 kW even with those improvements.…”