Abstract:
In the development of internal combustion engines, engineers and researchers are facing the challenge of improving engine efficiency while reducing harmful exhaust emissions. Previous research has shown that lean combustion is one of the viable techniques that can improve engine efficiency while effectively reducing exhaust emissions. Lean burn engines operate at low burned gas temperatures and can achieve high thermal efficiency based on favorable mixture thermodynamic properties. However, under high dilution levels, a lean misfire limit is reached where the combustion process becomes unstable and incomplete combustion starts to occur. Instability significantly affects engine efficiency, driveability, and exhaust emissions, which limit the full potential of lean burn engines. The lean misfire limit is not only dependent on engine design but also on fuel properties. Therefore, fuels that are conducive to lean combustion can provide the opportunity for enhanced efficiency and reduced emissions. Spark ignited (SI) combustion with conventional gasoline has shown to have relatively narrow range of fuel-air equivalence ratio; therefore, it is desired to explore the lean limit of SI combustion by using alternative fuels, which can also contribute to the reduction of greenhouse gas emissions from transportation and power generation. Experiments were conducted on a Cooperative Fuel Research (CFR) engine with varying fuel-air equivalence ratio (phi) to assess the engine performance and emissions with three alternative fuels, natural gas, ethanol, and syngas, at compression ratio of 8:1 and engine speed of 1200 rev/min. Equivalence ratio was varied by decreasing the mass of fuel while keeping the mass of air the same. The lean misfire limit was defined as the equivalence ratio where the CoV of IMEP across multiple consecutive engine cycles was greater than 5%. It was found that syngas can maintain stable combustion at extremely lean conditions and has the lowest lean misfire limit. Natural gas combustion achieved a lower lean misfire limit than gasoline and ethanol. Gasoline and ethanol had similar lean misfire limits, but it was found that gasoline helped the engine to achieve higher load and fuel conversion efficiency compared to the three alternative fuels.
