A comparative study on the combustion and emissions of dual-fuel engine fueled with natural gas/methanol, natural gas/ethanol, and natural gas/n-butanol

The effects of the addition of primary alcohol fuels (methanol, ethanol, and n-butanol) on the combustion characteristics and performance of natural gas engines are examined by experimentation with a dual-fuel test engine, followed by a comparative analysis of the test results. The test engine was operated under a light load, at a fixed speed of 1600 rpm, and with a brake mean effective pressure of 0.387 MPa. The air-fuel ratio was held constant at the stoichiometric equivalent ratio. The engine performance was analyzed for different natural gas/alcohol fuel mixtures by using four alcohol fuel energy substitute ratios (AESRs): 0% (pure natural gas), 19%, 44%, and 60%. The experimental results indicate that faster burning rates can be obtained by mixing any of the three alcohol fuels with natural gas, compared with pure natural gas. Further, by increasing the AESR, faster flame development and flame propagation times can be obtained, and the crank angle when 50% of the mass is burned approaches the top dead center position. These outcomes lead to an advance in combustion phasing. The addition of primary alcohol fuels in the dual-fuel mode can also reduce total hydrocarbon and nitrogen oxide emissions. Moreover, the brake thermal efficiency of natural gas can be improved significantly by adding methanol, whereas negligible improvements in this aspect are obtained by adding ethanol and n-butanol. The comparative analysis indicates that methanol addition has advantages over ethanol and n-butanol additions in increasing the brake thermal efficiency and reducing the regulated emissions of natural gas engines.