Comparative study of combustion process and cycle-by-cycle variations of spark-ignition engine fueled with pure methanol, ethanol, and n-butanol at various air–fuel ratios

The combustion characteristics and cycle-by-cycle variations of a spark-ignition engine fueled with pure methanol, ethanol, and n-butanol were comparatively analyzed. All experiments were performed using a natural gas/alcohols dual-fuel engine with the alcohol fuel energy substitution ratio of 100%. The engine was kept under a low load with a brake mean effective pressure of 0.387 MPa. Engine speed was kept constant at 1600 r/min and air–fuel ratio (λ) was set in the range of 1–1.5 and varied in intervals of 0.1. The results showed that the pure methanol fuel yielded the highest peak cylinder pressure (P_max) and peak heat-release rate (HRR_max) of the engine, followed by ethanol and then n-butanol. Moreover, P_max and HRR_max decreased with the increase in λ, and the corresponding crank angles were reduced for the three primary alcohol fuels. The flame-development and flame-propagation periods of methanol were shorter than those of ethanol and n-butanol. The indicated mean effective pressure (IMEP) was distributed in a wider range with the increase in λ for the engine fueled with ethanol and n-butanol. However, for methanol, the IMEP was distributed in a relatively narrow range under all conditions of λ. When λ was increased from 1.0 to 1.5, the coefficient of variation in IMEP (COV_IMEP) of methanol increased from 1.36% to 2.65%, the COV_IMEP of ethanol increased from 1.71% to 10.46%, and the COV_IMEP of n-butanol increased from 2.06% to 15.66%. Thus, methanol had a higher burning rate, lower cycle-by-cycle variations, and a better lean-burn capability than that of ethanol and n-butanol.