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 (lambda) 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 (HRRmax) of the engine, followed by ethanol and then n-butanol. Moreover, P-max and HRRmax decreased with the increase in lambda, 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 lambda 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 lambda. When lambda was increased from 1.0 to 1.5, the coefficient of variation in IMEP (COVIMEP) of methanol increased from 1.36% to 2.65%, the COVIMEP of ethanol increased from 1.71% to 10.46%, and the COVIMEP 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.