Parametric study on effects of excess air/fuel ratio, spark timing, and methanol injection timing on combustion characteristics and performance of natural gas/methanol dual-fuel engine at low loads

A parametric investigation of the effects of excess air/fuel ratio (lambda), spark timing (ST), and methanol injection timing (MIT) on the combustion characteristics and performance of a natural gas/methanol dual-fuel engine was conducted. The operation was carried out at 1600 rpm and a low load. lambda was varied from 1.2 to 1.6, in increments of 0.1. The ST was kept close to the optimized value of brake thermal efficiency (eta(et)) and varied from advanced by an 8 degrees crank angle (8 degrees CA) to retarded by 8 degrees CA. The results indicated that the flame development period (CA0-10) and the flame propagation period (CA(10-90)) were prolonged when. increased from 1.2 to 1.6. The prolonged CA(0-10) and CA(10-90) could be shortened via the addition of methanol. eta(et) increased with an increase in lambda and the methanol energy substitution ratio (MSR). The ST corresponding to the maximum value of.et was delayed with an increase in the MSR. When the MIT is changed from -720 to 0 degrees CA BTDC, two types of air-fuel mixtures can be formed: a stratified-like mixture (when the MIT is set at the timing of intake only) and a homogeneous mixture (when the MIT is set at the timing of the intake valve closing). The most suitable MIT proved to be approximately 60 degrees CA after the intake valve closed. When the MIT was set at this point, CA(0-10) and CA(10-90) were shortened, and the coefficient of variation in the indicated mean effective pressure was reduced.