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 (λ), 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. λ 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 (ηet) and varied from advanced by an 8° crank angle (8 °CA) to retarded by 8 °CA. The results indicated that the flame development period (CA0-10) and the flame propagation period (CA10-90) were prolonged when λ increased from 1.2 to 1.6. The prolonged CA0-10 and CA10-90 could be shortened via the addition of methanol. ηet increased with an increase in λ and the methanol energy substitution ratio (MSR). The ST corresponding to the maximum value of ηet was delayed with an increase in the MSR. When the MIT is changed from −720 to 0 °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 °CA after the intake valve closed. When the MIT was set at this point, CA0-10 and CA10-90 were shortened, and the coefficient of variation in the indicated mean effective pressure was reduced.