Effect of excess air/fuel ratio and methanol addition on the performance, emissions, and combustion characteristics of a natural gas/methanol dual-fuel engine

To investigate the effect of excess air/fuel ratio (λ) and methanol addition on the combustion characteristics, performance, and emissions, a heavy-duty, turbo-charged, six-cylinder natural gas engine was modified into a natural gas/methanol dual-fuel engine with a methanol port-fuel injection system. The engine speed was maintained at a constant value of 1600 r/min, the engine load was kept at a low value with the brake mean effective pressure at 0.387 MPa, and the spark timing was maintained near the optimized brake thermal efficiency (η_et). The results indicated that the addition of methanol decreased the flame development period (CA_0–10) and flame propagation period (CA_10–90), leading to an increased η_et and a decreased equivalent brake specific fuel consumption (BSFC), along with reduced total hydrocarbon emissions. Additionally, with an increase in λ, the burning rate of natural gas decreased owing to the decreased energy density and combustion temperature and pressure. However, η_et increased and the equivalent BSFC decreased at the lean-burn condition. The total hydrocarbon emissions increased while the nitrogen oxide emissions decreased with an increase in λ. The decreased burning rate of natural gas at the lean-burn condition can be increased by adding methanol, particularly when λ = 1.5 and 1.6. The lean-burn capability of natural gas can be improved by adding methanol.