Abstract: To reduce NO_x emission and improve the trade-off relationship between fuel consumption ratio (be) and NO_x under lean-burn operation, the influences of three strategies: retarding ignition timing, Exhaust Gas Recirculation (EGR) and leaner air/fuel mixture on the engine combustion performance and NO_x emission were experimentally investigated at light load on a port-injection and spark ignition methanol engine, which was modified from a diesel engine. In the experiments the engine speed, fuel consumption, air excess coefficient and ignition advance angle were fixed at 1400 r/min, 4.53 kg/h, 1.5 m and 210 °CA BTDC respectively. Results show that all three strategies can achieve the effect of NO_x purification inside the cylinder, and the NO_x emission approach zero. At the same level of NO_x emission, compared with the air/fuel mixture strategy, the combustion delaying and extension of the EGR strategy are weakened, and the coefficient of cyclic variation is decreased. The combustion of the retarding ignition timing strategy is more delayed than the leaner air/fuel mixture strategy, but the duration of combustion and the coefficient of cyclic variation are decreased. In contrast, the EGR strategy can get better be-NO_x trade-off than retarding ignition timing; however, the rise of the exhaust temperature by retarding ignition timing is helpful to improve the conversion efficiency of the catalytic converter at light load. Therefore, retarding ignition timing has the application potential to reduce emissions.

Key words: power machinery and engineering, lean-burn methanol engine, retarding ignition timing, exhaust gas recirculation, leaner air/fuel mixtures, NOx emission