A constant-volume chamber was used to research the effects of electric fields on the combustion of rich/lean methane-air flames. The input voltages were 0kV、5kV and 10kV

the excess air ratios(λ) of mixtures were 0.8 and 1.4

representing the two mixed gas states of rich combustion and lean combustion

respectively. The results show the flame propagation in the opposite direction of electric field is improved

and the flame developing velocity and the flame stretching rate are increased as the electric field becomes more intense and these behaviors are more pronounced for lean mixtures. For the mixtures of λ=0.8 and 1.4 at 10kV

the mean flame speed is increased by 36.4% and 49.5%

respectively. The improvement of flame speed enhances the combustion process

and this leads to reduction of the initial period of combustion and advancement of the timing of peak pressure. Meanwhile

the stretching effect of the flame propagation is increased. The joint functions of the two respects make the peak pressure for lean mixture increase

while the value keeps almost the same for rich mixtures. Two alternative mechanisms to explain the applied electric field enhanced effects are proposed：ionic wind effect due to negative ion neutral molecule collisions in the field direction

especially for O-2

and direction action of positive electric field on chemical kinetics via excited nitrogen molecule due to collisions with electrons.