Fig. 8(b) displays the variations of CO emissions. It is observed
that CO emission was dropped sharply from λ = 1.0 to 1.2 and then
almost kept the same as the mixture further leans out. This trend
is consistent with [27]. Another phenomenon is that CO emission
of engine fueled with methanol was slightly lower than that of gasoline
at each λ. This is possibly due to the more complete combustion
of methanol–air mixture as mentioned above.