The variations of HC emissions of engine fueled with gasoline
and methanol are shown in Fig. 8(a). It is clearly showed that engine
fueled with methanol emitted less HC than fueled with gasoline at
each λ. This is partly explained by the improving homogeneity of
air–fuel mixture as mentioned above, which results in more complete
combustion and consequently reduces HC emission. Moreover,
due to its faster flame propagation speed, methanol’s flame quench
distance is believed to be shorter than that of gasoline [24], making
the flame propagate closer to the combustion chamber wall and
further into the thin volumes in cylinder, thus reducing the HC emissions
originated from the flame quenching and crevices. It is also
seen from Fig. 8(a) that HC emissions of methanol as well as gasoline
were decreased first fromλ = 1.0 to 1.2 and then were increased
from λ = 1.2 to 1.4. This is mainly because appropriate lean burn condition
(λ = 1.2) benefits the complete combustion, but with further
dilution of air–fuel mixture (λ = 1.4), the instability of combustion
would be amplified, leading to the more HC emission