experimental start position in this study ranged from a
40–90 !CA BTDC; in these experiments, the coolant temperature
was 80 !C, and the experimental equivalence ratio around spark
plug was 0.8. A larger volume in the cylinder could increase the
fuel evaporation ratio and therefore improve the combustion.
Combustion at a 40 !CA BTDC and at a 50 !CA BTDC were far worse
than at other CAs because a greater proportion of the fuel must be
injected into the cylinder to increase the mixture concentration
and to ensure ignition reliability. Therefore, the combustion was
inhibited because of a too thick fuel concentration. Combustion
at a 90 !CA BTDC had the largest total released heat, but the large
volume of the cylinder increased the area of heat transfer and
made the cylinder pressure upgrade slowly.
Fig. 10 shows the maximum engine speed and the maximum
accelerated engine speed with different start positions. Although
the total released heat at a 90 !CA BTDC was the largest, the maximum
engine speed and the accelerated engine speed were not the
highest. There were two main reasons for this. First, because the
engine was static during the direct-start process, the maximum
cylinder pressure at a 90 !CA BTDC was not larger than at other
CAs, and it was even a little smaller than at an 80 and 70 !CA
BTDC. This led to relatively poor work capacity at a 90 !CA BTDC
in comparison to an 80 and 70 !CA BTDC. Another reason was that
the piston position at a 90 !CA BTDC was much closer to the button
dead center; thus, there would be additional compression resistance
from the cylinder during the power stroke, and more explosive
energy would be exhausted into the intake manifold during