characteristics. Therefore, the idling stop position of a
first-combustion cylinder should be controlled to be
between approximately 70–80 !CA BTDC to yield better
combustion performances. A higher coolant temperature
reduced the gas density and oxygen quality, and therefore,
the explosive energy was increased with lower coolant temperatures.
Because there was almost no hindrance to the
success of a direct start with different coolant temperatures
for the same equivalence ratio around the spark plug, it was
better to have a higher coolant temperature to promote fuel
evaporation.
(3) The exhaust HC emissions of the first-combustion cylinder
could be detected at the middle portion of the exhaust
stroke because there was a negative pressure difference at
the end of the power stroke. The more fuel injected into
the cylinder, the higher the exhaust HC emissions became.
Therefore, fuel could not be excessively enriched to increase
the explosive energy of the first combustion. Because the
exhaust HC concentrations at a 70–90 !CA BTDC was relatively
close to each other, the optimal start position to
reduce emissions was approximately 70–90 !CA BTDC.
Higher coolant temperatures would have less exhaust HC
emissions, and therefore, it was better to maintain a higher
idling stop temperature.
Acknowledgements
Supported by the National Natural Science Foundation of China
(Grant No. 511006098) and the National High-tech R&D Program
of China (Grant No. 2012AA111702).