3.2.3. Effect of coolant temperature on combustion characteristics
Fig. 13 shows the results of the combustion characteristics with
different coolant temperatures. Fig. 14 shows the maximum engine
speed and the maximum accelerated engine speed after the first
combustion with different coolant temperatures. The experimental
coolant temperature ranged from 60–90 !C to mimic the engine
temperature after an idling stop. The start position of these experiments
was at an 80 !CA BTDC, and the equivalence ratio around
the spark plug was 1. Higher temperatures could promote the
evaporation of the fuel injected into the cylinder, so less fuel was
needed with a coolant temperature of 90 !C than with that of
60 !C. On the other hand, the higher the coolant temperature
was, the smaller the gas density and oxygen quality were.
Although the combustion speed at 60 !C was the slowest, this
speed released the most heat. Additionally, the maximum engine
reversal speed at 60 !C was larger than at other temperatures.
Therefore, lower coolant temperatures may produce better direct
start performances.
3.3. HC Emissions characteristics of first-combustion cylinder
Fig. 15 shows the exhaust HC concentration with different
experimental equivalence ratios. The fuel had been enriched to
reach the desired equivalence ratio around the spark plug. The larger
the experimental equivalence ratio was, the greater the fuel
mass that needed to be injected. From Eq. (1), it could be concluded
that the amount of unburned fuel increased with an increase in the
equivalence ratio and therefore produced more exhaust HC emissions.
In the premise of ensuring the reliability of a direct start,
the injected fuel quantity should not be too large because a too
thick fuel concentration worsened combustion and increased the
HC emissions. The optimal injected fuel mass produced an equivalence
ratio around the spark plug of approximately 0.6–0.8.
Fig. 16 shows the exhaust HC concentration with different start
positions. The HC emissions increased with a decrease in cylinder
volume, but the emission at a 90 !CA BTDC was not the lowest.
The burning efficiency of a 60–90 !CA BTDC did not have large differences,
but the absolute injected fuel mass at a 90 !CA BTDC had
a slightly higher efficiency and a slightly higher unburned fuel
mass. In consideration of the uncertainty of the emissions