measurements, it seemed that the exhaust HC concentrations at a
70–90 !CA BTDC were relatively close to each other, and the optimal
start position for the emissions was at an approximately 70–
90 !CA BTDC.
Fig. 17 shows the exhaust HC concentration with different coolant
temperatures. A 60 !C coolant temperature had the highest HC
emissions due to the largest injected fuel quantity and the poorest
evaporation. Although the combustion performance and engine
piston movement seemed to be optimal with a 60 !C colorant temperature,
far more HC emissions would seriously pollute the environment.
Raising the coolant temperature did not cause too many
problems on the reliability of the direct start, and there were not
large differences in the direct-start performances with different
coolant temperatures. Therefore, an appropriate increase in the
coolant temperature after an idling stop would be beneficial to
reduce the HC emissions during the direct-start process.
4. Conclusion
In this study, the combustion and emission characteristics of
the first-combustion cylinder during the direct-start process were
investigated by experiments on a 2.0 L DISI engine. The in-cylinder
mixture concentration, cylinder pressure, engine speed and
exhaust HC concentration were used to analyze mixture evaporation,
combustion, engine movement and engine emissions, respectively,
and the optimal control strategies under different operating
conditions of a direct start were obtained. The following conclusions
based on the experimental results are presented here.
(1) Fuel should be enriched and injected into the
first-combustion cylinder to ensure that the mixture concentration
around the spark plug results in a successful ignition
and that at least one-third of the injected fuel would not
contribute to the air–fuel mixture and participate in combustion.
With a large volume of the cylinder injected fuel
evaporated, only 30% of the fuel could form the mixture
when the start position was at a 40 !CA BTDC. A higher coolant
temperature promoted fuel droplet evaporation.
(2) The piston started to move at almost the end of the first
combustion, and the cylinder pressure reached atmospheric
pressure when the intake valve opened. To ensure a
sufficient ignition mixture concentration around the spark
plug, more fuel needed to be injected, which resulted in
worse combustion. The optimal injected fuel mass should
be as small as possible without causing misfire. Although
the combustion at a 90 !CA BTDC was optimal, the start
position at 70–80 !CA BTDC produced better movement