| 摘要 |
PROBLEM TO BE SOLVED: To reduce an amount of unburned HC exhausted in the atmosphere by connecting each branch pipe of an exhaust manifold to a corresponding engine exhaust port, adding sub-fuel by substantially fully closing an exhaust control valve when unburned HC is to be reduced and reducing exhaust pulsation within the engine exhaust port and the exhaust manifold. SOLUTION: An exhaust port of each cylinder #1, #2, #3, #4 is connected to a branch pipe 19a of an exhaust manifold 19 and an upstream end within the branch pipe 19 is communicated by a communication pipe 20. When amount of unburned HC exhausted in the atmosphere is to be reduced, pressure (back pressure) within the exhaust port, within the exhaust manifold 19 and within an exhaust pipe 21 at an upstream of an exhaust control valve 24 becomes high when the exhaust control valve 24 arranged within the exhaust pipe 23 is substantially fully closed. Because back pressure is high, exhaust gas stagnates within an exhaust passage under a high temperature for a long time and oxidation reaction of unburned HC is promoted. Because unburned HC is lowered within a combustion chamber to reduce unburned HC to be exhausted in the atmosphere, sub-fuel is additionally injected in addition to main fuel.
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