单项选择题
The exhaust valve opens before bottom dead center in a four stroke engine to（）
Ⅰ.allow for blow down
Ⅱ.Reduce pumping losses
A.Ⅰ only
B.Ⅱ only
C.both Ⅰ and Ⅱ
D.neither Ⅰ nor Ⅱ
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Which of the turbo-charging systems listed operates with the least average back pressure in the exhaust manifold?（）
A.Constant volume
B.Constant pressure
C.Pulse pressure
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Which of the following conditions indicates the dilution of diesel engine lube oil by fuel oil?（）
A.Water discharging from the water outlet of the lube oil purifier
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If a single cylinder relief valve on a diesel engine lists frequently while the engine is running, the cause may be an （）
A.excessively late injection timing for each cylinder
B.incorrectly adjusted intake valve timing
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