单项选择题
A.up and draw in a fresh air charge
B.down to burn off fuel
C.down to compress the fuel air charge
D.up and force out the exhaust gases
单项选择题 A fuel injection valve opening at pressure lower than normal will result in（）.
单项选择题 In a diesel engine, exhaust valves open before the intake ports are uncovered to （） Ⅰ.reduce pumping losses Ⅱ.reduce back pressure
单项选择题 Which of the engine components listed increases air charge density and helps to improve engine operating efficiency?（）