单项选择题
Persistent knocking in one cylinder of a diesel engine could be caused by（）.
A.a low load on that cylinder B.sluggish piston ring action C.excessive piston crown cooling D.excessive clearance of piston pin
单项选择题 Operating conditions in the crank chamber are very （） those in the cylinder.
单项选择题 Opening of the cylinder （） will admit compressed air into the cylinder.
单项选择题 Once all cylinders are firing regularly, （） is shut off.
