In slow speed two-stroke engines,（）the fresh air entering begins when and stops as（）

单项选择题Usually we take the cooling effect of the air flown through the cylinder during the overlap period as（）

A.inevitable
B.harmful
C.helpful
D.unacceptable

单项选择题When the air charge contacts with the cylinders and pistons and mixes with the residual gases, it will（）

A.increase in temperature
B.lose its energy partly
C.expand and do work
D.begin to be compressed

单项选择题“This increase in charge air density is accomplished on most modern diesel engine types by use of exhaust gas turbo-charging, in which a turbine wheel driven by exhaust from the engine is rigidly coupled to a centrifugal type air compressor” According to this sentence, we can know that increases density（）, drives the compressor directly（）

A.a modern diesel engine；a turbine wheel
B.a modem diesel engine；exhaust gases
C.a compressor；an exhaust gas turbocharger
D.a compressor；a turbine wheel

单项选择题Gas exchange, which is a basic part of the cycle of an internal combustion engine, is（）

A.the supply of normal air and removal of compressed air
B.the supply of fuel oil and combustion of the atomized oil
C.the supply of fresh air and removal of exhaust gases
D.the supply of fresh air and removal of the extra heat

单项选择题“The use of a poor quality oil can lead to rapid deterioration of its properties with the resultant formation of deposits in the piston crown which in turn results in a considerable reduction in heat transfer with subsequent overheating and thermal cracking” According to this sentence, we can know that the principle reason of the overheating and thermal cracking is（）

A.the use of a poor quality oil
B.the deterioration of its properties
C.the formation of deposits
D.the reduction in heat transfer ability