In order to improve the working conditions of the crosshead bearings, the bearing pressure is made and the peripheral speed is made in later designs（）

单项选择题In order to control thermal stresses, some types of modern engines use pistons with（）

A.heavy wall and intensive cooling
B.heavy wall and no cooling
C.thin wall and no cooling
D.thin wall and intensive cooling

单项选择题In order to keep the oil film on the cylinder liner during the running-in period, the piston rings are slightly rounded（）

A.on both the external and internal top edges
B.on both the external top and bottom edges
C.on both the external and internal bottom edges
D.on all the four edges

单项选择题In some type of two-stroke engines, besides some small holes for（）, starting valve, safety valve, indicator cock, and central opening in each cylinder cover for mounting the（）

A.fuel valves；exhaust valve
B.exhaust valves；inlet valve
C.fuel valves；inlet valve
D.exhaust valves；fuel valve

单项选择题In a two-stroke engine, there are always a series of openings known as （）, and in some circumstances, there are also openings known as （）, in the part of the cylinder liner.

A.scavenging air ports；exhaust ports
B.inlet ports；scavenging air ports
C.scavenging air ports；inlet ports
D.exhaust ports；inlet ports

单项选择题If the engine is naturally aspirated, or is a small high-speed type with a centrifugal turbocharger, the period of valve overlap（）,ie when both valves are（）, will be and the exhaust valve will close some 10°top dead canter（）.

A.open；short；after
B.closed；short；after
C.open；long；before
D.closed；long；before