英语翻译

英语翻译
Valve Gear
The valve gear provides timely admission of the fresh charge into the cylinders and exhaust of spent gases from them.The valves at definite moments open and close the intake and exhaust ports in the cylinder head,through which the cylinders communicate with the intake and exhaust manifold.Valves in modern car engines are located in the cylinder head at the top of the engine.This is known as an overhead valve (OHV) configuration.There are several different arrangements of camshafts on engines.
Single overhead camshaft (SOHC)
This arrangement denotes an engine with one camshaft per head.So if it is an inline 4-cylinder or inline 6-cylinder engine,it will have one camshaft; if it is a V-6 or V-8,it will have two camshafts (one for each head).The camshaft actuates rocker camshaft that press down on the valves,opening them.Springs return the valves to their closed position.These springs have to be very strong because at high engine speeds,the valves are pushed down very quickly,and it is the springs that keep the valves in contact with the rocker arms.If the springs were not strong enough,the valves might come away from the rocker arms and snap back.This is an undesirable situation that would result in extra wear on the cams and rocker arms.
Double overhead camshaft (DOHC)
A double overhead cam engine has two camshafts per head,one for each set of inlet and exhaust valves.These engines are known as dual overhead camshaft (DOHC) engines.So inline engines have two camshafts,and V engines have four.Usually,double overhead cams are used on engines with four or more valves per cylinder -- a single camshaft simply cannot fit enough cam lobes to actuate all of those valves.
The camshaft must revolve at exactly half the speed of the crankshaft.This is accomplished with a 2:1 great ratio.The gears are linked in one of three ways:
（1）Belt drive
Such belts are made of synthetic rubber and reinforced with internal steel or fiberglass strands.The belts have teeth,or slotted spaces to engage and drive teeth on gear wheels.A belt typically is used on engines with overhead-cam valve trains.
（2）Chain drive
On some engines,a metal chain is used to connect the crankshaft and camshaft gears.Most push-rod engines and some OHC engines use this driving.
（3）Gear drive
The camshaft and crankshaft gears can be connected directly,or meshed.This type of operating linkage commonly is used on older six-cylinder,inline engines.
The valve gear is composed of timing gears,a camshaft,tappets,valves,spring with fasteners and valve guides.