1、Unit TwoEngine ConstructionPassage B Valve Train and Its Main Components The function of the valve train or valve gear is to open and close the intake and exhaust valves at the exact time to let the air-fuel mixture enter the combustion chamber and let the exhaust gas get out of the cylinder accordi
2、ng to the firing order and requirement of working cycle of the engine The valve train contains the following main parts as shown in Fig.2-6:Valves are usually made from steel alloy They are divided into the intake valve and exhaust valve There are always at least one intake and one exhaust valve in
3、each cylinder In many modern cars,there are two intake valves and two exhaust valves,or three intake valves and two exhaust valves in each cylinder They are held in the cylinder head The intake valve opens,letting the fuel-air mixture into the combustion chamber In the compression stroke,the intake
4、valve closes,and the fuel-air mixture is compressed and ignited by the spark plug Then the exhaust gases get out of the cylinder when the exhaust valve opens The exhaust valves must withstand extremely high temperature without damaging the valves The valve face must seat evenly against the valve sea
5、t in the cylinder head to prevent leaking from the cylinder during the compression and power strokes Many valve faces and seats have an angle of 45 degrees(see Fig.2-7).Valve springs return the valves to their closed position after the valve opening These springs have to be strong enough because the
6、 valves are pushed down very quickly at high engine speed Camshaft plays a very important role in the valve train,which controls the valve timing When it rotates,the camshaft opens and closes the intake and exhaust valves in time with the motion of the piston In most modern cars,the camshaft is fixe
7、d in the cylinder head rather than in the cylinder block in the traditional engine If there is only one camshaft in each cylinder head,it is called the single overhead camshaft(SOHC)as shown in Fig.2-8a If there are two camshafts in each cylinder head,we call the double overhead camshaft(DOHC),one r
8、esponsible for intake,the other for exhaust as shown in Fig.2-8b We know that it is the crankshaft that drives the camshaft through the timing chain or timing belt In the 4-stroke engine,each time one working cycle is completed,the crankshaft rotates 2 revolutions,and the intake valve and exhaust va
9、lve in each cylinder respectively open once That is,the camshaft rotates at a half speed of the crankshaft Timing belt and chain are used to deliver the power from the crankshaft to the camshaft,making the valves open and close at the right time in order to ensure the proper intake and exhaust of th
10、e engine Timing belt is made from rubber It is low in noise,small in resistance,cheap in cost and easy in replacement Timing chain is made of steel With the development of auto technology,timing chain is fast to take the place of the timing belt Comparing with the rubber belt,the timing chain is rel
11、iable,durable and space-saving Whats more,it is long-life free maintenance.New Words and Expressions requirement rikwaimnt n 要求 withstand wistnd vt 经受,抵挡 extremely ikstrimli adv 非常,极其 temperature temprt n 温度 evenly ivnli adv 均匀地 respectively rispektivli adv 分别地 ensure in;inu vt 保证,确保 proper prp adj
12、适当的,合适的 rubber rb n 橡胶;橡皮 resistance rizistns n 阻力 reliable rilaibl adj 可靠的 durable djurbl adj 耐用的 space-saving speisseivi adj 节省空间的 long-life llaif adj 长寿的 maintenance meintnns n 维护,维修 steel alloy合金钢 fix in安装在 rather than而不是 beresponsible for对负责 take the place of代替,取代 freemaintenance免维护Common Automobile Terms valve train配气机构 valve face气门锥面 valve seat气门座 valve spring气门弹簧 valve timing气门正时 overhead camshaft顶置凸轮轴 timing chain正时链 timing belt正时带 SOHC单顶置凸轮轴 DOHC双顶置凸轮轴
