新编汽车专业英语第3版黄汽驰电子课件Unit2PassageB.pptxVIP

Unit Two　Engine Construction;Passage B Valve Train and Its Main Components;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 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 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 seat 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 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 fixed 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 responsible 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