1、Unit OneClassification of Enginesand Their Operating PrinciplesPassage B Engine Operating Principles The piston goes up and down inside the cylinder,causing the crankshaft to turn through the connecting rod Each movement of the piston is calleda stroke Four strokesintake stroke,compression stroke,po
2、wer stroke and exhaust strokecomplete one working cycle When the fourth stroke is completed,the cycle begins again Currently,almost all cars use the four-stroke working cycle to make the engine run However,the four-stroke working cycle varies slightly from the gasoline engine and the diesel engineHo
3、w the gasoline engine works The first stroke is the intake stroke The piston starts down,when the intake valve opens up,letting the fuel-air mixture into the cylinder When the piston reaches the bottom of the cylinder,the intake valve closes,trapping the fuel-air mixture in the cylinder During this
4、stroke,the exhaust valve stays closed The second stroke is the compression stroke At the bottom dead center(BDC),the piston moves back up,compressing the trapped fuel-air mixture in the cylinder with both intake and exhaust valves closed When the piston reaches the top of the cylinder,the pressure r
5、ises The third stroke is the power stroke Near the end of the compression stroke or at the top dead center(TDC),the spark plug fires,igniting the compressed fuel-air mixture that produces a powerful pressure The combustion process pushes the piston down again with great force,turning the crankshaft
6、to transmit the power to the drive wheels Thefourth stroke is the exhaust stroke With the piston at BDC,the exhaust valve opens up,and the piston moves up again and forces the burned gases out of the cylinder into the exhaust manifold From there,the burned gases travel through the exhaust system int
7、o the environment The piston goes up to TDC,exiting all the exhaust gases in preparation for starting the four-stroke cycle over again(see Fig.1-7).How the diesel engine works Most diesel engines also use a four-stroke working cycle,which is similar to,but not the same as,that of a gasoline engine H
8、eres how the diesels four-stroke working cycle works:Intake stroke The piston goes downward,the intake valve opens,letting the air into the cylinder Compression stroke The intake and exhaust valves are closed as the piston moves upward and places the air under extreme pressure As the pressure increa
9、ses,the air heats up to flash point Just before the flash point is reached,fuel injectors spray fuel into the combustion chambers at the precise instant when ignition is to take place Power stroke This stroke is the same as that of a gasoline engine The diesel fuel ignites,and combustion forces the
10、piston down This driving power is transmitted through the transmission to the wheels,causing the car to move Exhaust stroke The exhaust valve opens as the piston rises and pushes the burned gases out of the cylinderNew Words and Expressions principle prinspl n 原理 movement muvmnt n 运动 stroke struk n
11、冲程,行程 vary veri vi 变化 slightly slaitli adv 轻微地,略有 trap trp vt 使密封于 environment invairnmnt n 环境,外界 similar siml adj 相似的 downward daunwd adv 向下 upward pwd adv 向上 spray sprei vt 喷射 precise prisais adj 精确的,明确的 instant instnt n 瞬间 working cycle工作循环 in preparation for为准备 heat up加热,变热 flash point燃点Common A
12、utomobile Terms transmission trnzmin n 变速器 connecting rod连杆 intake stroke进气行程 compression stroke压缩行程 power stroke做功行程 exhaust stroke排气行程 four-stroke working cycle四冲程工作循环 intake valve进气门 exhaust valve排气门 bottom dead center(BDC)下止点 top dead center(TDC)上止点 exhaust system排气系统 exhaust manifold排气歧管 fuel injector燃料喷射器,喷油器 exhaust emissions
