新编汽车专业英语第3版黄汽驰电子课件Unit1PassageB.pptxVIP

Unit One　Classification of Enginesand Their Operating PrinciplesPassage B Engine Operating PrinciplesThe 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 strokes—intake stroke, compression stroke, power stroke and exhaust stroke—complete 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 engine．How the gasoline engine worksThe 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 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 rises．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 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 into 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 di