大学英语Lesson 3 Engine Constructions.ppt

Lesson 3 Engine Constructions Internal combustion gasoline engines run on a mixture of gasoline and air.?The ideal mixture is 14.7 parts of air to one part of gasoline (by weight).?One part of gas that is completely vaporized into 14.7 parts of air can produce tremendous power when ignited inside an engine. Air enters the engine through the air cleaner and proceeds to the throttle plate. You control the amount of air that passes through the throttle plate and into the engine with the gas pedal.?It is then distributed through a series of passages called the intake manifold, to each cylinder.?At some point after the air cleaner, depending on the engine, fuel is added to the air-stream by either a fuel injection system or, in older vehicles, by the carburetor. Once the fuel is vaporized into the air stream, the mixture is drawn into each cylinder as that cylinder begins its intake stroke.?When the piston reaches the bottom of?the cylinder, the intake valve closes and the piston begins moving up in the cylinder compressing the charge.?When?the piston reaches the top, the spark plug ignites the fuel-air mixture causing a powerful expansion of the gas, which pushes the piston back down with great force against the crankshaft. Engine Types The majority of engines in motor vehicles today are four-stroke, spark-ignition internal combustion engines.? There are several engine types which are identified by the number of cylinders and the way the cylinders are laid out.?Motor vehicles will have from 3 to 12 cylinders which are arranged in the engine block in several configurations. The most popular of them are shown on the above.?In-line engines have their cylinders arranged in a row. The V arrangement uses two banks of cylinders side-by-side and is commonly used in V-6, V-8, V-10 and V-12 configurations. Flat engines use two opposing banks of cylinders and are less common than the other two designs.?Flat engines are also used in some Ferraris with 12 cylinders. Most engin