【正文】 18th century when the first automobile was put on road, this industry has developed tremendously. Now there are thousands of factories all over the world manufacturing numerous types of automobiles. This industry employs crores of men and women directly and indirectly in allied industries. The automobile engines are also being used in engine powered machines for agriculture, construction and manufacturing processes. Various types of small engines are also being used in lawn movers, power saws, snow removers and similar equipment. The automobile industry is a developing and demanding industry which does not find its end or saturation point. There is a great demand for varied types of automotive products, vehicles and engines. There is also a great demand for trained and experienced persons in this industry for diagnosing motor vehicle troubles, repairing and replacing engines ponents, transmissions, propeller shafts, differentials, axles, steering system ponents,brake system ponents, suspension ponents, airconditioners, heaters, body and glass work. The automobile consists of five basic mechanisms, or ponents. These are :(1) The engine, which is the source of power, including the fuel, lubricating, cooling, and electric systems(2) The frame, which supports the engine, wheels, steering and brake systems, and body(3) The power train, which carries the power from the engine(through the clutch,transmission, drive shaft, differential, and axles)to the car wheels (4) The car body (5) carbody accessories, including heater ,lights, windshield wipers, and so forthThe engine is the source of power that makes the car move. It is usually called an internal bustion engine because gasoline is burned inside the engine cylinders, or bustion chambers. This is in contrast to externalbustion engines(such as steam engines),where the bustion takes place outside the engine. The burning of gasoline in the engine cylinders produces the power. The power is then carried from the engine through the power train to the car wheels so that the wheels turn and the car moves. The fuel system plays a vital part in the power producing process since it supplies the gasoline to the engine cylinders. In each engine cylinder, a mixture of gasoline vapor and air enters the cylinder, the piston pushes up into the cylinder to press the mixture, and then an electric spark ignites the pressed mixture so that the piston is forced downward. Of course, in the engine the piston is not blown pletely out of the cylinder, the piston simply moves up and down in the cylinderup to press as the mixture burns. The piston straightline motion must be changed to rotary motion before it can be used to make the car wheels rotate. A connecting rod and a crank on the engine crankshaft make this change. The engine valves get the burned gasoline vapor out of the engine cylinder andbringing fresh changes of gasoline vapor and air into the cylinder. There are two openings ,or ports, in the enclosed end of the cylinder ,each containing a valve. The valve are accurately machined plugs on long stems. When they are closed, or seated (that is, moved up into the ports), the ports are sealed off and gas cannot pass through the ports. When the valve is open, gas can pass through the port. The valves are opened by cams on the engine camshaft. The cam has a high point,or lobe。 every time the cam rotates, the lobe es around under the valve lifter and move it upward. The lifter then carries this upward movement through the pushrod to the rocker arm. The rocker arm pivots on its support and pushes down on the valve stem, causing the valve to move down, that is, to open. After the cam has turned enough to move the lobe out from under the lifter, the heavy valve spring pulls the valve back into its seat. The spring is attached to the upper end of the valve stem by a spring retainer and lock. There is a cam for each valve (two cams per cylinder) on the engine camshaft. The camshaft is driven off the crankshaft by gears or by sprockets and a chain. When the entire cycle of events requires four piston strokes (two crankshaft revolutions), the engine is called a fourstrokecycle engine, or a fourcycle engine. The four strokes are intake, pression, power, and exhaust. On the intake stro