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 مقدمة عن محرك الإحتراق الداخلى

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Important Eng
Important Eng
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ذكر عدد الرسائل : 2997
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تاريخ التسجيل : 20/10/2007

مُساهمةموضوع: مقدمة عن محرك الإحتراق الداخلى   الإثنين فبراير 18, 2008 12:11 am

The automobile is a familiar object to all of us. The engine that moves it is one of the most fascinating and talked about of all the complex machines we use today. In this chapter we will explain briefly some of the operational principles and basic mechanisms of this machine. As you study its operation and construction, notice that it consists of many of the devices and basic mechanisms covered earlier in this book.

COMBUSTION ENGINE

We define an engine simply as a machine that converts heat energy to mechanical energy. The engine does this through either internal or external combustion.

Combustion is the act of burning. Internal means inside or enclosed. Thus, in internal combustion engines, the burning of fuel takes place inside the engine; that is, burning takes place within the same cylinder that produces energy to turn the crankshaft. In external combustion engines, such as steam engines, the burning of fuel takes place outside the engine. Figure 12-1 shows, in the simplified form, an external and an internal combustion engine.

The external combustion engine contains a boiler that holds water. Heat applied to the boiler causes the water to boil, which, in turn, produces steam. The steam passes into the engine cylinder under pressure and forces the piston to move downward. With the internal

Figure 12-2.-Cylinder, piston, connecting rod, and crankshaft for a one-cylinder engine.

combustion engine, the combustion takes place inside the cylinder and is directly responsible for forcing the piston to move downward.

The change of heat energy to mechanical energy by the engine is based on a fundamental law of physics. It states that gas will expand upon the application of heat. The law also states that the compression of gas will increase its temperature. If the gas is confined with no outlet for expansion, the application of heat will increase the pressure of the gas (as it does in an automotive cylinder). In an engine, this pressure acts against the head of a piston, causing it to move downward.

As you know, the piston moves up and down in the cylinder. The up-and-down motion is known as reciprocating motion. This reciprocating motion (straight line motion) must change to rotary motion (turning motion) to turn the wheels of a vehicle. A crank and a connecting rod change this reciprocating motion to rotary motion.

All internal combustion engines, whether gasoline or diesel, are basically the same. They all rely on three elements: air, fuel, and ignition.

Fuel contains potential energy for operating the engine; air contains the oxygen necessary for combustion; and ignition starts combustion. All are fundamental, and the engine will not operate without any one of them. Any discussion of engines must be based on these three elements and the steps and mechanisms involved in delivering them to the combustion chamber at the proper time.

DEVELOPMENT OF POWER

The power of an internal combustion engine comes from the burning of a mixture of fuel and air in a small, enclosed space. When this mixture burns, it expands; the push or pressure created then moves the piston, thereby cranking the engine. This movement is sent back to the wheels to drive the vehicle.

Since similar action occurs in all cylinders of an engine, we will describe the use one cylinder in the development of power. The one-cylinder engine consists of four basic parts: cylinder, piston, connecting rod, and crankshaft (shown in fig. 12-2).

The cylinder, which is similar to a tall metal can, is closed at one end. Inside the cylinder is the piston, a movable metal plug that fits snugly into the cylinder, but can still slide up and down easily. This up-and-down movement, produced by the burning of fuel in the cylinder, results in the production of power from the engine.

You have already learned that the up-and-down movement is called reciprocating motion. This motion must be changed to rotary motion to rotate the wheels or tracks of vehicles. This change is accomplished by a crank on the crankshaft and a connecting rod between the piston and the crank.

The crankshaft is a shaft with an offset portion-the crank— that describes a circle as the shaft rotates. The top end of the connecting rod connects to the piston and must therefore go up and down. Since the lower end of the connecting rod attaches to the crankshaft, it moves in a circle; however it also moves up and down.

When the piston of the engine slides downward because of the pressure of the expanding gases in the cylinder, the upper end of the connecting rod moves downward with the piston in a straight line. The lower end of the connecting rod moves down and in a circular motion at the same time. This moves the crank; in turn, the crank rotates the shaft. This rotation is the desired result. So remember, the crankshaft and connecting rod combination is a mechanism for changing straight-line, up-and-down motion to circular, or rotary, motion.

BASIC ENGINE STROKES

Each movement of the piston from top to bottom or from bottom to top is called a stroke. The piston takes two strokes (an upstroke and a downstroke) as the crankshaft makes one complete revolution. When the piston is at the top of a stroke, it is said to be at top dead center. When the piston is at the bottom of a stroke, it is said to be at bottom dead center. These positions are rock positions, which we will discuss further in this chapter under "Timing." See figure 12-3 and figure 12-7.

The basic engine you have studied so far has had no provisions for getting the fuel-air mixture into the cylinder or burned gases out of the cylinder. The

enclosed end of a cylinder has two openings. One of the openings, or ports, permits the mixture of air and fuel to enter, and the other port permits the burned gases to escape from the cylinder. The two ports have valves assembled in them. These valves, actuated by the camshaft, close off either one or the other of the ports, or both of them, during various stages of engine operation. One of the valves, called the intake valve, opens to admit a mixture of fuel and air into the cylinder. The other valve, called the exhaust valve, opens to allow the escape of burned gases after the fuel-and-air mixture has burned. Later you will learn more about how these valves and their mechanisms operate.

The following paragraphs explain the sequence of actions that takes place within the engine cylinder: the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. Since these strokes are easy to identify in the operation of a four-cycle engine, that engine is used in the de******************ion. This type of engine is called a four-stroke-Otto-cycle engine, named after Dr. N. A. Otto who, in 1876, first applied the principle of this engine
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