【机械类文献翻译】发动机的分类和总体结构
外文翻译
1 Engine Classification and Overall Mechanics
The automobile engines can be classified according to :1.number of cylinders ;2.arrangement of cylinders ;3.arrangement of valves ;4.type of cooling ;5.number of cycles (two or four );6.type of fuel burned ;7.type of ignition .
The engine is the source of power that makes the wheels go around and the car move .the automobile engine is an internal-combustion engine because the fuel (gasoline ) is burned inside it .the burning of gasoline inside the engine produces high pressure in the engine combustion chamber .this high pressure in the engine combustion chamber .this high pressure forces piston to move ,the movement is carried by connecting rods to the engine crankshaft .the crankshaft is thus made to rotate ;the rotary motion is carried through the power train to the car wheels so that they rotate and the car moves .
The engine requires a fuel system to supply it with a mixture of air and fuel .the fuel system does this by pumping liquid gasoline from tank into the carburetor a mixing device that mixes the gasoline with air .the mixture is delivered to engine where it is burned.
The engine also needs a cooling system, the combustion of the air-fuel mixture in the engine creates a very high temperature (as high as 2000 to 2700 C).the cooling system take heat away from the engine by circulating a liquid coolant (water mixed with antifreeze ) between the engine and a radiator .the coolant gets hot as it goes through the engine .it cools off as it goes through the radiator. Thus ,the coolant continually takes heat away from the engine ,where it could do damage ,and delivers it to the radiator. Air passing though the radiator takes heat away from the radiator.
The engine also includes a lubricating system. The purpose of the lubricating system is to supply all moving parts inside the engine with lubricating oil ;the oil keeps moving parts from wearing excessively .
The engine requires a fourth system ,the ignition system. The ignition system provides high-voltage electric sparks that ignite, or set fire to ,the charges of air-fuel mixture in the engine combustion chambers.
The fifth is starting system and its purpose is to change the electrical current into the mechanical energy to push the crank-shaft around. By means of this ,the engine can be started.
2 Four –stage –engine Operation
The action taking place in the engine cylinder can be divided into four stages ,or strokes. Store refers to piston movement ;a stroke occurs when the piston moves from one limiting position to the other. The upper limit of piston movement is called TDC (top dead center ). The lower limit of piston movement is called BDC (bottom dead center ). A stroke is piston movement from TDC to BDC or from BDC to TDC. In other words ,the piston completes a stroke each time it changes its direction of motion .
Where the entire of events in the cylinder requires four strokes (or two crankshaft revolution ),the engine is called a four-stroke-cycle engine ,or a four-cycle engine. The four piston strokes are intake ,compression power ,and exhaust .
Intake stroke. On the intake stroke ,the intake valve has opened ,the piston is moving downward ,and a mixture of air and vaporized gasoline is entering the cylinder through the valve port. The mixture of air and vaporized gasoline is delivered to the cylinder by the fuel system and carburetor .
Compression stroke. After the piston reaches BDC ,or the lower limit of its travel ,it begins to move upward. As this happens ,the intake valve closes. The exhaust valve is also closed ,so that the cylinder is sealed. as the piston moves upward (pushed now by the revolving crankshaft and connecting rod ),the air-fuel mixture is compressed. By the time the piston reaches TDC ,the mixture has been compressed to as little as one-tenth of its original volume ,or even less. This compression of the air-fuel mixture increases the pressure in the cylinder. When the air-fuel mixture is compressed ,not only does the pressure in the cylinder go up ,but the temperature of the mixture also
increases .
Power stroke. As the piston reaches TDC on the compression stroke ,an electric spark is produced at the spark plug. The ignition system delivers a high-voltage surge of electricity to the spark plug to produce the spark.. The spark ignites, or air-fuel mixture. It now begins to burn very rapidly ,and the cylinder pressure increases to as much as 3-5MPa or even more. This terrific push against the piston forces it downward ,and a power impulse is transmitted through the connecting rod to the crankpin on the crankshaft .the crankshaft is rotated as the piston is pushed down by the pressure above it .
Exhaust stroke. As the piston reaches BDC again ,the exhaust valve opens. Now ,as the piston moves up on the exhaust stroke ,it forces the burned gases out of the cylinder through the exhaust-valve port. Then ,when the piston reaches TDC ,the exhaust valve closes and the intake valve opens. Now a fresh charge of air-fuel mixture will be drawn into the cylinder as the piston moves down again toward BDC. The above four strokes are continuously repeated.
3 Two –stage-engine Operation
In the four-stroke-cycle engine ,already discussed in Lesson 1,2,the complete cycle of events requires four piston strokes (intake ,compression ,power and exhaust ). In the two-stroke-cycle ,or two-cycle ,engine ,the intake and compression strokes and the power and exhaust strokes are in a sense combined. This permits the engine to produce a power stroke every two piston strokes ,of every crankshaft rotation .
In the two-cycle engine ,the piston acts as a valve ,clearing valve ports in the cylinder wall as it nears BDC .
A fresh air-fuel charge enters through the intake port ,and the burned gases exit through the exhaust port. The complete cycle of operation is as follows: as the piston nears TDC, ignition takes place. The high combustion pressures drive the piston down ,and the trust through the connecting rod turns the crankshaft. As the piston nears BDC, it passes the intake and exhaust ports in the cylinder wall. Burned gases ,still under some pressure ,begin to stream
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