翻译发动机

EngineThe engine acts as the power unit. The internal combustion engine is most common: this obtains its power by burning a liquid fuel inside the engine cylinder. There are two types of engine: gasoline (also called a spark-ignition engine) and diesel (also called a compression-ignition engine). Both engines are called heat engines; the burning fuel generates heat which causes the gas inside the cylinder to increase its pressure and supply power to rotate a shaft connected to the transmissionAll engines have fuel, exhaust, cooling, and lubrication systems. Gasoline engines also have an ignition system. The ignition system supplies the electric spark needed to ignite the air-fuel mixture in the cylinders. When the ignition switch is turned on, current flows from the 12-volt storage battery to the ignition coil. The coil boosts the voltage to produce the strong spark of 20,000 V needed to ignite the engine fuel.The automobile supplies all the electricity it needs through its electrical system. For example, the electrical system supplies electricity for the ignition, horn, lights, heater, and starter. The electricity level is maintained by a charging circuit.The fuel system stores liquid fuel and delivers it to the engine. The fuel is stored in the tank, which is connected to a fuel pump by a fuel line.The cooling system removes excessive heat from the engine. The temperature in engine combustion chambers is about 2,000℉( 1,094℃). Since steel melts at around 2,500℉(1,354℃), this heat must be carried away to prevent engine damage. Air and a coolant are used to carry away the heat. The radiator is filled with a coolant. The water pump circulates this coolant through the hollow walls of the engine block and head. Constant circulation of the coolant through the engine and the radiator removes heat from the engine.Heat also is removed by the radiator fan, which draws air through the narrow fins of the radiator. This system also supplies heat to the passenger compartment and the window defrosters.The lubrication system is important in keeping the engine running smoothly. Motor oil is the lubricant used in the system. The lubrication system has four functions:1. It cuts down friction by coating moving parts with oil.2. It produces a seal between the piston rings and the cylinder walls.3. It carries away sludge, dirt, and acids.4. It cools the engine by circulating the motor oil.To keep this system working efficiently, oil filters and motor oil must be changed regularly.The automotive engine is essentially a heat engine. The heat engines used in automobiles are internal combustion engines.1. Principle of OperationThe spark-ignition engine is an internal-combustion engine with externally supplied in ignition. The gasoline engine is a kind of spark-ignition engines.The four-stroke-cycle gasoline engine cycle is spread over four piston strokes. The operation strokes areThe first stroke in your engine is called the intake stroke. Instead of opening the intake valve after you have drawn the piston down, you will find it better to open the intake valve as the piston starts down [1]. This allows the air to draw fuel in the entire time the piston is moving down. Remember, the intake stroke starts with the piston at the top of the cylinder (intake valve open and exhaust valve closed) and stops with the piston at the bottom of its travel [2]. This requires one-half turn ofthe crankshaftAs the crankshaft continues to move, the piston is forced up through the cylinder. If you keep both valves closed, the fuel mixture will be squeezed, or compressed, as the piston reaches the top. This is called the compression stroke. It, too, requires a half turn of the crankshaft.The compression stroke serves to break up the fuel into even smaller particles. This happens due to the sudden swirling and churning of the mixture as it is compressed.When the air-fuel mixture is subjected to a sudden sharp compression force, its temperature rises. This increase in temperature makes the mixture easier to ignite and causes it to explode with greater force. As the piston reaches the top of its travel on the compression stroke, it has returned to the proper position to be pushed back down by the explosion.Remember, the compression stroke starts with the piston at the bottom of the cylinder (both valves closed) and stops with the piston at the top of the cylinder. This requires an additional half turn of the crankshaft.As the piston reaches the top of the compression stroke, the mixture is broken into tiny particles and heated up. When ignited, it will explode with great force. This is the right time to explode the mixture. A spark plug provides a spark inside the combustion chamber. The spark produced at the plug is formed by the ignition system.Just imagine that a hot spark has been provided in the fuel mixture. The mixture will explode and, in turn, force the piston down into the cylinder. This gives the crankshaft a quick and forceful push. This is the power stroke. Both valves must be kept closed during the power stroke or the pressure of the burning fuel will squirt out through the valve pods. Remember, the power stroke starts with the piston at the top of the cylinder (both valves closed) and stops with the piston at the bottom of the cylinder. This requires another half turn of the crankshaft.When the piston reaches the bottom of the power stroke, the exhaust valve opens. The spinning crankshaft forces the piston up through the cylinder, pushing burned gases out. This is the exhaust stroke.Remember, the exhaust stroke starts with the piston at the bottom of the cylinder (exhaust valve open and intake valve closed). It stops with the piston at the top of the cylinder. This requires one more half turn of the crankshaft.If you count the number of half turns in the intake, compression, power, and exhaust strokes, you will find you have a total of four. This gives you two complete turns, or revolutions, of the crankshaft. While the crankshaft is turning around twice, it is receiving power only during one half turn, or one fourth of the time.Each complete cycle consists of four strokes of the piston, hence the name four-stroke cycle.2. Main Engine Components1）Engine blockThe engine block serves as a rigid metal foundation for all parts of an engine. It contains the cylinders and supports the crankshaft and camshaft. In older engines, the valve seats, ports, and guides are built into the block. Accessory units and the clutch housing are bolted to it.Blocks are made of either cast iron or aluminum. The lighter the block (providing it has sufficient strength), the better. The modern thin-wall casting process controls core size and placement much more accurately than the older casting process. This permits casting the block walls much thinner, reducing the weight of the block. Since the block wall thickness is more uniform, block distortion during service is less severe.2）cylinderThe cylinder is a round hole formed in the block. It forms a guide for the piston and acts as a container for taking in, compressing, firing, and exhausting the air-fuel charge. Cylinders have been made of both steel and cast iron. Cast iron is by far the most popular.When steel cylinders are desired in an aluminum block, they are installed in the form of cylinder sleeves (round, pipe-like liners), These sleeves may be either cast or pressed into the block. Some engines use removable cylinder sleeves. When the cylinder becomes worn, the old sleeves can be pulled out and new sleeves can be pressed in. The sleeves are pressed into oversize cylinder holes. Cylinder sleeves are widely used in heavy-duty truck and industrial engines. Sleeves can also be used to repair a worn or cracked cylinder in a cast iron block.3）PistonsThe piston must move down through the cylinder to produce a vacuum to draw a fuel charge into the cylinder. It then travels up in the cylinder and compresses the mixture. When the mixture is fired, the pressure of the expanding gas is transmitted to the top of the piston. This drives the piston back down through the cylinder with great force, transmitting the energy of the expanding gas to the crankshaft. The piston then travels up through the cylinder and exhausts the burned fuel charge.Pistons are usually made of aluminum. Often, aluminum pistons are tin-plated to allow proper break-in when the engine is started. Aluminum pistons can be forged, but they are more commonly cast. Cast iron is a good material for pistons used in a slow-speed engine. It has excellent wear characteristics and will provide good performance.4）Connecting RodsAs the name implies, connecting rods are used to connect pistons to the crankshaft. The upper end of the rod oscillates (swings back and forth), while the lower, or big end, bearing rotates (turns).Because there is very little bearing movement in the upper end, the bearing area can be reasonably small. The lower end rotates very fast, and the crankshaft bearing journal turns inside the connecting rod. This rotational speed tends to produce heat and wear. To make the rod wear well, a larger bearing area is required.The upper end of the rod has a hole through it for the piston pin. The bottom of the large end of the connecting rod must be removed so the rod can be installed on the crankshaft journal. The section that is removed is called the connecting rod cap.Connecting rods are normally made of alloy steel. They are drop-forged to shape and then machined.5）CrankshaftThe engine crankshaft provides a constant turning force to the wheels. It has throws to which connecting rods are attached, and its function is to change the reciprocating motion of the piston to a rotary motion to drive the wheels. Crankshafts are made of alloy steel or cast iron.The crankshaft is held in position by a series of main bearings. The maximum number of main bearings for a crankshaft is one more than the number of cylinders. It may have fewer main bearings than cylinders. Most engines use precision insert bearings that are constructed like the connecting rod bearings, but are somewhat larger. In addition to supporting the crankshaft, one of the main bearings must control the forward and backward movement.6）flywheelA heavy flywheel is attached to the rear of the crankshaft with bolts. The function of the flywheel is to smooth out engine speed and keep the crankshaft spinning between power strokes, in some engines, the flywheel also serves as a mounting surface for the clutch. The outer rim of the flywheel has a large ring attached with gear teeth cut into it. The teeth of the starter motor engage these teeth and spin the flywheel to crank the engine. When an automatic transmission is used, the torque converter assembly works with the flywheel.7）CamshaftThe camshaft is used to open and close the valves. There is one cam on the camshaft for each valve in the engine. Generally only one camshaft is used in most engines. Newer engines are increasingly equipped with two or more camshafts8）ValvesEach engine cylinder ordinarily has two valves. However, modern engines often use four valves per cylinder (two intake and two exhaust). A few engines used in smaller vehicles have three or five valves per cylinder: two intake valves and one exhaust valve or three intake valves and two exhaust valves.Because the head of an exhaust valve operates at temperatures up to 1300℉(704℃), valves are made of heat-resistant metal. In order to prevent burning, the valve must give off heat to the valve seat and to the valve guide. The valve must make good contact with the seat and must run with minimum clearance in the guide.9）Valve LiftersMechanical valve lifters are usually made of cast iron. The bottom part that contacts the camshaft is hardened. Some lifters are hollow to reduce weight. Most valve trains that contain mechanical lifters have some provision for adjusting clearance. Mechanical valve lifters were used in older engines.Hydraulic valve lifters perform the same job as mechanical lifters. However, hydraulic lifters are self-adjusting, operate with no lifter-to-rocker arm clearance, and uses engine oil under pressure to operate. Hydraulic lifters are quiet in operation.3. Engine Classification1)Cycle ClassificationEngines are often classified according to cycle. Most internal combustion piston engines use a two- or four-stroke cycle. All modern automobile engines use the four-stroke cycle engine.The two-stroke cycle engine performs the intake, compression, firing, and exhaust sequence in one revolution of the crankshaft.Cylinder ClassificationThe inline engine has its cylinders arranged one after the other in a straight line. They are in a vertical, or near vertical position. Most modern inline engines are four cylinders.A V-type engine places two banks or rows of cylinders at an angle to each other—generally at 60°or 90°. The V-type engine has several advantages: short length, extra block rigidity, a short, heavy crankshaft, and low profile that is conducive to low hood lines. The shorter block permits a reduction in vehicle length with no sacrifice in 2)passenger room.The horizontal opposed engine is like a V-type engine, except that both banks lie in a horizontal plane. The advantage here is an extremely low overall height, which makes this engine ideally suited to installations where space is limited.Cooling ClassificationAs you have learned, engines are either liquid-cooled or air-cooled. Most vehicles use liquid-cooled engines. Air-cooled engines are used in limited numbers on modern vehicles.3)Fuel ClassificationAutomobile engines can use gasoline, diesel fuel, gasohol (mixture of gasoline and alcohol), alcohol, LNG (liquefied natural gas), CNG (compressed natural gas), or LPG (liquefied propane gas). Gasoline powers the majority of vehicles, but diesel fuel is used in some vehicles. Gasohol, LNG, CNG, and LPG are beginning to see wider use. One of the principal differences in these engines is in method of fuel delivery and carburetion. Gasoline, LNG, CNG, and LPG utilize the same basic type of engine, but LNG, CNG, and LPG utilize a slightly different fuel delivery setup. Diesel engines do not use a carburetor or an ignition system.1.发动机发动机是汽车的动力装置。