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畢業(yè)設(shè)計_汽車發(fā)動機潤滑系的構(gòu)造與維修-文庫吧資料

2024-12-09 20:34本頁面
  

【正文】 ability. Vtype engines With Vtype engines, the cylinders are arranged in two blanks at an angle. This reduces the length of the engine and makes it more pact. This also reduces the length of the crankshaft, which can be designed to be more rigid than a long shaft. 2 . The Engine Assembly and Components The plete engine assembly consists of the mechanical ponents that make up the engine itself and also a number of associated systems. These are the systems that are needed to start the engine and also to control it and keep it running. The mechanical parts of the engine assembly can be broken down into a number of subassemblies, or groups of associated ponents, although these are usually referred to merely as assemblies, for example, cylinderhead assembly and piston assembly. Some parts of an engine are internal, other parts are external, this illustration identifies a number of external parts. The various ponents of an engine are assembled together during manufactured and most of these are secured by means of bolts, nuts and other types of fasteners. Some parts are internal and others are attached to the outside of the engine. The cylinder block is the largest part of engine. The other parts are either fitted into the block or attached to it. As its name suggests, it is basically a block of cast metal, usually cast iron, but it can be aluminum alloy with cast iron or steel liners. Aluminum is used to reduce the weight. The cylinder block has accurately bored cylinders to take the pistons. The lower part of the block is known as the crankcase and this has bearings that carry the crankshaft. The waterjackets that surround the cylinders are filled with coolant. The top of the cylinder block has a machined surface for the cylinder head and the bottom of the cylinder block is machined to provide a mounting for the oil pan, or sump. The cylinders can be arranged in the block so that they are in line, as shown, or they can be set at angle ( Vtype engine ). The crankshaft is mounted in bearings in the lower part of the cylinder block ( the crankcase ). The connecting rods connect the pistons to the crankshaft, which is rotated by the power strokes of the pistons when the engine is running. The upanddown, or reciprocating, 20 motion of the pistons is changed to rotary motion by the bination action of the connecting rods and the cranks of the crankshaft. The term crankshaft es from the word cranked, which also means bent. It is a shaft with a number of cranks or bends. With inline engines, there a crank for each cylinder, but with some Vtype engines, there is only one crank for each pair of cylinders. The flywheel is a heavy cast iron wheel attached to the rear of the crankshaft. It reduces engine vibrations by smoothing out the power impulses of the pistons. The flywheel absorbs energy during the power stroke and gives up energy during the other strokes and this helps to keep the engine running smoothly. A ring gear fitted to the rim of the flywheel so that the engine can be rotated by the starter pinion when starting the engine. With automatic transmissions, a drive plate and torque converter take the place of the flywheel and perform the same functions as a flywheel, as far as the running of the engine is concerned. The piston has grooves that carry the piston rings and these provide a seal between the piston and the cylinder wall. Compression rings are used as a gas seal and the oil ring is used to prevent excess oil from finding its way up past the piston into the bustion chamber. The connecting rod has a removable cap and a split bearing at its lower end where it is connected to the crankshaft. Its upper has a piston pin that provides a wristtype of action with the piston. Because of its action, the piston pin is sometimes referred to as a wrist pin. The cylinder head is made of cast aluminum alloy. It is bolted to the top of the cylinder block so that it encloses the cylinders. It has bustion chambers above the cylinders in which the airfuel mixture is burnt. Cylinder heads can be made of cast iron, which is more resistant to corrosion, but aluminum alloy is used for petrol engines because it has advantages of better heat transfer and lighter weight. The cylinder head has intake ports and exhaust ports. The intake valves open the intake ports to admit the fuel charge into the cylinder during the intake stroke. The exhaust valves open the exhaust ports to allow the burnt gases to leave the cylinder following the exhaust stroke. The camshaft and the valve mechanism are used to open and close the valves at the correct time. The camshaft is driven from the crankshaft at half the crankshaft speed. The valve cover, also called the cylinderhead cover, is fitted to the top of the cylinder head. This encloses the valve mechanism. An additional cover on top of the engine covers the ignition coils. This cylinderhead arrangement, for a fourcylinder engine, has two camshafts and sixteen valves. Other fourcylinder engines can have two camshafts and twelve valves, or one camshaft and eight valves. The timing belt and pulleys drive the camshaft at half the crankshaft speed ( engine speed ). A toothed drive belt is often used, but gears and chains are also usedin some instances a belt and chain are both used. On some engines, the timing chain is used to drive the oil pump. On diesel engines, a timing chain or a timing gear is used to drive the injection pump as well as the camshaft. The oil pan, or sump, holds the oil for the engine lubricating system. The oil pan is made of steel that has been pressed to shape. Other oil pans are made of aluminum alloy that has been cast to shape. The oil pan is bolted to the underside of the engine so that it closes off the crankcase. Only the ends of the crankshaft that extend beyond the cylinder block
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