【正文】 xit on the the valves in this way leaves room for relatively large valves and ports,and that helps the engine breather. Breathing refers to the engine39。s called wet because it has coolant against its outer helps speed up heat transfer between the sleeve and sleeve is sealed at the top to prevent coolant stops coolant entering the bustion chamber,and the bottom of the flange at the top of the sleeve fits into a recess in the lower end has 1 or 2 neoprene sealing rings. 3 With coolant in direct contact with the cylinder sleeve,corrosion can be a can even insulate the sleeve from the coolant,which reduces the main advantage of having a wet sleeve at all. The walls on wet sleeves are thicker than on dry don39。t easily be 39。 1 專業(yè)外文翻譯材料原文 Engines Cylinder Block The cylinder block is the largest past of the upper section carries the cylinders and ,the lower section forms the crankcase,and supports the can be cast in one piece from grey it can be alloyed with other metals like nickel or chromium. The iron casting process begins by making up the shapes of what will bee water jackets and cylinders as sand cores which are fitted into stops there parts being solid iron during iron is poured into sand moulds that are formed by patterns in the shape of the block. After casting,core sand is removed through holes in the sides and ends,leaving spaces for the cooling and lubricant holes are sealed with casting is then are bored and finished,surfaces smoothed,holes drilled and threads cylinder blocks are made with ribs,web and fillets to provide rigidity but also keep weight to a minimum. Cylinder Block Construction As more manufacturers try to make vehicles lighter and more fuel efficient,more and more engine blocks are being cast from block made of aluminium alloy is lighter than if it were made of cast if two engines are generating the same power,the alloy version would have a better weighttopower ratio than the cast alloy alloy blocks are made by various casting processes,including pressure method is gravity casting,where the molten metal is poured into molds. Cast iron liners are usually used in the cylinders of aluminium blocks,and sometimes in castiron sleeves are cast into the on the outside form a key that stops any movement in the also increase surface area to assist heat transfer from the sleeve to the block. Some blocks don39。s light and easy to cast,while the sleeve uses another kind that is better able to stand up to wear and tear. There are three main types of sleevesdry,flanged dry,and wet. The dry sleeve can be cast in or pressed into a new block, or used to recondition badlyworn or damaged cylinders that can39。s worn. With a wet sleeve,the outer surface is part of the waterjacket around the 39。squish39。,forcing the air to the center of the bustion causes turbulence as fuel is injected into the cylinder. In indirect injection,the piston is fairly flat,or has a shallow main bustion chamber is between the cylinder head and the top of the piston,but a smaller,separate chamber is in the is injected into this smaller can have various swirl chamber is spherical,and connected to the main chamber by an angled passage .Both the injector and glow plug are screwed into the glow plug preheats the air inside to help start the pression ,the spherical shape makes the air swirl in the helps make a better mixture of the air and fuel,which improves bustion .This bustion chamber is divided into a main bustion chamber and an air cell, joined by a throat. The injector is in the throat. When injection mences,bustion pressure forces the air to flow from the air cell where it mixes with fuel from the rush of air from the air cell produces a rotary motion of gas in the main chamber which helps make bustion more prebustion chamber is screwed into the cylinder injector is mounted in the upper end. Injection occurs near the top of into the pression stroke .Only part of the fuel is burned in the prebustion because of the limited amount of air high rise in pressure forces burning fuel into the main happens very rapidly,which helps make more efficient bustion. Intake and Exhaust Passages The size of passages in the head can affect engine intake and exhaust passages allow more torque at low engine is because smaller passages improve mixing of air and fuel fat low speeds ,which causes more efficient bustion .At high speeds however,these smaller passages restrict airflow .To reduce the effect of this ,this engine has two inlet opens at low speed and other operates at higher engine speeds .Larger passages produce greater power at high engine speeds. Each intake and exhaust passage can be formed separately in the passages for adjacent passages may have a mon,thin wall between them .This is called ports in the same head can also be siamesed. When all intake and exhaust ports are on one side,it is called a counter flow 6 can be cast separately or siamesed. When all of the intake ports are on one side and exhaust ports are on the other ,it is called a crossflow allows for straighter passageways and higher efficiency. Gaskets Gaskets form a seal by being pressed between stationary parts where liquid or gas could gaskets are made to be used only can be made of soft materials such as cork,rubber,paper, can also be made of soft alloys and metals such as brass,copper, aluminium or soft steel sheet metal. Choosing which material and design to use depends on the substance to be sealed,the pressures and temperature materials and mating surfaces to be sealed. Head gaskets seal and contain the pressures of bustion within the engine,between the cylinder head and also seal oil passages between block and control the flow of coolant between the