【正文】 d camshaft arrangement. Single overhead camshafts can use rocker cam can lift one end of the rocker arm,or it can press down on the rocker arm. On double overhead camshaft systems,the most mon arrangement is to use a bucket tappet or operates in a guide that protects the valve against side thrusts which it would receive if the cam operated directly against the valve. The adjustment of valve clearance is usually done by changing accurately machined are available in a range of thicknesses,and they39。 swell39。F or 1150℃ .Such materials are also designed to allow the cylinder head and block,some of which have considerable distortion rates,to move slightly on the head gasket as they expand during engine feature is vital for preventing head gasket failure. Some head gaskets also incorporate stainless steel fire rings to help to contain heat and pressure within the addition,many head gaskets also have an added silicone based outer coating on both sides of the side material layers to provide additional cold sealing ability during startup and gaskets also seal oil passages,and control the flow of coolant between the cylinder block and head and are fitted with beads or rings to prevent leakage and corrosion. Some joints between surfaces on modern engines are being sealed with special sealants which eliminate the use of gaskets in some rubber,or conventional cork rubber is unable to deal with the stresses and pressures in modern engines. Modern gasket manufacturers are producing improved material binations such as nitrile and cork blends to deal withhigh tech39。anisotropic39。O39。squish39。.That means squeezing of the gases to increase their velocity and turbulence. In gasoline engines,the three most popular bustion chamber designs are called hemispherical pent roof,bathtub and wedge. A hemispherical bustion chamber has the intake valve on one side of the chamber and the exhaust valve on the provides mixture enters on one side,and exhaust gases exit 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 39。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。s not a tight fit and it can be replaced if it39。t easily be 39。re often machined to give uniform thickness and allow free flow of air. Pistons The piston,with its connecting rod and bearing,transfers the force of the bustion and expansion of the power stroke to the piston itself,its rings,and the piston pin,also known as the gudgeon pin,are together called the piston cutaway shape on this piston allows it to clear the counterweights on this rotating crankshaft. The connecting rod connects the piston to crankshaft. It is fastened to the piston at its little end,by a piston or gudgeon big end of the connecting rod has a detachable cap,and carries 2 halves of the big end big end is attached to the crankshaft at the crankpin journal. Cylinder Sleeves Cylinder sleeves are used in engine blocks to provide a hardwearing material for pistons and piston rings. The block can be made of one kind of iron that39。 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。t need can be made of wear resistant material that makes a hardwearing surface for the pistons and piston the cylinder bore may have some sort of surface treatment to make it hardwearing. When the cylinders,block and crankcase are all cast together,it is called a monoblock horizontallyopposed block has a split two 2 engine blocks are joined together by the flanges of the crankcase. In aircooled engines,the cylinders are usually made as separate parts,then bolted to