轮机英语新版教材-专业阅读.pdf

Part One Reading第一篇专业阅读Unit 1 Main Propulsion Plant Lesson 1 Ships and Machinery ShipsShips are large,complex vehicles which must be self-sustaining in their environment for long periods with a high degree of reliability.A ship is the product of two main areas of skill,those of the naval architect and the marine engineer.The naval architect is concerned with the hull,its construction,form,habitability and ability to endure its environment.The marine engineer is responsible for the various systems which propel and operate the ship.More specifically,this means the machinery required for propulsion,steering,anchoring and ship securing,cargo handling,air conditioning,power generation and its distribution.Some overlap in responsibilities occurs between naval architects and marine engineers in areas such as propeller design,the reduction of noise and vibration in the ships structure,and engineering services provided to considerable areas of the ship.A ship might reasonably be divided into three distinct areas:the cargo-carrying holds or tanks,the accommodation and the machinery space.Depending upon the type each ship will assume varying proportions and functions.An oil tanker,for instance,will have the cargo-carrying region divided into tanks by two longitudinal bulkheads and several transverse bulkheads.There will be considerable quantities of cargo piping both above and below decks.The general cargo ship will have various cargo holds which are usually the full width of the vessel and formed by transverse bulkheads along the ships length.Cargo handling equipment will be arranged on deck and there will be large hatch openings closed with steel hatch covers.The accommodation areas in each of these ship types will be sufficient to meet the requirements for the ships crew,provide a navigating bridge area and a communications centre.The machinery space size will be decided by the particular machinery installed and the auxiliary equipment necessary.A passenger ship,however,would have a large accommodation area,since this might be considered the cargo space.Machinery space requirements will probably be larger because of air conditioning equipment,stabilizers and other passenger related equipment.Ships typesDepending on the nature of their cargo,and sometimes also the way the cargo is loaded/unloaded,ships can be divided into different categories,classes,and types,some of which are mentioned in Table 1.2The three largest categories of ships are container ships,bulk carriers(for bulk goods such as grain,coal,ores,etc.)and tankers,which again can be divided into more precisely defined classes and types.Thus,tankers can be divided into oil tankers,gas tankers and chemical tankers,but there are also combinations,e.g.oil/chemical tankers.Table 1 provides only a rough outline.In reality there are many other combinations,such as Multipurpose bulk container carriers,to mention just one example.Table 1 Ships typeDry CargoLiquid CargoPassengerUnit Cargo-Container vessel-Roll-on/Roll-off-Heavy cargo vessel-Refrigerated ships-Cattle shipBulk Cargo-Bulk carrier-Ore carrier-Crude carrier-Product tanker-Chemical tanker-LPG/LNC carriers-Passenger ship-Car and passenger ferries-Cruise shipMulti-purpose shipNavyFishingDredgers,etcWork ships-Aircraft carrier-Cruiser-Destroyer-Frigate-Submarine-Mine sweeper-Trawler-Other types of fishing vessels-Trailing hopper suction dredger-Cutter suction dredger-Rock-dumper-Crane vessel-Cable-layer-Buoy-layer-Oil-recovery vessel-Shearleg craneAuxiliary craftPleasure craftVariousOffshore material-seagoing tug-Harbor tug-Icebreaker-Pilot vessel-Coast guard vessel-Research vessel-Motor yacht-Sailing-Sailing yacht-Hydrofoil-Floating dock-Submersible platform-Pontoon,barge-Drilling rig/Jack up-Drill-ship-Pipe layer-Floating storage and offloading vesselShips sizeWhen a ship is in loaded condition,which floats at arbitrary water line,its displacement is equal to the relevant mass of water displaced by the ship.Displacement is thus equal to the total weight,all told,of the relevant loaded ship,normally in seawater with a mass density of 1.025 t/m.Displacement comprises the ships light weight and its deadweight,where the deadweight is equal to the ships loaded capacity,including bunkers and other supplies necessary for the ships propulsion.The deadweight at any time thus represents the difference between the actual3displacement and the ships light weight,all given in tons:deadweight=displacement-light weightIncidentally,the word ton does not always express the same amount of weight.Besides the metric ton(1,000 kg),there is the English ton(1,016 kg),which is also called the long ton.A short ton is approx.907 kg.The light weight of a ship is not normally used to indicate the size of a ship,whereas the deadweight tonnage(dwt),based on the ships loading capacity,including fuel and lube oils etc,often is.MachineryArrangementThree principal types of machinery installation are to be found at sea today.Their individual merits change with technological advances and improvements and economic factors such as the change in oil prices.It is intended therefore only to describe the layouts from an engineering point of view.The three layouts involve the use of direct-coupled slow-speed diesel engines,medium-speed diesels with a gearbox,and the steam turbine with a gearbox drive to the propeller.A propeller,in order to operate efficiently,must rotate at a relatively low speed.Thus,regardless of the rotational speed of the prime mover,the propeller shaft must rotate at about 80 to 100 r/min.The slow-speed diesel engine rotates at this low speed and the crankshaft is thus directly coupled to the propeller shafting.The medium-speed diesel engine operates in the range 250-750 r/min and cannot therefore be directly coupled to the propeller shaft.A gearbox is used to provide a low-speed drive for the propeller shaft.The steam turbine rotates at a very high speed,in the order of 6,000 r/min.Again,a gearbox must be used to provide a low-speed drive for the propeller shaft.Slow-speed dieselThe usual plan and elevation drawings of a typical slow-speed diesel installation are shown in Fig.1-1.A six-cylinder direct-drive diesel engine is shown in this machinery arrangement.The only auxiliaries visible are a diesel generator on the upper flat and an air compressor,below.Other auxiliaries within the machinery space would include additional generators,an oily-water separator,an evaporator,numerous pumps and heat exchangers.An auxiliary boiler and an exhaust gas heat exchanger would be located in the uptake region leading to the funnel.Various workshops and stores and the machinery control room will also be found on the upper flats.Geared medium-speed dieselFour medium-speed(500 r/min)diesels are used in the machinery layout of the rail ferry shown in Fig.1-2.The gear units provide a twin-screw drive at 170 r/min to controllable pitch propellers.The gear units also power take-offs for shaft-driven generators which provide all power requirements while at sea.The various pumps and other auxiliaries are arranged at floor plate level in this minimum-height machinery space.The exhaust gas boilers and uptakes are located port and4starboard against the side shell plating.Section looking to port Section looking to forwardFig.1-1 Low-speed diesel machinery arrangementpumpsEngine room layoutFig.1-2 Medium-speed diesel machinery arrangementA separate generator room houses three diesel generator units,a waste combustion plant and other auxiliaries.The machinery control room is at the forward end of this room.5Lesson 2 How Does a Marine Diesel Engine WorkThe diesel engine is a type of internal combustion engine which ignites The fuel by injecting it into hot,high pressure air in a combustion chamber.The marine diesel engine is a type of diesel engine used on ships.The principle of its operation is as follows:A charge of fresh air is drawn or pumped into the engine cylinder and then compressed by the moving piston to very high pressure.When the air is compressed,its temperature rises so that it ignites the fine spray of fuel injected into the cylinder.The burning of the fuel adds more heat to the air charge,causing it to expend and force the engine piston to do work on The crankshaft which in turn drives the ships propeller.The operation between two injections is called a cycle,which consists of a fixed sequence of events.This cycle may be achieved either in four strokes or two.In a four-stroke diesel engine,the cycle requires four separate strokes of the piston,i.e.suction,compression,expansion and exhaust.If we combine the suction and exhaust operations with the compression and expansion strokes,the four-stroke engine will be turned into a two-stroke one,as shown in Fig.2-l(a)-(d).The two-stroke cycle begins with the piston coming up from the bottom of its stroke,i.e.bottom dead center(BDC),with the air inlet ports or scavenge ports in the sides of the cylinder being opened(Fig.2-1(a).The exhaust ports are uncovered also.Pressurized fresh air charges into the 7cylinder,blowing out any residual exhaust gases from the last stroke through the exhaust ports.As the piston moves about one fifth of the way up,it closes the inlet ports and the exhaust ports.The air is then compressed as the piston moves up(Fig.2-1(b).When the piston reaches the top of its stroke,i.e.the top dead center(TDC),both the pressure and the temperature of the air rise to very high values.The fuel injector injects a fine spray fuel oil into the hot air and combustion takes place,producing much higher pressure in the gases.The piston is forced downward as the high pressure gases expand(Fig.2-1(c)until it uncovers the exhaust ports.The burnt gases begin to exhaust(Fig.2-1(d)and the piston continues down until it opens the inlet ports.Then another cycle begins.In the two-stroke engine,each revolution of the crankshaft makes one power or working stroke,while in the four-stroke engine,it makes two revolutions to make one power stroke.That is why a two-stroke cycle engine will theoretically develop twice the power of a four-stroke engine of the same size.Inefficient scavenging and other losses,however,reduce the power advantage to about 1.8.Each type of engine has its application on board ship.The low speed(i.e.90 to 120 r/min)main propulsion diesel operates on the two-stroke cycle.At this low speed the engine requires no reduction gearbox between it and propeller.The four-stroke engine(usually rotating at medium speed,between 2S0 to 750 r/min)is used for alternators and sometimes for main propulsion with a gearbox to provide a propeller speed of between 90 to 120 r/min.Working cyclesA diesel engine may he designed to work on the two-stroke or on the four-stroke cycle.Both of them are explained below.The four-stroke cycleFig.2-2 shows diagrammatically the sequence of events throughout the typical four-stroke cycle of two revolutions.It is usual to draw such diagrams starting at TDC(firing),but the explanation will start at TDC(scavenge).Top dead center is some times referred to as inner dead center(IDC).Proceeding clockwise round the diagram,both inlet(or suction)and exhaust valves are initially open.(All modem four-stroke engines have poppet valves.)If the engine is naturally aspirated,or is a small high-speed type with a centrifugal turbocharger,the period of valve overlap,i.e.when both valves are open,will be short,and the exhaust valve will close some 10 after top dead center(ATDC).Propulsion engines and the vast majority of auxiliary generator engines running at speeds below 1000 r/min will almost certainly be turbocharged and will be designed to allow a generous throughflow of scavenge air at this point in order to control the turbine blade temperature.In this 8case the exhaust valve will remain open until exhaust valve closure(EVC)at 50-60 ATDC.As the piston descends to outer or bottom dead center(BDC)on the suction stroke,it will inhale a fresh charge of air.To maximize this,balancing the reduced opening as the valve seats against the slight ram or inertia effect of the incoming charge,the inlet(suction)valve will normally be held open until about 25-35 ABTC(145-155 BTDC).This event is called inlet valve closure(IVC).The charge is then compressed by the rising piston until it has attained a temperature of some 550.At about 10-20 BTDC(firing),depending the type and speed of the engine,the injector admits finely atomized fuel which ignites within 2-7(depending on the type again)and the fuel bums over a period of 30-50,while the piston begins to descend on the expansion stroke.TDC(firing)At about 120-150 ATDC the exhaust valve opens(EVO),the timing being chosen to promote a very rapid blow-down of the cylinder gases to exhaust This is done:(a)to preserve as much energy as is practicable to drive the turbocharger,and(b)to reduce the cylinder pressure to a minimum by BDC to reduce pumping work on the exhaust stroke.The rising piston expels the remaining exhaust gas and at about 70-80 BTDC the inlet valve opens(IVO)so that the inertia of the outflowing gas,plus the positive pressure difference,which usually exists across the cylinder by now,produces a through flow of air to the exhaust to scavenge the cylinder.If the engine is naturally aspirated the IVO is about 10BTDC.The cycle now repeats.The two-stroke cyclesFig.2-3 shows the sequence of events in a typical two-stroke cycle,which,as the name implies,is accomplished in one complete revolution of the crank.Two-stroke engines invariably have ports to admit air when uncovered by the descending piston.The exhaust may be via ports adjacent to the air ports and controlled by the same piston(loop scavenge)or via poppet exhaust valves at the other end of the cylinder(uniflow scavenge).9Starting at TDC combustion is already under the way and the exhaust opens(EO)at 110-120 ATDC to promote a rapid blow-down before the inlet opens(IO)about 20-30 later(130-150 ATDC).In this way the inertia of the exhaust gases-moving at about the speed of sound-is contrived to encourage the incoming air to flow quickly through the cylinder with a minimum of mixing,because any unexpelled exhaust gas detracts from the weight of air entrained for the next stroke.The exhaust should close before the inlet on the compression stroke to maximize the charge,but the geometry of the engine may prevent this if the two events are piston controlled.It can be done in an engine with exhaust valves.At all events the inlet ports will be closed as many degrees ABDC as opened before it(i.e.again 130-150 BTDC)and the exhaust in the same region.Injection commences at about 10-20 BTDC depending on speed,and combustion lasts 30-50,as with the four-stroke.10Lesson 3 Diesel Engine Construction Bedplate and FrameThe bedplate,which in most cases is of the welded design,is bolted to the seating which forms a part of the double bottom of the ship.It is secured in the transverse direction by means of