Lesson Eighteenok

1、Lesson EighteenMarine EnginesMarine engine is an engine that propels a water-born vessel. Even in small craft the marine engine must have the following characteristics: reliability, light weight, compactness, fuel economy, low maintenance, long life, relative simplicity for operating personnel, abil

2、ity to reverse, and ability to operate steadily at low or cruising speed. The relative importance of these characteristics varies with the service performed by the vessel, but reliability is of prime importance.Steam engines Steam, used to drive the earliest powered vessels, is still a common type o

3、f propulsion for large ships. The diesel engine has gained wide acceptance in foreign merchant ships, but in the United States, the majority of seagoing vessels use steam propulsion. Reciprocating steam engines Early engines commonly used steam flowing in series through as many as four cylinders who

4、se pistons had the same stroke but were of increasing diameters. This system provided for an expansion or increase in steam volume which accompanied the decrease in pressure to the exhaust under a vacuum. The modern, multicylinder, uniflow marine steam engine, with complete expansion in each cylinde

5、r, shows better steam economy. Because it has the same diameter for all cylinders (two to six in number), it is preferable from a manufacturing viewpoint. Equal power is developed by each cylinder; units of four cylinders or more have good torque and balance characteristics. A steam rate of 10 lb/hp

6、/hr (1.7 kg/MJ) with 275 psi (1.90 Mpa) at 240°F （116°C）super heat is attainted. Uniflow engines as large as 5000 hp (3.7MW) have been used on shipboard. Normally, steam engines are double-acting; that is, steam acts on each side of the pistons. With superheated steam, piston-cylinder lubr

7、ication must be provided. Pure feed-water is required by modern, high-capacity boilers; thus, an effective oil filter is installed where the condensate must be returned to the boiler. Steam turbines The marine steam turbine has the advantages of direct rotary motion, little or no rubbing contact of

8、pressure-confining surfaces, and ability to use effectively both highly super heated steam and steam at low pressure, that is, at a high vacuum where specific volumes of over 400 ft3/lb are reached. For good efficiency of steam turbines, high rotative speeds are required. This requirement led to the

9、 introduction of the reduction geared turbine and turboelectric drive. These system give efficient turbine speeds and efficient propeller rmp. With geared turbine, for example, turbine rotor speeds range from 3000 to 10,000 rmp, while propeller rmp is reduced to the 80 to 400 range. Steam is general

10、ly supplied to the turbine at 850 psi (5.9MPa) and 950°F (510°C) by pair of oilfired marine water-tube boiler, and the exhaust from the turbine is usually at 1.5 in. lig(5.1kPa) absolute. Forced draft fans and other auxiliaries are usually motor-driven, except for the main feed pimps which

11、 are usually driven by an auxiliary turbine. Electric power is provided by a separate turbogenerator.In low-powered geared turbines, steam completes its expansion in one rotor and casing. Such a design has been used in geared turbines of up to 8000 shaft horsepower (ship) or a shaft power of 6 MW. H

12、owever, series flow through two or even three casings is preferable in most steam turbines. This arrangement provides more flexibility in turbine design, allowing for different and optimum revolutions for high-and low-pressure rotors. Also, in a seagoing vessel in case of casualty to one turbine or

13、its high-speed pinion, the vessel usually can make port with the remaining turbine in operation. A steam turbine is made up of fixed blades, usually called nazzles, and rotating blades. A stage is generally one stationary row and one moving row. Impulse staging has all the steam pressure drop taking

14、 place in the fixed blades. The moving row then absorbs the kinetic energy produced. Reaction staging results when some of the assume drop occurs in the moving blades, the degree of reaction depending on the design.Modern marine practice favors impulse staging in the high-pressure end of the turbine

15、 because of the steam progresses toward the low-pressure end where the value is much greater the reaction stage is more efficient. The arrangement has a cross compound system with a high-pressure unit of 7 to 10stages and a low-pressure unit of 6to 8 stages, each driving a pinion of a reduction gear

16、.A turbine is capable of operation in only one direction. In order to provide reverse power, a second turbine is installed on the shaft of the low-pressure ahead turbine. The astern turbine is usually not more than three moving rows of blades, but it may be only two. It produces about 40% of the nor

17、mal ahead horsepower. Since this unit is turning backward in normal ahead operation, it is located in the low-pressure end of the low-pressure turbine. The steam at this point has a very low density, and hence the astern turbine has a low windage loss.The propeller is reversed by closing the steam v

18、alue to the ahead turbine and opening the value to the astern turbine.Gas turbine The gas turbine is a relative newcomer to the marine field. It generally considered of an axial compressor discharging compressed air to a combustion chamber where fuel is burned, adding heat. The produces of combustio

19、n at high temperature and pressure then pass through a gas turbine that drives the compressor and load. Generally, the term “gas turbine” is applied to the entire plant. If lower pressure ratios (final pressure leaving the compressor divided by initial pressure entering) are used, a large amount of

20、heat is available in the exhaust gas which may be recovered by heating the compressed air before it enters the combustion chamber. This is done in a regenerator. With higher pressure ratios the expansion through the turbine is so great that the exhaust gas temperature is insufficient to heat the com

21、pressed air. Two distinct types of gas turbines are appearing in the marine field: the aircraft-derived type and the industrial type. The aircraft-derived type uses a jet engine as a gas generator, which discharges to a gas turbine driving the load. This type of plant offers simplicity and light wei

22、ght but must burn high-quality fuel. The industrial gas turbine is a more rugged machine designed for long life and is capable of using low grades of fuel, properly washed. This plant usually uses a regenerator. The gas turbine offers simplicity, ease of control, and efficiency, but requires special

23、 fuel or special treatment of the fuel .Large amounts of air and exhaust gas are used, and as a result uptakes and air supply are a special problem. The aircraft-derived gas turbine seems destined to drive a large number of naval combatant ships. Selections of this type for a new class of naval dest

24、royers has been announced. The industrial type will be used for merchant vessels where its greater weight will be of little disadvantage.Internal combustion engines Both diesel and gasoline internal combustion engines are used in marine applications. Many moderate and lowpower marine installations u

25、se automotive or locomotive engines designed for variable load and intermittent service. High-power marine propulsion units normally are called on to operate continuously under load. Therefore, the brake horsepower (bhp) rating of units selected for marine service should be conservative. The gasolin

26、e engine is the most common power plant for pleasure craft. It is inexpensive to buy and maintain. Because of its widespread use in automobiles, most parts are readily available. In most areas gasoline costs slightly more than diesel fuel, but the cost differential is usually insufficient to make up

27、 the difference between the cost of gasoline and diesel engines. Gasoline presents an explosion and fire hazard, which is its major disadvantage. Direct-drive diesels For typical commercial freight vessels, direct-drive diesels provide economical service. For good propeller efficiency, the propeller

28、 rpm should be under 120±. Such a top limit on engine revolutions results in a large, heavy, bulky, slow-rpm engine. However, the direct-drive diesels have a lower fuel oil consumption than do higher-rpm units, and with suitable fuel treatment they will operate on the better grades of the cheap

29、er fuel oil burned in boilers. Slow-speed, direct-drive diesels are favored by many European owners and shipbuilders. Turbo-charged, two-cycle, single-acting diesel engines of 50 000 bhp (37MW of brake power) are now available; such engines weigh more than 100 lb/bhp (60kg/kw of brake power). For hi

30、gh horsepower the total machinery weight for diesels is more than the weight of geared turbine machinery, including boilers and auxiliaries. Moderate-speed diesels Diesel engines of 250-500 rpm are available in two and four cycle, single-acting types, generally with trunk pistons. In some marine app

31、lications they are connected directly to the propeller and thus fitted only with reverse gear. However, they are also employed with geared diesel and diesel-electric drive. The weight of such engines runs about 35-70 lb/bhp (21-43 kg/kw of brake power). High-speed diesels Many high-speed diesel engi

32、nes of 600 rpm and more (some types originally developed for truck and locomotive service ) are available for marine propulsion. Opposed piston types have been developed; other manufacturers favor a V type to reduce weight. Such engines are of two-and four-cycle types and usually weigh 10-40 lb/bhp

33、(6-24kg/kw of brake power). Because of less efficient scavenging, breaker power, and other factors, their fuel and lubricating oil rates are higher than for large, low-speed diesels. Except for direct drive in moderate or fairly high-speed craft, marine applications of diesel engines or with diesel-

34、electric drive to provide good propeller efficiency. Because their pistons, valves, and other components are small, standardized, and carried in stock, repairs are readily made, with the result that engines of this type are popular for nonoceangoing services. Oil consumption and starting Lubricating

35、 oil consumption of diesel engines is high because of the cylinder-piston lubrication that must be provided and the contamination of the crankcase oil with residues blown by the piston rings. In large engines this contamination is avoided by using piston rod-crosshead construction so that the cranks

36、haft, connecting rods, and crossheads operate in a closed casing separated from the working cylinder. These engines are started and maneuvered by pressure from one or more reservoirs filled with air at about 250 psi (1.7 MPa). To make it feasible to start and readily reverse, two-cycle, single-actin

37、g marine engines should have at least four cylinders; four-cycle engines should have five or more cylinders.Nuclear power Very successful installations of nuclear power have been made in submarines and a few surface ships. Operation of the first nuclear merchant ship, the Savannah, was successful te

38、chnically but not commercially. (The operating crew required special training, and it would be difficult to replace with only one commercial nuclear ship in service.)Mechanical reduction gears Reduction gearing for diesel and gasoline engines allows the use of a relatively high engine speed and lowe

39、r, more efficient propeller speed. Speed reduction ratios of 1.8:1 to 4:1 are common, preferably with helical teeth to give better wear and quieter performance. A reverse gear device often is incorporated in lowpower gears for astern operation. Other methods for providing reverse rotation use a dire

40、ct reversing engine or a controllable-pitch propeller. One, two, three, or four engines may drive the same gear through individual pinions. The use of a friction, electromagnetic, pneumatic, or hydraulic coupling serves to disconnect any engine. By reversing one or more engines, ready maneuvering, i

41、ncluding astern operation, is provided for by the use of the respective coupling. The high rpm (3000-9000) of modern marine steam turbine and the low revolutions of an effective propeller (as low as 80 rpm) require the use of two-stage gearing. Gear teeth of harder material than the gear. The gear t

42、rains are of the double helical, type to avoid heavy axial thrust. Double reduction gears are constructed with flexible couplings between the high-speed train and the low-speed elements. Mechanical reduction gears are carefully constructed to close tolerances. They have forced lubrication in sprays

43、ahead of the meshing teeth, to the bearings, and to the flexible couplings. Tests have shown that bearings represent at least half the power loss of the entire gear set.Turboelectric drive This type of drive, comprising one or more steam turbine generators and ac propulsion motors, is also used for

44、ship propulsion. It was installed in many United States tankers during World War II because of available manufacturing facilities. The synchronous motors are provided with an induction winding for starting and reversing. Relatively large changes in propeller revolutions are made by alteration of the

45、 turbogenerator speed. Motor, generator, exciter, and cooling equipment losses result in several percent lower efficiency than with geared steam turbines. Weights and costs are generally 25-30% higher than for the comparable turbine gear arrangement. Electric drive is not employed unless it offers s

46、ignificant operational or design advantages. These include flexibility of control and the independence of the location of the turbo generator relative to the propeller shaft or propulsion motor.Diesel-electric drive This type of drive, composed of one or more dc diesel generator sets and often a dou

47、ble armature propulsion motor. is used in tugs, dredges ,Coast Guard cutters, and icebreakers, where maneuvering and a wide range in propeller speed are necessary. For slow-speed operating during maneuvering, the engine speed is often reduced and the generator field excitation is altered to provide

48、wide variation in the motor output and propeller speed.Control arrangements Bridge or pilothouse control of the ship propulsion unit, without action by the engineer on watch, s used with diesel-electric drive and for small, low-powered, direct-drive, and mechanically geared diesel installations. Thi

49、s is the customary arrangement for tugs and dredges. Modern practice for large diesel and steamships is to provide pilothouse control of the main engines and no engine-room watch except day workers and when entering and leaving port. Propulsion plant monitoring is usually provided in a central contr

50、ol space, with chart recorders or a data storage system. All levels of automation are now being used, from simple manual surveillance of all systems in a fully manned engine space to a completely unmanned system recording data and monitoring trends to pinpoint possible trouble.Gonernors Above the op

51、erating rpm, ship propeller torque increases faster than engine or turbine torque, and thus ship propeller drive is inherently stable. Because of the ships pitching, the propeller may lift partially out of the water and the engine may tend to race. To allow for this situation, or for propeller shaft

52、ing failure. American Bureau of Shipping regulations require that a governor be fitted to limit overspeed to 15% above the rated speed. A common type of governor uses oil pressure developed by small pumps incorporated with the main turbine rotors to activate the governor; low lubricating oil pressur

53、e will also shut off the steam supply. With turboelectric and diesel-electric drive, there is no mechanical connection between the generator set and the propulsion motor and propeller. The operating governor holds generator speed at the set value by throttling turbogenerator steam or the amount of f

54、uel injected in the diesel engine cylinders. (From McGraw-Hill “Encyclopedia of Science and Technology”, Vol.8, 1982) Technical Terms1. marine engine 船舶發(fā)動機2. waterborne vessel 船，水上運載器3. cruising speed 經(jīng)濟 航速（民船）；巡航速度（軍艦） 4. steam engine 蒸汽機5. steam engine 汽輪機6. diesel engine 柴油機7. reciprocating engin

55、e往復(fù)式發(fā)動機8. cylinder 氣（汽）缸9. double-acting 雙作用10. piston 活（柱）塞11. feed-water 給水12. condenser 冷凝器13. boiler鍋爐，蒸汽發(fā)生器14. pressure-confining surface 受壓面15. gearing齒輪裝置，傳動裝置16. rotor 轉(zhuǎn)子，電樞17. rotating blade旋轉(zhuǎn)葉片18. oil-fired燃油的19. marine water-tube boiler 船用水管鍋爐20. forced draft fan 強力通風(fēng)機21. turbogenerator 汽

56、輪發(fā)電機22. casing 外殼，箱殼23. reduction gear 減速齒輪/器24. nozzle 噴咀，排氣管25. stationary 穩(wěn)定/固定的26. impulse stage 沖動級27. reaction stage 反動級28. kinetic energy 功能29. cross compound system 交叉復(fù)合系統(tǒng)30. parasitic loss 無功/附加損失31. windage loss氣體阻力損失32. gas turbine 燃氣輪機33. regenerator 回?zé)崞?4. aircraft-derived type 航空派生型35.

57、 rugged machine 笨重機械36. uptake 煙道37. destroyer 驅(qū)逐艦38. internal combustion engine 內(nèi)燃機39. gasoline engine 汽油機40. locomotive engine 機車發(fā)動機41. power plant 動力裝置42. direct driver diesel 直接驅(qū)動的柴油機43. turbo-charger 渦輪增壓器44. two-cycle 二沖程45. single-acting 單作用的46. moderate-speed diesel 中速柴油機47. trunk piston 筒形活

58、塞48. diesel-electric drive 柴油機電力傳動49. opposed piston type 對置活塞型50. scavenging 掃氣，喚氣51. crankcase 曲拐箱52. contamination 污染53. piston ring 活塞環(huán)54. crankshaft 曲軸55. connecting rod 連桿56. crosshead 十字頭57. reverse gear device 容器，儲器58. nuclear power 核動力59. helical teeth 斜齒，螺旋齒60. reverse gear device 倒車齒輪（傳動）裝置61. electromagnetic 電磁的62. pneumatic 氣動的 63. hydraulic 水力的，液壓的64. involute 漸開線65