In a two-stroke cycle compression occurs on the first or upstroke; combustion and expansion occur during the down-stroke; exhaust, scavenging and recharging with air occur during he latter part of the down-stroke and the beginning of the
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next upstroke. These sequence of events is made possible by substituting ports in the bottom of the cylinder wall for one or more exhaust valves. There are two groups of these ports, one for exhaust and the other for scavenging air, usually on opposite sides of the cylinder, but on some designs both groups are arranged on the same side. The exhaust ports connect with the exhaust manifold, while the scavenging ports communicate with the scavenging air receiver in which low-pressure air is stored.
A two-cycle engine must be provided with the scavenging compressor for supplying scavenging air.
Text 7. Operation of propulsion turbine unit
Preparations for getting under way consist in starting the necessary auxiliaries and warming up the main turbines. During this warming-up process the temperature of the various part of the installation is raised from that of the surrounding atmosphere to approximately that reached during the early stages of operation. While this change is taking place, the metals in the various parts of the installation expand. In order to prevent inefficient operation and damage due to distortion, the procedure should be such that all parts of the turbines are evenly heated. If the rotor and casing are not evenly heated, unequal expansion, resulting in distortion of the rotor or casing will take place.
At any time that the sound of rubbing or grinding is detected in a turbine, it should be stopped immediately, and the trouble shot in order to prevent serious damage.
A reasonable time should be allowed for warming up the turbines before applying load. During this period the turbine should be inspected carefully to be sure that it will be in operating condition when the vessel starts.
This inspection should include the oil supply, the throttle and the governor valves, the bearing temperatures, turbine clearances and a general inspection of all moving parts about the governing mechanism.
Text 8. Instructions for starting and shutting down
Starting
1. Measure clearances where indicators are installed.
2. See that the turbine rotors and gears move freely. This may be done by turning the units manually by means of a ratchet wrench or a motor driven turning gear.
3. All valves and cocks for draining water from the main steam pipe, manoeuvring valves and turbine casings should be opened.
4. All steam valves at the manoeuvring gear and about the turbines should be closed, but eased slightly to prevent jamming when hot.
5. The cocks to the pressure and vacuum gauges on the turbine and condenser should be open.
6. Inspect the lubricating system carefully.
7. Start the main oil pump, and make sure that the oil is flowing freely to all bearings, flexible couplings and nozzles. With oil circulating in the system, again check the oil level in the reservoir.
8. Start the main circulating pump.
9. As soon as the vacuum begins to rise, open manoeuvering valve sufficiently to start turbines rolling immediately. Listen for any unusual sounds and be assured that there is no rubbing.
10. Warm up turbine slowly, alternately ahead and astern.
11. When the warming- up period is completed, the turbine may be brought up to half speed.
Shutting down
1. When finished with engines, shut down the air ejector.
2. Shut off the gland-sealing valve.
3. Shut down the main circulating pump.
4. Shut off the main steam stops valve.
5. Open all turbine and manoeuvering drain valves.
6. Keep the condenser pump working until the turbine is thoroughly drained. Then shut down the pumps. To keep the turbine interiors dry, it is important that the air pumps or ejectors be run for about 1/2 hour every second day to draw fresh air through the turbine.
7. Connect the turning gear and roll unit for about 2 hours.
8. After the turning gear has been disconnected, shut down the lubricating pump.
9. Every day start the oil pump and force a fresh supply of clean oil through all bearings, and to the gear spray nozzles if gears are fitted. While the oil is being circulated, turn the turbine rotors through 1 1/2 revolutions of the propeller in order to oil all the gear teeth and to let the rotors come to rest in a new position.
Text 9. Free-piston gas generator turbine as a power plant for ship propulsion
In a gas generator, the whole of the air from the compressor is delivered to the engine cylinder, which is therefore highly supercharged, the large excess of air
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passing out to the exhaust as scavenging air. The hot exhaust products join with the heated scavenging air and the whole constitutes the "power gas" that is delivered to the turbine. The power gas consists of at least 75 per sent of unburnt air, the remainder being the products of combustion of the fuel.
The overall compression and expansion ratio of the cycle is very high, which is well- known to be one of the essentials for high thermal efficiency. The first part of the compression takes place in the compressor part in the engine cylinder, and the high pressure part in the engine cylinder. Similarly, the high pressure part of the expansion takes place in the engine cylinder and the low pressure part in the turbine.
With the outward compressing type of gas generator it is necessary to employ stepped piston assemblies having additional pistons of smaller diameter mounted on the outsides of the compressor pistons to provide a cushion. This means that not only are the overall dimensions and weights greater, but the manufacture is almost certainly more expensive.
The maximum output obtainable from an outward compressing type of gas generator must be less than that obtainable from an inward compressing type, because the inherent greater weight of the moving parts will reduce the speed of oscillation.
Piston oil- cooling which can be arranged extremely easily in the case of the inward compressing type of gas generator becomes very much more difficult and complicated in the outward compressing type.
The inward compressing type is simpler with regard to maintenance; for instance, the pistons can be easily withdrawn.
Two sizes of free-piston gasifier have been developed for marine and industrial purposes: the GS-34 1.250 gas h.p. unit in France, and the CS-75 420 gas h.p. unit in England.
Text 10. Boilers
Boilers are used on board the ship for producing steam. This steam may be used for driving the main engines, when steam turbines are fitted, or for driving auxiliary machinery such as the windlass. There are two basic types of boilers in use in ships: the fire-tube boiler, and the water-tube boiler.
The fire-tube boiler consists of a cylindrical steel shell, which contains a furnace at the bottom. Two or more furnaces may be fitted, depending on the size of the boiler. The furnace is connected to a combustion chamber, situated in the middle part of the boiler. The furnace, the combustion chamber and the tubes are all surrounded by water. Boilers are now mainly used for auxiliary purposes on board ship.
Water-tube boilers have replaced fire-tube boilers for generating steam for main engines. They have a steam drum at the top, which is partly filled with water, and water drums at a lower level. These drums are connected by banks of tubes, which also contain water. The furnace is located at the bottom and the whole system is contained in a fire-proof casing. Downcomer tubes are placed outside the gas system to act as feeders to the water drums.
Gases are heated in the furnace and pass upward, transferring their heat to the water in the tubes. Because the steam drum provides a reservoir of relatively cool water, convection currents are set up causing the water to circulate round the system. Superheaters are added to the system to increase its efficiency. These are located between the rows of tubes.
Various valves and gauges are fitted to the boilers. For a water-tube boiler these include the following: safety valves, which are needed to release any excess steam from the boiler; a main stop valve in order to control the passage of steam to the engines; feed valves to add water into the boiler; water level indicators to show the level of water in the boiler; thermometers and pressure gauges for showing the temperature an pressure inside the boiler. In order to be able to drain water from the system drain valves are fitted. Chemical dosing valves are also necessary so that chemicals can be added directly into the boiler.
Text 11. Functions of auxiliary machinery
Besides running and maintaining the main propulsion machinery of the ship, the engine officer has a great deal of auxiliary machinery to look after. Auxiliary machinery covers everything mechanical on board ship except the main engines and boilers. It includes almost all the pipes and fittings and the equipment needed to carry out a number of functions. These functions may be summarized as follows: to supply the needs of the main engines and boilers. Air compressors are used to supply compressed air for starting engines. Coolers are used for cooling either oil or water. Water for the boilers is also heated before being admitted into the boiler by feed water pumps. This increases the efficiency of the boiler.
- To keep the ship dry and trimmed. This is done through the bilge an ballast pumping systems. Bilge pumping system removes water which has gathered in machinery, cargo and other spaces. Ballast pumping system pumps water into and out of ballast tanks. In general cargo ships, these systems are usually interconnected and served by the same pumps. In tankers an other bulk carries, these systems are completely separate, because these ships may need to ballast at 12,000 tons/hour and therefore need larger pumps.
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- To supply domestic needs such as fresh and sea water, sanitation; heating or cooling of air, ventilation.
- To apply the main power of the engines for propulsion and manoeuvring. The engine power is transmitted to the propeller by a line of shafting. This is made up of the thrust shaft, intermediate shafts and the propeller shaft. Steering gear is also necessary to operate the rudder for manoeuvring.
- To supply the ship with electrical power and lighting. This is done by steam of diesel-powered generators. To moor the ship and handle cargo. Deck machinery is extensive and varied. It can be divided into anchor-handling machinery (windlasses and capstans ), mooring machinery (winches and capstans), and cargo-handling machinery ( winches and cranes ). It also includes cargo oil pumps.
- To provide for safety. Firefighting and fire detection equipment, lifeboat engines and launching gear are also included.
- To prevent oil and sewage pollution.
Text 12. Maintenance schedule of marine diesel engine
Engine builders supply detailed instructions on the operation and maintenance of their machinery so that regular maintenance work can be carried out and breakdowns can be kept to a minimum. These instruction manuals are usually kept by the Chief Engineer, but are made available to all members of the engine-room staff. The intervals at which an engine and its parts must be inspected will vary from make to make( зависят от модели двигателя ) and will depend on the use the engine has been put to.
At frequent intervals, fuel pumps should be examined and adjusted, if necessary. When the engine is running, this will be shown by comparing engine indicator cards and by exhaust temperatures. Pistons should also be examined frequently for cracks.
At intervals of six weeks, the fuel valves should be taken out and carefully inspected. Atomizers and filters can be washed with clean diesel fuel oil. Cleaning rags must not be used because they leave behind small pieces of fluff, which may block the holes. Valve seats should be tested and if they are pitted or scratched, the surface should be reground.
At intervals of six months piston heads if cooled, must be inspected for deposits of carbon in cooling spaces and cooling pipes. When new piston rings are fitted, care must be taken to ensure there is sufficient clearance to allow for the expansion of the rings. Exhaust valves and manifold must also be examined and excessive carbon deposits removed. All carbon deposits should be removed from
cylinder ports. Cylinder liners must be examined externally for deposits of scale. If these deposits cannot be removed by flushing with water, then the liner must be removed for cleaning. The liner should also be measured for wear and renewed, if the limit for wear has been reached. The clearance of connecting-rod top and bottom ends should also be examined and adjusted if necessary.
At intervals of one year the manoeuvring gear must be examined for wear at the joints of levers and rods. The alignment of the crankshaft should be checked and any incorrect alignment corrected.
The main bearings must be examined and readings taken for wear. The clearances of all crankshaft bearings must be maintained at the figure recommended by the makers. Finally, starting air piping and air bottles must be cleaned, and the lubricating oil system thoroughly examined and cleared of deposits.
It must be emphasized that the above-mentioned parts are only some of the items which must be regularly maintained to ensure the efficient working of the machinery.
Text 13. Repairs
CYLINDERS. - Cylinder liners are a source of work for repair yard. Liner casualties occur either from wear or cracks. Wear is a normal result of engine use.
Liners that are originally fitted so that they will drop into place in the jackets with little or no pressure, sometimes require from 200 to 300 tons hydraulic pressure to remove them. This is caused by the carbon or scale accumulation that gets into the seating. After this seating deposit is once broken, no great pressure is required to move the liner the rest of the way.
Cracked liners are encountered occasionally, but in practically every case cracking is due to no fault in the liner itself. It usually results from piston seizure, and such seizure usually result from inadequate cooling of the piston. No attempt is ever made to repair cracked liners, renewal being the only remedy.
CYLINDER COVERS ( HEADS ) - The cover is usually the most complicated casting on the engine, and as such suffers most from heat trouble. To make a successful cover repair, electric welding must be done skillfully.
Once an emergency repair to a cracked cover was made on shipboard by the crew. Both bridges, between injection valve and inlet and exhaust valves cracked on bottom face of cover. The injection valve hole was bored out and threaded and a cast iron bush screwed in. a hole large enough to remove the cracks was then drilled on each side of the injection valve opening and threaded, after which a copper plug was screwed tightly into each one. Although this type of repair is not to be recommended, it did work well in this case.
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Text 14. MARPOL 73/78
The International Convention for the Prevention of Pollution from Ships, 1973 as modified by its Protocol of 1978, is the most important anti-pollution treaty ever adopted by IMO.
The convention is usually known as MARPOL 73/78.
Although pollution resulting from tanker accidents caused some concern, the convention was primarily aimed at pollution resulting from routine tanker operations, which was the greater cause of pollution from ships.
When a tanker has discharged its cargo it has to fill some of its cargo tanks with ballast water in order to provide the necessary stability.
Mixtures of oil and water also result from tank cleaning, which is generally done by spraying the tank walls and bottom with water.
Pollution can also come from engine room bilges of all ships (not only tankers), since bilge water is always contaminated by oil. In 1954 the normal practice was to pump these mixtures of oil and water into the sea.
The convention deals not only with oil but with all forms of marine pollution.
Most of the technical measures are included in five annexes to the convention which deal respectively with the following:
Annex I - Oil
Annex II -Noxious (вредный, ядовитый) liquid substances carried in bulk (e.g. chemicals)
Annex III - Harmful (вредный, опасный ) substances carried in packages (e.g. tanks and containers)
Annex IV - Sewage
Annex V - Garbage
MARPOL 73/78 requires:
1. An initial survey before the ship is put into service or before an International Oil Pollution Prevention Certificate is issued.
2. Periodical surveys at intervals not exceeding five years.
3. A minimum of one intermediate survey during the period of validity of the IOPP Certificate.
4. Unscheduled inspection or mandatory (обязательный ) annual surveys must be carried out.