Text 5. Four strokes of diesel engines

The first stroke is suction. The piston is moving downwards an the air inlet valve has been opened by the engine while the others remain closed. Air from the engine room is being drawn into the cylinder, and when the piston reaches the bottom of the stroke, the cylinder will be full of fresh air and the inlet valve will

close.

The second stroke is compression. The piston is now being driven upwards, all valves are shut and the air charge is compressed to a pressure of about 500 lbs. per square inch, at which pressure its temperature is 1200 deg. F. The third stroke is firing. Just before the beginning of the stroke the fuel valve is opened and oil is sprayed into the cylinder in the form of a fine mist. The hot air causes it to burn and this air is further heated by the combustion of the fuel. The fuel valve remains open only for a short period at the beginning of this stroke. The gas expands and the piston is driven downwards and so supplies power to the shafting through the connecting rod and crank.

The fourth stroke is exhaust. The piston is again travelling upwards and the exhaust valve has been opened, the waste or burnt gases are driven out to the silencer through the exhaust valve. The next stroke recommences the cycle of operations, with the admission of a fresh air charge.

Text 6. Two stroke engines

Two-stroke engines are also used for propulsion of ships. These, if single-acting, provide one power-stroke per revolution, while a double-acting 2-stroke engine develops power one every stroke.

A two-stroke engine develops almost twice as much power as a four-stroke engine with the same size and number of cylinders. Since the four operations, i.e. suction, compression, firing and exhaust have to be completed during two strokes of the piston, more than one operation must be performed per stroke. This complicates the engine. The piston is made to control the admission of air and release of the exhaust gases by opening and closing ports or passages in the cylinder walls through which the air and gases pass. The fresh air-charge is pumped into the cylinder at low pressure by means of a scavenging pump which may be driven either by the engine itself or by a separate auxiliary engine or electric motor. This air not only provides the air charge necessary for the proper combustion of the fuel but assists in clearing the burnt gases rapidly out of the cylinder.

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 1/2revolutions 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.