5. Diesel Engines
4-cycle and 2-cycle diesel engines are commonly used in marine practice. They may be single or double acting, trunk-piston type, crosshead type, opposed-piston type.
In the 4-cycle diesel engines the first stroke is called the suction stroke. The piston moves downward and a charge of air is drawn into the cylinder through the open inlet valve.
The second stroke is called the compression stroke. The piston moves upward. The charge of air is compressed into the clearance volume. At the end of the stroke the fuel injection valve opens and finely atomized fuel oil is sprayed in to the combustion volume under high pressure. The fuel oil is ignited and burns, generating hot gases. Hot gases expand and push the piston down on the third stroke.
The third stroke is called the working stroke. The piston moves downward. Near the end of the stroke the exhaust valve opens and part of the hot gases escape.
The fourth stroke is called the exhaust stroke. The piston moves upward. All the products of combustion are forced out. The exhaust valve closes, and the cycle is repeated.
In the 2-cycle diesel engine instead of an exhaust valve there is a ring of exhaust ports. In place of air inlet valves there are scavenging ports. Here one complete cycle requires only two strokes of the piston or one revolution of the crank-shaft.
6. Steam Turbines
The turbine is a heat engine which consists of a rotor carrying moving blades, a casing in which the rotor revolves, and stationary nozzles through which the steam is expanded or directed. Glands, bearings, throttle valve, governor and other devices are necessary for operation of the unit. In the steam turbine, two steps are required to convert the potential energy of the steam into useful work. First step, the pressure energy is converted into kinetic energy as the steam expands through the nozzles and the pressure drops. Second step, the kinetic energy of the jet is converted into useful work.
There are two types of turbines: impulse and reaction turbines. In impulse turbine the steam expands only through stationary nozzles with a decrease in pressure and an increase in velocity. In this process the potential energy of the steam is converted into kinetic energy. In reaction turbines the steam drops in pressure and at the same time expands as it passes across both the moving blades and stationary blades.
There may be high, intermediate and low pressure turbines. There may be single-cylinder turbines and compound. Compound turbines are tandem-compound and cross-compound.
As to the direction of steam flow the turbines are axial-flow and radial-flow. There may be condensing and non-condensing.
7. Power Plant Automation
Automation is a subject which receives considerable attention in marine spheres; indeed it is becoming rare to find a ship that does not have some form of automatic means remote control for engines, power plants navigation and other equipment. Automatic control is more accurate and continuously watchful than manual control can ever be. Its installation, besides giving engine-room personnel more time to deal with maintenance problems, results in saving of fuel, maintenance costs, and out-of-service time due to failure. The major benefit of shipboard automation is the saving in operating cost resulting from a reduction of crew. This can readily be achieved through the installation of remotely operated valves in the ship's flow systems, the installation of sensors and the grouping of control and monitoring devices for convenient operation in the central control station.
The remote control room is air-conditioned and insulated. The main items which can be controlled from the room are: main engine, steam plants generators, auxiliary machinery, pressures, levels, temperature regulation and readings, alarms, communications, boiler service.
The latter includes visual instruments, status signal lights, alarms, graphic displays, and automatic data-logging equipment. Data logging improves supervision efficiency and often spots incipient faults before the human eye.