Governors
The principal control device on any engine is the governor. It governs or controls the engine speed at some fixed value while power output changes to meet demand. This is achieved by the governor automatically adjusting the engine fuel pump settings to meet the desired load at the set speed.
Governors for diesel engines are usually made up of two systems: a speed-sensing arrangement and a hydraulic unit which operates on the fuel pumps to change the engine power output.
Mechanical governor
A flyweight assembly is used to detect engine speed. Two flyweights are fitted to a plate or ball head which rotates about a vertical axis driven by a gear wheel.
The action of centrifugal force throws the weights outwards; this lifts the vertical spindle and compresses the spring until an equilibrium situation is reached. The equilibrium position or set speed may be changed by the speed selector which alters the spring compression.
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As the engine speed increases the weights move outwards and raise the spindle; a speed decrease will lower the spindle. The hydraulic unit is connected to this vertical spindle and acts as a power source to move the engine fuel controls. A piston valve connected to the vertical spindle supplies or drains oil from the power piston which moves the fuel controls depending upon the flyweight movement.
If the engine speed increases the vertical spindle rises, the piston valve rises and oil is drained from the power piston which results in a fuel control movement. This reduces fuel supply to the engine and slows it down. It is, in effect, a proportional controller. The actual arrangement of mechanical engine governors will vary considerably but most will operate as described above.
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Electric governor
The electric governor uses a combination of electrical and mechanical components in its operation. The speed-sensing device is a small magnetic pick-up coil. The rectified, or d.c., voltage signal is used in conjunction with a desired or set speed signal to operate a hydraulic unit. This unit will then move the fuel controls in the appropriate direction to control the engine speed.
Cylinder relief valve
The cylinder relief valve is designed to relieve pressures in excess of 10% to 20% above normal. A spring holds the valve closed and its lifting pressure is set by the appropriate thickness of the packing piece. Only a small amount of lift is permitted and the escaping gases are directed to a safe outlet.
The valve and spindle are separate to enable the valve to correctly seat itself after opening. The operation of this device indicates a fault in the engine which was discovered and corrected. The valve itself should then be examined at the earliest opportunity.
Crankcase oil mist detector
The presence of an oil mist in the crankcase is the result of oil vaporisation caused by a hot spot. Explosive conditions can result if a build-up of oil mist is allowed. The oil mist detector uses photoelectric cells to measure small increases in oil mist density. A motor-driven fan continuously draws samples of crankcase oil mist through a measuring tube.
An increased meter reading and alarm will result if any crankcase sample contains excessive mist when compared to either clean air or the other crankcase compartments. The rotary valve which draws the sample then stops to indicate the suspect crankcase. The comparator model tests one crankcase mist sample against all the others and once a cycle against clean air.
The level model tests each crankcase in turn against a reference tube sealed with clean air. The comparator model is used for crosshead-type engines and the level model for trunk piston engines.
Explosion relief valve
As a practical safeguard against explosions which occur in a crankcase, explosion relief valves or doors are fitted. These valves serve to relieve excessive crankcase pressures and stop flames from being emitted from the crankcase. They must also be self-closing to stop the return of atmospheric air to the crankcase.
Various designs and arrangements of these valves exist where, on large slow-speed diesel, two-door type valves may be fitted to each crankcase or, on a medium-speed diesel, one valve may be used. One design of the explosion relief valve. A light spring holds the valve closed against its seat and a seal ring completes the joint.
A deflector is fitted on the outside of the engine to safeguard personnel from the outflowing gases, and inside the engine, over the valve opening, an oil-wetted gauze acts as a flame trap to stop any flames leaving the crankcase.
After operation, the valve will close automatically under the action of the spring.
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