NABTEB GCE 2025 Marine Engineering Craft – Typ A

NABTEB GCE 2025 MARINE ENGINEERING CRAFT ANSWERS (TYPE A)

MARINE ENGINEERING CRAFT OBJ (TYPE A)
01-10: CCDCAADCDA
11-20: DCDCBBCDDB
21-30: AAABCADCCA
31-40 ABCDDCACDD

COMPLETED

INSTRUCTIONS: ANSWER FIVE QUESTIONS ONLY.
(1a)
(i) Rain
(ii) Rivers
(iii) Wells
(iv) Springs

(1b)
(i) Springs
(ii) Rain

(1c)
When rainwater passes through regions containing carbon gases in the cloud, it absorbs these gases and forms weak acids. This makes the rainwater slightly acidic, increases its corrosive nature, and reduces its purity, making it less wholesome for direct consumption.
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(2a)
(Draw the diagram)

The pipe carries steam from the steam production plant through a main distribution line toward the consumer. Along the route, the pipe may pass through components such as steam separators or receivers to ensure dry steam. The pipe then continues and bends downward to reach the steam consumer, delivering the steam for use.

(2b)
(i) Steam pipe: Red
(ii) Cold water pipe: Blue
(iii) Acetylene gas pipe liner: Maroon
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(3a)
(Draw the diagram)

The plan shows four engines arranged in pairs, with each pair feeding power into a common reduction gearbox so that two engines jointly drive one propeller shaft. The left pair of engines transmits its combined output through a shared gearbox to the port-side propeller, while the right pair does the same for the starboard-side propeller, allowing increased power, redundancy, and balanced propulsion for the vessel.

(3b)
The clutch provides a smooth connection and disconnection between the engine and the propulsion system. It allows the engine to run smoothly while engaging or disengaging the propeller shaft, preventing shock or damage. This helps in starting, stopping, changing speed, and reversing the vessel smoothly during operation.
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(4a)
(Draw the diagram)

(4bi)
Flash point temperature of a fuel:
It is the lowest temperature at which a fuel gives off enough vapour to ignite momentarily when exposed to an external flame.

(4bii)
It can cause poor combustion, leading to loss of power, engine knocking, overheating, and increased fuel consumption.
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(5a)
(i) CI engines use diesel fuel, while SI engines use petrol.
(ii) CI engines ignite fuel by high compression heat, while SI engines ignite fuel using a spark plug.
(iii) CI engines operate at higher compression ratios, while SI engines operate at lower compression ratios.
(iv) CI engines use a fuel injector for fuel delivery, while SI engines use a carburettor or fuel injection system.
(v) CI engines are generally more fuel-efficient, while SI engines are less fuel-efficient.

(5bi)
It opens and closes the engine’s inlet and exhaust valves at the correct timing during operation.

(5bii)
(i) They provide better cooling because they are in direct contact with the coolant.
(ii) They are easier to replace without machining the engine block.
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(6a)
(i) Pistons: The piston must be replaced because reboring increases cylinder diameter, requiring an oversized piston for proper sealing and compression.
(ii) Piston rings: New oversized rings are needed to match the enlarged cylinder bore and ensure correct compression and oil control.
(iii) Cylinder head gasket: A new gasket is required because the old one cannot reseal properly after the engine has been dismantled for reboring.
(iv) Connecting rod bearings: These are replaced to restore correct clearances and ensure smooth crankshaft rotation after major engine work.
(v) Main bearings: They are replaced to maintain proper support and alignment of the crankshaft following engine overhaul.

(6bi)
(i) Loss of power
(ii) Engine knocking
(iii) Piston seizure
(iv) Increased wear

(6bii)
(i) Low coolant level: This prevents adequate heat removal from the engine, causing the temperature to rise excessively.
(ii) Faulty thermostat: A thermostat stuck closed restricts coolant flow to the radiator, leading to overheating in both SI and CI engines.
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(7a)
The 12-volt battery supplies low voltage to the ignition coil, where the primary winding receives the current and creates a magnetic field around the coil. When the contact breaker opens, the sudden collapse of this magnetic field induces a very high voltage in the secondary winding, which has far more turns of wire. The ignition coil, therefore, steps up the battery’s 12 volts to several thousand volts. This high voltage is then directed by the distributor to the appropriate spark plug, where it produces the spark needed for combustion.

(7b)
(i) Worn armature bearings: When the bearings wear out, the armature no longer stays centered, causing it to shift sideways and rub against the field coil.
(ii) Bent or misaligned armature shaft: If the shaft becomes bent due to improper handling, impact, or wear, the armature rotates out of line and makes contact with the field coil.
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COMPLETED