NABTEB 2024 MOTOR VEHICLE MECHANIC WORKS ANSWERS

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MOTOR VEHICLE
01-10: CABCBDCBAD
11-20: BBABDCABCA
21-30: CBADCBBABC
31-40: DACBDCDDAB

COMPLETED

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ANSWER FIVE(5) QUESTIONS ONLY

(1a)
A two-stroke petrol engine operates by completing a power cycle in two strokes of the piston, unlike a four-stroke engine. The process begins with the intake port opening, allowing the piston to move down and create a vacuum that draws an air-fuel mixture into the cylinder. The intake port then closes, and the piston moves up, compressing the air-fuel mixture. The spark plug ignites the mixture, causing a small explosion that drives the piston down, rotating the crankshaft. This motion is repeated with each stroke of the piston, completing the two-stroke cycle. The engine’s design uses the crankcase as a reservoir for the air-fuel mixture and the piston as a valve to manage the flow into the cylinder, resulting in a simpler, more compact engine with a higher power-to-weight ratio.

(1b)
(i) Lighter and more compact: Two-stroke engines are generally lighter and more compact than four-stroke engines, making them ideal for small applications like chain saws, leaf blowers, and motorcycles.
(ii) Simpler design: Two-stroke engines have fewer moving parts and no valves, making them simpler in design and easier to maintain.
(iii) Higher power-to-weight ratio: Two-stroke engines produce more power relative to their size and weight, making them popular for applications where power and portability are crucial.
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(2a)

(2b) The firing order of a four-cylinder engine is: 1 – 3 – 4 – 2

(2c)
(i) Overhead Valve (OHV) mechanism
(ii) Overhead Camshaft (OHC) mechanism
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(3i)
=Knock=
(i) Low Octane Fuel
(ii) Carbon Deposits

(3ii)
=Lack of Power=
(i) Faulty Spark Plugs
(ii) Restricted Air Filter

(3iii)
=Overheats=
(i) Low Coolant Level
(ii) Clogged Radiator

(3iv)
=Vibration=
(i) Unbalanced Crankshaft
(ii) Worn Engine Mounts
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(5)
(i) Fan:
Circulates air through the radiator to cool the coolant, helping to dissipate heat.

(ii) Radiator:
Transfers heat from the coolant to the air, using fins and tubes to maximize heat transfer.

(iii) Thermostat:
Regulates engine temperature by controlling coolant flow to the radiator, maintaining a optimal temperature.

(iv) Pressure Cap:
Maintains cooling system pressure and prevents coolant loss, opening at a set pressure to release excess pressure.

(v) Hoses:
Transport coolant between components, withstanding high temperatures and pressures.

(vi) Antifreeze (Coolant):
Absorbs and transfers heat, protects the engine from freezing and corrosion, and lubricates the cooling system, requiring a proper concentration to ensure effectiveness.
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(6a)
A pressure-relief valve is fitted in the lubricating system to prevent excessive pressure buildup, which can damage the engine or cause the lubricant to leak out. The valve opens at a set pressure, allowing excess pressure to be released and ensuring the system operates within a safe pressure range.

(6b)
(i) Friction reduction
(ii) Heat transfer
(iii) Contamination removal

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