NABTEB GCE 2025 ADVANCED MOTOR VEHICLE MECHANICS WORKS ANSWERS (ESSAY – TYPE A)

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ADVANCED MOTOR VEHICLE MECHANICS WORKS ANSWERS (TYPE A)

SECTION A: ANSWER ALL QUESTIONS

(1)
(i) Check engine oil level.
(ii) Check coolant/water level in the radiator.
(iii) Inspect tyre pressure and condition.
(iv) Check battery electrolyte level and terminals for cleanliness.
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(2)
(i) To improve traction on rough, muddy or slippery terrains.
(ii) To enable the vehicle carry heavier loads by distributing driving torque to more wheels.
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(3)
(i) Always add acid to water, never water to acid.
(ii) Wear protective equipment such as goggles, rubber gloves and apron.
(iii) Prepare electrolyte in a well-ventilated area.
(iv) Avoid open flames or sparks near the electrolyte.
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(4)
(DRAW THE DIAGRAM)

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(5)
(DRAW THE DIAGRAM)

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(6)
(i) It does not require a battery to generate ignition spark – works independently.
(ii) It is more reliable at high engine speeds and suitable for small engines.
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(7)
During the induction stroke, the inlet valve opens and the piston moves downward from top dead centre, drawing only clean air (not fuel) into the cylinder. The air enters due to the vacuum created by the downward movement of the piston.
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(8)
Backlash is the free movement or play between the meshing teeth of gears in the steering gearbox before the gears actually engage. It results in slight steering wheel movement without corresponding road wheel movement.
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(9)
(i) To increase load-carrying capacity of heavy vehicles.
(ii) To improve vehicle stability and traction during off-road operation.
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(10)
(i) Worm and roller gearbox.
(ii) Recirculating ball steering gearbox.
(iii) Rack and pinion steering gearbox.
(iv) Worm and sector steering gearbox.
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SECTION B: ANSWER FIVE(5) QUESTIONS ONLY

(11)
(i) Decarbonization: Decarbonization is the process of removing hard carbon deposits formed on engine parts such as the piston crown, valves, and combustion chamber. These deposits accumulate during fuel combustion and can reduce engine efficiency. The purpose of decarbonization is to restore normal compression, improve fuel economy, and reduce engine knocking, thereby ensuring smooth engine performance.

(ii) Overhauling: Overhauling refers to the complete dismantling, inspection, cleaning, repair, and replacement of worn or damaged parts of an engine or machine. It restores the equipment to near-original working condition. An overhaul may involve replacing piston rings, bearings, valves, and seals. The aim is to extend the lifespan of the engine, improve reliability, and prevent unexpected breakdowns.

(iii) Dilution: This is the contamination or thinning of engine oil by fuel, water, coolant, or other foreign substances. This reduces the viscosity and lubricating strength of the oil, leading to increased friction and wear. Dilution often occurs due to faulty carburation, frequent short trips, or worn piston rings, and it can cause overheating and premature engine damage if not corrected.

(iv) Synchronization: Synchronization is the process of properly timing and coordinating the operation of two or more mechanical components so that they work smoothly and accurately. In engines, synchronization ensures correct ignition timing, valve timing, and gear engagement, preventing mechanical conflict. For example, synchronized gears allow smoother gear shifting without grinding.

(v) Thermo syphon cooling system: A thermo syphon cooling system is a pump-less engine cooling method where coolant circulates naturally due to temperature differences. Hot coolant rises from the engine into the radiator, while cooler and denser coolant descends back into the engine. This system relies on gravity and convection currents. It is simple, requires no moving parts, and is used in small engines and older vehicles.

(vi) Reboring: Reboring is the process of machining an engine cylinder to a slightly larger diameter to correct wear, scoring, or out-of-roundness. After reboring, an oversize piston and piston rings are fitted to restore proper compression and efficiency. This procedure helps to extend engine life, improve power output, and reduce oil consumption.
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(12)
(I) Ignition Method: The SI engine ignites the air-fuel mixture using a spark plug, which requires an external electrical source, while the CI engine does not use a spark plug but relies on the heat generated by compressing air to ignite the fuel spontaneously.

(II) Type of Fuel: The SI engine operates on petrol, which is highly volatile and evaporates easily, in contrast, the CI engine runs on diesel, a heavier fuel that ignites under high pressure and temperature.

(III) Air and Fuel Mixing: In the SI engine, air and fuel are mixed thoroughly before entering the combustion chamber during the intake stroke, while in the CI engine, only air is drawn in during the intake stroke, and fuel is injected directly into the cylinder near the end of the compression stroke.

(IV) Compression Ratio Levels: The SI engine has a lower compression ratio typically between 6:1 and 10:1 to avoid knocking, while the CI engine has a much higher compression ratio ranging from 14:1 to 22:1 to achieve the high temperatures necessary for auto-ignition.

(V) Engine Speed and Weight: The SI engine functions at higher speeds and is lighter in weight due to lower internal pressures, while the CI engine operates at lower speeds but is heavier as it must withstand extremely high pressure and temperature conditions during combustion.

(VI) Thermal Efficiency and Fuel Economy: The SI engine generally exhibits lower thermal efficiency because it operates at lower compression ratios, whereas the CI engine benefits from higher thermal efficiency and better fuel economy due to its high compression and leaner fuel mixture combustion.
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(13a)
(i) Disconnect the battery, drain coolant, and remove the air cleaner, manifolds, and spark plugs to access the cylinder head.
(ii) Loosen and remove the cylinder head bolts in the correct sequence and lift off the cylinder head.
(iii) Inspect the pistons, valves, and combustion chambers to determine the level of carbon deposits.
(iv) Apply a chemical decarbonizing agent to soften the deposits, then scrape and brush off carbon from piston crowns, valves, and cylinder head surfaces.
(v) Remove the valves, clean their heads and stems, and lap them if necessary for proper sealing.
(vi) Wash all cleaned components with petrol/kerosene, dry them thoroughly, and clean piston ring grooves.
(vii) Reassemble the cylinder head with a new gasket, tighten bolts to the specified torque, refit all components, and reconnect the battery.
(viii) Start and run the engine to check for smooth operation and absence of leaks.

(13b)
(i) Scraper or carbon remover tool
(ii) Wire brush set
(iii) Valve spring compressor
(iv) Torque wrench

(13c)
(i) Allow the engine to cool fully before starting work to avoid burns.
(ii) Avoid scratching piston crowns, cylinder walls, or valve seats while scraping carbon.
(iii) Keep all small parts (nuts, bolts, springs) properly arranged to prevent loss or incorrect assembly.
(iv) Ensure no carbon particles or dirt enter the crankcase or oil passages; cover openings during cleaning.
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(14a)
(i) Remove the differential cover and any components obstructing access to the pinion bearings.
(ii) Assemble the pinion with its bearings in place, using the pinion nut but without fully tightening.
(iii) Attach a torque wrench or suitable measuring instrument to the pinion shaft.
(iv) Rotate the pinion slowly while observing the torque reading on the instrument.
(v) Adjust the pinion nut until the torque reading matches the manufacturer’s specified pre-load value.
(vi) Recheck the torque after adjustment to ensure correct bearing pre-load is achieved.
(vii) Once correct, reassemble the differential cover and any removed components.
(viii) Test-rotate the differential to confirm smooth operation without excessive play or binding.

(14b)
(DRAW THE DIAGRAM)

=TORQUE WRENCH=

(14c)
(i) Increased noise and vibration during operation due to improper gear meshing.
(ii) Accelerated wear and potential damage to the gear teeth, reducing the lifespan of the differential.
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(15a)
(i) Battery circuit: Red (positive), Black (negative/earth) cables.
(ii) Generator circuit: Red or Brown (positive output), Black (earth/negative).
(iii) Headlamp circuit: Yellow or White (main supply), Black or Green (earth/return).
(iv) Side and tail lamp circuits: Brown or Blue (supply), Black or Green (earth/return).

(15b)
(i) Switch off the ignition and disconnect the battery to ensure safety.
(ii) Isolate the circuit to be tested by disconnecting relevant wires or components.
(iii) Set a multimeter to resistance (ohms) or continuity mode.
(iv) Connect one probe to the positive wire of the circuit, and the other to the chassis or ground.
(v) A low or zero resistance reading indicates a short circuit or leakage, while high or infinite resistance suggests no fault.
(vi) If a short is detected, test individual wiring sections to localize the fault.
(vii) Inspect wiring for damage such as exposed or melted insulation and repair or replace faulty sections.
(viii) Reconnect battery and test circuit operation after repairs.

(15c)
The instrument used is called a Growler. It creates an alternating magnetic field around the armature, acting like the primary coil of a transformer. When a short circuit exists in the armature winding, it induces currents that cause a distortion in this magnetic field. A metal strip or a hacksaw blade held near the armature vibrates due to this alternating magnetic field, indicating the presence of a shorted coil. The vibration and a growling noise confirm the winding fault. This method effectively detects short circuits by using magnetic induction and physical feedback.
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(16a)
(DRAW THE DIAGRAM)

=MECHANICAL GOVERNOR USED IN A COMPRESSION IGNITION ENGINE=

(16b)
(i) Spindle: The vertical shaft that rotates and supports the governor mechanism.
(ii) Fly-ball: It the rotating weights that move outward as engine speed increases, generating centrifugal force.
(iii) Bevel gear: It transfers motion from the fly-ball assembly to the fuel control linkage.
(iv) Throttle valve: It regulates the amount of fuel entering the engine based on governor action.

(16c)
(i) Hydraulic governor: uses hydraulic pressure to control fuel delivery automatically.
(ii) Electronic/Electrical governor: uses sensors and an electronic control unit (ECU) to regulate engine speed.
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(17a)
(i) Switch off the engine: Always ensure the tractor engine is off before coupling or uncoupling to prevent accidental movement.
(ii) Engage the handbrake: Apply the handbrake firmly to prevent the tractor from rolling.
(iii) Use proper hitching points: Always use designated coupling points and avoid standing between the tractor and implement.
(iv) Check the implement: Ensure the implement is stable and properly aligned before attaching or detaching.

(17b)
PTO stands for Power Take-Off, a mechanical device on tractors that transfers power from the tractor’s engine to an attached implement. It allows implements such as mowers, balers, or tillers to operate without needing their own engines. The PTO consists of a rotating shaft, clutch, and gearbox, which allows the operator to engage or disengage power safely as required. PTO shafts are designed to rotate at standardized speeds, usually 540 or 1000 RPM, so that various implements can be used universally across different tractors. This system is crucial in mechanized farming because it reduces labor, increases efficiency, and allows a single tractor to power multiple implements safely and effectively.

(17c)
(i) To handle high torque: Larger threads can withstand the high twisting forces transmitted from the tractor to the implement.
(ii) To ensure durability: Large threads reduce the risk of stripping or wear, ensuring a secure and long-lasting connection.

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