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MOTOR VEHICLE MECHANICS OBJ
01-10: BCBDDCABAC
11-20: CBADCBCBCD
21-30: BCBCDCADCA
31-40: BBCABCCCBB
COMPLETED
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INSTRUCTION: ANSWER FIVE(5) QUESTIONS ONLY
(1a)
(i) Carbon monoxide (CO)
(ii) Nitrogen oxides (NOₓ)
(iii) Hydrocarbons (HC)
(1b)
(i) Carbon monoxide (CO):
Formed from the incomplete combustion of fuel (petrol or diesel) in the engine when there is insufficient oxygen.
(ii) Nitrogen oxides (NOₓ):
Produced when nitrogen and oxygen in the air react under the high temperature and pressure conditions inside the engine.
(iii) Hydrocarbons (HC):
Unburnt or partially burnt fuel released into the exhaust due to engine inefficiency or fuel evaporation.
(1c)
(i) Carbon Monoxide (CO) is a colorless, odorless gas that binds to hemoglobin in the blood, reducing oxygen delivery to organs like the brain and heart. It causes symptoms such as fatigue, headaches, dizziness, confusion, chest pain, and at very high levels can lead to unconsciousness or death. It is especially harmful to people with cardiovascular disease and unborn babies during pregnancy.
(ii) Hydrocarbons contribute to the formation of ground-level ozone (smog), which can cause respiratory problems, eye irritation, and exacerbate asthma and other lung diseases.
(iii) Nitrogen Oxides contribute to smog and acid rain formation, and cause respiratory issues such as inflammation of the airways, reduced lung function, and increased susceptibility to respiratory infections.
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(2)
(i) An air-cooled system uses air to cool the engine, whereas a water-cooled system uses water or coolant to remove heat.
(ii) The air-cooled system relies on fins and airflow, while the water-cooled system uses a radiator, water pump, and hoses.
(iii) Air-cooled engines are generally lighter and simpler, whereas water-cooled engines are heavier but more efficient.
(iv) Maintenance is easier in air-cooled systems, while water-cooled systems require regular checks of coolant and related parts.
(v) Air-cooled systems are suitable for smaller engines like motorcycles, whereas water-cooled systems are used in cars and larger machines.
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(3)
(i) Friction:
Friction is the resistance to motion when two surfaces slide against each other, causing energy loss mainly as heat. It is essential in engines to provide grip but excessive friction leads to wear and reduced efficiency.
(ii) Viscosity:
Viscosity measures a fluid’s resistance to flow, indicating how thick or thin it is. Higher viscosity oils provide better lubrication under heavy loads but may cause more resistance at low temperatures.
(iii) Oiliness:
Oiliness refers to the ability of an oil to adhere to metal surfaces and form a protective film that prevents direct metal-to-metal contact. This property reduces wear and extends the life of engine components.
(iv) Atomization:
Atomization is the process of breaking fuel into fine droplets to mix thoroughly with air for efficient combustion. Proper atomization improves fuel burning, engine performance, and reduces emissions.
(v) Combustion:
Combustion is the rapid chemical reaction of fuel with oxygen releasing heat and light, which in engines produces expanding gases to move pistons. This process converts chemical energy into mechanical energy to power vehicles.
(vi) Flash Point:
Flash point is the lowest temperature at which a fuel vapor ignites momentarily when exposed to a flame or spark. It indicates the fuel’s flammability and safety during handling and storage.
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(4a)
(i) Direct Injection (DI) Combustion Chamber: In this type, fuel is injected directly into the main combustion chamber, which is usually located in a cavity on the piston crown. The combustion occurs entirely within the main cylinder. It provides better fuel economy and higher thermal efficiency due to less heat loss and faster combustion.
(ii) Indirect Injection (IDI) Combustion Chamber: In this type, fuel is injected into a separate pre-combustion chamber connected to the main cylinder. Initial combustion occurs in the pre-chamber, creating high-pressure gases that then enter the main chamber to complete combustion. This design allows for smoother and quieter operation but results in lower efficiency compared to DI systems.
(4bi)
(Draw the diagram)
(4bii)
(Draw the diagram)
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(5ai)
Axle Noise:
(i) Worn or damaged bearings.
(ii) Lack of proper lubrication in the axle housing.
(iii) Worn or damaged differential gears.
(5aii)
Gear Out of Mesh:
(i) Worn or broken gear teeth.
(ii) Misalignment of differential components.
(iii) Loose or damaged gear mounting or retaining parts.
(5aiii)
Noise When Cornering:
(i) Worn or damaged CV (constant velocity) joints or universal joints.
(ii) Broken or worn differential side gears.
(iii) Inadequate or contaminated differential oil.
(5b)
(i) Semi-floating axle shaft
(ii) Full-floating axle shaft
(iii) Three-quarter floating axle shaft
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(6)
(DRAW THE DIAGRAM)
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(7)
(i) Thermal Efficiency:
Thermal efficiency is the ratio of useful work output produced by an engine or system to the total heat energy input supplied to it. It measures how effectively the engine converts heat from fuel into mechanical work, usually expressed as a percentage or decimal fraction.
(ii) Bore:
Bore is the diameter of the cylinder in an engine, measured inside the cylinder walls. It determines the size of the piston and affects the engine’s displacement and capacity.
(iii) Stroke:
Stroke is the distance the piston travels inside the cylinder from its topmost position (Top Dead Center) to its lowest position (Bottom Dead Center). It influences the engine’s swept volume and power output.
(iv) Displacement:
Displacement is the total volume swept by all the pistons inside the cylinders during one complete engine cycle. It is calculated from the bore and stroke and indicates the engine size or capacity.
(v) Engine Capacity:
Engine capacity, also called engine displacement, is the total volume displaced by all pistons inside the cylinders during one full stroke cycle. It is often expressed in cubic centimeters (cc) or liters and reflects the engine’s power potential.
(vi) Compression Ratio:
Compression ratio is the ratio of the total volume of the cylinder when the piston is at the bottom dead center to the volume when the piston is at the top dead center. It indicates how much the air-fuel mixture is compressed before ignition, affecting engine efficiency and power.
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