NABTEB 2023 MOTOR VEHICLE MECHANICS ESSAY AND OBJ ANSWERS

(1b)
(i)Gear Slipping
(ii)Hard Shifting
(ii)Strange Noises

(1c)
Sketch Coming

A brief description of the operation of a 3-speed sliding mesh with first gear engaged:

When the driver engages first gear on a sliding mesh gearbox, the countershaft gear and first gear on the main shaft are meshed together. This results in the transmission of power from the engine to the wheels at a low speed and high torque. The other two gears on the main shaft remain in mesh with their corresponding countershaft gears but do not rotate due to the method of sliding mesh engagement.

The sliding mesh gearbox operates by using a set of gears (usually three or four), each meshed to a corresponding gear on the output shaft. The different gears on the output shaft are selected by means of a gear stick that moves a selector fork. This, in turn, engages with a synchroniser sleeve located on the output shaft. The selector fork moves the sleeve into place, completing the meshing between the selected gear and the output shaft.

In summary, the engagement of first gear on a 3-speed sliding mesh gearbox results in the transmission of power from the engine to the wheels at a low speed and high torque, while the other gears remain in a neutral position until selected by the driver.
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(2a)
(i)Heat removal: The condenser is responsible for removing the heat from the refrigerant as it changes from its gaseous state to a liquid state. As the high-pressure refrigerant gas flows through the condenser coils, it comes into contact with cooler ambient air or water, and the heat is removed through the condenser walls. The cooled refrigerant condenses into a liquid, and the process is repeated in the evaporator coil.

(ii)Pressure reduction: The high-pressure refrigerant gas leaving the compressor is condensed into a high-pressure liquid in the condenser. The pressure of the liquid refrigerant is then reduced by the expansion valve, which allows the refrigerant to boil and evaporate in the evaporator coils. This pressure reduction is necessary for the refrigeration cycle to work since the lower pressure in the evaporator allows the refrigerant to absorb heat from the surrounding air or water.

(2b)
Dwell angle refers to the period of time (measured in degrees of distributor shaft rotation) during which the ignition points are closed in an internal combustion engine. It is the number of degrees that the distributor’s cam keeps the ignition points closed, allowing the ignition coil to build up maximum energy to create a spark when the points open again. The dwell angle determines the amount of time that the ignition coil has to build up energy, which affects the strength and duration of the spark. The correct dwell angle is important to ensure proper ignition timing and optimal engine performance.
(2c)Coming.
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(3)
(a)Octane Number: Octane number is a measure that indicates the resistance of a fuel to knocking in a spark-ignition internal combustion engine. It is a numerical value that represents the percentage of isooctane in a blend of isooctane and n-heptane that has the same knocking characteristics as the fuel being tested. The higher the octane number, the better the fuel’s resistance to knocking.

(b)Cetane Number: Cetane number is a measure of the ignition quality of diesel fuel. It is a numerical rating that indicates the delay between injection and ignition of the fuel in a compression-ignition engine. The higher the cetane number, the shorter the delay and the more efficient the combustion process, leading to reduced emissions of pollutants.

(c)Flash Point: Flash point is the lowest temperature at which a liquid fuel gives off enough vapors to form an ignitable mixture with air. It is an important safety parameter, as fuels with low flash points are more volatile and have a greater risk of fire or explosion. Flash point is often used to classify flammable liquids into different hazard classes.

(d)Swept Volume: Swept Volume is the volume displaced by the piston as it moves from top dead center (TDC) to bottom dead center (BDC) in a cylinder. It is a measure of the total amount of air or fuel-air mixture that can be drawn into the cylinder during the intake stroke.

(e)Brake Horse Power: Brake Horse Power (BHP) is a measure of the power output of an engine. It is the power delivered to the output shaft of the engine, minus the losses due to friction and other factors. BHP is often used as a means of comparing the power output of different engines.

(f)Indicated Horse Power: Indicated Horse Power (IHP) is the power developed inside the engine cylinder as a result of combustion. It is calculated by measuring the pressure within the cylinder during the power stroke and the volume displaced by the piston. IHP is often used as a means of determining the efficiency of an engine and comparing different engine designs.
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(4a)

(4b)
(i)Preparing the Vehicle: Before bleeding the brakes, ensure that the vehicle is in a secure and stable position. Park the vehicle on a flat surface with the parking brake engaged and chock the wheels to prevent movement. Then, locate the brake bleed valve located on the caliper or wheel cylinder.

(ii)Preparing the Brake Fluid and Tools: Gather the necessary tools and equipment required to bleed the brakes, including a brake bleeding kit, a wrench, and a container to catch the brake fluid. Also, ensure that you have enough fresh brake fluid to fill the reservoir after bleeding the brakes.

(iii)Removing Air from Brake Lines: Start with the brake farthest from the master cylinder and attach the bleeding kit to the brake bleeding valve. Invite an assistant to depress the brake pedal slowly and release it slowly to create pressure and release pressure in the brake system. This action will release the air bubbles from the system and eventually, clear brake fluid will start coming out of the valve.

(iv)Repeat the Process: Repeat the bleeding process for each brake starting from the farthest brake to the brake closest to the master cylinder. After completing each brake, check the master cylinder fluid level and top off with fresh fluid as needed.

(v)Testing the Brakes After Bleeding: To test the brakes after bleeding, release the parking brake and slowly apply the brakes. Check for a firm brake pedal, smooth braking action, and no sponginess or mushiness. If any abnormalities are noticed, repeat the process until the brakes function correctly.
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(5a)
Diagram

(i)Fully Floating Axle Support Shafts: In fully floating axle support shafts, the axle shaft is completely detached from the vehicle’s differential housing and is suspended by two bearings on the outside of the axle tube. The axle shaft provides only rotational force to the wheel, while the bearings support the weight of the vehicle and transmit the driving force to the wheels. These supports are most commonly used in heavy-duty applications, such as trucks and commercial vehicles, where the ability to isolate the load-bearing function from the driving function is important.

(ii)Three-Quarter Floating Axle Support Shafts: These types of axle support shafts use a bearing that is located in the differential housing. The wheel hub and the drum or rotor are attached to the end of the shaft. The shaft is not fully detached from the differential housing, but the weight of the vehicle is supported by the wheel bearings. These support shafts are commonly used in light-duty and passenger cars.

(iii)Semi-Floating Axle Support Shafts: In this type of support shafts, the wheel is attached directly to the end of the axle shaft. The axle shaft extends from the differential housing but is supported by a single bearing, which is located inside the axle tube. The weight of the vehicle is supported by the axle housing, and the shaft transmits the driving force to the wheels. Semi-floating supports are commonly used in light-duty and passenger cars.

(5b)
(i)To transmit the rotational force from the vehicle transmission to the drive wheels: The final drive unit converts the rotational force from the transmission into the torque that is used to turn the drive wheels. Depending on the specific vehicle design, this torque may be transmitted through a differential or a transfer case to either two or all four wheels.

(ii)To provide gear reduction: The final drive unit also provides a gear reduction that allows the engine to work more efficiently by translating high-speed, low-torque engine output into low-speed, high-torque output at the wheels. This increase in torque makes it easier for the vehicle to accelerate, tow heavy loads, climb hills, and perform other tasks that require a large amount of force.
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