cameroon gce intermediate level 2025 mechanical technology 2

 

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3. DIFFERENTIAL

• a) State the function of the differential unit.
  • Function: To allow the two output shafts (axles) to rotate at different speeds while still transmitting power from the driveshaft. This is crucial when a vehicle is turning, as the outer wheel must travel a greater distance than the inner wheel.
• b) Copy in your answer booklet and fill the table below referring to figure 3.

  | Ref | Name of Parts | Function |

  | :— | :— | :— |

  | 1 | Ring gear | Receives power from the pinion gear and transfers it to the differential case. |

  | 2 | Differential case | Houses the internal gears (pinion and side gears) and rotates the pinion gears around the side gears. |

  | 3 | Side gear | Splined to the axle shafts and drives the wheels. |

  | 4 | Drive pinion gear | Drives the ring gear, transferring power from the driveshaft. |

  | 5 | Spider gears/Pinion gears | Allow the side gears and axles to rotate at different speeds. |

  | 6 | Axle shaft | Transfers power from the side gears to the wheels. |

• c) Draw a schematic diagram of the differential unit above and on it; indicate the power flow using arrows.
  • A schematic diagram would show the power flowing from the driveshaft, through the drive pinion and ring gear, into the differential case and spider gears, and finally to the side gears and axle shafts.
• d) List two faults commonly associated with differential units and their remedies.
  • Fault 1: Whining or howling noise. This is often caused by incorrect gear mesh, worn gears, or worn bearings.
    • Remedy: Adjust gear backlash and alignment, or replace worn components.
  • Fault 2: Leaking lubricant. Caused by a damaged gasket or seal.
    • Remedy: Replace the gasket or seal.

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4. BRAKING SYSTEM

• a) State the role of a braking system in a vehicle.
  • Role: To slow down or stop a moving vehicle and to hold it stationary when required.
• b) Identify the following parts; 1, 2, 3, 4 and 5.
  • Figure 4A: Drum Brake System
    • 1: Brake drum
    • 2: Wheel cylinder
    • 3: Brake shoe
    • 4: Adjuster
    • 5: Backing plate
  • Figure 4B: Disc Brake System
    • 1: Disc brake rotor
    • 2: Caliper
    • 3: Brake pad
    • 4: Bleed screw
    • 5: Piston
• c) Briefly explain the operating principle of figure 4B.
  • Operating principle: When the driver presses the brake pedal, hydraulic pressure from the master cylinder forces the piston (5) within the caliper (2) to clamp the brake pads (3) against the rotating disc brake rotor (1). This friction converts the kinetic energy of the wheel into heat, slowing the vehicle down.

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2. PETROL INJECTION

• a) Identify the type of petrol injection system.
  • Type: Multi-Point Fuel Injection (MPI) or Electronic Fuel Injection (EFI).
• b) Copy and fill the table below.

  | Ref | Name of Parts | Function |

  | :— | :— | :— |

  | 4 | Fuel tank | Stores the fuel for the engine. |

  | 6 | Fuel rail | Distributes fuel under pressure to the individual injectors. |

  | 9 | Throttle body | Regulates the amount of air entering the engine in response to accelerator pedal input. |

  | 10 | Air flow sensor | Measures the amount of air entering the engine. |

  | 15 | Fuel injector | Sprays atomized fuel into the intake manifold or directly into the combustion chamber. |

• c) State two parameters required for the ECU to determine the quantity of fuel to be injected.
  • Parameters:
    1. Engine load/air mass (measured by the air flow sensor).
    2. Engine speed (RPM).
    3. Throttle position.
    4. Engine temperature.
• d) In case of failure of the part numbered 11, how will the engine be affected.
  • Part 11 is the idle air control (IAC) valve. If it fails, the engine’s idle speed will be affected. The engine may have a high idle, a low and rough idle, or may stall completely when idling, as the ECU cannot correctly regulate the amount of air bypassing the closed throttle plate.

 

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1. DIESEL ENGINE COMBUSTION CHAMBERS

• a) Identify the type of combustion chamber.
  • Type: Pre-combustion chamber (Indirect Injection)
• b) Give one advantage of forming a combustion chamber in the piston head.
  • Advantage: It allows for a more compact design and improved thermal efficiency by creating a squish effect which promotes better mixing of air and fuel. This is typically a direct injection design, and the image shows an indirect injection system with a separate pre-combustion chamber. The question seems to be based on a different type of engine than the one shown in the figure, so an answer based on the provided figure is a more direct interpretation. A pre-combustion chamber allows for the use of lower injection pressures and a less powerful injection pump.
• c) Copy and fill the table below referring to figure 1.

  | Ref | Name of Parts | Function |

  | :— | :— | :— |

  | 2 | Intake valve | Allows fresh air to enter the combustion chamber. |

  | 3 | Piston | Reciprocates in the cylinder, compressing air and transmitting power. |

  | 4 | Piston rings | Provide a seal between the piston and cylinder wall, preventing gas and oil leakage. |

  | 5 | Pre-combustion chamber | A small chamber where initial combustion occurs to ignite the main charge. |

  | 7 | Injector | Sprays atomized diesel fuel into the pre-combustion chamber. |

• d) During the operation of an engine equipped with the system above, a slapping noise is heard between the part 5 and 7;
  • i. What could be the cause?
    • The noise could be caused by the injector (7) nozzle hitting the wall of the pre-combustion chamber (5) due to improper mounting or a broken nozzle tip.
  • ii. Which instrument is used to adjust this problem?
    • A torque wrench would be used to ensure the injector is tightened to the correct specification, and specialized tools may be needed for injector alignment.