Kenya Certificate Of Secondary Education(KCSE 2013) Power Mechanics Paper 1 with Marking Scheme

SECTION A: (40 marks)
Answer all the questions in this section

1. List three factors to be considered when putting up a motor vehicle spare parts shop. (3 marks)
2. Explain two reasons why it is important to study power mechanics. (2 marks)
1. State the full terms represented by the following engineering drawing abbreviations:
  1. CL; ..........................................................................................................................
  2. Ø; ............................................................................................................................
  3. CSK; .......................................................................................................................
  4. A/F. ......................................................................................................................... (2 marks)
2. Name two classes of fire and for each class, identify one appropriate commercial fire extinguisher. (2 marks)
1. State two advantages of self-tapping screws over ordinary screws. (2 marks)
2. 1. Sketch an adjustable spanner. (1 mark)
  2. State where long nose pliers may be used in a small engine. (1 mark)
1. Explain one purpose of each of the following energy convertors in a motor vehicle:
  1. alternator; (1 mark)
  2. photo voltaic cells. (1 mark)
2. State two effects of adding each of the following alloying materials to carbon steel:
  1. Nickel; (1 mark)
  2. Molybdenum. (1 mark)
With the aid of sketches, differentiate between a 4 cylinder in line and a V-4 cylinder engine block. (4 marks)
Figure 1 shows a sectional view of a Wankel engine. Describe one cycle of its operation with reference to C and D. (4 marks)
1. Name the main components of the power transmission system of a motor vehicle. (2 marks)
2. Explain the reason why modern vehicles are designed with collapsible steering columns. (2 marks)
1. Briefly explain the process of hard soldering. (3 marks)
2. Explain the following terms as used in drum brake operation:
  1. leading shoe;
  2. trailing shoe. (2 marks)
1. State the purpose of the ply-rating of a tyre. (2 marks)
2. State two advantages of an independent suspension system over rigid beam suspension system. (1 mark)
Sketch a dipped beam light path having an offset filament and label its parts. (3 marks)

SECTION B: (60 marks)
Answer question 11 and any other three questions.

Figure 2 shows an isometric view of a Vee block resting on one side.

Draw full size, in first angle projection, the following views:
1. front elevation in the direction of arrow W;
2. end elevation in the direction of arrow X;
3. Plan.
  (Use A3 paper provided) (15 marks)
Figure 3 shows a component of the power transmission system of a motor vehicle.
1. Name the component. ( 1 mark)
2. Name parts labelled A to G. (3 1 marks)
3. Explain how the component operates. (11 marks)
With the aid of a labelled diagram, explain the operation of an overhead valve engine train whose camshaft is in the engine block. (15 marks)
With the aid of labelled diagrams, explain the operation of a four-stroke compression ignition system. (15 marks)
1. State three advantages of disc brakes over drum brakes. (3 marks)
2. Sketch a sectional diagram of a disc brake assembly and label six parts. (12 marks)

MARKING SCHEME

SECTION A (40 marks)

1. Factors to consider when putting up a motor vehicle, spare parts shop.
  - Areas/size the spare shop.
  - Number of motor vehicles within the chosen locality/demand for spare parts.
  - Nearness to garages.
  - Type of customers (small scale or large scale).
  - Nature of competition.
  - Accessibility.
  - Locality of suppliers.
  - Availability of staff/ labour.
  - Infrastructure in the locality.
  - Local authority incentives eg. tax rebate
  - Cost of building.
    Any 3 x 1 = 3 marks
2. Reasons for studying power mechanics.
  - It provides learners with knowledge, basic skills and attitudes used in industrial world to enable them to be market ready.
  - Prepares learners to be better consumer or producers of industrial goods and services.
  - To develop learner interest and talents that may lead to a vocation.
    Any 2 x 1 = 2 marks
1. Abbreviations
  1. CL - centre line
  2. Ø - diameter
  3. CSK - counter sink
  4. A/F - across flats. = 2 marks
2. Fire Extinguishers
  Class
  1. water hose
  2. carbon dioxide/ dry powder
  3. carbon dioxide
  4. foam extinguisher
    Fire class
    Extinguisher
1. Advantages of self-tapping screw
  - They cut their own thread, when screwed into drilled holes. Therefore they can be used in holes with blind threads.
  - They don’t get loose easily as ordinary screws do.
  - Can be used to drill holes on soft materials.
    any 2 x 1 = 2 marks
2. Uses of long nose pliers
  - Removing value retainer gudgeon pin circlip.
  - Any other insertion in closed areas i.e. small clips and washers insertion.
    any 1 x 1 = 1 mark
1. 1. Purposes of an alternator
    - Converts mechanical energy from the engine into electrical energy.
    - Keeps battery charged and handles electrical loads while engine is running.
      Any 1 x 1 = 1 mark
  2. Purposes of photo voltaic cells
    - They are used as detectors (e.g. flame detectors) because they are operated through illumination. Any 1 x 1 = 1 mark
      Nickel - improves strength
2. 1. - improves ductility
    - improves toughness
  2. Molybdenum
    - increases hardness
    - increases resistance to acids
Operation
1 - When only the exhaust port is open (as in the figure) the exhaust gases are forced out.
1 - Further rotation of the rotor opens the inlet port and closes the exhaust port.
1 - When ‘C’ closes the inlet port there is maximum intake of air/ fuel mixture the mixture is compressed into a very small volume.
1 - At maximum compression the mixture is ignited and the combustion force turn the rotor. The process is repeated for every face of the rotor.
(4 marks)
1. Components of power transmission system.
  - Clutch
  - Gearbox
  - Propeller shaft
  - Differential unit
  - Half shafts
  - Final drive
    Any 4 x = 2 marks
2. Modern vehicles are designed with collapsible steering columns as a protective measure. The steering column is made of two parts which are fitted together in such a way that they can ‘telescope’ as the steering column collapses during a head-on collision, which throws the driver forward. In the process, the steering column will absorb the energy of this forward movement and reduce the possibility of injury. (2 marks)
1. Hard soldering also called silver brazing is the type of soldering done using solder which melts at high temperatures √ 1 (≈ 600º C and 750º C)
  - Clean the surface
  - Apply flux.
  - Preheat the soldering bit
  - Apply the correct soldering rod and heat dipped in flux.
  - Allow to cool and remove unwanted solder.
2. 1. ‘Leading’ shoe - it rotates in opposite direction to the brake disc. It provides 75% of the braking. √ 1
  2. ‘Trailing’ shoe - it rotates in same direction to the brake disc. It provides 25% the braking force. √ 1
    2 x 1 = 2 marks
1. Purpose of ply-rating of a tyre.
  It indicates the load range √ 1 and inflation limit √ 1 of the tyre. 2 x 1 = 2 marks
2. Independent suspension system.
  - provides more stability.
  - lowers engine position or centre of gravity.
  - effects on one wheel are not transmitted to the other wheel.
  - Provides a more comfortable ride.
    (= 1 mark)
- FRONT ELEVATION
  Hidden details = 4 marks
- END ELEVATION
  1 face @ 1 = 1 mark
  2 T-sections = 2 marks
  V section = 1 mark
  = 4 marks
- PLAN
  3 faces @ 2
  Hidden details = 3 marks
  Correct angle of projection = 2 marks
  Proportionality = 1 mark
  Neatness = 1 mark
  = 4 marks
Solution
1. Differential unit ( 21 mark)
2. 1. Pinion/ or propeller shaft
  2. Crown wheel
  3. Differential casing
  4. Planet wheel or differential pinions
  5. Spider shaft
  6. Sun wheel or side gears
  7. Half shaft
    (21 marks)
3. - When vehicle is running straight propeller shaft turns the ring gear pinion 1 shaft. The pinion turns the ring gear, which in turn revolves the differential casing 1 . When the casing turns, the differential pinion shaft turns with it 1 .
    As the differential pinions are mounted on this shaft, they are forced to move with the case 1 . Being meshed with the axle side gears, the pinions will pull the axle side gears along with them. Therefore, when the car is moving straight, the ring gear is spinning. The differential unit is moving like a solid unit 1 because there is no movement between the teeth of the pinions and the axle side gears 1 .
  - When vehicle is taking a turn. The outer wheel has to move a greater radius than the inner wheel and must therefore be speeded to take the same time as the inner wheel 1 . As the inner wheel is slowed when vehicle is turning, this increases resistance of its sun wheel 1 . The spider shaft is still being turned end over end at the crown wheel speed, and as the inner sun wheel slows the planet wheels are forced to rotate (or 1 walk) on the spider shaft and about the inner sun wheel. In so doing, the speed of the outer sun and the outer road wheel is increased in the same proportion as the speed of inner sun is reduced 1 . The torque is still divided equally 1 between the two half shafts but their speeds are different.
  - When one wheel slips while the other is stuck to the ground.
  - The casing continues spinning the pinions but they will merely walk around 1 the stopped axle gear and impart the torque 1 to the spinning axle.
Drawing = 4 marks
Labelling any 6 = 3 marks
7 marks

Operation
The crankshaft turns the camshaft. √ 1
As the camshaft is turned, the camlobe reaches an upright postion 1 and lifts the tappet 1 .
The tappet then pushes up the push rod 1 . The pushrod movement makes the rocker arm to shivel 1 in its pivot at the rocker arm shaft thus pushing the valve downwards to open it 1 at the same time compressing the valve spring 1 .
As the valve opens, gases are allowed to either enter or exit the cylinder.
Further rotation of the camshaft makes the cam to release pressure on the tappet and so both the tappet and the push rod and the valve are returned to normal position 1 by the pressure of the released valve spring. (8 marks)
A (intake)
- piston moving down
- intake valve open 1
- exhaust valve closed
- air drawn in 1 (3 marks)
  
  B (compression)
- Both valves close 1
- Piston moving up
- Air compressed 1
  (2½ marks)
  
  C (power)
- Fuel injected in 1
- Both valves closed
- Piston moves downwards
  
  Exhaust gases expelled 1 (2 ½ marks)
  Diagram - 4 parts at 4 x ½ = 2 marks
  Labelling 4 x ½ = 2 marks TOTAL 11 marks
1. Advantages of disc brakes
2. Exposed to the atmosphere hence better at heat dissipation.
  - Are self-adjusting.
  - Are light in weight.
  - Are self cleaning on the surface due to the centrifugal forces.
  - Slower wear.
    (Any 3 x 1 = 3 marks)
    Correct and complete diagram - 8 marks
    (with 8 main component s)
    Labelling 6 parts - 6 x ½ = 3 marks
    Neatness and proportionately 1 mark
    12 marks