KCSE 2015 Power Mechanics Paper 1 with Marking Scheme

SECTION A (40 marks)
Answer all the questions in this section in the spaces provided.

1.    
   1. Define a “machine”. (1 mark) 
   2. State two reasons for having a first aid box in a vehicle.(2 marks)
2.    
   1. State three functional characteristics of a road wheel. (3 marks)
   2.      
      1. Define a business plan. (1 mark)
      2. List four components of a business plan. (2 marks)
3.      
   1. A piston ring fitted in a cylinder has a gap clearance of 0.25 mm at room temperature, Explain what happens to the gap clearance when the ring is hotter than the cylinder. (2 marks)
   2. Name the parts labelled A, B, C and D on the fastener in figure 1. (2 marks)
      
      
      1. .............
      2. .............
      3. .............
      4. ............. 
4.    
   1. State the purpose of each of the following tools:(1 mark)
      1. reamer;
      2. tap (1 mark) 
   2. State two factors to consider when selecting a spanner for a motor vehicle repair task. (2 marks)
5.    
   1. State four applications of a photovoltaic cell. (2 marks)
   2. List four types of lighting circuits of a motor vehicle. (2 marks)
6.    
   1. State two functions of the final drive. (2 marks)
   2. Sketch the symbols used to represent the following in assembly drawings: (2 marks)
      1. a square;
      2. a countersunk.
7. Outline three properties of soldering flux. (3 marks)
8.    
   1. List four types of manual steering gearboxes. (2 marks)
   2. Name four types of springs used in vehicle suspension systems. (2 marks)
9. State four purposes of lubricating oil additives. (4 marks)
10. Outline eight components of a pressurized water cooling system of a motor vehicle. (4 marks)

SECTION B (60 marks)
Answer question 11 on A, paper and any other three questions from this section in the spaces provided. Candidates are advised to spend not more than 25 minutes on question 11.

11. Figure 2 shows a machine block drawn in isometric projection.
    
    Draw full size in 3rd angle projection the following views:
    1. front elevation in direction "F";
    2. end elevation in direction “E”;
    3. plan. (15 marks)
12. The figure 3 shows a disc brake assembly:
    
    
    1. Name the parts labelled a, b, c, d, e and f. (3 marks)
    2. Explain the operation of the system when: (5 marks) 
       1. brakes are applied
       2. brakes are off.(7 marks)
13.    
    1. State three functions of the exhaust system. (3 marks)
    2.        
       1. State the cause of back pressure in an exhaust system. (1 mark)
       2. Explain the consequence of excessive back pressure. (2 marks)
    3. With the aid of a labelled cross-sectional sketch, describe the construction of an absorption silencer. (9 marks)
14.      
    1. State three operational differences between petrol and diesel engines. (3 marks)
    2. Compare a petrol and a diesel engine of the same size with reference to:
       1. weight;
       2. running costs;
       3. emissions;
       4. thermal efficiency;
       5. compression ratio;
       6. operating temperatures.(12 marks)
15.    
    1. State four functions of oil in an engine. (4 marks)
    2. Describe the following lubrication methods used in different engines: (11 marks)
       1. splash feed;
       2. pressure feed.

MARKING SCHEME

SECTION A

1.      
   1. A 'machine' is a device used to make work easier. (1) 1 mark
   2. Two reasons for a first aid box in a vehicle (2 marks)
      1. To enable the occupants attend to emergency treatment like dressing a patient.
      2. To Avail emergency drugs for common diseases and reduce.
         • pain
         • effect of injury
      3. Legal requirement in a country. (any 2 x 1 = 2 marks)
2.    
   1. Three functional characteristics of a road wheel:
      1. be strong to withstand weight of vehicle
      2. be able to absorb road shocks/make the ride comfortable
      3. be able to maintain grip on road surface
      4. be light in weight
      5. be easy to remove and clean
         (any 3 x 1 = 3 marks)
   2.    
      1. a business plan is a document that shows a road map to owning and operating a business.
         or
         a document that describes the goals and objectives of a business and clearly shows how and when they will be achieved. (1 mark)
      2. Four components of a business plan
         • Title
         • Executive summary
         • Business description
         • Marketing plan
         • Organization and management plan
         • Production/operation plan
         • Financial plan
           (any 4x1 = 2 marks)
           (2 marks)
3.    
   1. hotter than cylinder
      - gap reduces thus
   2.      
      1. diameter
      2. pitch
      3. threads
      4. head
         (any 4x1 = 2 marks)
4.      
   1.      
      1. Reamer is used to enlarge or shape holes(1 mark)
      2. Tap is used for cutting internal threads (1 mark) 
   2.      
      1. Strength/torque to be used on task
      2. Size of bolt/nut to be removed
      3. Location of bolt/nut to be removed
      4. Material of spanner
      5. Possibility of combination of the spanner
         (Any 2 x 1 = 2 marks)
5.      
   1. Four applications of a photo voltaic cell
      1. in light meters
      2. in space craft power supplies
      3. in automatic operated switches
      4. in burglar alarms
         (4x1 = 2 marks)
   2. Four types of lighting circuits
      1. parking
      2. tail
      3. braking
      4. directional
      5. back-up
      6. number plate
      7. panel instrument
      8. headlight
      9. courtesy
      10. beef and bonnet
      11. spotlights
          (any 4 x 1 = 2 marks)
6.    
   1. Functions of a final drive:
      1. To change direction of the drive through 90°
      2. To increase the torque of the drive by reducing the speed
         (2 x 1 = 2 marks)
   2.      
      1. square
         
      2. countersunk
         
         (2 x 1 = 2 marks)
7. Three properties of soldering flux
   1. should be fluid at soldering temperatures
   2. should remove any oxide filing from surfaces
   3. should prevent re-oxidation during soldering
   4. should be easily displaced by molten solder
   5. should form a non-corrosive residue when the soldering process is complete
      (any 3 x 1 = 3 marks)
8.    
   1. Four types of manual steering gear boxes
      1. worm and sector
      2. rack and pinion
      3. recirculating balls
      4. worm and wheel
         (any 4 x < = 2 marks)
   2. Four types of springs used in vehicle suspension system
      1. leaf
      2. coil
      3. torsional bal
      4. air
      5. stabilizer bar
      6. rubber
         (any 4x1 = 2 marks)
9. Four purposes of lubricating oil additives:
   1. to prevent carbon formation
   2. to prevent oxidation
   3. to suppress foam
   4. to prevent corrosion
   5. to prevent rusting
   6. to prevent acid formation
   7. to prevent wear
   8. antifreeze
      (any 4 x 1 = 4 marks)
10. Eight cooling system components
    1. radiator cap
    2. radiator
    3. hoses
    4. water pump
    5. fan
    6. thermostat
    7. water jackets
    8. temperature gauge
    9. overflow tank
    10. sensor
        (any 8x1 = 4 marks)

SECTION B

11. 
    
12.      
    1.    
       1. disc
       2. piston
       3. inlet pipe
       4. caliper
       5. pad
       6. Seal
          (= 3 marks)
    2. Operation
       1. Brakes applied (5 marks)
          • 1 - Driver applies the brake pedal
          • 1 - Brake fluid forces the piston to move towards the disc
          • 1 - On driver applying further pressure leads to distortion of the seal
          • 1 - The piston is used to push the brake pads to make contact with the disc
          • 1 - This slows/stop the vehicle
       2. Brakes off (7 marks)
          • 1 - The driver releases the brake pedal
          • 1 - The seal regains its original shape
          • 1 - The piston is withdrawn from disc
          • 2 - Maintaining a light contact between the pad and disc due to static head of brake fluid
          • 2 - When wear occurs the piston moves slightly through the seal and takes new position in the cylinder (7 marks)
13.    
    1.      
       • To clear exhaust gases from the engine to the atmosphere
       • To muffle or reduce exhaust noises
       • To remove a lot of heat from the engine
       • To convert poisonous gases to less harmful gases
         (any 3 x 1 mark = 3 marks)
    2.    
       1. back pressure is caused by restriction of the free flow of exhaust gases (1 mark)
       2. Excessive restriction causes high back pressure resulting in incomplete scavenging of exhaust gases (1). This in turn causes loss of engine power and increased fuel consumption (1) (2 marks)
    3. 
       
       Sketch - 2 marks
       Labelling - 4 x ½ marks
       Direction of exhaust - 1 mark
       5 marks
       • Silencer consists of a perforated inner pipe (1)
       • Enclosed in and welded on a cylindrical casing (1)
       • Which is about 3 times larger in diameter (1)
       • The casing is filled with absorbent material like wool, asbestos fibre or (1) glass (4 marks)
14.       
    1.         
       • Petrol intake stroke takes in air fuel mixture while diesel takes in air
       • Petrol engine compresses air-fuel mixture while diesel compresses air alone
       • Diesel engine fuel is used to ignite hot compressed air while in diesel engine a spark ignited the compressed air fuel mixture (3 x 1 = 3 marks)
    2. 
       

       	Petrol	Diesel Engine
       (i)Weight	lighter	heavier
       (ii) Running cost	expensive	cheaper
       (iii) Emissions	less pollutant	more polutant
       (iv) Thermal efficiency	less	higher
       (v) Compression ratio	lower	higher
       (vi) Operating temperature	lower	higher
       	6 x 1 mark	6 x 1 mark

       (12 marks)
15.      
    1.      
       • oil lubricates the engine
       • oil cools the engine
       • oil cleans the engine
       • oil seals the engine (4 x 1 mark = 4 marks)
    2.    
       1. Splash feeds
          • It consists of either an oil dipper or oil slinger
          • It is commonly used in single cylinder engines with the dipper fastened to the connecting rod to ensure adequate splash lubrication
          • The rotating crankshaft splashes oil on the bearing surfaces as it rotates in the bath of oil in the crank case
          • In case of a slinger, it is driven by the cam gear situated in the oil bath and sling the oil to lubricate the required areas (5 marks)
       2. Pressure feed
          • The oil pump delivers oil under pressure into the gallery pipe which extends the full length of the crank case
          • Each main bearing is connected to this gallery by holes drilled through the crankcase webs and similar drilled holes are used to connect the crankshaft bearings to the gallery
          • All these drilled holes enable oil under pressure to lubricate all these parts at the same time
          • Connections are made from the gallery to the oil pressure gauge or warning lamp switch, and to the rocker shaft of the overhead valve gear. (6 marks)