Automechanical Works Paper 2 WASSCE (SC), 2021

SECTION A

(a)        The table below shows the firing order of a complete cycle for a four-
                        cylinder, four-stroke engine.                                     

 

Cylinder

 

Firing Order

 

1

3

4

2

1

Power

 

Induction

 

2

 

Induction

 

Power

3

Compression

 

 

Induction

4

Induction

Compression

 

 

 

 

 

 

 

 

 

 

Complete the cells under the firing order with the missing strokes.

(b)        Explain the following terms, as applicable to four-stroke compression ignition engine:
(i)         induction stroke;         (ii)        power stroke.
(c)        What is the name of the automobile engine valve that has larger diameter?

Observation

 

Candidates who attempted this question did appreciably well. The attempt on the   
question was good; however, the respondents could not
fare well in 4(b).The solution to the poorly answered question is given below.

  1. (i)        Induction Stroke
    During this stroke, the inlet valve opens the piston
    move down to bottom dead centre, creating a partial
    vacuum that draws air into the cylinder.

                                                                                                                  

(ii)            Power Stroke
In the power stroke of a compression ignition engine, both valves are closed and  fresh air charge is compressed, which results to increased temperature and pressure. The fuel is sprayed at high pressure and combustion starts. The high increase of temperature and pressure forces the piston down during the power stroke.