Introduction to the Piston Engine and the Otto CycleAircraft & Engines — DGCA CPL practice questions

All 5 questions — Introduction to the Piston Engine and the Otto Cycle

Aircraft & Engines · DGCA CPL. The correct option is marked on each.

1. Q1. One complete four-stroke Otto cycle requires the crankshaft to rotate:

   • A.180°
   • B.360°
   • C.720°✓
   • D.1080°

   Why: Four strokes (induction, compression, power, exhaust) = two full crankshaft revolutions = 720°.

2. Q2. The piston engine is called a 'constant volume engine' because:

   • A.Cylinder volume never changes
   • B.Combustion occurs while the piston is momentarily stationary at TDC✓
   • C.The fuel/air ratio is fixed
   • D.Induction occurs at constant pressure

   Why: Ignition just before TDC means combustion takes place at nearly constant volume — unlike the gas turbine's constant-pressure Brayton cycle.

3. Q3. Valve overlap is the period when:

   • A.Both valves are closed
   • B.Both inlet and exhaust valves are partially open together✓
   • C.The inlet valve is open twice
   • D.Ignition occurs

   Why: Around TDC at the end of the exhaust stroke both valves are open; exhaust-gas momentum helps draw in the fresh charge, improving volumetric efficiency.

4. Q4. Brake Horsepower (BHP) is:

   • A.IHP plus FHP
   • B.IHP minus FHP✓
   • C.FHP minus IHP
   • D.Theoretical cylinder power

   Why: BHP = Indicated HP − Friction HP: the usable power actually delivered to the propeller shaft.

5. Q5. To improve efficiency, the inlet valve opens:

   • A.Exactly at TDC
   • B.Slightly before TDC and closes after BDC✓
   • C.Slightly after TDC and closes before BDC
   • D.Only during the power stroke

   Why: Valve lead and lag extend the induction period, increasing the charge drawn into the cylinder.