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Difficulty: EasyEngine Principles and Internal Components

During which stroke of a four-stroke internal combustion engine do both valves remain closed while the spark plug ignites the compressed air-fuel mixture to push the piston downward?

  1. A
    Intake stroke
  2. B
    Compression stroke
  3. Power strokeAnswer
  4. D
    Exhaust stroke

Answer

The power stroke is the phase where ignition occurs and expanding combustion gases force the piston downward with both valves closed.
During the power stroke (also called the combustion or expansion stroke), both the intake and exhaust valves are closed to seal the combustion chamber. Ignition of the compressed air-fuel mixture creates rapid expansion, forcing the piston down the cylinder bore and transferring mechanical power to the crankshaft.

Step-by-Step Solution

1
Identify the specific mechanical events described in the question
Both valves are closed, ignition occurs at the spark plug, and high pressure drives the piston downward.
This step converts the fuel's chemical energy into usable mechanical work.
2
Match these operating conditions to the correct stage of the four-stroke cycle
Power stroke
The four strokes are intake (intake valve open, downward travel), compression (valves closed, upward travel), power (valves closed, ignition, downward travel), and exhaust (exhaust valve open, upward travel).

Key Concept

Four-stroke internal combustion engine cycle phases
Estimated Time:45s
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Topics

Engine Principles and Internal Components

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