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Difficulty: MediumEngine Principles and Internal Components

During the compression stroke of a standard four-stroke internal combustion engine, what are the positions of the intake and exhaust valves, and what happens to the trapped air-fuel charge as the piston moves upward?

  1. A
    The intake valve stays open to regulate cylinder pressure while the air-fuel mixture is partially vented.
  2. Both valves remain tightly closed while the piston compresses the air-fuel mixture into the combustion chamber.Answer
  3. C
    The exhaust valve opens briefly to release residual exhaust gases as the air-fuel mixture expands.
  4. D
    Both valves stay open simultaneously to allow a richer fuel mixture to enter before ignition occurs.

Answer

Both valves remain tightly closed while the piston compresses the air-fuel mixture into the combustion chamber.
During the compression stroke of a four-stroke internal combustion engine, the crankshaft rotates to push the piston upward from Bottom Dead Center (BDC) toward Top Dead Center (TDC). To compress the trapped air-fuel mixture into the combustion chamber at the top of the cylinder, both the intake valve and exhaust valve must remain fully closed to maintain an airtight seal.

Step-by-Step Solution

1
Identify the four-stroke cycle phase described in the question.
The phase is the compression stroke, which occurs immediately after the intake stroke and immediately before the power stroke.
Determining the specific stroke defines the mechanical motion of the piston and the synchronized state of the valvetrain.
2
Determine the required valve positions for compression.
Both the intake valve and the exhaust valve must be closed and firmly seated against the cylinder head.
A sealed cylinder chamber is necessary to prevent the escape of the air-fuel charge and allow cylinder pressure to build as the piston moves upward from bottom dead center (BDC) to top dead center (TDC).

Key Concept

Four-stroke internal combustion engine cycle events and cylinder sealing during compression.
Estimated Time:45s
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Topics

Engine Principles and Internal Components

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